CHAPTER 2
What Do All the Parts Do?
This chapter covers the basic concepts of remote-controlled (RC) aircraft as seen through the lens of Brooklyn Aerodrome’s quick-to-build, inexpensive, and tough Flack flying wing. This chapter is directed at the complete novice. This is where terms such as elevon, speed control, and trim tab get explained. While the chapter generally should be useful for other kinds of RC aircraft, I am focusing on what is needed to get flat-plate-based flying wings working in your world. There are lots of ways to get flying—this is my way.
The Flight Cycle of a Flack
Once the Flack is built and the pilot has basic control of the plane, flights proceed as follows:
1. The flight battery is charged. This can take 3 hours on a low-power charger to 20 minutes depending on the capabilities of the charger and battery. It is a good idea to have more than one battery charged.
2. The transmitter is always turned on first with the throttle down.
3. The Flack is powered up by connecting the flight battery via the main power leads. There is no power switch on the Flack. The charging connector is not connected to anything.
4. A quick preflight check verifies that up/down/left/right commands work.
5. The throttle is advanced to two-thirds (depending on conditions).
6. The Flack is hand launched into the wind.
7. The Flack flies for 5 to 8 minutes depending on throttle settings.
8. The Flack should be flyable 1,000 feet away, but cheaper transmitters may not work that far. It is more fun to fly close in anyway. The Federal Aviation Administration (FAA) requests that model aircraft stay below 400 feet of altitude.
9. The cone of crashing potential dictates the pilot’s actions at all times—see Chapter 5.
10. Once the pilot knows how to fly, the Flack can do loops and rolls and fly inverted.
11. It can carry 4 to 6 ounces of payload, such as a camera or a parachute drop, or it can tow a streamer.
12. When the pilot notices that 4 minutes have passed or notices that the Flack seems to be lacking power, then he or she should begin to plan the landing.
13. Landing, like takeoff, is optimally done while flying into the wind.
14. The Flack is brought in under reduced power to land, with the last 5 feet or so of altitude being a pure glide with the motor off.
15. The Flack lands on its belly.
16. The pilot should check the motor for overheating by putting a finger on the motor housing.
17. Then repeat these steps with a fully charged flight battery.
The Pilot
My method starts with a person with good vision (corrected is fine), awareness of his or her surroundings, and a need to fly, as shown in Figure 2-1. If the pilot is a beginner, then he or she should have spent a good deal of time practicing flying on a simulator. Beginners can learn to fly without a simulator, but a simulator really helps with the basic dynamics of flying in a consequence-free environment. Chapter 12 tells where to get free/cheap flight simulators and how to set them up. The pilot is responsible for all that happens regarding the aircraft, even if events are out of his or her control. A healthy dose of worse-case-scenario reasoning is very helpful.
FIGURE 2-1 Pilot and assorted equipment.
The Builder
The builder (Figure 2-2) is often the pilot, but different skills are required for each task. The builder is patient and reads all the instructions once all the way through and thinks about what is happening before building. If the builder doesn’t understand what a step in the instructions is achieving, then he or she thinks about it until it is clear. Understanding what the goal of a step is makes a huge difference in execution because instructions cannot cover all contingencies. This skill is hugely valuable when planes need repair (they will) and when innovation is desired. If there are two or more builders, then one should read the instructions out loud and discuss with the other what they are going to do.
FIGURE 2-2 Builder and assorted equipment.
The Airframe
The airframe is where the Brooklyn Aerodrome style begins to express itself. The quick-to-build-from-scratch requirement forces us to work in the realm of flat plates of foam and tape and to minimize the number of parts. A traditional airframe has a separate fuselage, main wing, and tail. This means a lot of cutting, and the end result is not likely to tolerate damage well.
Without further ado, let’s get to know the core design of the Flack, as shown in Figure 2-3.
FIGURE 2-3 The core design of the Flack with major components called out.
The Wing
This big triangle of foam is where all the lift comes. The delta shape is a classic flying form that existed long before the invention of supersonic fighters. Alexander Lippisch was designing and flying very similar shapes before World War II. Nature even predates him with a flying-wing seed that has existed for 56 million years, as shown in Figure 2-4.
FIGURE 2-4 Alsomitra macrocarpa seed. (Photo courtesy of Scott Zona.)
The Elevons
The rear surface of the wing has two control surfaces that handle pitch and roll control. They are independent of each other, and both are used for aileron inputs and elevator inputs. Because both elevator and roll control are integrated into both control surfaces, the surfaces are called elevons, which is a portmanteau that works out as “elevator” + “aileron” = “elev + on.”
The Stabilizers
These surfaces function much in the same way as rudders on traditional airplanes or a skeg on a surfboard. But because they have no movable control surfaces, they are called stabilizers. They control yaw and keep the nose of the plane pointed straight into the airstream. Chapter 10 provides more detail about the aerodynamics of aircraft.
