If I was in an elevator in a sky-scraper and the cable broke, I would free fall and feel weightless until hitting the ground. When I cause a stall on an airplane (power-ff) and the wings stop producing lift, why doesn't the same effect occur?
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16$\begingroup$ No if the elevator cable breaks the cabin brakes will engage to stop it and keep the occupants safe. $\endgroup$– ratchet freakCommented Mar 31, 2020 at 9:58
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2$\begingroup$ With some stalls, you might feel weightlessness. But as an important safety point; reduced "g" is not a reliable stall symptom and should not be considered as one. It may not occur at all in shallow stalls, and may occur in lowering-attitude situations that aren't stalls. The "low, slow, ready to spin" situation will usually be 1g. $\endgroup$– Toby WilsonCommented Mar 31, 2020 at 10:22
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2$\begingroup$ Note that some aircraft can "fly" in stall mode. The maneuver is known by various names such as "high alpha" and "cobra". Although this is usually done via thrust vectoring engines radio-controlled model aircraft without thrust vectoring have been known to fly in the stall regime via aerodynamic forces (usually aided by prop wash) $\endgroup$– slebetmanCommented Mar 31, 2020 at 14:43
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4$\begingroup$ The free falling elevator is an idealization in physics, not engineering reality. It would have to have no air resistance, no resistance from the shaft, no resistance from safety equipment. $\endgroup$– Ross MillikanCommented Mar 31, 2020 at 15:30
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19$\begingroup$ @ratchetfreak, Elevators in thought experiments do not have cabin brakes. Providing them with cabin brakes might save the lives of imaginary victims, but it would virtually always defeat the purpose of the thought experiment. $\endgroup$– Solomon SlowCommented Mar 31, 2020 at 20:44
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4 Answers
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If the cables break on an elevator (and the safety brakes fail), you won't be in true freefall. You'll still have friction from wind resistance, from the guide rollers on the rails, etc. The same is true in an airplane. Even if you're falling straight down, you'll still have wind resistance.
In addition, lift doesn't just drop straight to zero when the wing stalls (no matter what it might feel like to the pilot). Here's the lift-to-angle diagram for a typical wing:
This wing stalls at about 16° AoA. But notice that the coefficient of lift doesn't simply drop to zero, it just starts descending at that point. So even in a stall, the wings are still generating some lift, just not enough to overcome the weight of the plane.
answered Mar 30, 2020 at 22:43
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10$\begingroup$ Also, at some point beyond the stall angle, the lift coefficient starts increasing again, to a peak at 45°, where the lift coefficient is even higher than at the stall angle (but the vastly-increased drag coefficient prevents most aircraft from maintaining steady-state flight at this high an attack angle). $\endgroup$– VikkiCommented Mar 30, 2020 at 23:05
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1$\begingroup$ Is that curve for some GA aircraft? Aren't the wings of most GA aircraft intentionally designed so that the stall starts at the wing root, and progresses out toward the wing tips as the AoA increases? In other words, when you say, "even in a stall the wings are still generating some lift," wouldn't it be more accurate to say, Even when the wing root is stalled, the wing tips may still be generating some lift? $\endgroup$ Commented Mar 31, 2020 at 20:57
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$\begingroup$ @Solomon Even when flow is separated, there is still lift produced by the separated portion of the wing. $\endgroup$– JZYLCommented Mar 31, 2020 at 21:48
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3$\begingroup$ @SolomonSlow Yes, a lot of aircraft (not just GA) are designed like that. The wing will have a different AoA, or different camber, or something to delay stall. But that's only to try and maintain smooth airflow over the ailerons for as long as possible. The stalled section(s) of the wing will still produce some lift. Thus, each section of the wing will have a similar lift-to-AoA diagram (just with the stall point in different places), and the overall wing will also have a similar lift-to-AoA diagram for its overall (average?) AoA. $\endgroup$ Commented Mar 31, 2020 at 22:08
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5$\begingroup$ The stall feels like a sudden free fall because the back side of the lift curve is an unstable zone. The slight decrease in lift after the peak causes an increase in vertical decent speed which in turn increases the AoA which further decreases lift... and this loop very quickly compounds to give the sensation that lift suddenly disappeared. $\endgroup$ Commented Apr 1, 2020 at 2:59
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Is a “stalled” aircraft free-falling?
