I am just busy reading Wolfgang Langewiesche book "Stick & Rudder"
And he dismiss's Bernoulli's Theorem.
An excellent book, but he does not "dismiss" Bernoulli! He is making a different point. He actually says (my bold):
... Bernoulli's Theorem doesn't help you the least bit in flying.
While it is no doubt true, it usually merely serves to obscure..."
He then goes on to say that Lift is produced when the wing pushes the air down, which is fair enough for piloting purposes, just don't expect to calculate much that way, at least not on on the back of an envelope! He also talks a lot about angle of attack, which is obviously very important for pilot training, and also lets you do quite decent calculations.
The key issue is what the student wants from "Lift Theory".
Some example motivations:
- Understanding how lift is even possible, and why it seems to need airspeed and wings.
- Regurgitating what is needed to pass one particular exam.
- Understanding how to keep a particular aircraft type flying safely.
- Wanting to "design" an aircraft, for a Flight Simulator, perhaps.
- Wanting to design an aircraft that somebody could actually fly!
Each requires a hugely different level, and even area, of understanding.
In my view these "competing" theories e.g. "Bernoulli", "Newton", "Downwash", "Circulation Theory", are not competing at all, they all describe the effect in different ways, and their main difference is their ability to produce numerical results, with or without massive computational effort.
As a simple example of the cross-linkage, (and sticking resolutely to motivation #1), you could note that Lift does seem to require wings, which produce an upwards force, but are essentially horizontal plates in contact with just the air. You could further surmise that the pressure above the wing must be lower than the pressure below, there is no other force available.
There is no flow through the wing, but the above-atmospheric air below the wing will induce a flow downwards toward the atmospheric air below it. Similarly the sub-atmospheric air above the wing, will induce a downwards flow from the atmospheric air above it. Bingo! We have "Newton", conservation of momentum and "Downwash". There will also be some leakage around the wingtips, so you should expect trailing vortices. Unless you are near the ground...
Thinking about that pressure difference above and below the wing, you could also say that Bernoulli predicts that the airspeed above the wing must be higher than the airspeed below it. This means that from a ground observer's view point, (if he could somehow see the air), the air is mostly still, but as the aircraft passes, the air above the wing has a backwards ground speed, and the air below the wing has a forward ground speed. Viewed from the ground, the air appears to "circulate" around the wing as the wing moves forward. And that circulation joins up very neatly with those trailing vortices... Bingo! We arrive at the "Circulation Theory" of Lanchester and Prandtl, about one century old now. I say "arrive", because there is a lot more to it, but I will stop there because I'm only trying to show how this stuff all links up, and also it gets quite hard quite quickly.