Flat-plate model airplanes have been around for quite a while. Materials vary from foam board to Coroplast to, my favorite material, Dow fan-fold. We all owe a great debt to those who innovated this approach to building park fliers.
The Deck
The deck is where all the electronics are attached, which brings me to another key feature of aircraft—revivification.
A brief story: Mark “Splinter” Harder had built an early version of the Flack with all the radio gear attached directly to the Dow blue foam that we currently use at Brooklyn Aerodrome. After a night of learning to fly, his delta wing was destroyed, and he was frustrated that he would have to redo all the work of attaching gear to another airframe. He suggested that the radio gear should be easily removed and reattached to the foam airframe. I brought out a piece of Coroplast that the radio gear could be attached to independent of the airframe, and the modern Flack was born. Figure 2-5 shows the initial drawing on the destroyed airframe from that conversation. The deck (Figure 2-6) does a few things.
1. It allows rapid transfer of the fussy radio gear to a fresh airframe in very little time.
2. It usefully stiffens the foam around the prop hole.
3. It firmly attaches all the bits that want to fly off in a crash to a very strong substrate via zip ties and/or tape.
FIGURE 2-5 An early destroyed delta wing with the first design of the deck drawn on it.
FIGURE 2-6 Deck with major components called out.
Moving on, we then have to control the Flack from a remote position. Enter the transmitter.
The Transmitter
Figure 2-7 shows a typical transmitter with labeled controls for Mode 2 flight, as used in the United States. The right stick is meant to mimic the traditional yoke of an aircraft with the stick perpendicular to the ground. Yoke control is standardized in full-scale aircraft so that pulling back on the stick raises the nose—think of movie fighter pilot scenes where someone is screaming “Pull up! Pull up.” This is the opposite of typical game controls. Novice pilots with a healthy dose of experience with games struggle with reversing their notions of up/down control. Try to sort this out on a simulator. This will be covered again in Chapter 5, where the reversal of roll control will be addressed as well.
FIGURE 2-7 Typical transmitter with Mode 2 controls.
How the Transmitter Works
The pilot’s eyes via the brain connect the pilot’s thumb/finger control inputs to the sticks of the transmitter. Those sticks communicate the desired position of both control surfaces and how fast the motor should be turning to the receiver. The underlying technology is not very important to understand, but the result is—if you move a stick on the transmitter 50 percent of its possible travel, then the motor or a servo will respond similarly and hold it. This is an idealized version of what is going on, but it is the general concept—variables include transmitter mixing of controls, programmable end points, etc.
The range of the transmitter should be at least 1,000 feet in clear air, but manufacturer claims vary wildly, and actual performance depends on conditions. Transmitters provide a range-test capability that reduces the power of the transmitter so that you can test the sensitivity of the receiver at smaller distances—consult the transmitter manual. For lesser equipment, the range can be a few hundred feet.
How to Control the Sticks
There are two major approaches to applying hands to the control sticks—thumb style and pen style. Pen style (control both sticks like a pen) is very popular among precision aerobatic/helicopter flyers, and the thumb style is popular with most everyone else (control sticks with thumbs on top of sticks). It’s a personal choice. Note that pen style generally requires a neck strap to support the transmitter’s weight.
Pilots should endeavor not to let the sticks autocenter by releasing them and allowing the centering springs to zero the controls. Flight simulators reinforce this behavior by having aircraft that are very stable and do not require constant corrective inputs. The real world is not so forgiving.
Mixing
On a typical RC airplane layout, there is one servo that controls the elevator, which, in turn, is responsible for pitch control. With a flying wing, both elevons must move up to provide up control. One control must control two servos. How does this happen?
Lots of approaches to this have been cooked up over time, but the modern solution is that the transmitter mixes inputs from the elevator control to the two elevon servos so that both servos respond with 50 percent of their upward/downward direction throw coming from the elevator control. Likewise, the transmitter mixes 50 percent of the aileron input into aileron outputs. Mix ratios may vary based on pilot preference or presets. Consult your radio manual for instructions on how to do this.
The correct elevon to control mapping is shown at the end of Chapter 4.
Throttle
The throttle is the “gas pedal.” Figure 2-7 shows the correct mapping of the throttle. Be careful about running 100 percent throttle other than when you are flying, and even then use full throttle judiciously because Brooklyn Aerodrome designs are overpowered.
Rudder or Yaw Control
The rudder control (left stick, left to right) is not used in Brooklyn Aerodrome designs and can be ignored. The rudder is usually used to help control the yaw of the airplane.
Trim Tabs
Notice the labeled trim tabs in Figure 2-8. The trim tab adjusts where the “center” of the controls is located. If you ever had a car that pulled to the left or right when you took your hands off the wheel, in airplane terms, you would adjust the left/right trim tab to change the center of the wheel.
