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Guest

Why Main Landing Gear Is Tilted During The Landing

Sun Sep 09, 2001 5:31 pm

I have noticed that the main landing gear is tilted upwards or downwards during the landing. Who knows if this is intentioned and if yes, why ?
 
777236ER
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RE: Why Main Landing Gear Is Tilted During The Landing

Sun Sep 09, 2001 6:23 pm

The torque produced by the straightening gear helps to flatten the plane out.
Your bone's got a little machine
 
JG
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RE: Why Main Landing Gear Is Tilted During The Landing

Sun Sep 09, 2001 11:16 pm

The gear is tilted to fit in wheel wells. That is all
 
Klaus
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JG

Mon Sep 10, 2001 9:49 am

JG: The gear is tilted to fit in wheel wells. That is all

Is it true that the 777´s MLG is tilted the other way before it can be stowed in the wheel wells?
Otherwise, wouldn´t it be sensible to design the wheel wells so they can take the gear the way it is tilted for other reasons?  Wink/being sarcastic

If the tilt actuator should be strong enough, it would help softening the touchdown impulse. Maybe that´s one of the reasons. No landing gear design engineers present?  Wink/being sarcastic
 
delta-flyer
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RE: Why Main Landing Gear Is Tilted During The Landing

Mon Sep 10, 2001 12:27 pm

I do not have any materials here with me, so someone should check this out --- the 777 has some crazy arrangement that helps prevent a tail-strike during take-off.

Cheers,
Pete
"In God we trust, everyone else bring data"
 
AJ
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RE: Why Main Landing Gear Is Tilted During The Landing

Mon Sep 10, 2001 3:34 pm

The main gear tilt on the B747 also provides air/ground sensing. All four main gear bogeys tilt when the aircraft is airborne, so on touchdown any systems relying on air/ground logic can activate.
As the tilt is driven by hydraulic pressure a hydraulic failure can lead to the gear not tilting and a loss of air/ground logic affecting those systems, including the upper deck door locks!
Finally a benefit of forward bogey tilt is the tendancy to smooth the touchdown, a benefit the 'back to front' B767 lacks.
Many years ago the Airbus A300 suffered many return to lands in cold weather when the gear would not retract. This was eventually traced to the undercarriage remaining in the rotation tilt position due to cold hydraulic fluid, thereby not moving to the retraction position! Smile/happy/getting dizzy
 
air2gxs
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RE: Why Main Landing Gear Is Tilted During The Landing

Mon Sep 10, 2001 6:38 pm

The gear is tilted to fit into the wheel well. The air/ground issue is a happy coincedence (sp?). Air/ground could easily be achieved using a squat switch such as a B727.
It is much easier to tilt the gear than to design a wheel-well to accomadate an un-tilted gear.

 
Klaus
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Air2gxs

Tue Sep 11, 2001 12:31 am

Air2gxs: It is much easier to tilt the gear than to design a wheel-well to accomadate an un-tilted gear.

Why would that be more difficult? I don´t see it. (Some airliners don´t tilt their bogeys; So...)
 
Staffan
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RE: Why Main Landing Gear Is Tilted During The Landing

Tue Sep 11, 2001 12:37 am

Klaus,
I think it is to save space, if the gear was retracted untilted, the gear would come too high up conflicting with fuel tanks and cargo area.

Staffan



 
777236ER
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RE: Why Main Landing Gear Is Tilted During The Landing

Tue Sep 11, 2001 1:38 am

The 777 is a good example of all of the tilted-gear issues. When coming out (or going into) the gear bays, the gear is tilted at 5 degrees DOWN ti fit into the bays (the 777 MLG is HUGE!). When down (in flight) the MLG is tiled 13 degrees up. This is for a number of reasons. Firstly, the gear will straighten to the horizontal before the full weight of the aircraft is placed on the gear strut, so the landing is softened. Secondly, when the gear goes to the horizontal upon touchdown, the torque produced helps level the aircraft. Admittedly, this torque isn't much, but it helps.

