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"can't be done without imposing additional drag."
There is no necessary relation here. It might well be true -- ot it might not. But aerodynamic laws don't require either result. |
Guys
The impact on drag of the lift spoiling devices, like the Carrera duck tail or the 917K body configuration depend on the individual body shapes before and after the changes. There is not a simple relationship between lift and drag that applies for all cases. The interaction between the flow separation zones and the potential flow streamline pattern is very complex and even today it is difficult to model and predict. The test data clearly shows that the addition of the duck tail and the front lip spoiler reduced the drag and increased the top speed for the 911 configuration while at the same time reducing the lift and stabilizing the car. The test data for the 917K configuration vs the 917 longtail showed a reduction in top speed but improved lap times at LeMans. Take a look at the body changes between the 917 longtail and the 917K and you'll see they are very different than the changes to the 911. |
Tom Butler is entirely correct ( you too RandyWebb)....
The "base case" is the shape / aero signature of the car itself.... spoilers will alter the flow-over profile of the airstream. Wings..as such...are another matter entirely.... The 917K ( kick-up tail tested first during the August , '69 trials in Austria....led by John Wyer's Jon Horsman)..... created...FOR THIS CAR...increased overall drag while increasing rear or overall downforce. The result was a more stable and confidence inspiring ride...that made the lap times come down at Lemans..... ...even though top speed was cut down dramatically...something like 30 mph ( from 240+ mph for the LH long tail version to about 210 or so, mph, for the "K" version). - Wil |
Frère has a lot of very sopecific aero data in his books, I urge you all to buy a copy
But in the interests of killing a snowy afternoon 2 days prior to turkey day(In other words I have nothing better to do) I'll summerize some of the #S Production cars 356 C <sub>d</sub> = 0.36 356 B C<sub>d</sub> = 0.40-0.41 original 911 A series C<sub>d</sub> = 0.381 S = 1.6853m<sup>2</sup> 911 B series C<sub>d</sub> = 0.408 S = 1.71m<sup>2</sup> 911 C series C<sub>d</sub> = 0.408 S = 1.6853m<sup>2</sup> C<sub>l</sub>f =0.190 C<sub>l</sub>r =0.264 @230kph(143mph) L<sub>f</sub> = 83kg(183#) L<sub>r</sub> = 115.5kg(255#) side wides increase both f/r lift #s max is a@ 22.5° 911 C series C<sub>d</sub> = 0.408 S = 1.6853m<sup>2</sup> C<sub>l</sub>f =0.190 C<sub>l</sub>r =0.264 @230kph(143mph) L<sub>f</sub> = 83kg(183#) L<sub>r</sub> = 115.5kg(255#) 911 E series w/ S front spoiler C<sub>d</sub> = 0.408 S = 1.6853m<sup>2</sup> C<sub>l</sub>f =0.107 C<sub>l</sub>r =0.261 @230kph(143mph) L<sub>f</sub> = 46.5kg(102.5#) 911 F series Carrera RS w/ front RS and ducktail C<sub>d</sub> = 0.397 C<sub>l</sub>f =0.070 C<sub>l</sub>r =0.084 @245kph(152mph) L<sub>f</sub> = 35kg(77.3#) L<sub>r</sub> = 42kg(93#) 911 F series Carrera RS w/ front RS and nomal lidC<sub>d</sub> = 0.409 C<sub>l</sub>f =0.064 C<sub>l</sub>r =0.290 @245kph(152mph) L<sub>f</sub> = 32kg(70.6#) L<sub>r</sub> = 145kg(320#) 911 F series Carrera RS w/ front RS and ducktail and 190mm extension on the bottom of the front spoiler C<sub>d</sub> = 0.375 C<sub>l</sub>f = -0.055 C<sub>l</sub>r =0.113 undesireable rocking moment is induced by by front down force and rear lift 911 H series w/ front air dam(rubber chin) and rear spoiler(early whale) C<sub>d</sub> = 0.414 C<sub>l</sub>f = 0.010 C<sub>l</sub>r =0.025 911 H series w/o front air dam(rubber chin) or rear spoiler(early whale) C<sub>d</sub> = 0.423 C<sub>l</sub>f = 0.112 C<sub>l</sub>r =0.246 911 H series w/o front air dam(rubber chin) and w/ rear spoiler(duck) C<sub>d</sub> = 0.414 C<sub>l</sub>f = 0.113 C<sub>l</sub>r =0.062 964 moveable spoiler raised S = 1.79m<sup>2</sup> C<sub>d</sub> = 0.32 C<sub>l</sub>f = -0.01 C<sub>l</sub>r =0.02 964 Turbo fixed spoiler S = 1.88m<sup>2</sup> C<sub>d</sub> = 0.35 C<sub>l</sub>f = 0.00 C<sub>l</sub>r =0.01 993 moveable spoiler raised S = 1.86m<sup>2</sup> C<sub>d</sub> = 0.33 C<sub>l</sub>f = 0.03 C<sub>l</sub>r =0.07 964 Turbo fixed spoiler S = 1.93m<sup>2</sup> C<sub>d</sub> = 0.34 C<sub>l</sub>f = 0.01 C<sub>l</sub>r =0.03 total drag is the product of S and C<sub>d</sub> |
Race cars
1973 Martini Monza RSR w/ full width rear spoiler 10.5" & 14" wheels C<sub>d</sub> = 0.413 S = 1.7966m<sup>2</sup> C<sub>l</sub>f = -0.019 C<sub>l</sub>r = -0.065 @250kph(155mph) L<sub>f</sub> = -10kg(-22#) L<sub>r</sub> = -35kg(-77#) 911 H series Carrera 3.0RS road trim w/ front air dam(rubber chin) and rear spoiler(early whale) C<sub>d</sub> = 0.414 C<sub>l</sub>f = 0.010 C<sub>l</sub>r =0.025 @ 152mph L<sub>f</sub> = 5kg(11#) L<sub>r</sub> = 13kg(28.5#) 911 H series Carrera 3.0RS race trim w/ front air dam(rubber chin) and rear spoiler(wide IROC whale), lowered wide fender for 10.5 & 14" wheels C<sub>d</sub> = 0.421 C<sub>l</sub>f = 0.094 C<sub>l</sub>r =0.073 S = 1.91m<sup>2</sup> @ 152mph L<sub>f</sub> = 47kg(103#) L<sub>r</sub> = 38kg(83#) 993RS street front, side and fixed rear spoilers C<sub>d</sub> = 0.33 C<sub>l</sub>f = 0.03 C<sub>l</sub>r =0.00 @ 270kph L<sub>f</sub> = 178kg(80.9#) L<sub>r</sub> = 0kg(0#) 993RS CS front, side and fixed rear spoilers variable #s due to variable wing angle of attack C<sub>d</sub> = 0.34 - 0.36 C<sub>l</sub>f = 0.00 - 0.03 C<sub>l</sub>r =-0.02 - -0.12 @ 270kph L<sub>f</sub> = 0 - 178kg(0 - 80.9#) L<sub>r</sub> = -119 - - 713kg(-54 - -324#) |
