vinwow
Posts: 21
Joined: Thu Mar 16, 2000 9:08 pm

Stick Push After V_rotation (Vr)

Tue Dec 17, 2002 3:09 pm

Hi,
Normally, the stick position at Vr would be aft such that the airplane gets some pitch rate to gain angle of attack. What do the pilots do as soon as the nose wheel is off the ground (Main gears still on ground)? Do they immediately start pushing the stick OR they wait till the aircraft is fully airborne (V_Liftoff) to push the stick?
My question is with regard to civil transport aircraft.
thanks for your answers,
vinwow
 
Ivo21
Posts: 25
Joined: Sun Jan 13, 2002 8:52 pm

RE: Stick Push After V_rotation (Vr)

Tue Dec 17, 2002 7:18 pm

Hi Vinwow

I think the stick is held in this aft position until just a little before the aircraft is at its best rate of climb. Because when the angle for best rate of climb is achieved, and then the stick is being pushed forward, the inertia of the plane itself will cause an overshoot. For every configuration there is another climb angle of course.

I hope this helps

Ivo 21
 
411A
Posts: 1788
Joined: Mon Nov 12, 2001 10:34 am

RE: Stick Push After V_rotation (Vr)

Wed Dec 18, 2002 12:08 am

Normally the column movement is aft to facilitate a controlled rate of rotation to achieve the desired body angle. Some long body aircraft (DC8 60/70 series for example) require the rotation to be stopped until the aircraft is definately airborne to eliminate the possibility of a tailstrike.
Recall years ago in an HFB320 Hansa Jet, the stick pusher was activated just after rotation and liftoff at about 100 feet agl. It took the combined efforts of both pilots pulling very hard aft to keep the aeroplane airborne.
This resulted in several (more) grey hairs for all concerned, and was quite enough excitment for one day.
 
AAR90
Posts: 3140
Joined: Fri Jan 21, 2000 11:51 am

RE: Stick Push After V_rotation (Vr)

Wed Dec 18, 2002 2:58 am

At AA, the desired technique is to smoothly rotate at a 2-3 degree per second rate until reaching initial climb attitude. If done properly, there should be no stopping of rotation rate until reaching the climb attitude. AA does not teach (and frowns upon) the "2-phase" rotation.
*NO CARRIER* -- A Naval Aviator's worst nightmare!
 
ramper@iah
Posts: 236
Joined: Mon Dec 24, 2001 7:23 am

RE: Stick Push After V_rotation (Vr)

Wed Dec 18, 2002 5:43 am

The EMB-145 requires the pilots to push forward as soon as the aircraft is off the ground. Since the main gear is aft of the CG, excessive back-pressure is needed to lift the nose. The back-pressure must be relieved and the column pushed forward to prevent the pitch from increasing after lift-off.
 
vinwow
Posts: 21
Joined: Thu Mar 16, 2000 9:08 pm

RE: Stick Push After V_rotation (Vr)

Wed Dec 18, 2002 8:03 pm

Thanks for your observations.
So can we say that the stick is held aft from Vr to the moment angle of best climb is achieved OR V_liftoff, whichever occurs earlier ?
AAR90 has talked about 2-3 deg /second rate until reaching climb attitude. Is this number approximately valid for aircraft with 20-40 passenger capacity?

411A mentioned that some long body aircraft require rotation to be stopped until the aircraft is definitely airborne. I could not understand this. If the upward nose rotation does not take place before lift off (becoming airborne), then it would require very large ground distance to lift-off.
Thanks for your clarifications,
vinwow
 
411A
Posts: 1788
Joined: Mon Nov 12, 2001 10:34 am

RE: Stick Push After V_rotation (Vr)

Thu Dec 19, 2002 12:17 am

Vinwow

The DC8 60/70 series (except 62 series) aircraft are very long body machines and require the rotation to be stopped at approximately 10 degrees and "flown off" before additional pitch up is achieved to desired climb attitude. Otherwise, tailstrikes occur and the respective fleet managers/chief pilots have a sense of humor failure, big time.

Other types (747-400's for example) have experienced tailstrikes due to too rapid a rate of rotation.

Do it be the numbers...no problemo.
 
AAR90
Posts: 3140
Joined: Fri Jan 21, 2000 11:51 am

RE: Stick Push After V_rotation (Vr)

Thu Dec 19, 2002 3:58 am

>AAR90 has talked about 2-3 deg /second rate until reaching climb attitude.
>Is this number approximately valid for aircraft with 20-40 passenger capacity?

I have no direct knowledge yes/no; however, the 2-3 degree per second rotation rate has become the defacto industry standard.

>411A mentioned that some long body aircraft require rotation to be stopped
>until the aircraft is definitely airborne. I could not understand this. If the
>upward nose rotation does not take place before lift off (becoming airborne),
>then it would require very large ground distance to lift-off.

This is known as the "2-step" or "2-phase" rotation technique. Some aircraft/airlines utilize this technique to reduce the chance of a tailstrike. One simply begins rotation (normally at 2-3 degrees per second) until reaching "lift-off attitude" or "initial takeoff attitude" then hold that attitude [phase one] until reaching a specified altitude AGL or positive rate of climb is verified. Once that parameter is met, rotation is resumed [phase two] until reaching desired climb attitude.

