Question to landing gear experts

[Roger Gaemperle]

Hello,

I have a question for landing gear experts: what is the exact purpose of the scissor link on nose gears? Was it always used as a shimmy damper? As far as I understood the scissor links were not always equipped with shimmy dampers.

Regards
Roger

[Bill Walker]

The main purpose of the scissor link is to keep the the wheel(s) pointed in the desired direction. The shock absorber piston that the wheels are attached to would be free to rotate otherwise. These links provide a convient place tro mount a number of accessories, including shimmy dampers, weight-on-wheel switches, and steering devices. Engineers often refer to these links as "torque links", since they resist any torque that would rotate the wheel(s). On many modern airliners the nose wheel scissors can be disconnected, to permit greater nose wheel steering angles while towing the aircraft.

On some struts, the scissors are also the extension stop, which means they keep the strut internal pressure from launching the strut out of the end of the cylinder.

[Roger Gaemperle]

Hello Bill,

Thanks a lot for your interesting post.

Is it right then that a nose wheel with scissor link cannot be rotated and keeps the straight direction unless one of the scissor link attachements is a shimmy brake that allows rotation but dampens it?

Or in other words: every nose wheel that was able to rotate +/- a certain angle for taxiing AND had a scissor link must have had a shimmy brake in one of the scissor links attachement? As otherwise it couldn't rotate and would keep direction.

Regards
Roger

[Tony Kambic]

I studied the P-38 nose gear at last year's GML2007 and noticed it had pistons on each side at the top of the scissors link of the nose gear. I am not sure if they are to enable steering only or act as a shock damper during steering also. The page below is a P-38 walk-around where you can clearly see them.

http://www.cybermodeler.com/aircraft...-38_walk.shtml

Some aircraft use a drag-link strut which does not need the link at all. The B-52's out-board single wing strut is an example of this. The wheel fork is angled backwards and thus the wheel turns almost any direction that the aircraft is rolling.

As a volunteer as Smithsonian, I have learned and visualized that most of our aircraft on static display either use 'strut collars' over the piston part of these struts, or have internal machinings replacing the fluid inside, otherwise the pistons would collapse over time so the link would be completely closed. Also some aircraft had strut collars as part of their regular maintenance procedures to eliminate any possibility of collapse. The DO-335 on display has original (1945) strut collars in place.

[Bill Walker]

Hi Roger;

There are as many different designs of nose gear steering systems and shimmy dampers as there are landing gear designers.

In general, if the nose gear has a scissor, the top of the upper link will be attached to a collar that can rotate around the outside of the strut housing. If there is powered steering, this collar can be rotated by a hydraulic cylinder, an electric actuator, or cables that are connected to the rudder pedals. If there is no powered steering (usually called a castoring nosewheel) the collar's movement will be restricted by some combination of springs, rubber or fluid dampers, or carefully controlled friction between the collar and the strut. This restricted motion prevents shimmy loads from being felt in the aircraft, and prevents divergent shimmy of the nose tires.

Many older aircraft, and some current small aircraft, use a shimmy damper that consists of a spring and rubber pad stack up at one of the scissors joints - usually the middle joint, or sometimes the joint between the top of the upper link and the strut. This package will allow a very small steering movement, but the purpose is not to steer the aircraft, but just to dissipate the shimmy energy at critical frequencies (like the natural frequencies of the nose gear strut or the forward fuselage).

To add to the confusion, older aircraft may not have any powereed steering or castoring of the nosewheel. Steering is accomplished by scuffing the tires (that is, dragging the tires sideways as the aircraft turns). These aircraft may still need a shimmy damper, as the free play in the scissors joints may allow enough movement for divergent shimmy to develop at some speeds.

All this is a roundabout answer to your question. Almost all nose gears that can steer or castor (and many that cannot steer or castor) will have some method of controlling shimmying. It may be a separate spring pack or damper called a "shimmy damper" in the aircraft parts list, or it may be built into the steering mechanism, the castoring mechanism, or the joints of the scissors.

Hope you followed all this. There will be a quiz next week.

[Roger Gaemperle]

Hi Bill,

Thanks a lot for the detailed explanations. I think I have a much better picture now of all the different types that were possible.

I have read about a nose landing gear with scissor links but with internal damper (i.e. insice the cylinders). I could make out small springs at the scissor link collar and I guess that the springs just kept it in the straight direction (under normal conditions; i.e. small amplitude and not at the natural frequencies) and that the inner shimmy damper was the one to prevent shimmy in critical cases.

Hi Tony,

Thanks for the link and the explanation. That's very interesting. I have visited the NASM museum (downtown) about 3 times and the new one at the airport 1 time and it was each time an exciting experience!

All the best
Roger

[Tony Kambic]

One other comment about the static displays that I wanted to confirm before posting. While some aircraft have strut collars and other aircraft have an internal piece of steel machined to place inside the gear to mimic a pneudralic supported strut, if you look closely at some other aircraft, you will visualize a finely machined collar of stainless steel fiitted around the piston. From a distance it looks just like the piston. Ways to tell are at some angles you can see screw slots to hold it in place and it also may appear to be wider than you would expect the piston. Among aircraft configured like this at Udvar-Hazy are the F8 Crusader and the Lockheed Constellation.

Tony

[Graham Boak]

One other point worth noting: Jet Provost/Strikemasters use nose tyres with two raised rims and a central valley, to act as shimmy dampers. The Mosquito tailwheel was the same, I believe.

[PeterVerney]

You are correct re the Mosquito tailwheel.