Stalling

[tony_hk]

Hi fellow forumers,

I am confused with the concept of stalling after reading several books. Here are the abstracts:
____________
"In fact an airfoil stalls at a certain angle, not at certain speed."
_______________
(A) Am I correct to say that when we cruises above the stalling speed, the occurance of stalling will depends only on the angle of attack (no matter what airspeed we have)?

(B) Can we find an angle of attack at which the airplane will not stall below stalling speed? (If I am correct, the answer is "no". Because if such angle existed, it would exceed 15 degree, right?)
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_____________________________
"Q: What affects indicated stall speed?"

"Ans: Weight, load factor, and (to some extent) power. At a given flap setting an airplane will always stall at the same angle of attack. The three factors listed above are the only variables that we as pilot can change."
______________

(C) What is load factor? and how can it affect the indicated stalling speed?

(D) How can power affect the indicated stalling speed?

(E) After certain amount of fuel is used, the weight of the airplane is reduced. Is it ture that the stall speed will decrease because of that?

I am a self learner who have just started for a few months. As I want to build up a solid foundation before I proceed further, I will appreciate if anyone can help me to have a better understanding on these fundamental concepts.

Thanks in advance.

[747FOCAL]

Search around the net and you will find more info on stalling than you can swallow. If somebody has not answered your question in greater detail I will take a stab at it later tonight.

I will tell say that after being in many hundreds of stalls in commercial aircraft it don't get much cooler doing them. Noises you never thought you would hear coming from all directions. :E

[Mad (Flt) Scientist]

(A) Am I correct to say that when we cruises above the stalling speed, the occurance of stalling will depends only on the angle of attack (no matter what airspeed we have)?
Not quite. Generally speaking the speed itself will not affect the angle of attack at which the airfoil stalls, but Mach number will do so. Therefore if you have an aircraft cruising at a speed well above the 1'g' stall speed (we'll get to that shortly), then the angle to which the wing will remain unstalled will vary with Mach number. So if the stall speed is the equivalent of M0.4, you may find that the wing stalls at 10 degrees at M0.70, but only 8 degrees as M0.80. (In both cases, the aircraft would be pulling 'g' when the wing stalled.

(B) Can we find an angle of attack at which the airplane will not stall below stalling speed? (If I am correct, the answer is "no". Because if such angle existed, it would exceed 15 degree, right?)
Not quite sure what you mean here. Stall speed is the speed at which the aircraft stalls; by definition any speed below that in 1'g' (level) flight will result in a stall. If you were to 'unload' the wing you could delay the point of stall, by decreasing the amount of lift you needed from the wing, and so 'fly' at a lower speed. But it's not really 'flying' in the normal sense.

C) What is load factor? and how can it affect the indicated stalling speed?
Load factor is the multiplier of the aircraft's weight being supported aerodynamically. So it's just a facncy name for what people commonly call "gee". A plane flying a loop will have a load factor of greater than one at the bottom of the loop as it pulls up.

(D) How can power affect the indicated stalling speed?
Directly: the engine generates thrust which, as the angle of attack increases, begins to directly support aircraft weight, so reducing the need to generate lift from the wing. This allows the stall speed to be lower (stall generally occurs at maximum lift coefficient - for the same lift coefficient, smaller actual lift means a lower speed). In the extreme case, "stall speed" of a VTOL aircraft is an almost meaningless concept (in the traditional sense).
Indirectly: the engine may be providing lift through the propwash or jet efflux passing over the wing; more power means more "jet lift" so the stall speed "power off" will be much greater than that "power on" for some aircraft.

(E) After certain amount of fuel is used, the weight of the airplane is reduced. Is it ture that the stall speed will decrease because of that?
Yes.
0.5 * density * (Stall speed)-squared * maximum lift coefficient * wing area = (weight of aircraft * load factor)

(excuse the approximations for aircraft with unusual behaviour, and ignoring e.g. Mach effects for simplicity; in practice maximum lift coefficient is not a constant for all conditions)

So if I increase weight or load factor, stall speed rises as an inverse square function. Conversely, reduced weight means lower stall speed.

