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Old 08-12-2009, 11:08 AM   #1
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Default Any aero engineers on here?

I have a question I can't seem to find the answer to, and I'm hoping someone can help me.

Let's say I have a wing shape. We'll keep it simple in terms of the shape - one side flat, one side curved.

Now, lets say I have a way of artificially injecting a stream of air so the wing sees more flow than just being pushed through the air while attached to the car.

My question is this - what side of the wing would I want to inject the air on to to see a greater drop in pressure along the curved surface?

Supplementary question would be as follows -

Would it be best to inject the air into the stream along one surface of the wing or would it be best to inject the air in front of the wing and let it do it's thing as is?
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Old 08-12-2009, 11:26 AM   #2
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I think there are some discussions and tests on this in the right places..


Jon
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Old 08-12-2009, 11:32 AM   #3
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Quote:
Originally Posted by Turn in Concepts View Post
I have a question I can't seem to find the answer to, and I'm hoping someone can help me.

Let's say I have a wing shape. We'll keep it simple in terms of the shape - one side flat, one side curved.

Now, lets say I have a way of artificially injecting a stream of air so the wing sees more flow than just being pushed through the air while attached to the car.

My question is this - what side of the wing would I want to inject the air on to to see a greater drop in pressure along the curved surface?

Supplementary question would be as follows -

Would it be best to inject the air into the stream along one surface of the wing or would it be best to inject the air in front of the wing and let it do it's thing as is?
First of all, the velocity of the air is going to be just as important as the volume your putting out (Reynold's Equation). Im not sure what the application in this case is but if your trying to increase downforce across a wing or air splitter like on an STi you usually tweak the angle of attack and redesign the air foil's shape (chord length, AR, coeff of drag of surface materials). You would increase your fluid volume, in this case air, by going faster. Although it would be pretty impractical in the case of a Subie, blasting a jet of air at a higher velocity (much higher to see any real difference) than the air naturally moving over the wing would increase its downforce. The air jet would have to evenly distribute the air or you risk compromising structural integrity. So short answer long, yes this would work but it would be very impractical.
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Old 08-12-2009, 11:34 AM   #4
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Might be some useful info here, we had thought about using similar ideas but scrapped them.

http://www.wikipatents.com/6109565.html

Doug
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Old 08-12-2009, 11:36 AM   #5
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Based on the wording of your question, I suspect you know the answer already.

The short answer is, inject the air at the leading edge such that the accelerated flow travels over the curved section. By speeding up the flow, you drop the pressure, and bingo, more lift (downforce depending on orientation of the wing).

Just run a search on "incompressible Bernoulli equation" to be able to do your own back of the envelope stuff. How you inject that flow without disrupting the existing flow and killing your lift is an interesting engineering problem. You'd at a minimum want some separate device placed ahead of the leading edge of the wing doing your flow injection for you.

What you're talking about, by the way, is powered lift via the Coanda effect. This takes enormous amounts of energy input to produce any really meaningful benefit, so if this is for a racecar, I daresay your time and money would be better spent on other things...

Another interesting approach is active sucking to keep the flow attached at angles of attack beyond the stall limit of your airfoil. The drag penalty here would be huge, though.
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Old 08-12-2009, 11:37 AM   #6
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Originally Posted by TopSpeed View Post
Might be some useful info here, we had thought about using similar ideas but scrapped them.

http://www.wikipatents.com/6109565.html

Doug
^^^ You can tell the people who have already been thinking in this direction...
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Old 08-12-2009, 11:39 AM   #7
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Originally Posted by saintsfan1483 View Post
First of all, the velocity of the air is going to be just as important as the volume your putting out (Reynold's Equation). Im not sure what the application in this case is but if your trying to increase downforce across a wing or air splitter like on an STi you usually tweak the angle of attack and redesign the air foil's shape (chord length, AR, coeff of drag of surface materials). You would increase your fluid volume, in this case air, by going faster. Although it would be pretty impractical in the case of a Subie, blasting a jet of air at a higher velocity (much higher to see any real difference) than the air naturally moving over the wing would increase its downforce. The air jet would have to evenly distribute the air or you risk compromising structural integrity. So short answer long, yes this would work but it would be very impractical.
Lets say that length, pitch, and chord are all fixed, and cannot be changed.

Lets also say that I just happen to have some extra air that right now is just being dumped outside the car. The air is going to be dumped regardless as there's no choice in the matter, but I'm wondering if there's any way that I can use this air to enhance the flow over a wing so that it sees more than just the air that's pushed over it by the car moving forward.

