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Wheel Bogey Droop (on 4 Or 6 Wheel Gear)  
User currently offlinebeechtobus From United States of America, joined Nov 2011, 324 posts, RR: 0
Posted (1 year 7 months 1 week 2 days 14 hours ago) and read 5896 times:

A dumb curiosity crossed my mind driving by LAX the other day. Does any engineering types know why certain aircraft manufacturers build certain inflight droop angles into their main landing gear quad or more wheel bogeys? For example the 767 has a downward droop, the A330 has an upward droop, the MD-11/DC-10 has pretty much no droop (parallel to fuselage), and the 747 has outboards that droop more than the inboards, etc.

Obviously this doesn't seem like its just happenstance and is for some kind of landing/take off/ aerodynamic reason. If so, why such a difference between the bogey angles on these aircraft?



18 replies: All unread, jump to last
 
User currently offlinebeechtobus From United States of America, joined Nov 2011, 324 posts, RR: 0
Reply 1, posted (1 year 7 months 1 week 2 days 14 hours ago) and read 5895 times:

Haha, wow, those came out really tiny, but you guys catch my drift.

User currently offlineDTWPurserBoy From United States of America, joined Feb 2010, 1729 posts, RR: 7
Reply 2, posted (1 year 7 months 1 week 2 days 13 hours ago) and read 5706 times:

It is called cantilevering. It smooths airflow, allows the aircraft to touch down with the rear tires first and then as the speed starts to slow, the rest of the main gear takes the weight of the aircraft, followed by the nose gear.

A330's are notorious for " firm" landings, especially since it sits in a slight nose down attitude on the ground. As the main gear takes the weight of the airplane and the nose gear drops, new pilots on the 330 are used to seeing the horizon at a certain spot on the windshield and assume the nose gear is down when it is still 3-4 feet in the air and it slams into the ground hard enough to reset your fillings. Of course the rest of the crew rags on them all the way to the hotel.



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User currently onlinebohica From United States of America, joined Feb 2004, 2742 posts, RR: 0
Reply 3, posted (1 year 7 months 1 week 2 days 11 hours ago) and read 5479 times:

When the airplane takes off the bogies are angled that way so they fit properly into the gear well when the landing gear is retracted. When the gear is lowered for landing the bogies stay at the same angle they were stored in the gear well.

User currently offlinewilliam From United States of America, joined Jun 1999, 1318 posts, RR: 0
Reply 4, posted (1 year 7 months 1 week 2 days 11 hours ago) and read 5408 times:

Quoting bohica (Reply 3):
en the airplane takes off the bogies are angled that way so they fit properly into the gear well when the landing gear is retracted. When the gear is lowered for landing the bogies stay at the same angle they were stored in the gear well.

I have read this is the case.

Quoting DTWPurserBoy (Reply 2):
It is called cantilevering. It smooths airflow, allows the aircraft to touch down with the rear tires first and then as the speed starts to slow, the rest of the main gear takes the weight of the aircraft, followed by the nose gear.

A330's are notorious for " firm" landings, especially since it sits in a slight nose down attitude on the ground. As the main gear takes the weight of the airplane and the nose gear drops, new pilots on the 330 are used to seeing the horizon at a certain spot on the windshield and assume the nose gear is down when it is still 3-4 feet in the air and it slams into the ground hard enough to reset your fillings. Of course the rest of the crew rags on them all the way to the hotel.

This is a first time I have read this reasoning.............interesting.


User currently offlineRoseflyer From United States of America, joined Feb 2004, 9709 posts, RR: 52
Reply 5, posted (1 year 7 months 1 week 2 days 10 hours ago) and read 5310 times:

The landing gear truck tilts significantly to allow the tires and brakes to be changed. Typically it is more than 30 degrees of rotation allowed. The wheel well is a tight space, so there is a specific angle required. Every airplane is different, so that is why you see different angles. There is a truck tilt actuator that controls the position of the gear in the air and allows the airplane to be jacked up.

For Boeing planes, there is a pressure relief valve that allows the gear to move when the airplane hits the ground. The effect of the angle of the gear prior to contact to the ground is relatively meaningless because the amount of force from the truck tilt actuator is very small compared to the force of the plane landing.



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User currently offlineN766UA From United States of America, joined Jul 1999, 8341 posts, RR: 23
Reply 6, posted (1 year 7 months 1 week 2 days 10 hours ago) and read 5297 times:

One purpose is to absord the shock of landing! One set of wheels touches down, which stabilizes and supports a percentage of the aircraft's weight before the rest of the gear touches. Finally, the shocks compress and the airplane settles down.


