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Aero Question..........

 
Old 02-10-2010, 12:53 PM
  #61  
Tom W
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Bob: I have the Getty hood that eliminates the front air scoop and believe it helps (but no data to prove it). One of my competitors has a standard shaped hood and he finds a significant benefit if he uses tape to cover the inlet and provide a more smooth air flow over the front windshield.
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Old 02-10-2010, 01:02 PM
  #62  
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Originally Posted by mark kibort View Post
Well, there you go . conditions. yes, give all the details and we can see if it is possible.

glide ratio is not lift to drag of the wing

you said, " Mark: READ CAREFULLY: I did not say it changed downforce by 10% - I said it changed Lift/Drag by 10%."

maybe its early in the AM, but isnt that the same thing

200lbs downforce , 20lbs of drag = 10:1 lift to drag
lift to drag changing to 10% less, or going to 9:1
180lbs (down 10% or 20lbs and drag still 20bs)

point is, the claim is substantial. for our wings, with naca numbers similar to the one Ive shown from one of my aero texts, it would be very unlikely that a piece of scotch tape could cause a 10% change in Lift/drag ratio.

originally, it was on the flat side of the wing, from what I remember. and I brought up the point of most early wings not even having "bottom" of wings. only the tops to fly effectly.

as you said, it was at only one particular angle of attack. sure, this is possible. it could be anywhere. extreme, and unique situations or conditions doesnt help the discussion here. we dont run our wings at max L/D ratios and all other points were "negligible". so, the claim has little or no point or value, does it?
At constant speed glide ratio and L/D are the same.

Changing just L is not the same thing as changing L/D. Maybe instead of a 10% increase in lift, there was a smaller change in drag and a smaller change in lift.

I don't remember the NACA number of the wing. The tests were done at Reynolds numbers of 100k, 200k and 300k (low for a car).

I didn't say "putting tape on your wing will change performance by 10%"- I said that aerodynamics is a very sensitive, often counter-intuitive and complex field of physics and as a consequence of that people shouldn't worry too much about small changes in downforce or drag on our production based cars. As an example of that I used experimental data that I happened to hear about yesterday, that for this specific wing, at/near max L/D angle of attack (which I think was about 7 degrees - not unreasonable to have on a car) leading edge tape changed performance by close to 10%. The other angles showed less difference with or without tape. The wing in question had max L/D in the range of 40-50. It is used on a popular motor glider.

Now lets all be friends :-)
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Old 02-10-2010, 02:19 PM
  #63  
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Originally Posted by mark kibort View Post
why would it increase downforce? I guess, due to its shape in the rear, you could get a slight drop in presure, which would increase downforce (or reduce the pressure on the bottom, in that area, to change the pressure differential which would increase downforce slightly). and yes, you are right, this downforce comes at no drag cost, similar to the concepts of the splitter in the front.

Now, ferrarri??? A whole 'nuther set of concepts. think venturi tunnels under the car. that can suck the car right to the road. (very low pressure , and that pressure change is directly proportionate to downforce).

Actually I think you're both saying the same thing ... A proper diffuser gives this high pressure air a controlled escape route from under the car (i.e. the venturi, assuming a smooth underbelly placed close to the ground), and in the process creates suction under the car. And lots of it if properly designed. The downforce can of course be enhanced dramatically by having a properly placed wing over the back of the car.


Originally Posted by mark kibort View Post
actually, aspect ratio doesnt change efficiency.
Actually it does ... wings are less efficient near their ends even with end plates. So a wing that's twice as long will produce somewhat more than double the downforce. Other things being equal, it is typically more efficient to have a longer wing with less chord, rather than the other way around.

Scott
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Old 02-10-2010, 03:46 PM
  #64  
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I have to think about that one for a minute. the L/D, Cp, etc all stay the same for a given air foil shape. sure, at the ends, they all have issues with wing tip vortecies that spoil lift as the pressure differential wraps around, and winglets (end plates) correct this and make the entire wing more efficient, regardless length. a longer wing with less cord, would be a DIFFERENT wing, and that does make sense. we are not talkng effiency if we are getting the same lift for the same drag, even if we get more lift for lower angle of attack due to a longer wing. more effectivness, yes, efficiency no. So, in effect, i think we are saying the same thing.

all things being equal...... Like talking about the same air foil???

Originally Posted by stownsen914 View Post




Actually it does ... wings are less efficient near their ends even with end plates. So a wing that's twice as long will produce somewhat more than double the downforce. Other things being equal, it is typically more efficient to have a longer wing with less chord, rather than the other way around.

