Mr Spaceman From Canada, joined Mar 2001, 2780 posts, RR: 15 Posted (12 years 2 months 17 hours ago) and read 12652 times:
Hi guys. My question is, Why does the "Main" Landing Gear on a Boeing 767 have the front wheels hanging "lower" than the rear wheels on the boggie assembly? This looks strange to me. However it must serve some purpose. The Boeing 757's "Main" Gear wheels hang like most of the other Airliner jets, with the trailing wheels hanging the lowest. [Apparently, pilots can transition between a 767 and 757 quite easily because their systems are so alike], but why the difference in the Gear? I've been wondering about this for years!
Wardair Canada From Canada, joined Dec 2000, 28 posts, RR: 0 Reply 1, posted (12 years 2 months 15 hours ago) and read 12601 times:
I am not too sure 100% on the facts that I'm about to say about the 767 gear stance...but I heard it was due to the fact that Boeing engineers realized too late that the gear bay would not fit the 767's bogies so they had to articulate it, or the fact that to eke additional range out of the 767 without doing major redesign by adding more fuel tanks and thus the bay size was reduced and they decided to articulate the bogies. I am not 100% certain, but somebody else is....I'll leave that answer to them.
But what I can tell you is there is an answer for the A330/A340's unique articulated bogies. It is not because the bay is too small or the gears are too heavy that they hang rear wheels down first.
When Airbus first designed the A330/A340, the main gear assembly was designed to give optimal ground clearance and control while the main gear is still on the ground. It provides better pitch control and additional ground clearance. In other words, the gear remains on the ground a bit longer. This is helpful especially in landings.
Fanoftristars From United States of America, joined Jul 2000, 1573 posts, RR: 5 Reply 2, posted (12 years 2 months 14 hours ago) and read 12578 times:
I remember this topic being discussed before, and the general consensus was clearence in the wheel bay.
What is not clear to me is how this affects the aircraft landing. Do all four wheels touch together, or do the front wheels hit first? I also heard that smooth landings are more difficut on the 76s because of this, but I havn't heard a reputable source back that up.
Wardair Canada From Canada, joined Dec 2000, 28 posts, RR: 0 Reply 4, posted (12 years 2 months 13 hours ago) and read 12563 times:
I agree with you, the A330 looks lively with that kind of gear stance....it also gives it a "tippy-toe" kind of attitude when landing. The plane would tip toe for a few seconds then settle on all 4s.
The A340-500 & -600 will have a 4 wheel center gear bogey..yes 4 wheels instead of the usual 2 found on the A340-300. It will articulate like the main gear bogies so it should be an interesting sight to see.
Cdfmxtech From United States of America, joined Jul 2000, 1338 posts, RR: 29 Reply 6, posted (12 years 2 months 9 hours ago) and read 12553 times:
For the record...the B757 gear WILL ENTER THE WHEEL WELL not tilted. It has been proven..recently at my airline. The B757 gear has something call a truck tilt positioner that with weight OFF wheels will position the gear with the fwd most wheels elevated. Well, there was a malfucntion that I won't go into too much detail about, (becasue this not the tech forum), but the truck couldn't tilt. Now the flightcrew could not normally raise the (that is without using the gear override button) because the B757 gets its Air/grnd sensing from those tilt sensors. But it will go into the wheelwell physically.
Cdfmxtech From United States of America, joined Jul 2000, 1338 posts, RR: 29 Reply 7, posted (12 years 2 months 9 hours ago) and read 12530 times:
OK....I just realized I WAS IN THE TECH FORUM. There is a hydraulic fuse right off the truck positioner shuttle valve. It went bad, and there was no hyd pressure downstream (worked as advertised) to position the truck positioner.
Quoting Cdfmxtech (Reply 7): There is a hydraulic fuse right off the truck positioner shuttle valve. It went bad, and there was no hyd pressure downstream (worked as advertised) to position the truck positioner.
Only on the LH shuttle valve side for both MLGs.
Vikkyvik From United States of America, joined Jul 2003, 8194 posts, RR: 28 Reply 9, posted (7 years 1 month 3 weeks 4 days ago) and read 11906 times:
AIRLINERS.NET CREW PHOTO SCREENER
Every thread that I've seen on this subject (and there have been a few!) has come to the consensus that various bogey positions are due to the way the gear fits into the wheel well. I'm not an expert on this, but this is what I've read on this forum. Hey, here's a thread on this very subject:
Mr Spaceman From Canada, joined Mar 2001, 2780 posts, RR: 15 Reply 10, posted (7 years 1 month 3 weeks 3 days 22 hours ago) and read 11889 times:
Quoting Vikkyvik (Reply 9): HAWK21M, glad to see you post again. Where'd you go?
EDIT: D'oh, I just noticed that this thread was 5 years old! I saw that it was posted in March, and just assumed it was March 2006! Oh well, at least it wasn't me who brought it back
>> Hi Vikkyvik,
I was really surprised to see a topic near the top of the page about the 767 versus the 757 .... with my username as the thread starter! I thought there was a malfunction with a.net because I knew I didn't post anything about landing gear for a Long Time.
