4.1.1 The Content of this Chapter

Section 4.2 presents a number of important design considerations and discusses their advantages and disadvantages. This is intended to help the designer to develop a keener eye for the implications of selecting a particular configuration. This awareness can avoid costly mistakes for any company designing a new aircraft.

4.1.2 Requirements, Mission, and Applicable Regulations

As stated in Step 1 of the Conceptual Design Algorithm for a GA Aircraft presented in Section 1.3.1, the design process begins by the execution of the statement: “Understand requirements, mission definition, and the implications of the regulations to which the airplane will be certified.” This means:

These three have a profound impact on the airplane configuration.

4.1.3 Past and Present Directions in Aircraft Design

It may strike many as a surprise that aircraft design would be affected by fashion. It would seem that something as vain as style would be beyond engineering, but a review of the history of aviation reveals this is not the case. It is actually vibrant with shapes and components that were popular at one time, but later became a part of history, although others stuck around and became the norm. Table 4-1 lists a few fads that are clearly visible by observing the evolution of the aircraft from early times to modernity.

4.1.4 Aircraft Component Recognition

In the discussion that follows, it is imperative the reader recognizes the terminology used. Figures 4-2 through 4-5 are intended to familiarize the reader with the external parts of typical aircraft. All aircraft feature many parts and components that are directly exposed to airflow and affect not only the performance and operation of the aircraft, but also the cost of manufacturing and maintenance. The location of most of these components (e.g. Pitot tubes, static sources, antennas, and others) is usually the consequence of hard work which involves the various design groups coming to an agreement on the most suitable location. For instance, a static port must be installed in an area where surface pressure remains relatively constant with angle-of-attack. This area, on the other hand, may be prime real-estate for an antenna, which requires an unobstructed signal path in order to work well, or the installation of a NACA duct for an inlet of cooling air for the avionics or other components. Having an understanding of where specific components may have to be placed on the aircraft will help the designer anticipate and avoid possible detail design conflicts.

The main components of an aircraft are: wings; fuselage; nacelle; empennage; horizontal and vertical tail; power plant; and landing gear, to name a few. These components can be broken down further into subcomponents. For instance, it is possible to break the wing into a main element, flap, aileron, spoiler, wingtip, and so on. Sometimes it is convenient, if not necessary, to break these down further. For instance, the flap consists of a spar, ribs, skin, attachment brackets, access panels, and so on. Of the above primary components, three need some further definition:

A fuselage is a structural body not intended to generate lift (although it may) whose purpose is to contain engine, fuel, occupants, baggage, and mission-related equipment, although not always simultaneously. A fuselage is always mounted to lifting and stabilizing surfaces, if not directly, then through structural members.

An empennage refers to the horizontal and vertical tail of a conventional aircraft configuration. The word is of French origin, where it refers to the tail feathers of an arrow. Sometimes it is taken to mean the general region or assembly of the fuselage that contains both the horizontal and vertical tail.

A nacelle is a fuselage that does not carry an empennage. Nacelles usually carry an engine, but may or may not house occupants. Nacelles can be mounted to a lifting surface, such as a wing, or to a non-lifting geometry like a fuselage.

4.2.1 Vertical Wing Location

One of the most prominent features of any aircraft is the vertical placement of the wing. This section presents five common such configurations and presents arguments for and against each. The vertical wing location may end up being based on a number of factors such as:

Ideally, the designer weighs each factor and decides the best position for the wing. It is important to avoid succumbing to biases such as “low-wing airplanes are always faster” or “high-wing airplanes have better stall characteristics.” There is no law of nature that says that one or the other is superior. It all depends on other details, such as overall drag, airfoil selection, geometry of the airplane, and so on. It is the interaction of the components constituting the complete aircraft that matters. In fact, a case can be made that a high wing will be faster as it has less destructive effect on wing lift over the fuselage. There are fast and slow examples of either configuration. Examples include the low-wing P-51 Mustang (fast) versus Evans VP-1 Volksplane (slow) and the high-wing Mitsubishi MU-2 (fast) versus the Piper J-3 Cub (slow). The most common vertical wing placements are shown in Figure 4-6. The designer is urged to consider the consequences of the selection that are detailed below.