[cwatters]

"In fact an airfoil stalls at a certain angle, not at certain speed."

Yes that's correct. There are lots of ways to stall but the most common goes like this...

There you are flying along S&L... the instructor asks you to demo a stall so skipping the usual checks you start to reduce power and slow down... As a plane slows down you have to increase the angle of attack in order to maintain the same amount of lift. Otherwise you start descending. At some point you can't get anymore lift out of the wing because the max angle of attack has been reached. The airflow over the wing breakes down and the wing stalls. The speed at which this happens is usually called the stall speed (at least it's ONE stall speed because you can stall at any speed).

eg You can also stall higher speeds... all you need to do is get the wing to exceed the critical angle of attack by applying too much back stick too quickly. You can do this even in a glider.

"B) Can we find an angle of attack at which the airplane will not stall below stalling speed? (If I am correct, the answer is "no". Because if such angle existed, it would exceed 15 degree, right?)"

The answer is yes - if you allow the aircraft to descend. You can fly below the stall speed (as defined above) provided you reduce the angle of attach = reduced lift = descending.

[Mad (Flt) Scientist]

Just to re-weigh-in on (a)...

cwatters' explanation is valid for a low enough Mach number - which can be surprisingly low, say 0.20 and below. Above this many aerofoils exhibit Mach-dependency on their stalling angles-of-attack.

This is the 'usual' environment under which stalls are encountered - low speed and (relatively) low altitudes in small-ish aircraft, all of which tends to mean a fairly low Mach number. The stall angle of attack will appear remarkably low at high Mach to someone whose experience is confined to that regime.

[mstram]

"B) Can we find an angle of attack at which the airplane will not stall below stalling speed? (If I am correct, the answer is "no". Because if such angle existed, it would exceed 15 degree, right?)"


The answer is yes - if you allow the aircraft to descend. You can fly below the stall speed (as defined above) provided you reduce the angle of attack = reduced lift = descending


Ok ...

But can you actually get a plane to fly below descending 1g stall speed while making it descend? :)

Mike


C) What is load factor? and how can it affect the indicated stalling speed?


Load Factor
http://142.26.194.131/aerodynamics1/Lift/Page8.html
http://142.26.194.131/aerodynamics1/Lift/Page10.html

The Stall

http://www.av8n.com/how/htm/vdamp.html#sec-stall-intro

[747FOCAL]

Not really sure it counts for anything, but I have been in a 727(not flying) at around 88 kts indicated, but that was false because the angle of attack was so great the pito tubes weren't seeing actual clean air. I suppose you could hold it and descend in stall, but if one wing flashes you will be on your back in the wink of an eye.

Some collegues of mine have been up performance testing a lear the past few weeks and they had one stall that lasted like 40 seconds before he got the nose down. bucking and kicking the whole way down.

[AerocatS2A]

You can find yourself well below stalling speed while doing aerobatics without actually stalling. E.g. at the top of a loop you may only be pulling 0.5g which means you can fly below the stall speed. The reason is that the wings are only carrying half of the aircraft's weight and have a low angle of attack. You are still flying in the sense that the control surfaces are all effective, but you are not flying in that the aircraft is no longer fully supporting its own weight.

In a similar vein to the 727, we used to do stalling exercises in a Pitts which involved stalling with some power on (about 1500rpm from memory) and holding the stick back as if entering a spin. However you use rudder to prevent either wing from dropping, lots of very quick precise rudder. I\'ve never looked at the ASI while doing it because if you don\'t look out the front you very rapidly lose it.

[tony_hk]

Thanks men. I've got it, although I need some more research on certain terms. Anyway, the information is more than enough for me.

[cwatters]

But can you actually get a plane to fly below descending 1g stall speed while making it descend?

Humm thats an interesting question...

If it's descending at a constant rate (so the pilot feels 1g) then the wing must be providing the roughly the same lift as it does in S&L flight. In which case the speed will be roughly about the same but it can'r be slower.

Therefore if you ARE flying slower than the S&L stall speed you are not only descending but your rate of descent is increasing. You could argue that this isn't flying - it's falling!