Wind Tunnel results
11-07-2011, 04:50 PM
#1
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Thread Starter
Wind Tunnel results
Our trip to the A2 Wind Tunnel was very informative and useful.
It was the usual mix of satisfaction that some of our design work was spot-on and confirmed, and then surprise that other areas were dead-nuts counter-intuitive and we had it exactly backwards of the way it should be. But, that's what you test for.
Our first wind tunnel test was "as-raced" at Bonneville to see what we had, and to establish a baseline. The Cd (Coefficient of Drag) was .30 We were pretty pleased with that - thats a nice low number, and reflects the work we did on the belly pan and removing the headlights, fog lights, mirrors, and the small things the rules permitted.
Then we started mocking up cardboard and foam to "try this" with the help of Dave Salazar, the BSME that was helping us from A2. He'd make suggestions from his experience with the NASCAR cars, and we'd try it. He was usually right, but in two specific cases the 928 proved to react exactly opposite of the coupes he was used to. Hatchbacks have their own chapter in vehicular dynamics - they are significantly different than other body styles.
In the end, we got the Cd down to .25 This is a gain equivalent to adding 95 HP to the engine. I need to push the math around to convert the drop in Cd to probable increase in MPH, and have not done that yet. (A shout out to Glen Evans)
Then we started adding rake and spoiler adjustment to reduce the lift in the rear axle at speed. We were never able to get the rear lift completely out of the back end, but we were able to reduce it by 3/4ths. Now we know that the rear spoiler is too small, and will make a larger one. When the control surface of the rear spoiler is bigger, we will be able to get into negative lift (down force) like we want to.
The last thing we did was to fire up the smoke wand. They let me stand in the booth and take video while the fans were on. What a rush! Its great video and I will post it once I have it edited (its over 2 Gb now).
All-in-all, well worth the time and money spent. We found a great deal of improvement, learned a lot (both personally and about the unique 928) and our trip next year back to Bonneville will be more productive because of it.
NOTE: although I am sure it will happen anyway, I must caution anyone from using my drag or frontal area numbers for their car. This 928 is highly modified for aero and my numbers do not reflect stock very much at all.
It was the usual mix of satisfaction that some of our design work was spot-on and confirmed, and then surprise that other areas were dead-nuts counter-intuitive and we had it exactly backwards of the way it should be. But, that's what you test for.
Our first wind tunnel test was "as-raced" at Bonneville to see what we had, and to establish a baseline. The Cd (Coefficient of Drag) was .30 We were pretty pleased with that - thats a nice low number, and reflects the work we did on the belly pan and removing the headlights, fog lights, mirrors, and the small things the rules permitted.
Then we started mocking up cardboard and foam to "try this" with the help of Dave Salazar, the BSME that was helping us from A2. He'd make suggestions from his experience with the NASCAR cars, and we'd try it. He was usually right, but in two specific cases the 928 proved to react exactly opposite of the coupes he was used to. Hatchbacks have their own chapter in vehicular dynamics - they are significantly different than other body styles.
In the end, we got the Cd down to .25 This is a gain equivalent to adding 95 HP to the engine. I need to push the math around to convert the drop in Cd to probable increase in MPH, and have not done that yet. (A shout out to Glen Evans)
Then we started adding rake and spoiler adjustment to reduce the lift in the rear axle at speed. We were never able to get the rear lift completely out of the back end, but we were able to reduce it by 3/4ths. Now we know that the rear spoiler is too small, and will make a larger one. When the control surface of the rear spoiler is bigger, we will be able to get into negative lift (down force) like we want to.
The last thing we did was to fire up the smoke wand. They let me stand in the booth and take video while the fans were on. What a rush! Its great video and I will post it once I have it edited (its over 2 Gb now).
All-in-all, well worth the time and money spent. We found a great deal of improvement, learned a lot (both personally and about the unique 928) and our trip next year back to Bonneville will be more productive because of it.
NOTE: although I am sure it will happen anyway, I must caution anyone from using my drag or frontal area numbers for their car. This 928 is highly modified for aero and my numbers do not reflect stock very much at all.
11-07-2011, 04:55 PM
#3
Race Director
SUPER COOL
amazing that "squaring" off the nose dropped the CD by that much!!! .05 is HUGE......however I am bit worried that you might need a larger opening for your radiator-motor
amazing that "squaring" off the nose dropped the CD by that much!!! .05 is HUGE......however I am bit worried that you might need a larger opening for your radiator-motor
11-07-2011, 05:06 PM
#4
Developer
Thread Starter
No canards?
I am bit worried that you might need a larger opening for your radiator-motor
11-07-2011, 05:18 PM
#5
Happily Amused
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11-07-2011, 07:43 PM
#11
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Great work Carl, keep it up!
11-07-2011, 08:35 PM
#12
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Very cool!
Aerodynamics is truly fascinating. What speed did you run it at?
And unlikely that you could have done it with out ripping the rear wing off, but it would have been a hoot to turn the car around and "run it backwards"
Aerodynamics is truly fascinating. What speed did you run it at?
And unlikely that you could have done it with out ripping the rear wing off, but it would have been a hoot to turn the car around and "run it backwards"
11-07-2011, 08:41 PM
#13
Three Wheelin'
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Hi Carl 
Two questions....
Firstly just out of interest, what did your day cost you at the tunnel as when I enquired in the UK, they wanted £17,000
Secondly, you mentioned rear downforce with a bigger wing would improve things, but no mention of the front, does your front splitter stop enough lift as it is ?
My runs it was pretty even in too much "lift" at both ends....
All the best Brett
Two questions....
Firstly just out of interest, what did your day cost you at the tunnel as when I enquired in the UK, they wanted £17,000
Secondly, you mentioned rear downforce with a bigger wing would improve things, but no mention of the front, does your front splitter stop enough lift as it is ?
My runs it was pretty even in too much "lift" at both ends....
All the best Brett
11-07-2011, 11:15 PM
#14
Race Director
http://www.rbracing-rsr.com/aerohpcalc.html
Did some math....I think Carls engine is near 800whp......using the above calc at .25 with the standard 21.28sqft equals nearly 250mph..... DAMM
Did some math....I think Carls engine is near 800whp......using the above calc at .25 with the standard 21.28sqft equals nearly 250mph..... DAMM