4.2.2 Wing Configuration

Airplanes are also categorized based on the number of wings they have. The monoplane is by far the most common configuration as it is without a doubt the easiest to make aerodynamically efficient. The primary advantage of the biplane or triplane configuration is the large wing area that can be packed in a small wingspan. This allows for very maneuverable airplanes with relatively low stalling speed without flaps. The drawback of the configuration is the aerodynamic inefficiency that stems from placing the low-pressure region of the lower wing close to the high pressure region of the upper wing. This reduces the production of lift, requiring higher AOA to generate the same lift coefficient and, consequently, higher lift-induced drag.

The difference between a sesquiplane and a biplane is the shorter span of the lower wing (Figure 4-8). This improves the efficiency of the outboard part of the upper wing by enabling higher pressure to be generated on its lower surface. It also results in a phenomenon that makes the configuration ideal for agricultural aircraft: the generation of four distinct wingtip vortices that help spread fertilizer or insecticide more effectively. This book primarily focuses on monoplanes, but details of biplane design are provided in Appendix C1.1.3, Conceptual Design of Small Biplanes.

The difference between a canard and a tandem plane is in the size of the forward wing. Generally, the elevator is installed in the forward lifting surface. Both lifting surfaces generate upward-pointing lift vectors in level flight and both forward surfaces are highly destabilizing, longitudinally.

4.2.3 Wing Dihedral

The dihedral angle is the angle the wing plane makes with the horizontal. It allows the aircraft designer to provide the airplane with roll stability and a way to affect the severity of dynamic modes such as Dutch roll. Its primary effect is on the stability derivative C_lβ (dihedral effect). In addition to the dihedral angle, the magnitude of the dihedral effect depends on the vertical location of the wing and sweep angle. Ultimately, the designer must predict the dynamic stability characteristics of the airplane design and evaluate the appropriate dihedral angle.

Figure 4-9 shows an airplane at airspeed U, banking through an angle ϕ. The banking results in a side-slip, which effectively results in a yaw angle β. The yaw angle results in side flow component amounting to U·tan β, which when combined with the change in vertical flow due to the roll causes a net change in angle-of-attack, Δα, on each wing. The subsequent change in lift (ΔL) on each wing is shown in the figure, and it generates a restoring rolling moment (one that tends to rotate the aircraft back to level flight), here denoted by the letter M.

Common dihedral configurations are shown in Figure 4-10. Of these the three on the left are most commonly used. The cranked dihedral is used extensively on the French Jodel and selected Robin aircraft, as well as on some sailplanes. However, it is also featured on the Argentinian FMA IA-58 Pucará twin turboprop ground attack aircraft and of course the McDonnell-Douglas F-4 Phantom.

The gull-wing configuration is fairly rare, being most famously used on the Vought F4U Corsair, where its purpose was to increase the propeller clearance for carrier operations. It was also used for various reasons on the Blohm & Voss BV-137, Caproni Ca-331b Raffica, Dewoitine HD-780, Fairey AS-1 Gannet, Heinkel He-112 B, and Junkers Ju-87 Stuka.

The inverted gull-wing configuration is often used for twin engine seaplanes, where it helps take wing-mounted engines and propellers away from the spray of water. It is featured on the Beriev Be-6, Be-12, Chyetverikov MDR-6, and Moskalev 16 amphibians and seaplanes. It is also used on the Göppingen Gö-3 Minimoa sailplane, and the PZL P-1, PZL P-11, Piaggio P-166, and Supermarine 224 landplanes. The Stinson SR-10 is an example of an aircraft that could fit into this class, featuring a wing whose upper surface has a distinct gull-wing break. However, the lower surface forms a straight line and the spar does not have a break, rendering it more of a transitional form.

