What is the immediate cause of liftoff?

Henning Makholm
  • What is the immediate cause of liftoff? Henning Makholm

    If I understand correctly, when a plane transitions from takeoff roll to being airborne, it is not something that happens "by itself" when the airspeed is high enough, but is caused by deliberate pilot input somehow.

    Which control surfaces are involved in causing the plane to lift off?

    1. Is it an ordinary nose-up movement of the elevators? That is, the elevators create negative lift that pushes the tailplane down, which makes the entire aircraft pivot around the main gear and increases the wings' AoA enough to create lift that takes the plane off the ground.

    2. Or is it something that increases lift with unchanged attitude, such as a symmetric aileron movement, or an additional flaps extension? And then after the plane is airborne it is rotated to climbing attitude?

    The descriptions I can easily Google up point towards 1. However, while that makes perfect sense for tricycle gear I don't see how that would work with taildraggers. Pushing down on the tail would just increase the downward force on the tail wheel rather than change the AoA.

    1. As the elevators rotate to get a negative pitch angle to the horizontal (moving the tip of the elevator upwards), a pressure difference is created causing it to produce a downwards force, and as a result the aircraft pivots around the landing gear, rotating upwards.

    elevator

    1. As it does so, the wing starts to generatore more lift, as can be seen on the graph below. The higher the angle of attack- the more lift, since the lift coefficant becomes higher, up to where stall starts.

    2. When the lift exceeds the weight, the aircraft will lift off.

    lift

    For taildraggers, the tail is normally rotated up at a low speed while the aircraft continues to accelerate, and once a sufficient speed is reached, the tail surface is again lowered, causing the wing to increase the AoA and to lift. Footnote: If i'm not mistaken, this is more of an act to improve control and reduce drag, since at some point the aircraft will lift off on it's own with it's tail-down angle of attack.

    taildragger

  • I can't answer for tail-draggers, but this is how it works for a tricycle landing gear.

    1. Initiate takeoff roll - power is applied
    2. $V_1$ - At this point, the pilot must proceed with takeoff, even in event of an engine failure.
    3. $V_R$ (Rotate) - at this point, the pilot pulls back on the stick, causing the aircraft to pitch up, rotating on the main gear.
    4. $V_2$ - Rotate at this speed in event of an engine failure.

    Also, if you keep accelerating down the runway, at some point you will have so much lift that the plane will take itself off.

  • The answer is indeed (a). For tailwheel aircraft, what typically happens is typically the following:

    1. With all 3 wheels on the ground, you start gaining speed by increasing thrust.
    2. Very soon (e.g. much sooner that the minimum take-off speed), you push the stick to lift the tailwheel off the ground and put the aircraft at more or less zero AoA.
    3. When you reach the rotate speed, you pull to increase the AoA and to lift off (as with tricycles). The required pitch up angle is much smaller than what would be needed for the tail wheel to touch the ground again.

    Alternatively, you may keep the tailwheel on the ground all the way until liftoff. Since with the tailwheel on the ground your AoA is positive, you just need to reach enough speed to generate the required lift for take-off. This may not be possible with all tailwheel aircraft or may increase take-off run length, though.

  • When you take off in a tail dragger, you let the tail rise so you’re only on the main gear as you accelerate. Then, when you want to take off, you use normal elevator movements to increase the angle of attack. Yes, the tail will fall slightly when this happens. Increased angle of attack means more lift, so you take off.

    If you hypothetically held the tail wheel on the ground with the elevator, the main wheels could, for a theoretical plane, lift off with the tail wheel still on the ground. But in reality, the main wing would be eventually generating more lift than the tail and so the plane would rise into the air.

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