------------- Which is it? Redirecting flow leading to negative pressure? Or creating less pressure over AND greater pressure under? --------------

Both. All of the characteristics of the airfoil that we think about happen at the same time.

Think of the 3 blind men telling about the elephant by just touching one part of it. One touched the trunk and said the elephant was like a large snake, one the tail and said it was like a rope, one the leg and said it was like a tree. Don't think that way.

The rate of increase of negative pressure is probably greater than the rate of increase of positive pressures but that depends on a host of configuration variables and there are probably lots of exceptions.

---------- So tell me, if lift=downwash in a propeller, what's the difference between a propeller and a wing? Isn't a propeller blade a wing that's traveling in a circle? I think it is, but I could be wrong. -----------

Did I leave that out? There is no difference except that a particular airfoil is optimized for the intended purpose. A prop is a just a wing going in little circles, a lot like some of us. It becomes most obvious when you consider the ultra light indoor microfilm covered models. The rpm is about 30-40-50 or so and the props can easily be thought of as lifting the airplane forward rather than pushing. We tend to feel the propwash and think of that as pushing forward.

--------------- As far as turbine engine planes having control lag, I'm not speaking of lag due to tubbine planes having smaller control surfaces relative to the plane. You could take two P-51 Mustangs, one with a prop in the front, another with a turbine thrusting out the back, and the one with the prop would have better maneuverability at slow speeds because the prop is blowing air over the control surfaces. The control surfaces of the turbine-powered airplane, however, are dependent on the speed of the airplane to provide air passing over them at adequate speed for deflection. An F-18, whose elevators and rudders are well in front of the rear "nozzles", doesn't have jet wash going over its control surfaces. Does it? --------------

Turbine planes don't necessarily have smaller control surfaces than prop planes. The F-15, -16, -18 etc. all have all moving tails. It is hard to get bigger than that.

Toward the end of the last writing I mentioned that with jetwash or propwash the dynamic pressure over the surfaces increase and so does the control effectiveness. Blowing air over a surface is doing that, the effectiveness does increase. A prudent designer does not depend on that because the motor might quit and you would still have to control the airplane. With extreme control throws it becomes more like a true deflection. It is a gradual variation from one mode to another with deflection angle increases.

There is jet induced flow around the F-18 tail surfaces but the magnitude is pretty small. It is however strong enough that when we did low speed wind tunnel tests (I worked at McDonnell Douglas St. Louis) that duplicating the jet with a piped in flow of air scaled to give the proper mass flow characteristics of the engines is given a lot of attention and test time. When trying to put the airplane on a deck you take all the help you can get.

-------------- If everything I've said above is correct, then we can come to the conclusion that a propeller, which is essentially a cambered wing, produces foreward motion by 1. increased pressure "underneath" the blade, and 2. wash. -------------------

No, you are telling about that part of the elephant that you want to see.

The blades (which are airfoils) rotate, the pressure in front of the blade drops and at the same time the pressure underneath increases and at the same time propwash (downwash) is generated. All at the same time, all 3. Incidently the work put into the prop by the motor should be close to the acceleration given the air by the prop when including friction loses. The forces from the pressures are carried through to the prop hub which accelerates the airplane etc. But the main thing is that the action of negative pressure, positive pressure and downwash are all simultaneously happening. It is just the way a wing works. There are some nice math that points this out and says things like vortex and circulation and bound things but at the end of the math it says that it all happens at the same time.

-------------Consider this as well: A propeller with a greater pitch is harder for the engine to spin at a given RPM than the same size propeller with a lesser pitch. But, when spun at the same RPMs, the higher-pitched prop will provide more thrust, regardless of the shape of the prop. Agreed? Similarly, an airfoil with a greater angle of attack is harder to push through the air than one with a lesser angle of attack, yet, if pushed through the air at the same speed, the greater angle of attack will produce more lift (until stall occurs, of course), REGARDLESS of the shape of the airfoil. Agreed? ------------

Yes, this is a fairly reasonably fundamental fact of aerodynamics though. Be careful of the use of the statement, "REGARDLESS of the shape of the airfoil." If you stall the airfoil with too much angle of attack it will lose lift.

That reminds me, all the discussions have been about an airfoil/wing at an unstalled, in the linear range of lift curve slope, condition. Stalling is a good indication of the relative importance of upper vs lower pressures in wing design. When a wing stalls the lower pressures don't change (for most practical purposes), the upper negative pressures just die and down comes the airplane.

--------------So, would you say that I've successfully and accurately compared a propeller to a wing?-------------

Yes for the most part but no prizes. This has been known since the Wright Brothers (very smart fellows) put props on their airplanes.