J richard

Or you could of added a gurney and taken out less wing, reduced the drag the same, gotten more DF in slower corners and faster lap times overall....just sayin....

Krokodil

Mark, remember that the complete Cd is a combination of the base Cd (skin friction and from) and induced drag. Induced drag is combination of Cl, wing plane shape (e.g., square or elipse), and aspect ratio (Length/chord) - and is not related to wing angle, only lift, and is therefore constant for a given Cl.

This results in a final Cd estimated by the following equation:

Cd = Cd0 + Cl^2 / ( pi * Ar * e)

Cd0 = base Cd (skin friction and form)
Cl = lift coefficient
Pi = well, Pi
Ar = aspect ratio (length / chord for a square wing)
e = efficiency factor (1.0 for ellipse, 0.7 for square)

So, looking at the Xfoil Cd0 charts and concluding that drag increases by X00% beyond "stall" is not correct as the induced drag stays nearly constant and acutally drops with addtional angle. We must look at total drag as described by the equation above.

I went back and re-calculated the E423 drag data (similar to, but the not the same as the Cup wing) for AoA beyond max lift. Below is an sample of the data and resultant calculations:

Assume: Ncrit = 5, Re = 1,000,000, Speed = 125 mph, large end plates (11.4% efficiency gain), 300 mm chord, 1700mm length, no Gurney

AoA=10 degrees (near max lift)
Cl = 1.91
Cd = 0.31
Lift = 356 lbf
Drag = 51 lbf
Power required = 17.1 HP

AoA=18 degrees
Cl = 1.90
Cd = 0.39
Lift = 353 lbf
Drag = 65 lbf
Power required = 21.6 HP

So, running the E423 wing at 6 - 8 degrees beyond max lift provides nearly the same downforce, but requires an additional 4.4 HP to create - a 26% increase in drag.

As you say, this is most likely why people see a drag reduction by adding a GF. They are at an AoA beyond max lift so adding the GF increases the effective AoA range of the wing, improving L/D at AoA beyond 10-12 degrees.

Also remember that not all of the wing is exposed to the same AoA, so the calcuations above are relative and not actual. Although I am curious as to whether the DF at a given speed and AoA are close.

Do you have any emperical data you can share?

Cheers,

mark kibort

based on the curves and all the simulations, the drag at near max downforce of the cup car wing, is near the same with and without the GF. slightly less without it. I don't want any more downforce as it seems to be balanced in the lower speed and high speed turns . the only way I could increase downforce to see if I needed any more, would be to put on the gurney flap and take out 3 degrees or so. (currently at 3 degrees angle of incidence which is near 11 degrees AoA.) putting on a GF and putting wing at 8 AoA, (0 Angle of incidence) , would give 20% more downforce. and proportionately higher drag based on the GF curves.

mark kibort

Yes, we went over this earlier in the discussion. its where I was questioning the Cd0 figures.......(parasitic and profile drag, sans drag due to lift).
and yes, the simulation curves all point to if lift continues to go down past max lift the only thing increasing drag past max lift, is Cd0. and this is how the simulators are programed... HOWEVER< in talking to the Head of the Manned Space Division at NASA, and discussing his simulator (that came up with the same results, he said , "remember, this is simulator, its not relatity".. "Based on the data we got in the wind tunnel, there is an error past stall. the actual data doesn't fit the sim tools (foilSim and probably Xfoil).

So, given the actual data from the Wing tested in the cal poly tests, you can see how they map out to the calculated data. (which is the area we are talking about , past max lift)...... it doesn't fit there either. It clearly shows the drag falling off in the simulator and skyrocketing in the actual wind tunnel. Tom (from NASA) says this is where the error is and they are going to correct it somehow if possible.

