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When a plane dives down and then pulls up (assuming that the plane is not in a stall), how exactly does the plane keep its speed when leveling out?

SpiralingTessaract8164
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    By adding power? Not sure what you mean otherwise... (if a skateboarder drops down a ramp onto level pavement how does he keep his speed?) – Michael Hall May 27 '23 at 00:37
  • Yes, throttle control. "When a plane climbs and levels out, how does it prevent its self from going too fast" would be a corollary. Gliders do speed control by adjusting pitch angle relative to the ground (gravity is the "throttle"). – Robert DiGiovanni May 27 '23 at 09:30

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For most airplanes, lift is an order of magnitude higher than drag. This means that lift acts in a direction not exactly, but mostly, orthogonal to the direction of movement and an axis parallel to wingspan.

Lift is normally created to oppose gravity so the plane stays aloft. However, during maneuvering, the pilot will point the lift vector into the newly desired direction of flight. This is done by both changing the magnitude of the lift vector and the attitude of the airplane.

Consider a turn: The pilot will roll the airplane such that the lift vector now points partially sideways into the desired direction. Also, by changing the magnitude, the pilot ensures that enough lift is left pointing up so the airplane can maintain its altitude. Now the sideways component of lift will pull the airplane into the new direction and also decelerate it in the old direction.

In a dive and subsequent pull-up, the same happens, only in the vertical plane.

graphical representation

Glider in a pull-up. Own work, DFS Habicht by Wolfram Gothe (source).

Note how initially lift will accelerate the airplane horizontally, so it becomes faster as it dives down, and on the upward path will decelerate it so it slows down again. All the time, lift will also pull the airplane into the circular movement by being larger than gravitational acceleration.

Also, the slight backward tilt of the lift vector relative to the longitudinal axis of the plane will make sure that it will not climb as high as from where it started the dive. Thrust will be needed to compensate for this backward tilt which we call drag.

Peter Kämpf
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  • The lift in all of your pictures is directed a little more than 90 degrees from the flight path. Then, in all three situations it is slowing the glider down. In a dive, it's the weight that acts in a direction to speed up the glider, not the lift. – Chris May 27 '23 at 16:55
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    @Chris however, the lift vector does help it accelerate it in a new direction. "Pulling out" of a dive involves a change in flight path. – Robert DiGiovanni May 27 '23 at 17:13
  • @RobertDiGiovanni Yes, it changes the direction. With the typical definition, it doesn't speed it up or slow it down. If you include "induced drag" in the definition of lift it also slows it down a bit always. – Chris May 27 '23 at 17:42
  • @Chris Did you notice the word "horizontally" that comes with accelerate? At the bottom of the maneuver the airplane is faster and that speed does not come from gravity. I might need to remind you that gravity only acts in the vertical direction. It can only give the plane a downward speed component. – Peter Kämpf May 27 '23 at 22:12
  • @PeterKämpf Horizontal velocity or momentum component would make sense. At every point, the lift is acting to slow the instantaneous speed (and also to change the direction of the velocity vector). The speed/kinetic energy comes solely from gravity- if you imagine "turning off" gravity while keeping the other forces constant, I'm sure you agree that the glider cannot in any way gain speed, yes? – Chris May 28 '23 at 00:16
  • @Chris So in a turn it is gravity which makes an airplane change direction? This does not make sense. Please reconsider. Start here: Speed is a vector, not a scalar! – Peter Kämpf May 28 '23 at 05:18
  • @PeterKämpf Speed is a scalar, velocity is a vector. Lift makes an airplane change direction (so can gravity, of course- that's essentially what happens in a stall), but it never makes it faster. By diving, a glider can gain speed (gravity does work on the glider), and then it can use its lift to change the direction of its velocity. If lift was directly causing the glider to gain speed, one would expect that pulling sharply up, and thus dramatically increasing lift, would cause a sudden large increase in speed. This, of course, is the opposite of what happens. – Chris May 28 '23 at 05:51
  • @Chris Now I understand your nitpicking. It is the wrong word that I used, not the wrong physics. – Peter Kämpf May 28 '23 at 07:30
  • @PeterKämpf The wrong word, but also the wrong physics. Forces pointed "backwards" relative to the direction of motion simply do not speed things up. Draw the free body diagram along with the direction of motion of a glider in a dive. Compute the work done by each force. It will be negative or zero for lift (depending on your definition of lift), negative for drag, and positive for gravity. – Chris May 28 '23 at 07:54
  • @Chris This is my last comment. You seem to have forgotten what the question is about. It even uses your preferred terminology, so it should be straightforward to understand. – Peter Kämpf May 28 '23 at 10:53
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In order to pull out of a dive, the pilot has to pull back and pitch up the aircraft. This increases the angle of attack, and thus the lift. This acts in a direction perpendicular to the flight path, and so acts to change the aircraft's direction without affecting its speed much.

It also may increase the drag somewhat, which will cause the speed to slow somewhat. And, of course, once the aircraft is in level flight its speed will change if the thrust is insufficient or too much to maintain unaccelerated flight.

Chris
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