RE: [RCSE] Why does dive CG test work?
It's all about the trim. When flying a nose heavy plane at regular thermal
cruise speed, you have to add up trim to get it to fly straight and level.
The "up" trim setting is causing an aerodynamic force on the tail is pushing
down to compensate for the extra nose weight. The amount of aerodynamic
down force required to stay level is constant for a given amount of nose
weight. The amount of aerodynamic force created by a flying surface varies
with the speed of the airplane. As it goes faster it is more effective
(better Reynolds numbers).
So what you start with is a nose heavy plane that is trimmed for level
flight at cruise speed. The tail forces balance the nose heaviness of the
model. Now increase the speed, and what happens? The nose heaviness
remains constant, but the downforce on the tail increases, causing the model
to pitch up.
The inverse is true on a tail heavy model. To trim for straight and level
at cruise speed, the tail requires down trim (an upward force on the tail to
compensate for the lack of weight in the nose). Increase the speed, and the
upforce on the tail increases, causing the model to pitch down. That's a
From: invicta421 [mailto:email@example.com]
Sent: Wednesday, April 02, 2003 12:51 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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