Multiple element wings

[dave kumar]

Well I don't know anything about aerodynamics, but I'm puzzled by these alternative explanations for lift - Newton's Third Law (equal and opposite reaction), Bernoulli's principle (pressure differences), turning moment...

Well if in doubt consult Wikipedia.
http://en.wikipedia.org/wiki/Lift_(forc ... ished_flow

There are several ways to explain how an airfoil generates lift. Some are more complicated or more mathematically rigorous than others; some have been shown to be incorrect. Explaining lift while considering all of the principles involved [Newton's laws of motion, conservation of mass, fluid stresses] is a complex task and is not easily simplified. As a result, there are numerous different explanations of lift with different levels of rigour and complexity. For example, there is an explanation based directly on Newton’s laws of motion and an explanation based on Bernoulli’s principle. Neither of these explanations is incorrect, but each appeals to a different audience.

We are dealing with flow at low Mach numbers (under the current regulations I can't see that changing) and so we can safely assume that flow viscosity and flow compressibility can be ignored.

In incompressible and inviscid flow the Bernoulli equation is just an integration of Newton's second law—in the form of the description of momentum evolution by the Euler equations—along a streamline.

Just an interesting aside I had a look at hypersonic (Mach 5+) fluid dynamics where the flow is compressible and flow viscosity becomes important considerations.

The waverider design was evolved from work done in the U.K. in the 1950's and early 1960's on winged atmosphere re-entry vehicles. Terence Nonweiler had first published work on the waverider concept in 1951, when he suggested the use of a waveriding wing shape for atmospheric re-entry vehicles. By the late 1950`s, Nonweiler, then at Queen`s University, Belfast, was working on the mathematics concerning basic 'wedge' flow for a manned re-entry vehicle developed by Armstrong-Whitworth Aircraft Ltd, to be launched off the nose of the British Blue Streak rocket (sadly cancelled).

To ease his calculations, he started by assuming a 2-D flow as seen from the side, i.e. no spillage. Nonweiler, while lamenting the fact that in 3-D, the underside flow would spill over the sides, causing cross-wise components of flow complicating his calculations, and causing loss of lift, decided to find a way of preventing the spillage.

He did this in order to keep his 3-D equations essentially 2-D, but realised that real hypersonic vehicles could utilise this principle, known as (shock) Wave-riding to improve their lifting performance.

http://www.aerospaceweb.org/design/wave ... ider.shtml for some more information for the curious.



Now there are some incorrect demonstrations of the generation of lift by an aerofoil in steady flow, for example using the Coanda Effect (which describes the tendency of a fluid jet to stay attached to an adjacent surface and is applicable to the blown wing explanation)

The way F1 cars generate downforce is by spoiling the air over the car slowing it down relative to the underside. To demonstrate hold a piece of paper vertically so that it is parallel to your chest, then blow across the surface nearest to you. Make sure that you don't blow directly on the surface of the paper but rather across it's surface. You will notice that the paper moves closer to your body, this is because the pressure on the opposite surface of the paper is higher, than on the side you are blowing across. The harder you blow the more the paper moves towards your chest.

http://en.wikipedia.org/wiki/Coand%C4%83_effect

More broadly, some consider the effect to include the tendency of any fluid boundary layer to adhere to a curved surface, not just the boundary layer accompanying a fluid jet. It is in this broader sense that the Coandă effect is used by some to explain lift. Jef Raskin, for example, describes a simple demonstration, using a straw to blow over the upper surface of a wing. The wing deflects upwards, thus supposedly demonstrating that the Coandă effect creates lift. This demonstration correctly demonstrates the Coandă effect as a fluid jet (the exhaust from a straw) adhering to a curved surface (the wing).

However, the upper surface in this flow is a complicated, vortex-laden mixing layer, while on the lower surface the flow is quiescent. The physics of this demonstration are very different from that of the general flow over the wing. The usage in this sense is encountered in some popular references on aerodynamics. In the aerodynamics field, the Coandă effect is commonly defined in the more limited sense above and viscosity is used to explain why the boundary layer attaches to the surface of a wing.

