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Landing Gear Curiosities  
User currently offline777DadandJr From United States of America, joined Feb 2005, 1516 posts, RR: 12
Posted (9 years 7 months 1 week 5 days 6 hours ago) and read 6312 times:

Just wondering what is the reason for the hang angle of the main landing gear?
Some a/c, the landing gear hangs forward, some hang rearward and some, such as the A340-600 the center main gear hangs opposite of the outboard main. why is this?
Also, on a Tupolev 154, the main gear is mounted in pods on the wing and retract backwards. On most western a/c the main gear folds sideways into the belly. Would not the TU-154 setup have an advantage of providing more space underneath for cargo/fuel tanks?


My glass is neither 1/2 empty nor 1/2 full, rather, the glass itself is twice as big as it should be.
36 replies: All unread, showing first 25:
 
User currently offlineNewark777 From United States of America, joined Dec 2004, 9348 posts, RR: 29
Reply 1, posted (9 years 7 months 1 week 5 days 6 hours ago) and read 6237 times:

I've always wondered this myself, but never got around to asking it myself. The one aircraft I seem to notice it on a lot on is the 777, probably because of the large six-wheel bogey.


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Harry



Why grab a Heine when you can grab a Busch?
User currently offlineTinPusher007 From United States of America, joined Feb 2005, 977 posts, RR: 1
Reply 2, posted (9 years 7 months 1 week 5 days 6 hours ago) and read 6221 times:

I have read that with regard to the 767 which has forward-tilting MLG, that in order for them to fit in the wheel well, they have to be tilted.


"Flying isn't inherently dangerous...but very unforgiving of carelessness, incapacity or neglect."
User currently offline777DadandJr From United States of America, joined Feb 2005, 1516 posts, RR: 12
Reply 3, posted (9 years 7 months 1 week 5 days 6 hours ago) and read 6217 times:

Hey Newark777
It has also been a curiosity of mine, and my son asked the same question, soooo...........
After I posted, I thought that I should have posted in "Tech/Op" but I guess I'll find out.
What about the Tupolev? Do you agree about saving space in the hold?



My glass is neither 1/2 empty nor 1/2 full, rather, the glass itself is twice as big as it should be.
User currently offlineNewark777 From United States of America, joined Dec 2004, 9348 posts, RR: 29
Reply 4, posted (9 years 7 months 1 week 5 days 6 hours ago) and read 6197 times:

I wouldn't be surprised if that was the reason in the Tupolev, since the gear have to fit in those funny pods on the wings.


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Harry



Why grab a Heine when you can grab a Busch?
User currently offlineCitjet From United States of America, joined Nov 2004, 97 posts, RR: 0
Reply 5, posted (9 years 7 months 1 week 5 days 5 hours ago) and read 6141 times:

YYYYYup. It all has to do with how the designer wanted the gear to fit into the wheel well when retracted. The TU154 retracts rearwards and kind of pulls the main wheel bogey up laterally rather than swining 90 degrees towards the center like most other airliners. Landing gear are one of the coolest parts of the aircraft in my opinion.

User currently offlineAfay1 From United States of America, joined Oct 2001, 1293 posts, RR: 2
Reply 6, posted (9 years 7 months 1 week 5 days 3 hours ago) and read 6030 times:

The Tu-154 also has wide spaced gears allowing for the explicit need for it to operate from "austere" landing strips, especially during the earlier age of jet travel in the USSR.

User currently offlineNudelhirsch From Germany, joined Oct 2003, 1438 posts, RR: 18
Reply 7, posted (9 years 7 months 1 week 5 days 3 hours ago) and read 6002 times:

TinPusher is right, this is an "old" (yet interesting) question. On the 67, the bogie must fit in the wheel well.

Other odd wheel thingies:
37: wheel exposed to air inflight...

