Gear

[Beardyman 604]

Have technical question - why outside bogies hang differently to inner ones ( angle ) and what is the reason to keep outside bogies under such high angle ?

[downscc 1,327]

Skipping the technical issues, you certainly get a smoother landing when landing on tilted gear assemblies.  Another consideration is how they stow, they must be tilted to stow in the gear boxes.  FCOM 14.20.1 might give you some insight.

[Beardyman 604]

Thanks Dan

 

[Chock 17,769]

The tilted bogie set effectively cushions the landing somewhat and does actually help with getting the aeroplane to point straight down the runway upon touchdown before all the weight is on the wheels if there is any crab angle still on the thing from a crosswind approach (which is why it is the outer set which is titled).

But since it also triggers detection of when the aeroplane is meeting the ground, it means the aeroplane's systems which trigger upon sensing that the wheels are on the ground - such as the spoilers - go a little bit earlier than if it was firmly planted on the deck, which helps to stick the thing on the runway and keep it down, by killing some of the lift from the wings early because the 747 has quite a lot of ground effect from the lifting body contribution of its flat underside centre section.

In the tilted position, the tilting bogies stow snugly alongside the non-tilting bogies, which interestingly means you actually can't retract the gear on a 747 once airborne if the main bogies fail to fully tilt when the weight comes off them, because the gear lever is inhibited from being moved if the 747's systems don't detect the bogies have tilted fully.

[skelsey 765]

The short answer is that they are tilted to enable them to fit in the gear bays!

As Alan says, the tilt sensors have some secondary applications as well.

[Beardyman 604]

Thanks Alan for detailed answer