Drag reduction question

[FormulaVee]

Hi,

Can anyone help with some "rules of thumb"?

I am building a number of parts for an amateur race car. This series has very limited power and I want to minimize drag as much as possible.

Mirrors are essentially a half sphere, directly in the air stream. I believe this shape has a coefficient of drag of 0.42. If I place a lip around the outside to detach the air will that reduce CD? Is there a better way?

There are two round tubes that are also in the air stream as part of the front suspension. If I surround them with body work, what shape is best (ellipse parabola)? How do I select the best shape? The tubes are 52mm spaced at 120mm on center.

I appreciate your input!

Tom Hussey

 

[GregLocock]

You'll find that the flow from the back of the mirror is already detached.

Look at old planes to see what fairing shapes they used. You are in the tricky tradeoff between detached flow with the cylindrical shape, and increasing the surface area. I'd go for a 5:1 thickness ratio ellipse, but would not be surprised to see it making things worse, due tot he increased surface area.

You could do an interesting experiment at the relevant sped. Fit a roof rack to a car, then mount a vertical axle to it. With two arms sticking out, one faired and the other not, you should be able to rapidly determine which is better. Adding a means of measuring torque will allow you to develop the shape further. If this sounds like a Wright brothers experiment, you'd be exactly right.





Cheers

Greg Locock

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

 

[FormulaVee]

Hi Greg,

The idea of testing the shape seems like a good idea. It is easy to try with just a fish scale and a few cardboard tubes. Thanks!

On the mirror though... I understand that the air will detach at the edge of the mirror and produce a low pressure and vortex behind it. If the edge of the mirror were to be shaped sort of like a spoiler all the way round, could that be used to help?

 

[GregLocock]

Try it and find out. But I'd expect to see no benefit.

Cheers

Greg Locock

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

 

[IRstuff]

In of itself, boattailing objects will help the drag coefficient. The question is whether that tiny little bit would make ANY difference to the overall performance.

TTFN

 

[MikeHalloran]

You could probably get away with removing the mirrors entirely, and subsituting rear-facing lipstick cameras and small monitors ... for one race. Then they will change the rules, unless someone has done it already.

OR, you could continue to use passive mirrors, but move them toward the driver's eyepoint and make them physically smaller, maintaining the same field of vision.

When you start pursuing reductions in Cd, it's easy to lose sight of the fact that Cd is a ratio, and big improvements can be made by reducing the frontal area, even if the Cd gets a little worse.

The extreme case would be the sort of mirrors that bicyclists mount on their helmets. That might also get outlawed quickly.

But if you made up normal looking mirrors in say 7/8 scale, maybe nobody would notice for a few races.



Mike Halloran
Pembroke Pines, FL, USA

 

[rb1957]

instead of round tubes, there are (were) streamline tubes that were used as airplane wing struts in the (long distant) past ... maybe elliptical would be just as good ... maybe buying round tube and "slightly" crushing it would be cheaper (even allowing for a heat treat to recover material propeties).

i like the camera idea (to replace rear-view mirrors)

good luck

 

[WKTaylor]

formulavee...

I suspect that you need to get the SAE books on race-car aerodynamics... lots of good/practical info on "details".

Regards, Wil Taylor

 

[MikeHalloran]

A transparent boattail is a very interesting idea. Pretty tricky to get the optics right, but maybe possible. Something pointy like a cone would be impossible, but if it had a sort of ducktail shape, so the optical path went through material of uniform thickness...



Mike Halloran
Pembroke Pines, FL, USA

 

[sprintcar]

The amount of Cd reduction gained by the mirrors (unless they were borrowed from a diesel truck) is really not that significant in your class of motorsports.
Focus on reducing unsprung, rotating weight which has about a 10:1 return - ie every lb you remove helps the car corner like it was 10lb lighter.
Things like titanium wheel fasteners, shaved tires, lighter wheels all make the largest difference.
Look for other aero drag items, like tabs and fastener heads under the car. Work on ducting to place the air where it does the most good. Polish the body work with something like ShowShine that reduces air drag.
Then put the driver on a diet........

"If A equals success, then the formula is: A = X + Y + Z, X is work. Y is play. Z is keep your mouth shut."
-- by Albert Einstein

 

[vortexman]

Hi Tom,

You might be able to get some drag reduction at the mirror by blowing air out the back. If there is a gap between the actual mirror and the housing, and a source of air, possibly even a small opening on the front of the mirror housing, air blown out that annular gap could give some drag reduction.

 

[Spice52]

Hi Tom

No do not put a lip or similar on the mirror, it will have no + effect

Round tubes are poor in airflow, and yes fair them in, a teardrop is about as good as you can guess at, you can get downforce from faired in wishbones by using certain profiles but normally this is illegal and not worth the hassle at club level.

Aero is normally a complete pacakge and is very very complex, but you can find gains in certain areas that are worth it.

Cheers

Marcus

 

[carla00]

A lip on the mirror could increase or decrease drag depending on how you position it, ether way it's probably quite insignificant and the side and front of the car may also effect the airflow in the area behind the mirror.

What type of racing are you looking at ?? if you are looking at less long straights and more corners, downforce is quite an important thing to look at too.

 

[Hydra70]

I have to agree with the weight reduction approach. While drag reduction is a good thing, there can (and will be) a point of diminishing return. Weight reduction always works.

If you look at your mirrors in terms of Drag Area, they are miniscule when compared to the car as a whole. Unless you are running a molded body fairing, there are probably some other snags that lend themselves more easily to drag reduction.

I have seen flow former/deflector type devices run in wind tunnels, and the net reduction is typically measured in low single digit %'s of improvement - if any. They are just as likely to add drag as to subtract it.
As to full enclosure like a boat-tail fairing, probably the best idea aerodynamically, but the optics would be a full booger.....

 

[sprintcar]

If you're going fast enough, you don't need mirrors......

"If A equals success, then the formula is: A = X + Y + Z, X is work. Y is play. Z is keep your mouth shut."
-- by Albert Einstein