Walter, to your Post #14:
Bob Whitely used an on-board controller that:
-- was switched on by the launching buddy upon release for takeoff...
-- waited a timed few seconds to start retraction...
-- kept the servo circuit either locked out, or in retract mode, until things got quiet (i.e., engine stopped)...
-- then switched to gear-down mode, and put them down.
I seem to remember some anxiety as the gear-down system didn't kick in immediately the engine stopped making noise. Sort of the "does she or doesn't she?" getting-up-on-tiptoes kind of thing as the glide continued to descend...
... and descend ...
and then, FINALLY! - wheep/click, and the wheels were once again ready to save that expensive prop.
Worked consistently, in the flights I saw. But, then, Bob Whitely is not just your ordinary builder/flyer.
RAM Electronics, I think t'was, made the actuator controller, with an adjustable delay from switch-on to gear-up triggering, and a sort of microphone that kept it in gear-up mode while it could hear the engine roar, then dropped to gear-down mode when things got (relatively) silent.
I picked up one of the units back about then, and seem to remember the instructions suggested that you could test the setting of the retract delay by making a loud noise for the 'microphone', with your voice, for the delay time. When you stopped after the gear retracted, you could prove the system by watching the gear extend...
Bob's P51 was BEAUTIFUL!! in the air!
Still leaves unanswered what you have to put aboard that will receive and act on the controller's triggering signals... Seems to require:
1) A servo strong enough to operate a...
2) gear retracting mechanism, and the ...
3) linkage to both mains, or both+tailwheel, or all three (mains and nose wheel), and a ...
4) battery with enough oomph to drive the servo.
5) And, some provision to have access to most of the above for trivial little things like recharging the battery, adjusting (or reconnecting) the linkages (making sure they do not stay at high-drain loaded condition because of binding or obstruction), tuning the controller, general maintenance and testing, and inspecting the works.
It is also nice to have a gear retraction gizmo that mechanically locks the gear in its full-down and full-up positions, so that the servo isn't loaded to do that. (Can you say, "dead battery"?...within one flight?)
With today's (arsonist-inspired) LiPO batteries, the weight of power aboard may be much reduced. With some of the silly, but functional, things being done for Indoor (f'Gawd's sake!) RC and "Park Flier" toys, some of the other elements may be available, if you wear a ski-mask or balaclava and hit the - hack, kaff - RC section of the "Hobbye Shoppes" that gobbled up our once beloved LHS's.
You'd still need a fairly robust retract/extend gizmo, and a servo with the grunt to do the loads to it.
AND factor for the 3+g sideways that goes with our circular flight regime.
Worth it? Most likely. BUT you'd need to get so involved in the minutia that a lot of the joy of flying stunt becomes second priority.