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Wheels In The Air And Where They Point?  
User currently offlineWeb500sjc From United States of America, joined Sep 2009, 749 posts, RR: 0
Posted (4 years 9 months 4 weeks 1 day 7 hours ago) and read 6134 times:
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Why on airplanes do the wheels point a certain way in the air, such as on the 767, they point downward, and on the 777 and a330 they point upward, also on the a34o-5/600's the outboard gears point up, and the center points down. is there any rhyme or reason, does it help airflow?


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25 replies: All unread, showing first 25:
 
User currently offlineBoeing767mech From United States of America, joined Dec 2000, 1030 posts, RR: 3
Reply 1, posted (4 years 9 months 4 weeks 1 day 7 hours ago) and read 6138 times:



Quoting Web500sjc (Thread starter):
Why on airplanes do the wheels point a certain way in the air, such as on the 767, they point downward

The 767 tip tops because of the way the main landing gear is swept back. If you look at the Main Landing Gear as the aircarft sitts on the ground you will see the sweep on the strut...


David



Never under-estimate the predictably of stupidty
User currently offlineAjd1992 From , joined Dec 1969, posts, RR:
Reply 2, posted (4 years 9 months 4 weeks 1 day 7 hours ago) and read 6137 times:

Usually just because they have to fit a certain way in the bay. Apparently the B767 is a hard plane to get a smooth landing out of because of the gear hanging forwards.

User currently offlinePGNCS From United States of America, joined Apr 2007, 2839 posts, RR: 45
Reply 3, posted (4 years 9 months 4 weeks 1 day 1 hour ago) and read 6041 times:



Quoting Ajd1992 (Reply 2):
Usually just because they have to fit a certain way in the bay. Apparently the B767 is a hard plane to get a smooth landing out of because of the gear hanging forwards.

The gear is designed to fit as compactly as possible into the wheel wells. Much of the gear geometry is based on the space and dimensions available for the wheel wells.

I have heard that rumor about the 767, and don't find it to be particularly difficult to land, although I dramatically prefer flying the 757. The 767 is certainly not even close to being the most difficult aircraft to land that I have flown, an honor that, without question, goes to the 727.


User currently offlineWeb500sjc From United States of America, joined Sep 2009, 749 posts, RR: 0
Reply 4, posted (4 years 9 months 4 weeks 22 hours ago) and read 5984 times:
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I thought on the 777 its desingned to piont like that when the gear is down, but not on the ground, and then it goes to a right angle when its pulled up


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User currently offlineHAWK21M From India, joined Jan 2001, 31702 posts, RR: 56
Reply 5, posted (4 years 9 months 4 weeks 19 hours ago) and read 5945 times:

The tilt on the B757 bogie provides the angle to retract into the Wheel well.....the tilt also provides the Air-grd sense.
regds
MEL.



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User currently offlineWeb500sjc From United States of America, joined Sep 2009, 749 posts, RR: 0
Reply 6, posted (4 years 9 months 4 weeks 10 hours ago) and read 5830 times:
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So does anyone know hat is different about the 777 gear


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User currently offlineUAL747 From , joined Dec 1969, posts, RR:
Reply 7, posted (4 years 9 months 4 weeks 10 hours ago) and read 5817 times:

The 777 gear, when down, has an angle tilted back. As it is retracted, before the strut actually moves and while it moves, the landing gear is tilted forward, leading me to think that there is more to having the gear tilted back than it just "being able to fit into the wheel wells as compactly as possible."

http://www.youtube.com/watch?v=HhWKxJGfS3U

Can someone explain that?

