yet another cp/cg question

[Jonny Hotnuts]

For stability purposes does the aerodynamic frontal down force change the CG. Or does the statement "have your CG in front of you CP" only apply to a non moving vehicle?

Say you have a car whos CG is well behind the CP at 0 but because of an inverted wing at the nose caused a considerable amount of downward pressure on the nose making the CG move forward. I know this would be true if the car was setup on a balance point in a wind tunnel and then it was turned on.


I have never heard anyone speak of if frontal down force contributes to stability.

Just wondering if it applies.


-JH

[JackD]

A device that contributes to the CG forces acting on a vehicle can be ballast that is constant or aerodynamics that depend on the flow of air that is variable.

[Jonny Hotnuts]

Thanks JD.
I thought frontal down force would change the center of gravity and agree that it is a variable (but predicable) force.

Having a mid engine car most of the weight is in the back half of the car. I built the nose to generate down force hoping that I would not have to add ballast and at worst just add the water tank for the turbo motor next year and keep the 2 batteries I have up front for weight.

-JH

[Sumner]

A device that contributes to the CG forces acting on a vehicle can be ballast that is constant or aerodynamics that depend on the flow of air that is variable.

Jack my good friend, I might have to disagree with you on this one, but this is only a guess.  My guess would be that we are using the CG/CP to keep the car straight that as soon as it isn't straight then the aero part of the cg is going to diminish quickly and the relationship of the two will change quickly, whereas if you are using weight to accomplish the cg it will always be there for you.

This is just a gut feeling and I'm open to other opinions and maybe this is what you were saying in the "flow of air that is variable" part of your post.

c ya,

Sum

[JackD]

Air is a fur shur variable and while you think you have it figured out, it or you changes, then it is a whole new ballgame.
Vehicles that really fly across the salt, REALLY FLY, when the apparent direction of the wind changes.
Ask the "Frequent Fliers" that use ground effects for suck.
Airplanes can correct the point of the craft and still go the direction they want with some loss in efficiency..
Vehicles in contact with the ground don't have that luxury without some way loss in stability.
I had a Corvair Turbo that could beat the Pushes of the day in part because I had two 80lb. sacks of cement in the front trunk and didn't have to be afraid to drive it.