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In Stick and Rudder, the author warns pilots against the unreliability of judging airspeed and hence “buoyancy” — his term for how far the airplane is from aerodynamic stall — by throttle position, engine noise, and nose attitude.

Perhaps the most deceptive of these factors is g load. When an airplane, flying at a certain speed, goes into a turn and loads itself down with g load it assumes larger Angle of Attack and this gets closer to the stall. That has been described earlier in this book. But it isn’t the whole story. At the larger Angle of Attack, the wings have more drag, and thus the airplane will slow up, unless the throttle is opened wider. The airplane assumes a still higher Angle of Attack and gets still closer to the stall! Few pilots realize how strong and dangerous this effect is.

Langewiesche, Wolfgang. “The Flying Instinct.” In Stick and Rudder: An Explanation of the Art of Flying, 58. New York: McGraw-Hill, Inc., 1944.

He then gives a surprising example.

The average small airplane, fully loaded and with its throttle set at cruising, is actually unable to hold indefinitely any turn banked much more than 45 degrees! The effect just described will slow it down gradually, as it circles, so that the pilot’s stick comes farther and farther back; until finally, after perhaps twenty turns have been completed, it will stall: stall, mark you, out of level flight with cruising throttle!

ibid.

Why doesn’t nosing over to leave the steep turns for straight-and-level attitude with open throttle produce sufficient airspeed to avoid the stall?

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    $\begingroup$ I think what is described is entering the regime of speed instability on the backside of the power curve. Additionally, the described scenario assumes the pilot holds altitude by pulling back no matter what. Trading altitude for sufficient airspeed in descent will restore normal flight at previously trimmed cruise conditions eventually. $\endgroup$ – Cpt Reynolds Mar 26 '18 at 15:06
  • $\begingroup$ I think the author's comments assume that the pilot has not taken action to maintain airspeed with the increased drag, by either increasing the throttle, (or, if the aircraft does not have sufficient excess power to hold airspeed at the higher bank angle, initiated a descent). [Obviously], if you don't maintain sufficient airspeed, eventually the aircraft will slow down and stall. $\endgroup$ – Charles Bretana Mar 27 '18 at 14:21
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A prolonged series of steep turns will not produce a stall in subsequent straight and level flight.

"after perhaps twenty turns have been completed, it will stall: stall, mark you, out of level flight with cruising throttle!"

In this case "level flight" means not climbing or descending while still in a steep turn.

Stopping the turn by rolling level would unload the wings and prevent the stall. Nosing down would also unload the wings and increase airspeed, also preventing a stall.

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  • $\begingroup$ Throttling up from cruise power to climb or TOGA power would also increase airspeed and prevent a stall. $\endgroup$ – Sean Aug 29 at 1:53
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Well, lowering the pitch attitude to reduce the AoA during the turn will nullify the onset of the stall at the expense of loss of altitude. Langeweisch’s point is that for a smaller aircraft ie an airplane with little reserve power available to climb and with the throttle setting for cruise power and not increased during the turn, holding a continuous steep turn at a constant altitude results in so much additional induced drag that the airplane will begin to decelerate. Doing so requires additional pitch up to hold altitude which again increases drag, creating a vicious circle. Sooner or later the airplane will be so slow that the pilot will exceed the critical AoA using elevator pressure and stall the airplane. This will occur at a speed much higher than it would in straight and level flight. The point here is that power settings have to be adjusted in order to maintain constant altitude and airspeed in a steep turn, as would be expected.

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