Friday, December 16, 2011

Everything old is new again


This is Vol Lebra in which there is a YouTube video of a folding wing model wherein the wings fold back toward the tail end., all FAI style. Looks successful. 


Where have we seen this before?

 December 18, 2009

The video does remind me of the A. J. Walker catapult glider with wings folded straight back in climb and snap out for glide, first introduced in 1939. But the physics of the two are different.

About 30 years ago I built a 1/2A prototype, with TD .049 engine, made with wings fixed in the fold-back position -- just to try to master a controlled straight-up climb. I gave each of my five kids a pillow and told them to try to catch it when it came down. Each time I tried, it was all over the sky -- none ever caught it -- just as well! Whereas the A. J. Walker catapult glider susccessfully goes straight-up, the engine powered likeness of it does not.

After deep thought, I reasoned that the catapult glider version followed Newton's First Law of physics: "A body at rest remains at rest and a body in motion continues to move at constant speed along a straight line, unless the body is acted upon in either case by an unbalanced force." In other words, the CG of the glider is set in motion along a straight line and the tail feathers follow. It goes in the direction pointed, usually straight up, until the sum of the drag energy and potential energy overcome the initial kinetic energy and then it stops and the wings pop out for glide. 

In the case of the power model, energy is being pumped into it through out the flight and then "---- unless the body is acted upon ---" applies. In this case it is acted upon primarily by the engine pulling it in a new direction initiated by any disturbance, wind, turbulance. In otherwords, whereas the glider's trajectory is a stable straight line (curved by gravity) the power model is unstable -- the engine thrust magnifies disturbances.

How did the Ukranian flyer get his F1C to go straight up? My bet is he has a gyroscope in it that regulates the tail feathers and makes it fly straight. If this is the case, it is illegal for FAI competition because feed back circuits are not allowed in FAI free flight models.

Sorry for bending your ear. I wanted to make it short :-)


No comments: