Wing debate

[Chuck 98 RT/10]

I've heard a couple of things...

1. Wings in fact, slow a car down.

2. Wings are only functional above 160mph.

If either of these are true then why do the big boys have them, especially when average track speeds for most tracks are well under 160mph?

 

[Sean Roe]

Hi Chuck,

A long answer should be given to your question, but i only have a few minutes, so here's the short one.

It all comes down to airfoil design and the angle that it's set at on the car. Poor design = drag = slower top speed. Excessive angle/downforce (good design or bad) = drag and / or understeer = slower lap times. Right design at just enough angle / downforce to balance the car in high speed corners = faster cornering speed and lower lap times.

A useful wing design should work well (ie. makes a positive effect on the handling) at speeds of 70 mph and up (based on my personal experience).

 

[Wing King]

I can't resist jumping in for a few quick comments on this subject. I have almost four years and a lot of money invested now in aero package development for our Vipers. I feel like I'm getting pretty close to "state of the art" through my latest design incorporating what I call my "active bi-plane" project.

Sean's pretty much right on, although I might disagree on quoting any effective "speed limits". There really isn't much rocket science involved here to start with. Although there are a few other small variables, it's basically all about angle of attack and surface size. Give me a 45 degree angle of attack on a 4x8 sheet of plywood, and I'll show you all the downforce anyone would EVER want for your Viper at 30 mph!!!! Speed, angle, wing shape and surface size ALL affect "pounds per square inch" and your downforce.

The adjusting variable for wing performance mainly involves the track it's going to run on. Imagine two equally prepared cars, one with a balanced wing and splitter package, and one without. Now select your track. Let's assume track 1 has two, one mile straights, with only 1 tight 180 hairpin turn at each end. The winged car is going to have MORE air drag, even at a supposedly neutral angle of attack. The non-wing car is therefor going to be faster on the straights. The short, slow hairpins are not going to be fast enough to allow any benefit for the winged car, so it's going to be slower every lap. But now, instead of those hairpins, let's envision a track 2, and turn them into a long, rounded, oval type corner.... or just think of something like Indy. The non wing car is still going to be a few mph faster down the straight, but when they get to the corners, the winged car is going to pass the non wing car so quickly it will be like it's almost standing still. As an example, in testing my first design of the the old "Multi-Axis" wing, we estimated it cost a Viper somewhere in the vicinity of 6-8 mph of top end speed near 200 mph, but could add as much as 20-25 mph in higher speed corners (using approximately 7 degree attack). A lot of the game for an Indy car is finding the perfect balance of straight speed and cornering downforce that results in the lowest laptime. For Vipers, Indycars, or anything else, it's purely a game of physics..... and finding the best setup and adjustment for the specific track. My car, with it's old single plane, GTSR style wing, loved longer 3+ mile tracks with several high speed corners. My new wing, with WAY more usable surface area, already seems to work much better on shorter 1.5-2.5 mile tracks. But..... there's MUCH more to it than just "a wing". How the air is handled at the front, with a splitter..... down the sides with front dams or "winglets"...... or underneath, through belly pans and rear diffusers..... has EVERYTHING to do with how fast you are going to get around even a shorter, lower speed track. Oh yea..... one more thing...... all this stuff not only WORKS, but it LOOKS really cool and **** too!!

 

[Sean Roe]

Hey Kingman, thanks for taking the time to give Chuck the long answer
Here's a little better description of what speed you want aero aids to be capable of helping out at (don't want Kingman to disagree with me because I didn't have time to explain

).

Most dedicated road courses (not ovals and not ovals with infield road courses) have corners that range from 45 MPH to 100 MPH. Because vehicles without aerodynamic aids will naturally oversteer at higher cornering speeds, we tend to tune the chassis (toe and rake adjustments) so that the car is neutral in theses higher speed corners (safer and easier for the driver to control). With the chassis tuned this way, the car will naturally tend to understeer in the slower corners on the track, which the driver has to compensate for by trail braking or pitching the car (or slowing down more, god forbid). With aerodynamic aids, you can tune the chassis to make the car neutral in the low to medium speed (45 to 70 MPH) corners, but then use the adjustable rear wing to add enough downforce to keep the handling stable, where otherwise it would naturally oversteer in the medium to high speed (70 to 100 MPH) corners.