What this means, is that if I was 18 degrees AoA, even though the wing was set at 9 degrees angle of incidence, there was a lot more drag, and less lift than at max lift of 11 degrees......... so much so, that even at a much lower AoA of 8 degrees, where the lift (downforce) might be the same , the drag would be 4X (.2Cd to .8Cd) at the "over" angled wing setting, even 4 degrees over (12 plus 4 degrees = 18 degrees).

in my case for example......

calculated and empirical data shows:
300lbs of downforce at 100mph at 11 AoA Drag should be 60lbs
at over angle of 18 degrees AoA, the drag might be 200 + lbs with (275lbs) downforce. this means that by putting the wing angle at an angle of incidence of 3, I lowered the AoA to 11, gained 25lbs of rear downforce and saved 140lbs of drag. (or about 5 hp in 4th gear)
Had I lowered the wing to 0 angle of incidence, that would have provided the same downforce as the original setting, but with a 1/4 the drag.
in other words: 275lbs of downforce at 0 angle of incidence, 8 degrees AoA is the same as 18 degrees AoA., but the drag is 60 vs 240lbs. (4x the drag)

if im operating at max lift, which it seems I am at a near max lift AoA. then, putting on a GF means I have to put the wing at -7 degrees, where the GF would then put the wing at max lift vs clean wing. However the data is not clear that the wing at -7 degrees wouldn't have any other odd drag abnormalities. common NACA wings, seemed to calculate ok, but im not sure about the e423

Funny, I was putting my data in and didn't closely look at yours first.... yours all makes sense..... a little differnet, but, close. the reall point here is what the lift does past max lift with increased angle. your data is consistant with the simulator. however, in actual wind tunnel tests of a similar wing, the lift in actual tests fell off faster than calculated data, and the drag continued to increase with increased angle, which is not consistant with the calculated data. im still talking to Tom at NASA as to why that might be.

I think worst case you nailed it. over angle has a cost of HP for the same hp and that's my take-away here. folks don't realize the defection factor of 8 -12.5 degrees for our porsches (depending on the model) they also don't realize that max lift is near 10-12 degrees for the cup wing.
However, the point is moot if there is a gurney flap on it, as it extends the downforce (range) well past clean wing angles, and they might have more downforce than they expected, but if it works, and they figure out ways to get the front to stick, all is good... they are not experiencing any excess drag.
However, in the case of the Ferrari, a flat wing setting at RA or RAtlanta, would be a cost of near 20% in drag vs a clean no Gurney flap wing because the true angle of attack for that wing is near 0-3 degrees with a 0 angle of incidence.

my empirical tests show about 288 lbs of downforce at 120mph. but there is some error due to my test methods ... compared to my over angled wing, it seems to be a little more. the drag is almost impossible to test for, so that's why the reliance on several of the wind tunnel tests and trying to get clarity of what happens near the limits.

mark kibort

This is not the same wing as the e423, but its a wing that does show what happens with GF added and the effect of drag past max lift.
I put this wing into the airfoil simulator and everything comes out close , until max lift. drag doesn't match the actual wind tunnel data here.
Try it on the XFoil simulation.

clearly it shows drag continuing to increase with increased angle past the point of max lift. even though drag is falling off as well.... there is a factor that is somehow left off of the simulation too. this is what tom was talking about in our conversations. Cd0 doesn't go up enough when induced drag starts to fall to account for this increased drag for excess AoA. there might be a factor of the Cd0 that goes up exponentially during the stall phase that isn't in the sim modeling.

cetom

sorry to revive this thread but i need to ask a question. . lets say that a rule change allows you to raise the height of your rear wing that is on a 911. prior to this thread i would have taken advantage of this ,raising the wing haven thought that raising it was a good thing since now the wing is in "cleaner" air making more downforce for a given degree of AOA. Or to put it differently i would be able to raise the maximal possible downforce
But is this true? mark points out that the wing at its porsche factory height is kind of taking advantage of the 12 degrees of roof line air flow that allows you to set the angle of incidence lower but achieve a better AOA. i am not saying this exactly right but you get the point . is raising the wing one of those things that sound intuitively right but in fact not advantageous . are you better off just leveling it a bit where it is now and just add a gurney flap?
thanks

flink

Complicated.

http://www.fem.unicamp.br/~phoenics/...304.092016.pdf

Figure 6 on Page 38.

