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An Experienced Pilot Explains "Stall Speed"

By       Message Paul Craig Roberts       (Page 1 of 3 pages)     Permalink

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Reprinted from Paul Craig Roberts Website

The following article was written by a pilot who wishes to remain anonymous

From youtube.com/watch?v=KFqN4ASWL10: Russian Air Force Su-24 military aircraft
Russian Air Force Su-24 military aircraft
(Image by ArmedForcesUpdate)
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In the aftermath of the downing by Turkish F-16 fighter jets of the Russian Sukhoi tactical bomber Su-24 over Syria, some have pointed out that Turkish claims the Russian jet was in Turkish airspace for 17 seconds, but covered a distance of only 1.15 miles, would imply the Russian jet would have had to be flying at only 243 mph, which some say would be slower than the airplane's stall speed, and therefore impossible.

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Others dispute this stall speed figure and say an airplane of this type would have a stall speed closer to 150 mph. Who is right, and how can we find some objective answers? We can and we will.

The most important point is that there is no such thing as a single stall speed for any airplane, as Dr. Roberts pointed out. In fact wing stall is not a function of airspeed, but of the wing's angle of incidence to the oncoming airflow, which is called the angle of attack (AoA) or simply alpha (this angle is obviously a function of the airplane's "attitude" or pitch-up angle).

If wing angle of attack exceeds the critical point, the airflow starts to separate from the top surface of the wing (the suction side), and the wing will begin to lose lift and a stall recovery maneuver is required.

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The stalling angle of most airplanes is on the order of 15 to 18 degrees of angle of attack, although some combat aircraft have special design features, such as leading edge root extensions (or LERX) that may increase critical AoA to about 30 degrees. Engine power plays a big role too -- a fighter aircraft with very powerful engines and low airplane weight can sustain high angles of attack by using its engine power to overcome the loss of lift due to wing stall.

It is also useful to know that wing lift increases nearly linearly with an increase in the angle of attack. So if we want to double lift, we must double the angle of attack.

For example when taking off, the pilot will pull back on the control yoke and increase the pitch-up angle of the airplane, thus increasing angle of attack and thereby total wing lift. The extra lift launches the airplane into the air!

This maneuver is called takeoff "rotation" which refers to the pilot in effect rotating the aircraft about its pitch axis (which can be thought to run horizontally from wingtip to wingtip).

It is an old truism that every student pilot is taught early on, that any airplane will stall at any speed, at any attitude, and any bank angle. So when we say that an airplane has a certain stall speed, it is simplifying things to the point of uselessness.

As Dr. Roberts pointed out, an airplane that is maneuvering can have a higher stall speed than if it is simply flying straight and level. When an airplane is in a banked turn the lift created by the wing is also tilted, since lift is always perpendicular to the wing. If the airplane is banked at 45 degrees, it means its lift vector will be pointing 45 degrees from the vertical (or horizontal if you prefer), as seen in the figure below.

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The wing's lift in a bank decreases by the cosine of the bank angle. If the airplane is banked 45 degrees, the cosine is 0.707 and the amount of vertical lift is only about 70 percent of the total lift that is now pointing at 45 degrees.

In order to maintain the airplane's altitude in a turn, the pilot must then pull back on the yoke and increase the angle of attack, so as to increase diagonal lift and thereby provide enough vertical lift component to maintain the aircraft at the same altitude. That vertical lift component is shown in the diagram.


It should be noted that the airplane loses lift exponentially with increasing bank angle. In a 60 degree bank the cosine is 0.5, (which is 0.707squared). At this bank angle the vertical lift component is now only half of the total diagonal lift, and the airplane now needs double the lift it would normally need at straight and level flight. Again the pilot has to pull back the yoke and increase airplane pitch attitude to an angle of attack about double what it would be in straight and level flight.

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Dr. Roberts was Assistant Secretary of the US Treasury for Economic Policy in the Reagan Administration. He was associate editor and columnist with the Wall Street Journal, columnist for Business Week and the Scripps Howard News Service. He is a contributing editor to Gerald Celente's Trends Journal. He has had numerous university appointments. His books, The Failure of Laissez Faire Capitalism and Economic Dissolution of the West is available (more...)
 

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