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User currently offlinencfc99 From United Kingdom, joined May 2005, 750 posts, RR: 0
Reply 7, posted (1 year 7 months 1 week 2 days 10 hours ago) and read 5245 times:

The video below shows a 777 gear swing. The gear comes out from the gear bay 'flat', then droops backwards into landing position. As for why it does this, i have no idea, just something i've noticed on videos.


http://www.youtube.com/watch?v=sCxROf8htNo

Surely there is very little weight on the gear until all wheels are down and struts compressed, when the first wheels touch, I wouldn't think much support or stabalizing is happening at that point, at that speed the wings are still generating almost enough lift to keep the aircraft airbourne. When enough weight is on the gear, sensors allow spoilers and thrust reversers to deploy, destroying all the lift being generated by the wings. This is sort of how it was explained to me, someone else with more knowledge will be able to explain it much better than I.

[Edited 2013-04-23 10:41:44]

User currently offlineDeltaB717 From Australia, joined Jun 2012, 559 posts, RR: 1
Reply 8, posted (1 year 7 months 1 week 2 days 3 hours ago) and read 4813 times:
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I've heard in the past the 767 is set the opposite way to 777/A330/A340/747/etc so that the gear will fit in the bays around a fuel tank or something similar which makes the 'rears down' angle unworkable. Not sure how accurate that is?

User currently offlineTWA772LR From United States of America, joined Nov 2011, 2353 posts, RR: 1
Reply 9, posted (1 year 7 months 1 week 2 days 2 hours ago) and read 4690 times:

Quoting ncfc99 (Reply 7):
http://www.youtube.com/watch?v=sCxROf8htNo

Nice video!
One thing I noticed is that the left gear comes down before the right gear. Why is that?



Go coogs! \n//
User currently offlineStarlionblue From Greenland, joined Feb 2004, 17119 posts, RR: 66
Reply 10, posted (1 year 7 months 1 week 2 days 1 hour ago) and read 4653 times:

Quoting bohica (Reply 3):
When the airplane takes off the bogies are angled that way so they fit properly into the gear well when the landing gear is retracted. When the gear is lowered for landing the bogies stay at the same angle they were stored in the gear well.

        

They are kept that way by a mechanism. However once the weight of the aircraft is on the bogies the force of the weight is way higher than what the mechanism can exert. Once the plane leaves the ground again they return to the position they need for retraction.

Quoting DTWPurserBoy (Reply 2):

It is called cantilevering. It smooths airflow, allows the aircraft to touch down with the rear tires first and then as the speed starts to slow, the rest of the main gear takes the weight of the aircraft, followed by the nose gear.
Quoting N766UA (Reply 6):

One purpose is to absord the shock of landing! One set of wheels touches down, which stabilizes and supports a percentage of the aircraft's weight before the rest of the gear touches. Finally, the shocks compress and the airplane settles down.

AFAIK it does not absorb the shock of landing. It certainly is not cantilevering since the bogies pivot more or less freely once the weight of the plane is on them.

As Roseflyer says, the weight of the airplane on landing is way bigger than the pressure exerted by the mechanism that holds the gear in position. When only the rear or front wheels on the mains are on the ground, they don't hold much weight.

Look at these videos around the 1:00 mark and onwards. Notice how the wings are still supporting the aircraft until all the mains are on the ground. When only the rear wheels are down the plane is still flying. Once the mains are fully on the ground, the wings come down. You don't see the strut compress until all wheels are on the ground, indicating that the initial pair does not hold up the airplane much at all.

http://www.youtube.com/watch?v=GJCH2t3KnGM
http://www.youtube.com/watch?v=84iXB-sKd9A

The 773ER uses cantilevering on the mains to increase the angle of attack on rotation. It effectively extends the gear struts. However this is only used on take-off, not landing.

Quoting TWA772LR (Reply 9):
Quoting ncfc99 (Reply 7):
http://www.youtube.com/watch?v=sCxROf8htNo

Nice video!
One thing I noticed is that the left gear comes down before the right gear. Why is that?

Might be due to the fact that they're testing and haven't tweaked it yet.