[Interesting note, the 767 has the unusual tendancy to pitch down quite rapidly during landing, which is why the MLG on the 67 is tilted forwards. Upon touchdown, the MLG straightens and stops the pitching forward. The downside to this, however, is that the the forward gear on the MLG touchdown first, so the 767 landing is quite a bit rougher than the 757 or the 777, both of which have up-tilted gear]

The on-ground logic is usually triggered by oleo compression, so tilting the gear doesn't really make that much difference in that respect.

Regards.

Your bone's got a little machine
 
Guest

RE: Why Main Landing Gear Is Tilted During The Landing

Tue Sep 11, 2001 1:59 am

Thank you 777236ER, it is really great and detailed answer that cleared the whole picture about this topic.
 
musang
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RE: Why Main Landing Gear Is Tilted During The Landing

Tue Sep 11, 2001 2:11 am

777236ER I thought the reason for the 767 forward tilt on the main gear is that it retracts in a slightly forward direction, not directly sideways?? Same is true on the A310 to a lesser degree (pun intended!).

Someone mentioned tailscrape protection; I believe thats to be found on the 737-300, which automatically limits pitch attitude on landing (possibly on rotation too?).

A330/340 main gear is tilted rearward and serves as a shock absorber. Routinely, the rear axles touch down, the fuselage rotates almost to the horizontal, then the forward axles make contact. Interesting to watch.

Another benefit of highly rearward sprung gear trucks is that on take off, during rotation, they "boost" the aircraft off the ground, improving tail clearance. This allows benefits in gear design, e.g. height and fore/aft location of the main gear from the tailscrape viewpoint.

Regards - Musang
 
777236ER
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RE: Musang

Tue Sep 11, 2001 2:18 am

Yeah, the 767's MLG retract into their bays pitched down, like the 777, but the forward tilted MLG also help to prevent the 67 from pitching forward (as i mentioned).

The A330/340 is an excellent example of the torque created by the straightening gear helping to level out the aircraft.

Regrads.
Your bone's got a little machine
 
musang
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Unusual DC-10 MLG

Tue Sep 11, 2001 2:53 am

On the subject, I posted a reply to another topic which is now archived, a few days ago, about a postcard I have of HB-IHA, Swissair's DC-10-30, with the outboard main gears hanging "nose down" like a 767's.

I've been after an explanation for years. Its a good quality photo, not a ground shot superimposed on a background, because in the shot all three MLGs can be seen from below.

When I learn how to use this borrowed scanner I'll post the picture.

Regards - Musang
 
cdfmxtech
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RE: Whoaa, Easy There 777236ER

Tue Sep 11, 2001 4:08 pm

"The on-ground logic is usually triggered by oleo compression, so tilting the gear doesn't really make that much difference in that respect."

You sure about that?? The B757, B767 utilize title sensors for air/ground logic....not WOW sensors.

Off the head, I'm not sure about the B777.
 
VC-10
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RE: Why Main Landing Gear Is Tilted During The Landing

Tue Sep 11, 2001 5:28 pm

Could someone explain this torque effect, it's the first time I've heard of it.
 
musang
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RE: Why Main Landing Gear Is Tilted During The Landing

Thu Sep 13, 2001 3:53 am

Torque effect is new to me too.

The impression I get from above posts is that weight on the rear axle of a multi-axle bogie/truck rotates the nose towards the ground. I accept that it would, but is this the prime purpose or a by-product?

At Farnborough a couple of years ago I visited the A340/330 gear manufacturer's stand and asked a suit about the gear workings. He explained the shock absorbing/cushioning effect, but I'm pretty sure that was all.

Regards - Musang
 
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Bruce
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RE: Why Main Landing Gear Is Tilted During The Landing

Sat Jul 17, 2004 2:30 am

What about the 747? The 747's main gear are tilted up at a pretty good angle but the body gear in the middle is at a lesser angle. why is this?

bruce
Bruce Leibowitz - Jackson, MS (KJAN) - Canon 50D/100-400L IS lens
 
air2gxs
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RE: Why Main Landing Gear Is Tilted During The Landing

Sat Jul 17, 2004 3:08 am

So that the gear can fit in the gear well. I can't show it to you without a picture, but the wing gear retracts towards the centerline of the aircraft and the body gear retracts forward. When they are in their respective holes they are "nested". The big tilt on the wing gear is so that the body gear can come in behind it.
 