Scott
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Old 02-10-2010, 03:57 PM
  #65  
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Yes, i understand, but you are making some assumptions below. so, if L/D changed by 10% your saying the proportion of lift to drag changed. you got more drag and only slightly less lift. since most of the drag is due to lift, that is hard to imagine, but sure, under the right circumstances and air foil shapes, anything is possible, but with our air foils, not probable that a piece of scotch tape is going to change the L/D ratios by 10%, thats all. I think anyone with aero experience here would agree.

Yes, im totally in agreement that aero can be counter intuitive and little things can create big issues and visa versa. Point is, I think I have shown that the drag values due to lift (or downforce) are really not as important as folks would think they would be, especially for the duration at speed that we spend time at . (i.e. getting back to that 10:1 L/D common air foil with 200lbs of downforce at a cost of only 5hp at 100mph)

(EDIT corrected, sorry) Now, lift to drag ratios being maxed out at 7 degree is uncommon. can you draw me that shape? certainly most L/D ratios of our wings are closer to 0 AOA as my graph shows. max Lift happening around 18 degrees AOA. anything drastically different than this, will be an odd shaped wing. I have the books. I can find the oddball wing shapes and give you the data if you want, but it wouldnt be anything that we would be putting on our car if I remember correctly.

Yes, friends it is!!!

mk
Originally Posted by claykos View Post
At constant speed glide ratio and L/D are the same.

Changing just L is not the same thing as changing L/D. Maybe instead of a 10% increase in lift, there was a smaller change in drag and a smaller change in lift.

I don't remember the NACA number of the wing. The tests were done at Reynolds numbers of 100k, 200k and 300k (low for a car).

I didn't say "putting tape on your wing will change performance by 10%"- I said that aerodynamics is a very sensitive, often counter-intuitive and complex field of physics and as a consequence of that people shouldn't worry too much about small changes in downforce or drag on our production based cars. As an example of that I used experimental data that I happened to hear about yesterday, that for this specific wing, at/near max L/D angle of attack (which I think was about 7 degrees - not unreasonable to have on a car) leading edge tape changed performance by close to 10%. The other angles showed less difference with or without tape. The wing in question had max L/D in the range of 40-50. It is used on a popular motor glider.

Now lets all be friends :-)
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Old 02-10-2010, 04:05 PM
  #66  
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Why are you guys going back and forth on how to make your car slower or not, by screwing up the wing?????

Who cares???

I think we all want to know how to go faster, not slower, with what we put on the car. So we get it, don't put tape on the front edge of the wing.

Now how about getting to the bottom of chord vs length.
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Old 02-10-2010, 04:07 PM
  #67  
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I believe it was this airfoil
http://www.worldofkrauss.com/foils/1671

Efficiency is related to aspect ratio - the formula for Cd has a 1/AR term in it. The Cp and lift curves you see in the handbooks are calculated for a 2D wing -ie infinitely long. Based on these values you can compute the values a wing with the same profile but different aspect ratios. The profile itself is only the relationship between thickness and chord. Then you have to include aspect ratio effects for a "finite" wing.

Large aspect ratio wings are more efficient.
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Old 02-10-2010, 04:18 PM
  #68  
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Mark, could you put your energy and brain power back on the discussion? For instance you obviously made some decisions on what wing to run on your including the front splitter. ( we can see that big *** in your avatar). Any other observations. One thing I have learned is without actually trying the changes on a track with data to back it up every theory goes in the trash can. I think Theory in this case helps point you in a direction towards a goal. I remember taking a Elec Eng class on advanced physics and we had a guest Prof who worked on the Apollo moon shot. He was not one for curtailing his language. His first interesting statement was that "State-of-the-art" was building it before you certain how and if it would f(*(ing work.
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Old 02-10-2010, 04:22 PM
  #69  
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I think I am eventually going to run a diffuser, if not for the simple fact that I run straight pipes and have a serious parachute efffect in my rear bumper cover. Need to cover that up. Really my only concerns are the engine heat issue.

Beyond that, I'm trying to decide what size and shape wing to get. I've never liked big end plates, was going to go with RS ones, but after reading this, I guess I will have the big ones.
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Old 02-10-2010, 06:15 PM
  #70  
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Yes, in thinking about it further. You are right to a certain extent. I was in 2D-Land. Its not absolute, as you mention, there are many other factors.

you really want to use the wing with the best lift and drag coefficient for a desired absolute lift (or downforce). in otherwords, shorter wing with more angle will work better than a wider wing with no angle. you want to be in the sweetspot of the air foil. not too small, not too wide. *for the downforce you want at the speeds you want.

I think that is what we are all trying to get at.




