What causes the slight "pause" at around 10 degrees during rotation in the 767 and 73
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Not the case in the 747-400/-8. There is a distinct change in the force required around the 9-10 degree range. The tail moving into ground effect is the cause.
Proper technique is to rotate at 2.5deg/sec with no pause to target 12.5 deg. initial (which is tail strike territory). However, executed properly this ensure about 18 inches of tail clearance at max weights prior to liftoff. A two stage rotation is not required to protect the tail. Mostly, on the 747 at least, if you see it it's just poor technique.
Of course, none of this precludes the PF from adjusting rotation rates and pitch attitudes for the given conditions (W/S, gusty conditions, eng failure, etc).
Proper technique is to rotate at 2.5deg/sec with no pause to target 12.5 deg. initial (which is tail strike territory). However, executed properly this ensure about 18 inches of tail clearance at max weights prior to liftoff. A two stage rotation is not required to protect the tail. Mostly, on the 747 at least, if you see it it's just poor technique.
Of course, none of this precludes the PF from adjusting rotation rates and pitch attitudes for the given conditions (W/S, gusty conditions, eng failure, etc).
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too lazy to read the whole thread to check if this has been already explained.
That apparent "pause" is basically a change in the pitching characteristic as you transition from rotating the airplane about its main gear, to pitching it up about its longitudinal axis, that is forward. That´s all. If you´ve rotated the plane a few times already, you know that by the time you are passing by those 10 degrees, you apply about 2 inches more of aft elevator displacement, so that the 3 degrees per second are not interrupted.
That apparent "pause" is basically a change in the pitching characteristic as you transition from rotating the airplane about its main gear, to pitching it up about its longitudinal axis, that is forward. That´s all. If you´ve rotated the plane a few times already, you know that by the time you are passing by those 10 degrees, you apply about 2 inches more of aft elevator displacement, so that the 3 degrees per second are not interrupted.
Sudden I think that your theory is incorrect as the change of the pivot point moves further away from the force; ie stab. Therefore there is a greater moment on transition from gear to CofG.
The ground effect explanation gets my money.
If you want an even better mystery then why do some gliders hunt in pitch during a winch launch...stick on the aft stop?
The ground effect explanation gets my money.
If you want an even better mystery then why do some gliders hunt in pitch during a winch launch...stick on the aft stop?
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On the 737, it is the result of the mach trim system action after take off when the aircraft is in the FCNSE region.(within 10 secs after lift off).
The elevators will move to a neutral shift position and will allow the aircraft to have a larger nose up attitude for a specific reason.
This neutral shift is felt by you as a 'pause'...
You can pitch up through this 'pause' for the loss of a few knots...
No,proper training and reading/applying FCTM do.
The elevator neutral shift is to allow a higher attitude in case of engine failure.
The elevators will move to a neutral shift position and will allow the aircraft to have a larger nose up attitude for a specific reason.
This neutral shift is felt by you as a 'pause'...
You can pitch up through this 'pause' for the loss of a few knots...
The third answer is that the "pause" is designed into these aircraft to minimise tail strike if the rotation is too fast?
The elevator neutral shift is to allow a higher attitude in case of engine failure.
Last edited by de facto; 26th Feb 2013 at 13:13.
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Excuse me my Lord
but the Mach trim works in high speed flight regime (as you said above Mach 0.615 but also within 10 secs after lift off as described above.
The speed trim by the way moves the stabilizer not the elevator , directly at least.
but the Mach trim works in high speed flight regime (as you said above Mach 0.615 but also within 10 secs after lift off as described above.The speed trim by the way moves the stabilizer not the elevator , directly at least.
Last edited by de facto; 27th Feb 2013 at 16:00.
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That's why I ask as my tech manual doesn't mention the 10 seconds airborne in the Mach trim section only in the speed trim bit.
Classic not NG by the way, are they different?
Classic not NG by the way, are they different?
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I fly the NG and only have a few hours on the CL so not sure,but i dont see why not,both have mach trim and speed trim system.
The speed trim after take off is for speed stability when in a low speed-high thrust scenario,the FCC will command the stabilizer CPU to mive it downwards to induce an increase in speed.
The mach trim in our case moves the elevator to a neutral position so the elevator can be moved more upwards in case anenginefailsduring take off,a different function of the mach trim other than speed stability at mach .615 and above.
The speed trim after take off is for speed stability when in a low speed-high thrust scenario,the FCC will command the stabilizer CPU to mive it downwards to induce an increase in speed.
The mach trim in our case moves the elevator to a neutral position so the elevator can be moved more upwards in case anenginefailsduring take off,a different function of the mach trim other than speed stability at mach .615 and above.
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Not the case in the 747-400/-8. There is a distinct change in the force required around the 9-10 degree range. The tail moving into ground effect is the cause.
Proper technique is to rotate at 2.5deg/sec with no pause to target 12.5 deg. initial (which is tail strike territory). However, executed properly this ensure about 18 inches of tail clearance at max weights prior to liftoff. A two stage rotation is not required to protect the tail. Mostly, on the 747 at least, if you see it it's just poor technique.
Proper technique is to rotate at 2.5deg/sec with no pause to target 12.5 deg. initial (which is tail strike territory). However, executed properly this ensure about 18 inches of tail clearance at max weights prior to liftoff. A two stage rotation is not required to protect the tail. Mostly, on the 747 at least, if you see it it's just poor technique.
I had been wondering what the reason was for the force change. Quite a convenient aerodynamic feature to have if it is just the ground effect as the cause.
Last edited by JammedStab; 28th Feb 2013 at 01:23.
