Landing gear questions

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So this is something that's intrigued me for a while. I watch a lot of plane spotting videos and can't figure it out. The 767's main gear bogies naturally "hang" with the front lower than the rear. They tilt forward. I assume they were designed this way. Example: Most other aircraft with 4+ tire main bogies, including other Boeing aircraft, "hang" the bogies with the rear tires lower than the front. Example (757): Airbus A330s and A340s seem to REALLY drag the rear bogie tires: Why the differences here? It would seem to me that the way the 767's rear bogies hang, you'd get an oscillation at touch-down, where the bogie would rock fore-and-aft before settling down. It's curious that Boeing used "front hanging" bogies on the 767 and "rear hanging" bogies on most of its other aircraft. Is this simply a packaging thing for when the gear is stowed during flight?
 
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And to add to the confusion, notice here around the 1 minute mark how the main bogies on the 777 are intentionally leveled out right before retraction begins. Subsequently, around the 2 minute mark the main bogies are deployed and then tilted back near the down position.
 
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Related to this discussion, I've always wondered why there isn't a mechanism in place to have the wheels rotating before touchdown. It seems to me it would reduce tire wear considerably rather than having one spot on the tire take all the initial friction/shock load to spin up from 0 - 125 mph instantly. Maybe cups molded into the sidewalls of the tire that would scoop into the slipstream and set the tire rotating. I'm sure I'm not the first to think of such a thing and maybe it's been tried before - it's just one of those things that on the surface of it seems to make sense.
 
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Originally Posted By: chestand
Related to this discussion, I've always wondered why there isn't a mechanism in place to have the wheels rotating before touchdown.
Perhaps because it doesn't particularly matter to the tire whether that energy dissapation comes in one shock on touchdown or further down the runway when the brakes are applied... the aircraft's kinetic energy is being dissapated in the tire either way.
 
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Originally Posted By: chestand
Related to this discussion, I've always wondered why there isn't a mechanism in place to have the wheels rotating before touchdown. It seems to me it would reduce tire wear considerably rather than having one spot on the tire take all the initial friction/shock load to spin up from 0 - 125 mph instantly. Maybe cups molded into the sidewalls of the tire that would scoop into the slipstream and set the tire rotating. I'm sure I'm not the first to think of such a thing and maybe it's been tried before - it's just one of those things that on the surface of it seems to make sense.
Part of it is weight and complexity. But the antiskid systems (ABS) in many planes calculate sthe time from when weight is detected on the wheels to when the wheels are at speed, to adjust braking for the slipperiness of the surface. If the wheels were spinning at speed at touchdown, it would take longer for the system to adjust. A second or two when you are landing a couple hundred thousand pounds on a slippery runway makes a huge difference in landing distance. From a pilot's viewpoint, consistent performance in all conditions is more important -and safer - than reduced tire wear. OP, no clue why they are designed that way. But in the scheme of things, there isn't much time between the fronts touching down and the rears following. We are talking less than a second in most cases, and it takes more time than that to get something that large to oscillate.
 
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I fly the 767 shown in your example a lot and I can tell you that it is harder to get a smooth touchdown with forward tilting trucks than with a rearward tilting truck like the 757. I'm not saying the 767 is harder to land, but you never quite get those "greasers" like you frequently get on the 757. Sometimes all you hear on the 757 is a "click" when the landing gear handle locks after the main gear touches down. Rearward tilting trucks "roll on" the runway better. The 767 landing gear was designed with a forward tilt due to gear bay space limitations, i.e., it fits better with the space available. Gear tilt is controlled with hydraulic pressure during retraction and extension. The airplane goes into "ground mode" when the main landing gear tilt is removed during landing. Gear tilt sensing allows the spoilers to deploy automatically (provided they were armed), pressurization outflow valves to open to equalize cabin pressure to the outside air pressure, and allows engine thrust reversers to be deployed (prevents reverse thrust from being deployed in flight) and autobrakes to automatically begin braking. Also, if the gear does not tilt properly after takeoff, the landing gear cannot be retracted. You will then see a message indicating "Gear Tilt Sense" indicating a problem during retraction. Normally, main gear brakes are applied automatically during retraction. The nose gear doesn't have a brake so you often hear the nosewheel tire making some noise as it hits a snubber to stop rotation once it is in the front wheel well (which is located right underneath the cockpit). Hope that information helps... 757Guy
 
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Originally Posted By: stranger706
And to add to the confusion, notice here around the 1 minute mark how the main bogies on the 777 are intentionally leveled out right before retraction begins. Subsequently, around the 2 minute mark the main bogies are deployed and then tilted back near the down position.
The 777 gear are leveled out so that the assembly will fit into the gear bay as designed. Upon extension, hydraulic system pressure "tilts" the landing gear so that when the main landing gear touches down and is forced to a level position, the aircraft goes into ground mode for spoiler deployment, reverser activation, autobrakes, etc.
 
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Originally Posted By: Kuato
it takes more time than that to get something that large to oscillate.
I don't know, there certainly appears to be some "oscillating" (or"shudder" as they call it in the video) going on here. This is a test of the A380's landing gear, which happens to hang lower in the front like the 767.
 
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Originally Posted By: KD0AXS
Originally Posted By: Kuato
it takes more time than that to get something that large to oscillate.
I don't know, there certainly appears to be some "oscillating" (or"shudder" as they call it in the video) going on here. This is a test of the A380's landing gear, which happens to hang lower in the front like the 767.
Cool video. When I said oscillate I meant side to side, referencing an earlier post about the lower trailing wheel being more stable. Looked like there was a good bit of shuddering, but it was all up and down from tire deceleration.
 
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Yep, I love when a commercial pilot comment on aviation threads. I would soooo love to fly a 747, 757, 767, 777 or 787 for a living.
 

Hokiefyd

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Originally Posted By: 757guy
I fly the 767 shown in your example a lot and I can tell you that it is harder to get a smooth touchdown with forward tilting trucks than with a rearward tilting truck like the 757. I'm not saying the 767 is harder to land, but you never quite get those "greasers" like you frequently get on the 757.
Thanks for this excellent input. Watching lots of plane spotting videos, it appears to me that the 767 has a little more "violence" in the rear gear area during landing, which my simple mind attributes to the forward-tiling gear. You can see on something with some good rear-tilt to it, like a 757 or an A330, the pilot just lays those trucks down on the tarmac with nary a shudder (at least no visual shudder).
 
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