Which tail? - aerodynamics questions
01-12-2004, 10:54 PM
#16
Addict
Rennlist Member
Rennlist Member
Hey Fellas;
There is indeed a difference between getting downforce and reducing lift. Downforce is using air, reducing lift is managing it.
The old 911 body was one of the least aerodynamic shapes in racing, but Porsche was able to overcome it with gobs of HP and reliability. The thing that kills all of them is the upright windshield.
The air slams into that thing, is significantly reduced in flow rate, and slowly moves toward the rear of the roof where it tumbles in turmoil and turbulence down off the rear of the car. If you were to see the wake produced by a stock 911 without a tail, it would be clearly visible right at the top of the rear window. Picture the effect of your SUV or minivan rear window in winter. Filthy!
Here's your AeroCheater Car!
This loss of air velocity creates a signficant low pressure dead zone right behind the rear roof line. Low pressure there relative to the air flowing under the car means lift. Designers have come up with various ways to reduce this vortex effect.
A ducktail is effective at managing airflow because it breaks up the vortex that forms directly over the rear window and engine lid, and moves this air closer to the main flow higher off the rear body of the car. However, it is so minimal in size - not sticking up into the higher flow air stream - that it does not add hardly anything in the way of drag. It weighs very little as well!
The various Turbo tails are larger. They increase drag substantially over the ducktail because they stand right out in the direct airflow, but they also create downforce. They also collect more air for cooling the engine and AC condensers! Porsche no longer had to worry about lack of HP with the Turbos, and the compromise must have been deemed worthwhile.
The most recent designers have not only made use of the aerofoil effect to create free downforce, but have realized that it is most efficient to move the vortex as far off the rear of the body as possible. By acclererating air in the proper places they have managed to reduce drag while creating downforce.
Indeed, various silhouette rules restrictions govern how far into the airstream a wing can be. Most classes closer to stock will stipulate that the wing cannot be visible from directly in front of the car. Higher classes will state that the wing cannot exceed the outer dimensions of the body perimeter. It can stand up to the roof line, but cannot be wider than the wheels nor stand behind the rear bumper line. The more free the class, the higher you will see the wings go, all in search of that clean fast moving air.
So, the Ducktail is very efficient because it is light, does not stick up into the air too far (causing drag), but manages that dreaded rear body vortex fairly well. They work very well for cars in the <110mph range. For cars that can reach in excess of that, the 3.8RSR wing is likely the best. It is pretty decent in terms of drag because it keeps air moving pretty swiftly. By doing so, it also reduces the vortex lift effect quite well. It is also not hugely heavy, and will produce a fair amount of downforce to boot!
There! Hope I'm right!!
There is indeed a difference between getting downforce and reducing lift. Downforce is using air, reducing lift is managing it.
The old 911 body was one of the least aerodynamic shapes in racing, but Porsche was able to overcome it with gobs of HP and reliability. The thing that kills all of them is the upright windshield.
The air slams into that thing, is significantly reduced in flow rate, and slowly moves toward the rear of the roof where it tumbles in turmoil and turbulence down off the rear of the car. If you were to see the wake produced by a stock 911 without a tail, it would be clearly visible right at the top of the rear window. Picture the effect of your SUV or minivan rear window in winter. Filthy!
Here's your AeroCheater Car!This loss of air velocity creates a signficant low pressure dead zone right behind the rear roof line. Low pressure there relative to the air flowing under the car means lift. Designers have come up with various ways to reduce this vortex effect.
A ducktail is effective at managing airflow because it breaks up the vortex that forms directly over the rear window and engine lid, and moves this air closer to the main flow higher off the rear body of the car. However, it is so minimal in size - not sticking up into the higher flow air stream - that it does not add hardly anything in the way of drag. It weighs very little as well!
The various Turbo tails are larger. They increase drag substantially over the ducktail because they stand right out in the direct airflow, but they also create downforce. They also collect more air for cooling the engine and AC condensers! Porsche no longer had to worry about lack of HP with the Turbos, and the compromise must have been deemed worthwhile.
The most recent designers have not only made use of the aerofoil effect to create free downforce, but have realized that it is most efficient to move the vortex as far off the rear of the body as possible. By acclererating air in the proper places they have managed to reduce drag while creating downforce.
Indeed, various silhouette rules restrictions govern how far into the airstream a wing can be. Most classes closer to stock will stipulate that the wing cannot be visible from directly in front of the car. Higher classes will state that the wing cannot exceed the outer dimensions of the body perimeter. It can stand up to the roof line, but cannot be wider than the wheels nor stand behind the rear bumper line. The more free the class, the higher you will see the wings go, all in search of that clean fast moving air.
So, the Ducktail is very efficient because it is light, does not stick up into the air too far (causing drag), but manages that dreaded rear body vortex fairly well. They work very well for cars in the <110mph range. For cars that can reach in excess of that, the 3.8RSR wing is likely the best. It is pretty decent in terms of drag because it keeps air moving pretty swiftly. By doing so, it also reduces the vortex lift effect quite well. It is also not hugely heavy, and will produce a fair amount of downforce to boot!
There! Hope I'm right!!
01-15-2004, 01:54 PM
#20
Rennlist Member
Thread Starter
I had no tail prior to the duck. The change was very noticeable at higher speeds. I really wouldn't want to drive over 80 or 90 without one, even on the street, just for safety reasons. I've been up to 120 on the track and I still felt very planted with the duck on.
The weight savings are also significant. With 5/8 tank of gas the car weighs 2550 pounds. Swapping the steel lid for the fiberglass duck was a noticeable part of that weight reduction. I still have bits and pieces of heater and a/c components to strip out.....
The weight savings are also significant. With 5/8 tank of gas the car weighs 2550 pounds. Swapping the steel lid for the fiberglass duck was a noticeable part of that weight reduction. I still have bits and pieces of heater and a/c components to strip out.....