UAL


User currently offlinePhen From Ireland, joined Oct 2007, 318 posts, RR: 0
Reply 8, posted (4 years 9 months 4 weeks 4 hours ago) and read 5736 times:

I was under the impression that (in the case of the A330/40, 747 or 777) during landing, the tilt in the gear allows the aft wheels of the main bogey to touch the runway first for a few seconds before the forward pair(s) thus allowing the pilot to align the aircraft correctly with the centreline before all 4 (or 6) wheels of each main bogey are touching the ground. Is this pure nonsense??

http://www.youtube.com/watch?v=PBO8NAltx6k

EDIT: UAL747; my theory, if not incorrect, might explain what you described in your post...

[Edited 2010-02-03 15:06:41]

[Edited 2010-02-03 15:08:52]

User currently offlineUAL747 From , joined Dec 1969, posts, RR:
Reply 9, posted (4 years 9 months 4 weeks 2 hours ago) and read 5706 times:



Quoting Phen (Reply 8):
I was under the impression that (in the case of the A330/40, 747 or 777) during landing, the tilt in the gear allows the aft wheels of the main bogey to touch the runway first for a few seconds before the forward pair(s) thus allowing the pilot to align the aircraft correctly with the centreline before all 4 (or 6) wheels of each main bogey are touching the ground. Is this pure nonsense??

I'm not sure if this is correct or not. Certainly it would make sense, but it's very difficult to worry about keeping the plane aloft longer so that all your wheels are aligned with the centerline. I would think that you would want to be aligned after crabbing before any wheels were touching the pavement. Could be wrong though....

I have noticed however, that on the A330 you REALLY notice the time between the rear wheels on the main bogies touching down, and the front wheels on the main bogie. I'm wondering whether the bogies are spring loaded in some way?

UAL


User currently offlineweb500sjc From United States of America, joined Sep 2009, 749 posts, RR: 0
Reply 10, posted (4 years 9 months 3 weeks 5 days 21 hours ago) and read 5560 times:
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I've noticed the three sets of wheels hiring the pavement on the T7, it's kind of like a
drum roll. I was kind of a bit amazed to hear it actuall, in fact I didn't even see LHR T5 go by.



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User currently offlineViscount724 From Switzerland, joined Oct 2006, 25871 posts, RR: 22
Reply 11, posted (4 years 9 months 3 weeks 5 days 3 hours ago) and read 5404 times:

Quoting HAWK21M (Reply 5):
The tilt on the B757 bogie provides the angle to retract into the Wheel well...

Intuitively, it would seem cheaper to design the wheel well to fit landing gear that's parallel to the fuselage when extended, like the 707 and DC-8, than to add all the complexity to permit the gear to tilt, sometimes at fairly extreme angles.



User currently offlineDH106 From United Kingdom, joined Jun 2005, 626 posts, RR: 1
Reply 12, posted (4 years 9 months 3 weeks 5 days 2 hours ago) and read 5393 times:

Earlier designs had the gear 'hang' so as to match the geometry of retraction and so fit in the gear bays.
As someone has said above, the 767 hangs bogie front down in order to align the bogie as it enters the well as the retraction pivot line is angled away from the fore/aft slightly and the strut sweeps forward as it retracts.
The 777 has active bogie positioning - a jack positions the bogie correctly for retraction and after extention.

On the A340- 5/600 I imagine that the centre 4 wheel bogie hangs front down to allow the doors forward of it to open without fouling the front wheels.



...I watched c-beams glitter in the dark near the Tanhauser Gate....
User currently offlinetdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 13, posted (4 years 9 months 3 weeks 4 days 20 hours ago) and read 5345 times:

Quoting Viscount724 (Reply 11):
Intuitively, it would seem cheaper to design the wheel well to fit landing gear that's parallel to the fuselage when extended, like the 707 and DC-8, than to add all the complexity to permit the gear to tilt, sometimes at fairly extreme angles.

You have to have gear tilt equipment if you've got more than one axle, unless you use semi-levered gear. If you're going to allow any pivoting at all (for rotation and derotation), the gear needs to tilt, and you can't just have it flopping in the wind or else you can't guarantee it'll be lined up when it enters the wheelwell on retraction, which means you need a positioning system.

