How to make a very slow flyer plane with cling wrap and carbon fiber strips. RC. DIY. Homemade.
The distance from the trailing edge of the main wing to the leading edge of the horizontal stabilizer is 33 centimeters, the equivalent of 1.32 times the wing chord.
The electronics consists of an 18 06 brushless motor, with a speed constant of 2400. A three-cell battery with 500 milliamps per hour. A 12 amp ESC and two 3.7-gram servos. The propeller is a 5-inch diameter, 5-inch pitch tri-blade. The weight of the plane with all the electronics included is 242 grams. The wing loading is 0.5 kilograms per square meter. The thrust to weight ratio is greater than one. In the first tests, we noted that to achieve longitudinal stability, it is necessary to put a lot of weight on the nose to move the center of gravity forward. It was set at 7 centimeters from the leading edge of the wing, which corresponds to 28% of the wing chord. The tail buckles laterally and this makes the aircraft unmanageable. We believe that the spin of the aircraft is caused because the wings occasionally get negative dihedral angle due to their structural weakness.
We have moved the entire fuselage forward to shorten the tail arm and lengthen the nose arm, where the motor and battery are placed. This let us to move the center of gravity forward without adding any weight. The distance from the trailing edge of the main wing to the leading edge of the horizontal stabilizer is now 27.5 centimeters, the equivalent of 1.1 times the wing chord. We have bought fishing line 0.6 millimeters in diameter. Using this line and a thicker thread, we have tightened the wings to achieve a certain dihedral angle that provides lateral stability to the aircraft. This way, the wings will not get negative dihedral. We have also decreased the decalage of the horizontal stabilizer, since the too advanced position of the center of gravity in the previous version implied that the backwards moment of the tail was excessive. The weight has increased to 245 grams, and the wing loading is now 0.51 kilograms per square meter.
In the latest version, transverse strips of scotch tape have been placed to fasten the cling film on the wings. We did not want to use duct tape strips, as they weigh too much. We have removed the angle of the motor, so it is now completely horizontal. The rudder area has been increased, so we have almost doubled the area for the rudder. The width is now 21.5 centimeters. The structure has been further stiffened, triangulating almost all of it using the fishing line. To solve the tail lateral buckling problem, we have placed two carbon fiber strips arranged horizontally, so that the greatest moment of inertia of the section coincides with the lateral buckling plane. This way, we form a kind of H-beam that has torsional stiffness and stiffness in two planes of flexion. The stabilizers have been linked by wooden sticks, so they move together. We have managed to move the center of gravity backwards so that it is placed 10 centimeters from the leading edge of the wing, which corresponds to 40% of the wing chord.
This position is more suitable for a flat wing profile than the position of the previous versions. The dihedral angle has been increased, so that it has an average angle of 8 degrees. This improves the lateral stability of the aircraft, so that it will not spin easily. The weight has increased to 266 grams, and the wing loading is now 0.55 kilograms per square meter. Note that this plane cannot fly with any wind or breeze. It only have a lateral surface on the tail, which implies that when a small breeze push on the tail laterally, the turning moment it creates can cancel out the moment we expect to be created due to the incidence of longitudinal wind on the rudder. If a small breeze push the plane laterally, it will turn well in one direction, but it will not turn in the opposite direction. Therefore, it becomes unmanageable. Furthermore, the center of gravity has to be placed very precisely. The static margin is very small. It is also noted that large diameter propellers cannot be fitted to this aircraft, since the torque would spin the plane. Remember that torque is directly proportional to the propeller mass and angular velocity, but is proportional to the square of the propeller diameter. That is, if we increase the diameter twice, the torque will increase four times. In these light models, it is recommended to use small diameter propellers with three or four blades. We have also painted the struts in different colors, to figure out the orientation of the aircraft. The transparent wings make difficult to guess the plane`s orientation when it is far away. You can see the Images of the plane in flight. And this is all, thanks.
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