The air wing....

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Ogami musashi
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Joined: 13 Jun 2007, 22:57

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Ciro: i still don't get it right with the air wing, and i explain why:

You can't use the action-reaction law on soft bodies like air molecules.
when molecules collides they don't follow the rigid collision's law, You can have basically three type of collision, one like rigids where the molecules bounce off with less energy than before the collision, bounce with same velocity and sometimes even with more energy than before their collision.
All of this is governed by their degrees of freedom far more than with rigid bodies.

This gives you, at large scale the point i talk about: if you apply a force on a elastic body (like a fluid) the body doesn't react with an equal opposite force, it simply changes his disposition, and the force applied is absorbed by collisions and friction between molecules.

The only fact a wing is pushed on the ground (or lifted in the air) is because a wing is mostly a rigid body and because air exerts pressure differential on it.
By himself the air does nothing else (except for a % of cross sectional drag that have a component toward the ground, for example when a flap is tilted at high angle for high downforce).

So an air wing would probably do nothing pressure wise (if we aim for downforce) on the air, nothing interesting for our purpose(their would be of course interactions and corruptions for sure).


About vortex, please be aware that the problem is their existence alone.
A vortex is beneficial when it comes helping the boundary layer staying laminar.

when you follow a car, even if the vortex is stable what happens? you have the vortex coming alone and nothing else, this means you don't have you boundary layer.

What is the problem really are vectors of the vortex. If vortex are created and stay stable in direction that goes just where it is useful okay, but we're then another problem: when you create vortex you necessarily divert air in a direction either this direction is of no use but doesn't decrease the efficiency either the opposite but from a pure aerodynamic point of view it is almost impossible to have a vortex that stay from one car to another in the same direction.

That's why active wings are planned, they could use more stable vortex (the vortex will be more stable because of new diffuser and rear wing design and less aggressive front wing)and by lowering the front wing will take benefit of it.



About elliptical, the lift repartition is good....when the AOA is not too big, i unfortunately saw in live what happened of a spitfire a very low speed and high AOA (may the pilot RIP).

By the way you're totally right about lift distribution on wings, recent fighter planes have down twist about the wing tip to reduce the lift in order to achieve better induced drag specs.

one thing however that differentiates planes from F1: the Yaw, F1 cars turn by yawing, not planes so you need to have very good downforce potential on wing tip when in yaw condition so twists are maybe not the best choice there.


See ya.

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MMUK
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Joined: 08 Apr 2007, 05:35

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Just so there is no further disinformation...

Shark skin is not rough to reduce skin friction. IN FACT ITS QUITE THE OPPOSITE. The roughness actually increases skin friction drag.

Shark skin is not rough in order to maintain a laminar boundary layer. IN FACT ITS QUITE THE OPPOSITE. The roughness and so called riblets actually encourage a turbulent boundary layer.

The shark skin is rough in order to ENCOURAGE a turbulent boundary layer. This turbulent boundary layer has a larger velocity gradient close to the wall. This larger velocity gradient means skin friction drag will increase. HOWEVER, because the boundary layer is turbulent it will seperate much later. This means pressure drag will decrease.

Turbulent boundary layers are a GOOD thing if pressure drag is the dominant drag mechanism.

Its the same reason golf balls are dimpled. The dimples increase skin friction drag but decrease pressure drag.

Straight from the horses mouth...
The scientists discovered that the dermal denticles help to control turbulence and keep water flowing past the shark’s body. They reduce the drag forces by helping to create a ‘turbulent boundary layer’; a thin layer of disturbed water that surrounds the shark as it swims along.
I've noticed that turbulent boundary layers get a 'bad name' on this forum. They are actually desired in many forms of aerodynamic design, especially bluff body design. LIKE IN F1!

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MMUK
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Joined: 08 Apr 2007, 05:35

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Also turbulent boundary layers have the possibility of reattaching once seperated, which laminar boundary layers could never achieve.

Ogami musashi
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Joined: 13 Jun 2007, 22:57

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I stand corrected for the shark skin thank you.

On the topic of turbulent layer, never said it was bad for efficiency, i actually said it was bad for the one that follows, and also, if you have laminar flow this is good both for skin and pressure drag.

Saying that turbulent layers are better is just as false as if i say laminar are better, it all depends on the turbulent conditions(especially true with body likeF1 cars) and the same for laminar.

Also you can mix turbulent and laminar layer I.E you can have a turbulent layer help the laminar one staying attached, still it the boundary layer is NOT turbulent.

Fighter planes use that system a lot.

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MMUK
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Joined: 08 Apr 2007, 05:35

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yeah, i was only saying turbulent boundary layers are better for situations where pressure drag is the dominant drag form. ie where delaying separation would be beneficial.

Laminar boundary layers are great where skin friction is the dominant drag mechanism.

So usually for really really streamlined objects you would want a laminar boundary layer because pressure drag will be small anyway. However for big ugly bluff bodies separation is going to be contributing more to drag than skin friction, so you'd probably want a turbulent drag layer.

Like you say, all depends on the situation. :)

Ogami musashi
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Joined: 13 Jun 2007, 22:57

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Yeah actually i think in practical, you're right, in planes (passenger and cargo) laminar flows are used and searched, because at high speed skin friction represent a large portion of total drag, while in fighters vortex lift is aimed because high AOA bring a lot of pressure drag.

Do you have any figures for repartition of drag for F1 cars?

Thank you.