AN225: check pictures and say yourself... Big grin



Putana da Seatbeltz!
User currently offlineAr1300 From Argentina, joined Feb 2005, 1740 posts, RR: 3
Reply 8, posted (9 years 7 months 1 week 5 days 3 hours ago) and read 5971 times:

Always wondered the same thing!!!!

Mike.



They don't call us Continental for nothing.
User currently offline747NUT From Australia, joined Sep 2004, 78 posts, RR: 0
Reply 9, posted (9 years 7 months 1 week 4 days 21 hours ago) and read 5813 times:

The tilt of the wheels has got to do with getting the gears in the hole, the 747 for instance has a tilt sensor that prevents the wheels from retracting if they aren't tilted.


If it's not broken, don't fix it !
User currently offlineAeroWeanie From United States of America, joined Dec 2004, 1609 posts, RR: 52
Reply 10, posted (9 years 7 months 1 week 4 days 13 hours ago) and read 5566 times:
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Tupolev had a tradition of retracting the main gear into pods on the wing. You can see this on the Tu-95, Tu-16, Tu-22, Tu-28, Tu-104, Tu-114, Tu-124, Tu-134, Tu-142 and Tu-154. If I remember right, the Tu-22M Backfire A prototypes had them too. However, there is a drag penalty for these big pods and Tupolev stopped using them - they aren't used on the Tu-22M3, Tu-160, Tu-204, Tu-214, Tu-234, Tu-334 and Tu-354.

At one point, there was intense speculation when a Tu-95 Bear was spotted with larger gear pods. All kinds of theories flew as to what was in them. We finally learned that Tupolev had gone from four wheel bogies to six wheel bogies and the pods had been enlarged to contain them.


User currently offlineSpeedracer1407 From United States of America, joined Dec 2004, 333 posts, RR: 0
Reply 11, posted (9 years 7 months 1 week 3 days 22 hours ago) and read 5373 times:

This topic hasn't been brought up for a while, but has been discussed to death in several threads over the past few years. Which isn't to say at all that I'm not glad this came up again. Here's why: The overwhelming opinion in previous posts (and this one) is that MLG bogeys are angled primarily for stowage purposes. But some mentioned in previous threads that the direction and degree of this angle is crucial to landing comfort as well as landing gear longevity. 12 tires, as on the 777, touching down simultaneously would result in a rather sudden and severe shock to the landing gear, which, even if it could handle it time and again without problem, would at least produce some uncomfortable shudders. Same (or worse) with the 16 MLG tires of a 747, or 12 of the bigger A340s. The impression I got from a few posters was that, even if the MLG could be stowed with any bogey angle, it would still be deployed at an angle that allowed some wheels to touchdown before others, thus softening the blow. This seems like a fairly rudimentary concept, even if it's not true, but what piqued my curiosity was one post that suggested that 'better' designed MLG touches it's front wheels down first, unlike the 777. The reason, he gave, was that the torque produced by the sudden spinup of the front wheels allowed the aft wheels to touchdown with more grace and comfort. As on a 747 or 777, however, if the aft wheels touch down first, the spinup torque encourages the the bogey to hop and slam back down onto the pavement; an event surely perceived as rough by PAX. Having ridden on 747s and 777s, and noticed only greasy-smooth touchdowns, I'm not sure how much I trust this notion. But I'm still intrigued by the science of this, and would like to hear some fresh opinions about the engineering decisions that go into MLG angle besides stowage. BTW, I've referenced a lot stuff that was discussed in previous posts, but as you may have noticed, I'm too lazy to provide links, and for that, I'm sorry.

O



Dassault Mercure: the plane that has Boeing and Airbus shaking in their boots.
User currently offlinePilotaydin From Turkey, joined Sep 2004, 2539 posts, RR: 51
Reply 12, posted (9 years 7 months 1 week 3 days 16 hours ago) and read 5318 times:

im not an engineer, but do you think it is less force and structural tension for the gears to slowly take on the huge airplane's weight?