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlinejetmech From Australia, joined Mar 2006, 2699 posts, RR: 53
Reply 11, posted (1 year 7 months 1 week 2 days ago) and read 4617 times:

Quoting beechtobus (Thread starter):

My pet theory for landing gear bogey tilt with the gear extended has always being that it is used to absorb part of the initial landing shock. As you can imagine the mass moment inertia about the pivot point of a complete bogie beam with brakes and wheels is appreciable, thus, to my way of thinking, the rotation of this entire assembly during touch down goes someway to absorbing the landing shock.

However, a very credible Tech / Ops contributor pointed out that in his opinion, any shock absorbed by the rotation of the bogie beams would perhaps only be a small percentage of the overall vertical force absorbed by the landing gear due to the vertical deceleration of the aircraft mass.

Quoting bohica (Reply 3):
When the gear is lowered for landing the bogies stay at the same angle they were stored in the gear well.

I'd say this would depend on the specific aircraft type in question. The B777 has different tilt angles for stowage and extension. Due to the shortening of the main gear of the A330/A340 during retraction, an appreciable change in bogie tilt would also occur.

Quoting Roseflyer (Reply 5):
The landing gear truck tilts significantly to allow the tires and brakes to be changed. Typically it is more than 30 degrees of rotation allowed.

The bogie could easily tilt more than 30 degrees, but the amount of tilt needed to accommodate a wheel change wouldn't be anywhere near this.

Quoting Roseflyer (Reply 5):
The effect of the angle of the gear prior to contact to the ground is relatively meaningless because the amount of force from the truck tilt actuator is very small compared to the force of the plane landing.

Assuming my theory of bogey tilt being used to absorb some landing shock is true, it would be the mass moment inertia of the assembly that provides the resistive force.

Quoting DeltaB717 (Reply 8):
I've heard in the past the 767 is set the opposite way to 777/A330/A340/747/etc so that the gear will fit in the bays around a fuel tank or something similar which makes the 'rears down' angle unworkable. Not sure how accurate that is?

All the commercial types you have listed have a broadly similar structural arrangement, in that the main gear wheel well is located immediately behind the centre wing section. From this video, it appears that the 767 bogie angle flattens out slightly during retraction.

http://www.youtube.com/watch?v=2tFgNP5cJHM

What many people may not know is that with many wing gears, the geometry of the trunnion is such that that the gear swings in a plane that is not perpendicular to the longitudinal axis of the aircraft. You can see in the following picture of the 747 landing gear, that the trunnion axes of the wing gears is angled out slightly at the forward end.

http://i94.photobucket.com/albums/l118/Jet-Mech/BLG1.jpg

Thus, the entire gear strut / bogie / brake / wheel assembly actually moves slightly forwards with respect to the longitudinal axis of the aircraft during retraction. The tilt of the wing gear beam must of course, accommodate this and the other structural geometry specific to the 747.

I believe the following is the US patent for the 767 landing gear.

http://www.google.com/patents/US4328939

In this case, you can see that the trunnion axis has a compound angle relative to the longitudinal axis of the aircraft. This combined with the kinked main strut and the particular structural geometry of the 767 again requires the bogie beam to assume a certain position upon retraction.

Assuming my assertion that bogie tilt is used to absorb some landing shock, you must keep in mind that this would work whether the bogie is tilted nose up or down.

Regards, JetMech



JetMech split the back of his pants. He can feel the wind in his hair.
User currently offlineN243NW From United States of America, joined Jul 2003, 1639 posts, RR: 20
Reply 12, posted (1 year 7 months 1 week 2 days ago) and read 4614 times:

Quoting Starlionblue (Reply 10):
You don't see the strut compress until all wheels are on the ground, indicating that the initial pair does not hold up the airplane much at all.

Here's another great video showing this - especially on the 767, you can see how much the struts compress once the spoilers come out and the brunt of the aircraft's weight gets transferred to the gear.

http://www.youtube.com/watch?v=BKvY-6PaiNg



B-52s don't take off. They scare the ground away.
User currently offlineStarlionblue From Greenland, joined Feb 2004, 17119 posts, RR: 66
Reply 13, posted (1 year 7 months 1 week 1 day 23 hours ago) and read 4606 times:

Quoting N243NW (Reply 12):
Here's another great video showing this - especially on the 767, you can see how much the struts compress once the spoilers come out and the brunt of the aircraft's weight gets transferred to the gear.

http://www.youtube.com/watch?v=BKvY-...PaiNg

Great vid. Perfect for clearing up this issue. 

