# How does an inverted aerofoil work?

1. Jul 14, 2013

### threy

I understand that high velocity will create a region of low pressure and a net upward force will lift up the aerofoil. And does it mean that when the plane is about to board, the aerofoil will be inverted so that the higher pressure will push the plane down? How does inverted aerofoil work as spoilers on racing cars? Which region has low pressure?

2. Jul 14, 2013

### rcgldr

Absent external forces, a parcel of air will accelerate from a higher pressure area to a lower pressure area. As that parcel of air accelerates towards the low pressure area, it's velocity increases and it's pressure decreases in accordance with Bernoulli principle. Maintaining that pressure differential will require some type of external force, which goes beyond Bernoulli.

For a given angle of attack, an aerofoil diverts the relative air flow (air flow relative to the aerofoil). If it diverts the relative air flow downwards, the resulting lift is upwards (and vice versa in the case of wings used on Formula 1 type race cars).

3. Jul 14, 2013

### cjl

A spoiler on a racing car is just an inverted airfoil (and it makes a low pressure region on the bottom surface which helps push the car into the ground for better grip). As for your question about aircraft though, I don't understand what you're trying to say. Specifically, this part:

Ordinary aircraft never invert the airfoil - even as they are landing or taxiing, they are making positive lift and the airfoil is in its normal orientation. They do have a variety of methods to control how much lift they make (spoilers to decrease lift, flaps to increase lift, plus they can change the orientation of the entire aircraft relative to the oncoming flow once they are in flight), but the airfoil itself is never inverted.

4. Jul 14, 2013

### rcgldr

Note the type of "spoiler" used by Nascar type race cars, which is an upwards angled flap at the back of a race car, just reduces lift and/or creates some downforce by deflecting air upwards, but since it's attached directly to the "trunk" of a race car, it doesn't produce a low pressure area below the car. Instead at the front, an air dam and/or "splitter" produce low pressure under the front portion of a race car, and at the back, a "diffuser" is used to maintain low pressure under the rear portion of a car.

Other types of race cars use inverted wings to produce downforce (and also "diffusers"). Indy Race cars also use under body tunneling to further reduce pressure under the car.