In the takeoff roll, I'm pretty sure planes produce some lift before rotation, but how much? Is it insignificant compared to post-rotation, or is it large compared to the post-rotation lift?
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asked Feb 25, 2015 at 3:48
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2 Answers
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Check out this plot (taken from wikipedia), of the lift coefficient against angle of attack. During takeoff roll, lift will be increasing due to the speed increasing, however not enough (in general) for the plane to achieve takeoff without rotation.
Once rotation starts, the lift increases dramatically, for (a very rough) example, consider a plane on takeoff roll, then rotating to 15 degrees angle of attack. By looking at the lift/AoA plot, we can see that the lift coefficient increases from roughly 0.55 to 1.7, giving roughly 3 times as much lift as before rotation.
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2$\begingroup$ Just to add, during take-off high lift devices are deployed, making the curve above (depending on the devices) moved up, making less dramatic (relatively) the lift change. Also to take into account the ground effect. $\endgroup$ Feb 25, 2015 at 6:53
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1$\begingroup$ In high crosswinds in a taildragger, we keep the stick forward to keep the wheels in firm contact with the ground. As soon as you release the pressure it'll fly off the runway. Very little back pressure is required. $\endgroup$– rbpApr 10, 2015 at 20:46
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Rotation increases the angle of attack, and as a result, the lift force also increases. However, there are aircraft that do take off with very little angle of attack change (maybe a couple of degrees, as in the case of UAVs). For those aircraft the lift force before rotation can be 60-70 % of the weight of the aircraft. Remember, for take off to happen, the lift force must be at least equal to the take off weight.
One more parameter is, of course, the airspeed. The lift increases with the square of the airspeed. And at the time of rotation, comparing just before and just after liftoff, the lift generated are quite close to each other, depending on the change of alpha (angle of attack). At a stationary airplane, with no headwind, there is no lift (ignore engine flow).
To sum up, during the ground roll, from standby to rotation, the lift increases from zero to a large fraction of the weight. The fraction depends on the type and design of the aircraft, but cannot be equal to the weight.
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2$\begingroup$ Take the B-52: It doesn't rotate but accelerates until it flies. Here, lift is 100% of weight before take-off (on a level runway). $\endgroup$ Apr 9, 2015 at 19:51
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$\begingroup$ @Peter, I'm not sure the B-52 really allows no-rotation for takeoff; at a small wind gust or disturbance, the plane would lift and touch again, depending on the magnitude. However, touching on and off would probably not cause hazards, so it may be that way. $\endgroup$ Apr 9, 2015 at 20:05
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$\begingroup$ Having read a bit more about the B-52, and after watching a take-off video, i guess they just let it float into air. But a pilot has said somewhere that he tries to keep the rear wheels on the ground, this sounds like there is some rotation implied? $\endgroup$ Apr 9, 2015 at 20:16
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2$\begingroup$ The forward wheels of the B-52 are so much forward of the center of gravity that there is no chance in hell that the rear wheels will lift off while the front wheels are still on the ground. Pilots sometimes use strange rationalizations for their actions. $\endgroup$ Apr 9, 2015 at 21:27