chris walrod

If anyone is interested, here's a study that you may find interesting.

http://www.aoe.vt.edu/~mason/Mason_f/AIAA2007-4175.pdf

911SLOW

Very interested ! Thank you for the link!

John

SundayDriver

Interesting data - thanks Chris.

It would be very interesting to see what Gurney flaps do the the pressures - do they work mostly by increasing or decreasing pressure (top vs bottom). This would have a big potential effect on how a wing helps drive a tunnel or diffuser.

analogmike

I am an engineer but I have been an administrator for too many years now, so that makes my head spin... brief synopsis as it relates to a 911 rear wing?

SundayDriver

Gurney Flaps Good. Bigger is better (Gurney flaps and other things too).

Lemming

Figure 4 shows that you can get the same increase in lift as a 5% GF by increasing the angle of attack 7.5 degrees. Given that the 5% GF increase drag by 335%, I would like know the increase in drag by increasing the angle of attack of the wing (without) by 7.5 degrees.

Edit: I think I answered my own question. This page http://www.lets-go-fly.com/Lift%20an...the%20Wing.pdf suggests that increasing angle of attack by 7.5% equates to an increase in drag of 500%, which is clearly greater than the GF.

SundayDriver

I was being a bit glib in my response. You have to always take these studies with the understanding that they are using a wing in clean air and that is NOT what your car sees.

Gurney flaps, up to a reasonable size (typically 1/2 to 3/4") will generate more downforce with less drag in the real world - vs. the same wing with no flap. Obviously if you go really large on the Gurney (inches tall) you make a lot of drag and will mess up the wing flow.

They do have a later chart of CL vs CD but I am not really sure what I am seeing - it looks like that chart may be saying the opposite of what I am saying and see in data.

TRAKCAR

No idea what that all said, but here is the short version; About one second a lap faster with Cup splitter and Gurney flap VS stock.
Could not feel it, but laptimers was proof.

John H

^^^^ Pretty^^^^^

911SLOW

That is the nicest part of this paper IMO, it shows the difference in drag vs the proportion of the gurney lip to the wing's size.
Usually people play with a one size fits all type of lip.
I will try to make the percentages work for a standard 6 cup wing if I find some time.
One size should be ideal for the wing's size and the chosen angle of attack to produce as much downforce as possible with the lowest drag penalty.
It is interesting also that the larger Gurney lip's drag increment is not linear to its size.. That is puzzling..

stownsen914

Cool article ...
With reference to larger Gurney flaps producing a disproportionately larger drag increment, actually this is consistent with what Katz says in his book Competition Car Aerodynamics. Larger gurney flaps do make more downforce, but the drag cost is even higher. I'm going a little from memory here, but the Katz book has a graph showing that you really get diminishing returns (and a significant jump in drag) from gurney flaps larger than about .5"

Scott

SundayDriver

That chart (Fig 14) seems backwards or I am not reading it right. It seems to indicate the for the same CL the wing with Gurney has a higher CD.

mark kibort

there is not a 1 second difference in using a gurney flap vs not using one and using wing angle to create the same downforce. will that 2.5 degree effective wing position give you a 1 second a lap advantage or difference ? probably not. remember the drag for most of our tracks is not that great. maybe 1/10th of the downforce produced in most cases. that relates to reduction of accelerative forces by a very small amount. if you get 250lbs of downforce, thats 25lbs of drag which would be near 5ft-lbs of torque at the engine (TOTAL) if you got a 25% reduction in drag for the same downforce, that would be 1ft-lb of engine torque saved.

mark kibort

And this is just the point of all this right there. gurney faps increase downforce to the same degree that the angle of attack (AOA) can be changed in small increments. It does so with a sligtly lower drag value vs the greater angle of attack. but there are trade offs, and limits as well.
effectively, the new angle of attack becomes the angle of the top of the gurney flap to the front of the wing. It also changes some of the wings flow characteristics. with the flow of each car being unique, it all has to be looked at to see the net effect of the GF.

those high numbers of drag increase % doesnt look quite right.

Here is a basic wing profile for lift and drag vs AOA

mk

TRAKCAR

My last post on the subject; Yes there is, you are wrong I am right. This is the end of that story.