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Bruce
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RE: Why Main Landing Gear Is Tilted During The Landing

Sat Jul 17, 2004 4:18 am

I have a picture that shows the tilt really good from the side. Obvious to see how the body gear is not as tilted as the wing gear.

http://www.jetphotos.net/viewphoto.php?id=324000

they just look so wierd landing like that....

bruce
Bruce Leibowitz - Jackson, MS (KJAN) - Canon 50D/100-400L IS lens
 
kdalaggie
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RE: Why Main Landing Gear Is Tilted During The Lan

Sat Jul 17, 2004 4:36 am

I think the manufacturers have developed new uses for the tilting mechanisms too. I recall that Boeing put on 'semi-levered landing gears' on the 777-300 from an article or aviation show. From what I can remember that it does two things it changes the center of rotation rearwards and (stated from an engineer) it gives the mechanical advantage of the characteristics of a landing gear that is 1 ft. taller. However, I have yet to find printed material on the latter statement. (i.e. the tilt pushes back on the airplane at takeoff and landing where the weight of the aircraft must force the strut level.)

Here's some stuff I found....
http://www.boeing.com/news/releases/2003/photorelease/q2/pr_030618g1.html

KDALAggie
Gig'em Ags!!!!
 
air2gxs
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RE: Why Main Landing Gear Is Tilted During The Landing

Sat Jul 17, 2004 7:21 am

Good picture. The body gear will rotate 90 degrees forward and the wing gear will rotate 90 degrees inboard, You can visualize it by looking at the gear in the picture. Imagine the wing gear was not tilted so much. It would strike the body gear on the way in.
 
Mender
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RE: Why Main Landing Gear Is Tilted During The Landing

Sat Jul 17, 2004 8:03 am

Once a 757 or 767 has taken off the trucks are tilted to the maximum angle. The gears cannot stow with this much angle and have to be leveled off to during retraction otherwise they would not fit in the bay. I'm sure if someone could find a photo of one of these aircraft with the gears almost fully stowed it would prove my point.

The truck tilt angle therefore has nothing at all to do with the way they fit in the wheel wells on these aircraft.

As CDFMXTECH has already said, air/ground logic is provided by sensors on the truck beams which go into air mode when the truck is fully tilted and sense ground mode when the beam is roughly level.

It wouldn't suprise me if this was a partial reason to tilt undercarriage trucks in the first place. On an aircraft like the 737-200 which have a single axle they had to rely on a teleflex cable to measure when the oleo leg compressed. With a twin axle bogie you can use a cheap, light, reliable proximity sensor instead.
 
Mender
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RE: Why Main Landing Gear Is Tilted During The Landing

Sat Jul 17, 2004 8:15 am

Compare these pictures to see what I mean. I can't be bothered to look for anything better but you can clearly see the Eritrean aircraft gear is almost level whereas the Quantas gear is fully tilted.

https://www.airliners.net/open.file/482144/L
https://www.airliners.net/open.file/530332/L
 
kdalaggie
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RE: Why Main Landing Gear Is Tilted During The Lan

Sat Jul 17, 2004 8:28 am

Mender,
Here ya go......


View Large View Medium
Click here for bigger photo!

Photo © Marlo Plate - IBERIAN SPOTTERS
View Large View Medium
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Photo © Alexander Jonsson




View Large View Medium
Click here for bigger photo!

Photo © Wietse de Graaf - AirTeamImages
View Large View Medium
Click here for bigger photo!

Photo © Y.K. Cummins NG

Gig'em Ags!!!!
 
Mender
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RE: Why Main Landing Gear Is Tilted During The Landing

Sat Jul 17, 2004 5:31 pm

Thanks

I was talking about the 75 & 767. I know the 74 retracts fully tilted as, I'm sure many other aircraft do. Also the truck only levels off in the final stages of retraction so you'll not notice this affect if the gear isn't almost completely stowed.
 
air2gxs
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RE: Why Main Landing Gear Is Tilted During The Landing

Sat Jul 17, 2004 8:55 pm

From the B757 AMM:

Truck Positioner Actuator (Fig. 7) (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.

