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When mounting a different model or type of engine to the FireFly, one is confronted with how to trim out the engine mount so that the thrust vector is parallel to the flight path vector while the FireFly is in level flight. In the past I built a vane type angle of attack meter that was calibrated and temporarily mounted to the bottom of the left wing.
After landing one can fix the tail in a raised condition where the level is once again centered. In this position one can measure wing angle of attack for the test flight rpm and also check the rotational vertical plane of the propeller to see if it is at a right angle to the direction of flight. If not, one can washer the engine mount to bring the two into proper alignment. |
The mounting hardware was made from oak wood scraps, and the control mechanism is a guitar string tightener. The spirit level was taken from a string level that can be found in any hardware store. The spirit level was attached with JB Weld. To ensure that the level would remain it a fixed position, an "O" ring was placed between the brass gear and the mounting bracket. When tightening the Phillips screw the brass gear squeezed the "O" ring and it acts as a friction brake.
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At right is a full scale template for the wood components.
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With the MZ 34 engine, I decided to take a different approach and mounted an adjustable spirit level on the upper longeron just back of the throttle. The advantage of this method is that one can establish what one believes to be level flight and adjust the level until the bubble is centered. Then one can fly the bubble centered for five or so minutes and compare altimeter readings to determine if the flight is indeed level. If not, one can change the stick position and readjust the bubble until one has achieved true level flight for a given rpm.