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Subject: Re: Crosswind affects drag (and hence power req'd) on a car?

Date: Sun Feb 21 01:35:57 1999
Posted by Ricky J. Sethi
Position: PhD

Hi Eugene,

I think I'd have to side with your physics professor on this one. I guess you can look at it like this: any real crosswind will have a directly perpendicular component and a head on component. The head on component will definitely contribute to drag and hence serve as a power drain, as below:

|-------  ---> perpendicular component
|      /
| -----------> head on component 
|    /
|   /
|  /---------> crosswind
| /

If we idealize out to a strictly perpendicular crosswind, I think it would still negatively impact on the car's power requirements (albeit very slightly). Imagine a incredibly strong gust of wind that is directed exactly perpendicular to the car's length; if it's strong enough, it will push the car off-course; the driver has to compensate by turning the car back to maintain his original trajectory. Thus, extra power was required. I think the extra amount required is probably extremely small but it will increase the car's power requirements nonetheless. I hope that made sense... if it didn't, please feel free to drop me a line and I'll do a more involved and detailed analysis.

Regards,


Rick.

Moderator note: If air resistance is modeled as proportional to the square of the velocity, then mathematically the drag will depend on both the head on and the crosswind components of air velocity. The effect will be small for "normal" conditions.


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