Tom.


User currently offlinejetlagged From United Kingdom, joined Jan 2005, 2572 posts, RR: 25
Reply 14, posted (4 years 9 months 3 weeks 4 days 15 hours ago) and read 5306 times:

Quoting UAL747 (Reply 7):
The 777 gear, when down, has an angle tilted back. As it is retracted, before the strut actually moves and while it moves, the landing gear is tilted forward, leading me to think that there is more to having the gear tilted back than it just "being able to fit into the wheel wells as compactly as possible."

There is a small advantage in the rear wheels touching first if the aircraft is yawed (e.g. due to crosswind). The initial contact point is thus behind the gear strut and there is therefore a turning moment which tends to help align the aircraft with the runway. If the front wheel touches first the moment tends to turn the aircraft away from the runway. Normally it's only a small effect but every little helps, as they say. However as the 777 has a long triple axle truck the adverse yawing moment caused by landing front wheel first may have been significant.



The glass isn't half empty, or half full, it's twice as big as it needs to be.
User currently offlineWN700Driver From , joined Dec 1969, posts, RR:
Reply 15, posted (4 years 9 months 3 weeks 4 days 2 hours ago) and read 5192 times:

I'd always heard that the design on the 767 had to do with ETOPS coming into play. Though earlier versions came without, doesn't the ER have a center fuel tank that requires enough space so that the mains have to tilt out of the way?

Anyway, if this is not the case, maybe someone can correct my impression. I've believed this to be the case for about twenty years now. Sure hope I'm not wrong, but if I am, someone say so! (hint, hint tdscanuck or laddie, lol...)


User currently offlinetdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 16, posted (4 years 9 months 3 weeks 3 days 23 hours ago) and read 5159 times:

Quoting WN700Driver (Reply 15):
Though earlier versions came without, doesn't the ER have a center fuel tank that requires enough space so that the mains have to tilt out of the way?

The back of the center wing tank (and front of the wheel well) is the rear spar...I'm 99% sure they didn't change the spar location for the ER, so there shouldn't have been any impact to the size of the wheel well. However, it's possible they added some of the ETOPS-required equipment into the wheel well, which might have required some space adjustment, but I've never heard of this before. No other Boeing had to do this when it got its ETOPS rating (that I'm aware of).

Tom.


User currently offlineMusang From United Kingdom, joined Apr 2001, 872 posts, RR: 7
Reply 17, posted (4 years 9 months 3 weeks 3 days 13 hours ago) and read 5116 times:

We need to adopt a new abbreviation - THBDB (This has been discussed before)! But hey, so what? I'm as guilty as any!

Quoting Phen (Reply 8):
the tilt in the gear allows the aft wheels of the main bogey to touch the runway first for a few seconds before the forward pair(s) thus allowing the pilot to align the aircraft correctly with the centreline before all 4 (or 6) wheels of each main bogey are touching the ground. Is this pure nonsense??

Not nonsense, but I've never heard of this as a design feature. As jetlagged says, this effect might happen as a result of the gear geometry, rather than a pilot using first contact as a cue to start aligning. Usually in a crosswind landing, the crabbed approach scenario, there wouldn't be much time between rear axle and forward axle contact unless its a 330/340.

Quoting UAL747 (Reply 9):
I would think that you would want to be aligned after crabbing before any wheels were touching the pavement.

Ideally yes, so as to land level and aligned. We all sort of aim for this, but a couple of degrees bank into wind never goes amiss in my opinion. In types with underslung engines (except the 146/RJ series IIRC) the manual dictates wings level, but on a wild rainy night its not top priority! (What is? Land it level so as to get brakes/spoilers effective, on the centreline, in the touchdown zone. If its also pointed down the runway, thats a bonus, although the real jet-gods out there will claim they always achieve this anyway.....)

