gunsontheroof
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Question About MD-11 Nose Gear.

Wed Feb 14, 2007 1:10 pm

I was browsing through the ol' database today and this picture of an MD-11 caught my attention. If you enlarge it and look closely at the nose gear, it's bending noticeably to the right while turning. I'm sure that this isn't an undesired condition resulting from too much speed while turning, but I never realized that any aircraft's nose gear flexed/bent in this way. Can anyone provide further information on what we're seeing in this picture?


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474218
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RE: Question About MD-11 Nose Gear.

Wed Feb 14, 2007 1:47 pm

Its an optical illusion caused by MD-11 and DC-10 nose gear, which do not do extend a full 90 degrees.

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speedracer1407
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RE: Question About MD-11 Nose Gear.

Wed Feb 14, 2007 2:52 pm

You can see the very same thing on many cars; It's caused by the caster angle. On a car's front suspension, either the strut or control arms are arranged in such a way that the upper attatchment point (to the body) is further aft than the lower attatchment point (to the wheel). When the wheels are turned the caster angle essentionally causes an increase in negative camber. Find a parked BMW of Mercedes of any type with its wheels turned full, and you'll see what I mean. On an airliner like the MD-11, the effect is simply exaggerated because of the length of the gear and the fact that it has two wheels.
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gunsontheroof
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RE: Question About MD-11 Nose Gear.

Wed Feb 14, 2007 3:25 pm

Well I guess that explains it then. Thanks for the answers guys!
 
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jetmech
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RE: Question About MD-11 Nose Gear.

Wed Feb 14, 2007 9:02 pm

Quoting Gunsontheroof (Thread starter):

I believe that the arrangement is also very similar to the rake and trail geometry used on motorbikes and bicycles http://en.wikipedia.org/wiki/Rake_and_trail . Basically, with bikes and the DC10/ MD11, the idea is that the extended centre-line of the strut should touch the ground at a point forward of the tyre contact point. What this geometry tends to do is to try and centre the wheels when they are steered away from straight ahead. When ever the DC10 / MD11 nose-gear is steered away from neutral, a restoring couple is formed. The moment arm of this couple is the distance between the point on the ground where the extended strut centre-line touches and the tyre contact point. The forces of the couple are the rolling resistance of the nose wheels and the reaction of this rolling resistance at the strut. Having the extended strut centre-line ground contact point ahead of the tyre contact point means that this couple always tends to restore the nose-wheels to neutral when disturbed.

This restoring couple tends to increase in magnitude the further the nose-wheels are steered from neutral. Not only does the moment arm of the couple increase, but the rolling restance of the tyres (and the subsequent reaction in the nose gear strut) also increases the further you displace the steering. This is due to scrub forces.

The DC10 / MD11 achieves this geometry by tilting the strut slightly forward. Boeing uses a different approach. The nose-gear struts on most Boeings are usually completely upright. Placing the ground contact point of the extended strut centre-line ahead of the tyre contact point is achieved by offsetting the axle centre-line behind the centre-line of the strut. The offset on the 747 is approximately 4 or 5 inches. Airbus uses a similar arrangement of the nose gears of the A330 /A340. I suspect that the centre-line of the axle on the DC10 / MD11 co-incides with the centre-line of the strut. Thus, the DC10 / MD11 system is similar to the geometry of a car steering system as mentioned by Speedracer1407. The Boeing and Airbus system is similar to the wheels found on a shopping trolley.

Regards, JetMech

[Edited 2007-02-14 13:08:10]
JetMech split the back of his pants. He can feel the wind in his hair :shock: .
 
AmericanB763ER
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RE: Question About MD-11 Nose Gear.

Wed Feb 14, 2007 9:41 pm

Quoting JetMech (Reply 4):
Airbus uses a similar arrangement of the nose gears of the A330 /A340


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The A32x series seems to have the same arrangement too. But doesn't this layout add a considerable amount of stress to the nose gear strut as opposed to a perfectly straight one ?


Marco
 
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jetmech
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RE: Question About MD-11 Nose Gear.

Thu Feb 15, 2007 11:39 am

Quoting AmericanB763ER (Reply 5):
But doesn't this layout add a considerable amount of stress to the nose gear strut as opposed to a perfectly straight one ?

Possibly. In a static situation, a nose gear that is tilted slightly forward may have some bending loads imposed on it that are not present in a perfectly vertical nose gear strut. On the other hand, a nose gear strut that is tilted forward would have slightly less axial loads in the static situation compared to a perfectly vertical nose gear strut. With the tilted strut design, the drag brace is always loaded up in the static situation with compressive loads.

Regards, JetMech
JetMech split the back of his pants. He can feel the wind in his hair :shock: .
 
phollingsworth
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RE: Question About MD-11 Nose Gear.

Sat Feb 17, 2007 12:03 am

Quoting AmericanB763ER (Reply 5):
The A32x series seems to have the same arrangement too. But doesn't this layout add a considerable amount of stress to the nose gear strut as opposed to a perfectly straight one ?

It actually decreses the bending load on the gear when it encouters bumps or imperfections in the pavement, which are more of a hazard then the static loads. If you draw a fee-body diagram of the nose gear hitting a bump you will see why this occures.

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