As for impractical - that's what I'm trying to figure out. Given that I have no choice but to dump this air any gain in flow over a wing, no matter how small would be a benefit.
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Old 08-12-2009, 11:40 AM   #8
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I think there are some discussions and tests on this in the right places..


Jon
Hey Jon - problem is I don't know those places. I tend to be a loner on odd stuff like this.
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Old 08-12-2009, 11:40 AM   #9
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Originally Posted by Turn in Concepts View Post
Lets also say that I just happen to have some extra air that right now is just being dumped outside the car. The air is going to be dumped regardless as there's no choice in the matter, but I'm wondering if there's any way that I can use this air to enhance the flow over a wing so that it sees more than just the air that's pushed over it by the car moving forward.
There is info on this out there.. see PM.



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Old 08-12-2009, 11:41 AM   #10
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Originally Posted by TopSpeed View Post
Might be some useful info here, we had thought about using similar ideas but scrapped them.

http://www.wikipatents.com/6109565.html

Doug
Thank you for the link. I'll check it out.
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Old 08-12-2009, 11:44 AM   #11
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If you are just looking to dump air in a manner to help air flow or downforce; I'd place it at the base of the rear window/trunk in manner flowing along the trunk lid.

Been discussed and I actually kicked this idea towards Doug last summer when he had mentioned some Chapparal'esque aero ideas...but the sucker setup never came to being for whatever reason...

I know in the windtunnel we had done similar testing (just cockpit flow, not pumped air from undertray) on 1/4 scale models but never test any in either full scale tunnel.


My concern with injecting the high speed air at the front of the wing (in attempt to make a change in downforce) would be if you'd stall the wing. Testing and getting results for this in the real world would be rather difficult and would require pressure sensors or some pitot tubes.

Last edited by Homemade WRX; 08-12-2009 at 12:11 PM.
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Old 08-12-2009, 11:45 AM   #12
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Old 08-12-2009, 11:48 AM   #13
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I'm not an aero engineer but when I was growing up I read tons of books on airplanes since aviation ran in my family. So... I don't know what the hell I'm talking about, but its an interesting question to ponder. From what I remember, the design of an aircraft wing is such that (oversimplifying here) the top is curved and the bottom is flat. This makes the path of least resistance the bottom of the wing, so more air passes under rather than over which leads to greater air pressure under the wing and therefore lift. Obviously on a modern aircraft they get a lot fancier, but if you look at the cross section of a wing of a simple R/C plane it's easier to see what is going on. I assume wings on a car work similarly, but upside down and are designed to take into account the airflow around the vehicle body.

Now, specifically WRT your question, I'm not 100% sure what you mean. My guess is that the best approach is to work with the wing within the system it is designed to function, and increase the amount of air hitting the leading edge (and decrease the amount of turbulence if possible). Trying to turn the wing into something like a lever by directly applying an air stream perpendicular to the surface seems like trying to use one tool for another tool's function.

This is an interesting question, I'm looking forward to an answer from somebody who actually knows what they're talking about.



..and by the time I finish my long winded reply, other folks are posting the real info.

Last edited by samophlange; 08-12-2009 at 11:50 AM. Reason: I need to do more reading and less yapping
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Old 08-12-2009, 12:03 PM   #14
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This makes the path of least resistance the bottom of the wing, so more air passes under rather than over which leads to greater air pressure under the wing and therefore lift.
The top path is longer because of the curve, resulting in a higher speed airflow, dropping pressure and hence giving lift. It's not because there is more air.

Angle of attack also comes into play, until the airflow breaks away from the wing and causes a stall. Some air "injection" mechanisms (slats are common) try to reduce the breakaway by 'injecting' air along the top surface, to allow for higher angles of attack. I have no idea if that would help on a car.

I did find some links to powered high-lift designs using air bled off the engines, but again, I have no idea if that's helpful.

BTW, I'm not an aero engineer either.
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Old 08-12-2009, 12:07 PM   #15
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Some fighter jets used to employ a tactic with holes on the top of the wing surfaces. By pulling a vacuum on the wing using venturi's you could essentially pull air from the top of the wing and dump it out the back. This resulted in reduced top pressure. Invert this idea for down force.
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Old 08-12-2009, 12:44 PM   #16
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If you have air that will be blowing out of the car, wouldn't you get more down force from blowing the air upward, or maybe up and back, than you would from blowing it horizontally over a wing? The wing idea seems less efficient because the air is indirectly gaining you down force, where as if it were just blowing upward, it would directly increase down force.

