my235
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Takeoff Momentum

Thu Jul 26, 2012 8:25 am

I'm sure people have noticed a lot of times 30secs or so after takeoff, the airplane (mostly larger jets) will "settle" and you get a sinking feeling. This is caused by the loss of forward momentum built up from the takeoff roll correct?

Crazy question: What if large airliners would continue to "momentum-climb" up to cruising altitude? Would you burn less fuel? Some small birds use this technique. But they're small.  
 
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Starlionblue
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RE: Takeoff Momentum

Thu Jul 26, 2012 8:31 am

Quoting my235 (Thread starter):

I'm sure people have noticed a lot of times 30secs or so after takeoff, the airplane (mostly larger jets) will "settle" and you get a sinking feeling. This is caused by the loss of forward momentum built up from the takeoff roll correct?

No it is not caused by that. Momentum doesn't work that way. Unless you keep applying thrust your momentum will be killed by drag (and weight if you climb).

The feeling you get is because a while after take-off, the engines are typically throttled back from take-off thrust to the lower climb thrust. And so you get less acceleration.

Quoting my235 (Thread starter):
Crazy question: What if large airliners would continue to "momentum-climb" up to cruising altitude? Would you burn less fuel? Some small birds use this technique. But they're small.

That won't work either. Momentum is inherent in accelerated mass. Sure, if you have a faster take-off roll you will have more momentum as per Newton's Second Law of Motion (F=ma). But that's true in any take-off, for bird or plane.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
David L
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RE: Takeoff Momentum

Thu Jul 26, 2012 9:22 am

Quoting my235 (Thread starter):
This is caused by the loss of forward momentum built up from the takeoff roll correct?

Generally speaking, from the start of the take-off roll, the momentum will either be increasing or remaining fairly constant until you begin to slow down on the descent. Of course, as fuel is used the mass decreases so the momentum will decrease if speed is constant, since momentum = mass * velocity, but that happens relatively slowly.

Quoting Starlionblue (Reply 1):
The feeling you get is because a while after take-off, the engines are typically throttled back from take-off thrust to the lower climb thrust. And so you get less acceleration.

Plus a reduction in pitch to compensate and to begin further acceleration to climb speed, which gives the sensation of the aircraft falling away from beneath you as your own momentum tries to keep you going in the original, steeper direction.
 
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Starlionblue
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RE: Takeoff Momentum

Thu Jul 26, 2012 11:00 am

Quoting Starlionblue (Reply 1):
Sure, if you have a faster take-off roll you will have more momentum as per Newton's Second Law of Motion (F=ma).

I can't believe I wrote that. What I meant was that you will have more momentum as of Newton's First Law of Motion... Damnit...
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
Slcpilot
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RE: Takeoff Momentum

Thu Jul 26, 2012 11:35 am

It is my notion that the "settle" feeling the OP describes is actually the retraction of slats and flaps.

This change of configuration often results in this sensation as well as a slightly increased angle of attack.

Cheers!

SLCPilot

Ps. I have no idea how momentum applies here!
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my235
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RE: Takeoff Momentum

Thu Jul 26, 2012 12:08 pm

Quoting Starlionblue (Reply 3):
I can't believe I wrote that. What I meant was that you will have more momentum as of Newton's First Law of Motion... Damnit...

Hahaha!   Thanks for all the input.
 
David L
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RE: Takeoff Momentum

Thu Jul 26, 2012 12:08 pm

Quoting SLCPilot (Reply 4):
It is my notion that the "settle" feeling the OP describes is actually the retraction of slats and flaps.

That's true - a reduction in lift will also cause the aircraft to feel as if it's falling away from under us. I'll rephrase and say I think the "sinking" feeling is caused by a reduction in the (upward) vertical velocity of the aircraft combined with our own momentum trying to keep us going on our original trajectory. For a brief moment, our backsides and the aircraft are travelling in slightly different directions and it feels as if the aircraft is dropping away from under us.
 
my235
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RE: Takeoff Momentum

Thu Jul 26, 2012 12:15 pm

The reason I posted this is i'm out on the ramp at PDX and watch 8 or 9 heavy freighters depart every evening (767, A300, MD10, MD-11) Each one, especially the MD-11's takeoff at a tremendously steep pitch. But then very visibly shove the nose back down. Awesome sight.
Again thanks for the input
 
CosmicCruiser
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RE: Takeoff Momentum

Thu Jul 26, 2012 1:23 pm

Quoting my235 (Reply 7):
Each one, especially the MD-11's takeoff at a tremendously steep pitch. But then very visibly shove the nose back down.

I can speak for the MD-11 and it's no steeper than any other jet, about 25deg max pitch. Usually you won't see that pitch angle unless you're light. Since we now use NADP 2 for domestic t/o we do pitch over at a low 1000' to accelerate and clean up.
 