Aero aids are especially effective on vehicles that the chassis are already well sorted out. Keep in mind that a stock Viper front end has an aerodynamic fault that causes air to go under the front end and stack up under the hood, which causes lift at higher speeds. A front splitter or an under fascia duct kit will reduce this phenomena drastically.

Also, I have to clear up a misconception that most people have. Those little winglets on the outside of the front bumpers, those are not wings and are not for producing front end downforce. They are Vortex generators and are for controlling the airflow down the sides of the car.

 

[Tom and Vipers]

Wow,

I'm impressed that wings do not come with lift and drag coefficients versus angle of attach and stream velocity.

Now granted you have to know the flow angle at the wing location...

So with no engineering aerodynamic data, how do you select a good wing?

Further, once you buy one, how to you quantify the wing's drag -v- lift? Is it done by a talented driver's seat-of-the-pants opinion? Of do you use strain gages or some kind of tell tails (actually stress coat would work if you had a FEM model of the struts - shouldn't be too difficult!)

I would like to put a wing on Wayne's Bad Viper mostly for cosmetic purposes (I should be horse whipped.) Mainly, because I don't have the data or experience to do it right. And it ain't the experience that disappoints me, it is the data.

There are a number of published NACA (I think - or is it BAH [Blispinhof, Ashley, and Holt (?)]) airfoil profiles that have extensive aerodynamic data available - they are intended to allow airframers to design airplanes with them. Does anyone make wings with these profiles?

A flat plate might actually work pretty good.

One observation: I have heard it remarked that what was thought about airfoils is believed to be wrong and that it is all incidence angle. The thickness of the traditional airfoil may have been a ramification of the need for spar height to support the loaded wing. If you had infinitely strong material, you could make the airfoil thickness nil. I don't know if there are very thin NACA (of BAH) A/F's.

Another observation: Regarding wings with lots of turning (camber.) While these shapes occur in turbine blades (usually not compressor blades) in jet engines, they probably are NO GOOD for drag. They will produce dramatic turning - probably too much. They may be good for short, low speed tracks - hill climbs maybe? (Yep, I think I'll go with that one - max down force in the slow corners - top speed not that important. Better watch that wing mounted aft of rear axle will unload front axle [gulp!])

There you have it.

I've talked thru my hat again - ruin more that way .... get them wholesale though....

 

[Tom and Vipers]

"Oh drat! He found another hat...," They Muttered.....

How about one of those under the but diffusers with splitter vanes in lieu of the wing??

Actually, I think these are even cooler!

 

[Tom and Vipers]

OK so I have lots of hats!

What about Jim Chaparral's dynamically adjustable wing?

You could have a flat plate with no drag w/zero incidence angle and then mega lift or even BRAKING as needed(!)

The control system would be quite a challenge.

(I spend too much time at this darn box [computer now])

 

[Mike H]

The basics of drag and lift occur all the way down at speeds of 20 MPh.On production based cars aero lift is what at higher speeds a wing is used to compensate for, however the use of a wing can and will not always increase drag. A great book to read on the subject is a book called Competition downforce a practical guide. The main theory of a spoiler is not to induce drag but rather reduce lift.

 

[Tom and Vipers]

Boy I gotta get that book caus I am totally ignorant about this stuff.

And use those wings! Hubba hubba!

My favorite wing was Jim Hall's advice Chaparral and then of course the vacuum car.

Let's see, a 16' by 6' car is 96 ft**2 - 4 ft**2 for each wheel is about 80 ft**2 and at 144 in**2/ft**2 we get 11520 in**2.

Now if the suction fans can generate a 1/2 psi pressure differential we get 5760 pounds of down force. This would increase tire loading 150% and probably produce the same scale factor to cornering G's.

Let's see, with gummy tires that would be about 1.5 times (5760+3400)/3400 = 4.04 G's

Boy, that would CRUSH the Virginia City Hill Climb!!

Wonder what ever happened to those vacuum cars?

I want one!

