An aero note...

[Blue]

Somehow Cp is thought to be at the 50% point of the lateral area and keeping the CG forward of this is aerodynamically stable.  This is not the case.  Cp of most symetrical airfoil sections is at the 25% point.  This means (for anyone with a rational mechanical layout) that CG will trail the aerodynamic Cp.  It is necessary to add vertical stabilizer(s) to move the lateral Cp aft (more negative C_nb).  Higher aspect ratio tails will have a greater stabilizing effect than low aspect ratio tails of the same area.

Calculating actual Cp on shapes as un-aerodynamic as car bodies is a little tricky.  Let me know if this rattles any thoughts on lateral stability.

[John Burk]

Eric
Can a reasonable size tail fin make a rear wheel drive lakester of streamliner aerodynamicaly stable ?

[willieworld]

eric  is the cp 1/2 of the side area or is the round front half area (looking from the side ) different than the flat back half area ---in a side wind   willie buchta

[panic]

.

[panic]

.

[Blue]

Calculating Cp shapes on elevation profiles (car etc. with no depth dimension or detail) is very easy.

Actually that is calculating the center of side area, not Cp.  Yes, there is a BIG difference between the Cp contribution of a round nose and a flat vertical tail; that's why it's not a simple process and cannot be measured correctly by simply plotting side area.

And yes, a reasonably sized tail can straighten out a lakester.  The best example was Al Tegue's lakester-turned-streamliner where the rear wheel fairings served as VERY large tails.  People on the team told me that the car was very aerodynamically stable and would come back in line anytime the traction slipped and it went out.  The same thing was reported by another team that I spent some time with this year on a door-slammer that I knew to be unstable in yaw:  They added fairly small tails the first time out and the car drifted pretty significantly above 200.  This year they put on bigger tails and the can was so solid between 200 and 300 that the driver actually steered it around a little on purpose to make sure he still had traction and wasn't going straight on luck and an airborne front end.

Finding the aerodynamic neutral point can be done with CFD, but modeling anything by the simplest car is time consuming and expensive.  Doing it manually is a bit of an art, and I can recommend some good books on the subject.

[dwarner]

Please define "tails" in layman's terms.

DW

[panic]

.

[panic]

.

[John Burk]

I made a model of my streamliner and blew air over it with it hanging from various points . It pointed steady into the wind with the wire at 41% and was unstable at 43% . The rear half of the body is 40% taller than the front .

[Blue]

Please define "tails" in layman's terms.

DW

(completely serious, not trying to over-simplify)

Vertical plates or airfoils mounted near or at the rear of the vehicle.  These can be integral to the body (BUB 7), attached above it (Flatfire), part of the bodywork (Teague), or endplates on the rear wing (lots).

[Blue]

I made a model of my streamliner and blew air over it with it hanging from various points . It pointed steady into the wind with the wire at 41% and was unstable at 43% . The rear half of the body is 40% taller than the front .

Good test, it saves a lot of calculation.  Pictures I've seen of your car show lots of vertical tail, mostly integrated into the body and weight distribution well forward of your tested neutral point.