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I am trying to understand this topic that is not letting me sleep. I am a private pilot and I am studying for the CPL, if someone could give me a simple explanation of this I would be very grateful. Complex explanations are also welcome, thank you very much in advance. I understand that a stall is an abrupt loss of lift due to the detachment of the boundary layer of an airfoil. As the angle of attack of an aircraft increases, it becomes more "curved" for the air that faces it, causing the air to no longer follow the shape of the wing profile and detach. The capacity of a fluid to follow the curvature of an object depends to a certain extent on its energy, and this can be seen with the use of Slots in airplanes, which re-energize the air flow and allow it to follow more curvatures. pronounced, and for this reason the critical angle of attack increases with the use of these devices. However, when we talk about fluid energy we are referring to its kinetic energy, which in theory could be increased by increasing the speed of the aircraft, so this would make the critical angle of attack to increase, but this does not happen. It is known that the critical angle of attack is an inherent property of the wing design and does not change with the speed of the aircraft. The real question then is why does the critical angle of attack not increase with increasing aircraft speed?

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  • $\begingroup$ The standard explanation about how slots work is that they "re-energise" the airflow, whatever this means. But they simply work giving the following airfoil making up the flap a brand new boundary layer... as simple as that. This new boundary layer is more robust than the one coming from the wing. $\endgroup$
    – sophit
    Jan 20 at 20:28
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    $\begingroup$ But the slot/slat is a convergent duct, so the air exiting is slightly accelerated relative to the free stream. $\endgroup$
    – John K
    Jan 20 at 20:40
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    $\begingroup$ Just to verify: you are asking why isn't the critical AoA (stall AoA) 20° at 200kts if it is 15° at 150kts (numbers just as an example). To me that is even more counterintuitive, since at a higher speed air molecules have to follow wing curvature faster, so they will experience higher acceleration. My original assumption when I started studies in this subject was that critical AoA should be smaller at higher speeds. $\endgroup$
    – Jpe61
    Jan 21 at 8:39
  • $\begingroup$ Basically it seems like another restatement of your question could be, if we put a wing test section in a wind tunnel and vary the wind speed, why should the airflow streamlines remain the same (within reason--i.e. we aren't changing the wind speed enough to cause huge changes in Reynolds number). Especially near the stall a-o-a. $\endgroup$ Jan 21 at 14:41
  • $\begingroup$ Related: aviation.stackexchange.com/q/51128/34686 $\endgroup$ Jan 21 at 19:10

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We can see from an analysis of a Clark Y airfoil that lift is linear to Angle of Attack at angles of attack below stall.

Intuitively, this would mean that the higher suction, or lower pressure above the wing at higher airspeeds, would counter-act the increased momentum of the air molecules at higher airspeeds, keeping flow attached.

Beyond the scope of GA aircraft, higher Mach number will affect stall angles of attack, but for a plane that flies within the range of 50-150 knots, the old saw that stall AoA is always the same holds.

Note on the graph of coefficient of lift vs AoA, when approaching stall, lift is no longer linear to increasing AoA, indicating a collapse of the low pressure pocket above the wing leading to flow separation.

Also remember, for your exam, the bottom of the wing is still producing lift at stall AoA.

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Well if you increase the airspeed you increase the lift and so will climb. This changes the direction of the relative airflow (it now comes more from above), thus slightly reducing your angle of attack.

Now you may say that a slot or slat will also increase lift and result in a climb, and this is true. But the pilot action is then to reduce the airspeed, which allows for an increase in angle of attack beyond the clean critical AOA.

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  • $\begingroup$ You've complicated the picture by touching on pitch stability dynamics. This opens a whole other can of worms that we might explore further (phugoid etc) but might better simply be left unopened. Consider that if we've increased the airspeed and want to experimentally explore what the critical attack is now, one way to vary the angle-of-attack at will without climbing or pitching up into the start of a loop would be to slowly increase the bank angle while operating the controls (including throttle and elevator, but not flaps) as needed to hold both airspeed and altitude constant. $\endgroup$ Jan 21 at 15:12
  • $\begingroup$ Re above -- the other condition I should have mentioned is that we should keep the aircraft in coordinated flight, so that the wing is doing all the lifting, not the side of the fuselage. Also, at least for modest thrust/power levels, leaving the power or thrust setting fixed and allowing the a/c to climb or descend during the test would give essentially the same basic result, as long as the airspeed was held constant as the bank angle was increased. $\endgroup$ Jan 22 at 0:22

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