andyp
Senior Heliman
Location: New Zealand
| Actually, strictly speaking, a stall is NOT directly tied to airspeed. I am surprised anyone with any knowledge would think it is.
This is so fundamental to any discussion I thought it worth reproducing this from http://www.aerialpursuits.com/misc/stalls.htm
It looks to me few have an understanding of this most basic idea:
Stall Myths
© 1996-2001 Raptor Designs Pty Ltd. (Last Update 10th August 2001)
In early flight training, it's justifiably hammered into new students to keep their airspeed up lest they stall. Despite lectures to the contrary, this sometimes leads to the student inextricably linking the onset of a stall with a particular airspeed, and to the belief that it is lack of airspeed that actually causes the stall.
Lack of airspeed does not cause a stall. Excessive angle of attack causes stalls.
A stall occurs when the airflow over the top of the wing separates into eddies rather than smooth flow. This occurs on most wing aerofoils at an angle of attack of about 16 degrees.(Angle of attack is defined as the angle that the chord of the wing makes to the direction the wing is travelling though the surrounding air mass). Some special aerofoils with high lift devices can exceed this figure, but for most aerofoils, it's 16 degrees give or take a degree or so.
The interesting thing is that this is pretty much independent of airspeed under most conditions. If you put an aerofoil in a wind tunnel and put wool tufts on it along the chord, it largely doesn't matter what speed you run the tunnel at, the wool tufts will start waving about or reversing direction at about the same angle as you increase the angle of attack.
The other thing you can measure in a wind tunnel is the amount of lift and drag generated by the aerofoil. One of the other fallacies that is often touted is that lift disappears at the stall. (The layman believes this because he sees that when an aircraft stalls, it usually drops the nose and loses some altitude, but he is attributing this effect to the wrong cause. The aircraft drops the nose because of it's stability. If it did not, there would be a real problem with the design.)
(Lift is of course, defined as the force generated by the wing perpendicular to the direction of travel of the wing through the air mass. Drag is the resisting force acting parallel to the direction of travel. For now, by the way, I will abbreviate angle of attack as "aoa".)
If you start testing your aerofoil with about 4 degrees of aoa (which, incidentally, corresponds to the value which yields the best ratio between lift and drag for most aerofoils) and increase the aoa slowly, you will find that the lift will increase, as will the drag, but the ratio of lift to drag will get steadily worse. At about 16 degrees, the flow over the top of the aerofoil will separate, and drag will increase sharply, and lift reduce a bit but not disappear. If you continue to increase the aoa, the drag will get huge and lift will reduce quickly, although some lift will still be present almost until the aerofoil is perpendicular to the airflow.
"Stall Speed" varies with wing loading!
If you're flying along, straight and level and slow the aircraft by gently raising the nose, eventually you'll exceed the critical 16 degrees and you will stall. If you do this several times, you'll notice that the stall always seems to happen at about the same speed, so you might mentally tag this as the "stall speed" of your machine, but you'd be wrong to do so. If you were to take along some ballast, or perhaps a passenger and conducted the same experiment, you would find that the aircraft stalled at a higher speed. Why is this?
In straight, unaccelerated flight (ie: constant airspeed), your wing is generating exactly as much lift as the weight it is supporting. The wing is a static device. For the same airspeed and same angle of attack, it always generates the same amount of lift. If you were flying along at a particular speed and suddenly doubled your weight (don't ask me how, this is a thought experiment!), the wing would suddenly have only half the lift necessary to support you. In order to support you, at the same angle of attack, it would have to fly faster, that being the only way of increasing the available lift. This principle applies at all angles of attack, including stall, so the upshot of this is that if you increase the wing loading, the airspeed at which the critical angle of attack is reached, is greater. That is: the heavier you are, the faster your stall speed.
The main reason instructors insist on students thinking of a "stall speed" is because it's the most practical way to avoid stalls. Most aircraft are placarded with a "stall speed" at "Maximum all-up weight" which is the heaviest the aircraft is likely, or allowed to be. The instructor can be reasonably happy that if the student stays over this speed in straight and level flight, and clam conditions, he's not likely to have an embarrassing accident.
However, there are some conditions where the aircraft's wing loading can effectively change in flight. The most obvious of these is steep turns. In a 45 degree turn, for example, the effective weight of the aircraft is increased by a factor of 1.414, and this is the reason extra airspeed is required in these turns. The stall speed is greater than in straight flight.
An aircraft can also enter an "accelerated stall" by doing a very fast pullup (ie: raising the nose very quickly after a dive) even at quite high airspeeds. In this case, even though the aircraft has lots of airspeed, the pilot has been able to rotate the wing very quickly beyond the critical angle of attack, and it will be stalled.
So in full size aviation, AOA instrumentation is gaining popularity:
Optimum approach to landing speeds vary with weight, bank angle, CG and even relative humidity. With the AOA instrument, we can now do precision approaches just like the military, Navy aircraft carrier, and airline pilots eliminating the requirement to compute performance speeds. AOA tells you where the airspeed is going to be unlike the airspeed indicator that tells you where it was. Your aircraft stalls at the same AOA regardless of weight, temperature, altitude or center of gravity and bank angle. Your stall indicated airspeed varies significantly with all the above. For any given airfoil, other performance parameters such as best lift to drag, best glide, maximum endurance and maximum maneuvering performance also occur at known AOAs. |