There are some nice diagrams on the Wikipedia webpage by the way.

Now if we want to establish the benefits of a multi-element wing then a few diagrams would not go amiss. If you can show some approximation of the streamlines http://en.wikipedia.org/wiki/Streamline ... _pathlines it will give an indication of flow speed of each streamtube and the Bernoulli's principle can be applied to guess at the pressures around each element. Then I might start to understand some of your explanations.

[marcush.]

in my understanding the angle of attack is determined by the line throgh leading and trailing edge of the wing assembly.If you want to increase the angle of attack with a fixed maximum height of the wing assembly nothing else than reducing chord length will help?(if that is useful remains another question).

[shelly]

@marcush: it is true that in a givene rectangle - side view of the legality box - only a certain angle would fit and exploit all the rectangle.
But you have to take into account also camber. From a certain point of view, it is the camber of the surface that forces acceleration and produces downforce.

A three element configuration with two slots allows for a more aggressive design camber wise, so that, even if the leading edge of the first element and the trailing edge of the last element are in the same position( corners of the side view rectangle) the tangency of the last trailing edge of a three-element solution is more vertical.

[segedunum]

@segenundum: I think previous posts have made clear not only that surface is not the point but also that even if it would be, three-elements wing have more surface than two-elements wing.

The collective sum of air together tends to be greater than the whole, which is something that hasn't been looked at. It's not simply a case of adding up the total surface area.

[Pierce89]

Pure, simple but wrong.

tell that to Gary Anderson at autosport and former TD of Jaguar. He says it all the time.

[Pierce89]

@segenundum: I think previous posts have made clear not only that surface is not the point but also that even if it would be, three-elements wing have more surface than two-elements wing.

I agree on the fact that prediction of the behaviour on track of a front wing is a big challenge, because it involves a lot of issues which can not effectively be tested in windtunnel or in cfd. I take two examples:
- even now it is very difficult to model the shape of the real wheel as it is deformed in action(braking, cornering)
-vortex stability is very difficult to predict

seg also agrees with Gary Anderson. wow!! looks like F1 engineers might know something.But, you all know better.

[shelly]

@Pierce89: don't get upset. Maybe you misunderstood Gary Anderson too.

[ringo]

I'm telling you guys!!
Multi element has more down-force. I'll leave it at that until the discussion gets a little deeper. I like to save the knock out blows for last.

The thing with multi element as well is that it has more than one angle of attack.
The trailing element of a 2 element wing may not get away with the same angle of attack as the 3rd element or a 3 element wing.
Someone mentioned it already, about the camber and so on. The angles of attack have to be taken individually.

I also think there is a confusion between multi plane wings and multi element wings. And i think the Jaguar engineer was misquoted on it.

A multi plane wing is this:



more than 2 planes doesn't make sense for the F1 regulations. The wing would need to be really tall to isolate the flows between the planes. I think this is what the jaguar TD was referring to. And some here confusing elements with planes.

Now this is multi element:


and more is better. As long as it's practical and realistic to construct. Count them, that's five elements including the slot.

[segedunum]

Whether anyone agrees with Gary Anderson or not is beside the point. Look at the way development has gone collectively. There are few teams using two element wings now and certainly no one who wants to be at the front is using them - and it's beyond raw downforce. It's about rapid turn-in, stability under braking, 'driveability'.....

Shelly is right about how complex this is though. You've got pitch, yaw, braking, the effect of the tyres........ You can't CFD or even windtunnel test a lot of that stuff. It's really a lot of brain power, trial and error over tests and on Fridays before races. That's why I always get intirgued when I see new shape or leading edge changes on any part of a car.

[ringo]

That probably explains why with the Honda money gone, Mercedes are scared to change the wing.