Hey Bill
Neat data. Which of Frere's books are you citing? |
a lot of it - maybe all - is in the 911 Story
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the 993RS date cove from the 93RS supplements to the 993 Shop Manual I have the GT2 racing manual as well but did't bother to look anything up from that. Ernie Wilberg has the SC/RS manual and may be persudaded to look up the data from that. |
Bill
I just looked through my copy of Frere's book. Lot's of good stuff on aero development. |
Bill,
Great data - It will take me a while to digest it all. You may have had a typo in the last line of your first drag post - Total drag is the product of Cd * S * q q is the dynamic pressure, and in metric units, it can be approximated by (at sea level standard day temp): q = {(KPH * 0.27778)squared} * 0.0625 The units of q for this equation would be kg / (meter squared) Total drag units would be in kg, area in square meters I too am killing time - but in Langley Virginia, way across the country from my home - waiting to install a 777 model in a wind tunnel. It isn't snowing here yet, but it is threatening to. Rex |
Thanks for your reply. I presume one could draw some conclusion from the speeds at which Porsche has chosen to activate the rear spoilers on the 964 and later models ?
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The 964 issue has a bit more "fuzz" on it than simply aero...
The 964 fan pushes about 75% of the air volume compared to the earlier straight vane fans. The "hole" opening on the deck is smaller too, with the lid down. At some point, the car's appetite for cooling air will increase. Porsche chooses to activate ( up) at around 75 mph, I believe, but to avoid unnecessary cycling it lowers the spoiler when you decel to about 5-20 mph...and stays up within this range ( coming down from speed). I've also posted before how a Car and Driver test of the (similar) wing arrangement for a later version of the car...showed that the wing does almost nothing aero-wise in the up position ( here come the counter-posts !! ). ....but the driver did say the car felt a bit more stable at triple digit speeds with it up. To be clear..."no effect" means coast-down tests....which indicates very efficient drag change...the wing itself likely provided an element of downforce for the tester to say the car felt more stable. - Wil |
Will
Your are right. The 964 and later style movable spoilers should act similar to the duck tail providing no drag impact and improved stability due to reduced lift. Since coast down testing evaluates drag not lift, there would be no impact on the test results. BTW...Bill you cited some work by Prof R White in one of your earlier posts on this subject. I did my Masters thesis under Prof Bob (White Sox) White in '77-'78 at the Univ Illinois while he was consulting for Porsche at Weissach during the summers. Bob had some great stories about the visits of the factory engineers to his home and the rides through the Illinois corn fields in his various 911's. Great fun. |
I race a 1977 911 RSR clone with IMSA GTU bodywork. I changed from a Turbo tail, which is required in SCCA, to a IROC style tail, and picked up 3 mph at Daytona, with no other changes.
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The basic 964 design is so much better thanthe F series 2.7 Rs that the spoiler is almost a non issue.
I have some other stuff on the 993 that shows how outstanding the flow is at the front and sides but how terrrible it is in the back even w/ a fixed wing. I am hesitant to post because of copy right rules. For most of us the best/most efficient isn't what we put on our cars because of other reasons. It is an interesting academic exercise though. On my 993 I went w/ base 993RS f/r spoilers for appearance and I need to be able to get into my draiveway. The 993RS/CS front spoiler while more efficient would have made the latter a chore. On my 911 I went w/ the Ruf front and 964RSA fixed wing rear for appearance and weight, a targa is an aero disaster anyhow. |
"interesting academic exercise"
speaking of which -- did anybody see the MB protoype Fuel cell car modelled on the boxfish? I saw a pic with streamlines in a magazine somewhere... maybe NudesWeek... Bill - fair use allows you to post snippets for analysis and commentary -- or you could use it in a parody... |
If anyone wants the data that Bill posted in an Excel spreadsheet format, send me an e-mail, and I will get a copy to you. The spreadsheet calculates the lift at the front and rear, the total drag, and the induced drag (I told you I was bored).
Rex res1narh@verizon.net |
remember air follows form, so if the top is down the air is probably disapating be fore it gets to the rear spolier, which of course is probably why the leading edge of a 911 coupe is up a little to help creat a flow behind the windshield, no expert here i have just delt with a lot of top on top off race cars, Kevin
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