bsergonomics
Posts: 458
Joined: Thu Jan 31, 2002 5:07 am

Inverted Wing In Ground Effect

Thu Mar 14, 2002 8:20 pm

I have been asked to apply my aviation knowledge to a racing car. However, I have come across a particular issue that raises the following question:


While I am very familiar with the Wing In Ground Effect, I have always considered the wing to be the "right" way up. Since we are using inverted wings for the car, does anyone have any idea of what the result of this phenomenon will be? More/less lift (i.e., downforce)? Do you know of any links where I might find this information?

I would be extremely grateful for any information you could provide.
The definition of a 'Pessimist': an Optimist with experience...
 
FredT
Posts: 2166
Joined: Thu Feb 07, 2002 9:51 pm

RE: Inverted Wing In Ground Effect

Thu Mar 14, 2002 9:45 pm

At the forums associated with the racing simulator Grand Prix Legends, a few people who will certainly be able to point you to a few good references on race car aerodynamics hang out.

It's rather interesting!

Cheers,
Fred
I thought I was doing good trying to avoid those airport hotels... and look at me now.
 
lehpron
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RE: Inverted Wing In Ground Effect

Fri Mar 15, 2002 3:08 am

In other words, yes, there is a downward force, pushing down the back of the car while the hood pushes the front. It is prefered if one wishs to not go off course, painfully. Big grin

As of links, try looking up general aerodynamics of airfoils/aerofoils. I'll try to find some, I'm sure others could help, hope I did.
The meaning of life is curiosity; we were put on this planet to explore opportunities.
 
wingscrubber
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RE: Inverted Wing In Ground Effect

Fri Mar 15, 2002 4:27 am

Wing in ground affect is caused by the high pressure air beneath a wing being buffeted off the ground/sea, or reflected if you like, causing greater pressure beneath the wing, and subsequently more lift, on a racing car wing, the high pressure is on the top surface, not the bottom, pushing the wheels onto the ground for better traction, so there is no equivalent wing in ground affect.

Pete
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zionstrat
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RE: Inverted Wing In Ground Effect

Fri Mar 15, 2002 5:22 am

Wingscrubber-
Your description certainly helped me- I've always wanted an analogy to explain ground effect, and had never thought of 'reflected'. Thanks for a good mental picture.
 
FredT
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RE: Inverted Wing In Ground Effect

Fri Mar 15, 2002 5:47 am

Wingscrubber,
the cushion of "trapped" air underneath the wing contributes only a small part of the lift increase/drag reduction for a wing in ground effect. Most of the ground effect comes from the reduced upwash.

Most of the downforce on a race car comes from a low pressure underneath the car. That is why they have a flat bottom panel and skirts. Have air pushed in through a narrow gap underneath the front spoiler, then let the ground clearance increase towards the rear. Voila, divergent nozzle, accelerating air and decreasing static pressure.

Cheers,
Fred
I thought I was doing good trying to avoid those airport hotels... and look at me now.
 
FredT
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RE: Inverted Wing In Ground Effect

Fri Mar 15, 2002 7:11 am

Must not have had my head screwed on tight for that one.  Big grin Unless you're reaching supersonic speeds, what I said isn't true - air will slow down rather than accelerate in a divergent nozzle. Must have been thinking too much about fast movers as of lately.

Low ground clearance -> higher airspeed under the car as the design of the car forces air through the gap under the car -> lower static pressure -> downforce.  Smile

Cheers,
Fred
I thought I was doing good trying to avoid those airport hotels... and look at me now.
 
prebennorholm
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RE: Inverted Wing In Ground Effect

Fri Mar 15, 2002 7:48 am

Ground effect is a very serious issue for race cars. But then we are not dealing with the inverted wings, or spoilers. We are taking about the bottom of the car.

In my company, which is a major Ferrari sponsor, we had a couple of months ago a Ferrari F1 car parked in the launch room, and a Ferrari technician came with it in order to answer questions. (Yeah, we just had to celebrate that our fuel and lubricants had powered the victories).

The outmost is done aerodynamically to get the lowest possible air pressure under the car in order to get the heaviest possible load on the wheels.

Ferrari actually has two very buzy wind tunnels to develope the very best aerodynamic shapes of everything on the car.

The cooling radiators are the most important thing in this respect. It is of outmost importance to get the most air through them at the lowest possible drag.

It is an entirely different thing to stand next to such a car instead of watching it on TV - not to mention to sit in it.

First of all the car is a lot smaller than we imagine. But then there are so many small aerodynamic details - too small to be seen on TV. "Our" car came without engine and gear box, and two men easily lifted it and rolled it 90 degrees on its side when passing an ordinary door.

I asked the technician about some one inch wide strips on the side of the car which seemed to have no apparent function. He said that they were spoilers which were there to deal with some turbulences from the front wheels before the air hit the radiator intakes.

Other spoiler strips were there to manage airflow to cool the brakes.

We also had a company wide competition about which three men (women) team could change a wheel the fastest. I think the winning team used just over 7 seconds. You can't imagige how leightweight such a F1 wheel is. I asked the technician how rubber could be so leight, was it filled with helium? "Rubber?" he said. "Call that rubber again, there is very little rubber on that tire", he said. When asked what else it was made of he refused to reply. My guess is that aramid fibres (Kevlar) is a major component in those tires.

Regards, Preben Norholm
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bsergonomics
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RE: Inverted Wing In Ground Effect

Fri Mar 15, 2002 7:00 pm

Thanks for the tips. I know about the effects of reducing air pressure below the car. Unfortunately, this is a rally car (a VERY special rally car) where the front splitter (and general ground clearance) has to be quite high in order to avoid ground strikes.

We have a newly developed system under the car to reduce air pressure, but we also have a very large wing on the top and we are thinking about a front-mounted wing as well.

To complicate matters, the race starts at 9000 ft AMSL and finishes at 14000 ft, so the aerodynamics are not the typical ones found in the racing world.

The two major points that I am trying to work out are:

1. Will a wing located approximately 15cm (8 inches) above the ground at the front of the car create a pressure wave in front and at the bottom of the car, thus creating more drag?

2. Any ideas where I can find the formula for wing stalling speeds at different altitudes (assuming the same AoA)?

Thanks again for your thoughts so far and I will be eternally grateful for any further info.
The definition of a 'Pessimist': an Optimist with experience...
 
FredT
Posts: 2166
Joined: Thu Feb 07, 2002 9:51 pm

RE: Inverted Wing In Ground Effect

Fri Mar 15, 2002 7:28 pm

Haha, I know that race! Certified nutcases, all contestants - but certified nutcases whom I envy! Big grin

Wings stall at a given angle of attack, regardless of speed. The Reynolds number can have a small effect and will change with altitude but it isn't significant at all in this case.

I don't think that wing will give you much of a benefit. Even if the wing does create some upwash (inverted downwash), the air would have had to be deflected over the hood of the car anyway creating the same downforce.

I'd be interested in hearing more about the system under the car.

Cheers,
Fred
I thought I was doing good trying to avoid those airport hotels... and look at me now.

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