9
$\begingroup$

I recently learned that there is a locking mechanism on the nose wheel of a Robin DR-400. This mechanism locks the nose wheel in place when there is no weight on the wheel. In particular, the wheel will be locked when the plane is in the air. And at landing, when there is weight on the wheel again, it unlocks. But the nose wheel can also become locked if the center of gravity of the plane is too far back, or may fail to unlock upon landing. In this case, the aircraft becomes uncontrollable, and this can cause a runway excursion.

So I am wondering a few things:

  • How does this mechanism work?
  • What is gained by having the nose wheel lock itself? Rather than for instance a spring-like mechanism that just keep the wheel aligned with the plane when the plane is in the air.
  • Are there other planes with a similar locking mechanism?
  • Are there planes with a different mechanism to keep the nose wheel from moving in flight?

Here is what the landing gear looks like. It is not retractable and there are fairings on the wheels for better aerodynamics: Here is a picture I found with a good view of the landing gear:

$\endgroup$
  • $\begingroup$ I can see the reason if it is retractable (to align it so it fits in the compartment) $\endgroup$ – ratchet freak Nov 9 '14 at 0:38
  • 1
    $\begingroup$ @ratchetfreak The landing gear is not retractable. $\endgroup$ – usernumber Nov 9 '14 at 0:42
  • $\begingroup$ If I remember correctly, the DR-400 nose gear is prone to shimmy when lightly loaded. Could that be the reason for the lock? $\endgroup$ – Peter Kämpf Nov 9 '14 at 15:54
  • $\begingroup$ I feel like a locking nose gear could make a crosswind landing pretty dangerous. A better solution would allow the pilot to lock the gear manually (after take off, and unlock it before landing). $\endgroup$ – Keegan Nov 10 '14 at 1:39
  • $\begingroup$ @kjmccarx I hear there have been runway excursions due to this system. $\endgroup$ – usernumber Nov 10 '14 at 3:13
7
$\begingroup$

It looks like the "How do they work?" question has already been addressed pretty well, so I'll answer some of the other parts of the question:

What is gained by having the nose wheel lock itself?

  • The nose gear is prevented from turning during flight when the rudder pedals are actuated. If the nose gear turned when the rudder was being used in flight, it would add a lot of form drag.

  • The nose gear remains straight until it touches down on landing, which prevents it from landing sideways in a crosswind landing where the rudder is being used to maintain runway alignment. This allows the pilot to continue to use the rudder inputs to maintain the centerline through the landing roll without damaging the tires or having a sudden yaw in the direction that the tire is pointing when the nose wheel touches down.

Rather than for instance a spring-like mechanism that just keep the wheel aligned with the plane when the plane is in the air.

You mean like a Cessna-style spring linkage, where there is not a hard linkage between the rudder pedals and the nose gear? One advantage of not using the Cessna-style springs is that it's easier to taxi with direct linkages rather than spring linkages. Cessnas have a lag between rudder pedal input and the nose wheel actually turning because of the spring linkages, so you have to give directional inputs ahead of when you actually want to turn. With direct linkages, the nose wheel turns immediately when you push the pedals, which makes taxiing a little easier, especially when you're first learning.

Are there planes with a different mechanism to keep the nose wheel from moving in flight?

Yes. See above for Cessnas. Also, in larger aircraft (e.g. passenger jets and similar,) there are no mechanical linkages to the nose wheel at all and the nose wheel is turned (mostly) by a separate control from the rudder. This control is called a tiller and it's moved by hand instead of by foot. It looks like this:

B737 tiller tiller

$\endgroup$
6
$\begingroup$

I found this report by the French accident investigation bureau (BEA) that explains how the mechanism works.

General mechanism: tige=rod; verrou=lock; moyeu=hub

There is a rod attached to the bottom of the damper, and the rod has a protuberance at the top. When there is no weight on the wheel, the top of the rod goes down into a notch that is attached to the mainframe, and locks the wheel. So the wheel can no longer turn around the vertical axis, until the damper is compressed again, and the top of the rod comes out of the notch.

Close-up view of the top of the mechanism

So when the plane takes off, the nose-wheel aligns itself with the mainframe, disengages from the rudder pedals and locks itself in that position. Upon landing, the damper is compressed, the wheel unlocks and can be used to control the plane.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.