Originally Posted by claykos View Post
I believe it was this airfoil
http://www.worldofkrauss.com/foils/1671

Efficiency is related to aspect ratio - the formula for Cd has a 1/AR term in it. The Cp and lift curves you see in the handbooks are calculated for a 2D wing -ie infinitely long. Based on these values you can compute the values a wing with the same profile but different aspect ratios. The profile itself is only the relationship between thickness and chord. Then you have to include aspect ratio effects for a "finite" wing.

Large aspect ratio wings are more efficient.
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Old 02-10-2010, 06:21 PM
  #71  
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Lots of different ways to skin a cat -- and it's always a trade-off. Simon McBeath's 'Competition Car Aerodynamics' has some great testing numbers on finding the optimal height for end plates (which is almost always compromised by the benefits of getting the wing itself higher while staying under maximum height restrictions).

I've also experimented in a completely different direction:







No data on any benefits, though.
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Old 02-10-2010, 06:25 PM
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Yes, and I have been a HUGE fan of emperical testing. (almost as far as Jack goes, but not quite) I use tufts, and video, but not to his extent.

I was able to accurately measure diffential pressure top to bottom of the wing at speeds from 60mph to 130mph. Ive video taped the deformation of the splitter at that speed, to understand the forces and pressure differentials, as to be able to put support where it is needed and not where it isnt. I ended up with a very balanced car. I was easily able to measure downforce of the rear GT3 cup wing at settings from 0-10 degrees and most of the measured data, matched the theory. On the track, it the was easy to make changes to help handling issues.

In the end, the car didnt become a lot faster, but it did become more predictable and easier to drive faster.

As Jack mentioned, there are advantages at higher speeds, and not as much at the lower speeds. slow , meaning 60mph and fast being 90-130mph.

The answer is probably in areas of what has been done already on track proven cars. copy what has worked for others and you are probably going to be in pretty good shape. Fine tuning will be in setting the angle for tracks, tires and driver characteristics.



Originally Posted by bobt993 View Post
Mark, could you put your energy and brain power back on the discussion? For instance you obviously made some decisions on what wing to run on your including the front splitter. ( we can see that big *** in your avatar). Any other observations. One thing I have learned is without actually trying the changes on a track with data to back it up every theory goes in the trash can. I think Theory in this case helps point you in a direction towards a goal. I remember taking a Elec Eng class on advanced physics and we had a guest Prof who worked on the Apollo moon shot. He was not one for curtailing his language. His first interesting statement was that "State-of-the-art" was building it before you certain how and if it would f(*(ing work.
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Old 02-10-2010, 06:28 PM
  #73  
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I have always read, you want the enplates as straight down as possible, but maybe there are some subtle differences in keepign an angle. the fin on the top of the wing, shouldnt do much, but I have a little over hang there, just for looks.

that last picture has an optical illusion.Its hard to see the shape of the wing.

Originally Posted by JackOlsen View Post
Lots of different ways to skin a cat -- and it's always a trade-off. Simon McBeath's 'Competition Car Aerodynamics' has some great testing numbers on finding the optimal height for end plates (which is almost always compromised by the benefits of getting the wing itself higher while staying under maximum height restrictions).

I've also experimented in a completely different direction:
No data on any benefits, though.
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Old 02-10-2010, 06:33 PM
  #74  
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open the scoop up again and put a huge vent hole in the middle of the hood. I have the advantage of knowing where to put it, because I have the aero diagrams for my 928. that will provide some additional downforce up front, by routing the air through the hood and then over the car to the rear wing, by also taking air that might have been routed to the sides, via the splitter, and putting it over the car.

Originally Posted by Tom W View Post
Bob: I have the Getty hood that eliminates the front air scoop and believe it helps (but no data to prove it). One of my competitors has a standard shaped hood and he finds a significant benefit if he uses tape to cover the inlet and provide a more smooth air flow over the front windshield.
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Old 02-10-2010, 06:44 PM
  #75  
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Originally Posted by mark kibort View Post
I have always read, you want the enplates as straight down as possible, but maybe there are some subtle differences in keepign an angle. the fin on the top of the wing, shouldnt do much, but I have a little over hang there, just for looks.
Actually, McBeath's testing showed benefits from going as high up as you go down below with straight end plates.

My angled winglets were based on different research:


Source: McCormick, Aerodynamics, Aeronautics, And Flight Mechanics. Wiley, 1979. ISBN: 0-471-03032-5

Of course, you have to invert everything for a race wing. But you can see it in use more and more on commercial aircraft:



And I agree that I sometimes take empirical testing too far.



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