Making and Diagnosis of a Cheap RC Airplane Suitable for Kids Pilots Training

In this instructables, I am making a simple flying RC airplane using a foam glider, and a broken quadcopter. The airplane is suitable for first time pilots and small kids trying to discover the wonderful world of flying. The airplane is controlled by means of differential thrust, i.e. there exist two motors on the sides of the wing and the difference in motor thrust between these motors will control the yaw (or direction) of the airplane.

If you want to see the easy making of an airplane then you are in the wrong page! Here I will make the airplane and find why it doesn't fly as expected. I will be diagnosing the machine, checking its thrust direction, CoG, and different reasons affecting its flight. Let's find out…

In this instructable, I used a cheap foam glider and an old toy quadcopter.

First of all, you need to salvage the electronics of the quadcopter. We are going to use the front two motors of the quadcopter, so remove the aft two motors using a solder iron. The circuit and the motors are then removed from the quadcopter and mounted on the foam glider. The circuit is placed horizontally in the cockpit and the motors are hot glued hanged under the wing (in equal distances from the cockpit). The battery is also placed in the cockpit to make the center of gravity of the airplane located at the wing close to its leading edge.

It is now the time to check the thrust of the motors. Note that the motors should turn in opposite directions in order to cancel the gyroscopic effects of the two propellers (which is expected to be small for these propellers). With the throttle joystick set to its maximum, I found that the right motor’s thrust is going forward! It seems I need to reverse the right motor connections (which will makes its direction similar to that of the left motor => bad gyroscopic effect (moment of inertia)! But will it affect the biplane’s flight? Let’s see). 

The airplane flies well, and I managed to fly it about 30 meters (100 steps). However, controlling the direction of the airplane using the roll joystick has little effect on the steering (it is hard to turn the airplane, it takes a long time). On the other hand, using the yaw joystick has a severe effect on the airplane resulting in its banking and crash. The climbing of the airplane is not enough. Other problems also emerged:

-      The motors pop out with each crash

-      Propellers often slip off the motors’ shafts

-      The circuit has a gyroscope

Motors are placed at the trailing edge of the wing and cemented firmly. The propellers are used in the pusher configuration so they are always secured to the motors’ shafts. Motors are also mounted in positive angle with the wing, so the thrust is directed raise the nose of the airplane. In a small test, I realized that the airplane tends to turn right. This is because both motors turn to the right, so the resultant moment of inertia makes the airplane turn right. To accommodate for this, the center of gravity of the airplane was shifted to the left by placing the battery on the left bank. Weight distribution will counteract the effect of the moment of inertia.

The gyroscope ensures that the airplane stays horizontal to the ground. However, the gyroscope in the circuit is for the quadcopter, and may not work well for airplanes. With a small test, I realized that the two motors slow down when the nose of the airplane rises, and they speed up when its nose drops. This is good to maintain its level. Banking the airplane to one side slows one of the motors and speeds the other one. Right and left motors are placed accordingly to maintain the direction of the airplane in case of sudden wind gust. To use the gyroscope to my own benefit, I put the circuit with its rear end inclined forward. The gyro will try to recover the horizontal position of the airplane, it will increase the speed of the rear motors and we will get more thrust. Sure, now I need to use the rear motors of the quadcopter and not its front motors.

Tests reveal the following: The airplane stalls at take-off and whenever the thrust is increased to the maximum, but overall it flies well. 

To fix the stalling problem, a small weight is added in the cockpit to move the center of gravity forward. Cables are also embedded inside the foam. The airplane weights now 63 grams. A cockpit cover was also made crudely out of white foam.

- The airplane flies very well but you need to practice its control.

- The airplane is mainly a glider; it is very sensitive to wind gust

- If you ordered the airplane to turn right but it doesn't obey, it means that air gust is directing it left; make your turn to the left and use the wind gust to your own benefit

- The airplane is still going right under the effect of the moment of inertia; the weight added is not enough, but further weight adding is dismissed because it will increase the total weight significantly

- To increase the control effect, we need bigger propellers; however, this may push the motors to their limit as the current configuration heats the motors up rapidly

- The airplane is perfect for kids starters; they can crash and refly it several times. No fear to inflict a permanent damage

- Flying the airplane requires a wide open space (as turns are very wide) with no obstacles and a wind free weather (as the slightest wind gust renders the control ineffective)

- The right way to control the airplane is:

○ At take off, increase the thrust to the maximum and throw the airplane horizontally. The airplane flies straight and start taking height

○ Whenever you want to turn the airplane, decrease the thrust to the middle and apply the turn command via the roll joystick

○ Keep watching carefully while performing the turn; if the airplane starts to roll down turn the joystick to the reverse direction until it maintains its level

○ After performing the turn increase the throttle to the maximum to gain height

My kid (4 years old), my little girl (8 years old), my young niece (7 years old), and my elder niece (11 years old) were all able to practice/learn flying airplanes using this airplane. The airplane crashed severely several times with no damage. The little mister pilot was the first to make successfully a closed turn (return the airplane to its take-off point)! I wasn’t able to do so on the first day!!!

I summarized all my work in this video.