That's just my thought on it. I graduated from HS earlier this year, so I don't have any training in this area.
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Old 08-12-2009, 12:57 PM   #17
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If you have air that will be blowing out of the car, wouldn't you get more down force from blowing the air upward, or maybe up and back, than you would from blowing it horizontally over a wing? The wing idea seems less efficient because the air is indirectly gaining you down force, where as if it were just blowing upward, it would directly increase down force.

That's just my thought on it. I graduated from HS earlier this year, so I don't have any training in this area.
That would work great, but I don't have THAT much air to push, and figuring out how to push it out and an appreciable pressure over a large surface area would not be possible.
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Old 08-12-2009, 01:52 PM   #18
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I still say fill that turbulent zone directly behind the back glass. I should also help direct more flow to the underside (trunkside) of the spoiler...which should have an effect on the air coming off the roof as well, potentially helping to direct airflow much like the spoilers found on 06-07 sti's and VG's on the evo...
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Old 08-12-2009, 02:08 PM   #19
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You also have to think that at a certain velocity, the volume of air naturally flowing over the wing may exceed the volume of "pushed" air...would this be harmful at higher speeds? It may cause a lower pressure were the natural high velocity air and the "pushed" air meet so that you don't have the most efficient airflow over the wing...or not, haha just something to think about. If this problem exists it would be pretty easy to fix anyways. Just block the airflow at the velocity were it becomes detrimental to flow...

Its a good concept. F1 cars used to do something similar by routing exhaust flow under the car through the diffuser and accelerating flow.
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Old 08-12-2009, 02:46 PM   #20
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F1 uses exhaust flow out the back of the car in concert with the flow coming off of the rear diffuser. I think they're using it more as a flow shaping tool that an "air injection" device though. I had an article on it somewhere and some pictures.... I'll have to dig. I know I've got some close-ups of the 06 Redbull car's rear diffuser.
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Old 08-12-2009, 03:11 PM   #21
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F1 uses exhaust flow out the back of the car in concert with the flow coming off of the rear diffuser. I think they're using it more as a flow shaping tool that an "air injection" device though. I had an article on it somewhere and some pictures.... I'll have to dig. I know I've got some close-ups of the 06 Redbull car's rear diffuser.
There was something recently in Racecar Eng. about this and how some teams were protesting another team's exhaust tips coming out above the body work.
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Old 08-12-2009, 03:33 PM   #22
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yeah, I'm not sure if they've changed the rules since, but '06 has this going on for sure. Pictures are on the home computer unfortunately, but I'll post tonight.


If whatever you're working on has a diffuser you may see better results working there rather than the top wing. I'm going completely on the fact that we're talking exhaust flow here. To be perfectly honest, I don't think you're going to see returns either way, but it's an interesting exercise for sure.
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Old 08-12-2009, 03:42 PM   #23
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I wonder if a 600hp turbo engine would move enough air to put the intake under the car and use the engine as the sucker fan. I guess that wouldn't work too well, since you'd have to be full-throttle through the corner to keep your downforce. I suspect a similar problem would arise from trying to use exhaust to increase downforce.

The more useful strategy would probably be to try and use exhaust to reduce drag.

-Mike
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Old 08-12-2009, 04:03 PM   #24
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Look at the size of the fan on the Chaparral fan car. Turbo inlet isn't gonna do any good in that aspect
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Old 08-12-2009, 04:09 PM   #25
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F1 uses exhaust flow out the back of the car in concert with the flow coming off of the rear diffuser. I think they're using it more as a flow shaping tool that an "air injection" device though.
We studied a few cases where such things had been introduced in attempts for downforce and were trying to inject the exhaust right at the break between the undertray portion and the rear venturi. It had reduced downforce by filling venturi volume. They since moved it up to help shape the backlight angle of the car....then again this was also 4 years ago on an F1 car with released data.

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There was something recently in Racecar Eng. about this and how some teams were protesting another team's exhaust tips coming out above the body work.
I'm going to guess that they had exhaust breaking some minor rule in an attempt to use the exhaust for downforce...but then again, I'm spectulating.

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The more useful strategy would probably be to try and use exhaust to reduce drag.
or reduce drag and increase downforce at the same time
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