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Old Dec 30, 2001, 01:06 AM
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sguty's Avatar
Redmond, WA, USA
Joined Oct 2000
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TwinJet spin recovery/CG questions

Been trying to trim out a new TJ:

had severe ballooning problems with the the stock CG, so had followed some suggestions in earlier threads and moved the CG progressively back 1/2" from the dimples. Still need about 1/16" down trim at speed. Problem I'm having now is that it's showing a strong tendency to spin at the top of loops and/or tailslides, and I'm finding it difficult to recover cleanly--I wasn't able to stop the spin on an entry from 100+ ft up yesterday, and had a spin this afternoon from 150 ft up where I was able to reverse the spin direction, stop the secondary spin, and _almost_ recover, then snap in from about 20' up. No irreparable damage, but the nose is starting to have a distinct upward curve...
Have to confess that this is my first R/C plane which spins readily--most of my prior experience with spin recovery is from full scale soaring gliders and the HobbyLab SR-71 boost glider--but the prior techniques of keeping the nose down and opposite aileron (and rudder where you have it) just didn't work here. Had chopped throtlle on the first spin just prior to impact, but full throttle in the initial stages of the recovery attempt didn't seem to be making much difference. On today's spin, I think I cut throttle earlier, which in hindsight was probably not the best course of action--but I alway thought that propwash was of more help in tractor, rather than pusher, configurations?
Has anyone found a sweet spot for the CG which minimizes ballooning but also stays ahead of the spin problem?
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Old Dec 30, 2001, 02:05 AM
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Alberta, Canada
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I would think that moving the cg forward would stop ballooning not back? Spins and snaps I think would be happening more with a rearward c of g.
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Old Dec 30, 2001, 02:09 AM
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Put the CofG back to where it belongs - this will end your spin problems. Hey, if I am not having ballooning problems with my first series TJ on two Brushless motors - it can't be a CofG triggered symptom. Jürgen
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Old Dec 30, 2001, 02:22 AM
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The relationship between CG and ballooning, as I understand it is: too far forward, need a lot of up (or reflex in the case of flying wings) to keep the nose up at cruise. At higher speeds (full throttle, or coming out of dive, etc.) the amount of uptrim required for level cruise translates into rapid climb (ballooning) Correct CG (sweet spot) results in less sensitivity to airspeed in pitch trim?
Stock CG required nearly full downtrim for level flight at full throttle--and a disturbing tendency to try to auger in at launch speed. Had already set up a flap->elevator mix to crank in ~ 3/32" of up elevator (from cruise setting) at launch for the stock CG, just to keep it out of the grass
Jurgen--are you using the stock thrustlines? Found my TwinStar benefitted greatly from ~ 1/16" of downthrust, am wondering whether this would be worth exploring on the TJ as well (although it'd be upthrust in this case)
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Old Dec 30, 2001, 03:08 AM
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My TJ is a "first generation one" and everything is stock - except reduced reflex by bending the elevons straight, and different motors and ESCs. Jürgen
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Old Dec 30, 2001, 10:24 AM
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Thx, Jurgen! I'll try de-flex-ing the reflex-ing , along with moving the CG forward at least halfway to the original spot.
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Old Dec 30, 2001, 02:31 PM
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Mine is also an early one, the only spins it does are intentional ones and recovery is immediate upon centering the controls CG is approx. 1 cm behind the bumps.

I flew my PJ with a rearward CG a couple of times .. you could feel it starting to twitch when you pulled any 'up'. Eventually at a 'safe' height I tried a tight turn and it dropped straight into a spin, all the way down into the deck! Low descent rate, so hardly any damage.

But my TJ is completely docile, like Jurgen's it has the elevons flattened to reduce the reflex.
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Old Jan 02, 2002, 07:26 PM
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Redmond, WA, USA
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Thx, Bill--
Gonna flatten those elevons and give it a shot with the CG much closer to stock. The one thing that perplexed me was that the TJ didn't feel at all twitchy with the rearward CG--I'm familiar with pitch porpoising in conventional (wing+tail) airframes and didn't see any of that in the TJ.
It seemed rock solid right up the point it started spinning
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Old Jan 02, 2002, 07:50 PM
LJH
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I fly my TJ just behind the bumps, just enough so that my finger hit the bumps as it slowly goes nose down. I also derelexed the relex mine is pretty good with no tendancy to spin and the ballooning is not bad at all. My PJC on the other hand is a handfull if you get the CG to far back. Even a little back from the bumps and inverted is really tricky. and you can easily spin it if flow aggresively right side up.

Cheers,
Jim
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Old Jan 03, 2002, 09:16 AM
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Quote:
Originally posted by sguty
The one thing that perplexed me was that the TJ didn't feel at all twitchy with the rearward CG--I'm familiar with pitch porpoising in conventional (wing+tail) airframes and didn't see any of that in the TJ.
It seemed rock solid right up the point it started spinning
Yes, my PJ was the same with a rearward CG. It flew nicely ... apart from not being keen on me pulling up elevator!
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