fenders vs. no fenders 1

[hachikid]

which one is more aerodynamically efficient? and no speculation, either. I have a friend that says that no fenders are more efficient cause of more surface area, and stuff, while I can swear I remember reading about an aerodynamicist who said that open wheel designs are inferior because the 4 protruding wheels disturb the air flow severely. anyone know?



Joseph.

 

[dalcazar]

no fenders is more efficient because of more surface area??? no

more surface area = more drag

The only example I can think of with fast cars with no fenders are the F1 cars, and they're not meant to be fast in a straight line, they're meant to take corners at 3 Gs of lateral force, stop HARD and accelerate fast, and the best way to do that is to get rid of all possible weight.

seriously fast ground vehicles like jet powered cars for record attempts have wheel fairings (or fenders if you must) over their tires or wheels, that is if their wheels are exposed at all. Your friend has little common sense IMO.

 

[jetmaker]

I know that Cessna 172s are faster by the book by a couple of knots when they have the wheel/tires fairings on as opposed to exposed wheels/tires.

jetmaker

 

[btrueblood]

How many airplanes have exposed wheels?

 

[jetmaker]

btrueblood,

Can't tell if that is an actual question, or you are just pointing out that most airplanes retract their wheels to get them out of the airstream.

Anyways, it is a good point to show why most airplanes try to have retractable gear. The tradeoff in performance is often weighted against expense, weight, and system complexity.

jetmaker

 

[bf109g]

Fairings are great if all you plan on landing on are relatively solid surfaces.

Take it into the bush, and the fairings get clogged with mud.

 

[scotty7]

The answer is probably more down to how the fenders (or wheel fairings) are designed, and how this affects the airflow. For example, a typical road saloon (sedan) has a Cd of around 0.3 or less. An open-wheeled F1 car has a Cd of around 0.7. However, the Caterham 7 road car has faired wheel arches, and has a Cd of 0.7 too, because of the aerodynamic characteristics. Equally, the earlier LeMans cars of the 1990's were hyped as F1 cars with closed cockpits and covered wheels - and generated more downforce for less drag - more akin to a Cd of 0.3 to 0.4.

http://en.wikipedia.org/wiki/Automobile_drag_coefficients

Also note the huge flares on the Caterham shown here, which add to the drag:

http://www.caterham.co.uk/assets/html/preowned.html

 

[IRstuff]

The question is too general to really get a meaningful answer in general. 1/2 * Cd * area * rho * v^2 is the equation to look at. And the key is that you want to minimize the Cd * area product.

You can do silly things to a design, e.g., use a flat-front box for a fairing, which will result in cruddy performance even relative to a naked wheel. You can use a bicycle racing wheel, which will probably beat any fairing you might try to design for it.

That said, you can clearly relate a normal tire to the standard ball-nose projectile with a flat tail, and a fairing to a high performance ogive nose and boat-tail projectile and guess what, that larger frontal area ogive and boat-tail wins by a wide margin, because it does a substantially better job of reducing flow separation and drastically cuts Cd, while the tire has a Cd that's nearly 1. That's why there are no blunt-nosed supersonic ammunition.

TTFN

FAQ731-376