In last month's WING TIPS, I ended the "Downwind Turn" article with mention of AOA or Angle Of Attack. What is AOA? Briefly stated, AOA is the angle between the aircraft Wing and it's Relative Wind. In flight, the wing will have an acute angle (angled slightly upward) to the relative wind and this Angle Of Attack is a factor in a wing generating LIFT. Of course Lift generation is more complex than just AOA, (not to worry, I won't go there). For a given steady airspeed, an AOA increase results in a lift increase and if AOA decreases - you guessed it, lift also decreases. Now think of the AOA remaining constant - say a steady 5 degrees, what will happen to the amount of lift if airspeed increases? Right again - lift increases. So in order to maintain the same amount of lift as we slow the plane (maintaining altitude), we must compensate by adding UP Elevator and thereby increasing the AOA - from 5 degrees to 10 degrees (arbitrary numbers). Continuing to decrease airspeed we must continue increasing AOA (more UP elevator) to maintain altitude. Eventually the angle between the wing and the relative wind is so much that the smooth airflow over the top surface of the wing becomes turbulent and lift will drop sharply - Stall. And that's not all that drops sharply, since both wings rarely stall at the exact same time, we will likely see one wing drop first. I've heard the term "Tip Stall" which would suggest the stall begins at the wingtips - not true. What actually happens is the part of the wing attached to the fuselage (wing root) usually stalls first then as the stall deepens the turbulent air progresses outward toward the wingtip. Most wings are designed so that during the initial stall condition you will still have aileron control, but if you continue holding or increasing elevator pressure, a deep stall (full stall) will occur rendering the ailerons useless - all mush and no control. We know what can happen if all this occurs 10 feet above the ground during a landing approach... It's not pretty! So far our airplane has been flying at 1 g. I'd guess just about every pilot knows what a "g" is. For a quick review, we can relate a 1 g force as normal weight times 1; 2 g's - weight times 2 and so on. I would also guess that during at least one of your landing approaches you suddenly add too much elevator and the plane stalls - a wing quickly drops, and you thought you had more than enough airspeed! Needless to say, when you suddenly pulled up elevator, what was a 1 g condition changed to 1.5 g's or more. During this g-force increase, AOA has increased as well. If we could measure the AOA, we would find that the wing will consistently stall at the same Angle Of Attack - say 20 degrees. This stall angle or Critical Angle Of Attack, will always be the same for a particular wing design, and it doesn't matter what speed, attitude or aircraft weight! It turns out that stall airspeed will increase by the square root of the g-force (Load Factor). Example: In a 60 degree banked turn (maintaining altitude), there is a 2 g load on the plane. The square root of 2 = 1.4. If the wing stalls at 20 miles per hour at 1 g, the stall speed will be 28 mph at 2 g's (1.4 X 20 mph). And 3 g's: the square root of 3 = 1.7, therefore when you "pull" 3 g's the plane will stall at 34 mph! It's not unusual during some maneuvers to pull 6 g's, and with that much wing loading the stall airspeed is 50 mph! (compared to 20 mph at 1 g). And you're correct again, the critical AOA (20 degrees) will be the same. This is what is called an accelerated stall -- one that occurs at greater than 1 g. Ok, so what does all this have to do with flying a 9 pound airplane? And where's the TIP? Would you believe a radio controlled 40 size airplane flies the same fundamental way that a 600,000 pound 747does! They both obey the same aerodynamic laws - square roots and all. Therefore, be advised that all airplanes can stall regardless of attitude or airspeed. Most of the time though, a stall occurs when the airplane gets too slow. Over control or a sudden wind gust can cause the angle of attack to exceed that critical (fixed) amount and you know what happens next. The corrective action for stall: More than likely, the PILOT is FORCING the airplane to stall, so UN-FORCE this condition by relaxing whatever flight control pressure you are holding and to prevent too much altitude loss, go to full throttle. Obviously, you wouldn't want to relax too much elevator pressure within a few feet of the ground - this is where full engine power comes to the rescue. Engine power converts to altitude gain, or at least a minimal altitude loss. Up to this point, stalls have been considered not good, but they are not all bad. Many aerobatic maneuvers (such as spins, snap rolls, and 3D) are performed while the wings are completely stalled. One of the first things I do when flying a brand new airplane is to stall it on purpose (a couple of mistakes high) to see how it behaves and to get an idea of just how slow it will fly - for landing approach and other maneuvers. It would be good to know if the plane has any bad manners during a stall while it's at a comfortable altitude. Keep 'em flying.
Brad |