777DadandJr
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Landing Gear Curiosities

Wed Feb 09, 2005 10:41 am

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?
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Newark777
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RE: Landing Gear Curiosities

Wed Feb 09, 2005 10:53 am

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
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tinpusher007
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RE: Landing Gear Curiosities

Wed Feb 09, 2005 10:58 am

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.
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777DadandJr
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RE: Landing Gear Curiosities

Wed Feb 09, 2005 11:00 am

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.
 
Newark777
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RE: Landing Gear Curiosities

Wed Feb 09, 2005 11:09 am

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
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citjet
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RE: Landing Gear Curiosities

Wed Feb 09, 2005 11:43 am

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.
 
afay1
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RE: Landing Gear Curiosities

Wed Feb 09, 2005 1:19 pm

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.
 
nudelhirsch
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RE: Landing Gear Curiosities

Wed Feb 09, 2005 1:36 pm

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...

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AR1300
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RE: Landing Gear Curiosities

Wed Feb 09, 2005 1:48 pm

Always wondered the same thing!!!!

Mike.
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747NUT
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RE: Landing Gear Curiosities

Wed Feb 09, 2005 8:13 pm

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 !
 
aeroweanie
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RE: Landing Gear Curiosities

Thu Feb 10, 2005 3:32 am

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.
 
speedracer1407
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RE: Landing Gear Curiosities

Thu Feb 10, 2005 6:39 pm

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
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pilotaydin
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RE: Landing Gear Curiosities

Fri Feb 11, 2005 12:19 am

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!
 
air2gxs
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RE: Landing Gear Curiosities

Fri Feb 11, 2005 12:22 am

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.
 
B747FE
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RE: Landing Gear Curiosities

Fri Feb 11, 2005 1:03 am

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.

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B747FE
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242
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RE: Landing Gear Curiosities

Fri Feb 11, 2005 1:53 am

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?
 
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Buyantukhaa
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RE: Landing Gear Curiosities

Fri Feb 11, 2005 4:20 am

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.)
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air2gxs
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RE: Landing Gear Curiosities

Fri Feb 11, 2005 4:28 am

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]
 
sovietjet
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RE: Landing Gear Curiosities

Fri Feb 11, 2005 6:53 am

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.
 
AJ
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RE: Landing Gear Curiosities

Fri Feb 11, 2005 7:00 am

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!?
 
speedracer1407
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RE: Landing Gear Curiosities

Fri Feb 11, 2005 12:56 pm

"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
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Silver1SWA
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RE: Landing Gear Curiosities

Fri Feb 11, 2005 1:01 pm

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.
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aeroweanie
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RE: Landing Gear Curiosities

Fri Feb 11, 2005 3:19 pm

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.
 
air2gxs
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RE: Landing Gear Curiosities

Fri Feb 11, 2005 10:47 pm

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.
 
Klaus
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RE: Landing Gear Curiosities

Fri Feb 11, 2005 11:52 pm

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.
 
dl757md
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RE: Landing Gear Curiosities

Fri Feb 11, 2005 11:54 pm

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 truck positioner also tilts the truck 5 degrees fwd wheels down during retract for proper stowage. During extension this 5 degree fwd wheels down position is held until the gear are down and locked at which point the truck positioner actuator tilts the truck 13 degrees fwd wheels up.

The biggest reason that I know of for truck tilt other than stowage is air/ground info. When the trucks contact the ground their angle changes and this info is sent to the auto speedbrakes and autobrakes to actuate those systems if they are armed. This is used rather than weight on wheels because the trucks will attain ground attitude before weight on wheels occurs.

Dl757Md
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air2gxs
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RE: Landing Gear Curiosities

Sat Feb 12, 2005 7:06 am

Thank you Klaus and Dl757md. I stand corrected.
 
wilax
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RE: Landing Gear Curiosities

Wed Feb 16, 2005 3:04 pm

Silver1SWA,

You are 100 percent correct...

From a physics standpoint, tilted gear would always take up more space, laterally, than flat and parallel gear on Western designed aircraft. The 757, 767, and 777, when the gear is retracted, it is stowed in a perfect parallel fashion. Otherwise, if tilted, the extended parts of the bogies would contact each other when they met as they were almost completely retracted.

Here you can see the process as a 777 retracts its gear, how the gear must be de-tilted before it is stowed


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Its the same with the 767


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CCA
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RE: Landing Gear Curiosities

Wed Feb 16, 2005 6:48 pm

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 efficiently that's it.

There are modifications to the tilt such as on the 777 which can change the tilt, as it is mechanically operated but electrically controlled.

