downforce v weigt/drag - Page 2

Bill Verburg · Bill Verburg's Garage

"The duck tail reduces lift by over 100kg at 152mm" (not my words Will)
and when used w/ the matching front spoiler "creates ~equal lift front and rear, so that yhe road behavior remained consistant as the speed increased"(again not my words Wil)

it also reduced drag by 3%

* quoted from Fr&#232re
* data from Fr&#232re

Rex Walter · 11-06-2005, 05:27 PM

In both examples given, it seems like the drag reduction was with a combination of duck tail rear spoiler and a front air dam. I wonder if most of the drag reduction is from the front air dam, lift reduction in the front from getting the air out from under the car, and lift reduction from the duck tail at the rear?

Regarding the front oil cooler - it may add a small amount of drag, but the big benefit (with or without the cooler) is the front air dams ability to keep the air from getting under the car. I wouldn't think you would see much benefit from relocating the cooler. Anywhere you put a cooler that has airflow, will cause some drag, and if there is no airflow, the cooler will not work as well.

Rex

island911 · 11-06-2005, 05:49 PM

A dimpled golf ball is a fine example of spoiling the lift . . .which happens to be on a coincident vector with drag . .. unless the ball is spinning.

Quote:

Originally posted by masraum
and you may want to check out these threads
...
911 rake front/rear
. ..

oh man

I just re-read that thread on rake angle. -good times

That 2-D CFD . . silly.

randywebb · 11-06-2005, 05:52 PM

Uh oh - now we are appling terminology usually used for redirection of bulk flows to skin effects....

I foresee much mischief flowing from this...

Bill Verburg · Bill Verburg's Garage

Quote:

In both examples given, it seems like the drag reduction was with a combination of duck tail rear spoiler and a front air dam. I wonder if most of the drag reduction is from the front air dam, lift reduction in the front from getting the air out from under the car, and lift reduction from the duck tail at the rear?

That's correct, but there is also a component of lift and drag attributable to the shape and placement of the spoilers.

The Martini RSRSs of the era are a prime example of a very efficient front and rear aero package.

The forward projection of the front spoiler and the lower splitter greatly enhance the aero performance here. Also note the more efficient placement and shape of the brake cooling intakes

Rex Walter · 11-06-2005, 06:45 PM

Bill,

Thanks for taking the time to dig this stuff up and post it. Now that I look at the Martini RSRS I see what you are saying. The front air dam is out front, with a smooth transition onto the hood. There are NACA ducts in the rear 1/4 windows, and at the rear is a real wing. A real wing will produce downforce with the smallest penalty in drag. The airflow over the roof is not interrupted, and will cause more lift, but less drag. The wing is probably capable of making plenty of downforce to compensate for the lift over the roof. This seems like the best way to create downforce, with the least penalty of drag. Great looking car, and a well engineered example.

Rex

Eagledriver · 11-06-2005, 10:30 PM

Rex,

I think you are not correct in some of your statements/understanding. The spoiler adds parasite drag but reduces induced drag. If the spoiler were on top of the roof the parasite drag would be great enough to increase the total drag on the car. Because it is placed at the back it "spoils" the lift of the body of the car reducing lift and therefor reducing drag in the exact same proportion (induced drag). It adds some parasite drag back into the equation (from the separation/turbulent flow) but not enough to make up for the reduction in induced drag.

Using a wing to create downforce will increase drag in direct proportion to the downward force created. For example if wing 1 produced 100 lbs for downforce and 10 lbs of drag (at a particular speed) then wing 2 that produced 200 lbs of downforce must produce 20 lbs of drag. This assumes the airfoil is of the same efficiency. Induced drag is an inescapable product of lift (no matter up or down).

Using a wing to counter act the lift of the body is less efficient than using a spoiler (from a drag/top speed perspective).

-Andy

Rex Walter · 06:22 AM

Andy,

I'm trying to use what I know about wing design, and apply it to a car.

I think this is where we are going to disagree.

Quote:

it "spoils" the lift of the body of the car reducing lift and therefor reducing drag in the exact same proportion (induced drag).

This would be true if the airflow was attached - that is to say a smaller wing (or car) would produce less lift, and have less drag (parasitic drag). However, I think if you reduce lift by separating the flow, you are going to increase the drag (induced drag/turbulence). So, on a car, you can reduce lift and drag by raking back the windshield and lowering the roof, but by placing a spoiler in the back to cause early airflow separation, you will cause drag due to turbulence.

It's difficult to discuss technical stuff on a forum like this - much better in person, or relaxing somewhere with a beer.

Rex

island911 · 11-07-2005, 08:22 AM

Quote:

Originally posted by Rex Walter
. . However, I think if you reduce lift by separating the flow, you are going to increase the drag (induced drag/turbulence). .. .

hmmm . .. seems to me that Eagledriver addressed that (very well).

fwiw, the reason I mentioned a golfball is that a regular (smooth) ball develops a really nice 'air pocket' of low pressure right where you don't want it. AND a first order application of aerodynamics says that adding surface roughness adds drag to a body in flight. (much of what you are arguing here)

.. but we all know that golfballs fly farther/faster with surface roughness, and 911s have higher top speeds with tails.

Eagledriver · 11-07-2005, 10:13 AM

It is indeed difficult to have these discussions on line. The statement that "reduce lift by separating the flow you are going to increase drag (induced drag/turbulence)" is inaccurate IMO. The INDUCED drag will decrease, it is the parasite drag that will increase from the separating flow. This is similar to the golf ball argument. The dimples on the golf ball increase the parasite drag but decrease the induced drag by an even greater amount thereby decreasing the total drag. (I'm not an expert on golf ball aerodynamics so I'm not sure how accurate this comparison is).

