RE: Bob Cat Copy Falcon 120 Jet
The damper has 1.5mm rod attached to a brass piston at the bottom. The piston has 4 small holes @ 0-8mm, to allow the oil to flow through, these are a bit small and i should have opened them up. The holes are drilled randomly on purpose. There is a flutter plate on top of the piston and this is lightly loaded with a spring. As the piston moves down the oil is pushed through the holes and lifts the flutter plate, escaping to the other side of the piston. When the piston is now pulled back the other way, the flutter plate under spring pressure seals the holes, restricting the oils ability to flow back. Therefore the leg should compress quickly, but extend at a slower rate.
I had to guess the thickness of oil to use, but settled on 20/40 engine oil. The unit was filled with oil and the piston inserted, then the end cap (containing 2 miniture o rings) was pressed on. I then soldered on a brass endcap to the 1.5mm rob, which is threaded at the top to accept a 10 BA screw to attach the damper to the top of leg. The main body of the damper has a hole drilled right through it and picks up on the leg pivot pin.
I found another spring which just fits perfectly over the damper and inside the leg.
I assembled the model for a test and the dampers effectivness is obvious, works a treat. I ran out of time, before returning to work abroard, so will have to wait for a few weeks to try it out.
Just a note;
- the flutter was to be drilled with one hole to allow the oil to escape quickly to the top of the damper during compression, but when i tried it before assembly, it seamed good as it was.
- the flutter plate should have a stop postioned above it, to limit its movement as the spring is probably not strong enough to force it against the resistance of the oil, to quickly shut off the holes in the piston as the leg extends, reducing its dampening effect.
- It is hard to tell if the leg extension is slower than compression, only a flight test will tell to see if the model sticks to the ground with out bouncing, but i think even a piston travelling in oil with no flutter plate assembly would be better than just a sprung U/C.
- To help the model stick to the ground better, i had thought of using a small undercarriage phnematic door cylinder pulling a cable attached to the bottom of the leg, to pull the leg up @ 8-10mm to reduce the length of the nose gear, but leaving enough suspension travel and reduce angle of attack when the model lands. Linked to the landing position flap setting, this would give short nose leg for landing and long leg for take off. I think some military jets use this idea.
- due to the 1.5mm rod being attached to the top of the piston, the area of that side will be less than the bottom (unequal area pistons) so this will offset some of the effectivness of the flutter plate. ( if no flutter plate were fitted, due to different areas of piston, the leg would compress slower than when it extends, which is the wrong way round)