You can see how the strut does not compress at all before all the wheels are on the ground. This indicates there initial wheel pair is not supporting any weight apart from the wheel assembly itself and part of the bogie.



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlinejetmech From Australia, joined Mar 2006, 2699 posts, RR: 53
Reply 14, posted (1 year 7 months 1 week 1 day 1 hour ago) and read 4295 times:

Quoting Starlionblue (Reply 10):
You don't see the strut compress until all wheels are on the ground, indicating that the initial pair does not hold up the airplane much at all.

Besides the force needed to overcome the mass moment inertia of the entire bogie / wheel /brake assembly, I agree that the initial pair of wheels would hardly be supporting any weight, but what you must remember, is that the nitrogen in the oleo is still at several hundred psi when the strut is fully extended. Thus, once all the wheels are in contact with the tarmac, you need to apply an appreciable force to the oleo even before it begins to shorten.

I still feel that the bogie tilt seen on extended landing gear is used to absorb some of the landing shock, and even if this is not correct, the physics of the arrangement would make it so. It may only absorb a small part of the overall force, but the bogie needs to tilt for other reasons anyway, so adopting a tilt angle to make touch down a little more progressive does not really add too much more complication.

Regards, JetMech



JetMech split the back of his pants. He can feel the wind in his hair.
User currently offlineStarlionblue From Greenland, joined Feb 2004, 17119 posts, RR: 66
Reply 15, posted (1 year 7 months 1 week 1 day ago) and read 4266 times:

Quoting jetmech (Reply 14):
Besides the force needed to overcome the mass moment inertia of the entire bogie / wheel /brake assembly, I agree that the initial pair of wheels would hardly be supporting any weight, but what you must remember, is that the nitrogen in the oleo is still at several hundred psi when the strut is fully extended. Thus, once all the wheels are in contact with the tarmac, you need to apply an appreciable force to the oleo even before it begins to shorten.

Fair point.

Quoting jetmech (Reply 14):
I still feel that the bogie tilt seen on extended landing gear is used to absorb some of the landing shock, and even if this is not correct, the physics of the arrangement would make it so. It may only absorb a small part of the overall force, but the bogie needs to tilt for other reasons anyway, so adopting a tilt angle to make touch down a little more progressive does not really add too much more complication.

I guess we need a wheelgineer to answer the question.  



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineKC135Hydraulics From United States of America, joined Nov 2012, 322 posts, RR: 0
Reply 16, posted (1 year 7 months 6 days 23 hours ago) and read 4094 times:

On the KC-135 the landing gear has no tilt at any time. It is actually CRITICAL that the gear is straight during retraction because it only fits into the wheel well one way. We have a centerting cylinder on the truck which is basically a self-contained pneumatic/hydraulic actuator that maintains truck leveling. To ensure the gear does not retract inadvertantly without the truck level, there is a truck leveling switch that makes a contact when the truck is positioned correctly. If the truck is not level, the switch is not contacted and the lock solenoid in the gear handle will not allow the gear to be retracted. Of course, this lock solenoid can be manually overridden but it is HIGHLY inadvisible to do this in flight! We use that on the ground for maintenance purposes primarily. I can't think of a single reason why the pilots would manually override it to retract the gear; if it's stuck down, oh well, at least it's down!

User currently offlineMax Q From United States of America, joined May 2001, 4751 posts, RR: 18
Reply 17, posted (1 year 7 months 6 days 22 hours ago) and read 4083 times:

Quoting KC135Hydraulics (Reply 16):
I can't think of a single reason why the pilots would manually override it to retract the gear; if it's stuck down, oh well, at least it's down!

Well said, even so, if the circumstance was dire enough, say an engine failure after take off with high terrain close you may need that extra performance with the gear retracted despite the risk of not being able to extend it later.


Better a wheels up landing than flying into a mountain


Best wishes KC135



The best contribution to safety is a competent Pilot.
User currently offlineAesma From France, joined Nov 2009, 6847 posts, RR: 12
Reply 18, posted (1 year 7 months 5 days 8 hours ago) and read 3847 times:

Quoting Starlionblue (Reply 10):
The 773ER uses cantilevering on the mains to increase the angle of attack on rotation. It effectively extends the gear struts. However this is only used on take-off, not landing.

To be clearer, the wheel assembly is maintained at a 90° angle with the strut, so the plane rotates around the rear wheels, giving some more room for the tail, which needs every inch !



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