Another test flight, and another disaster narrowly averted! This time it was the tail-rotor drive coupling!
The Lark used a very crude, but reliable, coupling for the tail-rotor drive. The drive itself was just a piece of piano-wire. The secondary pulley on the mechanics had a square recess in it, about 1/2" sq. Into this loosely fitted a 1/2" sq rectangular piece of aluminium, drilled to take the piano-wire, with a grub screw to bite onto the piano-wire. This provided a very simple kind of universal joint, allowing a little bit of play, whilst ensuring a positive drive to the tail.
For this Mayfly, Peter Valentine tried something more sophisticated! He cut a circle of leather (!) to produce a coupling similar to the rubber donuts used on some cars in the 60s. Anyone who has changed the gearbox in a Hillman Imp will know what I mean!
Unfortunately the leather wasn't man enough for the job - or maybe it had just deteriorated with age. I had been suspicious of it from the start, so it came as no surprise when it let go with a bang!
Luckily (again!), I wasn't very high and managed to land safely!
The quick and easy fix would have been to simply fit a Lark pulley (which I had). Unfortunately, I didn't have the matching aluminium block!
After a bit of head scratching I tried a rubber tap washer, but it was too thick. Eventually, I settled on a small servo output arm:
Coupling by
Peter Christy, on Flickr
This fitted absolutely perfectly! Even the holes in the arms were in the right place! The plastic is a little on the rigid side, so I've left the mounting screws slightly loose (hope the Loctite holds!) and I've test flown it successfully in the back garden!
Proper "kitchen sink" engineering!
I have hit another problem though! The ESCs keep overheating! The model draws just over 20 amps in the hover, and I fitted a 40A speed controller. After 2 mins of operation, it throttles itself back, and the model sinks to the ground!
I tried a "higher tech" 30A speed controller, but that one cuts without warning! (I tested it tethered to the ground!.)
I think the motor I have has a too high kV rating (3000). Ideally I need around 2500 kV and 400 watts, but so far I've failed to locate anything suitable. I do have a very old, but higher rated speed controller in my junk box, but I can't remember just WHY it was in the junk box! Oh, well, if I don't try it, I'll never know!
Onward and upwards!
--
Pete