If they were parallel, to the fuselage, the airplane would have to pitch down less, to make both of them touch at the same time. Also if they were parallel, the front part of the boggie would remain in the air, until the plane's nose touched down, which might cause vibration and possibly be more dangerous to the structure.

With the current gear angle, the pitch of the airplane wouldnt have to "worry" about the position of the wheels while touching down, and it would be a more smoother application to the runway, rather than smacking down a large load flatter on 4 tires

maybe this has nothing to do with it, but this is my thought process, and maybe why im not an engineer lol....



The only time there is too much fuel onboard, is when you're on fire!
User currently offlineAir2gxs From , joined Dec 1969, posts, RR:
Reply 13, posted (9 years 7 months 1 week 3 days 16 hours ago) and read 5311 times:

The gear is tilted so that it can fit in the gear well. I believe the AMM of certain aircraft even states that. When I get to work on Saturday, I'll look for the pertinent passages.

User currently offlineB747FE From Hong Kong, joined Jun 2004, 230 posts, RR: 4
Reply 14, posted (9 years 7 months 1 week 3 days 16 hours ago) and read 5299 times:

As stated above, the gear truck positioning actuator tilts the wheel truck to a pitched up or down position after liftoff, thus assuring correct gear positioning for retraction.
Touchdown forces and taxi shocks are absorbed by the shock struts.

Regards,
B747FE



"Flying is more than a sport and more than a job; flying is pure passion and desire, which fill a lifetime"
User currently offline242 From United States of America, joined Oct 2000, 498 posts, RR: 1
Reply 15, posted (9 years 7 months 1 week 3 days 15 hours ago) and read 5274 times:

The 757, 767 and 777 all must tilt their main gear forward to stow them. All three aircraft have tilt actuators that allow the main gear to be tilted nearly any degree the designers desired. As most people know, the 767 is the only one of the three that lands with the gear tilted forward, so 'stowage' isn't the reason for the 767 gear orientaion. I've been told it's due to landing CG. Anyone have the true explanation?

User currently offlineBuyantUkhaa From Mongolia, joined May 2004, 2899 posts, RR: 3
Reply 16, posted (9 years 7 months 1 week 3 days 12 hours ago) and read 5279 times:

Landing CG considerations cannot really play a part here, in my opinion - it's a different story when the landing gear strut is also tilted backwards, as with the 727. But that's another issue.

I don't think storage position is too crucial, as the actuators should be able to put it in any position desired.

The way I see it is that when the main gear is tilted in the common way (aft wheels touching down first), the touchdown will rotate the truck (I suppose it's called like that) so that the front wheels touch down too. This truck rotation is restrained by the actuator, which will dissipate energy in the rotation process, thus damping the shock of the touchdown. The main strut already has a shock absorber, but it might help.


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Now, as to why some planes do it ithe other way around (Tu-154 etc.). Look at the retraction sequence in the next pics:


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The truck rotates almost 180º in the process. If it were to have the same angle as in other planes, it would travel more than 180º which is difficult to do.

A close-up:


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As you can see, the gear retracting actuator does not leave any room for the gear to be tilted the normal way. In conclusion, I think the Tu154 (and similar planes) do not have the optimal configuration for shock absorption, but that's the price they pay for placing the landing gear much further away from the middle (because of unpaved runways etc.)



I scratch my head, therefore I am.
User currently offlineAir2gxs From , joined Dec 1969, posts, RR:
Reply 17, posted (9 years 7 months 1 week 3 days 12 hours ago) and read 5236 times:

The B757 & B777 (from pictures, I've never worked on the B777) gear all tilt aft. The B767 tilts forward. The B767 gear does not change its tilt depending on whether it's landing or whether it's going in the hole. It is always tilted forward.

The reason I know this (other than the fact that I've jacked the airplane a few times) is the air/ground system. Gear tilted is one of the inputs for "air", not tilted is "ground". Changing the tilt in the air is not an option, as that would screw up air/ground sensing.

