Cd for F1 cars 3

[DukeGlacia]

So F1 cars have open tyres but why does this increase the drag. Secondly is the speed of the air above the tyre slow or fast relative to the car? Thirdly does streamlined vehicles reduce the downforce because air flow fastly and smoothly on top of the cat

 

[racookpe1978]

This happens with bicycles as well.
Simplifying a little bit:
The bottom of the tire is at 0.0 car speed = The tires is not skidding, and the tire is motionless.
The center of the tire, the axle, is at car speed = It's vector is directly forward.
The back side of the tire (at axle height) is at car speed, but its vector is straight up. It is shielded from still air by the rest of the tire in front of it - but with a lot of turbulence, this is only a loose approximation.
The front of the tire is at car speed, but its vector is straight down. With no deflector in front of the tire, the air resistance is proportional to car speed.
The top of the tire is at 2x car speed, and does hit the stagnant air. Resistance is 4x car speed, since resistance is proportional to the square of velocity. The open tire does increase air resistance this way.

A airfoil type "fender" over the top of the tire reduces resistance because the trapped air below the fender (between the fender and the tire) "rolls" with the tire's local direction and so there is less delta velocity between the tire tread and the local air speed. The top of the fender is now at car speed compared to the air, and so resistance is less. Also, instead of a "circle" of the tire, the fender can be aerodynamically shaped and has a much smaller coef. of resistance. Weight goes up, air cross-sectional area goes up, and you need braces and structural parts.

Changing tires gets harder - but that's in the "rules of the game" of each different type of racing as well. A dragster, though much faster than a NASCAR or Indy car or F1 at the start, can't compete with them at distances greater than a full mile. A 24-hour LeMan's style road car, would find it difficult to win at Daytona or Indianapolis.

In bicycles, a front air-foil fender catches the wind - cross-winds in particular - and makes steering more difficult at higher speeds. You'll tend to see bicycle fenders in back, and not in front because of this.



(The F1 cars DO have lower airfoils in front of the tires that deflect stagnant air (right where the high-speed moving airfoil hits the still air in front of the tire) that deflects the air up and over the top of the tire.)

 

[JohnRBaker]

Well for one thing, the top of the tires are moving at a much high speed, relative to ground, than the rest of the vehicle so this would seem to result in more drag per unit area than other aspects of the car. As for the reduced downforce, that's why there is such extensive use of spoilers and 'wings'.

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[DukeGlacia]

Ok so due to the higher relative velocity of the air on top of the tyres the pressure will be high due to turbulance right ? So thats the rreason it creates a downforce ?

 

[moon161]

A rotating cylinder is in fact, an effective wing. There are in fact a small number of ships which use rotors, sometimes with trimmable bits at the downwind separation point increase the Cl of the wing.

https://en.wikipedia.org/wiki/Rotor_ship

This paper puts Cl around .2 for the rotor/wind velocity ratio of 1, which it would be, identically in still air, but likely for a completely different reynolds number.

http://www.tsfp-conference.org/proceedings/2011/6c3p.pdf

So I wouldn't rule out some significant down-force from the wheels. I don't know the relative magnitude, but I expect that the leading and trailing wings operate at a higher Cl, have more area, and the huge area of the body pan, which I think is sucked down to a low pressure by venturi in the body, contribute very much more down-force. I'm sure there are experts on F1 aerodynamics for whom small aspects of this are there daily well-paid pre-occupation.

 

[moon161]

Scratch that.. a Flettner rotor has free stream on both sides, not the same as a wheel with a ground constraint on one side. It would be neat to see the chord-wise Cp of a rotating wheel, google does not turn that up as easily. I still think the body pan dominates, based on relative area.

 

[racookpe1978]

The body pan - overall - is much more significant.

Three things (actually about 7 or 8!) going on here, don't get them unduly confused. Har, har.

The air resistance itself.
Car body frontal area resistance (proportional to frontal area, air foil shape, air velocity = car speed)
Car body side walls resistance (like a ship, the length of the car, it's shape, the laminar or turbulent speed of the local air as it flow along the car.)
Car body "lift" - like an airplace wing, the air is flowing faster over the top of the car than the bottom, thus the "lift" of the car - a "lift" that MUST be minimized, but can't entirely be eliminated.
Car body back-end resistance (the "suck" of the air back in behind the car after the over-the-top and around the sides air flows try to re-combine.)
Airfoil resistance (wings and foils, acting exactly like an upside-down-airplane): These wings deliberately produce DOWNWARD force to squish the car down to the pavement and increasing its apparent weight on the tires. The airfoil drag is an unwelcome but necessary evil of the wing.
The "suck" of the car body DOWN induced by clever (and often secret) tunnels and flow paths from UNDER the car into the vacuum at the back of the car. These increase the air resistance somewhat, and dramatically increase the road resistance and suspension loads because they are several times the car body weight at high speeds! , but keep the car on the road and allow ,limited high-speed turning without flipping over.

Adding to these are the 2 front wheel air resistance loads (see above) and the two rear wheel air resistance loads - a little like the front wheels, but more complex because they interact with the body air flow, the rear vacuum being pulled along behind the car, and and the wind loads.

Wheel aerodynamic down forces are not very high compared to underbody "suck" forces (which are deliberately designed to be very, very high' and to wing and airfoil loads - again, designed to be low resistance but create great down forces. Wheel aerodynamic resistances must take a back seat to steering, shocks, and tire adhesion. Plus light weight, easy tire replacement, balance, and movement.

 

[DukeGlacia]

rocookpe1978

awesome. really solved all my doubts !!!

 

[Lou Scannon]

I second DG on the cogent and informative responses from racookpe.