Quoting UAL747 (Reply 9):
on the A330 you REALLY notice the time between the rear wheels on the main bogies touching down, and the front wheels on the main bogie. I'm wondering whether the bogies are spring loaded in some way?

They're highly sprung. Its to do with take-off performance. Basically, when rotation speed is reached in, say, a 707, it rotates around the pivot where the gear leg meets the bogie beam. On a 330/340/777, as rotation starts the weight on the gear suddenly begins to reduce, and the spring loading is able to pivot the bogie, effectively boosting the aircraft a little higher off the runway and increasing tail clearance. On landing the icing on the cake is that the same spring loading acts as a primary shock absorber. [This explanantion from Airbus and their gear manufacturers' reps at Farnborough a few years ago, and a Flight International (UK) article on the 777-300].

Quoting DH106 (Reply 12):
On the A340- 5/600 I imagine that the centre 4 wheel bogie hangs front down to allow the doors forward of it to open without fouling the front wheels.

Correct.

Regards - musang


User currently offlineMender From United Kingdom, joined Feb 2004, 240 posts, RR: 0
Reply 18, posted (4 years 9 months 3 weeks 3 days 9 hours ago) and read 5090 times:

Quoting WN700Driver (Reply 15):
I'd always heard that the design on the 767 had to do with ETOPS coming into play. Though earlier versions came without, doesn't the ER have a center fuel tank that requires enough space so that the mains have to tilt out of the way?

Different variants of 767 used more of the available space between the front and rear spars in the fuselage/wing box area to store fuel.

Some didn't use this area for fuel at all, some used part of it and some used all the available space but the area that the wheels fit into on ETOP's and Non-ETOPs 767's is exactly the same.

When designing an aircraft the manufacturer has to provide enough clearance between the engines and the ground and also make sure the C of G is correct whilst making the wings strong but light and able to carry the required amounts of fuel. Because of these conflicting requirements they sometimes have to place the landing gear in a 'less than ideal' position which means that the gear might have to swing slightly forward or backwards during retraction.

In the case of the 767 the wheels need to slightly aft of their own centre line (with the wheels down) hence they sweep forward during retraction thus the bogie tilts the 'wrong way'


User currently offlinerwessel From United States of America, joined Jan 2007, 2392 posts, RR: 2
Reply 19, posted (4 years 9 months 3 weeks 2 days 1 hour ago) and read 4941 times:
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Quoting jetlagged (Reply 14):
There is a small advantage in the rear wheels touching first if the aircraft is yawed (e.g. due to crosswind). The initial contact point is thus behind the gear strut and there is therefore a turning moment which tends to help align the aircraft with the runway. If the front wheel touches first the moment tends to turn the aircraft away from the runway. Normally it's only a small effect but every little helps, as they say.

The critical factor would be whether or not the wheel hitting first is forward or aft of the aircraft’s CG. And the effect is amplified the further the wheel is away from the CG. Its position on the bogie isn't really relevant, except how that causes the wheel to be physically positioned relative to the CG.

Now as to exactly where the CG is relative to the front wheel(s) of a multi-axle bogey is a good question, but it's probably pretty close to the CG on many aircraft.


User currently offlinetdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 20, posted (4 years 9 months 3 weeks 1 day 22 hours ago) and read 4919 times:

Quoting rwessel (Reply 19):
The critical factor would be whether or not the wheel hitting first is forward or aft of the aircraft’s CG.

It had better be aft. If not, you're dangerously close to the tip-up limit.

Quoting rwessel (Reply 19):
Its position on the bogie isn't really relevant, except how that causes the wheel to be physically positioned relative to the CG.

It's position on the bogie is extremely relevant. If the contact point is ahead of the strut and you don't touch down perfectly straight, the resulting torque tends to aggravate the misalignment. If the contact point is aft of the strut, it tends to align. It's the same reason that you can't drive a buggy (shopping cart) with the caster wheels pointed forward...they're only stable if they trail their pivot point.

