ejbjh

HI OLLIE. I think I finally got it about right on my sailplane with a fixed horizontal, as it indicates lift pretty well, will fly hands off for minutes at a time and I made my first one hour flight last Saturday. I have another slicker sailplane with a flying stab and I am wondering if the dive test works the same with a flying stab since you are not dealing with a fixed incedence. Right now I am in the process of removing nose weight, and I am under the recommended weight and it still pulls out of a dive immediately. I'll proceed slowly, but am curious if the fact that it has a flying stab could have a different affect. Thanks for your help. ejbjh

Jack Hyde

My Feedback: (1)

The short answer is the dive test works the same for flying stabs.
The reason is, as I understand it, like this. When the cg is fwd of the center of lift for the wing, the weight of the plane creates a nose
down pitching moment that must be balanced by the down force of the
stab. If the cg is way fwd the stab force is relatively large. The negative lift
of the tail is proportional to velocity^2, the cg moment is independent
of velocity. Therefore if the plane with way fwd cg is trimmed for level
flight at slow speed , as speed increases in a dive
the nose will come up. If the cg is set so zero down tail force is required at slow speed level flight,
trim is independent of speed, the plane will stay at the same dive angle it starts at. If the cg is a little father aft, the tail force is lifting which in a dive , increases as speed increases, lifting the tail and steepening the dive etc causing the dive to become very steep very quickly. When you trim the elevator or flying stab you change its anle of attack which changes its lift coefficient. Both types if tail planes act the same in the dive.

ejbjh

Thanks for the response. The logic of the increased speed effect on the down force of the stab makes sense and helps me understand what 's going on. The recommended nose weight of the plane is 100 gms. And I have already removed
25gms, it still pulls out rather quickly and I was beginning to wonder ? I appreciate the help. ejbjh

Soar Head

My Feedback: (5)

How quickly the dive test also depends on your initial elevator setting. i.e. how fast the plane is going when just cruising around with your hands off the sticks. If your default setting is very slow, then the pullout will be quicker.

If you have experienced pilots to help you out, that is a bonus.

drela

An all-moving tail is in practice no different than a good hinged tail. The dive test is done the same way with either one.

For more info on the dive test, see the article and sketches at www.polecataero.com