Introductionto the fundamentals of aircraft design is provided. These include the definition of the mission of the aircraft and various requirements, such as performance, handling, manufacturing, certifiability, upgradability, maintainability, and many others. This is followed by a general description of the aircraft design process and a general definition of a number of regulatory terms that are frequently encountered in industry. These are of particular importance to the entry level aerospace engineer who usually has a very limited exposure to this part of engineering. Then, a specific aircraft design algorithm is presented whose purpose is to guide the aircraft designer through the conceptual design process. This is prepared in a “what to do next” format and is based on actual industry experience and are not academic “cookbook” approaches. This information is followed by an introduction of selected project management tools. Many beginning project leaders are often at a loss as to how to manage a project, warranting the discussion of the tools. Project management revolves around knowing what to do and when to do it. Thus, the manager must construct a chronological order of the tasks that need to be completed. Tools, such as Gantt and Ishikawa diagrams and House of Quality, are described. Finally, helpful approaches to describing engineering ideas using graphics ranging from three-view drawings, solid modeling, FEA, CFD, engineering reports and drawings, as well as composite photo images are discussed.
Mission definition; regulations; FAA; EASA; FAR; CS; LSA; design process; design algorithm; Gantt; Ishikawa; project management; House of Quality; three-view drawings; solid modeling; FEA; CFD; composite images; cutaway drawings; engineering reports; engineering drawings
1.1.1 The Content of this Chapter
1.1.2 Important Elements of a New Aircraft Design
Performance Requirements and Sensitivity
Handling Requirements (Stability and Control)
Features and Upgradability (Growth)
Lean Engineering and Lean Manufacturing
Integrated Product Teams (IPT)
Fundamental Phases of the Aircraft Design Process
1.2 General Process of Aircraft Design
1.2.1 Common Description of the Design Process
Elementary Outline of the Design Process
Typical Design Process for GA Aircraft
1.2.2 Important Regulatory Concepts
Supplemental Type Certificate (STC)
Standard Airworthiness Certificate (AC)
Special Airworthiness Certificate (SAC)
Technical Standard Order (TSO)
Technical Standard Order Authorization (TSOA)
Parts Manufacturer Approval (PMA)
1.3.1 Conceptual Design Algorithm for a GA Aircraft
1.3.2 Implementation of the Conceptual Design Algorithm
1.4 Elements of Project Engineering
1.4.2 Fishbone Diagram for Preliminary Airplane Design
1.4.3 Managing Compliance with Project Requirements
1.4.4 Project Plan and Task Management
1.4.5 Quality Function Deployment and a House of Quality
Step 2: Technical Requirements
Step 4: Interrelationship Matrix
1.5 Presenting the Design Project
Images Using Finite Element Modelers
Images Using Computational Fluid Dynamics Software
What is a design? It is probably appropriate to begin a book on design by discussing the term itself, especially considering the concept is often erroneously defined and sometimes even characterized through zeal rather than a true understanding of its meaning. The author recalls a past interview with a renowned designer who, during a TV interview, was asked to define the term. The show that ensued was a disappointing mixture of superficial self-importance and an embarrassing unpreparedness for the question. Following an artful tip-toeing around the issue the concluding response could be summarized as; “well, everything is designed.” No, nothing could be further from the truth: not everything is designed. Some things are designed while other things are not. When self-proclaimed designers have a hard time defining the term properly it should not be surprising when laypeople misuse the word and apply it to things that are clearly not designed. The least we can expect of any designer is to accurately define the concept to laypeople, some of whom have openly demonstrated an inability to distinguish a regular pattern from a design.
Any attempt at defining the word properly requires an insight into how the brain perceives the geometry that surrounds us. It is a question of great intrigue. How can the brain tell apart a clamshell and a cloud, or a raccoon and a road? This is achieved using the brain’s innate ability called rapid pattern recognition, common to all animals. It is one of the most important biological traits in any species that relies on optics as a primary sensory organ. In fact, this ability, a consequence of a biological evolution lasting over hundreds of millions of years, is imperative to the survival of the species. Its most important strength is that it allows animals to make a distinction between the facial features of a mother and the silhouette of a dangerous predator (for instance, see ).
If you see a face when you look at the front end of a typical car, or the silhouette of people or animals when looking at rock formations or clouds, you should know that this is your brain’s pattern recognition subroutine working overtime. It is desperately trying to construct a recognizable image from any pattern that hits the retina to help you quickly identify friends from foes. The faster a member of a species can accomplish this feat, the greater is the chance it may escape a dangerous predator or identify a concealed prey, providing a clear evolutionary advantage. It helps a falcon see a rodent from great heights as much as it helps an antelope identify a lurking lion. However, just as rapid pattern recognition is capable of discerning predator and prey; it can also play tricks with the brain and cause it to assemble random patterns into images of easily recognizable things that are simply not there. This condition is called apophenia. The lack of public education on this elementary biological function is stunning and renders some laypeople altogether incapable of realizing that the interplay of dark and light areas on their toast or potato chip that looks like their favorite celebrity is not a design but only a random pattern the brain has managed to assemble into a recognizable shape. Deprived of knowledge to know any better, many yield to wishful thinking and allow the imagination to run wild.
In short, a regular pattern is a combination of geometrical, physical, or mathematical features that may or may not be random, but “appears” either repetitious or regular through some characterization, such as learning. In fact, our environment is jam-packed with regular patterns. The repetition (or regularity) of a pattern allows the brain to separate it from the truly random background. People, familiar with the term “design,” erroneously deduce that since a pattern appears to be regular it must be designed, when in fact it is not. A design is a pattern of geometrical, physical, or mathematical features that is the consequence of an intent and purpose. A design requires an originator who intended for the pattern to look a certain way so it could serve its proposed purpose. This way, a design is a subset of regular patterns and one that has a preconceived goal, requires planned actions to prepare, and serves a specific purpose.
Consider the natural shapes in Figure 1-1. The mountain range to the left was not designed but formed by the mindless forces of nature. There was no preconceived plan that the range should look this way and not some other way. It just formed this way over a long time – it is a random but repetitious pattern. The contrails that criss-cross the sky over the Yosemite National Park, in the right image, were not planned either. They are a consequence of random departure times of different airplanes in different parts of the USA, headed in different directions at different altitudes. While the arrangement of the airway system is truly designed it was not conceived with the contrails in mind but for a different purpose altogether. No one planned the airway system so this pattern would form over El Capitan in this fashion and not some other. No one was ever tasked with figuring out that this particular day the winds aloft would allow the pattern to stay so regular. Its appearance is nothing but a coincidence. The geometry of the contrails, just like the mountain range in the left image, is the consequence of random events that were not designed. Claiming these are designs, automatically inflicts a burden-of-proof obligation on the petitioner: Show the plans, the originator, and explain the purpose and, if unable to, simply call it by its proper name until such plans surface: a pattern is a pattern until it can be shown to be a design.
FIGURE 1-1 Examples of random patterns. The mountain range to the left is shaped by the random forces of nature. The pattern of contrails over the Yosemite National Park is the consequence of random departure times of the aircraft involved that are influenced by random decisions of air traffic controllers. There is absolutely no intentional intelligence that forms these shapes. They just appear that way. (Photos by Snævarr Guðmundsson)
With the philosophy of design behind us, we can now focus on the primary topic of this book – the design of aircraft, in particular General Aviation aircraft. According to the Federal Aviation Administration, the term General Aviation aircraft (from here on called GA aircraft) refers to all aircraft other than airlines and military operations . This includes a large body of aircraft, ranging from sailplanes and airships to turbofan jets. Most aircraft are designed to comply with strict regulatory standards. In the USA these are managed and maintained by the Federal Aviation Administration (FAA). In Europe the standards are set by the European Aviation Safety Agency (EASA). These standards are similar in most ways, which results from an effort between the two agencies to harmonize them. Table 1-1 lists a number of standards for selected classes of aircraft.
Certification Basis for Several Classes of Aircraft
|General Aviation||14 CFR Part 23 (USA)
|Commercial Aviation||14 CFR Part 25 (USA)
|Sailplanes||14 CFR 21.17(b) (USA)
|14 CFR 21.17(b) allows the FAA to tailor the certification on a need-to basis to sailplanes. Then, by referring to AC 21.17-2A, the FAA accepts the former JAR-22 as a certification basis, which have now been superceded by CS-22.|
|Airships||14 CFR 21.17(b) (USA)
CS-30 and CS-31HA
|14 CFR 21.17(b) allows the FAA to tailor the certification on a need-to basis to airships.|
|Non-conventional Aircraft||14 CFR 21.17(b) (USA)
|14 CFR 21.17(b) allows the FAA to tailor the certification on a need-to basis to non-conventional aircraft.|
|Light Sport Aircraft (LSA)||Consensus (USA)
|See discussion below regarding LSA acceptance in the USA.|
In the USA, a light sport aircraft (LSA) is treated differently from an aircraft certified to 14 CFR Part 23 or 25. Instead of a direct involvement in the certification process, the FAA accepts compliance based on so-called consensus standards. These standards are neither established nor maintained by the agency itself but by some other organization. Some of these are really “watered down” FAA rules that are far less burdensome to comply with than the originals. This can partially be justified on the basis that the airplanes they apply to are much simpler than regular aircraft.
The acceptance of consensus standards (LSA) is effectively based on the “honor system.” In other words, a manufacturer tells the FAA its product complies with the applicable standards and, in return, receives an airworthiness certificate. This is done as long as no “issues” surface. The system is a form of “self-regulation” and is designed to keep the FAA out of the loop. The LSA industry recognizes that responsible compliance is the only way to avoid more burdensome regulations. According to FAA officials in 2012, this system has been more or less problem free, excluding one instance .
