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Old Apr 02, 2003, 11:05 AM
George Voss
George Voss
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Re: [RCSE] Why does dive CG test work?

While I'm not an AE, I'll give it my best shot.

Think in terms of static and dynamic loads. You balance your sailplane
in a static mode using either your fingers, or some type of
machine/gadget. When you fly your plane it sees the addition of dynamic
loads. The static loads still apply, but now you have a dynamic load to
deal with.

As the plane speeds up, the dynamic loads have more affect than the
static load, i.e. the elevator trim setting has more authority than the
static balance. Sooooooo, if you have more lead in the nose than
needed, you need to carry some "up" elevator to make the airplane fly
level at normal speeds. As the speed increases, the elevator over rides
the balance and the plane pitches up. The opposite is also true and
that's why the airplane will tuck if the CG is too far out of range.

I hope that helps. gv



invicta421 wrote:

>I have been pondering this question for the last few weeks, but
>haven't satified myself with an explaination yet. Most aerodynamics
>seem to make intutive sense to me, but this doesn't.
>
>The test I am speaking of is when you put the sailplane into a 45
>degree dive and see if it pulls up, flys neutral or tucks under. A
>page on how to do this can be found here:
>
>http://www.polecataero.com/articles/cg_art.shtml
>
>But this page doesn't explain why... to me it seems that a nose heavy
>glider should tuck not rise when the CG is in front of the average
>lift on the wing. Any help understanding this?
>
>Thanks
>
>-Richard
>
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>
>



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