Originally Posted by
JayMatlock
Rotation is a bit slow.. It took about 6s from what I see, whereas it should be done at around 3°s per second, which would give about 3s between nlg and mlg takeoff
Jay, you can't get 3 degrees a second instantaneously, it takes a time to establish the rate, and then the rate has to be reduced to avoid over-pitching, and that means that the rate of 2.5-3 degrees a second only occurs for a part of the rotate, The actual pitch obtained vs time is an S curve, so the underlying rate is more or less a Poisson/normal curve, or in other words, an upside down U. Each part of the pitch.dot change takes time, and that adds to the overall time to achieve the required attitude which will also occur (AOTBE) at a target IAS.
Additionally, once you rotate and lift off, you then have to get to 35'. That takes around another 1.5 seconds to 2.0 seconds to occur.
You are going from 0 to 3 degrees a second rate, and to get that in one second would take a 6 degree a second rate to have been achieved at the end of the second, which will get your eyes watering. the pitch.dot.dot is closer to 3 degrees a second max, and that means it takes a couple of seconds to achieve 3deg sec. If there was a square wave input, (which we don't do) then that would add 2 seconds to achieve 3 sec, and at the end of 3 seconds you would have 6 degrees attitude. At that rate, you are now starting to wash out the input so the rate reduces, and so the next 6 degrees takes about the same time to achieve. Liftoff will occur passing through between 9-11 degrees depending on what brand of plane you are flying, if you are on speed. If you are fast, rougly each 5 KIAS fast will result in liftoff pitch being about 1 degree lower than on speed. Same for low speed.
As a note, be aware that while ground effect increases lift, it also
reduces stall AOA... excessively fast rotates can spoil your day. Not a major problem if you have leading edge devices, more interesting if you don't, or if your wing LE is contaminated. Slow rotation adds take off distance. Under-rotation adds lots of distance.
In the video, the A340 commences rotate at 18 seconds into the video, and is over the end of runway at 27 seconds, and the cliff at 28 seconds. That is 10 seconds maximum to the TODA, and that is beyond the DER which is 10.600'. The plane should have been at 35' 1,382' before the end of the runway, 92,17' from the start of the runway on all engines. From achieving 35' there should have been another 4 seconds of climbing at V2+xx, for additional height above 35', so that is around... another 100-120' (its a Babe...). At is got to the end of the runway (not the cliff) it should have been at around 135-155'.
taking the latest time that the aircraft could be at 35' from the end of the TODA (cliff) then the aircraft should be at 35' at 24 seconds. At that point, its main wheels are still WOG.