Moved CG, Got Strange Result

[green66]

Flew my Organic glider with CG 40% behind LE (35% is mfr's position). The plane was slightly tail-heavy: 1) squirrely in yaw, 2) tendency to tuck at high speed, 3) hard to establish pitch trim, 4) generally an effort to maintain straight & level.

So I added about 1/2 oz to the nose to shift the CG forward.

On the next flight, the plane was so (seemingly) tail-heavy that there wasn't nearly enough down-trim on the tx to compensate. I tried to hold down elevator to maintain control, but it went into a spiral dive (fortunately into tall brush, but didn't come out unscathed :( )

As far as I'm aware, the only thing that changed between the two flights was the added nose weight - no loose internals to shift around, no misaligned or damaged wing/tail panels, no control changes, etc.

Question - Could this "reversed" (going against expectation) effect of the changed CG have anything to do with the plane being near a neutral stability condition? Any explanations?

It's not the damaged plane that bugs me as much as not having a clue what caused the crash :confused:

[Sparky Paul]

What does the plane weigh?
A 1/2 oz might not be enough to make a noticeable difference in the c.g. or handling.
The crash might have been an aggravated tuck more than anything else.

[green66]

Hi Spark, What does the plane weigh? About 35 oz all-up flying weight. The crash might have been an aggravated tuck more than anything else. However it was flying slowly as I fought to hold it level in pitch against the ridiculous nose-up tendency, then it snapped into a straight-down spiral dive, as in "vertical corkscrew," :( . Yuk. Totally baffled :confused: :confused:

[rcjunkie]

Sounds like just too tail heavy. Flying a tail heavy Aircraft at low speeds will cause the problem you described. Plus any wind direction changes will affect the Aircraft more. (Tailwind?) I would move your CG to 30-35% and start there. You can always move it aft if you want.
I just had a simular experience with a Class A Sailplane (Hacker Powered) where I designed and built a wing and tried calculating the CG myself and set it at what I thought was 35%. Turns out that I was off on my Calculations by 3/4". The tapered wing threw me off. It flew just like you described. I moved the CG Forward 3/4" and it flys great now.

Ernie S.

[Viper Pilot]

Strange. If the plane flew fine before (slight indication of tail-heaviness) adding weight to the nose should have improve the flight characteristics. Who figures???

VP

[Tony Oliver]

Sounds like you need to very carefully measure your rigging angles between wing and tailplane.

The effect you describe could be explained if your tailplane was at +2 to +4 degs and the wing neutral.(wing at zero lift angle to the tailplane). If the small weight addition - and it is small as pointed out by SparkyPaul - has such a dramatic effect, you're doing the flying equivalent of leaning out of a window, and almost at the point of no return.

Tony