FlyLKU From United States of America, joined Apr 2006, 652 posts, RR: 0 Posted (5 years 4 months 4 days 8 hours ago) and read 32767 times:
I noticed these two photos in the DB and thought they provide an excellent visual example of how Boeing engineered the gear in the -300 such that on rotation there is a bit more Angle Of Attack provided than otherwise would be if the bogey rotated on the strut like it does in the -200 (and every other modern jet that I know of). OK, I may have the terminology about the gear wrong so maybe an A&P can straighten me out...but you know what I mean.
FlyLKU From United States of America, joined Apr 2006, 652 posts, RR: 0 Reply 3, posted (5 years 4 months 4 days 7 hours ago) and read 32767 times:
Quoting FLY2HMO (Reply 2): Now, is the system designed to just keep the truck level, like in the L-1011, or does it actually push down against the runway?
This is a question for someone more knowledgeable on the subject than I. I know that due to the length of the -300 Boeing was looking for ways to reduce takeoff distance without lengthening the aircraft's "legs" any more than they had to to avoid tail strikes.
I was unaware that the L1011 had a similar system. How about the A340-500/600?
FlyLKU From United States of America, joined Apr 2006, 652 posts, RR: 0 Reply 5, posted (5 years 4 months 4 days 7 hours ago) and read 32767 times:
Quoting MDorBust (Reply 4): I looks to me as if the SQ aircraft is just further into it's rotation and has begun to lift the landing gear.
I believe Boeing went ahead with their plans on the -300 whereby the landing gear strut locks into the bogey during takeoff. This is like locking your office chair from reclining and then leaning back. It results in a slightly short takeoff distance - for the aircraft, not your chair.
If Boeing did not ultimately implement this approach then someone please post as such. I thought these two photos were pretty good evidence that they did.
Starlionblue From Greenland, joined Feb 2004, 15102 posts, RR: 69 Reply 6, posted (5 years 4 months 4 days 6 hours ago) and read 32767 times:
Quoting FlyLKU (Reply 5): I believe Boeing went ahead with their plans on the -300 whereby the landing gear strut locks into the bogey during takeoff. This is like locking your office chair from reclining and then leaning back. It results in a slightly short takeoff distance - for the aircraft, not your chair.
You are correct. The -300ER gear is cantilevered on take off. I believe it is the ER only. In other words, the hinge point where the bogie joins the leg is locked during take off and the bogie is not allowed to tilt forward. During rotation, this raises the aircraft a bit more compared to a loose hinge. As described above, this allows a higher angle of attack since tailstrike is less of a concern. Thus take-off performance is improved.
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MDorBust From , joined Dec 1969, posts, RR: Reply 7, posted (5 years 4 months 4 days 6 hours ago) and read 32767 times:
Here's the sticking point though.
Take two sticky notes.
Place one so that the straight edge forms a line across the bottom of the port landng gear tires. Place the second so that it's edge follows the bottom of the forward fuselage. The two stickies will converge at an angle instead of having parallel edges. The starboard landing gear also checks out to be not parallel to the fuselage.
On that SQ 773 at least, the landing gear truck is hanging out of parallel to the fuselage.
MidEx216 From United States of America, joined Jun 2005, 650 posts, RR: 5 Reply 9, posted (5 years 4 months 4 days 3 hours ago) and read 32767 times:
I noticed that too, in the picture, and I thought, that must put a lot more stress on the gear because it puts the weight of the plane on that one part of the bogey rather than in the shock-absorbing piston of the strut.
Starlionblue From Greenland, joined Feb 2004, 15102 posts, RR: 69 Reply 10, posted (5 years 4 months 4 days 2 hours ago) and read 32767 times:
Quoting Brons2 (Reply 8): Wouldn't that method increase pavement loading on the rearmost axle during takeoff?
Yes. But this is at the point of rotation so the wings are taking most of the weight anyway.
Quoting MidEx216 (Reply 9): I noticed that too, in the picture, and I thought, that must put a lot more stress on the gear because it puts the weight of the plane on that one part of the bogey rather than in the shock-absorbing piston of the strut.