For a sedan-style car, downforce is maximized when the gap between the wing and the body is about 0.8x the chord. That's really low! Put the wing higher and the downforce drops - a lot.

What I don't know is whether this is due to some weird interaction with the bodywork, or whether it's simply because the air is traveling downwards at a steeper angle close to the body, so lowering the wing effectively increases its AoA.

cetom

thank you very much ..
also thanks for the chapter . now i have something to read on my way to sebring.

333pg333

Aero has progressed a Loooooong way in 20 years when that was published. Not saying he's therefore incorrect, but I'd be willing to bet that there is more recent evidence to suggest that you don't have to place a wing so close to the rear of the car as pictured. That gives very little room for the underside of the wing to work properly and that is just as important if not moreso than the top.

Steve113

Do they sell the "Cliff notes" of this post or the "For Dummy" translation?

Matt Romanowski

Yes. Go here http://a2wt.com/RaceCars.html, spend around $2,000 and have real answers instead of guesses.

stownsen914

Joseph Katz' book Racecar Aerodynamics is a great read, and explains well why putting the wing closer to the car can often result in better OVERALL downforce for the car. In short, it's something like this:
A - A wing IN ISOLATION works best in free air. Putting it closer to the car results in somewhat less downforce the closer it gets to the car due to dirtier air. You have to get less than a chord width away before the downforce of the wing (again, in isolation) drops off measurably, and it keeps a good portion of the downforce even down to like .5 chord width away.
B - The trick is that the wing interacts with the car's aero to increase overall downforce. Examples of reasons why include 1) helping draw air out from under the car in some cases. On cars with flat bottom or a diffuser, the wing's effect on overall downforce can be staggering, but in some cases there is advantage for "regular" cars; 2) cleaning up the air flow off the back of the car. Both of these help reduce the natural aero lift tendency that most cars have at the rear, which can easily result in reducing lift by hundreds of pounds at high speed (i.e. increase downforce). This effect obviously varies greatly based on the car's shape and also how it's set up.
C - As noted elsewhere in the thread, the downward flow of air off the back of most cars means you can run less angle on the wing. This doesn't really give more overall downforce, it just changes the angle of attack you run for a given downforce level.

B above is why mounting the wing lower can give more overall downforce for the car, though it may seem counterintuitive. Katz goes into some detail about this in his book with data from IMSA GTP and GTO cars for which he worked on the aerodynamics back in the early 90s. Yes, older info, but the principles are the same.

Steve113

Matt,
that place is very cool . Wish it was closer I think it would be an amazing tool to see how the newly approved 996 Cup upright work
$1000. for 2 hours is a great price. Wish I lived closer

Anyone want to test a 996 with the new uprights? I would help out

stownsen914

Yeah, seriously ... almost worth a trip to NC.

mark kibort

Cliff Notes:

If you run normally with a wing at near 0 angle of incidence (angle that the wing is set at as you look at the car) , it probably really has a angle of attack of about 8-10 degrees. (AOA is the true angle that the wing sees oncoming air flow due to roofline deflection) in which case, the wing will have lower drag without a GF than with it. (because the effect of the GF will increase the effective AOA to about 5-8 degrees on its own, and the drag will be greater than if the wing was set at 5-8 degrees AOA)

However, since most wings are set at between 5-10 degrees, (angle of incidence), they are effectively at an higher angle than the max lift point (usually around 15 degrees AOA) and are producing larger amounts of drag and slightly less lift (than max ) than if that point was actually known and the wing was set there.

Summary: a Gurney flap allows for greater downforce than that wing could produce by angle alone.

Since most folks have their wings set at a near max lift setting to start. (i.e. 5 degrees plus roof line of 8 degrees is 13 degrees effective AOA), a gurney flap is a good way to make any increases of downforce, while keeping the normal lift to drag ratios in line with pre-max lift levels. More importantly, its an easy way to increase the lift (or downforce ) capabilities of most any race car wing.