It requires 13 degrees fwd wheels down to fit in the wheel well and changes to 5 degrees fwd wheels up once down and locked, now I haven't got the MM for the 777 in front of me but I'll confirm the reason later but I do believe it's for landing comfort as some have said.

Now the 767 gear is mechanical controlled and operated and the tilt is fixed so the tilt cannot change like the 777.

Now the left shot gives a perfect view of the tilt actuator and shows it in full tilt. The wing gear on the 747 is designed to occupy the least fwd/aft space as possible.

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Now the A330/340 wing/main gear is designed to allow the centre gear to fit in between so has a completely different set up. It is tilted (which actually results in it being virtually untilted) so that when the gear retracts the shortening link pulls the inner cylinder into the oleo which causes the bogie to untilt and leave the gear as short as possible and occupy the least space laterally (opposed to the 747 wing gear) which therefore leaves space to create a wheel well for the center gear.


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Hope this answers the question.

Rgds CCA
P1 in A330, A340, A346, B742, B744, B748.
 
wilax
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RE: Landing Gear Curiosities

Thu Feb 17, 2005 1:13 pm

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--during retraction in order to make room for the center gear stowage.

All other aircraft that have tilted bogies (in landing config.) do not have the tilt for stowage purposes, because they flatten out right before retraction...
 
G4Doc2004
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RE: Landing Gear Curiosities

Thu Feb 17, 2005 1:45 pm

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 them in the wells.
"Failure to prepare is preparing to fail"--Benjamin Franklin
 
meister808
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RE: Landing Gear Curiosities

Thu Feb 17, 2005 1:52 pm

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 that, but it is sensible to save space.

-Meister
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air2gxs
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RE: Landing Gear Curiosities

Fri Feb 18, 2005 12:10 am

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. I found the information in the AMM last week but didn't post it because I thought the topic was dead. I will post the pertinent references on Saturday.
 
sovietjet
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RE: Landing Gear Curiosities

Sat Feb 19, 2005 6:25 am

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 in the fuselage because of the need for Tupolev planes to have a wide track for stability purposes. Also they are 2 and 3 axle bogies which would be rather hard to fit anywhere on that plane without reducing fuel/payload/cargo space dramatically. The pods extend out the back of the wing jutting out only very little on top and bottom of the wing and therefore creating little drag. Sure they create drag, but not at such a drastic and unacceptable level as some people think they do. Imagine on a B-747 those pod looking things attached under the flaps. I believe they are tracks for the flaps to follow when they deploy since they are slotted. Anyway, on one wing you have 4 of them. Their combined drag IMO would be much higher than the landing gear pod of a Tupolev airplane. The pods jut out the back and don't really disrupt the airflow over the wing either. It was IMO an excellent solution to the landing gear problem. On the other hand, plane like the Il-62, Il-86 didn't have them as they weren't likely to be operated out of grass strips. Tupolev patented the idea as his own.
 
2H4
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RE: Landing Gear Curiosities

Sat Feb 19, 2005 8:33 am

Quoting Sovietjet (reply 33):
Aeroweanie - The pods are there for a reason. There is no other way to stow the landing gear.



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 wing, the pods are quite inefficient. See below:

Quoting AeroWeanie (reply 22):
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.



2H4
Intentionally Left Blank
 
dc10hound
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RE: Landing Gear Curiosities

Sat Feb 19, 2005 1:54 pm

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. I found the information in the AMM last week but didn't post it because I thought the topic was dead. I will post the pertinent references on Saturday.

I agree. Once it's tilted in the air, it remains tilted.

Here's some of the relevant references:


B757 Main Landing Gear Extension and Retraction AMM 32-32-00-0

2. Component Details

H. Truck Positioner Actuator

(1) The truck positioner actuator is a hydraulic piston-type actuator. It operates hydraulically in one direction. This actuator applies the force to put the truck assembly at an angle to permit clearance with the structure when the landing gear moves to the extended or retracted position.The actuator is installed on the aft side of the shock strut near the truck assembly. The head end attaches to the shock strut inner cylinder, and the rod end attaches to the truck beam.

3. Operation

A. Functional Description

(1) Landing Gear Retraction

(a) The retraction cycle starts with the landing gear down and locked and the main landing gear door closed and locked. With the control lever in UP, hydraulic fluid flows through the selector valve to pressurize the hydraulic lines for landing gear retraction.

(b) Hydraulic fluid flows through the truck positioner shuttle valve to the truck positioner actuator. The actuator extends to put the truck the at an angle for entry into the wheel well.

