That 'drop' after take-off

[v6g]

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.

[almostahovva]

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.

[zerozero]

...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.

[Seat1APlease]

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!

[Tinstaafl]

Not quite correct. In flight the a/c rotates around its Centre of Gravity, not its 'centre of lift' as you call it. I

[WHBM]

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 !

[SLFguy]

It's coz ur crashing. :D

[AntiCrash]

That bump may have been the Langing Gear Oleo Struts hitting the stops when the weight is off. The same thing is often heard in many airliners.

[john_tullamarine]

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.

[SuperRanger]

correct me if i'm wrong? but i always thought the CoG is well ahead of the nose? or did i missed something during my loading classes :O

SR

[sky9]

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

[john_tullamarine]

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.

[Brian D.]

Couldn't it also be flap retraction ?

/back in my cage

[jtr]

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)

[chippy63]

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.

[spoilers yellow]

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.

[Groundbased]

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?)

[Pegasus77]

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

[Cool_Hand]

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).

[ft]

“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.

[jtr]

There won’t be an abrupt change of longitudinal acceleration.

abrupt is a matter of opinion, the acceleration will however undeniably go from about 1 to 2 kts per second, to zero on the climb out.

The aircraft is now climbing out at a constant V2+10, any acceleration has now ceased, the ac is in a steady state, non accelerating climb.

The rotational acceleration has ceased, as you are no longer pitching up.

[ft]

No, the longitudinal acceleration is still the same, as counterintuitive as it is. It is given by Newtons 2nd Law,

a = F / m

where

F = T - D

The mass of the aircraft, m, is constant and the force F acting in the longitudinal direction, thrust minus drag, is constant as the speed isn't changing*.

The airspeed stops increasing though, as you turn the nose up which means that a component of the gravitational acceleration will now counteract the airspeed increase.

The aircraft is still pushing you just as hard in the back. Before rotation, all of this pushing force (longitudinal acceleration) is making you travel faster. After rotation, part of the acceleration is keeping you from falling through your seatback back to earth due to gravity.

In fact, right where you are sitting now you are accelerating at roughly 10 m/s per second upwards, just to keep you stationary in spite of gravity! What a wild ride! :ok:

When you sit upright, the force to give you this acceleration is exerted on you by the seat of the chair. If you tilt your chair back, more and more of this force will be transferred to the back of your chair. This is what happens as the aircraft rotates.

As the seatback pushes you forward to keep you from falling down, you are pushing the seatback in the other direction, equal and opposite reaction. Newton again, his third law this time around.

Alas, you are using part of the force which previously went to accelerating the aircraft, helping to keep the airspeed constant even though the power setting didn't change.

There no escaping Newton, however much we like to think we do when we are in our flying contraptions!

Am I making sense? If not, ask and I'll try again. :)

Regards,
Fred

* Not going into the intricacies of induced drag here

[Pub User]

No, rotation of any rigid body, which includes aircraft, is always about the CoG. This is not correct. Rotation on the ground will happen around a pivot point (the undercarriage), regardless of where the CG is.

I suspect the answer to the original question lies in the comparison of rotation rate to climb rate. As mentioned by Zerozero, some airlines teach a two-part rotation, thus the rear cabin will experience a momentary sinking feeling as the aircraft is pitched up to its climb angle after leaving the ground.

[jtr]

Sorry ft, you have me puzzled (have just returned from a long haul tho, so its not difficult)

But it seems to me you are saying that an ac in a constant rate climb, at constant airspeed, is accelerating. Is this correct?

As far as the vertical acceleration component is concerned, I can not see how an R22 climbing vertically at 100'/min, or an f-16 climbing at 20,000'/min are going to experience anything other than 9.8m/s/s pushing the pilot into the seat.

Translate the same force required to do the above into a horizontal acceleration, and the result will be grossly different accelerations.

Off to bed!

[chippy63]

Thanks, Pub User, that is what I was trying to say!:ok:

[dicksynormous]

I think youll find its caused by the the auto pilot gear cogs meshing after the valves have been activated by way of a beam capture lever located just between the radio operater/navigator and the flight engineer.

Alternatively in newer flying machineswith chuuby serving staff, the sinking feeling may be caused by the collection of large proportioned female staff at the back all moving in unison as they lift a cheek to start polluting the air which they then blame on the passengers.
consider this formula.

too much crew food + 3 fat arses X 3 cheek lifts = assisted rotaion rate.

