Aero Question..........
 

Take two Cup wings, one 63 inches, one 55 inches. All else the same.



- Is it a given that the 63 inch wing will provide more DF at any given angle than the 55 at the same angle?

- Is it also a given that the 63 will provide more drag at any angle than the 55 at that same angle?

- If the answers to both of those are yes then is it possible to have a longer wing with less angle on it and reduce drag from a greater angle on a lesser wing while providing the same amount of downforce. Or will you end up with the bigger drag plane equaling that out?


Hope that makes sense.

Yeah, it makes perfect sense. I always felt that there is a range of angle of attack where the wing is working at it's most efficient, i.e. it is producing the most amount of downforce (or lift when frivolously used in airplanes) for the least amount of drag. Logically, if the 55" wing is at a greater angle, but still with-in its optimum range, there might be little difference in drag as compared to a 63" wing at a lesser angle. If it is beyond its efficient range, the answer is obvious.

I too though, would like someone with aerodynamic knowledge to chime in here.

Wings are generally rated by a coefficient of lift (Cl) and a coefficient of drag (Cd). The lift or drag force is .5*C(d or l)*density*area*velocity^2. So if 2 wings are identical - meaning the same Cl and Cd, then the one with more area is going to have more drag or lift. Now, if you have one wing with a better lift to drag ratio then you can make more downforce and less drag - but it's all proportional to the size. The Cl and Cd both change with angle of attack.

Mounted on a car further complicates the issue because the incoming flow is not uniform due to the shape of the car in front - so although a wing might have a better Cl or Cd in a wind tunnel with uniform flow - when it is mounted on a car it's not going to have those same coefficients. So maybe 2 wings with the exact same profiles but one is wider will behave quite differently on a car due to the flow around the greenhouse. This is why you see 3 dimensionally shaped wings on certain cars such as the 997 RSRs or C6R corvettes.

In general though: bigger wing = more downforce and more drag with all other factors remaining constant.

From what little I know the aspect ratio of the wing: length to the parallel surface area (chord) equals the efficiency of the wing. Increased raked or angle adds downforce, but with more drag. So two wings, one is wider and has a more efficient aspect ratio should require less rake to exert the same downforce. The question for me is when is it too much for the given car. In the case of a 60inch EVO wing versus say a banana style wing. I am sure at different tracks there would be an advantage to having one wing vs the other.

the real key to the width issue is like claykos said- the wider the wing, the cleaner the air hitting it on the wider parts...because we're not talking open wheel here- aka there's a big fat cockpit in the way of the airflow...so-

in street based cars the majority of the actual downforce is produced at the tips of the wings where the airflow is the cleanest.

aero drag is also exponentially higher at higher speeds, so if you're running a super high speed track you probably won't need the wide one as much since the same aerodynamic downforce can be generated not by more wing angle or surface, but just as a result of the higher speed.

so if you can just change out wings for no good reason/no rules: wide wing for slower/turn oriented tracks and narrow wing for higher speed tracks with longer straights.

Aspect ratio is one of many things that will affect a wings efficiency, but it is certainly not THE measure of efficiency.

Correct. Aerodynamics is quite complicated - especially on a car where the flow is very dirty coming onto the wing. Small aspect ratio, larger aspect, profile, chord length, Reynolds number, etc all have effects on efficiency. But yes - in general larger aspect ratios of less drag. However, aspect ratio can also make the wing easier to "stall" so it's always a compromise. There is not a general answer to most aerodynamic questions, but as I said as a rule of thumb, for 2 wings with the same profile, the one with the larger area is going to make more downforce and more drag.

Also - drag and downforce are not "exponentially higher" with speed - they go with the square of speed. Go twice as fast and you have 4 times the downforce and drag. And note that power to overcome that drag goes with the cube of speed - so twice as fast requires 8 times the power to push it through the air!

Given two wings with similar profiles, but one is wider and capable of more down force. How would one determine the proper size before they have too much wing? Is there a reasonable formula to calculate the benefits on critical fast corners of specific track or does it really require data logging/tuning day at the track? I would like to add an EVO wing to my 993 as I am just finishing the GT2 options. I ran against a 993 last year that had a complete aero/flare kit and I could not match his grip levels in the sweepers.

