Thrust & Pressure

[Nemos]

Hi there.
I am trying to create a realistic physics simulation for testing thrust based hovering vehicle designs and i am stuck.

I understand basic physics like moment of inertia and forces and the like but thrust is something i am having trouble with.
Currently if i place a vertical thruster at each corner of a rectangular craft, when the thrust force overcomes the weight of the craft it just kind of floats streight up forever.
As it is, the thrust force is just a variable being incremented and added to the rest of the force calculations.
The thrusters are supposed to be propeller style things expelling air at high speed.

The main problem is that i expected the craft to reach an equallibrium for a given amount of thrust rather than just floating on out into space. There is a huge hole in my knowledge of physics here and if you can provide any links to good resources or have any comments of your own, i would be most grateful for any help.

[russ_watters]

Originally posted by Nemos
Currently if i place a vertical thruster at each corner of a rectangular craft, when the thrust force overcomes the weight of the craft it just kind of floats streight up forever.
As it is, the thrust force is just a variable being incremented and added to the rest of the force calculations.
The thrusters are supposed to be propeller style things expelling air at high speed.

The main problem is that i expected the craft to reach an equallibrium for a given amount of thrust rather than just floating on out into space. There is an equilibrium altitude, the problem is that the atmosphere is (depening on your definition) about 450,000 feet high, so the pressure/density gradient isn't very steep on the scale of a tabletop experiment. It may well be that a 1.01 thrust:weight ratio at sea level will take your craft to 1,000 feet before it drops to 1.00. The loss of thrust with altitude, however, is different for every engine.

[enigma]

You will need to model an altitude control system.

If you don't mind oscillation about the desired point, something as simple as:

Thrust = (desired-actual)/desired * Full_Thrust

may do the trick.

[Nemos]

Hi there.
Thanks for you input chaps.

Very helpful indeed.

I will try some of this out tonight now that i have some idea of what should be going on.

Thanks again.:0)

[Cliff_J]

Would ground effect be a factor as well, or is that a factor only applicable to fixed-wing aircraft? I once watched a show on hovercraft and vertical takeoff craft on the history channel and believe it was mentioned but forget in what context.

Cliff