Electric Stunt > New electronic technology

Electronic bellcrank idea?

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fielding mellish:
Guys,

I'm curious about a variation on the electronic bellcrank concept.  Instead of handle movement commanding a certain elevator position, what if handle movement commanded (proportionately) a certain pitch RATE?  When the handle is neutralized, the model would maintain its current pitch attitude until another correction is received.  Would this be possible or practical, and would it make for smoother flying?  

Tim Wescott:
I've put some thought into this myself because with a little bit of custom software you could do this with a TUT board.  So, yes it's possible, probably even practical in the sense that you could make the model do what you specify, at least at speed.

As to whether it'd be practical for stunt -- maybe not.  Think how fast the airplane rotates in response to elevator with the airplane stopped.  That's because the sensitivity of the airplane's rate vs. elevator displacement is zero at zero speed, low at low speeds, and high at high speeds.  So you'd need some sort of a speed-sensitive gain from handle to rotation.  Also, stunt planes are generally designed to be quite stable, with way-far-forward centers of gravity compared to any other form of aviation.  This should translate into a marked tendency for the tail to weather-vane into the direction of the airflow, which you cannot do with a TUT or anything else unless you add vertical flow sensors to the mix (which you could do with a TUT, if you wanted to go there).

I think that if you did it you'd want to have a linear relationship between flap and handle, and a nonlinear relationship between handle and pitch, to give you very little rotation for the amount of up and down you get during level flight and straight segments, yet still get lots of rotation in corners.  This is more or less the "Igor flap" concept, just tunable in electronics.

fielding mellish:
Wow, it's more complex than I thought.  Thanks for your quick reply.

Tim Wescott:
I'm not trying to drive you off of the idea -- I think it's a worthwhile thing to try, particularly because it would allow you to move the CG back until the airframe's pitch stability is zero or even negative.

It's just not something you want to slap together.

Howard Rush:
It's something a controls engineer could do, but I would hope it's outlawed for stunt contests and that anybody who tries it is banished to RC.  I presume that fielding is not using his real name out of shame. 

If you have a good stunt plane and know how to trim it better than I do, you can do this aerodynamically with traditional control linkage.  I flew an Impact once (not mine) that looked like a poor simulation of a stunt plane. The engine made the same sound everywhere, the airplane went the same speed everywhere, and I saw only the profile of the plane: never any wing, never any oscillation.  A handle input resulted in a pitch rate proportional to input.  It was weird. 

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