(c) Hydraulic fluid flows through the uplock-operated and downlock-operated sequence valves to the door actuator. The mechanical lock in the door actuator is released. Then the door actuator extends to open the main landing gear door.

(d) When the door is approximately 90 per cent open, the door-operated sequence valve sends hydraulic fluid to the downlock and retract actuators. The downlock actuator retracts to move the downlock links out of the locked position. Then, the retract actuator retracts to retract the landing gear.

(e) The landing gear continues to retract and the door continues to open until the landing gear is up and latched in the uplock assembly. Then, the uplock-operated sequence valve again sends fluid to the door actuator. The door actuator retracts to close the door until the actuator locks the door closed.

(f) When the door is fully closed and locked, the control lever is moved to OFF. This changes the position of the selector valve to prevent the supply of hydraulic pressure to the landing gear system. The hydraulic lines are opened to return, which completes the retraction cycle.

(2) Landing Gear Extension

(a) The extension cycle starts with the landing gear up and locked and the main landing gear door closed and locked. With the control lever in DN, hydraulic fluid flows through the selector valve to pressurize the hydraulic lines for landing gear extension.

(b) Hydraulic fluid flows through the truck positioner shuttle valve to the truck positioner actuator. The actuator is pressurized to hold the truck at an angle as it moves out of the wheel well.



(c) Hydraulic fluid flows through the uplock-operated and downlock-operated sequence valves to the door actuator. The mechanical lock in the door actuator is released and the actuator extends to open the main landing gear door. Hydraulic fluid also flows to the downlock actuator which applies pressure to extend the actuator and lock the downlock links. But, at this time, the links cannot move since the landing gear is still locked.

(d) When the door is approximately 90 percent open, the door-operated sequence valve sends hydraulic fluid to the uplock actuator. The uplock actuator retracts to move the uplock hook from the latched position. This releases the landing gear from the uplock assembly.

(e) When the door is approximately 95 percent open, the door-operated sequence valve sends hydraulic fluid to the retract actuator. The retract actuator extends to lower the landing gear.

(f) The door opens fully. The landing gear continues to lower until it is down and the pressure applied to the downlock actuator extends the actuator to move the downlock links overcenter to the locked position. Then, the downlock-operated sequence valve again sends hydraulic fluid to the door actuator. The door actuator retracts to close the door until the actuator locks the door closed.

(g) When the door is fully closed and locked, the extension cycle is complete.



And the official version of the B777 retraction/extension cycle:



B777 MLG EXTENSION-RETRACTION - MLG TRUCK POSITIONER ACTUATOR - FUNCTIONAL DESCRIPTION

General

The MLG truck positioner actuator retracts to move the MLG truck to the TILT position and extends to move the MLG truck to the STOW position.


Tilt Position

When the landing gear is down and locked, hydraulic pressure goes to both sides of the floating piston and the rod side of the main piston. This retracts the truck positioner actuator which moves the truck to 13 degrees forward wheels up (airplane in the air).


Stow Position

During gear retraction, hydraulic pressure goes between the floating piston and the rod side of the main piston. This makes the pistons move apart until the floating piston touches the rod shoulder.

Since the floating piston has a larger surface area than the main piston, the actuator extends until the floating piston touches the stops on the cylinder rod end. This moves the truck to the 5 degrees forward wheels down position.

After the landing gear is up and locked, the pressure in the truck positioner actuator keeps the gear in the STOW position.

A relief valve opens to release actuator pressure increases caused by thermal expansion and touch down loads.


MLG EXTENSION-RETRACTION - RETRACTION SEQUENCE

General

MLG retraction is the same for both left and right main gears.

These are the start positions:

MLG - DOWN and LOCKED

MLG door - CLOSED and LOCKED

MLG truck positioner actuator - TILT

MLG drag brace-operated sequence valve - DOWN

MLG uplock-operated sequence valve - UNLOCKED

MLG door-operated sequence valve - NOT OPEN.


Control

When you move the landing gear lever to the UP position, the MLG selector/bypass valve moves to the UP position. Hydraulic pressure then goes to the retract lines.


Gear Unlock - Truck Tilt

The pressure goes to the MLG drag brace and MLG side brace downlock actuators. These actuators start to retract to unlock the gear downlocks.

Pressure also goes through the MLG drag brace-operated sequence valve and the MLG uplock-operated sequence valve. This pressure extends the MLG truck positioner actuator to the STOW position.