HaHaHa! It's neat to read a post you did over 5 years ago!
>> Hello HAWK21M, I'm also glad to see you post again. You must have been doing a good search to dig up this thread!
As many others in this forum, I too have been wondering where you went?
Fr8Mech From United States of America, joined Sep 2005, 4238 posts, RR: 12 Reply 11, posted (7 years 1 month 3 weeks 3 days 21 hours ago) and read 11876 times:
OK for the record, the gear on a B757/B767 is positioned (tilted) so that it will fit in the wheel well. Here is a quote from the B767 AMM:
"Truck Positioner (Fig. 9) (1) The truck positioner is a hydraulic actuator which tilts the truck to fit into the wheel well during gear retraction."
Cdfmxtech, who do you work for? I was just involved with a B757 that had the fuse on the shuttle valve set. The gear will go in the wheel well, after bouncing off the doors, as evidenced by the skid marks on the door.
Without pressure on the tilt positioners (both aircraft have them), the bogie is free to 'flop around' after takeoff. If the tilt sensors (sys 2) show 'not tilted' the gear safety override will not pull. The flight crew can override the switch and retract the gear, an untilted gear will be tilted as it rides along the door.
A point of note, when the gear is tilted by the door, the sensors will will show tilted and any messages that may have appeared will go away.
Quoting Flight152 (Reply 5): Well than on the Boeing 747 why are the outter bogies tilted and the two innner ones are not, is it space
The body gear are tilted on a B747, just not as much as the wing gear. I would be very reluctant to press the override on a Jumbo if the wing gear show untilted after take-off (the classics can tell you, don't know about the -400). The body gear should be able to fit in its hole, but the wing gear, due to its extreme, required tilt, will probaby run into structure and/or the body gear if an attempt is made to retract with an untilted wing gear.
HAWK21M From India, joined Jan 2001, 31201 posts, RR: 58 Reply 12, posted (7 years 1 month 3 weeks 3 days 8 hours ago) and read 11788 times:
Quoting Vikkyvik (Reply 9): HAWK21M, glad to see you post again. Where'd you go
Quoting Mr Spaceman (Reply 10): Hello HAWK21M, I'm also glad to see you post again. You must have been doing a good search to dig up this thread!
The Thread popped up on a search.Found it interesting.
I was attending Boeing Theory Training on the B752 by Alteon. It was a 45 day course with lots of studies.Another 3 day Engine differences [-535C & -535E4], 3 day Pax to Freighter conversion course & Practicals [2 weeks] are due in April end.
DH106 From United Kingdom, joined Jun 2005, 621 posts, RR: 1 Reply 13, posted (7 years 1 month 3 weeks 2 days 21 hours ago) and read 11744 times:
Quoting Vikkyvik (Reply 9): Every thread that I've seen on this subject (and there have been a few!) has come to the consensus that various bogey positions are due to the way the gear fits into the wheel well. I'm not an expert on this, but this is what I've read on this forum. Hey, here's a thread on this very subject:
But if a tilt actuator IS required to tilt the bogie during retraction to fit the gear well, that STILL doesn't answer the question as to why the bogie tilt is required when extended. Why doesn't the tilt actuator just position the bogie flat when extended, and to whatever required by the well geometry during retraction? To my mind, the issue of gear well shape & geometry is a red herring, the differential spin-up on touch down arguement seems more plausible.
...I watched c-beams glitter in the dark near the Tanhauser Gate....
Fr8Mech From United States of America, joined Sep 2005, 4238 posts, RR: 12 Reply 15, posted (7 years 1 month 3 weeks 2 days 20 hours ago) and read 11737 times:
Quoting DH106 (Reply 13): But if a tilt actuator IS required to tilt the bogie during retraction to fit the gear well, that STILL doesn't answer the question as to why the bogie tilt is required when extended
Why make the system more complicated by having multiple settings? Remember, these aircraft are 70/80's technology. I realize the newer aircraft have multiple settings dependant on phase of flight, but not these 2.
These 2 aircraft (B757/67) have fixed tilt. When the aircraft is in the air the tilt does not change. The bogie tilts and the gear fits in the well. To answer the question of why one is tilted one way vs. the other, take a look at the B767 main gear. The whole amin gear assembly is canted aft (pivot point forward), this may explain the reason for the forward tilt. Next time I'm atound a gear retraction, I'll look at it closer, but being at a line station, a retraction is a rare and bad thing thing.
DH106 From United Kingdom, joined Jun 2005, 621 posts, RR: 1 Reply 16, posted (7 years 1 month 3 weeks 2 days 20 hours ago) and read 11732 times:
Quoting Fr8Mech (Reply 15): Why make the system more complicated by having multiple settings?
But the arguement that the tilt is needed because of the geometry of the retraction angles/gear well geometry is only valid if there isn't a tilt actuator moving the bogie during retraction. If you do then add a tilt actuator (as in our aircraft), then only 2 tilt positions are required - those for gear extended & gear retracted. Since we seem to be agreed that these aircraft do have tilt actuators, the question still remains: Why were those gear extended tilt angles chosen.
