Landing gear lean question?

[underfire]

Why does the A380 landing gear lean forward, while the 747, 777, and 787 lean backwards?

https://s-media-cache-ak0.pinimg.com/originals/97/3d/fd/973dfd4467ed0fd77b919e3f1a0351ba.jpg
http://mediad.publicbroadcasting.net/p/kplu/files/201111/boeing_787.jpg

[Check Airman]

Most likely that's the geometry necessary to get it to fit in the bay. Space is expensive!

[pattern_is_full]

The question has been raised many times - since 2005, even on this forum. Most responses seem to be guesswork (sometimes educated). The quote-concensus-unquote seems to agree with Check Airman - to fit in the wheel wells.

I'm not sure that really matters these days, since most gear in recent planes have a "tilt actuator" (A380, see also 777), therefore there is no required relationship between the angle when stowed vs. the angle when extended. They don't hang simply according to gravity anymore.

The A380 wing gear seem to "untilt" to level/parallel with fuselage centerline as they are stowed, in this video:

https://youtu.be/nms0-x0wDGQ?t=2m3s

The fuselage gear retract by sliding back and up, like an escalator.

I've seen reference to the idea that tilted-down vs. tilted-up has to do with the airframe's natural aerodynamic tendency to pitch down or up on touchdown, with the main gear tilted to counter that tendency. The 767 tilts front-down, like the 380.

[Starbear]

I think this question has been around for a lot earlier then 2005. Working for an employer who was a B757 operator, attended briefing by Boeing pending our intoduction of B767, circa 1989, the question was asked, why does the 767 gear tilt forward? We were treated to a somewhat long BS explantion of softer touchdown and other clever reasons, the presenter was rather flummoxed to be asked why then does 757 tilt rearwards? (both types designed about the same time)

The answer most definitley as Check Airman almost stated is " so it fits in gear bay" nothing more nothing less. All the tilt actuators; gear shortening tricks etc on various types boils down to same. Bristol Britannia? Canadair CL44? anyone?

[bafanguy]

..." so it fits in gear bay" nothing more nothing less. All the tilt actuators; gear shortening tricks etc on various types boils down to same. Bristol Britannia? Canadair CL44? anyone?

CV880, IIRC. I do remember checking truck position cylinders on walkarounds. And a truck position indicator as part of the cockpit landing gear indication system.

[DaveReidUK]

The answer most definitley as Check Airman almost stated is " so it fits in gear bay" nothing more nothing less.

But that doesn't really explain it.

Most jet airliners have the landing gear retraction axis (ie the trunnions) aligned for-and-aft. Ditto the bogie when stowed is aligned longitudinally.

So simple geometry would imply that the bogie wouldn't have any tilt when the gear is lowered.

But it does, and that's clearly by design and for a reason.

[Junkflyer]

Different airplane, but good job of showing retraction.
https://www.bing.com/videos/search?q=747+gear+retract+vid&view=detail&mid=070B3242F5469ACB6A1D070B3242F5469ACB6A1D&FORM=VIRE

[EEngr]

But it does, and that's clearly by design and for a reason.

"Make it fit" is one reason. In order to make it fit, the bogies need to be rotated forward, backward, or level in order to minimize clear the gear wells. Some consideration needs to be made for that operating mechanism. Once the gear is dropped and contacts the runway, the hydraulics that stow it now have to allow it to rotate one way or another about it's "ankles". The travel involved may dictate one postition (toes up vs toes down) over another.

Once down, the A380 design appears to position the bogies flatter, once a landing flare is considered. The 747 "heels down" would require the bogies to rotate once the rear tires make contact.

A landing gear engineer (I'm not one) might also provide some insight into failure modes. Consider what might happen, for example, if a wheel bearing froze and applied a sudden torque to a bogie. "Heels down" would rotate the bogie into position. "Toes down" would act the other way, tipping the bogie further forward and result in higher stresses when it is eventually flattened out. There may be other failure modes and effects to consider should the extension mechanisms fail to operate completely.


Just be happy you don't have to worry about this one working properly (it didn't once, rather spectacularly): https://youtu.be/seWJa6SG3LQ?t=285

[RVF750]

IMHO, I guess getting all the wheels down and spun up quicker would give better braking performance if you inclined to make that a design priority?

We've all seen the grace an A330 sits on the back axle for an age slowly settling down onto the front axles of the main gear. Maybe the pilots are just showing off, but that distance gives you less braking and if limiting could be the answer....

[aerobat77]

just different design ideas . on the a 380 they designed it to be level with the surface at a typical touchdown nose up attitude and boeing designed it to touch with rear wheels first for a better damping effect .

[RVF750]

You say that but both Airbus and Boeing have products that lean both ways.... A380vs A330/340... B757/767 in point.

[DaveReidUK]

"Make it fit" is one reason. In order to make it fit, the bogies need to be rotated forward, backward, or level in order to minimize clear the gear wells. Some consideration needs to be made for that operating mechanism. Once the gear is dropped and contacts the runway, the hydraulics that stow it now have to allow it to rotate one way or another about it's "ankles". The travel involved may dictate one postition (toes up vs toes down) over another.

