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Comment by drewg123 | Hacker News Reader

I don't understand why an airplane being heavier than what was estimated by the software would cause the tail to drag. I would assume it would cause the plane to not have sufficient thrust to take off, or to use more runway. But having the tail drag makes it seem like it had too much thrust, which translated to a steeper take off than anticipated... Can somebody explain?

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Not an expert, but here's my best guess. When I learned to fly a small plane, you pulled back the yoke at a specific speed. This is called 'rotation' and lifts the nose of the plane off the ground, (wheels are still on the ground) and changes the attitude of the wings, creating substantially more lift for the speed. The plane then lifts quickly off the runway. Presumably different possible weights in a larger plane would cause you to choose different speeds at which to rotate. I also presume rotation is also a more gradual process in a larger plane (a few seconds long???), so the 'early' part of the rotation occurs, gets the plane off the ground, and allows the space for the angle to become steeper in the later part of the rotation. Would be happy for an expert to review and comment on this.

Let's say your target rotation angle is 10 degrees, so at the specified speed (called VR) the pilots 'rotate' the aircraft nose up by 10 degrees.

When they start the 'rotate' maneuver, the nose of the aircraft doesn't immediately go to (for instance) 10 degrees. It's a progression.

As soon as the nose starts pitching up (by 1 degree for instance), the aircraft is already generating lift to pull the aircraft up.

If the speed is set right (so you're fast), when you get to that 10 degrees angle of attack, your tail has already cleared the runway by a very safe margin, because you've been producing X lift from 0 to 10 degrees of rotation.

However, if your speed is set too slow, you're not generating as much lift, so the aircraft will be pulled up slower (you have been producing Y% less lift from 0 to 10 degrees of rotation). This means that by the time you reach your designated 10 degrees of angle of attack, you haven't cleared the tail yet, and it will hit the runway.

In order to achieve the required lift force for takeoff, a higher angle of attack was needed to compensate for the incorrect low rotation airspeed. In effect, pilots raised to a higher nose-high attitude to achieve lift off by the expected point, and this resulted in a tail strike.

I believe it's a question of angle of attack (orientation of the wings vs the direction of airflow). The more you angle the wings, the more upward lift they generate. So when the pilots commanded the plane to "rotate" (take off), they had to pull it back farther than expected to generate sufficient lift for the heavier-than-calcuated and slower-than-calculated plane to leave the runway. That means the nose of the plane was pointing higher than normal, which means the tail was lower than normal.

(Note that you can't keep increasing AoA indefinitely if you need more lift; at a critical speed, the airflow will separate from the wing and stop generating any lift. That's an aerodynamic stall.)

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