 

[Gerald]

Hey Chuck,

Wings LOOK cool at ANY speed on a Viper. Need we say more?

Gerald

 

[Bob Woodhouse]

It is difficult to ask anyone to give up their expensive hard earned research in aerodynamics as applied to Vipers but a few kind souls have contributed things to me, and I share them with you here.
As a general assumption, you can expect a well done rear wing to generate 300#s of negative lift at 100mph. As many of you already gathered, you cannot just bolt a wing on and make things happen, so to compensate, the front of the car must have a splitter. Yes a well done splitter will be capable of applying an equal amount of negative lift to the front end, thus giving you a balance.
We also know that the stock Viper hood, shaped like a wing, can create positive lift, amounting to about 450#s at 150mph. GTS-R hoods have a slightly different nose shape and some spoiler flaps on the wheel vents to offset this.
All aero comes at a price, if the wing, flaps' spoilers, underbelly etc. are done well, you will create 1# of drag for 6#s of negative lift. I doubt that we are that efficient, but to the other end, at worst we are costing ourselves 1 pound drag to 3 pounds downforce.
My personal experience with my #13 GT-2 Viper Days race car is that these aero devices do work, they cost 10mph or more at Road America on the long straights but overall drop the lap times. They create two things, higher corner speeds on corners above 70mph and shorter braking distances on high speed straights. The feeling you get in the carousel at 90 mph is really wierd, your head tends to want to lean out the left window. The slip rate doesn't increase as much as expected when you increase speed. A genuine rush. You find yourself thinking "I can't live up to the potential of this car".
Braking is different too. You can really stand on the pedal initially but need to be backing off as the car slows to keep from locking any wheels.
We run a fully enclosed underbelly as well, without good testing though I have no evidence of what it is contributing. A lot of thinking has been done underneath race cars and in terms of wings, this is the most significant of all, think of the square feet of surface available, and all of it in ground effect. Rake, and width of opening are critical. Someone mentioned the rear diffuser earlier. Their function is to keep the air from tumbling out from under the car and causing drag. As soon as you get all this to work well you have to go back and re-think your suspension, after all what is going to hold up all that extra weight and keep the alignment in check? Higher spring rates?
I hope you find this useful. I also hope you come out and play at one of the Viper Days events. These cars are getting developed and pretty fast.

 

[GTS Bruce]

Blah,Blah,Yadda,Yadda.So who's tested these viper down force devices in a wind tunnel?The best aero package for a street viper just might be what the One Lap winners are running.Bruce

 

[Tom and Vipers]

Ahhhhh, wind tunnel testing.....

There are perhaps 2 regimes of testing:

1) Establishing efficiency of wing, and
2) Establishing efficiency of wing installed on car.

Frankly, the flow field of the car is not "homogeneous" like a wind tunnel, so that 1) is probably not "the answer" anyway.

Seriously, a way to test or setup the wing may be with strain gages. The supports tend to be thin, rectangular cross-sections. If you put vertically aligned SG's on the LE and TE surfaces, you could back out tensile and bending stress and hence load.

Tensile is downforce, Bending is drag.

Then you could start plotting DF and DG as functions of speed given a wing angle. Each wing design probably has a "sweet" spot that may work to your advantage....

So much for wings, but what about splitters?

Here drag is probably of a lesser concern since drag is probably more of a function of the body aero.

So how to quantify splitter drag?

Perhaps, linear displacement gages on the shocks may be the approach. If done on all 4 corners, DF could be determined easily.

I don't know what the Pros do. Of course, a talented driver could go out and from feel, estimate the amount of downforce.

When I was street racing 25 years ago, I could tell if there was an extra 150# in my car real easy, so an active, experienced racer would probably be pretty good at this.

So that leaves me out...., however, I know with the right instrumentation and lots of time and money (mostly time though), I could develop an effective aero package. Stuff like this was right up my alley at Pratt and Whitney.

However, I think what I want to do is see how quick I can go around a track at Viper Days with a 101% stock car....

...because, sad but true, what makes a car feel like a race car on the track is.... race tires. This is really disappointing because I do not have the desire to start trailering cars. Besides, part of the fun of Viper Days is to drive my Viper there!