Yes. But as in the pavement loading issue most of the weight is on the wings at this point.
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Would it be possible for you to elaborate on this point MDorBust? I'm not exactly sure what is not parallel in your explanation . This photo shows the hydraulic ram - situated on the front of the main leg - used to lock the bogie beam upon aircraft rotation.
DH106 From United Kingdom, joined Jun 2005, 621 posts, RR: 2 Reply 12, posted (5 years 4 months 3 days 23 hours ago) and read 32767 times:
Hmmm interesting....
If the bogie IS locked on the -300 for takeoff, I would guess there would probably have to be special ops procedures for the takeoff - I imagine theoretically that the elevator would be powerful enough to rotate the aircraft at well below normal Vr and should the pilots (for whatever reason) pull back at a speed less than Vr - then that would put massive stress on the bogie/pivot joint & lock as the wings wouldn't be supporting the vast majority of the weight.......
Any pilots know?
...I watched c-beams glitter in the dark near the Tanhauser Gate....
CX flyboy From Hong Kong, joined Dec 1999, 6129 posts, RR: 57 Reply 13, posted (5 years 4 months 3 days 22 hours ago) and read 32767 times:
All very interesting. I have little information on this as we do not have our 777-300ERs yet, or any changes to our manuals yet, but I do know that as far as we are concerned, there should be no real differences to the way we fly the takeoff and rotation on the ER.
"The 777-300ER adds new features to mitigate this geometry limit. One is a 13-ft. extension of wingspan to create more lift. Another is "semi-levered" main landing gear. At rotation, normal gear will pivot where the gear leg meets the wheel bogie, as part of the tilt motion of the bogie. The 777-300ER locks the tilt actuator for takeoff, so the gear leg-to-bogie angle is fixed and the aircraft rotates instead about the rear set of wheels. This allows the fuselage to achieve a greater angle before tail strike, letting the wing produce more lift and reducing the liftoff speed and thus takeoff distance."
"That's so stupid! If they're so secret, why are they out where everyone can see them?" - my kid
Starlionblue From Greenland, joined Feb 2004, 15102 posts, RR: 69 Reply 15, posted (5 years 4 months 3 days 18 hours ago) and read 32767 times:
Quoting DH106 (Reply 12): If the bogie IS locked on the -300 for takeoff, I would guess there would probably have to be special ops procedures for the takeoff - I imagine theoretically that the elevator would be powerful enough to rotate the aircraft at well below normal Vr and should the pilots (for whatever reason) pull back at a speed less than Vr - then that would put massive stress on the bogie/pivot joint & lock as the wings wouldn't be supporting the vast majority of the weight.......
First of all, let's drop the "if". The -300ER has this feature as standard. There are no special procedures, just slightly higher allowed rotation angle than on the -300. This is transparent from the pilot's perspective.
If the pilot chooses to pull back on the yoke well before Vr that would indeed put a lot of stress on the ram used to lock the landing gear. But since Boeing had to do minimum unstick speed testing I imagine they went through that scenario too. The whole thing looks a bit weak but in truth it is just as strong as the "conventional" arrangement. Otherwise it would not have been certified.
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Starlionblue From Greenland, joined Feb 2004, 15102 posts, RR: 69 Reply 17, posted (5 years 4 months 3 days 18 hours ago) and read 32767 times:
Quoting HAWK21M (Reply 16): Is there a difference really.Or are the Pics taken at different position,so the Gear Load is varied.Anyone work on the B777 knows.
There is a difference. Check Vzlet's post above. The -300ER has semi-cantilevered main gear.
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EMBQA From United States of America, joined Oct 2003, 9177 posts, RR: 15 Reply 21, posted (5 years 4 months 3 days 16 hours ago) and read 32767 times:
Quoting MDorBust (Reply 4): I looks to me as if the SQ aircraft is just further into it's rotation and has begun to lift the landing gear.
It's clear in the top two photos that the Singapore 777-300 is just about to become airborne, where the Malaysia 777-200 is not quite there yet.
Looking at these two photos below taken at close to the same point in flights shows the landing gear to be in the same position. Also, in my photo search I saw gear tilted forward and tilted aft...