Currently, the American Society for Testing and Materials (ASTM) is the primary organization that establishes and maintains consensus standards for LSA. ASTM has developed a number of standards that apply to different types of aircraft. The FAA accepts some of these in lieu of 14 CFR. Which standard ultimately depends on the subclass of aircraft (aircraft, glider, gyrocopter, lighter-than-air, powered parachutes, and weight-shift control) and on a number of specific fields (design and performance; required equipment; quality assurance; and many others). For instance, for the subclass aircraft, design and performance is accepted if it complies with ASTM F2245, required equipment must also comply with ASTM F2245, but quality assurance must comply with ASTM F2279, maintenance and inspection with ASTM F2483, and so on. Gliders, gyroplanes, and other light aircraft must comply with different ASTM standards. The matrix of requirements can be obtained from the FAA website .
While this book will mostly focus on the design of new GA aircraft, other classes of aircraft will be discussed when needed. The designer of GA aircraft should be well rounded in other types of aircraft as well, a point that will be made repeatedly throughout this book.
GA aircraft certified under 14 CFR Part 23 are subject to a number of limitations as stipulated under 14 CFR Part 23.3-Airplane categories. The regulations place aircraft to be certified into four categories; normal, utility, aerobatic, and commuter. These categories must abide by the restrictions listed in Table 1-2. With the exception of the commuter category, an aircraft may be certified in more than one category provided all requirements of each are met.
Restrictions for Aircraft Classes Certified under 14 CFR Part 23
W = maximum T-O weight. Maneuvering loads are based on 14 CFR 23.337.
A whip stall may occur when the airplane is stalled while in a slip. This can cause the outer wing to stall first and drop abruptly .
New aircraft are designed for a variety of reasons, but most are designed to fulfill a specific role or a mission as dictated by prospective customers. For economic reasons, some aircraft (primarily military aircraft) are designed to satisfy more than one mission; these are multi-role aircraft. Others, for instance homebuilt aircraft, are designed for much less demanding reasons and are often solely based on what appeals to the designer.
No matter the type of aircraft or the reason for its design, specific tasks must be completed before it can be built and flown. The order of these tasks is called the design process. This process is necessitated by the fact that it costs a lot of money to develop a new aircraft. Organizations that develop new aircraft do not invest large amounts of funds in a design project until convinced it can perform what it is intended to. A design process makes this possible by systematically evaluating critical aspects of the design. This is primarily done using mathematical procedures, as well as specific testing of structural configuration, materials, avionics, control system layout, and many more.
The order of the tasks that constitute the design process may vary depending on the company involved. Usually there is an overlap of tasks. For instance, it is possible that the design of the fuselage structure is already in progress before the sizing of the wing or stabilizing surfaces is fully completed. Generally, the actual process will depend on the size and maturity of the company in which it takes place and the order of tasks often varies. However, there are certain steps that must be completed in all of them; for instance, the estimation of weight; sizing of lifting surfaces and the fuselage; estimation of performance; and other essential tasks.
In mature companies, the design process is managed by individuals who understand the big picture. They understand the scope of the project and are aware of the many pitfalls in scheduling, hiring, design, and other tasks, that many engineers consider less than glamorous. These people must be well-rounded in a number of disciplines: aerodynamics; performance analysis; stability and control; handling; power plants; weight analysis; structural layout; environmental restrictions; aviation regulations; history of aviation; and aircraft recognition, to name a few. Although not required to be an expert in any of these fields, their understanding must be deep enough to penetrate the surface. Knowing what to do, how to do it, and when to do it, is the key to a successful aircraft development program.
• Section 1.2 presents a general description of the aircraft design process.
• Section 1.3 presents two specific algorithms intended to guide the aircraft designer through the conceptual design process. If you are unsure of “what to do next,” refer to these. They are based on actual industry experience and are not academic “cookbook” approaches.
• Section 1.4 presents project management tools. Many beginning project leaders are often at a loss as to how to manage a project. If this is your predicament you need to study these tools. Project management revolves around knowing what to do and when to do it. Thus, the manager must construct a chronological order of the tasks that need to be completed.
• Section 1.5 presents helpful approaches to describing engineering ideas using graphics ranging from three-view drawings to composite photo images. These are extremely helpful when trying to “sell” an idea.
Before going further, some specific topics must be brought up that the lead airplane designer must introduce and discuss thoroughly with the design team. Among those are:
It is imperative that the mission of the new aircraft is very clearly defined. Is it primarily intended to serve as a cruiser? If so, what airspeed and cruising altitude is it most likely to see during its operation? Is it a cargo transport aircraft? How much weight must it carry? How fast, far, and high shall it fly? Is it a fighter? What energy state or loitering capabilities are required? The mission must be clearly defined because the airplane will be sized to meet that particular mission. An aircraft designed in this fashion will be most efficient when performing that mission. Clarity of this nature also has an unexpected redeeming power for the designer: It is very common during the development of aircraft that modifications to capabilities are suggested by outside agencies. In spite of being well meant, some such suggestions are often detrimental to the mission. A clearly defined mission allows the designer to turn down a disadvantageous suggestion on the basis that it compromises the primary mission.
Performance requirements must be clearly defined and are usually a part of the mission definition. It is imperative to quantify characteristics such as the take-off distance, time to cruise altitude, cruise range, and even environmental noise for some types of aircraft. But it is also important to understand how deviations from the design conditions affect the performance. This is referred to as performance sensitivity. How does high altitude and a hot day affect the take-off distance? How about the upward slope of the runway? How does having to cruise, say, some 5000 ft below the design altitude affect the range? How about if the airplane is designed for a cruising speed higher than would be permitted by air traffic considerations and, therefore, is consistently operated at a lower cruising speed? How will that affect the range? Clearly, there are many angles to designing an aircraft, but rather than regarding it as a nuisance the designer should turn it into strength by making people in management and marketing aware of the deficiencies. And who knows – perhaps the new aircraft is less sensitive than the competition and this could be turned into a marketing advantage.
How important is the handling of the aircraft? Is this a small aircraft that is operated manually, rendering stick forces and responsiveness imperative? Is it a heavy aircraft with hydraulic or electric actuators, so stick forces are fed back to the pilot electronically and, thus, can be adjusted to be whatever is considered good? How about unsuspected responses to, say, thrust forces?
The Lockheed SA-3 Viking, an anti-submarine warfare aircraft, features a high wing with two powerful turbofan engines mounted on pylons. When spooling up, the aircraft experiences a powerful nose pitch-up tendency that is captured by a stability augmentation system (SAS) that was not originally designed into the prototype. The Boeing B-52 Stratofortress uses spoilers for banking. When banking hard, the spoiler on the down-moving wing is deployed and this reduces lift on the outboard part of that wing. This, in turn, means the center of lift moves forward, causing a nose pitch-up tendency, which the pilot must react to by pushing the yoke forward (for nose pitch-down). Handling issues of this nature must be anticipated and their severity resolved.
Is it imperative that the aircraft will be easy to manufacture? Ease of manufacture will have a profound impact on the engineering of the product and its cost to the customer. A straight constant-chord wing can be manufactured at a lower cost than one that has tapered planform and compound surfaces, but it will be less efficient aerodynamically. Which is more important? The designer must have means to demonstrate why a particular geometry or raw material is required for the project. The concept of ease of marketing always looks good on paper, but this does not guarantee its success. For instance, it is simple to select composites for a new aircraft design on the grounds that this will make it easier to manufacture compound surfaces. But are they really needed? For some aircraft, the answer is a resounding yes, but for others the answer is simply no.
As an example, consider the de Havilland of Canada DHC-2 Beaver (see Figure 1-2). Designing this otherwise sturdy airplane from composites would be an unwise economic proposition. In the current environment it would simply be more expensive to build using composites and sell at the same or lower price than the aluminum version. To begin with, it is not easy to justify the manufacturing of an aerodynamically inefficient frustum-style fuselage1 and constant-chord wing featuring a non-laminar flow airfoil with composites. Composites are primarily justifiable when compound surfaces or laminar flow wings must be manufactured. They require expensive molds to be built and maintained, and, if the aircraft ends up being produced in large numbers, the molds have to be manufactured as well; each may only last for perhaps 30–50 units.
The interested reader is encouraged to jump to Section 2.2, Estimating project development costs, for further information about manufacturing costs (in particular see Example 2-3, which compares development and manufacturing costs for a composite and aluminum aircraft). Cost analysis methods, such as the widely used DAPCA-IV, predict man-hours for the engineering development of composite aircraft to be around two times greater than that of comparable aluminum aircraft. They also predict tooling hours to double and manufacturing hours to be 25% greater than for aluminum aircraft. Labor and material are required not only to manufacture the airplane, but also to manufacture and maintain expensive tooling. As a result, composite aircraft are more expensive to manufacture in spite of substantial reduction in part count.
This inflicts an important and serious constraint on the scope of production. Composites require heating rooms to ensure the resin cures properly so it can provide maximum strength. Additionally, vacuum bagging or autoclaves are often required2 to force tiny air bubbles out of the resin during cure to guarantee that the certified strength is achieved. The manufacturer must demonstrate to the authorities that material strength is maintained by a constant production of coupons for strength testing. Special provisions must be made to keep down moisture and prevent dust from entering the production area, not to mention supply protective clothing and respirators to all technicians who work with the material. All of this adds more cost and constraints to the production and all of it could have been avoided if the designer had realized that requiring composites was more a marketing ploy than a necessity. This is not to say that composites do not have their place – they certainly do – but just because composites are right for one application, does not mean they are appropriate for another one.
