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777 Landing Gear Question  
User currently offlineUAL747 From , joined Dec 1969, posts, RR:
Posted (5 years 5 months 2 days 4 hours ago) and read 18073 times:

So I know that the 767 gear is tilted down because it needs to fit in it's space when retracted, and the same goes for the 747 as well....

My question though is why the 777 landing gear is tilted upward, when it clearly has a mechanism which tilts it back down (toepoint) position before it goes into the belly of the aircraft.

There has to be some advantage to have the landing gear tilted back in such a way when extended, otherwise, why go to the added weight and cost of designing a landing gear which changes it's position from tilted back to forward before going into the hull?

Also, I was reading on Boeing's website where the 77W landing gear was being discussed and it said that the 77W has a new landing gear which helps it change the center of rotation to the aft part of the landing gear which helps it take off and rotate quicker and to a greater angle.

How exactly does this work?

Thanks,

UAL

12 replies: All unread, jump to last
 
User currently offlineTdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 79
Reply 1, posted (5 years 5 months 2 days ago) and read 17946 times:



Quoting UAL747 (Thread starter):

My question though is why the 777 landing gear is tilted upward, when it clearly has a mechanism which tilts it back down (toepoint) position before it goes into the belly of the aircraft.

I don't know the correct answer, but there are a couple of possibilities:
1) It's aerodynamically better (either for load or noise) to be toe-up, so it "flys" that way when it's down.
2) There's a loading benefit at touchdown to having the rear axle hit first. This would be pretty airplane dependant, so it might be better for the 767/747 to hit front axle first.
3) The gear naturally runs toe-up and they don't want to bother working the actuator harder than they have to, so they just pull it toe-down when they need to.

Quoting UAL747 (Thread starter):
Also, I was reading on Boeing's website where the 77W landing gear was being discussed and it said that the 77W has a new landing gear which helps it change the center of rotation to the aft part of the landing gear which helps it take off and rotate quicker and to a greater angle.

How exactly does this work?

They replace the truck tilt actuator with a much stouter lockable actuator. During the takeoff roll they lock the actuator. As a result, the truck can't rotate relative to the strut. This moves the rotation point from the truck bearing to the rear tire contact point (several feet aft). If you doodle a quick sketch of the geometry, you can see that this actually lifts the gear attachment point on the fuselage up considerably more than if you let it rotate around the truck bearing. This allows greater angle of attack without smacking the tail.

Tom.


User currently offlineWilax From United States of America, joined Jun 2002, 465 posts, RR: 3
Reply 2, posted (5 years 5 months 13 hours ago) and read 17705 times:

Your premise is actually incorrect.

The 747 outer main gear trucks tilt when stowed only to make room for the inner gear trucks which retract forward into the body cramping the space of the outer trucks.

The 777 main gear, on the other hand, does not remain tilted when stowed. Once packed in the bays, just before the doors close, the trucks are acually situated paralell to each other, just like the 757, 767, and A330/340. If the gear retracted at a tilt, either the leading or trailing axles of opposite bogies would actually be closer together. This can be seen on various photos in the database. The tilting of landing gear, other than on 747's, has nothing to do with stowage.


::::::::O::::|::::O::::::::
=====O::|::O===== NO! This configuration actually takes up more space
::::::::::::O|O::::::::::::


::::::::::O|O::::::::::
=====O|O===== YES! This is the way gear is stowed.
::::::::::O|O::::::::::


User currently offlineJush From Germany, joined Apr 2005, 1636 posts, RR: 3
Reply 3, posted (5 years 5 months 12 hours ago) and read 17693 times:

Just fo those of you who like me like pictures and videos for better understanding

777 gear retraction as described in the post from Wilax

http://www.youtube.com/v/hJ3i_QNYASY&hl


Comparison to a 747 (a 200 shamefully but I suspect that the mechanism is similar)

http://www.youtube.com/watch?v=AOekRQBmQzM

As you can see. The 777 main bogies are not tilted anymore when they enter the body. The 747 bogies enter the body tilted. I suspect they wouldn't fit in straight.