Rara
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RE: Takeoff Momentum

Thu Jul 26, 2012 1:38 pm

Quoting David L (Reply 2):
Plus a reduction in pitch

   That's it.
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KPWMSpotter
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RE: Takeoff Momentum

Thu Jul 26, 2012 1:45 pm

The "settling" you are feeling is a combination of factors, not exactly momentum related.

After takeoff a few things need to be reconfigured on most jets: flaps and slats need to be retracted, thrust needs to be reduced from takeoff to climb power, and the aircraft needs to accelerate to its climb speed. These three things usually happen at generally the same time and result in the deceleration/sinking illusion. In order to accelerate, the aircraft's nose needs to be lowered. While the aircraft may still be accelerating and maintaining altitude, the pitch-down maneuver often feels like "falling out of the sky" from the back.

I've noticed that this sensation is most pronounced in the MD-80 and DC-9 series aircraft, where the initial climb phase is accomplished at a stunningly nose-high attitude. The MD-80s aren't falling out of the sky any more than other types, the transition from nose-high to a level climb attitude is just more pronounced.

As for momentum, momentum is directly related to mass and velocity (p=mv). Since mass will be relatively constant, momentum is actually increasing during this pitch-down/clean-up maneuver (direct relation to velocity). Even though the plane may be climbing or accelerating less than on the takeoff roll, it is still climbing and accelerating.
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chimborazo
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RE: Takeoff Momentum

Thu Jul 26, 2012 7:15 pm

Noise abatement?

Most times when I fly I notice at- as OP said- about 30s after take-off, a light feeling as the pilot reduces pitch slightly and often backs off the throttles a tad.

Also, perhaps a slight decrease in pitch to acelerate to climb speed? The flaps start to come up later.

I drive the M6 (sit in traffic rather) past BHX a lot and it's quite noticable that planes depart at "regular" t/o angle and then drop the nose quite noticably- often before making a big turn. Always assumed the combination was noise abatement as it's quite heavily populated.
 
aklrno
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RE: Takeoff Momentum

Thu Jul 26, 2012 8:55 pm

Quoting Chimborazo (Reply 11):

Noise abatement?

Most times when I fly I notice at- as OP said- about 30s after take-off, a light feeling as the pilot reduces pitch slightly and often backs off the throttles a tad.

Also, perhaps a slight decrease in pitch to acelerate to climb speed? The flaps start to come up later.

I drive the M6 (sit in traffic rather) past BHX a lot and it's quite noticable that planes depart at "regular" t/o angle and then drop the nose quite noticably- often before making a big turn. Always assumed the combination was noise abatement as it's quite heavily populated.

That's very much the reason when departing SNA. Some pilots warn passengers before take-off about the nearly immediate throttle back until the coastline is passed. It seems as if the engines have died.
 
tdscanuck
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RE: Takeoff Momentum

Fri Jul 27, 2012 12:35 am

Quoting Chimborazo (Reply 11):
Most times when I fly I notice at- as OP said- about 30s after take-off, a light feeling as the pilot reduces pitch slightly and often backs off the throttles a tad.

A noise abatement profile will do that, but the normal cutback from takeoff to climb thrust will do the same thing (and, unlike noise abatement, the cutback always happens).

Climb rate is all about excess thrust. If the thrust comes back you must shallow your climb or you will start to slow down. Airliners don't decelerate on climbout, they only stay steady or accelerate.

Tom.
 
BEG2IAH
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RE: Takeoff Momentum

Fri Jul 27, 2012 1:05 am

Quoting my235 (Reply 7):
Each one, especially the MD-11's takeoff at a tremendously steep pitch. But then very visibly shove the nose back down.
Quoting tdscanuck (Reply 13):
Climb rate is all about excess thrust. If the thrust comes back you must shallow your climb or you will start to slow down. Airliners don't decelerate on climbout, they only stay steady or accelerate.

I understand you can't keep a steep climb rate for too long (you're not on a rocket), but does this reduction in pitch have anything to do with the "regulated" airspace around big airports? If you go too high too fast, wouldn't you cut path of some other aircraft? One good example is ORD where you level off pretty quickly and then it takes forever before you start climbing again for real.

Thanks.
 
tdscanuck
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RE: Takeoff Momentum

Fri Jul 27, 2012 1:47 am

Quoting BEG2IAH (Reply 14):
I understand you can't keep a steep climb rate for too long (you're not on a rocket), but does this reduction in pitch have anything to do with the "regulated" airspace around big airports?

Not really. The thrust cutback from takeoff to climb thrust happens in the first couple of thousand feet; you're usually cleared at least that high coming out of the takeoff. It is possible for ATC to level you off really low after takeoff but it's not common.

