Airplane Lift Vectors

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When an airplane is in level flight, i.e, constant altitude, lift equals gravity. Figure shows a plane in both straight and level flight, and in a turn. With wings level, all lift is exactly opposite the effect of gravity. This is not true when the airplane flies in a banked turn.

Notice how the banked wings direct the lift in the direction the plane is banked. If this total lift were divided into vertical and horizontal components, for ease of discussion, the vertical portion would continue to oppose gravity, while the horizontal component would turn the airplane. The total of these two would necessarily be greater than when the airplane had the wings level. For pilots, this means that while turning, the plane must produce more lift, depending upon the angle of bank, which is usually accomplished by an increase in angle of attack and some increase in power. Simply stated, the pilot has to hold back pressure on the yoke while the airplane turns.

The complexity does not cease there, however, as the mechanics of banking the wings may generate an imbalance of drag at the wingtips, which may work against the direction of the desired turn. The drag occurs because the lowered aileron generates more lift, and correspondingly more drag than the raised aileron. This asymmetrical drag causes the nose of the plane to yaw in the direction of the lowered aileron. For example, in a left turn the right aileron moves down, causing greater drag on the right wing. This drag causes the nose of the plane to yaw right; this is adverse yaw. Coordinated rudder input compensates for adverse yaw. For a left turn, left rudder is used; in a right turn, right rudder is used. When done correctly, the amount of rudder exactly cancels out adverse yaw from the aileron.

An instrument known as the inclinometer is found on the instrument panel of most aircraft and is generally referred to among pilots as “the ball.”

Figure shows the ball, a small black bead in a curved tube filled with liquid. In a coordinated turn the ball stays centered in the glass tube, but in an uncoordinated turn the ball slips to the inside of the turn or skids to the outside of the turn. How much rudder will be required to keep the ball centered depends on the airplane, its airspeed, the angle of the bank, and a number of other factors. Early in flight training, students will often stare at the ball during a turn, trying to get the correct amount of rudder input. With practice, though, they develop a feel for whether a turn is coordinated or not.

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