That 'drop' after take-off
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That 'drop' after take-off
What causes that dropping sensation?
During take-off in a big jet, immediately after the main gear leave the ground, there's a momentary dropping sensation. Any ideas what causes this? Is this an actual 'drop' or is it some kind of sensory confusion? I've noticed it particularly when sitting in cattle class (well behind the CofG).
As I am preparing for my CPL exams, I think I should know enough theory to work this out for myself but I can't ... At this point the aircraft has just established a positive rate of climb so it should feel like the opposite is happening.
My line of thinking so far is that it might be something to do with the wings becoming fully loaded (from a stalled/flared angle of attack), or something to do with the rate of rotation increasing such that the aft section is descending faster than the planes rate of ascent is increasing.
During take-off in a big jet, immediately after the main gear leave the ground, there's a momentary dropping sensation. Any ideas what causes this? Is this an actual 'drop' or is it some kind of sensory confusion? I've noticed it particularly when sitting in cattle class (well behind the CofG).
As I am preparing for my CPL exams, I think I should know enough theory to work this out for myself but I can't ... At this point the aircraft has just established a positive rate of climb so it should feel like the opposite is happening.
My line of thinking so far is that it might be something to do with the wings becoming fully loaded (from a stalled/flared angle of attack), or something to do with the rate of rotation increasing such that the aft section is descending faster than the planes rate of ascent is increasing.
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I guess once the a/c is established in a safe climb, the nose is lowered to a more efficient climb profile. Not exactly, but something like, Vx to Vy. Then of course the flaps are cleaned up and you may get a bit more dropping sensation.
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I know what you're talking about...
...I've felt it too.
I think it's due to a two-part rotation technique.
Some airlines teach their crew to pitch to an initial attitude of 9 degrees or so *until* a positive rate of climb is established and *then* continue to pitch up to 15 degrees or so...
...this technique is usually used to avoid striking the tail by an initial over-rotation and it's logical to seem like you're "sinking" in the rear of the aircraft when it's actually beginning a climb.
Techniques and numbers all vary with aircraft and company procedure.
I think it's due to a two-part rotation technique.
Some airlines teach their crew to pitch to an initial attitude of 9 degrees or so *until* a positive rate of climb is established and *then* continue to pitch up to 15 degrees or so...
...this technique is usually used to avoid striking the tail by an initial over-rotation and it's logical to seem like you're "sinking" in the rear of the aircraft when it's actually beginning a climb.
Techniques and numbers all vary with aircraft and company procedure.
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Geometry for a start, as the nose comes up the A/C rotates about the back wheels and the back goes down. On lift off the initial attitude/rotation rate is usually limited by SOP's to avoid a tailstrike.
Once the A/C is airborne with the engines both running the flight directors will tend to give a pitch up request to stop the speed from running away, the A/c then rotates about the centre of lift i.e the wings, so once again the back end goes down.
The fact that you are pushed back in your seat by the attitude may also give a sensation of sinking?
I once had a lady in first class complain that the pitch after take-off spilled her champagne, and she suggested I didn't do it again!
Once the A/C is airborne with the engines both running the flight directors will tend to give a pitch up request to stop the speed from running away, the A/c then rotates about the centre of lift i.e the wings, so once again the back end goes down.
The fact that you are pushed back in your seat by the attitude may also give a sensation of sinking?
I once had a lady in first class complain that the pitch after take-off spilled her champagne, and she suggested I didn't do it again!
My father (WW2 RAF Halifax crew) always used to speak about feeling "the bump" as the wings took the weight a few seconds AFTER leaving the ground. As a kid I used to argue with him that this was a nonsense and the wings must have fully supported the weight by liftoff. But he persisted, and he had the experience ! In later years I figured he might be speaking about leaving ground effect and that it was maybe a feature of that airframe you could feel at high weight. But it could also have been a feeling as the aircraft rotated.
Now in a commercial jet, if you are behind the centre of gravity, as the aircraft rotates your seat angle will change and you will feel you are being tipped backwards. That will give a sensation of being pressed into your seat as your relative g changes during the rotation, and you go down below the CofG, and it will feel like a drop. Up front you will rise up above the CofG
A more obvious "drop" happens during noise abatement where the rate of climb is reduced, and again there is a relative change of g you experience.
Interested to hear what others say.
BTW, back to the Halifax, I gather that if they managed more than a 1 degree climb angle (not certain about the associated attitude) it was a good set of engines !
Now in a commercial jet, if you are behind the centre of gravity, as the aircraft rotates your seat angle will change and you will feel you are being tipped backwards. That will give a sensation of being pressed into your seat as your relative g changes during the rotation, and you go down below the CofG, and it will feel like a drop. Up front you will rise up above the CofG
A more obvious "drop" happens during noise abatement where the rate of climb is reduced, and again there is a relative change of g you experience.
