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# Aerodynamic coefficients for a production car

## Aerodynamic coefficients for a production car

(OP)
Is it normal for the yaw moment coefficient to be negative in sign, and all the others are positive, assuming SAE conventions for axes? This is for a conventionally shaped sedan. It seems reasonable (ie the car weathercocks) but I want to be sure.

Also, where is the reference location usually situated in most wind tunnel software, for the single point method? I'm guessing half way between the axles at axle height, but that is only a guess.

Finally, what is the meaning of the yaw moment coefficient, CYM

After all, a torque of 1/2 CYM rho v^2 A is dimensionally incorrect, so that equation needs to be modified.

Yes I know I should look it up but the books are elsewhere.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

Replies continue below

### RE: Aerodynamic coefficients for a production car

Greg:

I don't know about SAE aero convention, but in NACA convention
CYx is a side force derivative.

As to the dimensional problem, remember that moments are nondimensionalized by an additional characteristic length that doesn't show up in the force coefficient dimensionalization.

Just curious, what is the M in CYM?

Doug

### RE: Aerodynamic coefficients for a production car

(OP)
CS is the sideforce coefficient, CYM is the yaw moment coeffcient. This is a single point method, so I have three forces and three moments, and, necessarily, an assumed point of application.

I'm guessing moments are normalized by the wheelbase.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

### RE: Aerodynamic coefficients for a production car

Thanks Greg:

It is interesting that the NACA and SAE symbology is so
different.  Even marine, SNAME, is close to NACA.

NACA yaw moment coef is CN and derivatives are CNx.

Lateral reference length is usually the wing span. Longitudinal point of reference is usually .25mac.

Doug

### RE: Aerodynamic coefficients for a production car

(OP)
Turns out it the assumed point of action is  half way between the axles, at ground level, on the centreline.

Wheelbase is the reference dimension used to normalise moments.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

### RE: Aerodynamic coefficients for a production car

As far as i know... the normal practise is to measure the forces at at each wheel (wheel/road contact).

### RE: Aerodynamic coefficients for a production car

(OP)
Yes, but you then have to reduce those measurements to a set of coefficients. The Europeans typically specify side force and lift at each axle (2 pt method), the USAns typically specify a force acting at a single point and a moment about that point.

They are mathematically identical.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

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