relic
Posts: 103
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Rotation

Sat Apr 28, 2007 6:35 am

Hi everyone
I feel really embarrased asking this question having loved civil aircraft for many years but here goes.
When V1 is reached and the stick is pulled back,what actually moves on the aircraft to pull nose up.
I have always thought that the wing config creates lift at a certain speed.V1 is when the nose wheel starts to lift and elevators create nose up.
I have mailed pilots to find out but with no reply........mind you would you reply to a probably obvious question like this.lol.
Please put me out of my misery
Thanks
Andy
 
FredT
Posts: 2166
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RE: Rotation

Sat Apr 28, 2007 6:48 am

Stick aft -> elevator (the hinged part of the horisontal stabiliser) goes up.

This is done at Vr. V1 is the go-no go speed, before which you will abort the takeoff if there is a serious problem (read: engine failure) and after which you go flying no matter what.

Cheers,
/Fred
I thought I was doing good trying to avoid those airport hotels... and look at me now.
 
thegreatchecko
Posts: 689
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RE: Rotation

Sat Apr 28, 2007 6:50 am

Technically, the nose is lifted up at VR not V1. V1 is the takeoff decision speed. Below V1, if an engine were to fail, the aircraft would stop on the runway, after V1, the takeoff is continued and the engine failure is an airborne emergency. VR is rotation speed.

What actually brings the nose up? To keep things on the simple side: The elevators on the horizontal stabilizer.

The horizontal stabilizer is shaped opposite to the wings of the aircraft and actually provide a downforce on the tail. When the pilot pulls back on the yoke, the elevators are deflected upwards, changing the angle of attack (the angle between the relative wind and a line drawn from the leading edge of the surface to the trailing edge, this being the back side of the elevator), which causes the elevator to create more downforce, pushing the tail down and the nose up.

Hope this helps, aerodynamics can be difficult to explain without a good picture, but this explanation should suffice. I'm sure someone will add to it.

Checko

[Edited 2007-04-27 23:51:03]
"A pilot's plane she is. She will love you if you deserve it, and try to kill you if you don't...She is the Mighty Q400"
 
vikkyvik
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RE: Rotation

Sat Apr 28, 2007 6:55 am

Quoting Relic (Thread starter):
I have always thought that the wing config creates lift at a certain speed.V1 is when the nose wheel starts to lift and elevators create nose up.

Just to clarify: the amount of lift the wing creates increases as speed increases. Technically, the wing is creating lift at 10 knots - just not anywhere near enough to lift the airplane.
I'm watching Jeopardy. The category is worst Madonna songs. "This one from 1987 is terrible".
 
relic
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RE: Rotation

Sat Apr 28, 2007 6:57 am

Thanks guys
so i wasnt far off then...lol
Thanks for your help.
 
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Starlionblue
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RE: Rotation

Sat Apr 28, 2007 10:53 am

In theory: If the pilots do not pull back on the stick/yoke, the aircraft will probably lift off anyway eventually as the wings produce more and more lift. However, runway length and tire max speed are limitations.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
FredT
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RE: Rotation

Sat Apr 28, 2007 3:42 pm

Quoting Starlionblue (Reply 5):
In theory: If the pilots do not pull back on the stick/yoke, the aircraft will probably lift off anyway eventually as the wings produce more and more lift. However, runway length and tire max speed are limitations.

Except for the fact that while on the ground, the wing of an airliner is at or near the zero-lift angle of attack. Thus it will not generate any significant lift until rotation.

If it will eventually raise the nose without pilot interaction depends on the trim setting of the aircraft, but with the MLG reaction force aft of the center of gravity it is no all that likely.

Wasn't this covered in another thread here recently?
I thought I was doing good trying to avoid those airport hotels... and look at me now.
 
speedracer1407
Posts: 330
Joined: Fri Dec 24, 2004 4:19 pm

RE: Rotation

Sat Apr 28, 2007 5:38 pm

It was covered with quite a few posts in this thread:
Why Does The Nose Go Up? (by Xbraniffone Mar 23 2007 in Tech Ops)
Dassault Mercure: the plane that has Boeing and Airbus shaking in their boots.
 