Quoting BEG2IAH (Reply 14):
If you go too high too fast, wouldn't you cut path of some other aircraft?

Possibly, although if ATC is on the ball that should be relatively rare. They want you out of their airspace as quickly as possible and the guys on approach should be sequenced so that they're that close to the takeoff path.

Quoting BEG2IAH (Reply 14):
One good example is ORD where you level off pretty quickly and then it takes forever before you start climbing again for real.

It's definitely not unusual to get levelled off at some intermediate altitude on the way out but it's rare that it's so low that the engines haven't pulled back to climb thrust already.

The general profile is to takeoff and climb the first few thousand feet at V2+ ~20 knots, then reduce thrust to climb thrust and pitch over to 250 knots, then climb at 250 knots to 10,000', then pitch over again to accelerate to best climb speed, then finish climbing to initial cruise altitude.

Note that the 250 knots below 10,000' restriction is not universal (it's how CONUS works)...other airspace may not have the intermediate speed restriction.

Tom.
 
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RE: Takeoff Momentum

Fri Jul 27, 2012 2:30 am

Tom, I really appreciate your detailed responses. I'm not sure if you ever had a chance to teach, but from someone who did it for 10 years, I can tell that you really know how to explain things in such a way that even a total amateur like me can understand.
 
FlyMKG
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RE: Takeoff Momentum

Fri Jul 27, 2012 4:23 am

In the 727 we have two main (and one rarely used) takeoff profiles which we fly, all of which have pitch and power changes throughout the maneuver. Occasionally when going to climb power we actually have to increase the pitch due to using the max derated T/O power.

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LONGisland89
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RE: Takeoff Momentum

Fri Jul 27, 2012 6:51 am

This "sinking feeling" is directly related to a change in vertical speed. Thrust setting, configuration, and angle of attack are all variables that do have an influence on vertical speed, but are not directly related to the sensation the OP describes. Example, I've been on some 757/767 flights where (depending on the takeoff and VNAV performance) climb thrust is about the same or even greater(not by much) than T/O thrust. In these cases there was still the settling sensation caused by pitching for 250. Instead of picking one out and saying "x is responsible for the feeling," I think you can say, without a doubt, that the change in vert. speed is the direct cause. Next time you ride an elevator going up, pay attention to how it feels when you stop at a floor.

Quoting tdscanuck (Reply 13):
Climb rate is all about excess thrust. If the thrust comes back you must shallow your climb or you will start to slow down. Airliners don't decelerate on climbout, they only stay steady or accelerate.

Eh, not really. Climb rate is all about time and altitude. You can have zero thrust and still increase your rate of climb.

[Edited 2012-07-26 23:55:32]
 
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Starlionblue
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RE: Takeoff Momentum

Fri Jul 27, 2012 7:30 am

Quoting LONGisland89 (Reply 18):

Quoting tdscanuck (Reply 13):
Climb rate is all about excess thrust. If the thrust comes back you must shallow your climb or you will start to slow down. Airliners don't decelerate on climbout, they only stay steady or accelerate.

Eh, not really. Climb rate is all about time and altitude. You can have zero thrust and still increase your rate of climb.

Yes, you can (at the expense of speed). However that wasn't what Tom was saying. He was saying that in airliner operations this is the behavior.
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Rara
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RE: Takeoff Momentum

Fri Jul 27, 2012 9:29 am

Quoting FlyMKG (Reply 17):

In the 727 we have two main (and one rarely used) takeoff profiles which we fly, all of which have pitch and power changes throughout the maneuver. Occasionally when going to climb power we actually have to increase the pitch due to using the max derated T/O power.

Interesting. The A380 does that too. Sometimes engines spool up after "reduction" to climb thrust, because flex t/o thrust is so low.
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vikkyvik
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RE: Takeoff Momentum

Fri Jul 27, 2012 3:02 pm

Quoting LONGisland89 (Reply 18):
Eh, not really. Climb rate is all about time and altitude. You can have zero thrust and still increase your rate of climb.

Yes, really. The climb rate you can achieve without reduction in speed is all about excess thrust. Sure, you can stop the engines, pitch up, and you'll climb.....until you slow down, stall, and start falling, which won't take very long to do without any thrust.

Zoom climbs by fighter aircraft take advantage of kinetic energy to add potential energy in this manner, but you can't maintain a climb rate that way. You're either trading speed for climb rate, or climb rate for speed.
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tdscanuck
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RE: Takeoff Momentum

Fri Jul 27, 2012 5:27 pm

Quoting LONGisland89 (Reply 18):

Quoting tdscanuck (Reply 13):
Climb rate is all about excess thrust. If the thrust comes back you must shallow your climb or you will start to slow down. Airliners don't decelerate on climbout, they only stay steady or accelerate.

