Re: [RCSE] Why does dive CG test work? - Aerodynamics
Aerodynamics forces like lift and drag increase with the square of the
airspeed. The static forces - like lead in the nose - do not.
When a plane is nose heavy, it must be counteracted with up trip. A heavier
nose means more up trim. When you speed up the up trim causes more force
and lifts the nose. If you have a lot of up trim, it will generate more
lift at a lower speed and pull up more quickly. Conversely, the more
neutrally balanced a model, the less up trim and the slower it will pull
out. In this scenario the plane is self correcting because as the nose
rises above horizontal again, the speed begins to drop and the up begins to
lessen until it re-neutralizes itself or starts another cycle when the plane
slows too much.
If a plane is tail heavy it will be trimmed with down elevator. As it
speeds up, the tail generates even mored down and forces the nose more down,
which causes it to speed up and causes more down, which causes more speed
and causes more down until the plane disintegrates, does an outside loop, or
you make a control adjustment.
All this assumes that the fuselage, control surfaces, and pushrods are stiff
enough not to deflect significantly under the large forces. This is not
always a good assumption.
Clarence Sailplane Society
----- Original Message -----
From: "invicta421" <firstname.lastname@example.org>
Sent: Wednesday, April 02, 2003 3:50 AM
Subject: [RCSE] Why does dive CG test work?
> 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:
> 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?
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