[ODE] Hovercraft?

[Nate W]

On Fri, 30 Aug 2002, Clay Larabie wrote:

> > "For a hovercraft, you might try putting damped springs at each corner
> > of the chassis."
> 
> Can ODE be used for the springs, or is this something we'd have to
> ourselves? For example, I looked through the documentation for "spring" and
> didn't find anything. Could a joint be used to do this?

You could maybe use joints, but it would probably be computationally
expensive.  Springs are pretty straightforward to implement yourself
though.  It would go something like this... 


float SpringRate = use_trial_and_error_to_find_this
float DesiredAltitude = this_is_up_to_you

For each corner of the hovercraft

	Vec3 vecChassisCorner = the xyz of the corner of the chassis
	Vec3 vecGroundContact = the point on the terrain directly below vecChassisCorner
	Vec3 vecNormal = terrain surface normal at vecGroundContactPoint

	// get the current altitude as a scalar
	Vec3 vecDistance = vecChassisCorner - vecGroundContact
	float Altitude = vecDistance.GetLength ();

	// if the craft gets below the desired altitude, use a spring
	// to push it back, in the direction of the terrain normal.

	if (Altitude < DesiredAltitude) 

		// compute the spring force
		Vec3 vecSpringForce = vecNormal * (Altitude - DesiredAltitude)
		vecSpringForce *= SpringRate

		// apply the spring force
		dBodyAddForceAtPos (vecSpringForce, vecChassisCorner)
	end if
end for

If would be even better to find the point on the surface closest to
vecChassisCorner rather than what's directly below it, but that is, as
they say, left to the reader. :-)  

It might also be a good idea to add damping - to reduce the spring force
by some value proportional to the vertical component of the velocity of
the hovercraft's chassis at vecChassisCorner.  That might not be necessary
though, I'd try without and see how it goes.

One last thing - you probably don't want to apply any springs if the
hovercraft flips inverted.  Perhaps the normal of the bottom of the
hovercraft should figure into those calculations somewhere... or the angle
between the hovercraft normal and the terrain normal.  I dunno... mess
with it and see what happens. :-)

I hope this helps.

-- 

Nate Waddoups
Redmond WA USA
http://www.natew.com