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757-type Main Landing Gear Bogies...  
User currently offlineUSAFHummer From United States of America, joined May 2000, 10685 posts, RR: 52
Posted (11 years 12 months 2 days 5 hours ago) and read 11735 times:

My question is...when the main gear bogies of aircraft with 4 or more wheels extend, they are obviously tilted as such in this picture...Im assuming that the reason for this is so that the gear bogie can fit in the main gear bay

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Photo © Miguel Snoep



...but what I would like to know is at what point of the approach phase does the gear bogie shift to this position at touchdown, or is the touchdown on the rear two wheels of the bogie...


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Photo © Justin Cederholm



Hope this is clear, if not Ill try to clarify...

Thanks,
Greg



Chief A.net college football stadium self-pic guru
8 replies: All unread, jump to last
 
User currently offlineFritzi From United Arab Emirates, joined Jun 2001, 2762 posts, RR: 2
Reply 1, posted (11 years 12 months 2 days 3 hours ago) and read 11695 times:

The 757 will touchdown on the rear two wheels of the bogies and they will tilt into the position as shown in the second picture when enough weight is put on them.

You can see in the picture below of the A332 that the bogies stay in the tilted position until the aircrafts sets its full weight on th ground during the touchdown.


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Photo © Andy Graf



/Fritzi



User currently offlineCdfmxtech From United States of America, joined Jul 2000, 1341 posts, RR: 26
Reply 2, posted (11 years 12 months 1 day 23 hours ago) and read 11642 times:

Common question.
I'll explain the B757 for you. There is a truck positioner actuator located on the aft part of the gear. When weight is off the wheels, hydraulic pressure positions the actuator to extend and drive the aft wheels down. Now the purpose as the Aircraft manual states is to permit clearance between the gear and aircraft structure during gear retraction and extension. But I remember an instructor once telling me that they testes a B767-200 some time ago to see if it would clear the structure if not tilted. He said it did. (The B767 has the actuator on the front part of the bogie and the forward wheels go down.).

I also remember another incident at my airline a couple of years ago. And aircraft had to do an air return due to the landing gear lever lock solenoid not releasing after the aircraft transitioned to air mode. Now, I believe the crew pushed the lever lock override button and raised the gear. I mention this because when the aircraft was troubleshot, it was found that there was a hydraulic leak upstream of the truck positioner actuator and the wheels were sort of dangling, meaning that you could tilt them with your hands up and down. There was no hydraulic pressure to position the gear. Maybe the gear just happened to be in the correct position when they retracted the gear that time, because there were no problems.

So the gears are tilted to allow for clearance into the gear (official reason), and a secondary function is to provide air/gnd mode logic, since sensors depend of gear angle and wows (This applies to the B757/B767/B777).

As for you other question as to when this happens. On the B757/B67, the gears are tilted whenever weight is off the wheels. The gears are always tilted. The B777 is different. The rear wheels are tilted downward for ldg (unless there is a high approach angle. At that point the gear will actually adjust to help compensate for a steep ladg angle) and the rear wheels are tilted up during retraction. The can only fit into the wheelwell this way.

Hope this helps, I need water now!

PS...Maybe someone can shed some light on Airbus. I'm going to ASS-U-ME it works in a similar manner on the A300/10/30/40. But u know what happens when you ASS-U-ME


User currently offlineUSAFHummer From United States of America, joined May 2000, 10685 posts, RR: 52
Reply 3, posted (11 years 11 months 3 weeks 6 days 21 hours ago) and read 11492 times:

Thanks for the response, it makes sense now...

Greg



Chief A.net college football stadium self-pic guru
User currently offlineRydawg82 From United States of America, joined Sep 2001, 867 posts, RR: 8
Reply 4, posted (11 years 11 months 3 weeks 6 days 14 hours ago) and read 11461 times:

Quick correction, and thanks for the great info so far. The 777 truck actually tilts back at takeoff and landing. When the truck is positioned for retraction, the truck tilts forward, and remains in this position until it has been lowered again. It's a rather interesting process to observe....Ryan


You can take the pup out of Alaska, but you can't take the Alaska out of the pup.
User currently offlineCdfmxtech From United States of America, joined Jul 2000, 1341 posts, RR: 26
Reply 5, posted (11 years 11 months 3 weeks 6 days 10 hours ago) and read 11450 times:

The rear wheels are tilted downward for ldg (unless there is a high approach angle. At that point the gear will actually adjust to help compensate for a steep ladg angle) and the rear wheels are tilted up during retraction.

That's what I said. Just said it a little differently


User currently offlineUSAFHummer From United States of America, joined May 2000, 10685 posts, RR: 52
Reply 6, posted (11 years 11 months 3 weeks 6 days 4 hours ago) and read 11422 times:

Just to check also, at takeoff then on aircrafts qith 4-6 wheel bogies, the front wheels will lift off first before the aft wheels? The aircraft won't pivot on the gear so that all 4 wheels get off at the same time?

Thanks again,
Greg



Chief A.net college football stadium self-pic guru
User currently offlineBio15 From Colombia, joined Mar 2001, 1089 posts, RR: 7
Reply 7, posted (11 years 11 months 3 weeks 2 days 19 hours ago) and read 11333 times:

That would be the case with the 767 I believe, which tilts the bogeys foreward. On the backward tilted bogeys, the foreward 2 wheels will lift off the ground first.

-Alfredo


User currently offlineAJ From Australia, joined Nov 1999, 2395 posts, RR: 24
Reply 8, posted (11 years 11 months 3 weeks 2 days 18 hours ago) and read 11329 times:

These are good photo's taken at the point of rotation/liftoff:
Until the wings are supporting the aircraft's weight all main wheels remain on the ground:

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Photo © Andrew Hunt
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Photo © Garry Lewis


Rearward tilting bogey's, and level bogey's, aft wheels leave the ground last:

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Photo © Rob Rindt
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Photo © Carlos Borda


Forward tilting bogey's forward wheels leave the ground last:

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Photo © Felix Sieder
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Photo © Carlos Borda


This also depends on whether the bogey tilt has hydraulic assistance to the tilted position. Notice these A310s bogey's remain at the angle at which the aircraft left the ground:

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Photo © Oliver Brunke
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Photo © Glenn Alderton

They will 'level' for retraction.
All in all it's a beautiful thing:

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Photo © Don Boyd




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