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Rob W
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Joined: 18 Aug 2006, 03:28

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Why am I thinking of this?
[IMG:500:400]http://img301.imageshack.us/img301/4400 ... iseuj0.jpg[/img]


Rob W

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syguy
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Joined: 22 Feb 2007, 04:06
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Riblets do reduce skin friction

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I have to disagree with MMUK and say that riblets DO REDUCE skin friction on streamlined bodies such as yacht hulls and airplanes.

Riblets are small, of the order of 20 microns (20.0 e-6 m) in depth, 200 times smaller than the dimples on golf balls which are of the order of a millimeter (1.0 e-3 m) in depth. This difference in length scales produces fundamentally different effects within a boundary layer. The dimples on golf balls trigger a turbulent boundary layer in order to reduce pressure drag as MMUK correctly described. However, riblets appear to interact with small scale turbulence within a boundary layer to reduce skin friction.

A shark has no need of golf ball like dimples to reduce pressure drag, as it has a streamlined after-body that does not suffer from the inevitable separation behind a golf ball. A shark's main concern (as it is for passenger airplanes) is the reduction of skin friction and hence the use of specialized riblets.
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Ogami musashi
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Joined: 13 Jun 2007, 22:57

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But, do shark skin can be really compared to riblets?

However the fact a streamlined object has to minimize friction drag is correct to me.

That's the point, what is an f1? i think it is both streamlined and for some part, having a high cross sectional area.

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syguy
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Few modern passenger airplanes use laminar flow wings

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With respect to accuracy I want to address one more issue - very few, if any, modern passenger airplanes use laminar flow wings.

Although laminar boundary layers do have lower skin friction compared to turbulent boundary layers there are a number of disadvantages that make them impractical for high speed use. Modern passenger airplanes (e.g. Boeing and Airbus) fly relatively fast (~0.8 Mach Number) which means turbulent Reynolds Numbers over the majority of a wing and therefore turbulent boundary layers. Also laminar boundary layers are notoriously fickle requiring perfectly smooth surfaces, so dirt, flies, rain etc. can ruin the effect. Where laminar flow wings have succeeded on powered airplanes they use either blowing or suction (diverting power from the engine) to maintain laminar flow, negating some of its power saving benefits.
Symscape, Computer-Aided Engineering for all

Ogami musashi
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Joined: 13 Jun 2007, 22:57

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You're right, both airbus and boeing in fact use vortex lift.

I may have forgotten to say that laminar is a prospect for many new generation planes, and you're right, many employ suction or blowing, but not all.
The Quiet Supersonic Platforms bizjets employ all for all laminar flow without suction or blowing.

The most ambitious being the northrop QSP platform aimed to run at mach 2.4 with L/D of 11.

However boeing has launched another project of a flying wing with laminar flow with circulation wing (which involves blowing).

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MMUK
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Joined: 08 Apr 2007, 05:35

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Point noted syguy,

i guess i shouldnt have emphasised the shark skins effect on pressure drag and skin friction drag as you have quite rightly pointed out.

What i was trying to emphasise though was that the skin actually encourages turbulent eddies (albeit on a small scale as you pointed out) as opposed to maintaining laminar boundary layer.
http://www.noisemakers.org.uk/modules/a ... .cfm?id=11

On a side note, keep an eye out for the suits that swimmers and drome cyclists wear. The seams on those suits are very scientifically placed, with the aim of tripping the boundary layer. Something i incorrectly related to shark skin.

Would be an interesting one to see the components of drag that make up the total drag for an F1 car. Induced drag would play a large part considering the vast amounts of vortices underneath and around the car. Pressure drag would be high due to the differences in pressure between the large wake and stagnation regions. Skin friction would be the interesting one though, how much would that contribute to drag in percentage?

Ogami musashi
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Joined: 13 Jun 2007, 22:57

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I think the dominant parameter is for sure parasite drag.
The nature of an open wheeler is very prone to that.
I would say that parasite drag materialize as skin friction mostly and i would say that what triggers this is induced drag(for parasite drag coming from wings) and pressure drag(coming from non streamlined like wheels.
I bet the induced drag is the most present. Today even suspensions are streamlined, and air forward of the wheels is deviated by end plates.


On a side note i think syguy was saying that shark skin in fact do provide laminar flow by reducing small scale turbulence, but i may have not well understood.

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syguy
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MMUK you are right in that it appears the shark skin effect is tailored for turbulent boundary layers - same goes for riblets. So your point is valid that in the riblet case it needs a turbulent boundary layer to see a benefit. I'd guess if the flow over a shark could remain laminar it would experience less drag than the riblet case, but the Reynolds number of the flow over the shark is likely too high and hence its use of riblets to counter the higher turbulent boundary layer skin friction.

Ogami, for clarity I wasn't advocated that the shark skin produces laminar flow.
Symscape, Computer-Aided Engineering for all

The Nutty Professor
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Joined: 23 Aug 2007, 01:47
Location: Savannah, Georgia USA

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How many of you have watched or heard of the American show "Mythbusters"? I was driving to work behind a good old American Pickup truck and remembered a episode involving the theory that a lowered truck tailgate gave better gas mileage. Well their test proved that wrong and the reason was along the lines of our discussion. The air flow over the cab of the truck basically was in two layer's. The top and bottom layer's. The bottom layer flowed over the top of the cab and into the bed of the truck. As it reached the tailgate in was forced up and back in a rolling motion. The second layer was pushed up by the roiling layer and flowed out further behind the truck lowering the drag Co. With the tailgate down the tailend of the airflow was closer to the rear raising Co? But I still think that was a low tech brick in a windstorm test and the higher rates generated by a F1 vehicle would shred the rolling wing.
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