My point is that total drag is made up of parasite drag plus induced drag. Induced drag cannot be reduced by any method other than reducing the amount of lift (up or down), it is an inescapable product of the lift production itself. You can actually draw it with a vector diagram. Draw a shape that changes the vector of the air from straight into the object to downward (upward lift). It is the turning of the air from straight in to downward that produces the drag (induced only). If the air is turned downward 30 degrees the force on the object is upward and backward 15 degrees from straight up. The upward component is lift and the backward component is drag (induced only).

In summary the only source of induced drag is from the angle that you are deflecting the air with the object. The more you deflect the air the more "backward" the force will be on the object.

-Andy

randywebb · 11-07-2005, 12:06 PM

Before we go any further, we'd better define parasit[ic] drag and induced drag.

BTW, the golf ball dimples break up the boundary layer ...

FWIW, whales have big problems with parasite drag...

Rex Walter · 11-07-2005, 12:23 PM

Again, I think we are running into problems with trying to communicate using this forum. It seems like we are all saying the same thing, just coming up with slightly different conclusions. Take the golf ball for example. The dimples are there so that the airflow does not separate too soon, and the air can travel over the ball and around the back side. This produces the smallest amount of drag – which is just like the roof of a car, where you want the airflow to remain attached, and travel over the top, and down the back, to get the smallest amount of drag. When designing for minimum drag, you always want the flow to remain attached to the surface as much as possible.

http://wings.avkids.com/Book/Sports/instructor/golf-01.html

Rex

randywebb · 11-07-2005, 02:16 PM

Here you go:

The aerodynamic forces on golf balls were studied by dropping spinning balls through the horizontal wind stream of the B. F. Goodrich wind tunnel. The lift, L, and drag, D, were calculated from the drift of the balls, rotating at speeds, N, up to 8000 r.p.m. while falling through a wind stream having a velocity of 105 feet per second. For a standard dimple or mesh golf ball weighing 0.1 lb., the lift varied with the rotational speed as L=0.064 ×[1–exp(–0.00026N)], with a maximum observed value of 0.055 lb. or more than half the weight of the ball. The drag increased nearly linearly from about 0.06 lb. for no spin to about 0.1 lb. at 8000 r.p.m.For a smooth ball the lift was negative at all rotational speeds below 5000 r.p.m. Above this speed, the lift was positive but was less than for the standard ball. The drag for these balls was nearly constant at about 0.08 lb. Balls with shallower dimples than standard gave intermediate results. Driving tests were consistent with the wind tunnel results. These results explain why a golfer cannot obtain long drives with a ball having a smooth surface and why the standard dimple or mesh surface gives him greater distance and better control of the ball.
John M. Davies, J. Appl. Physics 20: 821-828.

BReyes · BReyes's Garage

Here's two more, compare specifics of Moby Dick 1978/81 with Martini RSRSs.

Hope you have a great Holiday.

Eagledriver · 11-18-2005, 07:37 PM

If I read this right Randy's post shows about 30 percent more drag for a smooth golf ball compared to a dimpled golf ball.

I think that the idea of keeping airflow attached to decrease drag is deeply rooted in the past. Think about the ideas of the 30's with cars that had tapered rear ends and tapered fairings behined the driver.

My guess is that modern aerodynamics favor the wedge shapes that we see on modern cars. The idea being less frontal drag and less lift being more important than keeping the airflow smooth and attached. As I've previously said the lift generated by the smooth airflow over the car will produce lift proportional to the lift generated. No amount of smoothing can diminish that.

-Andy

Eagledriver · 11-18-2005, 07:39 PM

I meant DRAG proportional to the lift generated.

randywebb · 11-18-2005, 09:49 PM

"keeping airflow attached ..."

- It largely depends on the type of airflow. The flow breaks up behind the car, generating eddies that flow back forwards to the car...

- re last para., not really. The big change was the Kamm tail which breaks the flow off sharply to prevent eddies from 'sucking' the car back. That lead to the demise of the beautiful boattailed Alfas...

Watch the eddies in a river behind the rocks - they can suck a kayak backwards and underwater...

tom1394racing · 11-19-2005, 04:10 AM

With all of the weighty discussion on lift and drag I did not see any mention of stability.

When I installed the aero package (Carrera Tail & Front spoiler)on my 82SC a few years ago one big difference was the elimination of high speed buffeting. The difference was noticeable the first time I took the car over 100MPH.

Flow separation tends to be an unsteady phenomena with the separation point moving back and forth. This occurs without the spoiler and causes a buffeting as the lift and drag vary with the movement of the flow separation point up and down the rear window and deck lid. The tail spoiler fixes the separation at the trailing edge of the spoiler and eliminates the buffeting. It's very noticeable when you do a back to back with and without the spoiler.

Bill Verburg · Bill Verburg's Garage

The various aero packages also help w/ cross winds at all speeds

SC-targa · 11-19-2005, 06:10 AM

Quote:

Originally posted by randywebb
FWIW, whales have big problems with parasite drag...

Actually, a useless piece of drag trivia. Dolphins (like Flipper)have an active skin surface that actually reacts to minute pressure changes to maintain laminar flow and low drag. I remember reading of a Navy study in an engineering journal years back, where they were trying to mechanically duplicate the dolphin effect for low drag submarines.

Regards,

Jerry Kroeger