By John Forester of Silent Electric Flyers
Introduction and Equipment
Dual control has earlier been called “Buddy Box” instruction. I prefer the more accurate name of dual control. The system uses two transmitters, the master which controls the aircraft at all times and the slave, connected to the master, which controls the master only when the instructor permits, by holding down a spring-return switch.
In earlier times, the two transmitters had to have all their settings exactly matched and appropriate for the aircraft. This was tiresome to set up, so that once a pair of transmitters was set up for a particular training aircraft that was the only dual control setup used. In the new equipment, the cable between the two transmitters carries only the stick positions, so that a slave transmitter can control any aircraft for which the master transmitter is set up. This discussion assumes such a setup.
Dual control requires two compatible transmitters that connect together. Identical transmitters are ideal, but the manufacturer may list compatible models. In any case, the system must be tested before flight. Consult the transmitter instructions to see how to set one transmitter to master and the other to slave. Mark the slave transmitter for identification. Set the master transmitter to fly the training aircraft, which has been previously flight tested and trimmed, and then add the master setting as instructed.
Many medium-speed aircraft may be used as trainers. Fast aircraft will get the student into trouble too rapidly for the instructor to correct. The prime requirement is that the aircraft operate consistently, without bad habits. It has been said that stability is highly desirable, but that is not so. Stability refers to the tendency of the aircraft to return to some attitude similar to level flight. That means that the plane fights the student’s control movements, which is undesirable. In a full-size plane, the pilot feels the aerodynamic forces fed back from the control surfaces. But radio control has no such feedback; all the feedback is visual. The pilot must see and understand the attitude of his plane. If he wants to change the attitude, he must decide which way to move the controls to change the attitude. He wants a plane that goes where it is pointed, not one that wants to go elsewhere.
It is commonly thought that pitch stability is particularly important, as is achieved with a slightly nose-heavy balance. That is not so important. However, no plane that is tail heavy should be flown, because it fights the pilot in a particularly nasty way.
A plane tuned for aerobatic flight meets these requirements. Even a plane designed for 3D flight fulfills them when its control surface throws are limited to produce no more than normal primary flight maneuvers.
To turn a plane requires banking it toward the center of the turn, just like a bicycle and for the same reason. R/C planes employ two different systems to control banking. The simpler uses rudder and elevator, and steep dihedral or polyhedral wings. The rudder turns the plane a little crosswise to its course and the crosswind on the up-tilted wing ends produces more lift on one tip and less lift on the other, so the plane banks for the turn. The more complicated control system uses rudder, elevator and ailerons for direct three-axis control.
The difference may seem important, but it is not in primary training. For each system, banking is controlled by right-and-left movement of the right stick. The student does not notice whether the banking is produced indirectly, by the rudder and polyhedral wing, or directly, by the ailerons.
Determine the throttle setting useful for cruise flight, for all the early instruction will be done at cruise power. Set the speed control without brake, so the prop will windmill when gliding. (This gives instant response after power off, when that becomes necessary.)
The instructor needs to lend the student an adjustable neck strap with a fitting to fit the transmitter’s holding loop.
It is advisable for the instructor to print out a set of student logs, one for each student, to record the progress of each student.
Check Out Equipment
The training aircraft must be given the standard pre-flight check to determine that the master transmitter is controlling it properly.
Then connect up the slave transmitter to the master and retest the master control functions. For an electric plane, make sure that the slave’s throttle is closed. Then hold down the master’s dual control switch and test that the slave’s stick positions control the plane. Then release the dual control switch to determine that control returns immediately to the master. If all functions work properly and transfer between transmitters, you are ready to fly. Repeat this test for each time the connection is remade.
This may be given before checking out the equipment, or after. And these points need to be frequently repeated as necessary during the entire instructional sequence.
“This is a safe dual control system. You (the student) will have control only when I give it to you, which will be only after the plane is at a safe height. When you do something that will crash the plane or fly it beyond the field limits, I will take back control. You don’t have to do anything. But I will tell you that I now have control, just to reduce your worries.”