A346Dude
Posts: 1161
Joined: Tue Nov 30, 2004 11:23 am

RE: Rotation

Sat Apr 28, 2007 8:41 pm

Quoting FredT (Reply 6):
Except for the fact that while on the ground, the wing of an airliner is at or near the zero-lift angle of attack. Thus it will not generate any significant lift until rotation.

Not true. Unless the lift vector is parallel to the ground (and it's not even close), there will be at least some vertical component of lift.
You know the gear is up and locked when it takes full throttle to taxi to the terminal.
 
FredT
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RE: Rotation

Sun Apr 29, 2007 2:51 am

A346Dude,
the definition of the zero-lift angle of attack is the angle of attack where the lift coefficient is zero, thus there is no lift, the lift vector has a magnitude of zero and thus no direction at all. Not parallell to the ground, not perpendicular to it.

If the lift vector has a non-zero magnitude on the ground roll it is by definition perpendicular to the ground in the aircraft xz plane, as it is perpendicular to the freestream velocity vector.

In real life, there will be positive lift along some parts of the wing and negative lift along some parts, due to washout, but on the whole no significant lift is generated. Lift generates induced drag, and if you have the option to design away the induced drag while accelerating on the take-off roll it'd be pretty daft not to do it.

Cheers,
/Fred
I thought I was doing good trying to avoid those airport hotels... and look at me now.
 
vikkyvik
Posts: 11909
Joined: Thu Jul 31, 2003 1:58 pm

RE: Rotation

Sun Apr 29, 2007 3:54 am

Quoting FredT (Reply 9):

In real life, there will be positive lift along some parts of the wing and negative lift along some parts, due to washout, but on the whole no significant lift is generated. Lift generates induced drag, and if you have the option to design away the induced drag while accelerating on the take-off roll it'd be pretty daft not to do it.

Is this specifically due to some effect of the ground on the airflow? I'd think that if the aircraft were flying in level flight (at a deck angle of zero), it would be nowhere near zero-lift angle of attack.

Thanks.

~Vik
I'm watching Jeopardy. The category is worst Madonna songs. "This one from 1987 is terrible".
 
FredT
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RE: Rotation

Sun Apr 29, 2007 5:11 am

No, in flight at zero deck angle the AoA will be that of the angle of incidence, which is above the zero-lift AoA. In real life, most airliners tend to cruise with a degree or two of positive pitch attitude.

On the ground, most airliners are slightly nose down to keep the aircraft from generating lift until you want it - at rotation.

Rgds,
/Fred
I thought I was doing good trying to avoid those airport hotels... and look at me now.
 
A346Dude
Posts: 1161
Joined: Tue Nov 30, 2004 11:23 am

RE: Rotation

Sun Apr 29, 2007 6:53 am

Quoting FredT (Reply 9):
the definition of the zero-lift angle of attack is the angle of attack where the lift coefficient is zero, thus there is no lift, the lift vector has a magnitude of zero and thus no direction at all. Not parallell to the ground, not perpendicular to it.



Quoting FredT (Reply 9):
In real life, there will be positive lift along some parts of the wing and negative lift along some parts, due to washout, but on the whole no significant lift is generated.

 Confused

I don't get it? How could a wing that is roughly parallel to the ground create no net lift? After all, the cruise attitude is only slightly more nose-up than the attitude on the ground.

Furthermore, if the wing is not creating lift, how does the tail alone generate enough force to rotate the airplane?
You know the gear is up and locked when it takes full throttle to taxi to the terminal.
 
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Starlionblue
Posts: 17212
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RE: Rotation

Sun Apr 29, 2007 9:13 am

Quoting A346Dude (Reply 12):
Furthermore, if the wing is not creating lift, how does the tail alone generate enough force to rotate the airplane?