4.2.4 Wing Structural Configuration

Nowadays, the wing’s structural layout is based on either a cantilevered or a strut-braced methodology (see Figure 4-11). Both have their pros and cons, with strut-braced having a higher drag than the cantilevered configuration. However, the maximum shear and bending loads of the strut-braced wing are much less than those of the cantilevered wing, resulting in a lighter wing structure.

As an example, consider Figure 4-12, which shows a strut-braced (top) and cantilevered wing (bottom) subject to an equal aerodynamic load, represented by the simplified trapezoidal lift distribution. The lift distribution of real wings is not trapezoidal, but the accuracy of its shape is not important to the point being made here.

The upper part of the figure shows where the maximum shear and bending moment occurs on the strut-braced configuration. Their corresponding magnitudes are given by V_max and M_max, respectively.

The lower image shows shear and moment diagrams for the cantilever configuration. It shows the maximum shear is 2.3× greater than that of the strut-braced wing and the moment is 4× greater. Conversely, although not shown, there is a substantial compression load that must be reacted between the wing-to-fuselage and strut-to-fuselage attachment points. It follows that the structural weight of the strut-braced wing will be much less and it should be given a serious consideration if aerodynamic efficiency is not a factor.

4.2.5 Cabin Configurations

Here, the discussion of cabin configuration will be limited to light aircraft only, as cabins for passenger aircraft are presented in more detail in Chapter 12, The anatomy of the fuselage. Typically there are two kinds of cabin styles: canopy and roofed (see Figure 4-13). An advantage of the roofed cabin is increased protection in the case of a turnover accident. This configuration requires an entry door to be added, preferably one on each side. These may present some fit and function issues in production, although similar arguments can be made against the canopy. The roof also limits the field-of-view.

A canopy offers exceptional field-of-view to the pilot, which is very desirable for many travelers, in addition to reducing the risk of a mid-air collision. However, turnover mishaps are of considerable concern for such aircraft. This is reflected in the regulation 14 CFR 23.561, General. The applicant must demonstrate compliance by reinforcing the window frame to which the windscreen is attached to prevent a collapse that would harm the occupants. Often called the A-pillar, this frame effectively becomes a rollover cage, increasing its girth and weight.

Excessively high cabin temperatures due to greenhouse effects are a drawback of the canopy. The configuration should allow for a canopy that can be left open during ground operations (while taxiing) for cabin cooling. This is particularly important if the airplane does not have air conditioning (such systems are not common in small aircraft). Reduction in green-house effect makes the roofed cabin configuration a viable candidate.

The acrylic canopy must be installed and operated with care (if flexible), as cracks may develop around fastener holes. The configurations with the canopy should feature an appropriate mechanism to prevent it from opening in flight due to aerodynamic forces. For instance, should the latching mechanism fail in flight, an aft-hinged or side-hinged canopy is at risk of opening due to aerodynamic forces. If the canopy departed the aircraft, it might damage the HT or VT, possibly rendering the vehicle uncontrollable. If the canopy opened up and stayed with the vehicle, a substantial asymmetry in loads could result rendering the aircraft uncontrollable as well. Of course, either canopy-mounting technique makes it easier to board the airplane than a forward-hinged canopy.

4.2.6 Propeller Configuration

There are two fundamental ways to mount a propeller to an engine; as a tractor or as a pusher (see Figure 4-14). Either configuration is practical for piston engine, gas turbines, and electric motors.

The pros and cons of these configurations are discussed in detail in Section 14.1.2, Propeller arrangement.

The tractor configuration is a proven arrangement that is generally suitable for most applications. It provides undisturbed air for the propeller although the higher airspeed and lower quality air increase the drag of the body immersed in the propwash.