In bicycles, a front air-foil fender catches the wind - cross-winds in particular - and makes steering more difficult at higher speeds. You'll tend to see bicycle fenders in back, and not in front because of this.

Perversely, a front fender, even though a smaller arc typically than a rear fender, does the majority of the job of keeping tire splash off of the bike and rider, especially when equipped with a decent mudflap, drag-inducing though it may be. I wager though, that the time lost to cleaning and otherwise properly maintaining a non-front-fender equipped bike more than offsets the shorter trip cumulative elapsed times, all other things being equal, and assuming riding in the rain is the standard mission profile. Not to mention cleaning and drying footwear and other gear.
Naturally, when one accepts the weight and drag of a front fender, it is a no-brainer to have a rear fender as well, especially since they're normally sold in pairs... .
Moral of the story for me is, I have fendered bikes for rain riding, and non-fendered bikes for the rest of the time.


"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

 

[KENAT]

"Car body "lift" - like an airplace wing, the air is flowing faster over the top of the car than the bottom, thus the "lift" of the car - a "lift" that MUST be minimized, but can't entirely be eliminated."

Racookpe, could you clarify this for me. The car body should only be lifting if the effective angle of the effective 'chord line' is positive (nose up).

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[racookpe1978]

could you clarify this for me. The car body should only be lifting if the effective angle of the effective 'chord line' is positive (nose up).

Well, that is the immediate effect you see when the nose of the car lifts even a tiny bit: The car flips over (more often earlier before the automatic dive brakes on the roof of NASCAR vehicles were made mandatory) as soon as the nose lifts up or the car reverses.

A normal streamlined car (say a Porshe or similar - definitely NOT your typical pickup truck) has a low nose, slightly sloped back from the vertical. The radiator inlet is recessed slightly, the front air dam is forcing air sideways, not allowing a lot to go under the car. The hood is nominally flat, but is slightly rising from the low curved nose back towards the bottom of the windshield, then a strong rise over the windshield then the generally flattened roofline, then the lower tail and trunk. Certainly not a pure airfoil lifting body (Cessna wing) but an approximation thereof.

End approximation is close that of a wing: sloped upper surface longer than the flat, straight airflow underneath and on both sides. The "lift" at most normal speeds is very low compared to the usual car weights, and as you point out, at normal car designs, the nose is pushed down slightly by the impact of the air getting pushed by the hood. The modern air damn reduce the air flow under the car as well, and today's underbodies are actually pretty well streamlined. Both the air dams and the nose design reduce the airfoil effect, but don't eliminate it entirely. But when race cars get "lifted" even a little bit? They go flying up.

Rolling Wheel air resistance: Look at a the new semi-trailer "V" air dams now found under the trailers. Those are specifically intended to reduce the relative air speed of air hitting the tops and fronts of the big wheels under the trailers.

 

[KENAT]

racookpe, thing is the upper surfaces being longer is not fundamentally what leads to lift on a wing airfoil/aerofoil or similar, before making this point I wanted to make sure I understood what you were saying. It's about the relative angle of incidence to the air flow - hence flat plates will generate lift & why stunt aircraft etc. can fly for prolonged periods inverted.

Done properly, nose down with flat underpan will create down force though whether you consider this true negative lift may be debatable - though that particular aero lecture was a long time ago and I won't claim to recal specifics.

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[gruntguru]

A few corrections to various points made so far.
1. The front and rear faces of a tyre at axle height are travelling at 45 deg to the vertical - down at the front and up at the rear. (Every point on the tyre is rotating about an instant centre located on the ground directly below the axle line.)
2. The additional drag of an "open wheel" is due to its shape (form-drag) not the fact that it is rotating. A tyre is quite a blunt object compared to a car body which encloses the wheels.
3. A car body with a half-sausage or tear-drop shape will experience lift even though the angle of attack is zero. Likewise an airfoil with flat bottom and curved top will produce lift at zero AOA.

je suis charlie

 

[KENAT]

gruntguru - 3. Hence my use of the term effective chord line, i.e. the chord line adjusted to allow for effect of camber. My point being that fundamentally lift is not directly related to the airfoil/aerofoil shape.

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[tbuelna]

The tires of F1 cars are not totally exposed to the oncoming airflow. The tires are shielded from much of the airflow by the front wing airfoil/endplate shapes and side bodywork at the rear tires.

 

[gruntguru]

True but my point was about rotation - not form-drag.

je suis charlie

 

[tbuelna]

Yeah, I got that. It's still impressive how much effort F1 designers put into details such as the airflow in and thru the wheels used to cool the brakes, wheel bearings, etc.

 

[GregLocock]

A nice (and amazingly cheap) overview of race car aero is Katz,

http://www.amazon.com/Race-Car-Aerodynamics-Engineering-Performance/dp/0837601428

In the 2005 edition Fig 6-23 shows the difference in flow over a rotating wheel and a stationary one, basically the rotating one has full separation at 12 o'clock, whereas the stationary one separates at about 2 o'clock.
Fig 6-24 shows the resulting pressure distributions, basically the rotating wheel has less drag and less lift, if I am reading it right.

Of course this is in the absence of any bodywork.

Most of the downforce on a modern open wheeler car is generated by the floorpan especially the venturi in the vicinity of the rear axle. The incremental L/D of the underfloor is about 10, whereas the incremental L/D of more wing is about 1 (!).

Peter Wright's smashing coffee table book on the Ferrari 2000 car

http://www.amazon.com/Ferrari-Formula-Under-Championship-Winning-F1-2000/dp/0768013410

has some nice data on its aero


Cheers

Greg Locock


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[racookpe1978]

On my Amazon Wish List now. Thank you.