Fr8mech From United States of America, joined Sep 2005, 4238 posts, RR: 12 Reply 17, posted (7 years 1 month 3 weeks 2 days 12 hours ago) and read 11650 times:
Quoting DH106 (Reply 16): those for gear extended & gear retracted
There is only one tilted position. Anytime the aircraft is in the air with hydraulics on the bogie is tilted. Any position other than tilted is untilted. This is not a commanded position. It is a position that is induced, usually, by the weight of the aircraft on the wheels. When the gear is extended or retracted the tilt remains the same.
Okay - perhaps I've misunderstood the function of the tilt actuator. Does it merely return the bogie to a preset angle on leaving the ground - there's no 'dynamic' repositioning behavior on retraction on the 757/767 ?
The 777 certainly does this - normally the 6 wheel bogie is positioned 'rear wheels down', but as the main gear doors open, the bogie is tilted to approximately flat before the gear legs pivot in.
Mender From United Kingdom, joined Feb 2004, 224 posts, RR: 0 Reply 23, posted (7 years 1 month 17 hours ago) and read 11248 times:
The reason the main gear tilts forward IS to fit in the wheel well. Conventional thought is that the main leg pivots at 90 degrees to the airframe thus it is hard to understand why the gear is tilted at all.
The REAL reason the gear is tilted is that the main leg DOESN’T pivot at 90 degrees to the airframe. It actually pivots at an angle such that it moves forward in an arc. Thus if the truck beam was parallel to the main leg the rear wheels would be closer to the keel beam of the airframe. To reduce the track of the gear they tilt the truck beam so that it stays parallel to the keel when it is retracted.
When the gear is retracted in the bay and the gear lever selected to off the gear rests on a linkage attached to the hinge line of the door on the keel beam. Part of this linkage is attached to the door/keel beam of the airframe and part of it is attached to the truck, on the pivot point of the truck beam to main leg. If you were to view the aircraft on the ground with the gear doors open looking at the uplock fitting on the truck pivot point you can see that it is aft of the uplock link on the door/keel beam. The uplock link on the door is actually slightly forward of the front axle with the gear down, that’s how much the gear arcs during retraction.
When you look at the top of the leg it’s easy to see that it is “bent” over at the top but it is also “twisted” in relation to the truck beam. Whilst this is not as easy to see, the pivot point (or trunnion) is further inboard at the rear trunnion bearing than it is at the forward trunnion. This makes the gear arc forward during retraction.
I speculate that the design engineers at Boeing had a structure to fit the gear into and a position to place the gear in so that the centre of gravity didn’t make the aircraft fall over on the ground. These two didn’t match unless the gear was to arc forward, thus they needed to tilt the gear against popular convention and that was that!
Kaddyuk From Wallis and Futuna, joined Nov 2001, 4125 posts, RR: 29 Reply 24, posted (7 years 1 month 16 hours ago) and read 11221 times:
Quoting DH106 (Reply 19): Okay - perhaps I've misunderstood the function of the tilt actuator. Does it merely return the bogie to a preset angle on leaving the ground - there's no 'dynamic' repositioning behavior on retraction on the 757/767 ?
I'm Not sure in regards to the B75/76 actuator however the A340 gear is set up so that if there is no Weight On Wheels the gear will naturally tilt of its own accord, however the reason for the tilt actuator is to force the bogie beam into the tilt position if for any case, it doesnt tilt naturally. This is because of how the gear fits into the well... There is also a shortening link which actually shortens the landing gear and then actually extends it so that the gear will fit in the bay correctly AND all the doors close correctly.
The reason the A346 CLG is rotated 180 degrees to the rest is because that landing gear flips upside down and the wheels sit in the roof with the Leg Underneath, its such a complex method of retracting the gear.
It seems that manufacturers design the aircraft, then send the specs to whoever makes the landing gear and says "Make It Fit".
Whoever said "laughter is the best medicine" never had Gonorrhea
25 DH106: It's all to do with 'relaxed stability' - moving the CG rearward to unload the tailplane with the aim of reducing drag, whilst letting computers worr
26 David L: Makes more sense than having the gear manufacturer telling the aircraft manufacturer to "rotate your aircraft around this".
27 Lfutia: Namaskaar Mel. I believe the restrictions were uplifted from airports that dont have military presence in them. Leo
28 Gt1: It's taken me 8 years of very close association with this airplane to figure out what Mender told us in a few paragraphs! Great post! If I may, I'd li
29 A3204eva: Don't have time to read every reply atm so if what I am about to say has been previously said above, I appologies Boeing didn't intend for the main ge
30 Fr8mech: So they installed the tilt actuator for giggles?
31 HAWK21M: True.But not on the Tarmac.Only as a Scheduled Pax from Inside an aircraft at Non Military Airports & Terminal Buildings. AIC 8of2005 regds MEL[Edite