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-   -   Drag coefficient (http://www.physicsforums.com/showthread.php?t=574919)

Wouter Feb6-12 03:19 PM

Drag coefficient
 
Hi

I am a mechanical engineering student currently busy with my final year project on spoilers I need to design manufacture and test a device to test different spoilers it must determine the drag coefficient of the spoiler and the down force generated by the spoiler without the use of an windtonnel can anybody help me with ideas to determine these 2 aspects but I have limited resources it would help me alot.

Thanx you

boneh3ad Feb6-12 03:53 PM

Re: Drag coefficient
 
Look into xfoil. A spoiler is essentially an airfoil anyway.

JaredJames Feb6-12 05:07 PM

Re: Drag coefficient
 
Without a wind tunnel? Do you mean you'll be driving around with it on a car?

I suppose you could model it, but then the software will do it all for you (not necessarily very well though).

So far as down force goes, you could simply attach it to a rig which is capable of moving (pivoting / compressing) and use some form of compression sensor (clever use of a strain gauge if you want it cheap) to record the downforce produced. (If you want really simple and cheap, just hook up a simple spring Newton Meter - a spring with a gauge in Newtons - to it so that it compresses and gives you the force downwards.)

Wouter Feb7-12 10:33 AM

Re: Drag coefficient
 
Yes it must fit onto the back of n pickup truck but I think the most difficult part of all is a way to successfully determine the drag coefficient of the spoiler

boneh3ad Feb7-12 01:09 PM

Re: Drag coefficient
 
Quote:

Quote by Wouter (Post 3750727)
Yes it must fit onto the back of n pickup truck but I think the most difficult part of all is a way to successfully determine the drag coefficient of the spoiler

XFoil will do it fairly accurately. Seriously. Unless you are specifically told to do it experimentally, I don't know why you don't just do this.

http://web.mit.edu/drela/Public/web/xfoil/

Wouter Feb7-12 01:43 PM

Re: Drag coefficient
 
I can use if for theoretical values but I must determine it experimentally

JaredJames Feb7-12 01:50 PM

Re: Drag coefficient
 
Have a look at this, http://en.wikipedia.org/wiki/Drag_coefficient.

It's a bit basic, but it should help.

You can calculate the drag force roughly using the equation shown (http://en.wikipedia.org/wiki/Drag_co...ent#Background) and then use that to get the coefficient. All you'd need is the velocity of the vehicle and an approximation for the flow type.

xxChrisxx Feb9-12 06:41 AM

Re: Drag coefficient
 
Quote:

Quote by Wouter (Post 3751028)
I can use if for theoretical values but I must determine it experimentally

Wirecut profile from foam.
Measure surface finish.
Test it in a wind tunnel.
Compare to X-foil output.

Your aerofoil won't have a single Cd value, it'll alter with angle of attack.
There is little point in testing unless you can get meaningful data.

Why would testing it in a windtunnel be better than testing it on the back of a truck?


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