Interested to hear what others say.
BTW, back to the Halifax, I gather that if they managed more than a 1 degree climb angle (not certain about the associated attitude) it was a good set of engines !
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It's a bit hard to interpret exactly what you are concerned about but, I suspect, you are looking at the short period during the rotation when you appear to be pushed down into the seat ?
Consider that, during the takeoff flare, the aircraft is doing a mini loop segment and the load factor (g-load, if you prefer) will increase for a few seconds until the aircraft achieves the steady state climb. During this time you experience the same sensation as you would during an inflight pull up .. ie minor g increase.
The comparatively high acceleration of the jets (during the transition to a steady state climb) contributes to this perception.
Without doing some measurements and based on my own observations, I don't think that body pitch rate is high enough to contribute more than a minor variation to the basic process.
Consider that, during the takeoff flare, the aircraft is doing a mini loop segment and the load factor (g-load, if you prefer) will increase for a few seconds until the aircraft achieves the steady state climb. During this time you experience the same sensation as you would during an inflight pull up .. ie minor g increase.
The comparatively high acceleration of the jets (during the transition to a steady state climb) contributes to this perception.
Without doing some measurements and based on my own observations, I don't think that body pitch rate is high enough to contribute more than a minor variation to the basic process.
Last edited by john_tullamarine; 4th Sep 2004 at 11:11.
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SuperRanger,
I think that you might wish to delete that question before your mates think that you were either sleeping in your loading classes or are thick.
The whole point of your loading classes was to calculate where the C of G was relative to the "Mean Aerodynamic Chord". or MAC
I think that you might wish to delete that question before your mates think that you were either sleeping in your loading classes or are thick.
The whole point of your loading classes was to calculate where the C of G was relative to the "Mean Aerodynamic Chord". or MAC
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SR ... think you might be confusing CG with datum. The latter often is located well to the fore although it can be any old place you like .. provided you are consistent and measure everything with respect to the one, same datum position of your choosing.
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I am confident John_tulla has it nailed.
The "drop" is actually the unloading of the centripetal (here goes that discussion again) force that is experienced during rotation.
For the entire T/O roll, up until the completion of the roation, an acceleration is being experienced.
During the T/O roll it is a longitudinal acceleration, and then from Vr to V2+10 (all things working as they should) it is longitudinal, plus rotational. Then at V2 (more or less the time the wheels leave the deck-give or take), the acceleration stops. i.e. steady state climb, constant speed, and thus the sensation is an unloading, or "drop"
Never really noticed it before when operating, but paxed today on a short RWY departure, and did notice it.
(All opinions expressed in this post are those of the poster, and in no way represent the views of my employer)
The "drop" is actually the unloading of the centripetal (here goes that discussion again) force that is experienced during rotation.
For the entire T/O roll, up until the completion of the roation, an acceleration is being experienced.
During the T/O roll it is a longitudinal acceleration, and then from Vr to V2+10 (all things working as they should) it is longitudinal, plus rotational. Then at V2 (more or less the time the wheels leave the deck-give or take), the acceleration stops. i.e. steady state climb, constant speed, and thus the sensation is an unloading, or "drop"
Never really noticed it before when operating, but paxed today on a short RWY departure, and did notice it.
(All opinions expressed in this post are those of the poster, and in no way represent the views of my employer)
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tinstaafl,
doesn't the rotation take place centred on the top of the main u/carriage struts? That is, it is a mechanical rather than an aerodynamic movement. Once in flight, then the centre of lift becomes the datum.
doesn't the rotation take place centred on the top of the main u/carriage struts? That is, it is a mechanical rather than an aerodynamic movement. Once in flight, then the centre of lift becomes the datum.
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I`m not sure how long after rotation you are talking about, but in our company on (almost) every departure at 1000aal there is a significant decrease in engine power from T/O thrust to climb thrust, this in turn will (on an A/C with under wing engines), cause a slight nose drop, this coupled with the nose lowering anyway for the acceleration through flap retraction may cause the sensation you feel in the cabin.
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I've felt this sensation. I'll try to explain what it feels like to me, though since I am a complete layman on the technical aspects, I'll let someone else explain what is actually going on.
As a passenger it feels like the element of the aircrafts weight that is still supported by the gear at the point of liftoff is transferred to the wings and the slight drop is the sensation that is felt as this occurs. i.e its the change from one state to another.
(presumably the load on the gear reduces as lift begins to be generated during the takeoff roll?)
As a passenger it feels like the element of the aircrafts weight that is still supported by the gear at the point of liftoff is transferred to the wings and the slight drop is the sensation that is felt as this occurs. i.e its the change from one state to another.
