Wing design question - funny sticky-out bit on trailing edge

[PaulPerkins]

Hi All,

I'm not an aero expert in any way and I have done absolutely no research about my question before posting it, but my mind has been puzzling over this one...

I was looking at a front F3 wing at the Oulton meeting and I noticed that on the adjustable section of the front wing (the bit on eith side of the nosecone, just in front of each front wheel) is what appears to be a small slither of something like 2mm mild steel riveted to the trailing edge.

The section is then turned upwards so that the very rear edge of the wing has a, perhaps, 5mm protrusion at the normal to the plane of the adjustable section.

On further inspection, I also found this to be common on the rear edge of some wings.

Now, I understand the principle behind how wings work (faster air over the bottom , causing lower presure than above, hence downforce), but this section (on the front wing in particular) puzzles me in that surely it will make the air flowing over the front wheel more turbulent.

Any thoughts?

Thanks for any enlightenment,

Paul.

[Onlooker]

It is called a Gurney lip or Wicker strip.It increases downforce but I am unsure excatly how it works

[PaulPerkins]

Thanks, Onlooker.

So I guess it is exaggerating the profile of the wing right at the very edge by causing an extreme change in airflow.

Still, I would have thought that this would cause more turbulence?

[Warwick]

http://www.insideracingtechnology.com/techstart.htm

if you run a high wing angle the airflow seperates from underneath the wing the Gurney flap helps reattach it,I would think it would only be used on a high downforce track.

[PaulPerkins]

Wow! That's fantastic! Thanks, Warwick!

[CFD-9]

I wrote a paper on 'Gurney flaps' some time ago. I did a lot of CFD analysis in Fluent on the effect of Gurney flaps in ground effect and their effect on the velocity and pressure distributions over a wing surface.

It would be fair to say that we are still not 100% sure why the Gurney works so efficiently. You are correct to say that it does create more turbulence and so increase drag but the lift/drag ratio is improved vastly and for this reason you will see it used in some shape or form on most competition wings.

There is a low pressure region created in the 'wake' behind the flap and this does reduce the wing's chance of stalling, as was mentioned earlier...

I doubt I will be able to find any straight away, but when I can I will post some CFD pictures of the pressure profiles over wings with and without Gurneys, in and out of ground effect.

[PaulPerkins]

Thanks, CFD-9 - that would be excellent.

[Warwick]

CFD-9 I would be interested in seeing that as well.

I was under the impression they were only used at high attack angles or do they use a different size for a lower angle?

[CFD-9]

There is an optimum size for the Gurney flap depending on the wing profile (shape)... This is between 1% and 10% of chord - typically 2% of chord is a good guessing point, but this should be optimised in CFD or a wind tunnel!

I messed around with NACA 0012 section wings at a 0° angle of attack - these are completely symmetrical about the chord line, so should not produce lift... In ground effect and with a Gurney, the wing can actually be made to produce a fair amount of downforce.

In this case - the Gurney cannot really be considered an extension to the profile as it is a very suuden 90° change in wing profile... What I am saying is - a lower drag section witha Gurney can be made to produce as much lift as a higher drag section without a Gurney - in other words the Gurney improves the L/D ratio.

[Garp]

Now i'm puzzled about this subject.
If the gurney makes the wing more efficent (improved L/D ratio) why don't planes use a gurney. As far as i know the guerney is used because the regulations decide the wing area. What is the importance of angle of attack in this subject? Is the gurney still L/D efficient on low angles of attack?

[MulsanneMike]

Aircraft operate in a different "regulations" regime in that they are allowed active aero. bits (moveable slats and flaps) and fixed doo-dads like gurneys are superceeded by these bits.

[CFD-9]

Mike is right but.... Some planes do use Gurney's!

Modern commercial aircraft have the benefit of moving flaps, so don't require them!

[GTV27]

Originally posted by CFD-9
Mike is right but.... Some planes do use Gurney's!

Modern commercial aircraft have the benefit of moving flaps, so don't require them!

