12 in a wind tunnel

Flat 4 · Post by **Flat 4** » Mon Sep 14, 2009 9:19 pm

I had a chance to take some high level wind tunnel data on my car (SWB 912) the other day. With the main aim to baseline the car for lift and drag and also to get some undertstanding of small things that could be changed which may effect both. In summary opening holes on the front of the car (fog lamp covers or horn grilles) increases drag and front lift, although reduces rear lift. With the fog lamp covers open there is an increase in flow to the brakes so this also needs to be considered. Changing the front number plate angle to vertical (it is currently at the same angle as the bumper skin) gives a significant reduction in front lift without increasing drag but does increase rear lift.

The other thing we tested was yaw stability which doesn't seem to be to bad the lift figures do change but at fairly progressive rates so shouldn't have a Audi TT issue in a cross wind at high speed!

This trip was made even more interesting as the tunnel had measure and standard 1968 2.0L SWB 911 in 1969 which gave the below figures:

1968 911 measured in the same tunnel (on the same balance)
CD = 0.329
Combined front and rear lift = 0.275 (96.3 kg @ 130 mph)

My car Baseline:
CD 0.323
Front lift = 0.028 (10.7 kg @ 130 mph)
Rear lift = 0.246 (85.6 kg @ 130 mph)
Combined fronta nd rear lift = 0.275 (96.3 kg @ 130 mph)

Setups tested:

Best drag (Le Mans set-up!): fog lamp covers in place, horn grilles cover, bumper plate angle/height (5mm reduction in Z) change.
CD 0.320 - this is a 9 count improvement over the measure in 1969 (equivalent of 1.93 bhp)
Front lift = 0.005 (2.5 kg @ 130 mph)
Rear lift = 0.252 (87.7 kg @ 130 mph)

Best lift:
CD 0.329
Front lift = 0.005 (2.5 kg @ 130 mph)
Rear lift = 0.238 (82.7 kg @ 130 mph)

I have taken the performance effects figures out as they were all wrong, as you will see on the rest of this thread.....that is what you get for only reading the first page of a text book!

So setup (what I am going to run?) - I think best lift as the extra brake cooling is a good thing for road rallies and track work, also long runs at v.max are unlikely!

Ian Gunney · Post by **Ian Gunney** » Mon Sep 14, 2009 9:45 pm

John

won't pretend to understand all the technical bits - but the pics look great!

JAN · Post by **JAN** » Mon Sep 14, 2009 10:38 pm

Thats fantastic, how did you get the opportunity to go into MIRA?

Im a little dubious of some of those performance figures though, max velocity of over 170mph? Wouldnt you have to be revving at around 12k revs or something crazy?!? Not to mention 130mph wouldnt get you there in terms of hp.

I also doubt the Cd difference of 0.09 would knock half a second off your 0-60 time.

Fantastic and very interest pictures and reading though.

Jan

(MEng in Mechanical Automotive Engineering, although no longer an engineering so simply semi-educated musings

)

Gary71 · Post by **Gary71** » Tue Sep 15, 2009 7:38 am

Great information!

The rear lift figure shows why 911s have gained larger and larger rear spoilers over the years!

80kg rear on a 1000kg car, with no front lift is pretty significant, no wonder the back of our cars start to wander about once at 'high' motorway speeds.

The next questions on the effects of the 'S' front spoiler and RS ducktails are just waiting to be answered next time you get a chance to visit the tunnel.

GP911 · Post by **GP911** » Tue Sep 15, 2009 8:09 am

Very interesting and thanks for sharing your info, car looks superb.

Mr Pharmacist · Post by **Mr Pharmacist** » Tue Sep 15, 2009 8:21 am

very cool - I also havent a clue what it all about, but it looks very interesting!

how did you get the time in a wind tunnel? do you have contacts?

Flat 4 · Post by **Flat 4** » Tue Sep 15, 2009 9:43 am

Jan

I understand your a little dubious, I worked them out more than once to be sure, these are only high level formula so many things are assume to be constant or of no importance, so is indication only. I am also an engineer (did my degree about 10 years ago so forgot it all) and work in automotive design and development although aerodynamics is a side line of my work in aeroacoustics so I am no expert........so this could all be wrong.

Andy102 · Post by **Andy102** » Tue Sep 15, 2009 4:07 pm

Interesting reading, and a CD of 0.32 not bad.
I remember testing a 1970s Austin Princess once, that wasn't so good.

Flat 4 · Post by **Flat 4** » Tue Sep 15, 2009 4:14 pm

Gary71 wrote:Great information!

The rear lift figure shows why 911s have gained larger and larger rear spoilers over the years!

80kg rear on a 1000kg car, with no front lift is pretty significant, no wonder the back of our cars start to wander about once at 'high' motorway speeds.

The next questions on the effects of the 'S' front spoiler and RS ducktails are just waiting to be answered next time you get a chance to visit the tunnel.