That's indisputably true, but as an answer to the question it's back-to-front.

Yes, on some types the bogie needs to be rotated as the gear is stowed because when it's deployed it isn't level, as opposed to early jets like the Comet where it was and retraction was more straightforward.

But we haven't got any closer to explaining why bogies are tilted, let alone why the tilt is one way on some types and the opposite way on others.

[Uplinker]

In a crabbed landing, if someone doesn't decrab properly, a gear that lands front wheels first is going to put huge twisting strain on the gear leg and hinge bearings, because it will try to steer away from the runway centre line, and the forward motion will try to twist the gear round.

If the gear lands trailing wheels first, this strain will be much less, since the aircraft wheels will simply be pulled into line to follow the direction of travel as the aircraft follows the centreline.

Imagine pushing a wheelbarrow in front of you that is not straight, and the force required to keep it going in a line not along its axis, and then imagine a similar scenario but pulling an offset wheelbarrow behind you.

I have only flown types that land trailing wheels first, (apart from the SIM), and they are very smooth if you get it right.

As to why some tilt one way and others the other, perhaps it has something to do with minimising aquaplaning or something?

[CV880]

Further to RVF750's post, Convair also built the 880 and 990 with opposite bogie tilts (880 nose up and 990 nose down).
The basic reason for the different angles is as Check Airman wrote in post #2 is to make it fit in the least amount of space. The reason for nose down bogie tilt on some aircraft is this results in the bogie lying parallel to the centreline in the retracted position as the trunnions are skewed so the bottom of the gear moves forward as it retracts. The A380 wing gear is an example and probably the 767. The first step in a 777's retraction is to reposition the bogie to a nose down tilt angle so it lies parallel to the keel in the wheel well.
By mounting the actual leg as far aft in the trunnions as possible and skewing the trunnions you can get a longer landing gear into the same horizontal space. This is how Convair managed to jack up the 990 by 18 inches at the MLG compared to the 880 yet the trunnions are the same distance from the centreline. The MLG alone was 10 inches longer than the 880's with bigger tires but clever geometry enabled it all to fit.

[Vessbot]

In a crabbed landing, if someone doesn't decrab properly, a gear that lands front wheels first is going to put huge twisting strain on the gear leg and hinge bearings, because it will try to steer away from the runway centre line, and the forward motion will try to twist the gear round.

If the gear lands trailing wheels first, this strain will be much less, since the aircraft wheels will simply be pulled into line to follow the direction of travel as the aircraft follows the centreline.

Imagine pushing a wheelbarrow in front of you that is not straight, and the force required to

I thought about this post for a while, and don't see that this is true. Let's say you land with the nose cabbed to the left. If the front of the bogey touches down first, the gear leg will twist counterclockwise as seen from above. If the rear touches down first, it will twist clockwise. But in both cases the angle of contact will pull the gear leg to the left, which is behind the CG, therefore stabilizing the plane in yaw. And in neither case would the leg twisting moment be greater than the other, at last for any reason that I can see.

[Uplinker]

I reckon there will be more twisting force if the fronts land first than if the rears do. When the fronts land first, the inertia of the aircraft etc pushes forwards and downwards on the front wheels - more or less in line with the tilted angle of the bogey, pushing the tyre into the tarmac and making it 'dig in'.

If the rears land first, they will only have their own weight pressing on the tyres, (until the fronts have landed), so they will just 'skate along', imparting much less twisting force.

Any design engineers in the house?

[DaveReidUK]

While the above may well be true, it sounds more like a happy side-benefit. I doubt that mitigating the effects of a ham-fisted pilot neglecting to kick off drift is high on a designer's list of objectives.

[Uplinker]

In order for the design engineers to derive the strength, size and construction of the gear strut and torque links, gear twisting forces will have to be considered.

I would imagine though that any such twisting forces caused by a crabbed landing would be much less than those caused by a max steering angle taxiing turn at MTOW. So I agree with you :ok:

Still makes me wince when folk don't decrab though.

[mrfox]

My thoughts -
I think the gear toe orientation is a result of gravity - the heavier end hangs down . Demonstrated by the fact that most gear hang the same orientation during gravity extension w/o the help of any hydraulics. *since shown to be wrong
The amount of torque on the assembly should be about the same during a crabbed landing regardless of gear toe up or down - a free body diagram should show about the same amount of restoring force around the cg of the aircraft. In fact the torque should be slightly less in the toe down case due to the fact that the contact point is slightly closer to the cg - so shorter moment arm.
Standing by to be happily corrected!

[mrfox]

Just watch a 330 or 777 (non-er so no bogie lock) on take-off. The bogie stays level with the ground as the ac rotates around the center of the bogie - so relative to the ac it is nose down. Then as the aircraft unsticks the back wheels falls and the bogie hangs nose up due to the weight distrubition (*pitch trimmer positioning), then the gear pitch trimmer (or its equivalent in the 777) moves the bogie nose down for the retraction.

[underfire]

Thanks for all of the replies.....

Looking at the different ac's on final, and the angles with the fuselage, could it be to help prevent tailstrike?

or perhaps to help get the ac on the runway with the associated ground effects?