The pusher propeller is a good solution to some specialized mission requirements, for instance for single-engine reconnaissance or observation missions. This will remove the propeller from the field of view and allow a high-visibility cockpit to be designed. There are a number of issues the designer should be aware of. Propeller manufacturers are generally apprehensive about the arrangement as it introduces problems not always anticipated by the designer. Some of those are detailed in Section 14.1.2, Propeller configurations. However, the aspiring designer should not let this aversion influence the introduction of a particular configuration, as propeller manufacturers are happy to work on any such project. They only want the designer to recognize the shortcomings.

4.2.7 Engine Placement

As intuition would hold, the placement of any significant source of force on an aircraft is of great concern. Engine thrust is an example of such a source and, in magnitude, is second only to that of the wing lift. The moment generated by this force must be arrested by the stabilizing surfaces. If the engine thrust is placed above the CG of the airplane, the consequence will be a nose pitch-down moment that, on conventional aircraft, must be trimmed out using elevator trailing edge up (TEU) deflection (see Figure 4-15). The opposite holds for an engine whose thrustline is below the CG. The higher the thrust, the greater is the deflection required, although a better method is to enlarge the size of the elevator or increase the planform area of the horizontal tail, or a combination thereof.

In short, the larger the value of Δz is in Figure 4-15, the larger must be the elevator authority. The moment generated by the thrust must be taken into account during the design phase to prevent the dangerous potential of an undersized stabilizing surface. The possibility is serious enough to warrant a discussion in Section 23.3, GA aircraft design checklist.

Noticeable power effects are another consequence of engine placement. These can be quite complicated for propeller-powered aircraft, as is discussed in detail in Section 14.2, Propeller effects, and some of those are shared by the jet engine. While not necessarily dangerous, they can be particularly annoying to the pilot. If each power change calls for swift pilot reaction, the airplane will simply be less pleasant to fly. In spite of such nuisances, some aircraft require the thrustline to be high, such as seaplanes, as this protects the propeller from water spray. For such airplanes, pitch changes with power settings are accepted because it saves the propeller. For the operators, it is just something the pilot has to get used to.

Another important consideration for the layout of a propeller is the effect of propwash. If it flows over a control surface like the horizontal and vertical tail, it will improve the control authority at high power settings, but the higher airspeed also increases the drag of the surface, albeit modestly.

Figure 4-16 shows a number of common engine placements. Configuration A features the jet engines in pods (or nacelles) mounted to the aft part of the fuselage. This configuration was first introduced in the 1960s in the French Sud-Est Caravelle passenger jetliner. The placement results in modest, if any, pitch effects and is intended to reduce engine noise in the cabin, although noise in the aft-most part of the cabin is increased, if anything.

Configuration B mounts the engines below the wing, using pylons. This configuration will result in substantial pitch effects, although some kind of stability augmentation system (SAS) can be employed to reduce the effect. The configuration is vulnerable to foreign object damage (FOD) but, in spite of that, is the most common engine placement found on passenger jetliners. The placement is beneficial from a structural standpoint as the weight of the engines introduces bending moment relief, which ultimately reduces airframe weight. Additionally, its forward position has a favorable effect on the flutter characteristics of the wings.

Configuration C features the engine above the wing and will generate nose pitch-down moment at high thrust settings. The configuration was first introduced in the 1970s on the German WFV-Fokker 614 jet, but later adopted on the Hondajet, where the intent is to avoid the ground clearance problem of under-wing nacelles. However, it can introduce peculiar aerodynamic and flutter issues [2,3].

Configuration D is a twin-engine turboprop aircraft with propellers mounted in nacelles on the wing. This is the most common method to install turboprops in such aircraft. However, it can lead to serious asymmetric thrust condition in a one engine inoperative (OEI) situation. Also, the rotating propellers can cause a so-called whirl-flutter or subject the airframe to fatigue through life-cycle oscillations (LCO) of the engine-wing combination.

Configuration E is a tractor propeller configuration, which has limited thrust effects due to engine placement, but more due to the physics of propeller thrust generation.