Will the aircraft be certified? If the answer is yes, then the designer must explore all the stipulations this is likely to inflict. If no, the designer bears a moral obligation to ensure the airplane is as safe to operate as possible. Since non-certified airplanes are destined to be small, this can be accomplished by designing it to prevailing certification standards, for instance, something like 14 CFR Part 23 or ASTM F2245 (LSA aircraft).
Regulations often get a bad rap through demagoguery by politicians and ideologues, most of whom sound like they have less than no understanding of their value. In fact, regulations are to be thanked for the current level of safety in commercial aviation; commercial air travel is the safest mode of transportation because of regulations and this would be unachievable in their absence. As an example, commercial aviation in the USA, which operates under 14 CFR Part 121, operated with fewer than 1.5 accidents per million departures and no fatalities over the years 2007–2009 . Up until May 2012, there had only been one fatal accident in commercial aviation since 2007, the ill-fated Colgan Air Flight 3407 . Unfortunately, GA suffers from approximately 6 accidents and a little over 1 fatal accident per 100,000 flight hours – a statistic that has remained relatively constant since 2000.
Of course it is possible to make regulations so strict they smother industry; however, this is neither the intent nor does it benefit anyone. The intent is public safety. The modern aircraft is a very complicated machine, whose failure may have catastrophic effects on people and property. The early history of aviation is wrought with losses of life that highlight this fact. For this reason the flying public has the right to know the risk involved before embarking on a flight. Although admittedly extreme, a number of intriguing questions can be posed: would we board an aircraft knowing there was a 50% chance it would crash? No? What percentage would we accept? And generally, how do we know commercial aviation is safe enough to accept the risk?
The fact is that the flying public is completely oblivious to the risk they take when boarding an airplane. While most have heard that aviation is the safest mode of transportation, how do people really know? Aviation is far too complex for anyone but experts to evaluate the level of safety. So, what has convinced people that the risk is indeed very low? The answer is statistics; statistics driven by standards not intended to guarantee profits, but the manufacturing of safe aircraft; standards that all players must adhere to by law.
There is only one way to promote such adherence and it is to employ a body powerful enough to enforce the standards and prevent negligence that otherwise would be rampant. This body is the government. History is wrought with examples of industries that let operational and product safety take the back-seat to profits. After all, this is what spurred the so-called “strict-liability” clause of 1963 (see Section 2.1.2, A Review of the State of the General Aviation Industry). While, at the time of this writing, LSA is a form of self-regulation that seems to work, this cannot be extrapolated to other industries. It works for LSA because compliance is less expensive than refusing to comply. But this does not hold for all other industries. Rather than focusing a futile effort on getting rid of regulations, the focus should be on streamlining them to make sure they work for everybody. Regulations are akin to a computer code – they contain wrinkles that need to be ironed out. There is a golden medium between regulations that are too strict and no regulations at all. In the experience of this author, this is exactly what industry and the authorities are trying to accomplish. Aviation authorities are well aware of the effects burdensome regulations have on businesses, and for that reason, on a regular basis, review regulatory paragraphs for the purpose of streamlining them. Such reviews always include representatives from industry, who wield a deep understanding of the topic, and also benefit from making aviation regulations the safest and least burdensome possible. That aside, it is in fact very beneficial for industry that everybody must comply with the same set of rules – nothing is worse for industry than rules that favor one company over another. Other aspects of regulations are discussed in more detail in Section 1.2.2, Important Regulatory Concepts.
The weight of most civilian and military aircraft increases with time. It is not a question of if, but when and how much. There is not an example of the opposite, to the knowledge of this author. Requirements for added capabilities and systems raise the weight and often require substantial changes such as the introduction of a more powerful engine, and even wing enlargement. Additionally, it is often discovered during prototyping that the selected material and production methodology leads to a heavier aircraft than initially thought. The careful designer sizes the aircraft for a weight that is 5–10% higher than the projected gross weight.
In light of the above topics, looks may seem like a secondary concern. But it is one that should never be underestimated. While beauty is in the eye of the beholder, it is a fact of business that aircraft that have a certain look appeal to a larger population of potential buyers and, therefore, sell better, even if their performance is less than that of the competition. The so-called Joint Strike Fighter program is a great example of such appeal (even though difference in performance is not the issue). The purpose of the program was to introduce an aircraft for the US armed forces that simultaneously replaced the F-16, A-10, F/A-18, and AV-8B tactical fighter aircraft. Three versions of the aircraft were planned and in order to keep development, production, and operating costs down, a common shape was proposed for which 80% of parts were interchangeable. There were two participants in the contract bid; Lockheed Martin and Boeing. Lockheed’s entry was the X-35 and Boeing’s X-32 (see Figure 1-3). Both aircraft were thought to be worthy candidates, but on October 26, 2001, Lockheed’s design was announced as the winner. The reason cited by the Department of Defense, according to The Federation of American Scientists, an independent, nonpartisan think tank, was:
“The Lockheed Martin X-35 was chosen over the competing Boeing X-32 primarily because of Lockheed’s lift-fan STOVL design, which proved superior to the Boeing vectored-thrust approach.” 
FIGURE 1-3 Does the Boeing X-32 or the Lockheed X-35 look better? (Photos: (left) by Jake Turnquist; (right) by Damien A. Guarnieri, Lockheed Martin)
Apparently, in hover, the X-32’s engine exhaust would return to the intake, and lower its thrust. However, soon thereafter rumors began running rampant that the real reason was in fact the looks of the two proposals, a claim denied by James Roche, the then secretary of the Air Force . Rumor held that military pilots did not like the looks of the Boeing proposal, some allegedly referring to it as ‘the flounder.’ This rumor cannot be confirmed, but perhaps the reader has an opinion on whether the looks of the two aircraft in Figure 1-3 could have had an impact on its acceptance.
Another case in point is the Transavia PL-12 Airtruk, shown in Figure 1-4. It was originally developed in New Zealand as the Bennett Airtruck (later Waitomo Airtruk). It is a single-engine agricultural sesquiplane of all-metal construction. Among many unusual features is a cockpit mounted on top of the engine, twin tail-booms that are only connected at the wing, designed to allow a fertilizer truck to back up and refill the airplane’s hopper, and the sesquiplane configuration generates four wingtip vortices that help better spread fertilizer. It is a capable aircraft, with a 2000 lb fertilizer capacity, and can be used as a cargo, ambulance or aerial survey aircraft, as well. But a strange-looking beast it is, at least to this author.
Maintainability is the ease at which an airplane can be kept airworthy by the operator. Maintainability is directly related to ease of manufacturing. Complicated manufacturing processes can result in an aircraft that is both hard and costly to maintain. One of the advantages of aluminum is how relatively easy it is to repair. Composites on the other hand can be very hard to maintain. Maintainability also extends to the ergonomics of repairing. Are expensive tools required? Will the mechanic need to contort like an acrobat to replace that part? Will it take 10 hours of labor to access a part that will take 5 minutes to replace? These are all issues that affect maintainability. It cannot be emphasized enough that a novice engineer should consult with A&P mechanics and try to understand their perspective. Many valuable lessons are learned from the technicians who actually have to do the work of fabrication, assembly, and maintenance.
The concepts lean engineering and lean manufacturing refer to design and production practices whose target is to minimize waste and unnecessary production steps. For instance, consider the production of a hypothetical wooden kitchen chair. Assume that pride has the manufacturer attach a gold-plated metal plaque to the lower surface of the seat that reads: ‘World’s finest kitchen chairs, since 1889.’ Assume it takes five separate steps to attach the plaque: two drilling operations of pilot holes, one alignment operation, and two operations in which the plaque is screwed to the seat’s lower surface. Not only would the plaque have to be attached, but an overhead labor is required to order it from an outside vendor, transport it to the manufacturer, keep it in stock, and so on. Strictly speaking, the purpose of a chair is to allow someone to sit on it and, then, the said plaque is not visible. It can be argued the plaque serves no other purpose than to brag about the manufacturer and, as such, it brings no added value to the customer. In fact, it only brings up the cost of production; it certainly does not make the seating experience any more enjoyable. The plaque is therefore simply wasteful and from the standpoint of a lean production should be eliminated from the process.
The purpose of lean manufacturing is to refine the production process to ensure minimum waste. This increases the profitability of a business through efficiency. Since production processes either add value or waste to the end product, the purpose of lean engineering is to refine the design of a product so that simple, effective, and non-wasteful production processes may be employed. The scope of lean manufacturing is large and can entail topics such as optimizing the layout of templates for cutting material for clothing to minimize the amount of material that goes to waste; to the operation of the stock room, where parts are ordered from vendors just before they need to be assembled (so-called just-in-time philosophy), so the assembler won’t have to keep capital in parts in an inventory. The overall consequence of such practices is a far more efficient production, and therefore, less expensive products, both to the customer and Mother Earth.
The philosophy behind lean manufacturing is usually attributed to Toyota the car manufacturer, which is renowned for its adherence to it in its production processes. For this reason, it is also known as Toyotism, and the success of the company's philosophy has afforded it plenty of attention. An imperative step in Toyota’s approach is to identify what is called the Seven Wastes . The approach was originally developed by Toyota’s chief engineer Taiichi Ohno, who identified seven common sources of waste inside companies: (1) overproduction, as in the manufacturing of products before they are needed; (2) waiting, which occurs when parts are not moving smoothly in the production flow; (3) transporting, as in moving a product in between processes; (4) unnecessary processing, when expensive, high-precision methods are used when simpler methods would suffice; (5) unnecessary inventory, which is the accumulation of vendor parts and components in stockrooms; (6) excessive or unnecessary motion, which is the lack of ergonomics on the production floor, which may increase production time; and (7) production defects, which are inflicted on the production floor and are costly due to the inspection and storage requirements.