Eh, not really. Climb rate is all about time and altitude. You can have zero thrust and still increase your rate of climb.

I should have been clearer...*steady* climb rate *capability* is all about excess thrust. Instantaneous climb rate is, indeed, just about altitude and time. But in normal airliner operations you don't do the kinetic/potential energy trade very often (overspeed takeoffs being the only notable exception and then you're only using the extra KE for reserve).

At steady state, your ability to climb is all about excess thrust. I brought this up to counter the oft-heard-but-physically-incorrect idea that climb is about extra lift. In normal airliner operations, lift remains very very close to weight for the entire flight regardless of speed or altitude change. You climb by increasing thrust, not by increasing lift.

Tom.
 
LONGisland89
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RE: Takeoff Momentum

Sat Jul 28, 2012 6:06 am

I stand corrected. I was misunderstanding "excess thrust" to mean simply increasing the power setting. Excess thrust has nothing to do with the thrust setting (well, they aren't directly related I should say). Pitching for a higher airspeed in a climb phase causes the thrust vector to be greater than the drag vector (the increasing airspeed is evidence of this difference). This difference is called excess thrust. Once your target airspeed is attained, the thrust and drag vectors become equal and that excess thrust is transferred to the vertical component of lift. Aerodynamics 101, gotta love it.
 
vikkyvik
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RE: Takeoff Momentum

Sat Jul 28, 2012 7:32 am

Quoting LONGisland89 (Reply 23):
Pitching for a higher airspeed in a climb phase causes the thrust vector to be greater than the drag vector (the increasing airspeed is evidence of this difference).

Sort of. If you started out in a climb, then the thrust was already greater than the drag. The excess ( = amount greater than drag) thrust was going toward battling the component of gravity that is opposing a climbing aircraft.

Pitching down for a higher airspeed causes the gravity component to be reduced, so speed increases until drag makes up for the reduced gravity component. Again, since you're still climbing, thrust and drag are not equal; thrust is still greater.
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EGGD
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RE: Takeoff Momentum

Mon Jul 30, 2012 5:49 pm

Anyway, back to the original question!

Yes, the 'sinking feeling' is the reduction in climb rate, as result of a reduction in pitch 'nose up' attitude of the aircraft in order to accelerate. It is actually an increase in forward momentum as previously stated as the initial climb from leaving the runway is done at a constant speed (momentum=mass x velocity). By pitching down and increasing speed you are increasing velocity. The sinking feeling is actually a small amount of negative g depending on how aggressively the nose is lowered. I find this is worst on the 757 when the autopilot is engaged after moving to climb thrust, if the nose has been left high by the pilot flying (between 15 and 20 degrees nose up) it has a tendency to drop it quickly to avoid losing speed.

As far as the 'crazy' question goes, yes we would use less thrust if we used full power until cruising altitude. The reason why we don't is that the higher the thrust, the higher the temperature of the engine. Higher temperature leads to more wear and engines won't last for as long. A new engine costs more than the savings made in using slightly less fuel to get to our cruise altitude and that is why we do what we do!

Regards

Dan
 
David L
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RE: Takeoff Momentum

Mon Jul 30, 2012 7:01 pm

Quoting EGGD (Reply 25):
The sinking feeling is actually a small amount of negative g depending on how aggressively the nose is lowered

Do you mean negative g or a reduction in positive g?
 
tdscanuck
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RE: Takeoff Momentum

Tue Jul 31, 2012 1:28 am

Quoting EGGD (Reply 25):
The sinking feeling is actually a small amount of negative g depending on how aggressively the nose is lowered.

Any amount of negative g means things end up on the ceiling. Great way to clean out the crevices of the flight deck, not so great for the passengers.

Quoting David L (Reply 26):
Do you mean negative g or a reduction in positive g?

I really hope it was the latter.

Tom.
 
EGGD
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RE: Takeoff Momentum

Tue Jul 31, 2012 10:19 pm

Of course I mean a reduction in positive G, however I'm not ruling out that negative G has been achieved during the pitch reduction for acceleration!!
 
vikkyvik
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RE: Takeoff Momentum

Wed Aug 01, 2012 3:12 pm

Quoting EGGD (Reply 28):
Of course I mean a reduction in positive G, however I'm not ruling out that negative G has been achieved during the pitch reduction for acceleration!!

That would be a pretty severe pitch reduction....Given that you're not changing pitch by THAT much (what, maybe 5-10 degrees?), I'd wager you're likely to overshoot if you do it fast enough to achieve negative G!

Remember, there's also the range between 0G and 1G, where we weigh less than normal. You'll still get that roller-coaster feeling without going negative.
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