Dan From France, joined Aug 2005, 0 posts, RR: 0 Posted (17 years 1 week 21 hours ago) and read 1062 times:
When you see the 747 coming in for a landing, you can see the to outmost set of main wheels are in an almost up-right position whereas the two innermost boogies are almost level with the body. Why the difference? why aren't they all either in the up-right position or in level?
And another thing: I suppose the wheels are spinning before touchdown, otherwise the impact would be too rough, I guess. But I'm only guessing. Can anyone tell me if this is true? How many landings can the tires manage by the way?
And finally: after take-off the landing gear is pulled up, does the wheels spin or do you brake before retracting, and is it done manually or automatically?
A&P Mech From , joined Dec 1969, posts, RR:
Reply 1, posted (17 years 1 week 18 hours ago) and read 1062 times:
I have not worked on any 747s and can't answer specifically on their truck positions at touchdown, but I can give you some "generals" related to your other questions.
The wheels are not spinning before touchdown. They experience a high amount of friction at touchdown, obvious in the puffs of smoke at touchdown. Some systems rely on wheel speed to activate. In many aircraft, the anti-skid system is dependent on wheel speed and won't even activate until the wheels spin up over a certain speed (like 50 kts), and then disengage as the aircraft slows down to a ground speed of about 25 kts. When it detects a wheel is locking up (one wheel's speed compared against the others), it will relax the brake pressure for just a fraction of a second on that wheel to allow the wheel to regain traction and then re-engage normal brake pressure.
Many of the aircraft's systems are tied into specific wheel position. On highly automated aircraft, where the actions of a system depend on an "in air" or "on ground" condition, the aircraft could depend on a "squat" switch in the main gear, nose gear, or both to determine if the gear is extended (as in flight) or compressed (as on the ground). On an MD-11, I know of an incident where someone was refueling an empty aircraft, and began with the tail tank. As the aircraft became tailheavy and started to pivot back like it was going to sit on it's tail, the nose gear extended, making the fuel system think it was "in the air" and began transfering fuel forward out of the tail to get the aircraft back into proper CG for level flight. You see, the fuel system controller on the MD-11 is also responsible for CG management by controlling the fuel in the tail tank.
The groundspoilers and reverse thrust systems are usually also tied to either the wheel speed, OR the squat switch to prevent an in flight deployment.
After take-off, there are wheel brake pads located in the wheel wells (on most aircraft). As the gear is retracted up into the wheel wells, the wheels contact these pads which stops the spinning.
Know It All. From , joined Dec 1969, posts, RR:
Reply 2, posted (17 years 1 week 13 hours ago) and read 1062 times:
I can answer for the 747. The wheels are twisted to fit into the wheel bays. They are too big to retract them normally. You should try to get under one and see into the wheel bay. It's bigger than my car, and I don't drive a compact POS (don't yell at me for my disliking of small cars).
Hiker From , joined Dec 1969, posts, RR:
Reply 4, posted (17 years 6 days 18 hours ago) and read 1062 times:
Dan, good question,
The 747 is not unlike other airliners in the way the systems function, A&P and Know it All are essentially correct , but I know of no "wheel spin-up" ability on any of the airliners I've worked on. The truck tilt on the 47 actually levels the wing gear before it retracts into the wheel-well, Quite a complicated process of getting it all in.
As to why it's tilted one way or the other, that probably has something to do with spreading the load as the aircraft touches down. Look at the 757 and 767, both gears tilt in opposite directions when in the air.
Anybody from Boeing here?
Tires are another matter, some aircraft like the 47 can get upwards of 450 cycles through normal wear, others like the L1011 only get 200. It has alot to do with the manufactuer of the tire, and the weight of the aircraft per wheel assy. A tire assy can be re-capped up to 5-6 times and the operator usually pays per cycle, so it's of no benefit to run a tire to the cord. The normal tire pressure for the 747 is approx. 200psi depending on the model. Any leakage below 10% of the normal press. warrents a tire change, due to flexing of the sidewalls and exceccive heat build-up, which breaks down the rubber compounds.
Tires are prevented from rotating in the wheel-well to prevent throwing a cap inside the aircraft belly, this can have dissasterous effects, due to the proximity of fuel cells and essential flight control systems. Nose wheels are stopped normally with rup strips in the roof of the wheel-well, and some brake pressure is applied automatically to the mains as the retract sequence begins. You can see this on most aircraft as it lifts off, just as the gear doors open a puff of smoke from the brakes will appear. It's best viewed from behind as the aircraft moves away.