Originally Posted by
chornedsnorkack
True for a cambered airfoil.
For a symmetrical airfoil, at 0 degrees AOA, the air passes over top and bottom surface at equal speeds, so lift is 0 by symmetry. For an asymmetrical airfoil, lift does not have to be zero at exactly 0 degrees AoA - and for cambered airfoils, it is positive. However, for every cambered airfoil, there is an angle of attack where lift does equal zero, this being less than 360 degrees - but normally much closer to 360 than 270 degrees. Anyway, AOA of cambered airfoil is often defined so as to be 0 at zero lift... (though it seems to me that such definition cannot be independent of Mach).
And at the same time, a high pressure area forms under the wing, pushing the a/c into the air.
But there is still the high pressure area under wing. The only time the engine thrust remains the only force counteracting weight is when the AoA is about 90 degrees, so that the lift is directed horizontally.
The air hitting the actual underside of the wing is still squashed against the underside and forms high pressure area. Which is what you want under the wing.
Hi there-
Your thoughts describe a portion of newtonian lift, but a symetrical airfoil at high altitude at zero angle of attack in equilibrium is not likely
The book, "Fly the wing", in the high speed, high altitude section of the book, really explains the physics of high speed flight in a straight forward, yet more detailed manner than many sources.
Studi has it right for high altitude flight.
I suspect part of the original question asks the following clarification that has already been offered: For any speed flight , low or high, the wing will ALWAYS enter a traditional stall at the critical angle of attack at lower and mid altitudes.
At high altitude, the "perception" of an aerodynamic wing stall is blurred slightly by a few factors. First, a relatively "high" angle of attack, but not necessarily the "critical" angle of attack, may force the air over the wing to reach such a speed where sufficient shock waves form on the upper surface of the wing to seperate the local airflow. The significant buffeting, change of center of pressure, shift in downwash angle and lift distrubution change.. they closely give you the "indications" of a traditional, immenent stall. The difference in this case is that a shock wave is disrupting the airflow at a angle of attack that is often below the critical angle of attack.
The above scenario occurs at a low envelope speed, meaning it is the "low speed buffet" of the high altitude envelope. This is from the aircraft being too heavy for the altitude - and the required lift needed by the wing (usually higher in the root area where airfoil shape is typically the most dramatically cambered) accelerates the air to a supersonic speed in a more dramatic way than typical. (local, small areas of mach flow normally exists at normal cruise flight but here we have a much greater area of supersonic flow and widespread seperation).
The high speed buffet is where the aircraft reaches buffeting, generally, more consistantly from shock waves all over the airframe, like the tail, the fuselage, the wings, etc. This is from the aircraft simply flying too fast for the aerodynamic design.
The sources that speak about this subject in depth go into the shock wave induced stall buffet in detail. however, the majority of sources that do not break down the "whys and hows" simply call it a high altitude stall buffet.
