At 11:16 PM 7/5/04 -0400, you wrote: >So, does the critical angle of attack change or not? > >Which is it, "This angle is known as the critical angle of attack, and this >does not change for a particular wing." or "That changes the critical >angle of attack for that wing loading, referred to as an accelerated >maneuvering stall. "? (Colin Rainey) > >Ken Jones +++++++++++++++++++++++++++++++++++++++++++++++++++++
There is a key phrase that is being left out of this discussion that will help all to understand better. That phrase is "while maintaining altitude". Let's start a square one. A wing will only produce lift at an angle 90 degrees to it's span or straight up when the wing is level. It will also produce a given amount of lift depending on the speed through air and it's angle of attack (cord line to oncoming air). The air traveling across the top of the wing will always separate at a given angle or as we say, stall. If we bank the aircraft we take some amount of our vertical lift and vector it in a horizontal plane. This causes the aircraft to turn. At some point we vector enough vertical lift to the horizontal that the aircraft starts to lose altitude. As a pilot we increase the pitch angle (angle of attack) to create more lift to hold altitude. The additional lift creates more drag and the airspeed drops. We add more power to overcome the increased drag. As the bank angle increases the forces of the turn keep adding additional down forces that the wing lift must offset. To keep adding lift we can only add speed or increase the angle of attack. They each have their limit. The "angle of attack" is limited by when the air will separate on the top surface. The "speed" is limited by the wing structure and when the air loads would cause it to fail. I can put my wing at a bank angle of 90 degrees and low airspeed and not stall the wing if I keep the A of A below the speed where the air separates on the top surface. I can't maintain altitude with that bank angle but the wing won't stall. I'm sure you've all seen an airplane do "knife edge" flight. The wing is not stalled because it's being kept at or near a "zero" lift angle and the aircraft is kept in the air by lift generated by the fuselage and engine thrust and stored energy of speed. The reason you see a higher indicated airspeed at stall in a high banked turn is because you started at an airspeed higher than the stall speed for that bank angle " while trying to maintain altitude". The A of A at which it stalls is still the same. You reached that angle at a higher airspeed because you were trying to "maintain altitude". If I roll in to that same bank angle at five miles per hour indicated above level stall speed "and try to maintain altitude" I would get a stall immediatly but at a much lower indicated airspeed. The wing still stalled at the same A of A. Thus the advantage of the A of A indicator. I can fly every approach at the same A of A. My airspeed may be higher or lower depending on the aircraft weight on that flight and I can adjust rate of decent with power. The key is I won't be flying my approach at a fixed airspeed so when I'm really heavy I increase the A of A to generate enough lift at that airspeed and put the wing at near the stall A of A. In that situation if I increase my bank angle "and try to maintain altitude" by increasing the A of A of the wing it stalls, the ground rushes up to grab your a** and someone is left to pick up the rubble. Bottom line: The wing always stalls at the same A of A. One of my favorite maneuvers in the KR is what I call a wingover. I do a low G pullup into a rather steep climb. As the airspeed slows to about 80 mph indicated I roll into a near 90 degree bank. I release all back pressure on the stick so I don't feel any weight on the seat and let the nose fall through. I do a low G pullup from the dive and I've done a 180 degree turn. Some call this a "cotton patch" or "duster" turn. That's basicly how they do those fast turn-arounds without stalling when they are low and slow. They use stored energy of speed to keep them in the air and unload the wing in the turn. Once back to straight and level they increase the A of A to generate the lift they need to check their decent rate and return to level flight. If they don't have sufficent speed in the dive that A of A will be greater then the stall angle and the ground will reach up and grab their a** too. THE END Larry Flesner