The B767 gear issue you may be referring to is the question as to why there is no B767-100s. When I was in familiarization school the instructor said that the gear on a -100 created too small a footprint on certain runways (SFO & LGA). So, the gear was modified and the footprint widened. Look at the upper trunnion of the main gear of a B767, you'll see that it is offset, making the distance between the mains larger. This increased the footprint. This also required a small door be installed on the upper surface of the wing, so this offset could pass through during extension and retraction. The door prevented a major modification to the wing.

[Edited 2005-02-10 20:30:24]

User currently offlineSovietjet From Bulgaria, joined Mar 2003, 2607 posts, RR: 16
Reply 18, posted (9 years 7 months 1 week 3 days 10 hours ago) and read 5182 times:
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Actually the Tupolev gear pods are very aerodinamically efficient and cause very little drag. On the Tu-22M and Tu-160 they weren't used because those planes have a moving wing, not because of drag. That Tu-95 with the bigger pods was actually the Tu-142 prototype which had not 6 but 12 tires(3 rows like on Tu-154 but 4 tires per axle). The first early production Tu-142s also had those 12-wheel bogies however they were replaced by the standard 4-wheel ones because of the extra weight. The newer Tu-204 family doesn't have pods because of the wing mounted engines. The Tu-334 family has only one axle on it's main gear making the design of a whole pod for that small unit pointless. And in fact the Tu-154 lands very smooth compared to many other airliners. You almost can't feel it.

User currently offlineAJ From Australia, joined Nov 1999, 2391 posts, RR: 24
Reply 19, posted (9 years 7 months 1 week 3 days 10 hours ago) and read 5178 times:

I've been told that the 767's bogie tilt results in the lowest drag whilst extended on approach. this is due to the designers quest fo fuel efficiency in every part of the design.

If true my question would be why didn't they make the nose gear doors close flush after extension!?


User currently offlineSpeedracer1407 From United States of America, joined Dec 2004, 333 posts, RR: 0
Reply 20, posted (9 years 7 months 1 week 3 days 4 hours ago) and read 5113 times:

"Touchdown forces and taxi shocks are absorbed by the shock struts."

It's my understanding that VERTICAL forces are absorbed by the shock struts, but in my previous post, I questioned the effect of other forces that may play apart in MLG design. No doubt, MLG are stout pieces of kit, but surely the sudden friction of 4 or 6 tires touching down simultaneously and spinning up to well over 100 MPH in an instant is something engineers take into consideration when designing the angle at which the bogey touches down. And since some aircraft use actuators to adjust the MLG tilt, it seems like there must be some other considerations at play. I'm still hoping someone addresses my curiosity about this, though I could just be reaching for explanations about things that just don't exist.
O



Dassault Mercure: the plane that has Boeing and Airbus shaking in their boots.
User currently offlineSilver1SWA From United States of America, joined Mar 2004, 4811 posts, RR: 25
Reply 21, posted (9 years 7 months 1 week 3 days 4 hours ago) and read 5110 times:
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Has anyone ever payed close attention when a 777 raises it's gear? The tilt in the MLG is actually eliminated as the bogies rotate to a position where they are in a level position (relative to the aircraft) before the MLG begin to retract into the body of the aircraft. It's rather hard to explain...you'd have to see what I'm talking about....but his is why I have to question the whole "they must be tilted in order to properly fit when stowed" explanation. It seems to me the 777's MLG are adjusting themselves so that is can properly fit into the body of the aircraft when retracted where as the degree of tilt (as seen on approach and landing) would not allow the MLG to fit.


ALL views, opinions expressed are mine ONLY and are NOT representative of those shared by Southwest Airlines Co.
User currently offlineAeroWeanie From United States of America, joined Dec 2004, 1609 posts, RR: 52
Reply 22, posted (9 years 7 months 1 week 3 days 1 hour ago) and read 5080 times:
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Sovietjet:

How can landing gear pods possibly be aerodynamically efficient? Given a choice of stowing the landing gear completely internally or stowing it in pods that increase the wetted area of the aircraft, pods are very inefficient. They also screw up the local spanloading on the wing.