AA's preferred technique of 2-3 degrees per second rotation rate will prevent tailstrikes for all aircraft in its current (and recent) fleet; however, rotation faster than 3 degrees per second greatly increases the probability of a tailstrike... especially in 757-200 and 737-800 aircraft. Virtually all of AA's takeoff tailstrikes have been caused by overly agressive rotation rates during takeoff at SNA -- where a rapid climb for noise abatement is required.
*NO CARRIER* -- A Naval Aviator's worst nightmare!
 
Rick767
Posts: 2613
Joined: Sun Jan 30, 2000 8:11 pm

RE: Stick Push After V_rotation (Vr)

Thu Dec 19, 2002 9:18 am

We have to stop the rotation on the (long) 767-300 if by 8 degrees nose-up the aircraft has clearly not yet become airborne.

Theoetically 2.5 degrees per second is the rotation rate, a while ago when I was still a junior as FP on a 763 EGCC-MDPP I over-rotated (maybe 4 degrees per second) and had to arrest at 8 degrees until we got airborne.

Problem is during a Flap 5 takeoff on the 763 the pitch control is much lighter than on the 752 with the typical Flap 15 takeoffs I had been doing for the many weeks before that flight!

All our rotations (on the 752/762 or 763) should be a single steady controlled movement to about 15 degrees, but if over-rotation occurs it is important to remember with the longer a/c especially that you must stop at some point and effectively make a 2-stage rotation if it's all happened too quickly in the initial stages.
I used to love the smell of Jet-A in the morning...
 
EssentialPowr
Posts: 1646
Joined: Wed Sep 06, 2000 10:30 pm

RE: Stick Push After V_rotation (Vr)

Thu Dec 19, 2002 3:36 pm

Ramper@iah, you wrote:

"The EMB-145 requires the pilots to push forward as soon as the aircraft is off the ground. Since the main gear is aft of the CG, excessive back-pressure is needed to lift the nose. The back-pressure must be relieved and the column pushed forward to prevent the pitch from increasing after lift-off."

GCE here (gross conceptional error)

Once the a/c is off the ground, it rotates about its center of gravity. The mains ARE aft of the COG; otherwise the a/c will do a tailstand sitting at the gate. Back pressure causes the rotation, forward pressure "AFTER ROTaTION Vr" is required:

Due to the fact that the a/c must be trimmed in the takeoff config to account for an engine loss "Slightly" Before (read at) V1. Without and engine loss, for any transport a/c, nose down trim will be required.

Most of those erjs use about 7-8 units of trim at takeoff, which accounts for the engine out scenario. The pilots may not know it, but that's why it tends to pitch up after rotation, along, of course, with the fact that the velocity of the a/c is increasing rapidly after rotation - which requires nose down trim to maintain a constant deck angle.

But that's another topic...

Cheers-
 
vinwow
Posts: 21
Joined: Thu Mar 16, 2000 9:08 pm

RE: Stick Push After V_rotation (Vr)

Thu Dec 19, 2002 3:47 pm

Essential,
What Ramper said seems to be correct without any GCE.

He said : "The back-pressure must be relieved and the column pushed forward to prevent the pitch from increasing AFTER (emphasis not Ramper's) lift-off."

This is exactly the same as your explanation that after lift-off, the airplane rotates about COG instead of main LG. This shift of moment reference point drives the requirement of stick to be pushed forward for stable aircraft.
thanks,
vinwow
 
EssentialPowr
Posts: 1646
Joined: Wed Sep 06, 2000 10:30 pm

RE: Stick Push After V_rotation (Vr)

Fri Dec 20, 2002 2:48 am

My apologies; I misintrepreted his statement.

Suffice it to say that the moment arm at rotation is shorter than in flight, and the a/c's speed is rapidly increasing through Vr.

Therefore:

1. less down force is required from the tail, as the a/c rotates
2. increasing speed causes the nose to pitch up for a constant elevator trim setting (set to account for the V1 cut)



cheers
 
vinwow
Posts: 21
Joined: Thu Mar 16, 2000 9:08 pm

RE: Stick Push After V_rotation (Vr)

Fri Dec 20, 2002 3:00 pm

OK, now what's clear to me is the following :

1. The stick is kept pulled (no change) at the position corresponding to Vr to achieve best ROC or Lift-off whichever occurs earlier.
2. The achived pitch rate should be 2-3 deg/sec at lift-off ?
3. After lift-off, except for long bodies aircraft, stick may need to be pushed (vigorously ?).

Please comment if the above is not approximately correct.

There is another point linked to the pitch rate, and this arises from some calculations which I made starting with rotation and then on to Lift-off. I start with pitch acceleration (3 deg/sec2 or 9 deg/sec2). By the time lift-off is achieved (3.3 sec later for low pitch acc. and 1.4 sec later for 9 deg/sec2 pitch acc.), the pitch rate in both cases was same : 15 deg/sec.
Does that kind of high pitch rate get achieved in flight for civil aircraft? I want to judge my calculation with your experiences.


Another point for Essential : " Suffice it to say that the moment arm at rotation is shorter than in flight".
It seems the opposite is true. At rotation the MLG is the point of rotation which is always aft of CoG.

thanks,
vinwow

 
EssentialPowr
Posts: 1646
Joined: Wed Sep 06, 2000 10:30 pm

RE: Stick Push After V_rotation (Vr)

Sat Dec 21, 2002 12:11 pm

The pitch moment arm at rotation is from the (approx) longitudinal center of the horizontal tail to the MLG. Since the COG is fwd of the MLG, in flight the pitch moment arm is longer, as it runs from the tail to the COG.

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