DH106 From United Kingdom, joined Jun 2005, 621 posts, RR: 2 Reply 22, posted (5 years 4 months 3 days 15 hours ago) and read 32767 times:
I'm guessing that the locking of the bogies only prevents 'back tilt' past the normal ground sit position, and you wouldn't see an different hang angle when airbourne.
[Edited 2007-01-23 17:47:11]
...I watched c-beams glitter in the dark near the Tanhauser Gate....
JetMech From Australia, joined Mar 2006, 2445 posts, RR: 54 Reply 23, posted (5 years 4 months 3 days 2 hours ago) and read 32767 times:
Quoting DH106 (Reply 12): then that would put massive stress on the bogie/pivot joint & lock as the wings wouldn't be supporting the vast majority of the weight.......
Quoting Starlionblue (Reply 15): If the pilot chooses to pull back on the yoke well before Vr that would indeed put a lot of stress on the ram used to lock the landing gear
Quoting DH106 (Reply 22): I'm guessing that the locking of the bogies only prevents 'back tilt'
If you have a closer look at the hydraulic ram set-up for the 773ER, you will see that in actual fact, the bogie beam is not "locked" at all.
When the hydraulic ram "locks", all it does is fix the distance between the upper (on the main gear leg structure) and lower (on the forward end of the bogie beam) mounting points of the hydraulic ram. The actual pivot between the bogie beam and the MLG inner cylinder fork is not locked and is always free to rotate.
Imagine the extreme situation of the B773ER somehow being rotated in a static situation with the hydraulic ram locked. The distance between the upper and lower ram mounting points would be fixed, but rotation between the bogie beam and the MLG inner cylinder fork would still be allowed as the entire MLG strut can still compress.
This of course would have highly undesirable consequences. In the extreme case, the wheels on the forward and middle axles would be off the ground with all weight upon the rear wheels. The strut would most likely be bottomed out with the bogie beam being subjected to massive bending loads.
In this case, the B773ER landing gear is acting like a cantilevered design found on the MLG's of many business jets. Even is this extreme situation, the pivot between the bogie beam and inner cylinder fork is still free to rotate.
I have no doubt that the system designed by Boeing is much more sophisticated that this. I dare say the system uses such information as take off weight, weight on wheels, and take off speeds (Vr etc.) and aircraft attitude, to calculate when to lock the actuator. I would also assume that the actuator may have a hydraulic relief valve to prevent it locking when subjected to extreme loads.
Regards, JetMech
JetMech split the back of his pants. He can feel the wind in his hair.
DH106 From United Kingdom, joined Jun 2005, 621 posts, RR: 2 Reply 24, posted (5 years 4 months 3 days ago) and read 32767 times:
Quoting JetMech (Reply 23): If you have a closer look at the hydraulic ram set-up for the 773ER, you will see that in actual fact, the bogie beam is not "locked" at all.
But that's a landing shot.
According to the above the bogie is only 'locked' for takeoff.
I take in what you're saying though about the lock/bogie pivot - the bogie can still move to some extent - but constrained by the oleo travel.
...I watched c-beams glitter in the dark near the Tanhauser Gate....
25 JetMech: Sorry about the confusion DH106. I only included that photo to illustrate the hydraulic ram and the ram mounting points. My explanation was with resp
26 DH106: No problem at all JetMech. I think we're in agreement
27 Max Q: JetMech. I have wondered also how the Aircraft logic is set up to lock the bogie for take off and theorize it would be simplest to use flap position.
28 JetMech: G'day Max Q , I think the landing situation would be quite easy to cater for. Perhaps the box controlling the hydraulic rams gets a message from the
29 DH106: How about: On power up, if weight on wheels (squat switches) then bogies locked until retraction sequence unlocks bogies. On extension, bogies unlocke
30 Pygmalion: The oleo are compressed at takeoff with weight on the gear, during this phase the actuator for the semi-levered gear is active. In the air, the oleo i
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. This photo shows the hydraulic ram - situated on the front of the main leg - used to lock the bogie beam upon aircraft rotation. 
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