The above discussion barely scratches the surface of lean manufacturing, but is intended to whet the appetite of the reader.
An integrated product team is a group of people with a wide range of skills who are responsible for the development of a product or some feature. The formation of IPTs are very common in the aviation industry, as the modern airplane is a compromise of a number of disciplines. To better understand how IPTs work, consider the development of a pressurization system for an aircraft. An example IPT could consist of the following members:
(1) A structural analyst, whose task is to determine pressurization stresses in the airframe and suggest airframe modifications if necessary.
(2) A performance analyst, whose responsibility is the evaluation of the benefits of the higher cruise altitude and airspeed the pressurization will permit.
(3) A power plant expert, who solves engine-side problems, such as those associated with bleed air, heat exchangers, and liaison duties between the engine and airframe manufacturers.
(4) An interior expert, who evaluates the impact of the pressurization system on the interior decoration, such as those that stem from the requirement of sealing and condensation.
(5) An electrical expert, who evaluates the electrical work required to allow the pilot to operate the pressurization system.
(6) A systems expert, to work on the pressurization system ducting layout, interface issues with heat exchangers, cabin pressure relief valves, cabin sealing, and so on.
Such a group would meet, perhaps once a week, to discuss issues and come up with resolutions, often with the inclusion of representatives of the manufacturers of the various systems.
In general, the aircraft design process involves several distinct phases. These are referred to as:
(5) Proof-of-concept aircraft construction and testing phase.
These will now be discussed in greater depth. It should be stressed that these differ in detail from company to company. Some tasks that here are presented in the conceptual design phase may be a part of the preliminary design phase in one company and a part of a different phase in another. The exact order of task is not important – its completion is.
From a certain point of view, requirements are akin to a wish list. It is a list of expectations that the new design must meet. It specifies the aircraft capabilities, such as how fast, how far, how high, how many occupants, what payload, and so on (in other words, its mission). The requirements may be as simple as a few lines of expected capabilities (e.g. range, cruising speed, cruising altitude, and number of occupants) or as complex as documents containing thousands of pages, stipulating environmental impact, operating costs, maintainability, hardware, and avionics, just to name a few. It is the responsibility of the design lead to ensure the airplane has a fair chance of meeting the requirements and this is usually demonstrated during the next phase, the conceptual design phase.
The conceptual design phase formally establishes the initial idea. It absorbs just enough engineering to provide management with a reliable assessment of likely performance, possible looks, basic understanding of the scope of the development effort, including marketability, labor requirements, and expected costs. Typically, the following characteristics are defined during this phase:
• Type of aircraft (piston, turboprop, turbojet/fan, helicopter)
• Mission (the purpose of the design)
• Technology (avionics, materials, engines)
• Aesthetics (the importance of “good looks”)
• Requirements for occupant comfort (pressurization, galleys, lavatories)
• Ergonomics (occupants and occupant ergonomics)
• Special aerodynamic features (flaps/slats, wing sweep, etc.)
• Certification basis (LSA, Part 23, Part 25, Military)
• Ease of manufacturing (how will it be produced)
• Maintainability (tools, labor, and methods required to maintain the aircraft)
The conclusion of this phase is an initial loft and a conceptual design evaluation, which allows management to make a well-reasoned call as to whether to proceed with the design by entering the preliminary design phase.
The preliminary design ultimately answers whether the idea is viable. It not only exposes potential problems, as well as possible solutions to those problems, but yields a polished loft that will allow a flying prototype to be built. Some of the specific tasks that are accomplished during this phase are:
• Detailed geometry development
• Evaluation of special aerodynamic features
• Evaluation of certifiability
• Evaluation of mission capability
Ideally, the conclusion of this phase is a drawing package and a preliminary design evaluation. If this evaluation is negative, this usually spells a major change to, if not a cancellation of, the program. If positive, a decision to go ahead with the fabrication of a proof-of-concept (POC) aircraft is usually taken.
The detail design process primarily involves the conversion of the loft from the preliminary design into something that can be built and ultimately flown. Of course, it is far more complicated than that, and a limited description of the work that takes place is listed below:
• Detail design work (structures, systems, avionics, etc.)
• Study of technologies (vendors, company cooperation, etc.)
• Subcontractor and vendor negotiations
• Design of limited (one-time use) tooling (fixtures and jigs)
• Avionics and electronics detail design
• Maintenance procedures planning
The conclusion of this phase is the final outside mold line and internal structure for the POC. This is generally the beginning of the construction planning, although it almost always begins long before the detail design phase is completed.
The construction of the POC aircraft or prototype, begins during the detail design phase. For established companies that intend to produce the design, this is a very involved process, as the production process, with all its paperwork and quality assurance protocols, is being prepared at the same time. Some of the tasks that are accomplished are listed below:
• Detail design revisions (structures, systems, avionics, etc.)
• Application of selected technologies
• Tooling design and fabrication
• Aeroelastic testing (ground vibration testing)
The culmination of this phase is maiden flight of the POC. This is followed by the development flight testing as discussed below.
A development program follows a successful completion of the preliminary design. The development of this phase usually begins long before the maiden flight and is usually handled by flight test engineers, flight test pilots, and management.
• Establish aircraft operating limitations (AOL)
• Establish pilots’ operating handbook (POH)
• Prepare master flight test schedule (MFTS)
• Envelope expansion schedule (or Matrix)
• Flight support crew training
• Group roles must be trained prior to flight – not on the job
• Establish emergency procedures
• Establish group responsibilities
The conclusion of this phase is a certifiable aircraft. This means the organization understands the risks and scope of the required certification effort and is convinced the certification program can be successfully completed.
A lot of work still remains, even though the development program comes to an end in a successful manner. A viable aircraft design continues in development when customers begin its operation and discover features that “would greatly benefit the design”. There is the advancement of avionics. New equipment must be installed and these must be engineered. A broad scope of various post-development programs is listed below:
• Development flight test/structural/systems/avionics program
• Certification flight test/structural/systems/avionics program
• Aircraft is awarded with a type certificate
• Production tooling design and fabrication
• Delivery of produced aircraft
This section presents several views of the general process of airplane design, from the introduction of a request for proposal (RFP) to a certified product. Since great effort is exerted in designing the airplane to comply with civil aviation regulations, a brief introduction to a number of very important regulatory concepts is given as well.
A general description of the aircraft design process is provided in several aircraft design textbooks intended for university students of aerospace engineering. Understanding this process is of great importance to the aircraft designer, in particular design team leaders. An elementary depiction of the design process is presented in Figure 1-5. While the diagram correctly describes the chronological order of steps that must be accomplished before the POC is built, it is somewhat misleading as the overlap between phases is not presented. In a real industry environment there really is not a set date at which conceptual design ends and preliminary design begins. Instead there is substantial overlap between the phases, as this permits a more efficient use of the workforce. Instead, the conceptual design stage is slowly and surely phased out.
As an example, in the form presented in Figure 1-5, the engineers responsible for the detail design would effectively be idle until the design reached the detail design stage. This would present a costly situation for any business. Instead, the preliminary design takes place in various stages that are parallel to the detail design stage. This way, the preliminary design of the fuselage might take place after the wings, whilst the detail design of the wing takes place at the same time.
In his classic text, Torenbeek  discusses the process in detail and presents a depiction, reproduced in Figure 1-6. This diagram demonstrates the process in a realistic manner, by showing overlapping activities. There is really not a specific date beyond which the previous phase ends and a new one begins. It also shows important milestones, such as a configuration freeze, go-ahead approval, and acceptance of type
certificate. A configuration freeze is a set date after which no changes are allowed to the external geometry or the outside mold line (OML), even if a better geometric shape is discovered. It marks the date for the aerodynamics group to cease geometric optimization, as the “frozen” configuration is adequate to meet the requirements. The go-ahead approval is the date at which upper management gives the green light for the design team to proceed with the selected configuration and develop an actual prototype. In other words, it marks the readiness of the organization to fund the project. A type certificate is described in Section 1.2.2.
The diagram correctly shows that the preliminary design begins before the conceptual design is completed. Of course, most major geometric features (wings, fuselage, tail, etc.) have already been sized by then, but many others remain as work in progress. Evaluation of the effectiveness of winglets, control surface sizing and hinge moment estimation, landing gear retraction mechanism, and many others, are examples of such tasks.
Torenbeek’s diagram also shows that detail design begins during the preliminary design phase, and even manufacturing overlaps the other two. The manufacturing phase includes the design and construction of production tooling, establishment of vendor relations and other preparatory tasks.
The following process is based on the author’s experience and explains the development of typical GA aircraft. It parallels Torenbeek’s depiction in most ways, but accounts for iteration cycles often required during the preliminary design phase. This reflects the fact that during the preliminary design phase, issues will arise that may require the OML of the configuration to be modified; in particular if the design is somewhat unorthodox. Such an issue might be a higher engine weight than expected, requiring it to be moved to a new location to maintain the original empty-weight CG position. This, in turn, calls for a reshaping of the engine cowling or nacelle, calling for other modifications. Such changes are handled by numbering each version of the OML as if it were the final version, because, eventually, the one with the highest number will be the configuration that gets frozen. This allows the design team to proceed with work on structures and other internal features of the aircraft, rather than waiting for a configuration freeze.