[DaveReidUK]

Looking at the different ac's on final, and the angles with the fuselage, could it be to help prevent tailstrike?

No - the bogie would have to be locked relative to the leg in order to be able to do that.

Only the 777-300ER has that facility, and it's only used on takeoff, not on landing.

[Chu Chu]

Does anyone know how the bogies are actually held at a specific tilt angle? Gravity seems a little iffy -- I assume the pivot point has to be centered between the axles to distribute weight evenly on the ground, so presumably the bogie's balanced pretty well. It seems like a fairly simple spring could bias it to one stop or the other without too much fuss, though.

The tilt angle must make an aerodynamic difference. Maybe it can be reversed or adjusted during design if there's excessive buffeting (or something like that) with the gear down?

Finally, though I doubt this is a significant design consideration, I'd guess that toes up or down affects wheel spin-up (and perhaps tire wear). The tires that touch down first are under very little load until the other axle touches, so it seems like they'd spin up more slowly.

[Jet II]

On the 777 the gear is positioned by the tilt actuator to 5 degrees nose down for retraction and to 13 degrees nose up for landing - gravity has nothing to do with it, just good old hydraulic pressure.

[Roj approved]

same on 787:ok:

[mrfox]

My question is - if this is the case - why does it go to the same nose up position initially on takeoff? Seems to be a lot of unnecessary acrobatics for it to pitch nose nown on rotation, then intentionally nose up 13 degrees, only to go 5 degs nose down again for retraction.

[Jet II]

When the gear is selected up then Hyd pressure is diverted one side of the actuator to position the truck to -5°. If the gear is selected down then pressure goes to both sides of the actuator to position it to +13° (due to differential areas). This pressure stays there and the truck simply moves to desired position when weight is removed on takeoff.

Hope thats clear.

[mrfox]

Good info - thank you.
Just to satisfy my curiosity, what would it look like then if we do a gravity drop with no hydraulic pressure? Is this something thats ever done in mx on jacks?

[Jet II]

You do a gravity drop every time you lower the gear - the 777 doesn't have hyd gear lowering.

If you lost Centre system hyd pressure then you lose truck positioning.

[mrfox]

Any idea what the bogie orientation would be in this case?

[Jet II]

It stays around -5 due to the friction in the actuator and bearings.

[mrfox]

Thank you. And digging into the books for the A330 it appears the pitch trimmer here does the same thing - positions the bogie nose up on gear down selection. So the nose up orientation is very much intentional - and not because of gear bay fitment (or weight).
Always a good day to prove myself wrong and learn something!

[Chu Chu]

If you port pressure to both ends of an ordinary hydraulic cylinder, it will extend (because the rod occupies some of the effective area on its side of the piston.) So if all else is truly equal, nose-up or nose-down tilt might be driven by whether it was more convenient to mount the actuator toward the forward or aft end of the bogie.

[CV880]

Re Mr.Fox's posts, gravity extensions are done on jacks in the hangar usually after major gear maintenance. Without hydraulic pressure and air flow to influence bogie position it usually tilts to the heaviest end as you might expect.
The A330/A340 main gear is a special case. The nose up tilt is indeed intentional and is meant to act a bit like the 777-300ER's semi-levered gear in improving aft fuselage clearance. However, on retraction the gear actually shortens and this process removes the bogie tilt as it retracts so the bogie is parallel to the fuselage centreline when in the wheel well. The shock absorber strut slides inside the main fitting (the undercarriage leg) and is pulled up inside the leg as it retracts which removes the bogie tilt as well as shortening the length of the whole landing gear.
See video https://www.youtube.com/watch?v=-_z-KBTZRVk

[fantom]

The 330 main leg-gear retraction is PFM.

Probably witchcraft too.

[k3k3]

I was always impressed by the small space the Victor landing gear fitted in to, and the contortions it went through on the way.

[galaxy flyer]

The 330 main leg-gear retraction is PFM.

Probably witchcraft too.

Go look at a C-5 retraction!

https://m.youtube.com/watch?v=fySoPgHITeY

[Uplinker]

The 330 main leg-gear retraction is PFM.

Probably witchcraft too.

.....except when the leg shortening cam shears off, as happened to us once. Sorry, I have no pics, but it makes a mess as you can probably imagine!
.

[stilton]

I thought about this post for a while, and don't see that this is true. Let's say you land with the nose cabbed to the left. If the front of the bogey touches down first, the gear leg will twist counterclockwise as seen from above. If the rear touches down first, it will twist clockwise. But in both cases the angle of contact will pull the gear leg to the left, which is behind the CG, therefore stabilizing the plane in yaw. And in neither case would the leg twisting moment be greater than the other, at last for any reason that I can see.

That’s the best explanation to something
I pondered for some time

However, having flown the 757 with a
trailing bogie and the 767 with forward
‘trail’ I can tell you the latter is far less
forgiving

The design seems to magnify touchdown
forces!

I suspect the same happens on the A380
and perhaps the A350

[underfire]

stilton,

Thanks for that, was wondering, as it seems they should be different, and you have confirmed....thanx