No!
If I was in an elevator in a sky-scraper, and the cable broke, I would free fall and feel weightless (until hitting the ground of course). When I stall an airplane(power-off) and the wings stop producing lift, why doesn't the same effect occur?
Because in a stall, the aerodynamic force component acting against the direction of the weight vector is not actually zero. In fact, after a very short time it becomes equal to the weight vector, thus yielding a steady-state situation in the vertical plane. Lift and drag, as conventionally defined, both contribute to this aerodynamic force component; lift is by no means zero even in stalled flight. In a stall, you are definitely not free-falling. You are just flying along a very poor glide ratio or descent ratio. After the first few seconds, you are no longer accelerating. An aerodynamic force exists that not only limits the rate of downward acceleration, but also yields a specific downward terminal velocity, as well as as a specific forward speed.
answered Mar 30, 2020 at 22:34
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12$\begingroup$ Basically, you can look at a stalled aircraft as a very poor parachute. $\endgroup$ Commented Mar 31, 2020 at 4:20
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7$\begingroup$ @MikeBrass As a hang-glider pilot, that's exactly how my instructor described it. In hang-gliding, when you land you rotate the wing to give a rapid stall and turn it into an airbrake. If you've got too much speed though, you go up a bit. You then have to hold your nerve and ride it down like a "very poor parachute", because otherwise it tries to recover from the stall and you'll plant into the ground like a lawn dart. $\endgroup$– GrahamCommented Mar 31, 2020 at 8:03
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1$\begingroup$ "In fact, after a very short time it becomes equal to the weight vector, thus yielding a steady-state situation in the vertical plane." -- Is that not also true for our poor, doomed elevator? $\endgroup$ Commented Mar 31, 2020 at 8:32
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$\begingroup$ @WayneConrad Indeed, that would be true for any falling object that's not accelerating - it's reached terminal velocity. In reality, falling objects are only truly "weightless" at the very start of their descent when drag is near-zero, so a person in a falling elevator is not really weightless, just like a person in a stalled plane. $\endgroup$ Commented Mar 31, 2020 at 13:00
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$\begingroup$ @WayneConrad I am not sure if an elevator "flight level" is high enough to get even close to a terminal velocity before reaching the ground. $\endgroup$– SumaCommented Mar 31, 2020 at 18:52
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A stall is not a free-fall. It is a loss of laminar airflow over the wing, resulting in a loss of lift. The response to a stall is to stop the (usual) roll induced by one wing stalling before the other. Then drop the nose - most aircraft will do this by design - until you achieve a flyable speed with proper airflow over the wing. The problem here is that you may not have enough altitude to reach that speed >> crash. I once stalled and dropped from over 25000 (not looking at altimeter) to under 12000 before I had enough airspeed to pull up. With the resulting pucker factor I bottomed out at around 7000 ft.
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Speaking as a commercial pilot, pilot instructor and skydiver I think I can answer this.
An aircraft has a centre of pressure, which is the lift vector that comes out of the wing and opposes the weight. As you increase the angle of attack of the wing, up to around 16 degrees angle of attack, the centre of pressure (lift) moves slowly forward. When you stall, the boundary layer which is the area of low pressure air sucking the wing upwards, rips off and the centre of pressure moves rearwards, behind the centre of gravity. This drops the nose of the aircraft. The drop of the nose reduces the angle of attack and the wings start to fly again.
Even if you hold the aircraft in a stall at the buffet, the wing will still drop and attempt to fly again repeatedly. So, you're never in freefall, which would assume terminal velocity, you're just pitching up and down as you move forwards in the stall.
Freefall, as a skydiver, implies no forward movement and no lift, even though many skydiving diciplines do involve both such as tracking, wingsuiting and even formation, where you de-arch to slow yourself down and rise relative to others.