Regds
jush



There is one problem with airbus. Though their products are engineering marvels they lack passion, completely.
User currently offlineDH106 From United Kingdom, joined Jun 2005, 626 posts, RR: 1
Reply 4, posted (5 years 5 months 3 hours ago) and read 17608 times:

I believe the 777 is the only Boeing that has active bogie re-positioning as part of the retraction sequence. The 747 / 757 / 767 apparently all retract their gears with the bogies positioned as they'd freely hang.
There are several other threads concerning the 767's 'front down' bogie hang, and the consensus there is that this is to render the bogies parallel when retracted because the leg retraction pivot axis is canted outwards at the front so that it sweeps the gear forward as it retracts.



...I watched c-beams glitter in the dark near the Tanhauser Gate....
User currently offlineTristarSteve From Sweden, joined Nov 2005, 4007 posts, RR: 34
Reply 5, posted (5 years 5 months 2 hours ago) and read 17582 times:

Quoting DH106 (Reply 4):
The 747 / 757 / 767 apparently all retract their gears with the bogies positioned as they'd freely hang.

All bogie main gears have a truck position actuator which positions the truck for retraction. The gears do not hang freely. This actuator is pressurised to put the truck into the retraction position, but of course it can be overridden by the truck moving after ground contact.
The B777 actuator has two positions. One for tilt, and one for stow.
No idea why the stow position is not kept for landing.
The large B777s are further complicated by having a lock position for rotation.

[Edited 2009-04-21 09:05:25]

User currently offlineDH106 From United Kingdom, joined Jun 2005, 626 posts, RR: 1
Reply 6, posted (5 years 4 months 4 weeks 1 day 20 hours ago) and read 17529 times:

Yes - I didn't phrase it very well, but we're saying the same thing here Steve. Every 4/6 wheel bogie will have a jack to positively position it when in the air so that it doesn't just hang and be moved by the slipstream.
I'm not sure about the term 'actuator' - don't the other aircraft types (not 777) mentioned above just have a oleo type strut similar to a car shocker that merely applies force to position the bogie like a car shocker wants to extend the wheel down? I picture an 'actuator' as more dynamic - being positively controlled hydraulically to/from multiple positions, as in the 777 ? Just terminology perhaps.



...I watched c-beams glitter in the dark near the Tanhauser Gate....
User currently offlineJetMech From Australia, joined Mar 2006, 2699 posts, RR: 53
Reply 7, posted (5 years 4 months 4 weeks 1 day 9 hours ago) and read 17448 times:



Quoting Wilax (Reply 2):

The 747 outer main gear trucks tilt when stowed only to make room for the inner gear trucks which retract forward into the body cramping the space of the outer trucks

I suspect that the tilting of the wing gear bogies prior to retraction on the 747 is actually to prevent interference between the bogies, and the wing gear aft trunnion support beam carry through structure.

http://i94.photobucket.com/albums/l118/Jet-Mech/747-1.jpg

http://i94.photobucket.com/albums/l118/Jet-Mech/BLG1.jpg

Quoting DH106 (Reply 6):
I'm not sure about the term 'actuator' - don't the other aircraft types (not 777) mentioned above just have a oleo type strut similar to a car shocker that merely applies force to position the bogie like a car shocker wants to extend the wheel down? I picture an 'actuator' as more dynamic - being positively controlled hydraulically to/from multiple positions, as in the 777 ? Just terminology perhaps.

I'm pretty sure the bogie positioning device is a ram connected to the normal hydraulic supply. I'd say the main difference is that these rams can be back driven when there is sufficient force attempting to rotate the bogie beam.


