B747-400 Heavy Vibration After Take-Off
 

I am a crew member and I am asking what is it that causes major vibration just after take-off around the door 4 Area of the aircraft. It is a shuddering like vibration that occurs until gear up. Is this caused by the landing gear? And why is it sometimes really bad and others not noticeable? It does not seem to happen on other aircraft.
Cheers
:)

It could be a couple of things:

1/Landing gear brakes activate to stop the wheels spinning during retraction, perhaps it could cause a shudder as the wheels stop?

2/ Airflow noise around the open Gear Doors during the retraction. ( a 400 at heavy weight is doing around 180 knots when the gear is retracting and this can cause a bit of air noise )

Nothing to worry about, you should have heard the BAE146 air noise/vibration during Flap retraction!! It may have been really quiet outside but it sure wasn't inside!!

Qstar,
It is a longstanding problem with B744, it is the wheels, and seems to be a harmonic being set up.

Carefully set up bearings, correctly inflated wheels and tires withing balance tolerances, all are important, but even with everything "just right", it still happens at high rolling speeds, although I have had it once at only about 255T tow.

Boeing has never found an answer.

Gear actuation on B747-400
 

Hydraulic motors used to retract landing gear? News to me!!!

Quite right Old Fella.:) silly me, never was an Engineer!!

I removed the incorrect part.:ok:

Although with 3000 psi pushing and shoving the Gear and Doors there would be a bit of vibration?

Usually its due to wheels out of balance...probably due to uneven wear, if you look at the tyres some times they can be seen to be ovalised...I have changed them for this in the past.

Also at very high fuel loads vibration can come from fuel venting from the wing tip NACA duct....

Spannersat KL is 100% correct. It is the ovalised wheels creating an out of balance as the wheels slow down after takeoff, until the retract braking kicks in.
When the wheels are extended the heavy part of the wheel falls to the bottom. When it makes contact with the runway the same part of the wheel is the first bit to be worn every time and it just gets worse and worse until the wheel is changed. The airline I work at checks for this at every 'A' check and changes wheels usually with lots of tread on one side but not much on the other.:}

"When the wheels are extended the heavy part of the wheel falls to the bottom. When it makes contact with the runway the same part of the wheel is the first bit to be worn every time and it just gets worse and worse until the wheel is changed".

Think about this statement. If it was true, which it isnt, the worn part would be at the top and the side with the most thread would be the first to touch the runway.

gents

may be stupid question. Why not pilot applying a little of braking after take off in order to stop the wheels like it is done on some a lot smaller aircraft

Ah, this old chestnut. It isn't (present tense, for the 146 is still very much with us) that quiet outside either. If you are on Stratford railway station, on the downwind leg of the departure circuit out of London City, the sound of flap retraction can be heard as a 146 passes overhead at 3,000 feet. I do wonder if it will be audible on television when the 2012 Olympics are broadcast, as the stadium is directly underneath.

It's an aerodynamic flow between the flap inner edge and the airframe, and was apparently a surprise to BAe on the first test flight. They developed a modification to overcome it, but it weighs over 100 kg, equal to two passengers, so nobody took it up. I did hear cabin crew say it was a convenient alternative signal that they can start service.

Jomison

B747 has auto braking on the main gear as do all large aircraft. If they used the normal braking system, like small plane drivers do, the tyres could rotate around the wheels and break the valve stems.

Second, I'd guess it is the nose tyres, slightly out of round, being slowed by the snubbers in the well. Again, the snubbers cannot stop the wheels instantly for the same reason. Why "out of round"? Sitting on the ramp all night, heavy weights, out of balance.

Hi galaxyflyer,

It's not April 1st any more.

Tapping the brakes might normally be trying to stop 300-400T of rolling force.

A 747 tyre spins at approx 1,250 RPM during rotation at 160kts, tapping the brakes with zero load, I'd imagine, would stop the wheel instantly so yes I'd imagine the tyre would spin down on the rim, obliterating the bead seat and flattening the tyre, fortunately the valve stem is part of the wheel rim and not the tyre so it would be fine :}

Hi SMOC,

Surely the force on the tyre/rim is proportional to the brake pressure during the deceleration.

Consider an RTO with maximum brake pressure applied on a dry runway.
Force on tyre/rim = Huge mass * slow deceleration.

When airborne, the force to stop the relatively tiny mass (of wheel + tyre) from spinning - (even if it was done with the same maximum brake pressure) resulting in a huge deceleration surely can NOT result in a bigger force on the tyre/rim.

RudderRat

Would you believe gyroscopic forces as the gear turns out of plane on retraction? That was the reason for main gear braking on the C-5, the mains rotated 90 degrees before retracting. I have the bead seating problem.

GF

Hi GF,

Have you seen a racing motorcycle bank from one side to the other very quickly. Their tyres don't seem to have a bead seating problem with the gyroscopic effect.

I've seen the torque on aircraft tyres during tight turns on the ground and they seem to be able to withstand that sort of punishment.

I thought heavy vibs during gear up between doors 3 and 4 may be down to ALTN anti skid valves (used during this operation) that do not maintain correct pressure.

From memory, these valves should keep a pressure of about 600/900 psi on the brakes to get spin down in a suitable time.

Seem to remember, they may hold less than desired pressure, result is slow spin down, wheels tend to stop on hi spot in brakes and so instead of touch down at random tyre points, these tyres often touch down on same/similar spot and makes tyres/wheels more out of balance and even more vibs.

Apart from vib reports, if main tyres getting changed with these low spots and plenty of meat on rest of tyre, suspect ALTN anti skid valves.

I think the fault isol manual covers this well.

Rudderrudderrat,

Yes I concede, plus touching down at 160kts with 0 RPM would be a similar force, so the buggers are held on tight!

On a sidenote, Boeing actually has to redesign the original 747 gear doors on
the new 747-8. Tuned out they cause buffeting on the new wing design when in
flap 30 config due to less flap slots which my allow for a
turbulent layer to form.

M

Although the wheel might well stabilise at the same point each time, touchdown is a gradual process, as soon as the first fractional contact is made the wheel will rotate, before any substantial weight is applied.

Have a look, for example, at some first-ever touchdown marks on a brand new runway and you see just a little wisp of rubber, building up progressively to the heavier marks well ahead as the weight and resistance increase, by which time the wheel will have performed several full rotations.