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Knife edge is one of those maneuvers that looks easy, but you really have to jockey the sticks around to do it. What it is, is simply flying the plane on it's side. Knife edge can be done at almost any speed in the plane's envolope, as the only restrictions are power and rudder throw. Low speed knife edge is best done into the wind, and high speed is better down wind. To begin the maneuver, simply roll the plane on either of it's sides and apply opposite rudder. (Ha! That's only the beginning!) The amount of rudder you need is proportionate to the speed of the plane. The faster the speed, the less rudder you need. The slower the speed, the more you need. Rudder in this maneuver is important because when the wings are perpendicular to the ground, they don't do anything to keep the plane in the air. By the same token, when the rudder is parallel to the ground, it no longer acts as a rudder, but as an elevator. So rudder applied opposite the roll direction is the same as up elevator.

So, as of now, you are on your side with opposite rudder. Great. So why is the plane rolling out of a sideways attitude? Well, to make a long story short, when you apply rudder in knife edge, (think about this) the plane rotates to a nose high attitude. As this is happening, the low wing moves through the air faster than the high wing. Surely you know that the faster a wing moves through the air, the more lift it creates. So the low wing, with it's faster (rudder induced) speed, rises. And when the plane is on it's side, a rising wing translates to a roll. In order to counter this roll, hold a small amount of aileron in the same direction as the initial roll. So what this aileron does is counter the rudder needed to hold knife edge, and keep the plane in a 90 degree bank. (By the way, this paragraph ends my boring textbook session of the month. The next ones are exciting. I think.)

Now that you are holding the plane on a 90 degree bank with a small amount of aileron and opposite rudder, the only other control that needs to be taken care of is the elevators. Most planes, once held in knife edge with rudder and aileron, will tend to pull "towards the canopy". To better understand why this happens, we will go to the most basic aspect of flight, straight and level. When a plane is flying straight and level, the wings produce lift to keep it flying. Lift pulls the plane towards the top of the wings, which in straight and level flight is up. Both wings are traveling through the air at equal speed. The weight of the plane, however, counteracts this lift. This is why planes fly straight and don't shoot up into space. Well, when the plane is on it's side, and the wings are traveling through the air at equal speed (achieved by that small amount of aileron), they still produce lift. But since the weight of the plane is supported by the rudder, there is nothing counteracting the lift. The effect of uncountered lift is to make the plane rise. Rising on it's side is the same effect as a simple inside turn. To counter this rising action, apply a small amount of down elevator. This should keep the plane tracking straight along in a perfect 90 degree bank. (I guess I lied in saying that the last paragraph was my last textbook one.)

Now, if anything, I'm sure you at least appreciate the difficulty of a seemingly simple maneuver. Trainers are even harder to knife edge because of the dihedral. Some won't even do it. Once you can proficiently knife edge in both directions, you might try outside circles. First establish complete knife edge, then apply more down elevator. The plane will push around a circle. For a real challenge, try a knife edge circle with outside snaps at the three, six, nine, and twelve o'clock positions of the circle, stopping the snaps on knife edge and continuing the circle until the next snap is due. (Yeah right.) That wraps up another "Acrobatic How To", and until next month, see ya at the field.