User currently offlineAir2gxs From , joined Dec 1969, posts, RR:
Reply 23, posted (9 years 7 months 1 week 2 days 18 hours ago) and read 5042 times:

Silver, I'm not familiar with the B777 retraction sequence, but I don't see why Boeing would complicate the process by changing the tilt prior to or during the retract sequence. On all aircraft I'm familiar with the "tilt valid" signal is required before the gate will be pulled from the landing gear handle. And again air/ground sensing is an issue. Now both these issues can be address by software modifications, but why complicate the sequence?

Any B777 mechanics out there that can confirm.


User currently onlineKlaus From Germany, joined Jul 2001, 21461 posts, RR: 53
Reply 24, posted (9 years 7 months 1 week 2 days 17 hours ago) and read 5017 times:

Air2gxs: Silver, I'm not familiar with the B777 retraction sequence, but I don't see why Boeing would complicate the process by changing the tilt prior to or during the retract sequence.

Yet they apparently do. On the 777-300ER even to the point that they´re using the tilt actuators to improve tail clearance on rotation. And it might be somewhat difficult to stow the long 777 bogies in a tilted position (or at least a waste of precious space).

The A330 and A340 also change bogie tilt during retraction/extension. One forum member once reported an airbus engineer explaining that one reason for the bogie tilt was indeed to aid in a smoother touchdown.

That could be especially true in case of "one wing down" crosswind landings when the plane has to settle horizontally without a "bouncing roll"...


Air2gxs: On all aircraft I'm familiar with the "tilt valid" signal is required before the gate will be pulled from the landing gear handle. And again air/ground sensing is an issue. Now both these issues can be address by software modifications, but why complicate the sequence?

I don´t see the problem here. There´s a little added effort to effect and monitor the transitions between "stowed", "extended" and "leveled/ground", but it´s a very minor complication, well within the capabilities of reliable engineering, I would think.


25 Dl757md : The truck positioner actuator on the 777 gear positions the the truck 13 degrees fwd wheels up when the gear are in the down and locked position. The
26 Air2gxs : Thank you Klaus and Dl757md. I stand corrected.
27 Post contains links and images Wilax : Silver1SWA, You are 100 percent correct... From a physics standpoint, tilted gear would always take up more space, laterally, than flat and parallel g
28 Post contains images CCA : The Gear is tilted whether it be 0 degrees or 51.7 degrees (wing gear of a 747), TO FIT IN THE WHEEL WELL and occupy the available space most efficien
29 Wilax : CCA: Great job... I didn't even think about the 747. That is the only Western-built aircraft whose bogies retain their tilt--or do not tilt at all--du
30 G4doc2004 : On the topic of main gear bogey being "altered" before retraction, the Concorde main bogies were shortened when the retract sequence was begun to fit
31 Meister808 : That is interesting that the Airbus widebodies not only swing the gear in and undo the tilt, but they also literally 'retract' as well. I didn't know
32 Air2gxs : The B757/B767 tilts are fixed, they do not change before or during retraction. The gear (on these aircraft) is tilted so that it can fit in the well.
33 Sovietjet : Aeroweanie - The pods are there for a reason. There is no other way to stow the landing gear. It can't be retracted in the wing. It can't be retracted
34 2H4 : Sovietjet...Aeroweanie isn't arguing that. You said the pods are very aerodynamically efficient. He's making the point that, when compared to a clean
35 Dc10hound : The B757/B767 tilts are fixed, they do not change before or during retraction. The gear (on these aircraft) is tilted so that it can fit in the well.
36 Sovietjet : I agree, of course they add drag and make the wing inefficient however not to the extent that some people think they do.
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