This thread did a nice turn. Anyway the stabilizer generates enough downforce to rotate the plane, increasing the angle of attack of the wing from "little or no lift" to "lots of lift". The wing does the "heavy lifting".
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
FredT
Posts: 2166
Joined: Thu Feb 07, 2002 9:51 pm

RE: Rotation

Sun Apr 29, 2007 2:49 pm

Quoting A346Dude (Reply 12):
Furthermore, if the wing is not creating lift, how does the tail alone generate enough force to rotate the airplane?

The weight of the aircraft is still carried by the landing gear.
I thought I was doing good trying to avoid those airport hotels... and look at me now.
 
QFA380
Posts: 2013
Joined: Fri Jul 22, 2005 4:38 pm

RE: Rotation

Sun Apr 29, 2007 4:36 pm

Quoting Vikkyvik (Reply 3):
Just to clarify: the amount of lift the wing creates increases as speed increases. Technically, the wing is creating lift at 10 knots - just not anywhere near enough to lift the airplane.

Is speed-lift linear? So at the wing is creating 10 times as much life at 100knots than it is at 10 knots? With flaps and what not at the same settings.
 
FredT
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RE: Rotation

Sun Apr 29, 2007 5:32 pm

No, both lift and drag are proportional to the dynamic pressure, which is proportional to the (true) air speed squared. At 100 knots you will have 100 times more lift than you would at ten knots, all things else being equal.

Regds,
/Fred
I thought I was doing good trying to avoid those airport hotels... and look at me now.
 
rwy04lga
Posts: 1976
Joined: Mon Jul 18, 2005 7:21 am

RE: Rotation

Sat May 05, 2007 1:52 pm

If you've ever seen cockpit videos of takeoffs, notice that the pilot's (PF) hand is on the throttles up until 'V1' is called by the copilot (PNF). After V1, the airplane is committed to flight and cannot be stopped on the remaining length of runway. Therefore, the PF removes his hand from the throttles and grasps the control wheel. When the PNF calls 'Rotate' (VR), the PF will pull back on the stick until the aircraft has attained the proper attitude for unsticking from the runway.

PF pilot flying
PNF pilot not flying.

The two pilots sometimes switch roles for the purpose of letting the junior pilot gain practical experience. The Captain might not be the PF and the copilot might not be the PNF. Hence these specific terms.
Just accept that some days, you're the pigeon, and other days the statue
 
FredT
Posts: 2166
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RE: Rotation

Sat May 05, 2007 3:41 pm

Quote:

British Airways Operations Manual
*** British Airways Flight Operations Department Notice ***

There appears to be some confusion over the new pilot role titles. This notice will hopefully clear up any misunderstandings. The titles P1, P2 and Co-Pilot will now cease to have any meaning, within the BA operations manuals. They are to be replaced by Handling Pilot, Non-handling Pilot, Handling Landing Pilot, Non-Handling Landing Pilot, Handling Non-Landing
Pilot, and Non Handling Non-Landing Pilot.
The Landing Pilot, is initially the Handling Pilot and will handle the take-off and landing except in role reversal when he is the Non-Handling Pilot for taxi until the Handling Non-Landing Pilot, hands the Handling to the Landing Pilot at eighty knots.
The Non-Landing (Non-Handling, since the Landing Pilot is Handling) Pilot reads the checklist to the Handling Pilot until after Before Descent Checklist completion, when the Handling Landing Pilot hands the handling to the Non-Handling Non-Landing Pilot who then becomes the Handling Non-Landing Pilot.
The Landing Pilot is the Non-Handling Pilot until the "decision altitude" call, when the Handling Non-Landing Pilot hands the handling to the Non-Handling Landing Pilot, unless the latter calls "go-around", in which case the Handling Non-Landing Pilot, continues handling and the Non-Handling Landing Pilot continues non-handling until the next call of "land" or "go-around", as appropriate.
In view of the recent confusion over these rules, it was deemed necessary to restate them clearly.

