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SpeedbirdHeavy
Topic Author
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Joined: Wed Jan 14, 2004 3:39 pm

Landing Gear Sensitivity

Tue Aug 10, 2004 2:19 pm

I am curious about the over sensitivity of landing gear. I realize that there must be shock absorbers because of the force of a landing. But, why is the gear so sensitive? As a plane taxis, you can feel it every time the wheels go over a crack in the concrete. Even on a 747, the plane would shudder when it crossed over a crack.

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ArmitageShanks
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RE: Landing Gear Sensitivity

Tue Aug 10, 2004 3:13 pm

Someone correct me if I am wrong but you feeling every bump is due to the lack of shock absorption.

An aircraft isn't really made to travel along the ground for long, so a super smooth taxi isn't high on the list of design features.
 
dl757md
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RE: Landing Gear Sensitivity

Tue Aug 10, 2004 4:08 pm

Someone correct me if I am wrong but you feeling every bump is due to the lack of shock absorption.

Actually the landing gear strut or the main vertical tube on each gear is a huge air/oil shock absorber. But they are designed almost exclusively with landing in mind. They are as ArmitageShanks says not made to travel along the ground for long. Also the tires being 24-32 ply and inflated to 150-225 psi don't provide a smooth taxi. A landing gear that could effectively provide a car-like ride during taxi and handle the loads and stress of landing would be too costly, complex, and heavy to be practical.

Dl757md
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citationjet
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RE: Landing Gear Sensitivity

Tue Aug 10, 2004 10:08 pm

DI757md is correct.
The aircraft landing gear design is optimized for performance during landings and the dynamic loads associated with it, and not for the comfort of a slow taxi over concrete cracks.
The shock absorption characteristics in the oleo are designed to withstand the 12 foot per second reserve energy drop. The main concern in gear design for aircraft taxi conditions are the side loads due to ground turning, and braked roll conditions.
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Mr.BA
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RE: Landing Gear Sensitivity

Tue Aug 10, 2004 10:32 pm

Is it possible to explain in simple terms how the landing gears are designed to withstand such heavy loads especially during landing? It's just pretty fanscinating to know that these gears absorb quite a huge amount of force without problems and not puncture into the aircraft as well! We're not talking about 10 landings but thousands of it over years of service!

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Starlionblue
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RE: Landing Gear Sensitivity

Tue Aug 10, 2004 11:34 pm

This is just like a racing car. It has shock absorbtion galore, but it makes for a pretty rough ride. The shocks are hard because you get better handling that way (simplistic but basically true) and because you are focused on high speed bumps as opposed to low speed smoothness. Low speed taxi comfort would mean pretty soft shocks (think Lincoln Town Car) and that would mean bottoming out on every landing.

The pax would probably not appreciate it if the landing gear struts came through the floor during landing  Big grin
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air2gxs
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RE: Landing Gear Sensitivity

Wed Aug 11, 2004 3:01 am

Really quick and dirty. A simple shock strut design consists of 2 major parts: the inner cylinder and the outer cylinder. The outer cylinder is what you see of the landing gear and is fixed. The inner cylinder is the chrome. Attached to the inner cylinder are the bogies or axle depended on aircraft. This is the part that moves.

OK, so far? The shock strut is filled with a hydraulic fluid (Mil 5606 or equivalent is the most common), then an air charge is added. The air pressure introduced is depended on aircraft weight at the time of servicing. A simple chart is usually attached to the gear or nearby.

Inside the shock strut, attached to the inner cylinder is a tapered metering pin. This pin travels up and down with the cylinder. In the outer cylinder is a metering plate. Its fixed. When the aircraft lands the upward movement of the inner cylinder forces the fluid to pass through the metering plate. As the strut further compresses the pin enters the metering hole and begins to restrict the fluid flow. This snubs or absorbs the energy being exerted. The goal is to have the full eight of the aircraft on the gear before the pin (strut) bottoms out.

The air charge doesn't really have a role in landing except to maintain a head on the fluid (give it a little resistance). The aircraft exists to cushion the aircraft during taxiing. The aircraft does not exert enough pressure during taxiing to really bring the fluid into play. That's why the ride is so bumpy. Its only an air cushion.

Don't get me wrong, an air charge is required for a smooth landing, but it is the fluid charge that takes the brunt of the landing forces. I've taxies plenty of aircraft taxi with very low fluid and a good air charge. The ride is no different. (usually taxiing to hangar for repair of a leaking strut).

Again, that was quick and dirty. Numerous other components are installed within the cylinders to help the drivers make a smooth landing.
 
tbanger
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RE: Landing Gear Sensitivity

Wed Aug 11, 2004 7:09 pm

Is it possible to explain in simple terms how the landing gears are designed to withstand such heavy loads especially during landing? It's just pretty fanscinating to know that these gears absorb quite a huge amount of force without problems and not puncture into the aircraft as well! We're not talking about 10 landings but thousands of it over years of service!

Mr.BA
Keep in mind that the landing gear does not take the FULL brunt of a landing unless it is a really bad one.

The wings are still creating a fair a mount of lift at touch down and still has the majority of the weight of the aircraft. As the spoilers are applied and the aircraft slows down, so the aircraft becomes "Heavier" and sits down on the gear.

The best way to see this is if you ever have the chance of flying on a Saab 340, make sure you get a window seat in either of the last 2 rows. On touchdown, watch the torque link. It will be almost straight when the aircraft is in the air and as the aircraft touches down, it will compress a little. As you feel the aircraft slow down, so to will you see the torque link compress more and more.

This is why large aircraft have spoilers. Because there is buggerall weight on the wheels at touchdown. The spoilers force the weight of the aircraft onto the wheels to assist in braking.
 
FredT
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RE: Landing Gear Sensitivity

Wed Aug 11, 2004 9:42 pm

Even though the wings carry the weight of the aircraft, the shocks deal with the inertia as the aircraft touches down. That's their primary purpose. Which, as above posters stated, explains the bumpy ride.
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phollingsworth
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RE: Landing Gear Sensitivity

Wed Aug 11, 2004 10:17 pm

Also, one of the critical load times for landing gear is during a max weight taxi. This is because the combination of compression and torsion loads is quite high. This phase of operations is where a lot of trunion failures occur.
 
Mr.BA
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RE: Landing Gear Sensitivity

Thu Aug 12, 2004 6:17 pm

Thanks for the information  Smile
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MD11Engineer
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RE: Landing Gear Sensitivity

Thu Aug 12, 2004 7:44 pm

Just one correction.
The Nitrogen gas charge in the struts acts as a spring (gases can compress) and finaly stops the downward movement (if the pressure is properly serviced). It is like compressing a steel spring. Now, compressed, the spring wants to rebound. Already on compression, the oil being squeezed through a decreasing orifice, creates friction and heat, and this way reats up energy. On expansion the oil has to go through the orifice again, eating up more energy, which will slow down the expansion, so that the plane won´t jump.
If the nitrogen pressure is too low, the landing will be hard, and there is the danger of the strut bottoming out, because there is not enough opposed force to the compression of the strut. If the oil level is too low, the plane will tend to oscillate on the struts and to jump.

It is actually similar to a car suspension system. There the big metal spring takes the role of the nitrogen and the shock absorber contains the oil part.

Jan
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