It is the duty of the government of the country in which the aircraft is designed and manufactured to ensure it is built to standards that provide sufficient safety. If the aircraft is built in one country and is being certified in another, it is still the duty of the government of the latter to ensure it complies with those same standards. This task requires the standards (the law of the land) to be enforced – and may call for a denial of an airworthiness certificate (see below) should the manufacturer refuse to comply with those standards. This is why this important task should remain in the hands of a government – for it has the power to force the compliance.
From the standpoint of manufacturers, certifying aircraft in a different country can become contentious when additional, seemingly local, requirements must be complied with as well. Such scenarios can often inflict great frustration, if not controversy, on the certification process. International harmonization of aircraft certification standards remains an important topic. In effect, such harmonization would allow the demonstrated compliance in one country to be accepted in another, upon review of certification documents. This would be helpful to already cash-strained businesses that have clearly demonstrated they already side with safety. It underlines a fair complaint often leveled by industry that a serious review and justification for the presence of selected paragraphs in the regulatory code is in order.
The standards that aircraft are designed and built to have names like the Civil Aviation Regulations (CAR, now obsolete), Federal Aviation Regulations (FAR, current in the United States), Joint Aviation Regulations (JAR, European, obsolete as of September 28, 2003), or Certification Specifications  (CS, current in Europe). The current government agencies that enforce adherence to these standards have names like the Civil Aviation Authority (CAA, now obsolete), Federal Aviation Administration (FAA, current in the United States), Joint Aviation Authorities (JAA, European, obsolete as of June 30, 2009), or European Aviation Safety Agency (EASA, current in Europe). With respect to FAR, the convention is to refer to them as Title 14 of the Code of Federal Regulations, or simply 14 CFR. This way, a particular section of the regulations is cited by adding it to that code. For instance Part 23 would be written as 14 CFR Part 23, and so on.
Once the manufacturer of a civilian (i.e. non-military) aircraft, engine, or propeller has demonstrated that their product meets or exceeds the current airworthiness standards set by its regulatory agency, it is awarded a TC by publishing a type certificate data sheet (TCDS). The TCDS is a document that contains important information about operating limitations, applicable regulations, and other restrictions. This means the aircraft is now “officially defined” by the TC. TCDS for all civilian aircraft can be viewed on the FAA website .
Obtaining a TC is a very costly proposition for the manufacturer, but it is also very valuable in securing marketability of the product. It can be stated with a high level of certainty that a specific product without a TC (and thus considered experimental) is unlikely to sell in the same quantity or at the same price it would if it had a TC. The reason being that the TC guarantees product quality, which is imperative to the customer: it makes the product “trustworthy.” The reason why a TC is so costly is that it requires the product to undergo strenuous demonstration of its safe operation and quality of material and construction. Additionally, the TC serves as a basis for producing the aircraft.
Many owners of airplanes want to add features to the model. A replacement of a piston engine with a gas turbine is an example of a very common change made to existing certified aircraft. Another example is the conversion of an airplane to allow it to transport patients, something it was very unlikely to have been originally designed for. Such changes are possible, but require the aviation authorities to approve the installation or change. Once convinced the change does not compromise the continued airworthiness of the aircraft, a supplemental type certificate is issued. The STC specifies what change was made to the aircraft, details how it affects the TC, specifies new or revised operational limitations, and lists what serial numbers are affected. The list of serial numbers is called effectivity.
Once the TC has been obtained, each unit of the now mass-produced aircraft will receive a standard airworthiness certificate. This is only issued once each aircraft has been demonstrated to conform to the TC and be assembled in accordance with industry practice; is ready for safe operation; and has been registered (giving it a tail number). Each aircraft produced is tracked using serial numbers. The AC allows the aircraft to be operated, as long as its maintenance is performed in accordance with regulations.
A special airworthiness certificate can be issued for airplanes that, for some reason, must be operated in a specialized fashion (e.g. ferry flying, agricultural use, experimental, marketing, etc.), but precludes it from being used for commercial transportation of people or freight. An S-AC is issued in accordance with 14 CFR 21.175 in the following subclasses: primary, restricted, limited, light-sport, provisional, special flight permits, and experimental. Of these, the prototypes of new aircraft designs typically receive an experimental permit while they are being flight tested or for market surveys.
Once the manufacturer is nearing the completion of the certification process and it is apparent it will comply with the remaining regulations, the authorities often allow the manufacturer to begin delivering aircraft by issuing provisional permits. This helps the manufacturer begin to recover the extreme costs of developing the aircraft. The provisional permit inflicts limitations to the operation of the aircraft that are lifted once the manufacturer finally receives the TC. An example of this could be a GA airplane designed for an airframe lifetime of, say, 12,000 hours.3 Since fatigue testing is one of the last compliances to be demonstrated, it is possible the aircraft would receive a provisional S-AC with a 2000 hr airframe limitation. Since GA aircraft usually operate some 300–400 flight hours per year, the 2000 hr limitation will not affect the operator for several years, allowing the manufacturer to complete the certification while being able to deliver aircraft. Once the 12,000 hr lifetime is demonstrated, the 2000 hr limitation on already delivered aircraft is lifted, provided their airframe is deemed to qualify.
The use of an aircraft subjects it to wear and tear that eventually will call for repairs. Such repairs can be of a preventive type, such as the replacement of a component expected to fail within a given period of time, or the restorative type, such as the addition of a doubler to improve the integrity of a structural part beginning to show signs of fatigue. The aviation authorities require manufacturers to stipulate frequency and severity of preventive maintenance by instructing what tasks must be accomplished and when, in a maintenance program. If the owner or operator of the aircraft does not comply with this satisfactorily, the aircraft may lose its AC and is then said to be “grounded.”
Sometimes the operation of a specific aircraft type develops unanticipated issues that may compromise its safety. If such issues arise, the manufacturer is obligated to notify the aviation authorities. The authorities will issue an Airworthiness Directive (AD) to the manufacturer and to all operators worldwide. The AD is a document that stipulates redesign effort or maintenance action that must be accomplished to prevent the issue from developing into a catastrophic event. Compliance with the AD is required or the AC for the specific aircraft may be cancelled. ADs for different aircraft types can be viewed on the FAA website .
In due course the manufacturer inevitably gains experience from the operation of the aircraft. This experience results from dealing with individual customers as well as from the manufacturer’s sustaining engineering effort. This experience usually results in the improvement of the aircraft or its operation and is therefore very valuable. Consequently, it is important to share it with other operators. This is done by publishing service bulletins (SB). Although the recommendations in a SB are most often discretionary (i.e. it is up to the customer to comply), they will sometimes relay information required to comply with an AD.
An advisory circular is a means for the FAA to share information with the aviation community regarding specific regulations and recommended operational practices. This information is sometimes detailed enough to be presented in the form of a textbook (e.g. AC36-3H – Estimated Airplane Noise Levels in A-Weighted Decibels) or as simple as a few pages (e.g. AC 11-2A – Notice of Proposed Rulemaking Distribution System). A complete list of ACs is provided on the FAA website www.faa.gov.
A technical standard order is a minimum performance standard that particular materials, parts, processes, and appliances used on civil aircraft are subjected to. Effectively, a TSO is a letter to the manufacturers of a given product that states that if they (the manufacturers) wish to get their products TSOd, they will have to meet the performance requirements and submit a list of engineering documentations (drawings, specifications, diagrams, etc.) that are specified in the letter. Effectively, a TSO is an official certificate that confirms the part is safe for use in a specific aircraft. In other words: it is airworthy. This puts the manufacturer at a significant advantage over another one whose product is not TSOd. It is also essential for pilots to know that the equipment they are using is airworthy.
A technical standard order authorization is a document that authorizes the manufacturer to produce parts and components in accordance with a particular TSO. As an example, consider a battery manufacturer who wants to produce a battery for use in a particular type of aircraft. The TSO tells the manufacturer what the battery must be capable of (e.g. amp-hours, temperature tolerance, etc.). The TSOA tells the manufacturer that in the eyes of the FAA the product is qualified and can now be produced.
Parts manufacturer approval authorizes a manufacturer to produce and sell replacement or modification parts for a given aircraft. This way, the manufacturer can produce airworthy parts even if they were not the original manufacturer.
This section presents a step-by-step method intended to help the novice designer begin the conceptual design of a GA aircraft and bring it to the preliminary phase. As stated earlier, the conceptual design phase formally establishes the initial specifications of and defines the external geometry of the aircraft. It is imperative that proper analyses are selected during this phase, as this is an opportunity to design as many problems out of the airplane as possible.
The conceptual design process is one of iteration. The designer should realize this from the start of the project. A well-organized project is one that allows analyses to be conducive to iteration. This means that as the design of the aircraft progresses, it is inevitable that new things will be discovered that call for many of the previous calculations to be redone. For instance, it might be discovered that the wingspan needs to be increased, something that will affect all parts of the wing geometry. Aspect ratio, wing area, and so on, will need to be recalculated – often many times during this process. We wish to prepare the design process so the calculations of this nature are swift. Ideally, if we change the wingspan, all parameters that depend on it should be updated automatically, from the most elementary geometry to the most complex weight, drag, performance analyses.
The modern spreadsheet is ideal for such an analysis approach. This book is written to provide the designer with methods to make the implementation of this process easier for spreadsheet analysis – which is why many of the graphs also feature the exact equations used to generate them. When necessary, the author has painstakingly digitized a great number of graphs for which there are no data available other than the graphs themselves. Additionally, many methods are presented using computer codes. All of this is done to help the designer to more easily answer fundamental questions about his or her creation.
