Most people know that the wings' job is to provide lift. If the wings are perpendicular to the ground, the only lift they can provide is parallel to the ground. But that doesn't keep the airplane in the air. We know lift must be coming from somewhere, otherwise the airplane wouldn't be able to maintain its altitude as it flies by us drop-jawed spectators.
The lift comes from the fuselage. The next time you watch a knife-edge pass take a close look at the airplane's orientation. You'll notice the plane's nose is higher than its tail even when flying level. That's what allows the fuselage to act like a wing to provide enough lift to remain level.
Many aerobatic airplanes have wings with symmetrical airfoils. The upper and lower surfaces are identical. This airfoil generates lift because it moves through the air at an angle of attack (AOA), which is the angle between the relative wind and some fixed reference line on the wing. The reference line is usually its chord line, an imaginary line drawn between the wing's leading and trailing edges. The higher the AOA, the more lift the wing generates - up to a point. If the AOA gets too high, the air no longer flows smoothly over the wing, which reduces lift drastically - in other words, the wing stalls.
It works the same way with the fuselage during knife-edge flight. By keeping the nose higher than the tail, the fuselage flies through the air with a significant sideslip, meaning the relative wind is from the left (or right) of its nose from the pilot's perspective. When the airplane is on its side, this sideslip is the AOA of the fuselage. Because the fuselage is flying through the air at a positive AOA, it generates lift (Figure 1.)
The fuselage is a terribly inefficient wing, so the airplane usually needs to fly at a high AOA and fairly fast airspeed to create sufficient lift. Then again, your hand isn't a particularly good airfoil, yet you can still feel the lift it produces when you hold it out a car window at a positive AOA.
What Controls What?
When an airplane is in knife-edge flight, the elevator and rudder swap duties. Imagine your favorite air show airplane in right-wing-down knife-edge flight. Stepping on the left rudder pedal deflects the rudder trailing-edge-left, which yaws the airplane's nose to the left. With the airplane flying with its right side down, that left yaw raises the nose farther from the ground. If we just look at the airplane from the cockpit, left pedal yaws the plane nose-left. But from the spectator's viewpoint, left pedal lifts the nose higher.
Left pedal is necessary to keep the plane's nose up, creating the necessary fuselage AOA. For air show pilots, the pedal input should be fairly intuitive. Looking forward, they can see the airplane's nose in relation to the horizon, and they can readily surmise that to move the nose further above the horizon, they need left pedal.
A pilot has another thing to think about when he's using the rudder as an elevator. On most airplanes, including aerobats, the rudder usually doesn't extend below the fuselage. In knife-edge flight this is the equivalent of having an elevator on only one side of the fuselage. The result is that the deflected rudder also acts like an aileron and it tries to roll the airplane. In our example, left rudder tries to roll the airplane to the right.
You might think a little left aileron would be necessary to prevent the airplane from rolling to the right. It depends. Many airplanes have a strong dihedral effect, which would tend to roll the airplane to the left during right-wing-low knife-edge flight. (A positive dihedral effect rolls the airplane away from the sideslip, which is from the right in our example.) So, the pilot might need to apply a little right aileron depending on the relative strengths of the dihedral effect and the rolling tendency due to rudder deflection. Aerobatic airplanes sometimes are designed to have minimal dihedral effect. These airplanes may require left aileron during right-wing-down knife-edge flight.
In knife-edge flight the elevator takes on rudder duties as well. It still makes the airplane's nose pitch up and down, but that translates into left and right (or right and left) when the airplane is on its side. Unlike the rudder, the elevator has no asymmetry, so pulling and pushing on the stick shouldn't cause an appreciable rolling tendency. But elevator deflection can generate some rolling moment because the fuselage's high AOA partially blanks the high elevator. This tends to make the lower elevator more effective than the higher elevator.
The ailerons continue to function in their normal fashion in knife-edge flight.
In the Cockpit
We humans tend to want to see the world right-side-up. In knife-edge flight the horizon is tilted 90 degrees. This takes a little getting used to. On the brighter side, it's fairly easy to visually establish the 90-degree bank angle. At least it's easier than holding 70 or 50 degree banks when you rely solely on a visual reference.
The airplane's nose movement follows convention even in knife-edge flight. Push or pull the stick or displace the left or right pedal, and the nose moves in the direction of the applied control. In this fashion the control inputs are intuitive regardless of the airplane's orientation.
A level knife-edge pass is 1-G flight, but that 1-G is toward the side of the cockpit. We normally think of gravity pushing us down into our seats. that's because our seat usually is between us and the ground. During knife-edge flight the pilot's entire body weight rests on his (or her) side, and none of it acts through his seat. If the pilot were sitting on a scale, it would read 0 during knife-edge flight. Go ahead, lie on the floor on your side. That's the same lateral 1-G the pilot feels during knife-edge flight. Now imagine operating the stick, throttle, and pedals while flying your airplane 100 feet above the runway at an air show. Looks a lot easier from the bleachers, doesn't it?
Needless to say a proper restraint system is a must for any aerobatic flying. It has to keep the pilot firmly in place during all planned maneuvers.
Starvation Is Not an Option
Trying knife-edge flight in the typical general aviation airplane is not a good idea. They are not designed for such antics, and their flight control authority may not be sufficient for this maneuver. We have one other very important reason not to attempt it - the engine. Air show performers usually make their knife-edge passes at very low altitudes where an engine problem would be an immediate emergency compounded by the unusual attitude.
The fuel systems of most aerobatic airplanes are designed to feed the engine an uninterrupted fuel supply in all flight attitudes, including knife-edge flight. This is essential to prevent the engine from quitting because of fuel starvation. Engine lubrication is another important factor in knife-edge flight. If the airplane's sideways orientation uncovers the oil pump inlet, the high-revving, heavily loaded engine doesn't get any lubricating oil. A temporary lack of oil may not result in engine stoppage, but it will surely have a negative effect on engine temperatures and ultimately - engine life. Engines designed specifically for aerobatic use incorporate an oil scavenge and pump system that ensures continuous lubrication in all attitudes.
Engine cooling is another concern. With the relative wind coming from such an extreme sideward angle during knife-edge flight, it can disrupt engine compartment and oil cooler airflow significantly. Thanks to the short duration of the knife-edge pass, this cooling disruption may not cause any immediate problems, but rapid temperature changes of engine components are not good for the engine.
In perspective, the knife-edge pass is typically a small percentage of an air show performance. Certainly the snap rolls, inverted spins, and lomcevaks take a far greater toll on the entire airplane, but all these abuses combine to shorten the useful life of various components. Knife-edge flying is one of these abuses.
So there you have it. A small glimpse into some of the issues involved with knife-edge flight. You'll encounter others - lots of them, but now you know the basics. The next time you're watching a knife-edge pass at an air show and the stranger next to you says, "How does it do that?" make a friend. Have a little chat about it over a soft drink. Who knows, your new friend might even pick up the tab for your soda.
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