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.

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. It is mounted on the aft side of the shock strut between the truck and shock strut. The positioner retracts to tilt the gear during retraction; during landing an internal relief valve allows the positioner to be extended by the gear.

1) Gear Extension (a) To extend the main gear, the landing gear control lever should be moved to DN. Quadrants and cables then move the main gear selector valve to direct pressure from the center hydraulic system to the left and right main gears. (b) The truck positioner receives pressure from the selector valve to tilt the truck. This allows the gear to extend without the truck interfering with the wheel well.

The gear on B757/767 aircraft is tilted to clear and fit into the wheel wells. No other reason.

I've jacked both types of aircraft and been up close and personal with the gear during retraction and extension tests. With hydraulic pressure on; and weight off the wheels the trucks will assume the tilted position, i.e. will go to the position that will allow them to enter the wheel well.
 
Klaus
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Air2gxs

Sat Jul 17, 2004 10:58 pm

Air2gxs, while you´re at it... How is the extension/retraction sequencing controlled, precisely?

Is there any feedback from any of the components to the sequencing? Or is everything just assumed to have moved at a certain point?

In case of a minor fault with the gear doors, for instance, it might be a good idea not to bang the gear on the still-closed doors, right? Same goes for tilt actuators and maybe up/downlocks. How many position sensors are involved there, typically and which failure modes can result?
 
air2gxs
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RE: Why Main Landing Gear Is Tilted During The Landing

Sun Jul 18, 2004 12:16 am

Off the top of my head the sequencing is controlled by sequence valves. Very simply (and for the jumbo):

There are door operated sequence valve which sequence the uplocks and the gear, gear operated sequence valves which sequence the doors and the uplocks and uplock sequence valves that sequence the doors and gear.

Got that? Confused yet?

Lets look at extension. The first thing that happens is pressure is applied through the uplock valves to the door actuator and the upside of the gear actuator. The door opens, when it reaches its full stroke fluid is ported through the sequence valve to the uplock. The uplock rolls over and when it goes full stroke fluid is ported to the downside of the gear actuator. When the gear is fully extended (no downlock needed, since the arms go over center) fluid is ported back to the doors to close them. Extension is just the opposite, except that we also port fluid to the unlock actuators to bring the arms back from overcenter.

OK? The tilt actuators are always pressurized with hydraulics on. When the aircraft lands the prssure is relieved back to return or into the atmosphere through a relief valve if the hit was exceptionally hard (a common issue at the gear is a wet tilt actuator relief port).

As you can see, any failure in the gear sequencing system can result in the gear not coming out. That's why the alternate systems do not depend on hydraulics. Should the gear fail to extend (on a jumbo) the uplocks are rotated out of the way with an electric motor and the gear falls into the door and pushes them out of the way. The gear gravity falls to the over-center position and locks. The doors stay open.

Again, this is for the Jumbo, but most other aircraft are similar.

[Edited 2004-07-17 17:23:38]
 
Klaus
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Air2gxs

Sun Jul 18, 2004 12:56 am

Air2gxs, that was exactly the information I was looking for.

And from a systems design point of view, I had hoped it would work that way.  Wink/being sarcastic

This would probably mean that on models with changing bogie tilt those would have their security valves or sensors as well (or at least a mechanical forcing mechanism)... If a 777 or A330/340 bogie wouldn´t reach its proper retraction position in time, that would probably have to cause a sequence abort...
 
air2gxs
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RE: Why Main Landing Gear Is Tilted During The Landing

Sun Jul 18, 2004 2:54 am

I'm away from the computer with the AMM for the B757/B767 right now, but I know that on the B747 the tilt sensors (proximity sensors) sense when the gear is tilted to proper position. If they don't sense the tilt the landing gear handle gate will not pull out of the way and a "tilt" light illuminates.

I'll check on the B757/767 later today.
 