I will say one thing, the only racing venue that has captured my imagination is the Virginia City Hill Climb..... but boy, is Steve's Ruf a monster! I figure, that a Viper would have to have at least 2 Secret Weapons to beat Steve.

Oh, and one more thing, I don't like seeing how Viper brakes can be improved by Porsche air deflectors and Porsche Red Hat brakes. You see, this name, Porsche, keeps poping up! And then you read about this RenTech stuff..... This German Thing is showing up again!!

Driving Porsches, in Germany, on the Autobahn........

!@^&*%^!@#$*#$^!@&#

 

[Frank Parise]

***** is pretty much spot-on.

I rented a track in Tucson to test drag and downforce on one of the popular rear wings. All tests were done at 100 MPH as a reference point. Both drag and downforce can then be calculated at any other speed since they both vary precisely with square of speed.

Downforce was calculated by gently accelerating the car to 100MPH, then lifting immediately and returning to the pits to measure the rear suspension travel (how much the rear shock piston rods compressed). Measurements were to the nearest millimeter and, utilizing the known spring rates, we were able to determine rear downforce created by the wing at different angles of attack. We ran the car with the wing completely off to determine our base reference points for measurement.

We calculated drag by measuring coast-down times. Upon reaching 100 MPH, we immediately dropped the car into neutral and coasted. Utilizing a stop watch we measured the amount of time necessary to coast from 100 MPH to 80 MPH. The more drag, the less time it takes to decelerate.

All tests were made in a controlled environment and verified with multiple retests. There's too little room here to go into details, but I'd be happy to discuss this procedure in more detail.

There are nearly an infinite number of factors and variables that effect the efficiency and effectiveness of a wing. With everything else on the car remaining constant, the surface area, aerfoil shape, and angle of attack are the primary considerations. We determined that the angle of attack is critical. There becomes a point where drag increases significantly with little or no increase in downforce.

On a dual-element wing, the location of the top element relative to the bottom element is very important. What looks good isn't neccessarily the most efficient or effective.

An incorrect rear wing setting can put a first-place car at the back of the grid.

Our most efficient setting produced 282 pounds of downforce at 100 MPH. Other settings produced more downforce but a disproportionate amount of additional drag. Efficiency was measured in terms of relative changes in downforce compared to relative changes in coast-down times (drag). This translates into 138 pounds of downforce at 70 MPH and 634 pounds of downforce at 150 MPH.

With a high-downforce rear wing, your front tires will be unloaded by this same amount of weight, thus reducing steering ability considerably unless you address the natural lifting condition of the front end. That's why you see all the race cars with big hairy front splitters. They may not produce much downforce, but at least they reduce the amount of natural lift that is part of the Viper design. This helps you dial out some of the additional understeer ("push")that is created by an effective rear wing.

If you are serious about racing, get an adjustable rear wing....one where you can adjust the overall angles of attack through a range of 5 degrees to 20 degrees. On a dual-element wing, this may require rear flap angles of considerably more.
You can also utilze different height Gurney flaps to modify the effective angle of attack.

Finally, your lap times at the track will determine the optimum wing setting for that track. All wings will reduce your top speed, but the right wing and setting will reduce your overall lap time.

 

[Tom and Vipers]

I was just thinking that it might be possible to make a shock position telltale out of a choke cable clamped onto the shock.

The cable would be run into the interior where measurements could be made real time. This would eliminate countless trips under the car.

A wing setting could be documented w/o leaving the inside of the car.

Actually, a quickie might be to get a linear potentiometer with a linear scale (not log), make some brackets and clamp to the shock. A multimeter could be used to measure the resistance which would be calibrated to load.

If I put a wing on mine, I will do something that will keep me out from under the car during wing setup. I hate what is involved to jack up a Viper.

Does anyone make a rear wing where they try to cantilever it forward to reduce the negative effect on the front axle?

One other point about downforce being proportional to the square of the velocity.... Steve's Porsche rear wing alledgedly deflects some number of inches at speed which would result in reduced camber and twist. This change in geometry would result in a non-linearity and may offer a means to reduce downforce at speed.

Do you know what a Chaparral 2J was?