Transferring control from instructor to student will be done as follows. Before takeoff or launch, the student sets his throttle to the cruise power position, and doesn’t touch it again until instructed. The instructor controls the plane from takeoff or launch until a safe altitude is reached. The instructor then steers the plane straight and level across the field of view (right to left, or left to right).
Instructor: “Do you see the plane?” If the student doesn’t immediately recognize it, the instructor may maneuver it until the student does.
Student: “I see the plane”.
Instructor: “Do you want control?”
Student: “Yes.” If the answer is no, then the instructor maneuvers the plane into a better position.
Instructor presses the training switch and says: “You’ve got control”. Once control is handed over, I find it useful for the instructor to set his throttle for a bit more power than the cruise power student is using; that gives a margin for getting out of trouble as the left fingers move from the training switch to the left stick.
When the student gets the plane into a crashing height and attitude, and he will, or flies to the field’s limits, the instructor takes control and says, loudly, “I’ve got it!” The instructor then gets the plane straight and level at a reasonable position, and the questions and answers for transferring control are repeated.
That’s how it will be done during flight. But some more ground instruction is necessary before flight.
All movements of the sticks should be done gently, smoothly, consistently, and small. Don’t jerk the sticks. Grip each stick (only the right for first flights) between thumb and tip of forefinger.
The left stick controls the power, and the student won’t touch it until later in the course. The right stick controls the direction of the plane. Pull towards you, the elevator goes up, and the plane’s nose goes up. Push away from you, the elevator goes down and the plane’s nose goes down. Move right, and the plane banks right for a right turn, move left and the plane banks left for a left turn. (That’s how the Wright brothers discovered this secret of flight; they had been building bicycles.)
In level flight, the wings produce a straight-up lift force equal to the weight of the plane. When the plane is banked, the lift force is tilted over. That portion of the lift that is now working sideways is what pulls the plane into the turn. But the tilt also reduces the straight-up lift force that keeps the plane up. So when the plane is banked for a turn, the nose must be pulled up, just a little, with the elevator, to make the wings produce the original amount of straight-up lift. Then when the wings are returned to level after the turn, the extra lift produced by having the nose a little up must be canceled. So turning in level flight requires coordination of both banking and elevator so that the plane doesn’t lose altitude during the turn or bounce up as the turn is straightened. Developing this coordination is the first object of flight training.
An instructional sequence needs to start with simple and safe tasks that develop skills needed for more complicated and difficult tasks. Also, the instructional sequence needs to build confidence in the student, by repeated successful experience, as he learns new skills. The goal of primary flight training is to develop the student’s skills, confidence, and enjoyment so that he will be able to fly his own easy-flying plane without crashing so frequently that he gets disheartened and gives up.
The following is a useful sequence of instruction for electric powered model aircraft.
Assemble & Checkout Plane
Assemble plane and install battery, but don’t connect. If flight and battery logs are kept, insert information. Make sure battery is correctly placed and is strapped down securely. (If operating on 72 MHz check out and claim the appropriate channel according to field instructions.) Turn on transmitter and adjust to plane, see throttle is closed, connect battery, and close switch (if present). Check out operation of all control surfaces, start motor to idle, and then switch off. Set flight time timer (preferably Talking Timer).
The student needs to get used to this routine, so he will do it correctly later with his own plane.
Straight and Level Flight
The student does not take off or launch a plane until he has proved his ability at safe altitude. The student sets his throttle to cruise power, but it will not take effect until he has control. Therefore, the instructor gets the plane in the air, to a safe altitude, and straight and level from right to left (or left to right) before giving control to the student. The sequence for this is in section 3 of this document.
When the student first takes control, the plane will wander right and left, up and down, as the student learns how much of what does what. The instructor takes control only when the plane is doing something that might cause a crash or fly out of bounds. As the student’s plane wanders about, the instructor provides gentle guidance suggestions, trying to correct obvious flaws in handling the right stick. The student has to learn how to see what his plane is doing, then thinking what he needs to do to it, and then working his fingers to make the plane do what he wants. It isn’t simple.