Mender
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RE: Why Main Landing Gear Is Tilted During The Landing

Sun Jul 18, 2004 6:53 pm

Wow, now I'm sober, I read back my posting and it doesn?t make sense to me, let alone anyone else. I didn?t mean it was hydraulically levelled out because this would put the air/ground sense into ground. I was trying to illustrate the fact the tilt angle isn?t as pronounced as a 747.

Right. What I meant to say was unlike some aircraft the 767 doesn?t have any significant obstructions in the wheel well that require the gear to be in a specific position.

I believe the gear is not tilted to fit in the bay; it is tilted for other reasons. The gear pivot in the wing (trunnion) is parallel to the fuselage centre line (buttock line zero). The undercarriage leg is approximately vertical so when the gear is retracted I believe it is perpendicular to the fuselage centre line. If this is correct then there is no reason why the bogie has to be tilted because it goes into a rectangular bay with no obstructions in it like hydraulic reservoirs.

So why do I think the bogies are tilted for retraction.
The wheels and tyres have a great deal of inertia which needs to be controlled. If there was no truck tilt actuator the bogie would flap around violently. If the bogie was held level it would need substantial damping in both directions. Therefore it is much easier to tilt the gear as far as it will go in one direction with a truck tilt actuator. By doing this you control the bogie in the simplest manner possible and you know what position it is in when it is stowed so you can design the undercarriage bay around it. This is why I believe the gear is not tilted to fit in the bay, it is tilted for other reasons and it is convenient that it fits the bay.

As far as gear sensors are concerned the 767 there are sensors on the bogie, the downlocks and the doors.

The near/far sensor on the bogie to sense air/ground. If the truck is level then it is on the ground (the aircraft weight is overcoming the 3000 psi trying to tilt it). If the aircraft is on the ground a solenoid on the landing gear handle prevents it from being moved into the retract position.

The downlock sensors give the three green gear locked lights on the dashboard.

The door switches sense the body door position either open or closed (gear not up as the gear rests on the door in flight.)

The million dollar question that I can?t answer is why the 767 bogies tilt forwards whereas most bogies tilt backwards.

The Aircraft Maintenance manuals are written to be read by someone whose native language is not always English. They may not tell the whole truth but tell things in a manner to be as clear as possible, hence you get some fantastic statements such as this in the main wheel installation procedure AMM 32-45-01/409 para 3F - 4b ?? It is important to apply torque with only one hand because application of more than 5 foot-pounds of torque can CAUSE DAMAGE TO A PART THAT IS NOT ALREADY DAMAGED.
 
air2gxs
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RE: Why Main Landing Gear Is Tilted During The Landing

Sun Jul 18, 2004 8:59 pm

Mender,

I think we're talking the same thing, but looking at it differently. The gear is tilted to fit in the well. If there was no tilt mechanism it would flop around and hit things on the way up (as you stated). You're right, there does not appear to be a clearance problem, but the gear must be tilted in order to enter the well.

The AMM is fairly unambiguous in saying that it is tilted to fit in the well.
 
Mender
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RE: Why Main Landing Gear Is Tilted During The Landing

Sun Jul 18, 2004 9:21 pm

Air2gxs

Totally agree mate. Egg or the chicken? One thing I've learnt from this thread is to wait until you are sober before posting ;0)
 
fadec
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RE: Why Main Landing Gear Is Tilted During The Landing

Mon Jul 19, 2004 9:52 am

My bet is that it was a little of both reasons. If one engineer said "I would like to induce this characteristic to soften the landing and make it more stable." Another says "I want it to be within this cg zone when retracted and in a different zone when extended to help stabilize the aircraft." Both can find the same result... tilting the gear would comply with both engineers requirements.
 
miamiair
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RE: Why Main Landing Gear Is Tilted During The Landing

Mon Jul 19, 2004 11:09 pm

Aviaition Week & Space Technology had a good article on the 777-300 Levered Landing Gear. I will have to sort thru the back issues. Basically in this particular instance, this was done to give additional clearance before striking the tail, and allowing a greater angle of incidince on rotation. All these factors combined gave the airplane a shorter take-off roll.
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