Some operators actualy factor this in when planning increased v2 ops to ensure Assisted Cheek Raising Obsatcle Clearance leap.

(A CROC)

This is what causes the sinking feeling.

Hope this helps.now lets move on

[West Coast]

Perhaps its a bit simpler. The effects of spatial disorientation, something we as pilots suppress due to education and training and therefore overlook could be the answer.
While the effect described doesn't fit neatly into one of the classic illusions it could still be how this persons inner ear responds.

[Milt]

dicksynormous

Henceforth renamed dickminiscule.

Thanks for the erudite contribution.
Tell you mother about your problem and she may be able to help.

Throw him off moderator.

[ft]

This is not correct. Rotation on the ground will happen around a pivot point (the undercarriage), regardless of where the CG is.

Sorry, but it is correct. Physics is a wonderful science. :) If you could have an airliner on the ground and apply a large torque anywhere on it, pitching it up... the mains would move forward. This is due to the torque rotating the airliner around it’s CoG. The mains will also create an upward force, moving the CoG up. The same thing happens on rotation.

Any toy on wheels which has a bit of mass makes a good demonstration object, as does your bike (as the unicycle example wasn't enough to convince you). Pull up on the handlebars, lifting it up on the rear wheel. Which direction does the rear wheel go? You’re playing the part of aerodynamic lift in this situation. Co-starring as your partners in moment creation are gravity and the normal force exerted on your bike by the ground.

If one point on the body is held in place the rotation will still be about the CoG. However, the torque/rotation will create forces at the swivel point which will make the entire body accelerate as it rotates. For a non-accelerating rotation, these forces are what makes up the centripetal force.


The standard example is trying to drill in a board on the floor which has not been secured. If you drill in the middle of the board (the CoG) it will start to spin about the drill. So far, nothing unexpected.

If you drill at the end of the board, you could expect that the board should still rotate about the drill – but it doesn’t. Instead, as you apply torque with your power drill, it still will try to rotate around it’s CoG, kicking your drill sideways unless you hold it steady.

Put one end of the board on a fixed swivel and drill in the other end. The board will still try to rotate about its CoG, kicking your drill one way and trying to kick the swivel the other.

Go out in the garage and try it, but keep your feet clear! ;)



As for acceleration:
You are always accelerating at 9,82 m/s^2 upwards, unless your vertical velocity is changing. That’s what gravity does to you. The proper term is gravitational acceleration. Sitting in a chair, a helicopter or a fighter jet in a vertical climb at constant speed makes no difference.

Going straight up, this acceleration is given to you by the engine(s). Flying level, it comes from the lift generated by the wings.

As L/D is typically above 1, you’ll need less engine thrust to counter the (induced) drag created by generating the lift in level flight than you’ll need to counter gravitational acceleration in vertical (or climbing) flight.

In level flight, the lift equals mass times the gravitational acceleration g, L = m * g (m*g is the weight). This divided by the L/D ratio will give the drag. L/(L/D) = D. For a constant speed the thrust equals the drag, T_l = D. Alas, T_l = m*g/(L/D).

For vertical flight, the thrust has to equal m*g. T_v = m*g.

Divide T_v by T_l. T_v/T_l = m*g / (m*g/(L/D)) = L/D. For a typical airliner, L/D should be around 15-20. This means you’ll need considerably more thrust going vertically than in level flight.

Nature, at least the parts we’re affected by in everyday life, is as a rule continuous. This means that if you are somewhere between these two flight conditions, your thrust requirement to maintain airspeed will end up between the two extremes as well.

This is of course simplified. One major omission is the fact that while you are on the ground roll, you’re probably close to the zero-lift angle of attack and not generating much lift at all. On rotation, the wings begin generating lift and the drag increases. This will reduce the T-D margin and reduce the overall acceleration. However, picture what happens if you instead of climbing remained flying level along the runway. The acceleration decrease you’d experience is the same as the acceleration decrease due to loading the wings on rotation. It is certainly nowhere near the acceleration decrease you get when initiating a normal climbout, even if we assume being out of ground effect.

Regards,
Fred

[Tester07]

Definition of a boffin:

Someone who knows the square root of a jam jar but can't get the lid off one!