(presumably the load on the gear reduces as lift begins to be generated during the takeoff roll?)
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As I commute to work, I experience the said bump often. It appears to happen if you are sitting in the back of the a/c, and not in the rows in front of the wing.
It has nothing to do with thrust reduction or flap retraction. It happens much earlier, in the rotation, when the wings start producing lift and the aircraft just leaves the ground. There is ohwever a noticable difference between flights/ rotation rates etc.
I think Johnt T. is about right, but I cannot explain the effect myself at all...
P77
It has nothing to do with thrust reduction or flap retraction. It happens much earlier, in the rotation, when the wings start producing lift and the aircraft just leaves the ground. There is ohwever a noticable difference between flights/ rotation rates etc.
I think Johnt T. is about right, but I cannot explain the effect myself at all...
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I believe WHBM's father has it.
It is probably due to the aircraft flying out of ground effect. As the aircraft flies to a height greater than it's semi span above the ground it experiences a slight reduction in lift and hence the wings fully taking the weight of the aircraft. People near the CoG won't notice it as much as people at the extremes as the sensation is exacerbated by fuselage bending about the CoG and hence the tail and nose will dip, the tail dipping more than the nose (as it has a greater distance from the CoG).
It is probably due to the aircraft flying out of ground effect. As the aircraft flies to a height greater than it's semi span above the ground it experiences a slight reduction in lift and hence the wings fully taking the weight of the aircraft. People near the CoG won't notice it as much as people at the extremes as the sensation is exacerbated by fuselage bending about the CoG and hence the tail and nose will dip, the tail dipping more than the nose (as it has a greater distance from the CoG).
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“doesn't the rotation take place centred on the top of the main u/carriage struts? That is, it is a mechanical rather than an aerodynamic movement. Once in flight, then the centre of lift becomes the datum.”
No, rotation of any rigid body, which includes aircraft, is always about the CoG. When on the ground, one of the forces involved in creating any rotation is the force exerted on the U/C by the ground.
There will be no drop as the gear stops supporting the weight of the aircraft. After all, the aircraft is being pulled away from the gear by the wings. It isn’t being dropped onto the wings from an elevated position ontop the u/c.
Assume that you are 25 m aft of the CoG. For there to be a 1/3 g of acceleration created by increased angular velocity (rotation) of the airframe, the acceleration has to be 9.8/3 m/s^2. This translates to an angular acceleration of 7.5 degrees per second per second. Considering that a rotation rate of 2 to 3 degrees/second seems to be what is typically recommended, this means the rotation rate should be established in slightly less than half a second. Reasonable?
It is certainly not due to the aircraft leaving ground effect. The same lift is still generated by the wings, albeit in a different way. There’ll be a change in lift distribution which will mean a (minor) change in the flex of the wings, but the energy stored in that flex will be very small compared to the weight of the aircraft fuselage and thus will not give much in the way of acceleration which can be felt by pax.
If the fuselage bends enough for it to be felt in my stomach, I’ll want a parachute so I can step out through the hole where the rest of the fuselage was attached and drift down to safety.
There won’t be an abrupt change of longitudinal acceleration. The aircraft still weighs the same, the engines put out the same thrust.
My vote is that it is the reduction of +ve load factor as the aircraft turns to a straight climb path which is being felt, as John T stated.
No, rotation of any rigid body, which includes aircraft, is always about the CoG. When on the ground, one of the forces involved in creating any rotation is the force exerted on the U/C by the ground.
There will be no drop as the gear stops supporting the weight of the aircraft. After all, the aircraft is being pulled away from the gear by the wings. It isn’t being dropped onto the wings from an elevated position ontop the u/c.
Assume that you are 25 m aft of the CoG. For there to be a 1/3 g of acceleration created by increased angular velocity (rotation) of the airframe, the acceleration has to be 9.8/3 m/s^2. This translates to an angular acceleration of 7.5 degrees per second per second. Considering that a rotation rate of 2 to 3 degrees/second seems to be what is typically recommended, this means the rotation rate should be established in slightly less than half a second. Reasonable?
It is certainly not due to the aircraft leaving ground effect. The same lift is still generated by the wings, albeit in a different way. There’ll be a change in lift distribution which will mean a (minor) change in the flex of the wings, but the energy stored in that flex will be very small compared to the weight of the aircraft fuselage and thus will not give much in the way of acceleration which can be felt by pax.
If the fuselage bends enough for it to be felt in my stomach, I’ll want a parachute so I can step out through the hole where the rest of the fuselage was attached and drift down to safety.
There won’t be an abrupt change of longitudinal acceleration. The aircraft still weighs the same, the engines put out the same thrust.
My vote is that it is the reduction of +ve load factor as the aircraft turns to a straight climb path which is being felt, as John T stated.