[marekk]

Agreed that the main plane is now usually a multi-element setup, but there is also the second upper plane. I was reading a bit about bi-planes, and the relative positioning of the upper and lower planes can make a difference to the aero of the combined system.

If vertical distance between wings is comparable to wing span, they'll influence each other's flow - so clearly yes, F1 front wing is sort of bi-plane. Wich makes it even more complex to design and evaluate.

[godlameroso]

The way F1 cars generate downforce is by spoiling the air over the car slowing it down relative to the underside. To demonstrate hold a piece of paper vertically so that it is parallel to your chest, then blow across the surface nearest to you. Make sure that you don't blow directly on the surface of the paper but rather across it's surface. You will notice that the paper moves closer to your body, this is because the pressure on the opposite surface of the paper is higher, than on the side you are blowing across. The harder you blow the more the paper moves towards your chest.

I never realized how simple an F1 car really is...it's just like a piece of paper! Glad we finally put to rest all this nonsense about Newton, static pressure, and turning flow.

A knife is also simple, using it isn't, likewise aero is simple, using it to your advantage in the face of the huge compromises plagued by F1 engineering...not so simple.

Amazing how I get attacked for simply illustrating something...meh.

[shelly]

Don't get pissed off godlameroso.

Your statement about f1 generating downforce by spoiling upper flow and smoothening underbody floor is far too simplistic so it has been rejected, but it is not an attack towards you.

The point is that aerodynamics is not simple.

[godlameroso]

hmm it's not smooth airflow underneath the car that generates negative lift, it's the speed with which the air is moved across the underside relative to the airflow over the top of the car.

Regarding multi element wings, if you notice carefully, the first element closest to the ground has the highest surface area, but also the lowest AOA, and each succeeding element has a lower surface area, albeit with a higher AOA. This is at least true with the fastest cars in testing so far. The same is true of the rear wing. Now why would you want to do this? A clue might lie in the design of the 2008 cars.

The diffuser was more tucked into the chassis, along with the lower and wider rear wing. The effect of this was that the wake of the wing and the diffuser were positioned so as to combine their effects, and further facilitate the evacuation of the air underneath the vehicle. This was also true with DDF cars in 2009 and most cars in 2010, albeit to a lesser degree due to the higher rear wing.

Aiming the air as an airfoil does in fact create lift to a certain extent, although the majority of the lift is not caused by the direction the air is moving but rather the differential of pressure. Any way you slice it, any theory that you subscribe to the same process happens, the air sheds more energy on one side of the wing than the other, and if you can keep the air from separating the effect is magnified.

Another example would be a turbo charger, the exhaust pulses push the turbine blades, although only to a certain extent. The main driving force of the turbine is the exhaust pulses expanding into the turbine chamber and the resulting change in pressure. That is why exhaust manifolds on turbo cars tend to be shorter than on NA vehicles(well packaging has something to do with it as well), in order to preserve the kinetic energy as well as pressure of the exhaust pulses. Also, it's the reason why a large diameter exhaust (around 8cm diameter) reduces turbo lag.

My final example is a bit bigger, a hurricane. Why are the strongest winds right around the eye of the storm? Because that's where the biggest difference in pressure exists.

In conclusion aerodynamics is simply the manipulation of pressure differentials across surfaces.

PS I don't get pissed, maybe a little confused as to the sarcastic comments, but not pissed.

[horse]

Regarding multi element wings, if you notice carefully, the first element closest to the ground has the highest surface area, but also the lowest AOA, and each succeeding element has a lower surface area, albeit with a higher AOA. This is at least true with the fastest cars in testing so far. The same is true of the rear wing. Now why would you want to do this? A clue might lie in the design of the 2008 cars.

This is simply the design of a multi-element wing. Multi-element wings provide higher lift coefficients at a fixed angle of attack than single element wings. For a reference, see:

The aerodynamic design of multi-element high-lift systems for transport airplanes