The design algorithm presented below assumes the implementation of the conceptual approach using a spreadsheet. Of course, it is not the only way to take care of business. However, it is based on a careful evaluation of what went well and not so well in the industry design experience of this author. Another word for algorithm is process; it is simply a list of tasks that are arranged in a logical order. In addition to the algorithms, the designer should keep handy the information from Section 23.3, General Aviation Aircraft Design Checklist, which describes a number of pitfalls to avoid.
This algorithm treats the design process almost as if it were a computer program (see Table 1-3). First, a number of initialization tasks are performed, followed by a set of iterative tasks. The table provides a complete conceptual design process and several tasks to help bring the design into the preliminary design phase. Where appropriate, the reader is directed toward a section in this book that will provide solutions and analysis methods.
It should be pointed out that sketching the airplane is not suggested until Step 10. This may appear strange to some readers, however, the reason is simple: not enough information exists for an effective sketch until Step 10. This of course does not mean a sketch cannot be or should not be drawn before that – just that an accurate depiction of the airplane is not possible. For one, the wing and tail geometry are determined in Steps 8 and 9, so an earlier sketch is unlikely to represent those with any precision. For this reason, and in the humble view of this author, an earlier sketch is a bit like a shot in the dark. Of course, adhering to this algorithm is not the law of the land. It merely represents how this author does things. The reader is welcome to make modifications to the algorithm and bend it to his or her own style. What works best for the reader is of greater importance.
The implementation of this algorithm is best accomplished through the use of a spreadsheet. Organize the spreadsheet in a manner that is conducive to iteration. What this means is that if (or more precisely, when) any parameter changes, all parameters that depend on it will be automatically updated.
Consider the ways of the past when engineers didn’t have the powerful tool that the modern spreadsheet is. Months were spent on estimating performance; stability and control; structural analyses, etc., and any change to the external geometry of the aircraft would call for a major recalculation effort. So, let’s entertain the scenario in which the wing area has to be increased by 5% to reduce stalling speed. This would call for an update in drag analysis, because the change in area increases the drag, which in turn changes the performance. The geometric change also modifies the airplane’s stability. Additionally, greater area changes the distribution of the lift and the magnitude of the bending moments. So, all wing structural calculations will have to be revised as well. And this effort takes a lot of time, perhaps many weeks. Fortunately for the modern engineer, those days are gone, because, if properly prepared, a spreadsheet will automatically re-calculate in a heartbeat everything that depends on the value that is changed.
The spreadsheet is best prepared in the manner shown in Figure 1-8. Modern spreadsheet software such as Microsoft Excel or the free-of-charge Open Office Calc are three-dimensional, which means they allow multiple worksheets. Each cell in a worksheet can contain formulas that refer to any other cell in the spreadsheet, which means each worksheet can link to another worksheet within the same spreadsheet. It is ideal to use this capability to organize the spreadsheet in a manner particularly useful to aircraft design. This requires specific worksheets to act either as a hub or a spoke in a hub-and-spoke hierarchy. The hub, which is called the general worksheet, acts primarily as a data entry page, where, at best, only relatively simple calculations take place. For instance, the user would enter empty and gross weight and the useful load might be calculated by a simple subtraction. However, all the sophisticated analyses take place in subsequent worksheets, which should be considered the spokes. This is shown as the systematic hierarchy in Figure 1-8.
FIGURE 1-8 Organizational hierarchy of a spreadsheet (see Section 1.3.2 for explanation).
As an example of how this would work, consider the tail sizing worksheet. It requires the wingspan of the aircraft as a part of that analysis (see Chapter 11, The Anatomy of the Tail). However, rather than entering that parameter on the worksheet itself, the worksheet would fetch it from the general worksheet. The same holds for the stability and control worksheet. It also requires the wingspan in its calculations. That worksheet would also fetch its value from the general tab. This way, if for some reason the wingspan has to be changed, the designer can simply enter the new value on the general worksheet, and the tail sizing, and stability and control worksheets will be automatically updated. The only thing for the designer to be mindful of is to be rigorous in the application of this philosophy from the start of the project. It will save him countless hours of review work.
To give the reader a better insight into how this is implemented in a real spreadsheet, consider Figure 1-9, which shows how the hierarchy appears in practice. Note that two easily identifiable colors have been chosen for cells to indicate where the user shall enter information and where a formula has been entered. This reduces the risk of the user accidentally deleting important formulas and helps with making the spreadsheet appear better organized and more professional.
FIGURE 1-9 Organizational hierarchy implemented in an actual spreadsheet (see Section 1.3.2 for explanation).
The purpose of this section is to present a few tools that are at the disposal of the project engineer. The reader is reminded that this is not a complete listing; there is a multitude of ways to conduct business. Experienced engineers may not find anything helpful in this section, but that is all right, this section is not intended for them, but rather the novice engineer who is not sure where to begin or how to do things.
Any serious engineering project requires many activities to be managed simultaneously. Scheduling, communication, hiring, conflict resolution, coordination and interaction between groups of specialists are but a few tasks that are required to move the project along. Such projects usually require someone, frequently an experienced engineer, to perform these duties; this person is the project engineer.
In addition to the aforementioned tasks, there are multiple others that the project engineer is responsible for and some of those seem to have very little in common with the engineering the person was trained in. He or she serves as a liason between management of the company and the engineering workforce ensuring scheduled deadlines are met and, as such, plan overtime and effective delegation of tasks. He or she may have to evaluate, negotiate with and work with vendors, as well as organize training of the workforce and frequently help select between design alternatives and facilitate the resolution of development problems. He or she may even be required to help resolve personal problems between individuals in the workforce, through mediation or other means. Some of these are tasks the project engineer has never even heard about as a student.
Nowadays, project engineering is offered as an elective course in many universities. Typically, such courses emphasize various project engineering skills, such as time management, how to make meetings more effective, scheduling, leadership, effective communication, delegation styles, lean engineering, and engineering economic analyses.
People often ask what skills a project engineer must master in order to be a good project manager. Six important skills are often cited; communication, organization, team building, leadership, coping, and technological skills. Communication skills involve the ability to listen to people and being able to persuade them to act in a manner that favors the goals of the project. Having organization skills means you have the ability to plan, set goals, and analyze difficult situations. Team-building skills involve having the ability to empathize with other team members, demonstrating loyalty to the team, and the motivation to work as part of the team in order to ensure that the project is successful. Leadership skills involve setting a good example and the exercise of professionalism. Having good leadership skills means being enthusiastic, having a positive outlook, and involves being able to effectively delegate tasks. A good leader sees the “big picture” and can communicate it to the team members. Coping skills involve flexibility, patience, persistence, and openness to suggestions from others. It makes the leader resolute and able to adjust to changing conditions. Technological skills involve the utilization of prior experience, knowledge of the project, and the exercise of good judgment.
A Gantt diagram is a graphical method of displaying the chronological flow of a project and is a standard method used by the industry. The diagram breaks the project down into individual major and subtasks, allowing the manager to assign a start and end date to each, as well as multiple other information, such as human resources, and equipment. These appear as horizontal bars, as can be seen in Figure 1-10. It is possible to buy software that allows this to be done effectively once it has been mastered. Gantt diagrams also generally feature important project completion dates or milestones.
The Fishbone diagram, more formally known as an Ishikawa diagram or a cause-and-effect diagram, is named after Kaoru Ishikawa (1915–1989), a Japanese quality control statistician. At its core, the diagram focuses on effects and their causes by drawing them in a special manner. The causes are written along the perimeter of a page, with arrows pointing towards the effect or consequence, which is the horizontal arrow in the middle of the diagram. The resulting graph is reminiscent of a fish skeleton, which explains its name.
This diagram is used in a slightly modified fashion for aircraft design (see Figure 1-11). In this application the horizontal arrow is actually a timeline. It starts at the initiation of the project and terminates at its completion. This format can be applied to the entire development program, or sub-projects. The causes can be thought of as major tasks that are broken down into sub tasks. The causal arrows points to a milestone or a representative time location on the horizontal line, as shown in the figure. The advantage of this diagram is that it allows the project manager to (1) demonstrate the status of the project to upper management (for instance compare Figure 1-11 and Figure 1-12), (2) to anticipate when to ramp up for specific sub-projects, and (3) understand the “big-picture” of the project.
FIGURE 1-12 The same fishbone diagram at a later time during the design process. Completed tasks have been struck through. The diagram displays progress made at a glance and gives the manager a great tool to reallocate resources.
A fishbone diagram for a preliminary airplane design typically consists of the following categories:
Proper documentation (reports) should always be created along the way in a manner stipulated by the corresponding company. In small companies the designer may choose to use a system based on limited reporting; however, a more comprehensive reporting system may be employed to generate certification-style reports along the way. This may help free up resources during the actual certification program.
For project management purposes, it is imperative to list all design requirements clearly and then regularly evaluate the status of each, in order to indicate compliance. An example of such a list is shown in Figure 1-13. Some requirements are not as much “required” as they are desired, and these should be indicated in the ‘required’ column, as such. Note that some customer requirement may “invoke” other requirements the customer may or may not have a deep understanding of. An obvious example of this is federal regulatory requirements. Figure 1-13 lists one of the customer’s requirements as Certification in the Light Sport Aircraft (LSA) category. This invokes a set of complex new requirements, so important that they may compromise the viability of the entire project.
For a complex project with a large number of milestones, a milestone list should be prepared. It is helpful if it is based on the fishbone diagram as shown in Figure 1-11 and Figure 1-12. This will allow the manager to keep a close tally on how each set of tasks is progressing.
Figure 1-14 shows an example of a project milestone list, which is used to monitor the progress of a complicated project and help the manager relay to the workforce when certain tasks have to be completed. This way, target days can be set and proper scheduling of resources can take place.