Configuration F is a seaplane, with the engine placed on top to protect the propeller from sea spray. It is a pusher configuration. It suffers from substantial pitch effects but, as stated earlier, this detriment is accepted due to the protection the prop enjoys.

Configuration G is a pusher configuration, which has the thrustline sitting somewhat high, although this is not as much of a problem as one might first think. The propeller will help keep flow attached on the aft part of the fuselage. It is subjected to some drawbacks of the pusher propeller configuration, although the particular configuration shown improves safety by making it hard to accidentally walk into a rotating propeller.

Configuration H is a single-engine jet that features a turbofan engine on a pylon on top of the fuselage. It would suffer from substantial pitch effects, although this can be partly remedied by deflecting the nozzle a few degrees up. The advantage of the engine placement is that it places the inlet in the airstream so it has a high pressure recovery, even at high angles-of-attack (AOA).

Configuration I features a buried engine, which results in minimal pitch effects with thrust, if any. The drawback is that the bifurcated inlet reduces pressure recovery at the front face of the compressor, reducing maximum available thrust. The bifurcated duct is also problematic if the airplane is operated in icy conditions as ice will accrete in the bend of the inlet.

Configuration J is a small four-seat twin-engine propeller-powered aircraft, suitable as a light VIP transport or reconnaissance aircraft. Its piston engines are mounted on the wings and must sit high enough to prevent damage to the propellers due to small objects that might be thrown from the operation of the nose landing gear on unimproved runways, or if it is subject to a flat tire on any of the landing gear. The nacelles are designed to accommodate the retractable landing gear. It features an H-tail to help generate restoring yawing moment in the case of an OEI situation.

4.2.8 Landing Gear Configurations

A large number of different landing gear configurations have been developed for use in aircraft. Six examples are shown in Figure 4-17. It should be stressed it does not show all the options, only those that are used on 99.99% of all GA aircraft. The most widely used configuration is the tricycle, followed by the taildragger. One of the advantages of a taildragger is less drag than the tricycle. An example of improvements attained by a small aircraft is that of the Cessna 150. It is claimed that converting a tricycle version of the aircraft to a taildragger gave it an increased cruising speed of nearly 8 knots. [4]. The benefit is always a function of the airplane and its overall drag, but a 4–10 knot increase in cruising speed is reasonable.

The monowheel with outriggers is a popular design for sailplanes and motor gliders like the British-designed Europa XS and the German Scheibe Tandem-Falke. The monowheel configuration reduces the drag of the landing gear. The same is true of the tandem wheel configuration, although it is rarely used in GA aircraft. The British Hawker Harrier is the best-known example of tandem wheel configurations.

Tricycle, taildraggers, mono- and tandem wheel configurations may all be retractable. Fixed landing gear will increase the drag of the airplane and, if this is the case, the designer should strongly consider wheel fairings for drag reduction. The floats increase drag substantially, but allow operation on land and water. They are still popular among many pilots who are more interested in access to obscure wilderness retreats than high airspeeds. Floats and amphibious airplanes are dealt with in Appendix C3, Design of seaplanes.

The tricycle landing gear makes the vehicle dynamically stable on the ground and reduces the risk of a ground loop. For this reason, it is better for inexperienced pilots and, thus, better suited for trainer aircraft. Similarly, the taildragger configuration is dynamically unstable and more prone to ground looping. It is better for operation off unimproved runways. The pros and cons of and the conceptual design of the landing gear are presented in far more detail in Chapter 13, The anatomy of the landing gear.

