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?

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.

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.

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.

same on 787:ok:

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.

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.

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?

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.

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

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

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!

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.

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

The 330 main leg-gear retraction is PFM.

Probably witchcraft too.

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

Go look at a C-5 retraction!

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

.....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!
.

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

stilton,

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