Neutral Aero

[fastback33]

I really enjoy this board you guys have so much knowledge i like to ask my elementery questions and get great responses in return!

So here goes another one. I saw a show on Nat. Geo. about bonneville speedweek, and while looking at the streamliner's i was wondering: "Is it possible for a car to have neutral aerodynamics?". meaning the air going over and under the car does not create lift nore downforce? is it possible? Is seems like no matter what there would always be some kind of force acting on the car. Any help on this matter would be cool!

[sasquatch]

Yes. It is possible to design a car neutral lift. Many solar cars are designed not to create downforce because downforce or lift will create vortex induced drag on the body.

Take the new Aurora solar car as a good example of this. http://www.aurorasolarcar.com. The body is basically a wing but a wing near the ground will create downforce. As a result, a morelli camber line was applied to move the thickest section of the wing away from the ground until it was neutral. I found a quick explaination here. http://epubl.ltu.se/1402-1617/2007/007/ ... 007-SE.pdf

[slimjim8201]

Is it just me or does that masters thesis look like a high school report to anyone else? Nothing was testing, nothing was proven.

[Ogami musashi]

No, as long as your body displaces air, it will affect the following car.

once displaced, the air needs to reshape, which means motion which means pressure variations.

[sasquatch]

I think it was a thesis in industrial design, not aerodynamics. It was just a quick reference to work referencing Morelli.

Solar car still have a wake. If you consider the momentum equations, there are losses due pressure loss drag, vortex induced drag and longitudinal velocity loss. The vortex induced drag term is what is most effected by lift or downforce of the object. So minimizing this term will reduce the overall drag of the vehicle. The other losses will come from viscous effects and shear stress on the surface of the vehicle (important to reduce the wetted area as much as possible.)

[Ogami musashi]

oh sorry i didn't read the question right..i thought it was a question about the wake.


Well a zero forces body? maybe a symmetrical one would come close but now the problem is that we are talking of cars and they run in proximity of the ground.

[SoundMan]

from my simple understanding... the electric car mentioned above is aerofoil shaped in a sense. landing aircraft experience a reduction in drag as they get very close to the ground (at an altitude of about a 1/2 wingspan and below) called the 'Ground Effect'.

so with this thinking, would it be beneficial to have the aerofoil shaped car located at a 1/2 wingspan (or carspan!) above the ground to benefit from this effect that neutralises the trailing edge vortices?

if so, i suppose they already do it!

[Carlos]

The Morelli Body refers to the long tail profile - teardrop profile like some of those cars referenced. How the three converging angles(back-light/boat-tail/ramp angles) and length of the rear bodywork section relate to reducing drag. It reminds me of the long tail Lemans prototypes like the Porsche 908, 917 and the Ford P68. Morelli was looking at this in the early '80's and I found a reference to some aero work he has done in the last few years on the X43A. Here is a 8Pg. PDF Hybrid Method for Aerodynamic Shape Optimization in Automotive Industry which includes some reference sources that are interesting.

http://www.ann.jussieu.fr/~dumas/comp&fluids.pdf