All Things Mk5 > Performance Modifications
Improving aerodynamics with aftermarket parts
rex:
Yup, I know about that diffuser. That is my template also at the moment.
I know I have to get rid of the rear exhaust muffler, that is why I want a new complete exhaust.
Interesting read about the gurney flap.
As I can see it is even used on the diffuser above...
fuscobal:
Good findings on the gurney flap. MAybe I'll make one on my wing as well. Let's take my wing and make some small assumptions :
- Wing > Reverie 150mm chord with 1300mm span
- Desired gurney flap > 1.25% to 5% of chord, meaning 1.875mm to 7.5mm (these are quite small and barely visible on the car)
- Downforce @ 160kmh & 12deg AOA with no gurney > 45kg
- Downforce @ 160kmh & 12deg AOA with 1.875mm gurney > +20-30% wich means about 54 to 58.5kg ( 9 to 13.5kg increase !!!)
- Downforce @ 160kmh @ 12deg AOA with 7.5mm gurney > +40-50% wich means about 63 to 67.5kg ( 18 to 22.5kg increase !!!)
If these values (taken from the last link Bruce provided) are not highly optimistic, then the downforce addition from these small devices is tremendous and easy to achieve !
It seems that I did not pay attention while ordering the wing since they also have carbon 5mm and 10mm gurney flaps available for 90-130L)
the bruce:
According to Joseph Katz:
From an aerodynamic standpoint when on track with a production car rear downforce is
by far most important, then front DF and drag is neglicible. Even more with a Golf which
has more rear lift than most sedan style cars.
Rear DF allows for a more aggressive suspension setup which will help on small radius
corners (where you don't have DF worth mentioning).
Well, the gurney flap is one of the very few parts invented first in automotive racing (by
Dan Gurney) and then established in aviation. In the latter case such flaps are just needed
on take off and landing. On journey low drag and highest possible efficiency is what plane
designers are seeking for.
Contrary to this at car racing drag is nothing reace car designers worry that much. Most
prototype racers have a Cd of 0.7 to 0.8, open wheel cars have even more.
Even production car based racers have much more drag compared to it's stock brothers
(which as you all know are in the 0.25 to 0.35 area today).
Highest downforce you can get is faster than highest possible efficiency. That's the point.
This is why the same applies to a wing on a car. Downforce is more important than efficiency.
While you get the best efficiency with a wing without a flap a wing with a gurney flap will
give you more downforce at higher angle of attack without stall. Stall will occur later than
without the gurney.
Conclusion: It's better you fit a gurney if you want the most DF (and you will want DF).
There are some not-so-expensive CF lips on ebay, but any black plastic profile which is
not too brittle will do. Just take care to fit it to the upper (high pressure) side. No part of
it should disturb the flow on the lower (low pressure) side of the wing.
fuscobal:
Yes, seems pretty clear I should add it. At 1-2% of chord the gurney adds no drag enalty at all and at 2-5% it adds increasing drag but still nothing to worry about if you compare to what it brings. I've read some technical papers and it seems that a 4-5% of chord gurney can add up to 40% downforce. For me, even adding a conservative 25% would mean about 10-12Kg @ 160kmh ! Not bad at all !
fuscobal:
Did some approximations on the real downforce level with my wing at max downforce position. For this I used the following data :
- OEM car rear lift > 37kg @ 200kmh (as posted by Bruce some time ago from a wind tunnel test)
- Reverie wing downforce > 30kg @ 160kmh (could be 35 but better be conservative so let's say it's 30)
- I've considered a 25% downforce addition from a gurney flap wich is 4-5% of the wing's chord
- Formula used to extrapolate from the 2 values I have was DF @ new speed = DF @ old speed x (new speed ^ 2 - old speed ^ 2) wich mean practically downforce increases with the square of the speed
Speed in kmh > Stock lift > Wing DF > Total 1 (Wing DF - Stock lift) > Wing + gurney DF > Total 2 (Wing + gurney DF - Stock lift)
80 > 5.92kg > 7.50kg > 1.58kg > 9.38kg > 3.46kg
100 > 9.25kg > 11.72kg > 2.47kg > 14.65kg > 5.40kg
120 > 13.32kg > 16.88kg > 3.56kg > 21.09kg > 7.77kg
140 > 18.13kg > 22.97kg > 4.84kg > 28.71kg > 10.58kg
160 > 23.68kg > 30kg > 6.32kg > 37.5kg > 13.82kg
180 > 29.97kg > 37.97kg > 7.99kg > 47.46kg > 17.49kg
200 > 37kg > 46.88kg > 9.88kg > 58.59kg > 21.59kg
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