So lets consider that the stall angle (=Cl max) of a B747 is at 16° (in clean configuration). Does this mean, that, at whatever speed you are flying (i.e. 500 knots), you would stall if you go on a climb angle of more than 16°? So there is no need to always reduce the speed to the stall speed (which might lie at around 150 knots...). I am just wondering, since, if you fly in the microsoft flight simulator with a big jet, you can have an angle of attack of 30° and more, and not stall until you get to the stall speed.
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2$\begingroup$ related: aviation.stackexchange.com/questions/2903 aviation.stackexchange.com/questions/6366 $\endgroup$– FedericoCommented Apr 19, 2015 at 19:06
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15$\begingroup$ I think you are confusing Angle of Attack with pitch angle relative to the horizon. $\endgroup$– egidCommented Apr 19, 2015 at 19:08
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2$\begingroup$ AoA is the angle between the airstream and the wing (roughly, the vertical angle between the way the plane's pointing and the way it's moving). If you're climbing, your AoA is less than your pitch. $\endgroup$– cpastCommented Apr 19, 2015 at 19:44
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$\begingroup$ I am sure this is already explained somewhere around here, with pretty pictures, but the related question finder is not good enough to find it. And I can't find it with google either. $\endgroup$– Jan HudecCommented Apr 19, 2015 at 20:52
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$\begingroup$ this is by definition: "a stall occurs then the wing exceeds the critical angle of attack" $\endgroup$– rbpCommented Apr 20, 2015 at 14:37
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3 Answers
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Is there always a stall if you exceed a specific angle of attack?
Yes, stall depends only on angle of attack. However
Does this mean, that, at whatever speed you are flying (i.e. 500 knots), you would stall if you go on a climb angle of more than 16°?
No. Climb angle and angle of attack are completely different things.
This image from How It Flies shows the four different angles involved. Pitch is the angle between aircraft floor and horizontal, wing incidence is angle between aircraft floor and wing¹, angle of climb is angle between direction of flight (a.k.a “flight path” or “relative wind”) and horizontal and finally angle of attack is angle between direction of flight and wing.
The image shows that angle of attack + angle of climb = pitch + wing incidence.
Up to stall, lift depends approximately linearly on angle of attack and square of speed (and on air density). In straight flight the forces on the aircraft need to be balanced, so angle of attack will be such that they are. If you increase pitch, the angle of attack will increase, which will cause unbalanced force, which will cause upward acceleration and that will increase the climb angle at the expense of the angle of attack again.
So if you go on a climb more than 16°, the angle of attack will not significantly differ from what it is when you fly level at the same speed.
I am just wondering, since, if you fly in the microsoft flight simulator with a big jet, you can have an angle of attack of 30° and more, and not stall until you get to the stall speed.
No, you can't. You, however, can climb at 30° or more, for a while before you run out of speed. Which at low altitude is actually quite long; the jet engines are designed to have enough power at high altitudes where air is much thinner and to allow taking off when one engine fails late in the take-off roll. Therefore low with all engines operating at full power you have quite a bit of extra thrust available.
Also note, that stall does not mean loss of all lift. You only loose part of it. A significant part, but not all. Stalled aircraft is still controllable (though ailerons effect is reversed) and some aircraft (though this would be fighters, not airliners like 747) may even have enough lift and thrust to maintain altitude when stalled.
¹ Specifically, the zero-lift line of the wing. This coincides with the chord of symmetric wings, but cambered wings have it tilted up.
answered Apr 19, 2015 at 21:38
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1$\begingroup$ Would it be correct to say that the angle of attack is the angle between the wing and the relative wind? $\endgroup$– TerryCommented Apr 20, 2015 at 1:21
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$\begingroup$ I agree with Terry - 'relative wind' is the usual term to describe what you are calling 'flight path'. Both are jargon, to a degree, so I dunno if it matters much. $\endgroup$– egidCommented Apr 20, 2015 at 2:06
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$\begingroup$ "No you cant"? Well.. you certainly can with an SU27. In fact, you can fly with an angle of attack of 108 degrees (that's angle of attack, not angle of climb) $\endgroup$ Commented Apr 20, 2015 at 14:38
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2$\begingroup$ @slebetman: No, you can't. The Su-27 is deeply stalled when doing that. It can, however, still maintain horizontal flight, because it has enough thrust; an example of what I mention in the last paragraph. $\endgroup$ Commented Apr 20, 2015 at 14:42
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Short answer: No.
