Measuring In-Flight Deflection of Cable Actuated Flaps

The flaps of the Experimental Amateur Built Bearhawk were observed to have less deflection while airborne than on the ground with the same flap handle position. The difference was suspected to be caused by stretching in the actuating cable. Flap deflections were measured on the ground using a digital level. A theodolite was fabricated using a knotted cord and dry erase marks on a window. Flap deflection in flight was recorded using this theodolite. On the ground, the flap was moved to the theodolite position recorded in flight and measured with the digital level. A CAD model was used to determine the required cable stretch to reproduce the in-flight measurements. Expected elastic stretch and structural stretch of the cable were calculated and found to be within expected values, confirming that the difference in flap deflection in-flight from deflection on the ground was caused by cable stretch. The in-flight deflections were shown to be suitable for the aircraft mission.