Physics problem with dives

[benchaplin]

I've spotted a possible problem with the way gravity is modelled in FMS. By turning on the airspeed display, I've noticed that if you enter a vertical dive at full throttle, then reduce the throttle, the model's speed actually decreases - while it's still diving vertically. This doesn't happen in real life - models just get faster and faster when they're pointing straight down - whatever the throttle level.

Perhaps something to change in future versions. Great sim though.

[Roman Moeller]

Hello

I am not that sure wether models get faster and faster in real life. If there is enough height left, your model gets faster until gravity equals drag force. Maybe this happens a little bit too fast in FMS. But if you reached that point you can only increase your speed by adding a force, for exaple give full throttle. So I don't think there is a problem with gravity.
What I will change in the new simulation is the fact that the force caused by the engine is no longer constant. Because this leads now to increasing engine power with increasing speed at constant throttle.

Greetings Roman



Edited By Roman Moeller on Mar. 01 2002 at 13:15

[benchaplin]

Hi. Thanks for the reply.

What you say is correct - the model has constant speed (terminal velocity) when gravity + engine force is equal to the drag force. If the model is still in a vertical dive, it should not be possible to slow down from this speed by reducing the throttle. The speed should only decrease when the model hits the ground!

[Steve Lewin]

Are you sure ? The propeller has drag as well as thrust. If you reduce the throttle far enough the prop drag will slow the plane down (more often it will just reduce the acceleration). This is readily observed in electric planes without folding props and I understand it is deliberately used by pattern fliers to keep speed more constant in the vertical dive portion of a pattern (I don't fly well enough to tell myself ).

You're right that FMS is a little unrealistic in this area but I'm pretty sure a plane can slow somewhat without hitting the ground .

Steve

[CodyCBarnes]

The plane will slow down in a dive if it has enough power. When you remove thrust, the forward force may now be less than the drag. As a result, the model will eventually slow down until the gravity is equal to drag. Also as mentioned before, the prop does when stopped (or pinwheeling) induce a considerable amount of drag. It is afterall nearly a flat surface perpendicular with the relative wind.