Frontal area and Coefficient of drag questions 1

[EdDanzer]

Could someone explain how coefficient of drag and frontal area effect how much power is required to move a vehicle at a specific speed?

Where would I find frontal area and Coefficient of drag values for current vehicles especially larger cars and SUV’s?

Ed Danzer

http://www.danzcoinc.com

http://www.dehyds.com

 

[reidh]

If you do a free body diagram for a vehicle, it will become quickly apparent the effect of Cd and area if we know that:

Drag (Force)=1/2*Cd*rho*Area*velocity^2

Magazines like Car and Driver or Road and Track usually have Cd values for new vehicles.

-Reidh

 

[patprimmer]

Advertising brouchers have them if they are good. If it is not listed in the specs, you can presume it is bad.

Regards

eng-tips, by professional engineers for professional engineers
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[sreid]

Or in words, drag is directly proportional to both Cd and Area at constant velocity.

The power required, however, is proportional to the velocity cubed.

 

[thundair]

Ed
Some links

http://www.landracing.com/forum/

http://purplesagetradingpost.com/sumner/bvillecar/bville - LSR Thoughts-1.html

Cheers

I don't know anything but the people that do.

 

[sbrats]

If I continue the thought, if the frontal area was changed (ie lower the vehicle) then the drag would be lower and a potential for fuel savings would be some small value

 

[70AARCUDA]

...an 'edge-wise' piece of paper presents far less air-resistance than does the same paper positioned tangentally through the air.

...or as an analogy: a FULL parachute descends SLOWLY, but the same parachute when STREAMING falls like the proverbial rock.

 

[MacGyverS2000]

@sbrats: Lowering the vehicle doesn't (in itself) change the frontal area of the vehicle... go the other way and put the car on stilts, still roughly the same frontal area. It will likely change the airflow characteristics as you prevent air from running under the vehicle, but that's a different animal.


Dan - Owner

http://www.Hi-TecDesigns.com

 

[reidh]

To expand on what Macgyvers2000 said, the change in airflow characteristics seen by lowering the vehicle would be reflected in a change of Cd of the vehicle.

Reidh

 

[mullins]

Unfortunately, automobiles are complex. The flow field for a car is in incompressible flow unlike an airplane. Since you are so close to the ground, you also have to deal with ground effects. Defiantly a CFD problem that would not be too much fun.

 

[GregLocock]

Milliken, and Hucho, and Katz, have plots of aero drag vs ride height for some (mostly racing oriented) cars. For real cars your best bet is probably Hucho. Sorry, I haven't got them with me to check.

For any given shape and pitch angle there is an optimum ride height. Too low and you get downforce, too high and you get lift. You want zero vertical force, for minimum drag.



Cheers

Greg Locock

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

 

[patprimmer]

You get a very small change to frontal area as less tyre is exposed, but the change in ground effects far outweighs chang in frontal area due to tyre.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[GregLocock]

Oh, I was ignoring the air churning around the tire. That is a big effect.

Cheers

Greg Locock

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

 

[sbrats]

PatPrimmer, to clarify, if a given shape was static mounted in a wind tunnel - drag readings taken. 2nd run, say lowered 2" and drag reading taken - the drag value would be influenced more by the under vehicle interaction with the ground and the incompressible flow characteristics.

Is the effect of air churning from the tire a bigger influence than the ride hight effect? Expanding on this then - other than maybe the desire to use air flow through the wheel for brake cooling - if the wheel well was 'sealed' off from the underside of the shape and the underside leading area of the tire zone flared (to prevent or minimize the rotating surface exposure to the incoming air) not that this would be practical then you still need to deal with the ground effect and incompressibility.

Steve