# What is the significance of stall speed [duplicate]

Irrespective of the stall speed, the only reason aircraft stalls is when its control surface (elevator, aileron) exceeds critical AOA. Then what is the significance of the stall speed?

Does the aircraft stall at the "stall speed" even if its AOA is less than critical?

• "the only reason aircraft stalls is when its control surface (elevator, aileron) exceeds critical AOA" This is completely wrong, stalls have nothing to do with the elevators or ailerons. – Ron Beyer Apr 12 '16 at 18:29
• – BowlOfRed Apr 12 '16 at 19:51
• The word "stall" in general means the aircraft can no longer fly. Now, the reason for an aircraft to stall can either be that the wing can no longer generate adequate lift due to excessive flow separation (which, confusingly is also called a "stall" condition) or that the wing can no longer generate adequate lift because there is not enough air blowing over it (the aircraft is too slow). This is what is meant by stall speed when quoted by manufacturers: don't fly your plane slower than this speed. But be aware that you can also stall at almost any speed. – slebetman Apr 13 '16 at 5:48
• The "stall speed" should be renamed "the speed below which the aircraft cannot be maintained at the same altitude without exceeding the stall angle". It's a shortcut. When you slow the aircraft, the AoA must be increased to maintain the altitude, but at some point this is not possible any more, the aircraft stalls if the flight is maintained horizontal. – mins Apr 13 '16 at 6:16

There is not one stall speed, instead there are several, depending on aircraft configuration and flight conditions. They are used to determine speeds for the safe operation of the aircraft or for limit speeds.

For example: The stall speed in landing configuration and level flight with the actual weight is used for determining the approach speed. By multiplying with a safety factor of 1.3, you arrive at the recommended approach speed. Or: The stall speed in take-off configuration is used for determining the rotation speed. And so on.

Another example: The speed range in which it must be safe to deploy flaps is limited by 1.4 times stall speed of the clean aircraft, or 1.8 times the stall speed in landing configuration, whichever is greater. Or: The maneuvering speed v$_A$ is calculated by multiplying the stall speed of the clean aircraft with the square root of the maximum allowable load factor. And so on.

If you wonder what a stall is, maybe it helps to read the answers to this question. It is not flow separation or exceeding a specific angle on the control surfaces that determines the stall condition, but the reversal of the lift curve slope over angle of attack: Stall is when more angle of attack will no longer increase lift, so it is the angle of attack of the whole aircraft that counts. Physically, this means that large-scale flow separation appears over part of the wing. A good design will still have fully working control surfaces at this point.

No. If the angle of attack is not critical, then it does not matter what the relative air speed is, the plane will not stall. By definition the "critical" AOA is the angle the plane stalls at, so you are asking sort of a tautological question.

The stall speed is there for guidance only, to give the pilot a general idea of at about what point the aircraft will begin to stall in level flight.

In practice the speed at which the aircraft actually stalls will vary according to a lot of different conditions such as temperature, humidity, angle of bank, gust conditions and many other factors, even how clean the wings are.

• "By definition the "critical" AOA is the angle the plane stalls at" Not the plane, the airfoil. The angle of the aircraft is different depending on the wings and the AOA changes along the length of some wings that have a twist in them for any given pitch angle. – Ron Beyer Apr 12 '16 at 19:03
• @RonBeyer True, but I think you are kind of splitting hairs. The OP does not seem to be at the point where knowing the details of airfoil and wing geometry would be relevant. – Tyler Durden Apr 12 '16 at 19:09
• Either way I think the goal should be the accuracy of the answer, an airplane can stall at pretty much any pitch angle, but there is a critical AOA where the airfoil will stall, and the two are not interchangeable. Lets not add confusion by associating pitch angle with AOA. – Ron Beyer Apr 12 '16 at 19:15
• @RonBeyer Who said anything about pitch (besides you)? Neither me nor the OP said anything about pitch. – Tyler Durden Apr 12 '16 at 19:19
• Angle of the plane = pitch... – Ron Beyer Apr 12 '16 at 19:21