Inverted Flight

[+'ve ROC]

An engineer friend and I were having this conversation the other day and could not come up with a satisfactory answer.....

A 'normal' wing produces lift by generating lower pressure above it.

If we were to invert the wing, all other things being equal, we assumed it would produce a downward force similar to a rear wing on an F1 car.

If this is the case, how is it possible to fly inverted?

Forgive me if this is slightly 'lay-man,' but I'm not in the industry.

ROC

[Reverand Lovejoy]

+'ve ROC

Ok mate. Without boring you to tears this is quick roundup of how a wing produces lift.

There is only one lift producing process. It is made up of many parts, one of which you have touched on already. A wing produces lift because of:

1) It is flying at an angle of attack
2) Circulation
3) Bernoulli's principle
4) Newtons law of action and reaction.

All these parts contribute to lift.

The wing produces circulation in proportion to it's angle of attack and airspeed (not aerofoil cross section). Circulation means the air above the wing is moving faster.

THE FLOW DOESN'T MOVE FASTER ACROSS THE TOP OF THE WING BECAUSE IT HAS FURTHER TO GO!!!!! THIS IS A MISCONCEPTION AND THE TWO FLOWS DO NOT MEET AT THE TRAILING EDGE.

It travels faster across the top because of circulation. This in turn produces the low pressure in accordance with Bernoulli. The low pressure sucks up on the wing and pulls down on the air in accordance with all of the Newtonian Laws. This process as a whole is lift, but you must first start with two things......an angle of attack and airspeed. Take either of these two away and you have no lift.

Now think about an extra 300 or similar. It has a symetrical cross section and is fixed at an angle to the aircraft. This is the angle of incidence. This means that in level flight the aircraft already has an angle of attack relative to airflow. For this wing to produce lift inverted it must raise the pitch angle through the angle of incidence to make the angle of attack greater than 0 and be moving at enough airspeed to create circulation and the afore explained lift process.

Hope this hasn't confussed you too much. Throw some searches in and see what you get. But all in all for a wing to produce lift it must have airspeed and circulation, how you obtain these is down to wing design. If you have a very large leading edge and a thin trailing edge then you would need large angles of attack inverted to create the required angle of attack. Please don't shoot me down as I've only touched the surface and didn't want to confuse anyone.

Chow for now
The Reverand

[barit1]

You can think of it this way too:

Lots of airplanes have used symmetrical airfoils - My father instructed in the Vultee BT-13 during WWII and it flew rather well upside down. AOA is all that mattered.

A cambered airfoil optimizes lift in the "normal" direction - It can create some lift at zero AOA. But at some negative AOA value, the lift goes to zero, and a further negative AOA value means negative lift. That's what you need when inverted!

[Piltdown Man]

And obviously a non-symetrical can also develop lift when inverted. But this comes at a price. Firstly, a larger angle of attack when compared with operating the wing "the right way up", a higher stall speed (not forgetting that wash-out becomes wash-in) and a lot more drag.