Gary,

To ture the 80kg rear lift becomes even more scary when you consdier the rear axle load was only 519 kg in static air on the balance (total weight 910 kg, with a full tank).

Front lift needs to be keep low on these cars as the front axle load is only 2/3 of the rear @ 390kg (with a full tank), plush the COG is 150mm rearward of axle centre line. So even low levels of front lift can be a concern.

The yaw stability is good through so the Audi TT high speed issue shouldn't be there (which did have around 80 kg of rear lift @120 mph but the lift changed rapidly with yaw hence the recall).

Flat 4 · Post by **Flat 4** » Tue Sep 15, 2009 4:18 pm

Andy102 wrote:Interesting reading, and a CD of 0.32 not bad.
I remember testing a 1970s Austin Princess once, that wasn't so good.

No 0.320 is quite good, the latest turbo is only 0.295 (although the lift is much better). The main reason for being that low (in relative terms considering its age) is due to next to zero cooling drag as its all at the rear.

Gary71 · Post by **Gary71** » Tue Sep 15, 2009 4:39 pm

...and as they have such a low frontal area the CdA is really good on narrow early cars.

sd · Post by sd » Fri Sep 18, 2009 2:18 am

Thats very interesting to see your car in the wind tunnel.

I’m no auto engineer but I don’t think its quite that simple to calc Vmax !

Vmax depends largely on Aero drag which is proportional to the drag coefficient, frontal area and the square of the vehicle speed.

Also add in the rolling resistance which factors in the coeff, mass and speed, and presumably its power to the wheels rather than crank.

So if Vmax squared is proportional to 1/drag coefficient, and current Vmax is 120mph, a change in Cd from 0.329 to 0.320 might raise top speed by 1.7mph, or 1.6mph allowing for an increase in rolling resistance.
A 130bhp car might do 140mph, and it should go up 1.9mph, or so …

0-60 I’d have thought would be hardly any difference as aero drag is less important at lower speeds (less than 0.1sec)

Sorry to bore anyone who reads this far – and please feel free to correct it if you actually know what you are talking about !!

I think the actual formula is on the lines of:
Power (kW at wheels) = 0.5 x air density x frontal A x Cd x Vmax3 + mass x g x coeff resistance x Vmax

tim78 · Post by **tim78** » Fri Sep 18, 2009 9:31 am

sd wrote:I think the actual formula is on the lines of:
Power (kW at wheels) = 0.5 x air density x frontal A x Cd x Vmax3 + mass x g x coeff resistance x Vmax

Bang on! Thats just what I was thinking

Flat 4 · Post by **Flat 4** » Sat Sep 19, 2009 8:45 am

sd wrote:Thats very interesting to see your car in the wind tunnel.

I’m no auto engineer but I don’t think its quite that simple to calc Vmax !

Vmax depends largely on Aero drag which is proportional to the drag coefficient, frontal area and the square of the vehicle speed.

Also add in the rolling resistance which factors in the coeff, mass and speed, and presumably its power to the wheels rather than crank.

So if Vmax squared is proportional to 1/drag coefficient, and current Vmax is 120mph, a change in Cd from 0.329 to 0.320 might raise top speed by 1.7mph, or 1.6mph allowing for an increase in rolling resistance.
A 130bhp car might do 140mph, and it should go up 1.9mph, or so …

0-60 I’d have thought would be hardly any difference as aero drag is less important at lower speeds (less than 0.1sec)

Sorry to bore anyone who reads this far – and please feel free to correct it if you actually know what you are talking about !!

I think the actual formula is on the lines of:
Power (kW at wheels) = 0.5 x air density x frontal A x Cd x Vmax3 + mass x g x coeff resistance x Vmax

All good points, when you think about it properly and don't just read the first page of a text book.........lesson learnt, all performance effect figures removed from this thread as complete rubbish. I will read the rest of the book and re-post when (if ever i fully understand it). A classic case of a little bit of knowledge giving all the wrong answers !

Having said all that, the pictures look great on my garage wall.

Flat 4 · Post by **Flat 4** » Fri Sep 25, 2009 11:57 pm

Ok so I have now read a few books (past the first page) and noted the errors of not correcting the CD for frontal area an also missing out the loss from rolling road power figure to real world power at the wheels.

So the below I believe is correct:

v max (kph) = 100 * 3√P/(k*CD*A)

p = engine power at the flywheel (kw)
k = vehicle losses (rolling resistance, drive train loss, electrical system load)
CD = drag coefficient
A = Frontal area (squ m)

Mass is not considered as it has been assume there is not gradient.

Best drag set-up (0.320)

v max (kph) = 100 * 3√97.2/(14*0.320*1.75)

V max = 231.3 kph (142.8 mph)

Reduced lift set-up (CD 0.329)

v max (kph) = 100 * 3√97.2/(14*0.329*1.75)

v max = 229.2 kph (141.5 mph)

So that is about a 30 mph correction over my initial cal, which is quite large

Now I think this is correct having run it pass a aerodynamic engineer, but please feel free to correct as I am not to proud to admit a mistake