Long answer: The stall angle of attack varies with speed, altitude, Mach number and the rate of angle of attack increase, as discussed here and here. Since the lift curve slope of a wing does not change with the pitch rate, a high pitch rate will indeed increase the stall AoA by up to 50%.
Speed and altitude effects are expressed in the Reynolds number, and this will also shift the AoA up by several degrees when increased from, say, 200,000 to 5,000,000. The influence of the Mach number will really manifest itself above Mach 0.5, but if the leading edge radius is small, it can already make a difference of several degrees between incompressible conditions and Mach 0.3. Stall at higher Mach numbers is more complex, because before lift drops, the wing will experience increasing buffeting, which by itself will limit the operational AoA.
Next, pitch angle and angle of attack are not the same, but they differ by the flight path angle and the wind angle of attack, which is nonzero if you fly in an up- or downdraft. This is discussed in detail here.
If your engine or airspeed allows, you can fly a full loop without ever stalling the airplane.
answered Apr 20, 2015 at 7:28
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$\begingroup$ would you mind expounding on your conclusion that the critical angle of attack varies as you described? The documentation you cite only states that the maximum CL achieved by the wing varies with the above-mentioned factors; the actual angle of attack at the point of stall does not appear to have been measured during these experiements. The CL conclusion is a very interesting one indeed, which raises a few questions about the way we teach VG diagrams to pilots, but my reading of it suggests that this might be more of an artifact of wing or experiment design or the nature of fluid dynamics. $\endgroup$– habuCommented Apr 21, 2015 at 12:50
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$\begingroup$ It is worthwhile to note that the study mentions two other experiments that did not produce the same results - i will freely admit that I did not dig into those references due to lack of time, so i'm not familiar with their full conclusions. Also, a properly flown loop should have a constant load factor throughout, meaning that at no point during the maneuver should the aircraft enter a stall. $\endgroup$– habuCommented Apr 21, 2015 at 12:51
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$\begingroup$ @habu: The lift curve slope is unchanged, and the higher $c_L$ with higher pitch rate does indeed include a higher stall angle of attack. Note that when a wing stalls, recovery is also delayed with higher pitch rates. The mechanism is explained in the linked answer. Regarding the loop: When properly flown, the load factor should be 2 g less at the top - constant curvature at constant speed would create a constant load factor, and gravity, which changes with the cosine of the pitch angle, must be added to that. Realistically, speed is also less at the top, so centrifugal forces change as well. $\endgroup$ Commented Apr 21, 2015 at 13:36
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$\begingroup$ the second answer provides a good explanation of why, under certain circumstances, an aircraft might not immediately display expected stall behavior until after the boundary layer has had a chance to catch up with the aircraft's new attitude (side note: anyone else picturing Wile E. Coyote reading that?), but it does not change the fact that once things "settle" the aircraft will, in fact, enter a stall should it find itself at an AoA higher than the critical angle of attack. $\endgroup$– habuCommented Apr 21, 2015 at 15:36
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$\begingroup$ @habu: Yes, that is correct. But sometimes this short moment of unstalled extra lift can make all the difference. Don't count on static conditions to apply everywhere, especially when dimensioning wing spars and the like! $\endgroup$ Commented Apr 21, 2015 at 15:41
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For a given wing configuration eg leading/trailing flap, sweep etc, the stall will occur at the same AoA in incompressible flow (low speed, low alt flight) - that's assuming you can clearly define the stall as some fighter aircraft don't have a sudden classical lift break, rather their stall might be defined by rapid drag rise, controllability, handling issues.
For compressible flow, ie flight at high Mach number, the stall AoA will reduce: Stall AoA at 300KCAS/sea-level is greater than the stall AoA at 30,000ft/300KCAS.
Two effects at play here... Compressibility and Viscosity. The effect of compressibility is the predominant one.
Viscosity (Reynolds number). For a given wing design and configuration, as you go up in altitude, the density of the air decreases, while viscosity increases with lower temp. This has a small effect on Reynolds Number.
Compressibility: energy lost in compressing the air is significant at higher mach, which is why you see lower AoA for a given defined stall.
Incidentally, buffet onset is not always a defining attribute of the stall as some jet fighters will experience buffet at very low AoA eg 6-7 degrees, well before the defined stall.
answered Apr 21, 2015 at 5:55