Figure 1-15 shows an example of a project design plan. The design plan breaks the entire project into subparts and tasks, allowing the manager to keep track of the progress of various tasks. The hardest part of the plan is to keep the plan suitably detailed. Planning too many details will result in a plan that’s impossible to achieve on deadline (see rightmost column in the figure) and will absorb the manager’s time by requiring constant revisions. Remember, the plan is not the boss, the boss is. The plan is simply a tool to help run things smoothly. By the same token, a plan with too little detail is useless. This can be seen by considering the comparison in Table 1-4. As is so often required, a golden balance should be struck between the two extremes.
Examples of Plans of Different Detail
|A Simple (Useless) Plan||A Clear and Effective Plan||A Complex and Ineffective Plan|
|Buy a new car.||List what I want in my new car.
Look for a car that satisfies my list.
Get a bank loan.
Purchase my new car.
|List what I want in my new car.|
List what my spouse wants in my new car.
List what my kids want in my new car.
Compile a comprehensive list of every ones’ needs and wishes.
Start up computer.
Start Internet Explorer and go to Google.
Search for cars online.
Look for cars at dealerships.
Refill my current car with gas after driving to dealerships.
Go to three banks and talk to loan officers about specifics.
Evaluate which bank to choose.
Discuss the choice with spouse.
Go to the bank of choice and get loan.
Celebrate by taking spouse to dinner.
Get gas again after running all the errands.
Order car online or buy from dealership.
Celebrate again by opening a bottle of champagne.
An important requirement of sophisticated products is that they must simultaneously satisfy a large number of requirements. Among them are customer and engineering requirements. In order to enhance the likelihood that a product will satisfy the needs of the customer, it is necessary to survey what it is they know or think they need. Unfortunately, survey responses can often be vague and it is, thus, necessary to convert them to statements that allow them to be measured. For instance, a statement like “I don’t want to pay a lot of money for maintenance” can be translated to “reliability.” This, in turn, can be measured in terms of how frequently parts fail and require repairs. It is inevitable that some of these requirements conflict with each other, in addition to depending on each other. For instance, the weight of an aircraft will have a great impact on its rate-of-climb, but none on its reliability.
Quality function deployment (QFD) is a method intended to help in the design of complex products, by taking various customer wishes into account. This is accomplished using a sophisticated selection matrix that helps evaluate the impact of the various customer wishes on areas such as the engineering development. The output can be used to highlight which customer wishes to focus on. The primary drawback is that it can take considerable effort to develop and it suffers from being highly dependent on the perspectives of the design team members. It was developed by the Japanese specialists Dr. Yoji Akao and Shigero Mizuno and is widely used in disparate industries. One of the method’s best-known tools is known as a house of quality (also called a quality functional deployment matrix), a specialized matrix, resembling a sketch of a house, designed to convert customer requirements into a numeric score that helps define areas for the designer to focus on.
The preparation of a house of quality (HQ) is best explained through an example. Generally, the HQ consists of several matrixes that focus on different aspects of the development of a product (see Figure 1-16). This way, the impact of desired (or customer) requisites on the technical requirements and their interrelation is identified. Ultimately, the purpose is to understand which requirements are of greater importance than others and how this complicates the development of the product.
Below, a much simplified version of the HQ, tackling the development of a small GA airplane, is presented. The reader is reminded that the HQ can be implemented in a number of ways – and a form that suits, say, the textile industry does not necessarily apply directly to the aviation industry.
Assume that customer surveys have been collected for the design of a simple aircraft and the desired requirements are fast, efficient, reliable, spacious, and inexpensive (see Figure 1-17). An actual HQ would almost certainly have more than five requirements, but, again, this demonstration will be kept simple.
The survey has requested that potential customers rate the requirements using values between 1 (not important) and 5 (very important). This is placed in a matrix as shown in the figure [1-17]. This way, ‘fast’ has a rating of 3.0 (moderately important), ‘efficient’ has a rating of 5.0 (very important), and so on. Then, the ratings are added and the sum (18.5) is entered as shown. The column to the right shows the percentages of the ratings. For instance, the percentage associated with the requirement ‘fast’ is 100 × 3.0/18.5 = 16.2%.
The next step requires the designing team to list a number of engineering challenges that relate to the customer requirements. For instance, the requirement for ‘efficiency’ calls for special attention to the lift and drag characteristics of the aircraft. These have been listed in Figure 1-18 with some other engineering challenges, such as ‘size of aircraft’, ‘drag’, ‘weight’, and so on. These will be revisited in Step 4.
The roof (see Figure 1-18) is used to indicate interrelationships between the various engineering challenges. It must be kept in mind that the roof sits on top of the technical requirements matrix and the diagonals enclose the columns of engineering challenges. This arrangement must be kept in mind for the following discussion.
The roof consists of two parts: the roof itself and, for a lack of a better term, the fascia. The fascia is used to indicate whether the challenge listed below (e.g. ‘drag’ or ‘weight’) has a favorable effect on the product. This way, more ‘power’ has a favorable effect (more power is good) and this is indicated using the arrowhead that points up. ‘Production cost’, on the other hand, has negative effects on it, so the arrow points down.
The other challenges have been identified in a similar manner, except the last one (‘size of aircraft’). It is not clear whether or not a larger or a smaller version of the aircraft is beneficial to the product, so it is left without an arrow. Naturally, this may change if the team decides this is important; all parts of the HQ are ultimately decided by the design team and its consensus may differ from what is being shown here.
Next consider the roof itself, shown as the diagonal lines in Figure 1-18. It is used to indicate positive and negative relationships between the challenges. These are typically denoted with symbols (e.g. + for positive and − for negative), but here the following letters are used:
NN – means there is a strong negative relationship between the two engineering challenges.
N – means there is a negative relationship.
P – means there is a positive.
PP – means there is a strong positive relationship between the two engineering challenges.
Consider the columns containing ‘size of aircraft’ and ‘drag’. It can be argued that there is a strong negative relationship between the ‘size of aircraft’ and ‘drag’ (large aircraft = high drag). This is indicated by entering NN at the intersection of their diagonals. Similarly, there is a positive relation between ‘size of aircraft’ and ‘lift’, indicated by the P at the intersection. Some might argue there should be a strong positive relationship; however, if the size refers to the volume of the fuselage rather than the wings, then the relationship is arguably only positive. This shows that the build-up of these relationships is highly dependent on interpretation, requiring the design team to reach a consensus. Once complete, the example letter combinations are entered as shown in Figure 1-18.
The next step is to try to place weight on the engineering challenges as they relate to the customer requirements. This is accomplished using the inter-relationship matrix (see Figure 1-19). In other words, consider the customer requirement ‘fast’. It will have a strong influence on the engineering challenge ‘drag’. However, ‘lift’ will be less affected by it. Similarly, the customer requirement ‘reliable’ will not have any effect on the ‘weight’, and so on.
The design team must come up with a scale that can be used to indicate the severity of such associations. It is not uncommon to use a scale such as the one shown below:
9 – means the customer requirement has great influence.
3 – means the customer requirement has moderate influence.
For clarity, omit entering numbers in cells where no influence exists. Some people enter special symbols in the cells that mean the same, but in this author’s view it only adds an extra layer of confusion. Note that these numbers will be used as multipliers in the next step, which makes it much simpler to enter them directly.
The target matrix (see Figure 1-20) represents the results of a cross-multiplication and summation that is used to determine where to place the most effort during the development of the product. The operation takes place as follows:
Consider the percentage column of the customer requirements matrix (16.2%, 27.0%, etc.) and the first column of the technical requirements column (‘size of aircraft’, 9, 1, 1, etc.). These are multiplied and summed as follows:
The remaining columns are multiplied in this fashion, always using the percentage column, yielding 4.86, 3.24, 2.43, and so on.
The next step is to convert the results into percentages. First, add all the results (4.24 + 4.86 + …) to get 24.73. Second, for the first column, the percentage of the total is 100 × 4.24/24.73 = 17.2%, 100 × 4.86/24.73 = 19.7% for the second one, and so forth. These numbers are the most important part of the HQ, as the highest one indicates where most of the development effort should be spent. The results and the entire HQ can be seen in Figure 1-21. It can be seen that, in this case, the ‘production cost’ and ‘drag’ are the two areas that should receive the greatest attention.
It is often helpful to create a matrix to compare, perhaps, an existing company product to that of the competition. This helps to identify shortcomings in the company products and to improve them. A comparison matrix is shown in Figure 1-21, where they have been “graded” in light of the customer requirements, allowing differences to be highlighted. This way, while the customer requirement ‘fast’ has a score of 3.0, it is possible the design team values it a tad lower, or at 2.5. However, the team may also conclude that competitor aircraft 1 and 2 emphasize it even less. Such a conclusion should be based on hard numbers, such as drag coefficients or cruising speed, and not subjective opinions.
The purpose of this section was to introduce the reader to the HQ as a possible tool to help with the development of a new product (or the redesign of an existing one). The interested reader is directed toward the multitude of online resources that add further depth to this topic.
A picture is worth a thousand words. This old adage is particularly true in the world of engineering, where detailed information about complicated mechanisms, machinery, and vehicles, must be communicated clearly and effectively. While the topic of geometric dimensioning and tolerancing (GDT) and industry standards in technical drafting is beyond the scope of this book, saying a few words about the presentation of images is not. The practicing engineer will participate in many meetings and design reviews, where often a large number of experts in various fields gather and try to constructively criticize a new design. The process is often both exhausting and humbling, but is invaluable as a character-builder. At such moments, being able to adequately describe the functionality of one’s design is priceless and no tool is better for that than a figure, an image, or a schematic. Three-dimensional depictions are particularly effective. The modern aircraft designer benefits greatly from computer-aided design (CAD) tools such as solid modelers (Pro/E, Solidworks, CATIA, and others), which allow very complicated three-dimensional geometry to be depicted with a photo-realistic quality. Highly specialized software, for instance, finite element analysis (FEA) and computational fluid dynamics (CFD) programs, allow the engineer to describe the pros and cons of very complicated structural concepts and three-dimensional flow fields, and even add a fourth dimension by performing time-dependent analyses. It cannot be over-emphasized to the entry-level engineer to get up to speed on this technology. It not only helps with communication, but also develops a strong three- and four-dimensional insight into engineering problems.