Tom.


User currently offlinerwessel From United States of America, joined Jan 2007, 2392 posts, RR: 2
Reply 21, posted (4 years 9 months 3 weeks 1 day 22 hours ago) and read 4910 times:
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Quoting tdscanuck (Reply 20):
It's position on the bogie is extremely relevant. If the contact point is ahead of the strut and you don't touch down perfectly straight, the resulting torque tends to aggravate the misalignment. If the contact point is aft of the strut, it tends to align. It's the same reason that you can't drive a buggy (shopping cart) with the caster wheels pointed forward...they're only stable if they trail their pivot point.

I thought we were talking about the alignment of the aircraft, not of the bogie. jetlagged's post mentioned "turning moment which tends to help align the aircraft with the runway."

Certainly touching down on the front wheel of the bogie will resulting in a torque on the *bogie* further away from the direction of travel, but the torque on the *aircraft* as a whole will be into the skid (in the direction of travel), so long as that front wheel is still behind the CG.

Certainly if you put enough torque on the bogie to get it to point well off axis, you're going to have trouble, but the amount of force you'd typically see with only front wheels down will be relatively small, since the bogie is going to tip back with only relatively modest load on those wheels, limiting how much side force they can generate (relative to what the entire fully loaded gear would generate).


User currently offlinetdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 22, posted (4 years 9 months 3 weeks 1 day 2 hours ago) and read 4795 times:

Quoting rwessel (Reply 21):
Certainly touching down on the front wheel of the bogie will resulting in a torque on the *bogie* further away from the direction of travel, but the torque on the *aircraft* as a whole will be into the skid (in the direction of travel), so long as that front wheel is still behind the CG.

Ummm...yeah. I'm ashamed I didn't realize that.

Tom.


User currently offlineWN700Driver From , joined Dec 1969, posts, RR:
Reply 23, posted (4 years 9 months 3 weeks 23 hours ago) and read 4781 times:

Mender, thanks. I guess I've been wrong about that all this time!

Another general Q/hijack for the engineers & mx folks here. I've worked in a hangar or two over a number of years, off AND ON. But never on anything larger than a 752.

My question is this. What is done with regard to jacking procedures for AC like the 767, or even more extreme, like the 747, or A330/40 family, where the wheel bogies have a huge amount to tilt? I have always assumed that there are pins (like as in the gear doors) to hold the bogies in a "flat" position while jacking. Or is it that there is simply nothing done, and one must jack the AC all the way up off the ground until that last tire has clearance? (I hope not, as that would be a royal PITA for checking out a WOW switch!)

Anyway, forgive that insanely naive question, but it is always something that I've wondered about, but forgot to ask whenever someone in the know was actually available. Thanks!


User currently offlineRJ111 From , joined Dec 1969, posts, RR:
Reply 24, posted (4 years 9 months 3 weeks 14 hours ago) and read 4727 times:

I read in a book that they made an error with the 767 wheel-well design and thus had to design the gear this way so the wheels would fit.

User currently offlinejetlagged From United Kingdom, joined Jan 2005, 2572 posts, RR: 25
Reply 25, posted (4 years 9 months 3 weeks 13 hours ago) and read 4716 times:

Quoting rwessel (Reply 21):
Certainly touching down on the front wheel of the bogie will resulting in a torque on the *bogie* further away from the direction of travel, but the torque on the *aircraft* as a whole will be into the skid (in the direction of travel), so long as that front wheel is still behind the CG.

This is true, but the front wheel is closer to the CG than then rear one so this stabilising moment will be reduced touching front wheel first. There is a benefit to touching rear wheel first but it is very slight. Only the people who designed the 777 landing gear can definitively explain why the truck is designed to rotate to the rear wheel low position for landing, rather than leave it in the front wheel low position required for stowage in the wheel well.



The glass isn't half empty, or half full, it's twice as big as it needs to be.
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