FIGURE 2-8 Trim tabs.
The trim tabs allow for small adjustments to the corresponding control’s range. Around 10 percent of the control’s movement can be adjusted with the trim tab. If the plane is diving all the time, then the elevator (up/down) trim tab can be adjusted up a bit. Likewise with the aileron (roll). If the plane is always turning left, then you can dial a little bit of right in with the trim tab. In calm conditions, the airplane should be able to fly hands off for 50 yards without a problem. This is achieved by setting the trim correct. The trim tab on the throttle should be all the way down because some speed controls require the maximum negative value for the control to arm and power the motor. The rudder trim tab is irrelevant because Brooklyn Aerodrome designs don’t use rudders.
Trim-tab starting points are shown for the aileron/elevator (centered) and the throttle (down) in Figure 2-8. There are also digital trim tabs that use a rocker switch to achieve the same result—consult your radio manual.
Rechargeable Batteries
Higher-priced transmitters come with rechargeable batteries. The less-expensive end of the spectrum uses disposable AA batteries. The cheapest I have ever found for AA batteries is eight for $2, so it doesn’t take long for that to add up to more than the cost of a set of rechargeable AA cells.
The Receiver
The receiver sits on the airplane and takes signals from the transmitter and commands the attached servos/speed control. Figure 2-9 shows a receiver with the major parts called out. The antenna can be very fragile and difficult to replace, so take care that it is well secured on the airplane.
FIGURE 2-9 Detailed receiver connections.
Figure 2-9 shows a lot of wire coming out of a typical receiver in the form of three-wire connectors. Two of the wires are for power, as indicated by negative being black or brown and the positive being red. Positive is always the middle wire. If your receiver came with a manual, then use that to make sure that the three wire connectors have the correct orientation. Sometimes there is an indication of connector orientation on the receiver itself. In the absence of any information, the negative terminal tends to be toward the edge of a top plug receiver, as shown in Figure 2-9. See the section on the electronic speed control and battery for a discussion of how the receiver gets power to operate.
How Servos Get Information
Servo motors need power to move, and that power comes from the positive (+) and negative (–) wires. The power from the receiver is around 5 volts, but it can be higher for performance systems. Notice, however, that the Flack has a variable throttle and servos that hold a position—how do they get the information about where to be?
That information is relayed via the signal wire, which is either white or yellow. It conveys a position that by convention ranges over 200 percent. Zero percent is in the middle, –100 percent is one extreme, and 100 percent is the opposite extreme of servo movement. For the throttle channel, –100 percent is no throttle, 0 percent is half throttle, and 100 percent is full throttle.
The expected servo or throttle position is communicated by pulse-width modulation (PWM) that works as follows: Fifty times a second the receiver sends a pulse of voltage that lasts between one-thousandth and two-thousandths of a second. The short pulse indicates to the servo motor controller that it is to have the output at –45 degrees or –100 percent, the long pulse is +45 degrees (100 percent), and 1.5-thousands of a second indicates 0 degrees (0 percent). Figures 2-10 and 2-11 show the maximum extents of servo movement.
FIGURE 2-10 Full right aileron with corresponding servo arm throw.
FIGURE 2-11 Full left aileron with corresponding servo arm throw.
The servos do their best to achieve that instruction. Some specialty servos are capable of throws greater than 90 degrees total.
What the Radio Channels Do
Four- to six-channel radios are what I use in this book, but you will be using only three channels for the standard designs. Channels map to a single signal for a servo or throttle. There are ways to have more than one servo or motor on a channel, but they all will be getting exactly the same signals. The mapping I use in this book is as follows.
Channel 1
Channel 1 is traditionally the aileron channel, which controls roll. It is used for the right elevon. All left/right/nose/tail/front/back descriptions are from an imagined pilot’s point of view facing the direction of flight. A servo is connected to channel 1, which, in turn, is connected to a pushrod that connects to the right elevon control horn—it is not much of a horn, but that is its proper designation. That output arm connects to a rod that connects to a hinged control surface that is called an elevon, as shown in Figure 2-12.
FIGURE 2-12 The right servo connects to the control rod, and the control rod connects to the right elevon horn.
Channel 2
Channel 2 is traditionally the elevator channel, which controls pitch on traditional aircraft, but Brooklyn Aerodrome design uses it for the left elevon.
Channel 3
Channel 3 connects to the electronic speed controller (ESC), which, in turn, powers the receiver and servos and any other accessories, drawing from the receiver via the positive and negative wires. The signal wire communicates to the speed control how much power to provide to the motor. More about the ESC next.