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Photo © Connector
View Large View Medium
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Photo © Daniel Alaerts - AirTeamImages



Regards, JetMech



JetMech split the back of his pants. He can feel the wind in his hair.
User currently offlineDH106 From United Kingdom, joined Jun 2005, 626 posts, RR: 1
Reply 8, posted (5 years 4 months 4 weeks 1 day 8 hours ago) and read 17436 times:



Quoting JetMech (Reply 7):
I'm pretty sure the bogie positioning device is a ram connected to the normal hydraulic supply. I'd say the main difference is that these rams can be back driven when there is sufficient force attempting to rotate the bogie beam.

Ok, thanks JetMech.



...I watched c-beams glitter in the dark near the Tanhauser Gate....
User currently offlineFr8mech From United States of America, joined Sep 2005, 5438 posts, RR: 14
Reply 9, posted (5 years 4 months 4 weeks 1 day 6 hours ago) and read 17416 times:



Quoting DH106 (Reply 6):
don't the other aircraft types (not 777) mentioned above just have a oleo type strut similar to a car shocker that merely applies force to position the bogie like a car shocker wants to extend the wheel down?

The B747/757/767 all have actuators. They are plumbed to one of the main systems. These actuators provide positive pressure, whether the gear is selected down or up to maintain the proper bogie position. In fact, if the bogie is not in the proper position, the 'gate' will not pull and the gear will not move to the up position, under normal conditions.



When seconds count...the police are minutes away.
User currently offlineDH106 From United Kingdom, joined Jun 2005, 626 posts, RR: 1
Reply 10, posted (5 years 4 months 4 weeks 1 day 5 hours ago) and read 17399 times:



Quoting Fr8mech (Reply 9):
The B747/757/767 all have actuators. They are plumbed to one of the main systems. These actuators provide positive pressure, whether the gear is selected down or up to maintain the proper bogie position. In fact, if the bogie is not in the proper position, the 'gate' will not pull and the gear will not move to the up position, under normal conditions.

So - if there's a hydraulics failure (whichever system), then manual extension may involve a bogie perhaps not being positioned correctly?



...I watched c-beams glitter in the dark near the Tanhauser Gate....
User currently offlineFr8mech From United States of America, joined Sep 2005, 5438 posts, RR: 14
Reply 11, posted (5 years 4 months 4 weeks 1 day 5 hours ago) and read 17393 times:



Quoting DH106 (Reply 10):
So - if there's a hydraulics failure (whichever system), then manual extension may involve a bogie perhaps not being positioned correctly?

Yes, but the system that positions the bogie is also the system that retracts the gear. So, if the #1 system on a B747 fails, the wing gear (almost postive #1 powers wing and nose) will not position, but it will also not retract, even if the flight crew pulls the 'gate'.

Manual extension should not be a problem. Most gear free fall for non-hydraulic extension. The gear is already in the position to fall out having been put there during the retraction. Without hydraulic pressure, it is possible for the gear to bogie to flop around a little, but it should still come out. I guess it's possible for it to hang up, but I haven't heard of it happening.



When seconds count...the police are minutes away.
User currently offlineJetMech From Australia, joined Mar 2006, 2699 posts, RR: 53
Reply 12, posted (5 years 4 months 4 weeks 14 hours ago) and read 17307 times:



Quoting Fr8mech (Reply 11):
So, if the #1 system on a B747 fails, the wing gear (almost postive #1 powers wing and nose) will not position, but it will also not retract, even if the flight crew pulls the 'gate'.

I think the body and nose gears are powered by #1 hydraulic system, with the wing gears powered by #4. I remember years ago we had a Thai 744 return to SYD because the gear wouldn't retract. Apparenty, the bogie tilt actuators could not rotate the bogie beams into the proper position for retraction due to stiction from lack of grease in the truck pivot.


Quoting DH106 (Reply 10):
So - if there's a hydraulics failure (whichever system), then manual extension may involve a bogie perhaps not being positioned correctly?

How it’s meant to happen.

http://www.youtube.com/watch?v=TZNjrQfTP5s

How it’s not  Smile !

http://www.youtube.com/watch?v=-eCvwzOhwYo

Regards, JetMech



JetMech split the back of his pants. He can feel the wind in his hair.
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