How different would motorsport be now if in the sixties, instead of responding to breakages by outlawing moveable surfaces and unspring mountings, they just required better design and let the innovation take its course.

Even the most advanced road cars of today are only now getting into some limited forms of active aero - imagine where we'd be if development had of been accelerated by the competitive pressure of racing over that period.

sigh

[CFD-9]

Well said GTV!!!

Active control has always been and will always be the way forward! We would be a lot further than we are now with road cars if they had allowed Group C to continue and kept the F1 regs a lot less strict!

I did a bit of work with suction on wings to take off the boundary layer etc and the benefits you can get are huge!

If they wanted to slow cars down... Cut back on engine size - but thats prob a topic for another forum!

[Garp]

As far as i know, flaps and slats create a bigger normal wing shape. Nothing like the effect of a gurney.

If the L/D is improved by using a gurney on any wing (at any speed). Why don't you always use a gurney (like, airplane at cruise speed).

[MulsanneMike]

If you can use slats and flaps to generate even higher L/Ds...hence they superceed gurneys.

[KC]

One of the beauties of the Gurney Flap or Wickerbill (in dirt racing circles) is that they can be changed easily for wider or narrower without adjusting the entire wing. The IRL uses mandated Gurney Flaps on their rear wings to tailor downforce and drag for specific tracks. In sprint cars changing the wing angle makes a drastic change in drag because of the area of the wing and so they use wickerbills for fine tuning.

[avsfan733]

bingo kc...and if you watch CART pitstops, they sometimes change them for exactly the same reason during the race.

[Garp]

Flaps/slats on airplanes have a bad influence on the L/D ratio.
When they are used (while landing) drag is not an issue, only lift is
.
If Gurneys are used for their ease of change in dirt racing, ok. But in F1 there is enough money to develope a nice wing style ad-on that can be changed just as easily as a gurney.

[Bob Pearson]

Does anyone have any views on whether gurneys have a greater percentage increase on downforce at medium speeds than at higher speeds or the other way round.

[CFD-9]

It depends on the separation point on the wing's surface and therefore flow regime (laminar/turbulent) etc etc. The Gurney should different heights depending on these factors, but since you can't have a moving aero surface... Flaps and slats can alter the L/D detrimentally but... The main reason for using slats is to allow you to use a wing with a higher angle of attack without fear of separation and therefore losing lift. Think of the angles a planes wing moves through during takeoff and landing!!! They must cope with massive changes, a car's wing does not move relatively!! They are different things - a car wing does not need slats if it is producing enough lift at a given angle of attack - plus, in F1 we have a number of elements rule plus the fact we are not allowed moving aero surfaces! Don't get too hung up on aircraft wings - its a whole different ballgame! The Gurney is awesome for easy adjustment, each circuit effectively requires a wing with certain characteristics - the Gurney is the cheap and effective way round this!

Thanks, D

[Smokey 6 litre]

For my A2 Tech project im building a rear wing for a road car, i was wondering that because i assume the centre of the wing would recive less/slower/more disturbed airflow would a Gurney be a good idea for the part of the wing where there is this area of less/slower/more disturbed airflow?

[gttouring]

Hmmm...What excacly happens to a Wing when it stalls? particularly on a car, and I believe active aero is seen on many road going models namely the porsche boxster and 911 families and the Audi TT (and VW new Beetle turbo) as they have that little flap that moves up or the what looks like a gurney flap at right angles to another gurney flap grow out the back of the latter two.

[CFD-9]

When a wing stalls the air flowing over the surface effectively separates from the wing so that it becomes unattached flow... Basically the air no longer follows the shape of the wing which means it cannot generate lift. The Boxster et al 'wings' are spoilers and not wings at all really! They are just there to reduce lift at speed by deliberately upsetting airflow over a lift generating shape...

[gttouring]

ah good bit
Ta
and while we are on topic...Splitters and diffusers, underbody tunnels and such- what the hay?