The initial formal task is to fly straight and level the length of the field, turn around, and fly straight and level in the opposite direction, then make another turn. If the field has a set circuit pattern, try to fly the circuit as everyone else does.
This develops the skill to fly straight and level and to make mild turns.
Upwind & Downwind
Student pilots often have more difficulty in flying upwind than downwind; their flight paths wobble around more widely. This is not the fault of the student. In each case, the plane is wobbling about to the same extent, the extent dictated by the student’s level of skill. But when a plane is flying upwind, a given wobble appears to be much sharper, while when flying downwind, the same wobble appears to be smoothed out.
This is a visual illusion because the eye sees the path of the plane but does not see the flow of the wind past plane and pilot. Explaining this to the student pilot assists him in learning how to compensate.
The second task, to be started once straight and level is mastered, is lazy eights. These are circles in one direction followed by a circle in the opposite direction. These train the coordination between banking and elevator, first when entering the circle, then when straightening out, then again when entering the opposite circle. This should all be done at constant altitude, not losing altitude during the circle and not bouncing up when transitioning to straight before the next circle.
This maneuver will tend to drift downwind. If the wind is significant, the same training for transitioning between circling and straight can be obtained by zig-zagging upwind, like a sailing yacht beating to windward. So the plane does a one-third circle to the right, then straightens up, then does a one-third circle to the left, always turning in the upwind direction.
As the student gains coordination between banking and elevator, instruct him to bank more steeply to tighten the turns. He may well have to increase power to keep the plane in the air. If he can control circles when banked to 45 degrees, that is good.
Spiral Dive Recovery
Several popular types of model trainers (rudder-elevator only with polyhedral wings) are susceptible to spiral dive crashes. The plane gets into a spiral dive from which the pilot fails to recover. The student needs to be trained to deliberately put the plane into a spiral dive and then recover from it.
At a safe altitude, the plane is put into a spiral dive by putting the rudder hard over and leaving it there. If the rudder is to the right, the plane will spiral right, and vice versa.
If the spiral is to the right, the recovery is made by moving the stick somewhat to the left of center until the spiral turns into a dive, when the stick is returned to center. Then the dive is ended by pulling up to level flight.
The instructor demonstrates this for the student, and then gets the student to try to replicate this from a very safe altitude. The student needs to learn the recovery skill from both directions of spiral dive. And the student needs to be able to recognize the direction of the spiral that his plane is making if he inadvertently gets it into a spiral dive. So this takes repetition, and it doesn’t hurt, with sufficient altitude, to let the spiral run for more than one turn, so the student can learn to observe which way the plane is spiraling. Repeating the practice as part of later instructional sessions is desirable.
Spin and Recovery
For aileron aircraft, the equivalent is spin and recovery. Go to a very safe altitude. Close the throttle so the plane slows down, while pulling the elevator up to keep the nose up. The plane will stall, dropping the nose and probably one wing. Move the stick to one side, preferably the side to which the plane has banked. The plane will spin. To recover, center the stick until the spin stops, then pull back to come to level flight and return the throttle to cruise power.
This is not the aerobatic competition spin, which must be made with only elevator and rudder, but aerobatic planes have very large rudders.
The loop is an easy maneuver, but it provides training in both maintaining the wings level and in throttle control.
Start at a safe altitude in level flight at cruise power. Go to full power and allow time for the plane to build up speed. Then pull back to make the loop. At the top of the loop, reduce power to a low level. At the bottom of the loop, level off and return to cruise power.
If the plane tends to spiral out of the plane of the loop, the wings have not been kept “level”, meaning inadvertent sideways movement of the stick as it was pulled back.
If the stick is not pulled back sufficiently, the plane may make too large a loop and stall during the climb. This gives training in recovering from stalls.
It may be that the trainer has insufficient power to make a loop from level flight. Then gain additional speed by diving before pulling back, just as the early pilots had to do.
