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This question makes me wonder what happens at -1G.

When the load factor of an aircraft increases above 1, the stall speed increases. As the load factor comes closer to 0, stall speed decreases and becomes "infinitely" low (basically 0 kts) as an aircraft can not stall at 0G. But what happens at a negative load factor?

For example, during inverted flight the airfoil is still producing lift when the angle of attack is negative (enough). Thus the stall speed should change and as the efficiency of a inverted wing is (almost definitely) not as high, I am assuming that the stall speed decreases.

But is a negative G maneuver necessarily to be done in inverted flight? No!

As a glider pilot, I can refer to negative Gs during winch launches, or rather when they fail. You often practice cable fails, where your instructor disconnects you from the winch. To maintain speed and prevent a stall you push the nose down, ideally flying a small Zero-G parabola (as this will prevent a stall). But what if you push even more? Where is the stall speed going to be at?

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  • $\begingroup$ It is more correct to reference stall angle of attack than stall airspeed. A given airfoil can technically stall at any airspeed once AoA increases beyond the critical AoA. $\endgroup$
    – Rhino Driver
    Aug 23, 2015 at 19:38
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    $\begingroup$ AoA changes with the load factor as well. In fact, I believe AoA will instantaneously change with load factor. $\endgroup$
    – Rhino Driver
    Aug 23, 2015 at 23:43
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    $\begingroup$ In your question you said : >When the load factor of an aircraft increases above 1, the stall speed decreases. Well this is not true, saying that means that for example your stall speed in a 60 degrees turn will be less than the stall speed in straight and level flight. So it is obvious to say that when the load factor of an aircraft increases above 1, the stall speed increases $\endgroup$
    – Thomas
    May 22, 2016 at 17:53
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    $\begingroup$ I would correct some words in the first paragraph : When the load factor of an aircraft increases above 1, the stall speed increases. As the load factor comes closer to 0, stall speed decreases and becomes infinitely low as an aircraft can not stall at 0G. $\endgroup$
    – Thomas
    May 22, 2016 at 18:32
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    $\begingroup$ Yes, first two sentences are now correct after recent edit to fix another edit; stall speed increases as load factor increases, and is 0 at 0G. $\endgroup$
    – quiet flyer
    Nov 4, 2020 at 13:03

1 Answer 1

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Only in aerobatic aircraft is the inverted stall speed identical to the upright stall speed in level flight. Airplanes registered in the Normal and Utility classes have cambered airfoils which have a higher positive maximum lift coefficient. In inverted flight, their stall speed is higher than their upright stall speed. The same goes for gliders and airliners.

When I had my last aborted winch launch in a ASK-13, I involuntarily checked how well the cockpit had been cleaned last time. I had to briefly close my eyes to avoid getting sand and dust into them. I can confirm that it is fully possible to fly a negative maneuver in upright position, albeit only briefly. However, the negative acceleration had as much to do with the rate of rotation and my position ahead of the cg as with negative lift.

If I had pushed a lot more, I could had stalled the wing by undershooting the negative lift minimum. But this would had just limited the downward acceleration, and the negative lift would not had been sufficient for inverted flight, because the inverted stall speed in level inverted flight was most likely higher than the speed at the time of the tow line rupture.

The negative stall speed is just your flight speed, and the possible negative gs grow with the square of speed. Stall is a matter of angle of attack, not of speed. A negative stall in upright attitude is rather uneventful - the aircraft will be at a high rate of rotation and will just hit the limit of downward acceleration.

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  • $\begingroup$ Yeah, it is definitely a good way of checking the cleaning staff's work! Am I right to believe that as soon as you have a negative aoa, the lift will act as a downward force? And (I know this could be one on its own, but) does negative G Forces automatically mean a negative aoa? I think not...? $\endgroup$
    – Maverick283
    Aug 23, 2015 at 22:08
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    $\begingroup$ @Maverick283: No, the AoA must drop below the zero-lift value, which is slightly negative for most airfoils. But then, yes, the lift will act downwards. Also, the effect of pitch rate is considerable - a high nose-down rate might give the pilot the impression of negative gs while the wing is only around zero lift (and the whole aircraft around zero g). $\endgroup$
    – Peter Kämpf
    Aug 23, 2015 at 22:18
  • $\begingroup$ Symmetrical aerofoils were also common in the early days of supersonic flight. So too were tailless aircraft, usually deltas. For example the Fairey Delta 2 speed-record-breaker had a symmetrical tailless delta wing, which had identical characteristics either way up. $\endgroup$
    – Guy Inchbald
    Oct 14, 2020 at 15:06

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