Electronic Speed Control and the Battery Eliminator Circuit
Things are getting a little “acronymy,” but these are common usages and worth getting familiar with. I use electronic speed control (ESC) and speed control interchangeably. The speed control has two major components: (1) the circuitry to send pulses of energy to the motor so that it turns and (2) the battery eliminator circuit (BEC), which provides power to the receiver and connected items such as servos. Figure 2-13 identifies the inputs and outputs. Note that there are three motor power leads. A very useful property of this setup is that to reverse the rotation of the motor, you switch any two of the three leads.
FIGURE 2-13 Inputs and outputs of the speed control and what they connect to.
Power Limitations of ESC and BEC
The speed control is a good place to address the power limits of the various connecting components. The speed controls we use at Brooklyn Aerodrome have approximately the following specs:
1. The speed control will draw 18 amperes of power from the battery pack at around 7.4 volts (two cells). In reality, the battery is finished charging when it reaches 8.4 volts. That voltage will drop to 8 volts under any serious load, and over the course of a flight it drops to 6 volts. The ESC should limit the discharge to 6 volts while powering the motor. To calculate watts, you multiply amperes times volts. Thus 18 amperes × 8 volts = 144 watts, which is pretty impressive for such light gear—more than a typical lightbulb’s worth of energy. Speed controls will rate the maximum voltage that they can handle, usually expressed in how many LiPo cells for which they are designed. On our end of the scale, you will see two- to three-cell limits.
2. Most of the 18 amperes will go to turning the motor via sequenced ac bursts. Speed controls will be destroyed if a motor draws more amperes than the speed control can handle. The number of amperes a motor will draw is usually described in the motor documentation, and it is also a function of how big a propeller the motor is swinging—see the motor section below.
3. Modern speed controls will cut off the power to the motor at a predetermined voltage while keeping the receiver and servos powered to allow a controlled gliding landing. For two-cell batteries, the cutoff is around 6 volts. Speed controls generally ship with reasonable defaults and will automatically recognize the cutoff voltage, but it is worth looking at the manual to verify this. LiPo battery packs will be destroyed if they are discharged below 3 volts per cell under load. Some speed controls will allow a burst of motor power if you drop the throttle to zero and bring it back up, but use this feature sparingly. The BEC will function as long as possible, even if the motor cutoff has been hit, and can take the battery to less than 3 volts per cell, resulting in destruction of the battery. Do not leave the battery attached to the speed control after flying for this reason.
4. The typical speed control provides 2 amperes of power to the receiver, attached servos, and other power-drawing accessories such as night-flying electronics. Typical sub-microservos draw a maximum of 0.5 ampere and the receiver 0.1 ampere, so 1.1 amperes are needed for the typical two-servo setup. Night-flying gear can easily add 1 ampere, for a total of 2.1 amperes, which may lead to power brownouts on hard maneuvering. Some receivers will reboot on a brownout, which can take 1 to 2 seconds and is more than enough time to result in a crash. In addition, if the BEC limit is exceeded, it may burn out. If the energy budget is exceeded for the BEC, either select a speed control with more BEC power or get a separate BEC from the speed control. Castle Creations, purveyors of very high-quality gear, offers both a 10- and a 20-ampere peak BEC if lots of energy is required. You can disable most speed control BECs by cutting the positive wire if an independent BEC is being used.
Speed controls have a heat-sink side, indicated by a smooth surface, that is meant to be exposed to air for cooling. Install that surface in good airflow.
The Flight Battery
The flight battery has a simple but important role to play: It powers everything on the airplane. It is the descendant of a laptop battery crossed with a cell phone battery. You can expect it to last between 5 and 10 minutes depending on throttle use. It is also the volatile prima donna on the airframe that can burn your house down if you treat it badly. Refer to the warnings in Chapter 1—they need to be taken seriously. Get at least three batteries and a good charger to keep flying continuously.
The Motor
The motors we use at Brooklyn Aerodrome are the descendants of CD-ROM motors, and fancier versions are now on Mars driving the Curiosity Rover around. Brushless outrunners have high torque and high efficiency at low weight. This means that no gearboxes are needed, and a very small motor can swing a very big propeller. Repeating again from the speed-control section, to reverse the rotation of the motor, switch any two of the three motor leads. The enemy of motors is heat. It degrades the magnets, and if it melts the insulation on the windings, the motors short out and emit a puff of black smoke, stop turning, and may even destroy your speed control in an instant. Heat builds up from turning a propeller that is too big or from inadequate cooling. Test the motor temperature after every flight by placing a finger on the bell housing for at least 5 seconds. If the motor is too hot to touch, then seek the remedies covered in Chapter 6.
Conclusion
This chapter maps the equipment listed in Chapter 1 to how it is going to be used on an airplane. Before moving on, get on a first-name basis with all the parts of your airplane, and know what they do. It is time to start building a Flack. Chapter 3 covers building the deck, and Chapter 4 covers creating the airframe.