Landing Approach and Go-Around
Now that the student has developed control of the plane, it is time to train his distance perception and to improve his plane handling.
Student flies the plane around the typical landing circuit for this field, starting high and not getting near the ground. His initial task is to fly along and over the centerline of the landing strip at cruise power, going round and round the circuit until he can judge the distance away from him for making the turns that line the plane up with the landing strip.
As the instructor judges the student’s competence, he instructs the student to make his flight along and over the centerline at lower altitude.
From this point on, the student must go through the proper landing procedure as if for a touch-and-go. He has to make sure that no other plane is on landing approach, and, if possible, see that no plane or person is on the field. (Field rules should insist that such events are called out for all to hear.) When starting the final approach, he calls out “Touch and go!”
Then to approach, reduce power, and lose altitude as if to land, but then increase power and climb out for another circuit. Only when the student does this reliably can the student be trusted to attempt a landing, which is done later.
Avoiding Tip Stall
A plane needs speed through the air (airspeed) to generate the lift that keeps it up. As the speed drops, the nose has to go up to retain the normal amount of lift. When the nose goes up too far at low airspeed, the lift suddenly disappears. This is a stall. And with many planes, the lift first disappears near a wing tip, which is a tip stall. With no lift near one wing tip, the plane rolls that wing down and spirals or spins down. If this occurs near the ground, the plane has to crash; nothing the pilot can do will get the plane flying again before it hits the ground.
As you have learned, making a turn requires getting the nose up a bit to compensate for the loss of lift. Therefore, making a turn at low airspeed on a landing approach will increase the chance of tip stalling the plane into the ground. Therefore, while making a landing approach, don’t reduce power lower than cruise, or just a bit less, until after you have made the last turn so the plane is lined up with the runway. After that, you can use your judgment to decide when to reduce power and “glide” the plane into the landing strip.
Take Off or Launch
Taking off or launching is done depending on whether the trainer has landing gear or not. If launching is required, it is appropriate to give the student practice in throwing the plane while the instructor has control. This can be done conveniently during earlier instructional flights. This trains the student how to throw the plane without having the additional complication of controlling it, while the instructor is able to correct problems caused by bad throwing.
The student should already have observed many launches or take-offs. But he must go through the complete sequence entirely under his own control. Have the timer set for the desired flight time and the plane ready to fly at the appropriate gate to the strip, with the throttle closed (or at idle). The instructor switches control to the student.
Look around to see that the air is clear around the strip, no planes approaching a landing. Start the timer and check to see that it is running. Look around again. Shout “Taking off” or “Launching” as appropriate. Bring the power up to take-off power and steer the plane off the ground, or throw it, and fly into the normal circuit.
This is the most likely maneuver to crash the plane, so approach landing instruction cautiously. The student should already have been making good approaches and almost touching down before going around again. If he has been reasonably reliable in this, it is necessary to risk landings.
The instructor must have already determined the proper throttle setting for landing. Many slow aircraft land without power, just gliding in, but others may require some power to control rate of descent and maintain control. Climb to a safe altitude and instruct the student to shut off power and feel how the plane glides. (He will need this in case of power failure.) Then repeat with the power setting suitable for landing, to see how the plane descends at this power.
If the student has progressed to making good approaches from which a good landing would be possible, and when the battery flying time is close to being ended, I tell the student that when he makes a good approach I will shout “Landing” for him. Then he goes to landing power and brings the plane in. If he doesn’t make a good approach, I take over and land the plane on the next circuit.
(Landing is one reason for setting the speed control to “No Brake”. With the propeller wind milling during a glide, the speed control can immediately increase the power to go around.)
From then on, the student calls out his intention to land, or to touch and go.
A student who has demonstrated reliable skill in all of these tasks can be considered capable of flying his own easy-flying plane without crashing it so frequently that he loses confidence and ceases flying. With frequent flying of his own plane, he should be able to develop his flying skills in any direction he desires. But he should never be reluctant to ask for advice from those with knowledge and the ability to explain.