So if anyone has any questions please keep them to themselves.

Rather likely to be a spoof of course... but you never know! Big grin
I thought I was doing good trying to avoid those airport hotels... and look at me now.
 
chrisMUC
Posts: 54
Joined: Sat Jul 22, 2006 7:47 pm

RE: Rotation

Sat May 05, 2007 8:58 pm

Quoting Rwy04LGA (Reply 17):
The two pilots sometimes switch roles for the purpose of letting the junior pilot gain practical experience. The Captain might not be the PF and the copilot might not be the PNF.

I think (at least I hope so) that at most companies it's a fifty-fifty share between Cpt and F/O being PF.
At my company always CM1 (Cpt) has his hand on the thrust levers, either if he's PF or PNF. Call outs are done by the PNF.
This is because RTOs are very time and safety critical, so if the PIC (CM1) thinks that an aborted T/O is necessary, he/she can do it without consulting the F/O.
 
MidEx216
Posts: 547
Joined: Tue Jun 14, 2005 12:19 am

RE: Rotation

Sun May 06, 2007 1:54 pm

In reality, when you pull the stick back, the elevators go up, creating an inverted airfoil shape. Rather than creating nose up, it creates tail down. Given the main gear as a pivoting point, the tail does in fact go down, but then the nose comes up (like a see-saw), and the forward momentum of the engines, along with lessened back-pressure on the stick (which allows the wings to have superior lift over the elevators again) lifts the wheels off the ground.
 
rwessel
Posts: 2448
Joined: Tue Jan 16, 2007 3:47 pm

RE: Rotation

Mon May 07, 2007 3:34 pm

Quoting MidEx216 (Reply 20):
In reality, when you pull the stick back, the elevators go up, creating an inverted airfoil shape. Rather than creating nose up, it creates tail down. Given the main gear as a pivoting point, the tail does in fact go down, but then the nose comes up (like a see-saw),

This is more or less correct.

Quoting MidEx216 (Reply 20):
and the forward momentum of the engines, along with lessened back-pressure on the stick (which allows the wings to have superior lift over the elevators again) lifts the wheels off the ground.

This unfortunately is completely incorrect. First, just tilting the aircraft up does not change the momentum of the engines, which is still straight forward. Second, it's quite aircraft dependent on whether or not you need to reduce back pressure on the stick after rotation, but on almost all, you will have to maintain at least *some* back pressure, unless you want the nose to drop. You *might* have to reduce back pressure a bit *after* the wheel leave the ground, since the CG will be ahead of the main gear (assuming a trike), and after takeoff the aircraft will want to rotate around the more forward CG, increasing the lever arm of the tail, thus increasing the rotation. On the flip side, as you start climbing you need to increase your deck angle to keep the same pitch (since the relative wind will now be coming from somewhat above the aircraft), so you might not have to release back pressure.

Finally, what causes the aircraft to lift off the ground is in increase in lift by the main wing, since it's angle of attack in increased by the rotation. The horizontal stabilizer, under normal conditions (and assuming a conventional configuration), always produces "down" lift, since you always load the airplane such that it's a bit nose heavy (CG ahead of the center of lift). The main wing overcomes the tails downward force by the simple expedient of being vastly bigger.
 
SilverComet
Posts: 85
Joined: Wed Apr 18, 2007 3:04 am

RE: Rotation

Mon May 07, 2007 4:17 pm

Quoting Rwessel (Reply 21):
increase your deck angle to keep the same pitch

What's the difference between deck angle and pitch?

Quoting Rwessel (Reply 21):
since the relative wind will now be coming from somewhat above the aircraft

If the relative wind is coming from above the aircraft, chances are the AoA is negative. Not a good place to be at when you're low and slow. I do, however, agree that as you gain speed, AoA decreases for a constant pitch attitude and your flight path angle increases.

