Stall: Different angles of attack????

[Rez O. Lewshun]

Well, of course it does...but pitch is not a reliable indication of AOA in all circumstances. If it was, no airplane would be capable of a loop.

Agreed... just checking I guess. I flamed on another thread for suggesting that lowering pitch lowers AOA....

 

[Singlecoil]

Assuming you don't damage the airplane first(from pulling too many G's), you can stall any airplane at any speed at or above minimum stall speed.

Not to get technical on ya, but you can stall at ANY airspeed, i.e. 5 knots, 1 knot, etc. Minimum stall speed is straight and level 1g gross weight flight.

 

[91100 100 set]

Nobody has mentioned how ice accretion will change the AoA a wing will stall at.

 

[Goose Egg]

...........

 

[FracCapt]

Not to get technical on ya, but you can stall at ANY airspeed, i.e. 5 knots, 1 knot, etc. Minimum stall speed is straight and level 1g gross weight flight.

You're absolutely right. I didn't bring low G and negative G maneuvers into the equation because it would probably do nothing but confuse students. People that want to know more about this, and experience it, should do so under the supervision of an instructor(preferably in an aerobatic airplane). Negative G maneuvers bring a whole new set of problems.

 

[Goose Egg]

In staight and level flight, I reach over and turn the ignition off. The airspeed starts to drop to stall speed, but I do not touch the yoke. Instead, I just let the nose go over. Is this not a stall? I never touched the yoke, so I assume the angle between the wing chord-line and the relative wind never exceeded the citical limit? Is this not a stall at a low AOA???

Nope. Ever think of actually doing the reading? That might help. An aerodynamic stall is, by definition, whenever the critical angle of attack for that particular airfoil design has been exceeded. No critical angle exceeded, no stall. It's that easy. In your example, letting the nose go over is what reduces the angle of attack to prevent the stall from occuring.

And actually, if you turned off the ignition (which I don't recommend) without touching the yoke, the airplane would immediately pitch down to maintain the airspeed that it was trimmed for. The only way the airplane would be close to the published stall speed would be if the airplane was trimmed for it to begin with.

Just be aware that an airplane can stall at airspeeds other than the published reference stall speeds. Ask your CFI to demonstrate some accelerated stalls.

-Goose

 

[apcooper]

Here is the best way to think of it plain and simple. The difference in the direction of which the nose is pointed vs. where it is actually flying is the angle of attack. If this is over 17 degrees or so BINGO a stall will occur regardless of airspeed, power setting, pitch attitude or weight!!

 

[Tinstaafl]

Unanswerd, perhaps the following will help:

Some basics that you're probably already familiar with but it helps me to lay out my comments:

Angle of Attack is the angle between the relative airflow (relative wind, to some) and the chord line. The relative airflow is opposite in direction to the flight path through the air. The chord line, in turn, is defined as a straight line from the leading edge of the wing (ie the most forward bit...) and the trailing edge of the wing (its most rearward part). This is not the same as the pitch angle of the fuselage (what the pilot readily perceives). Think of an aircraft somehow descending vertically while in a normal flight attitude. The pitch angle is 'normal' but the relative airflow is from below.

Aerodynamically a stall is when the wing can no longer produce sufficient lift to counter weight as a result of exceeding its critical angle of attack (crit. 'AoA' or 'crit alpha'. Some texts refer to it as the stalling AoA. Note the wing doesn't suddenly stop producing all lift - there is still some amount but not sufficient for flight.

So, when the wing stalls, the angle between the chord line & the relative airflow has exceeded the point where increasing the angle of attack will increase lift. In fact the revers occurs: lift will reduce with increasing angle of attack. The corollary is that at the critical AoA the wing is producing its maximum amount of lift.

For a given shape an aerofoil stalls at a particular angle of attack. For all intents & purposes this doesn't change unless you change the wing somehow. Extending trailing edge or leading edge devices & ice accretion are all ways that change the shape of the wing. In effect it becomes a whole new wing. For the various aerofoils used on most aircraft the critical angle is about 15 or 16 deg.

If you were to look at the airspeed indicator just as the wing stalls you would see a particular speed on it. That speed isn't fixed. It varies depending on aircraft weight, load factor (think of it as apparent weight or 'g' force), configuration/damage/ice (the 'changed wing' thing) & power application (a small component of thrust helps support the a/c, reducing the load the wing has to support). What a pity we rely on airspeed indicators instead of AoA indicators!

In general trailing edge lift devices will slightly reduce the crit. AoA. The upside is that for any given speed the aerofoil can produce more lift than 'clean'(it has a higher Coefficient of Lift, CL) so the slightly reduced crit AoA is irrelevent. Leading edge devices tend to increase the crit. AoA while also increasing the CL). All these things also increase drag hence why we don't have them permanently extended...

If, while maintaining trimmed straight & level flight, you close the throttle the aircraft won't suddenly stall unless you make it. The aircraft is designed to be speed stable in normal flight conditions. It does this by the tailplane causing the aircraft to pitch nose up or down to maintain the trimmed speed, losing height to correct a speed reduction, and gaining height to correct a speed increase. Reduce power to less than what is needed for S&L flight and the aircraft will pitch ND to maintain the trimmed airspeed, losing height in the process. The AoA will be largely unchanged, except for bit of bobbing around while the aircraft stabilises in the new flight path.

If, on the other hand, you were to prevent the aircraft from pitching ND to maintain speed then you would have to cause the a/c to pitch NU. Do this enough and eventually the wing will reach its crit. AoA. Do it a bit more & the wing will stall. Depending on how much and the rate of NU pitch you cause to happen the flight path could be descending, level or even climbing.

In all of this, what's important is the angle of attack that the wing experiences, not whether the aircraft is climbing, descending, level or turning. Similarly, the pitch attitude is not what defines the stall. You can stall the wing in *any* a/c pitch attitude, if you make the wing exceed its crit. AoA.


Oakum Boy, the short answer is no, none of those things you mentioned is a stall. In aviation a stall is a specific thing that relates to an aerofoil as I discussed above. The term 'stall' is not used w.r.t. whether or not the engine is running. You can stop an engine, it can malfunction or fail, but it doesn't stall**

The engine sputtering or flames emanating from the aircraft are not necessarily anything to do with stalling - although a pilot could always cause one if s/he grossly mishandles things while trying to deal the with sputtering or fire.

Aircraft - of all sizes - glide without the engine(s) producing sufficient thrust to counter the drag resulting from moving through the air & as a byproduct of the wings producing lift. Some glide better than others eg purpose built sailplanes/gliders, some glide like well greased crowbars eg high wing loading fighter jets - but they all glide. Even passenger jets. BTW, a typical pax. jet has nearly double the glide performance of a typical light piston aircraft, using the glide ratio as the arbiter. Granted they travel forward & downwards at much greater rates than a lighty but they still travel further per unit of height. Of course the much, much greater speeds forwards & downwards in the jet means that the light aircraft is a rather better bet to be in when the earth eventually gets in the way....




**Caveat: Turbine engines work by having lots of compressor & turbine blades. These are aerofoils & therefore able to stall, so a jet engine can 'stall' - or at least it's aerofoil components can. It would usually be called a 'compressor stall' or similar to try to distinguish it from the common usage.