Aircraft don't rotate about the CG at 'rotate', that's why pitch characteristics sometimes change after take-off, as the moment arm to the elevator changes.

This effect can be significant for aircraft that have main gear well aft. The Embraer 145 springs to mind - during Vmu certification testing the aircraft pitched up significantly after lift-off (change of moment arm to CG) and immediately stalled, dropping a wing and striking the runway. Fortunately all landed safely, But Embraer learnt a great deal about the relationship between the main gear and the CG that day, and what you rotate around during and after the take-off!

[ft]

Edit: Attempt to carry on conversation with Tester07 deleted due to futileness of educational effort.

[R T Jones]

shortly after take off when the aircraft changes rate of climb and the engines rev down a bit, i think we are about to drop out of the sky. Awful feeling for a wannabe :). Last time I went on a plane that feeling of impending doom wasnt so overcoming. Still a little strange though, all that weight becoming airboune in 45 seconds.

[catchup]

Me thinks
dicksynormous is the only one who understood this complex aerodynamic phonemena. Respect!

regards

[Pub User]

ft

Sorry mate, you're still wrong. How about if you lifted the tail of the airliner on the ground, would it still rotate about its CG, or about its pivot (nosewheel)?

[ft]

The moment applied [ (force at tail - force at nosewheel)*distance tail to nosewheel ] will still serve to get the aircraft rotating about it's CoG. However, the upwards force applied to get it rotating will also serve to make the entire aircraft shift upwards, thus making the aircraft rotate while still within the constraint set by the fact that the nose gear cannot go through the surface of the earth (if the ramp is up to specs and the nose gear can take the load).

You have to remember that there is a difference between a moment and a force!

Why will the nose wheel move back, in your view of the world?

Edit: Another thing to ponder. If the aircraft in your tail-lifting example is hinged about the nose wheel, why aren't you and your unicycle hinged about the wheel of the unicycle in the example where you fall on your back riding it? If the hinge theory held, you'd fall backwards but the unicycle wheel would remain in place. This is not what happens though. The unicycle scoots off forward at quite a speed.

Ty the board and the drill, allright?

Regards,
Fred

[Pub User]

The nosewheel will move back (unless chocked, which would be an obvious measure if trying this!) because it will TRY to move arounds its CG, but will fail miserably because of the the (severe) constraint of the ground beneath it.

The unicycle is an irrelevant example.

[ft]

The nosewheel will move back (unless chocked, which would be an obvious measure if trying this!) because it will TRY to move arounds its CG, but will fail miserably because of the the (severe) constraint of the ground beneath it.

Exactly. Rotation about the CoG, as opposed to any point of the u/c, and then translation due to constraints. Two different things. You cannot set out assuming rotation about the u/c and expect getting correct results.

The unicycle is very relevant but I won’t stress the point. :)

Cheers,
Fred

Edit to get the vB code right.

[runawayedge]

All my trust in you commercial boys! .....as a regular regular flyer behind you boys and gals I think you all need to go back to basics....remember one of the effect of flaps...increasing lift etc...so when you take em away nose changes pitch.....it takes a second or two and it is very noticeable but I would bet heavily on this reason as being the correct answer

[Pub User]

runaway

The flaps aren't run in until after power reduction at around 1000'. You're right, this will be accompanied by a 'sinking' feeling as the RoC reduces, but it's not immediately after take-off as in the original question.


ft

Rotation about the CoG, as opposed to any point of the u/c, and then translation due to constraints "and then" ??

There is no 'and then', it's a piece of metal and it all happens at the same time.
You cannot set out assuming rotation about the u/c and expect getting correct results Oh yes you can.

[overstress]

Also, as has been alluded to earlier, the wings are not taking the 'full weight' immediately after rotate, as there is a vertical component of thrust present. the wings take the full weight only once the a/c has levelled off.

btw, you don't get this 'drop' sensation on the flight deck, so I reckon it is only felt by those sitting aft of the c of g, once tha a/c has left the ground and as further rotation takes place, the rear half (behind the c of g) rotates downwards. Try it on a see-saw sometime!

[zerozero]

I can't believe this question went on for THREE pages when I (pat on the back!) said the very same thing on the THIRD post of the FIRST page!!

Amazing.

[ChrisVJ]

59 years I have been happy to fly airline. Seeing the answers on this thread I am now seriously worried about my next flight.