The three-view drawing is a fundamental presentation tool the engineer should never omit. Airplane types are commonly displayed using three-view drawings, showing their top, side, and frontal views. Such drawings are an essential part of the complete submittal proposal package for any aircraft. Although such presentation images date back to the beginning of aviation, they are as important to any proposal as wings are to flying. Figure 1-22 shows a typical drawing, with the added modern flare in the form of a three-dimensional perspective rendering.
The modern solid modeler software (CAD) has revolutionized aircraft design. Long gone are the sloped drafting tables the technical drafter used to work with, as are the special architectural pens that deliver uniform line thicknesses and other tools of the past. These began to disappear in the late 1980s and early 1990s. Now, drafters are equipped with personal computers or workstations and model complicated parts and assemblies in virtual space. At the time of writing, programs such as Pro/E (Pro-Engineer), Solidworks, and CATIA are the most common packages used and pack an enormous sophistication in their geometric engines. They can be used on any desk- or laptop computer and allow mechanical linkages to be animated, photorealistic renderings of the design to be made, and some even offer limited finite element and computational fluid dynamics capabilities. These methodologies allow highly mathematical surfaces, referred to as NURBS, to be defined and modified on a whim. Such tools provide perfect mathematical definitions of complicated compound surfaces and, therefore, allow curvature-perfect OML to be created. Images from such programs can be quite persuasive and informative. Figure 1-23 shows an image of a twin-engine regional jet design from one such package, superimposed on a background image taken at some 18,000 ft. The resulting image can be of great help in engineering and marketing meetings.
The modern structural analysis often includes very sophisticated finite element analyzers, which are capable of producing very compelling images. While such images should be used with care (as their compelling nature tricks many into thinking they actually represent reality, which they may not), they can give even a novice an excellent understanding of load paths as well as where stress concentrations reside. While such images are usually available only after detailed design work has begun, images from previous design exercises can sometimes be helpful in making a point about possible structural concepts.
Figure 1-24 shows stress concentrations in a forward shear-web of the wing attachment/spar carry-through for a small GA aircraft, subject to an asymmetric ultimate load. The elongated diamonds in the center of the spar carry-through are corrugations intended to stiffen the shear-web, but these cause high stress concentrations on their own.
Computational fluid dynamics is a vibrant field within the science of fluid mechanics. Spurred by a need to predict and investigate aerodynamic flow around three-dimensional bodies, this computational technology has become the stalwart of the modern aerodynamics group. Similar advice as above should be given to the entry-level aircraft designer. The images generated by the modern CFD packages are often mindboggling in their sophistication (Figure 1-25). It is therefore easy to be lulled into trusting them blindly – but they may not necessarily show what happens in real flow. This is not to say they never resemble reality, only that they do not always.
Few visual representations are as capable of illustrating the complexity of an airplane as the cutaway drawing. Such images are normally extremely detailed and require a great depth of knowledge of the internal structure of an airplane to correctly prepare. A case in point is Figure 1.26, which shows a cutaway of the Dassault Falcon 7X business jet. While certified to 14 CFR Part 25 (Commercial Aviation) rather than 14 CFR Part 23 (General Aviation) the figure depicts the state of the art in civil aircraft design in the early twenty-first century.
The work of the engineer is primarily of the “mental” kind; it largely involves the process of thinking. This poses an interesting challenge for anyone hiring an engineer – how can this intangible product be captured so it does not have to be recreated over and over again? The answer is the engineering report and engineering drawing.
An engineering report is a document that describes the details of a particular idea. Engineering reports encompass a very large scope of activities. It can be a mathematical derivation of a particular formula, listing of test setup or analysis of test results, justification for a particular way to fabricate a given product, evaluation of manufacturing cost or geometric optimization. The list goes on. Engineering reporting can also be the completion of proposals or even the writing of scientific papers. Regardless of its purpose, the report must always be written with the emphasis on completeness and detail. Such technical reports are how a company retains the thinking of the engineer so it does not have to be ‘rethought’ next time around – it turns the intangible into something physical.
The organization and format of reports vary greatly. It is not practical to present any particular method here on how to write a report. However, what all reports have in common is that they should be objective, concise, and detailed. One of the primary mistakes made by entry-level engineers is to ignore the documentation of what appears trivial. The author is certainly guilty of making such mistakes. While working on a specific assignment, one effectively becomes an expert on that topic. Grueling work on such a project for a number of weeks or months can blur the senses to what needs to be included in the engineering report. The expertise, surprisingly, skews one’s perspective; complex concepts become so trivial in the mind of the engineer that their definitions or other related details get omitted from the documentation. Then, several months or even a few years later, one has become an expert on a different topic. The previous work is a distant memory, securely archived in the digital vaults of the organization. Then, something happens that calls for a review of that past work. It is then that one realizes how many important concepts one left out of the original report and these, now, call for an extra effort and time for reacquaintance.
Additionally, detailed and careful documentation is priceless when you have to defend your work in a deposition. It is what US companies use every day to defend themselves against accusations of negligence, saving themselves billions of dollars.
The modern engineering drawing has become a very sophisticated method of relaying information about the geometry of parts and assemblies. The details of what is called an “industry standard drawing” will not be discussed here, other than mentioning that such drawings must explain tolerance stack-ups and feature a bill of materials and parts to be employed. Today, engineering drawings are almost exclusively created using computers by a specialized and important member of the engineering team – the drafter. A competent drafter knows the ins and outs of the drafting standards and ensures these do not become a burden to the engineer.
Engineering drawings are typically of two kinds: part drawing and assembly drawing. The part drawing shows the dimensions of individual parts (a bracket, an extrusion, a tube, a bent aluminum sheet, etc.), while the assembly drawing shows how these are to be attached in relation to each other. A home-built kitplane may require 100-200 drawings, a GA aircraft may require 10,000, and a fighter or a commercial jetliner 50,000 to over 100,000 drawings. For this reason, a logical numbering system that allows parts and assemblies to be quickly located is strongly recommended. This way, all drawings pertaining to the left wing aileron could start with WL-A-drw number, while the right wing flap system would be WR-F-drw number. Such systems increase productivity by speeding up drawing searches – which are very frequent.
|Symbol||Description||Units (UK and SI)|
|AR||Wing aspect ratio|
|BHP/W||Brake horse power-to-weight ratio||BHP/lbf or BHP/N|
|CG||Center of gravity||ft, m, or %MAC|
|KCAS||Knots calibrated airspeed||ft/s or m/s|
|MAC||Mean aerodynamic chord||ft or m|
|TR||Wing taper ratio|
|VHT||Horizontal tail volume coefficient|
|VvT||Vertical tail volume coefficient|
|W||Weight||lbf or N|
|W/S||Wing loading||lbf/ft2 or N/m2|
1. Boyer Pascal. Religion Explained. Basic Books 2001.
2. In: http://faa.custhelp.com/app/answers/detail/a_id/154/kw/%22general%20aviation%22/session/L3RpbWUvMTMzNTgwOTk4MS9zaWQvSkxqTW9ZV2s%3D.
3. Zodiac CH. 601 XL Airplane, Special Review Team Report. January 2010.
4. In: http://www.faa.gov/aircraft/gen_av/light_sport/media/StandardsChart.pdf.
5. Anonymous. Airplane Flying Handbook. FAA-H-8083–3A 2004.
6. NTSB/ARA-11/01. Review of U.S Civil Aviation Accidents 2007–2009. NTSB 2011.
7. Anonymous. Aircraft Accident Report: Loss of Control on Approach; Colgan Air, Inc.; Operating as Continental Connection Flight 3407; Bombardier DHC-8-400, N200WQ; Clarence Center, New York; February 12, 2009. NTSB/AAR-10/01 2010.
8. In: http://www.fas.org/programs/ssp/man/uswpns/air/fighter/f35.html.
9. U.S. Department of Defense News Transcript. Briefing on the Joint Strike Fighter Contract Announcement. October 26, 2001; 4:30 pm EDT http://www.defense.gov/transcripts/transcript.aspx?transcriptid=2186; October 26, 2001.
10. Source http://www.emsstrategies.com/dm090203article2.html.
11. Torenbeek Egbert. Synthesis of Subsonic Aircraft Design. 3rd ed Delft University Press 1986; p. 499.
12. In: http://www.easa.europa.eu/agency-measures/certification-specifications.php.
13. In: http://www.airweb.faa.gov/Regulatory_and_Guidance_Library/rgMakeModel.nsf/MainFrame.
14. In: http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgAD.nsf/Frameset.
1A frustum-style fuselage is a tapered structure that does not feature compound surfaces. It is discussed in Chapter 12, The Anatomy of the Fuselage.
2Note that some manufacturers of composite structures claim that curing composites using vacuum “bagging” is equally effective as using an autoclave – it is certainly more economical. For instance see: http://www.gmtcomposites.com/why/autoclave.
3GA aircraft often specify airframe lifetime in terms of flight hours rather than cycles because they are operated in a much less rigorous environment than commercial aircraft.