The Boeing 747 has four main landing gear assemblies (cf. two main-gear assemblies on most other commercial airliners). When airborne, the two forward/outboard assemblies have their carriages tilted upward at the front (think “toes up, heels down”), while the two rear/inboard assemblies have their carriages oriented pretty much parallel to the roll axis of the fuselage; see in this video, at 1:26 or so.
Why the difference in angle? Is this an automatic spring-loading feature related to how the gear will retract into the fuselage for cruise, or is there some subtle but important difference in how these two sets of gear will contact the runway at touchdown?
I would assume it’s because if the front and back part touch the ground at the same time and the plane isn’t going exactly straight, the wheels are going to try to turn it in the directly that the plane is pointed, but since they can’t do that, they’ll get torn off or jostle the plane around more then necessary. With only the rear part of the gear down for that split second, it gives the plane a chance to find some comfortable medium between the direction it was flying and the direction it was pointed.
I swear, this makes sense in my head. To make this a little more extreme, just to make the point, imagine this plane, with lots of cross winds landing flat. If all the wheels hit the ground at the same time, it would have been launched off to the left of the screen. Instead, with just the back wheels on the ground the pilot can straighten out the plane before he sets the front wheel on the ground. I think that’s what’s going on here.
OTOH, maybe I’m totally wrong. Maybe it’s just for shock absorbing purposes or so the front set of wheels don’t wear out as fast.
They are, a little, but probably not enough to do anything. Maybe it’s what you said, and just how they’re stowed. They obviously get pushed down easily enough, so there’s no reason to add hydraulics to do that before landing and (if this is the case), something probably just forces them into the upward position when they retract (but doesn’t push them back down when they come back out).
Maybe Broomstick will help us out with this one. Googling for why planes land with their wheels up doesn’t yield the answers I was hoping for.
I was curious so I poked around on google a bit. I learned that the landing gear units are called “bogeys” and that the 747 has two “wing bogeys” and two “body bogeys” and one “nose bogey”.
All of the bogeys are intentionally tilted so that the rear wheels hit first, which makes them self-straightening. It also allows sensors to be placed within the bogeys so that monitoring systems can determine exactly when the plane touches down.
Additionally, the wing bogeys are tilted more than the body bogeys simply to make them all fit better when retracted.
No cites and the information just came from message boards, but that’s what I found.
Its said that it just allows the bogey to fit into the wheel well…
The outside ones tilt so that there is more space for fuel tank in the wings.
eg they say that the 767 has opposite tilt, just because thats the way the bogeys swing.
This shows them "toeing up" as soon as they're off the ground.It (IMO) appears to be a passive system that does that. Based on the two videos, I'm guessing that the plane has to be on the ground for the wheels to be level.
