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Project Final Report – CUDBF April 30 th , 2009 ASEN 4028: Aerospace Senior Projects Figure 123: Nose gear failures from flight test #2 and #4 13.2.2.3 Flight Test #3 The purpose of this flight was to test the takeoff distance using an added 5° incidence angle. The initial flights of the Buff-2A aircraft utilized a LiPo battery in order to test aerodynamic characteristics of the aircraft without the limitations inherit to NiMH batteries. Therefore, the takeoff distance will not exactly match the performance of the aircraft with a 24-cell competition battery pack. The aircraft was able to takeoff in just 25ft, proving that the new incidence angle did significantly reduce takeoff ground roll. During the flight, the aircraft experienced a left motor failure in flight, causing the aircraft to enter a spin and consequently crash. This caused the aircraft to enter a spin and crash. Fortunately, the resulting damage was minor: broken propellers, damaged motor mounts and damaged nose gear. Figure 124: Buff-2A motor failure during flight test #3 140
Project Final Report – CUDBF April 30 th , 2009 ASEN 4028: Aerospace Senior Projects In order to isolate the cause of the motor failure, the propulsion system was bench tested on the ground after the flight. It was determined that there were no issues with the motors, speed controllers, or wiring. The problem was determined to be a receiver issue. The receiver used on this flight was a 72 MHz receiver instead of a 2.4 GHz receiver, which will be used at competition. During bench testing, there was significant interference noticed with the 72 MHz receiver. The possible cause of the crash was suspected to be from the faulty receiver which caused the motor to cut out in flight. The 72 MHz receiver was replaced with the 2.4 GHz receiver. There were no receiver issues after replacing the receiver. 13.2.2.4 Flight Test #5 Flight test #5 was another flight dedicated to getting the pilot used to flying the aircraft. During this flight, the aircraft experienced servo travel on the right elevator. The right elevator was deflected up about 20° while the pilot commanded neutral elevator. The red arrow shown in Figure 125 shows the deflection experienced in flight. The pilot aborted landing twice before being able to force the aircraft on the ground during the third attempt. After the flight, it was determined that the servo travel occurred because the servo was over-torqued. Figure 125: Elevator servo travel experienced on flight test #5 During analysis, the worst case scenario torque calculated for the elevator servo was 92 oz/in. This would only occur at the aircraft top speed of 70 mph, and a full control surface deflection of 25°. However, it was determined that the pilot would never use full control deflection at top speed. Therefore, a lower torque servo was used in order to save weight. The servo torque of 48.6 oz/in was used since it still allows for 20° of deflection at 60 mph. However, this torque proved to be insufficient through flight testing. Therefore, the next size up in servos was chosen, providing 66.7oz/in of torque. This allows for full 25° elevator deflection at up to 60 mph, and up to 20° of elevator deflection at the top speed of 70 mph. 141
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