Door Opens

Pressure through the sequence valves also retracts the MLG door lock actuator to unlock the main gear door. The MLG door actuator gets extend pressure through the MLG door priority/relief valve. The MLG door starts to open.


Gear Retracts

When the MLG door is almost all the way open, the MLG door-operated sequence valve moves to OPEN. Pressure then goes to the MLG retract actuator to retract the gear.

When the gear starts to retract, the MLG drag brace-operated sequence valve moves to the NOT DOWN position.

As the MLG goes into the wheel well, a roller on the landing gear strut moves the MLG uplock mechanism to the LOCKED position. This moves MLG uplock-operated sequence valve to the locked position.

This removes pressure from the MLG truck positioner actuator. Pressure trapped in the MLG truck positioner actuator keeps the MLG truck in the STOW position.

Pressure then goes to the MLG uplock actuator to make sure the MLG uplock mechanism locks.

Pressure through the MLG door-operated sequence valve also goes to brake system components for the gear retract braking function. See the wheels and brakes section for more information ( SECTION 32-40).


Door Closes

Pressure also goes to the close side of the MLG door actuator and the lock side of the MLG door lock actuator. The door starts to close.

When the door is almost closed, a roller on the door starts to move the MLG door uplock mechanism to the locked position. Pressure in the MLG door lock actuator moves the uplock mechanism over-center to the locked position.


Final Position

Final main gear conditions are:

MLG - UP and LOCKED

MLG door - CLOSED and LOCKED

MLG truck position actuator - STOW

MLG drag brace-operated sequence valve - NOT DOWN

MLG uplock-operated sequence valve - LOCKED

MLG door-operated sequence valve - NOT OPEN.

Pressure stays in the retract lines until the auto-off function moves the MLG selector/bypass valve to OFF.



MLG EXTENSION-RETRACTION - EXTENSION SEQUENCE


General

Main gear extension is the same for both left and right main gear.

These are the start conditions:

MLG - UP and LOCKED

MLG door - CLOSED and LOCKED

MLG truck positioner actuator - STOW

MLG Drag brace-operated sequence valve - NOT DOWN

MLG uplock-operated sequence valve - LOCKED

MLG door-operated sequence valve - NOT OPEN.


Control

When you select DOWN with the landing gear lever, the MLG selector/bypass valve moves to the down position. This permits hydraulic pressure to go to the extend lines.


Door Opens

Pressure goes through the MLG drag brace-operated sequence valve and the MLG uplock-operated sequence valve. This pressure goes to the MLG truck positioner actuator to hold the MLG truck in the STOW position.

This pressure also goes to the MLG door lock actuator to unlock the MLG door. The MLG door actuator then gets extend pressure through the MLG door priority/relief valve. The MLG door starts to open.


Gear Extends - Truck Tilts

When the MLG door is almost all the way open, the MLG door-operated sequence valve moves to OPEN. Pressure then goes to the MLG uplock actuator to unlock the MLG uplock.

The MLG extends by its own weight and by airloads. It is not pressurized during extension.

When the main landing gear is 20 degrees from fully extended, the MLG drag brace-operated sequence valve moves to DOWN.

Hydraulic pressure then retracts the MLG truck positioner actuator which moves the MLG truck to the TILT position.

Hydraulic pressure also goes to the MLG drag brace and MLG side brace downlock actuators to lock the gear down.


Door Closes

Pressure from the MLG drag brace-operated sequence valve goes through the MLG uplock-operated sequence valve and the MLG door release/safety valve module. This pressure goes to the MLG door actuator and the MLG door lock actuator. The MLG main gear door closes.

When the door is almost closed, a roller on the door starts to move the MLG door uplock mechanism to the locked position. Pressure in the MLG door lock actuator moves the uplock mechanism over-center to the locked position.


Final Condition

These are the final conditions of the MLG:

MLG - DOWN and LOCKED

MLG door - CLOSED and LOCKED

MLG truck positioner actuator - TILT

MLG drag brace-operated sequence valve - DOWN

MLG uplock-operated sequence valve - UNLOCKED

MLG door-operated sequence valve - NOT OPEN.

These actuators stay pressurized when the MLG is down and the center hydraulic system is pressurized:

MLG door actuator

MLG door lock actuator

MLG truck positioner actuator

MLG side brace actuator

MLG drag brace actuator

MLG uplock actuator.
"Eagles soar. But weasels never get sucked into jet intakes.."
 
sovietjet
Posts: 2547
Joined: Sat Mar 15, 2003 12:32 am

RE: Landing Gear Curiosities

Sun Feb 20, 2005 7:39 am

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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