There isn't a simple math formula to answer that question - you could need to test it...but IMO you can't have too big a wing on our type of cars (assuming you have a reasonable amount of power). You can always flatten out the wing to decrease downforce and drag. The wing is too big when even at it's minimum downforce setting the car has too much rear grip in high speed corners which just isn't going to happen on cars like ours. Min downforce might not be at 0 degrees, it might be at a positive angle because the sloping roof on a 911 angles the air downwards toward the wing. If you are running your current wing at fairly significant angles I'm sure you could benefit from a larger one.

It would be hard to say if the 993 you ran against really had an advantage just because of the aero...you said the car was flared so was he running significantly larger tires than you? That makes a big difference. Maybe his spring/shock/bar settings were better suited to high speed corners than yours, maybe the driver is more experienced...lots of variables there.

One empirical point that I can offer was when I had my GT3 and changed the wing from the stock one to the RS version. The RS wing was wider and a little deeper and I felt that I had more grip and similar terminal velocities with the same wing settings using the RS wing. I definately had more speed at 4degrees (without giving up any grip) than the guys who had their stock wings at 8 degrees.

Okay a little more info. GTS3 class I can run just over 250whp to make weight. Engine is stock. I had a somewhat stock (from PCA stock class) aero wing at the time. Running 245/40s up front, 285/35 rear. Suspension was setup well with Chris Cervelli; Motons, RSR tilt kit, RSR sways etc. Car #2 has Rick Deman setup RSR flares, 275s up front , 315s rear, 3.8 wing with I believe an EVO60 wing. In the slow speeds, I carried more speed eg 50-60mph corners except in a very extended sweeper (max speed est at 85mph). Where I suffered was the fast turn onto the front straight where I had to slightly breath the throttle and T3 which was a kink where the same situation existed. Both turns very high speed 5th gear turns. Our best lap differentials was .8secs. Anytime I made up ground I lost it down the front straight and into the kink. I also have stock rain gutters which Car#2 I think were removed.
Chris has driven the car and felt the aero options were worth up to 2 secs a lap. I have everything installed, but have not ordered the additional wing yet. I also have vented the rear bumper now to get the air out from under the car.

In my opinion, drag is an issue if you're an airline and you're paying for jet fuel by the ton. More or less drag from a wing is not a big deal at all.

Jack, I saw how much work you put into your car in an older post. What was your final direction on the wing? If I recall correctly you even attached some tails to your wing to look at the flow. One warning I was given is to avoid not balancing the down force front to rear. eg front splitter needs to be consistent with what wing you install. My car is balanced pretty well suspension wise, but Chris used very high rate springs and it requires fast hands at times.

Neither one of your questions are a given, but I think I see where you are going with this. Drag and d/f #'s are unique to each shape of vehicle and wing element so nothing can transfer. If you think the wing is stalling then spray some WD-40 on the bottom edge and make sure it trails off the edge in a straight line, that's usually about 12-13 degrees of angle. Just because the wing is larger does not mean it makes more drag or d/f.
One item you have omitted are the wing endplates, which can have as much as a 20% impact on both the downforce and drag numbers and are probably the most important part of the wing other than the main element itself. Ditto on the wicker.
From what little bit I know, it is far more important to keep the center of pressure in the middle of the car than influence raw downforce and drag ratios. When you think of a 911, the wing angle is adjustable but there is very little you can do to influence the aerodynamics at the front of the car, so if you start playing around with larger wing elements then you move the pressure to the rear and create an imbalance. The wing angle operates in an acceptable range depending on the more fixed front items.
The new RS/RSR has a unique center element that maximizes the airflow over the roof (ie-the air going over the roof of the car hits the wing element at a greater angle than air passing by the side of the cockpit), but also has a larger front spitter, wider fenders and some tacked on wickers (which actually add very little in the way of downforce). The point here is these cars are designed in a tunnel and maximized to be within a certain range of acceptable balance and changing from that offers only problems.
If you have a car that can be readily fitted with stock components from the newer RS or RSR you are much better off using them as presented. Trust me, I stayed at a Holiday Inn Express last night, the difference between a well sorted car and one even slightly off in terms of center of pressure is diabolical.

As a side note, it's interesting to see a lot of the F1 cars go with a narrower rear wing. Sure they're not dealing with a caged cockpit but just an observation.

So higher and deeper to get cleaner air?