Pitch = AoA + FPA.
 
rwessel
Posts: 2448
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RE: Rotation

Mon May 07, 2007 5:19 pm

Quoting SilverComet (Reply 22):
What's the difference between deck angle and pitch?

Indeed I meant Angle of Attack and not pitch.

Quoting SilverComet (Reply 22):
If the relative wind is coming from above the aircraft, chances are the AoA is negative. Not a good place to be at when you're low and slow. I do, however, agree that as you gain speed, AoA decreases for a constant pitch attitude and your flight path angle increases.

I should have been clearer about the frame of reference. I was referring to the path of the relative wind as seen by an outside observer, which in climbing flight appears to be coming from above.

All in all, not very clear.  Sad
 
relic
Posts: 103
Joined: Sun Apr 10, 2005 3:14 am

RE: Rotation

Mon May 07, 2007 7:49 pm

Well....I didnt expect this much info.Many thanks to everyone who has left comments.
I thought i had the basics of "Nose up",but the info and calculations on this topic have really put me straight.
Many thanks
Andy
 
vikkyvik
Posts: 11909
Joined: Thu Jul 31, 2003 1:58 pm

RE: Rotation

Tue May 08, 2007 10:03 am

Quoting Rwessel (Reply 21):
First, just tilting the aircraft up does not change the momentum of the engines, which is still straight forward.

Actually, it will change the momentum of the engines. Momentum is a vector, with a magnitude and a direction. Tilt the engines, and you've changed the direction (at rotation, by maybe 10 degrees?).

Quoting MidEx216 (Reply 20):
Rather than creating nose up, it creates tail down.

I suppose that's a way to visualize it, but strictly speaking, they're the same thing. It's a force with a lever arm, meaning a moment around the CG (or in the case of takeoff, around the MLG). This moment could be located anywhere along the longitudinal axis of the airplane, provided it has the same value.

~Vik
I'm watching Jeopardy. The category is worst Madonna songs. "This one from 1987 is terrible".
 
rwessel
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Joined: Tue Jan 16, 2007 3:47 pm

RE: Rotation

Tue May 08, 2007 10:08 am

Quoting Vikkyvik (Reply 25):
Quoting Rwessel (Reply 21):
First, just tilting the aircraft up does not change the momentum of the engines, which is still straight forward.

Actually, it will change the momentum of the engines. Momentum is a vector, with a magnitude and a direction. Tilt the engines, and you've changed the direction (at rotation, by maybe 10 degrees?).

Are you saying that if I have a moving object, and I rotate it, it's going to change direction? Sorry, no.
 
vikkyvik
Posts: 11909
Joined: Thu Jul 31, 2003 1:58 pm

RE: Rotation

Tue May 08, 2007 11:23 am

Quoting Rwessel (Reply 26):
Are you saying that if I have a moving object, and I rotate it, it's going to change direction? Sorry, no.

Ah, I see what you were saying. I thought you were talking about the force it was applying to the aircraft (changing the aircraft's momentum).

Just misunderstood.

~Vik
I'm watching Jeopardy. The category is worst Madonna songs. "This one from 1987 is terrible".
 
FredT
Posts: 2166
Joined: Thu Feb 07, 2002 9:51 pm

RE: Rotation

Wed May 09, 2007 3:02 am

Quoting Vikkyvik (Reply 25):
It's a force with a lever arm, meaning a moment around the CG (or in the case of takeoff, around the MLG).

And to be really strict about the semantics and physics, you have a moment applied to a rigid body* (i e not about any specific point of the rigid body, neither CoM nor MLG). This sets up a rotation and an acceleration... and the mains are not a hinge.

I think this has gotten to the point where it is OK to further muddy the waters. Big grin

*) Let's leave the flexing for next weeks class. Homework assignment until then: Set up the FEM model...
I thought I was doing good trying to avoid those airport hotels... and look at me now.

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