Taildragger aircraft have a number of advantages that make them more attractive for “bush-plane” operations. The primary advantage is the high AOA that can be generated at a low airspeed (in fact at zero airspeed). This way the airplane can be allowed to accelerate and then the pilot can quickly “drop” the tail, allowing the airplane to lift off in ground effect. Such techniques give the configuration markedly shorter runway requirements and, thus, make it better suited as a bush-plane. Interestingly, this is not always reflected in the data. For instance, the Cessna Model 180 (taildragger) and 182 (tricycle) are effectively identical excluding the landing gear. Jane’s All the World’s Aircraft 1970–71 [5] reports each having identical T-O and landing distances at the same weight, but the 180 is favored as the bush-plane option, providing evidence that pilot technique is imperative. The configuration is better suited for operation off unimproved landing strips because two wheels on the ground reduce the possibility of the landing gear hitting ground obstructions (large rocks or other obstructions) during the T-O run (two wheels versus three).

The taildragger configuration is generally thought to be harder to land and maneuver on the ground than a tricycle gear due to a high deck angle, which makes it harder to see over the nose of the airplane. This configuration is usually used on small aircraft, although it has been featured on large aircraft. The largest taildragger ever built is the eight-engine Soviet Tupolev ANT-20 Maxim Gorky, with a gross weight of 116,600 lb_f. The Curtiss C-46 Commando is another large taildragger, although its gross weight of 48,000 lb_f is dwarfed by the ANT-20.

The total structural weight of the float is the highest, but least for the monowheel. The structure required to react the main landing gear impact load will weigh less than the structure required to react the impact loads of both tricycle and taildragger. The configuration is also the least expensive to manufacture. It is a drawback that it is vulnerable to crosswinds and too much taxiing on the ground. The same holds for the tandem wheel.

4.2.9 Tail Configurations

A number of possible tail configurations are shown in Figure 4-18. Configuration A is a conventional tail, B is a cruciform tail, C is a T-tail, D is a V-tail, E is an H-tail, G a Y-tail, H an inverted Y-tail, and Configuration I is an inverted V-tail. The pros and cons of these tails are detailed in Chapter 11, The anatomy of the tail, and will not be further addressed here.

4.2.10 Configuration Selection Matrix

Many manufacturers of aircraft know up front what configuration is to be designed. Regardless of the internal debate that may take place, and to which we are not privy, the history of aviation shows there are certain themes that present themselves in these designs. All single-engine Cessna aircraft are high-wing (except the Ag Wagon and Ag Cat agricultural aircraft). All Piper and Beech are low-wing (except the Piper Cub and Tri-pacer), as are all Mooneys, which also feature their signature straight LE horizontal and vertical tails. However, this is not always the case and the resulting configuration is a consequence of internal debate. In this section, a method to help with the configuration selection is presented (see Table 4-3). Often, the selection is compounded by the fact that all candidate designs can be shown to meet the performance and operational requirements. An observation of modern-day regional jets reveals that a number of dissimilar configurations can clearly perform the design missions effectively. Aircraft as disparate as the Dornier 328 (high wing, engines on wing), Bombardier CRJ200 (low wing, aft podded engines), and Embraer 175 (low wing, engines on wing) are examples of this. Granted there is a difference in fuel efficiency and the economics of each, however, what are the reasons behind these configurations? Why would these three manufacturers come to such different conclusions about their particular configuration?

If many different candidate configurations are being considered, it is prudent to evaluate each based on a list of desirable and undesirable characteristics. The selection of a configuration is something that, in particular, baffles the student of aircraft design and the less experienced (some would say less-opinionated) designer. The weighted tabulation of Table 4-3, which considers the development of a two-seat trainer aircraft, can be very helpful when down-selecting a configuration. The trick is to phrase features such that a beneficial one receives the high score and an unfavorable one a low score and to allow the same score to be given more than once.

The number and selection of “questions” should be the result of an internal evaluation, where everyone who has a stake in the outcome has an opportunity to incorporate their concerns and goals. Note that in this case, configuration B beats the other configurations although all score well, indicating they are all plausible candidates for the particular mission. The greatest drawback of this approach is that, in addition to the nature of questions included, it is very susceptible to the weighting factors, and an honest debate about each should take place prior to the evaluation. The reader should keep in mind it is presented here as one, and not the only approach.