P-51 Mustang - Tailwheel Door System

11 May 07 - Animation added of motion.

As I have been asked a couple of times about the tailwheel door system for my P-51 Mustang, I decided that it was time I add some more information about it. This tailwheel leg is a Robart Micro Mechanical Nose Leg Retract (part #124). The easiest way is to include a number of pictures to illustrate the system rather than explain it by words.

This picture shows the tail wheel mechanism down and the steering linkage on the lower side of the aperture. The door micro-switch arm can be seen to the left of the shot. The 2 screws (#2 x ") secure the plate the micro-switch is fitted to. This allows the micro-switch to be removed if necessary for repairs.
This picture again shows the tail wheel mechanism down, but this time includes the retract snake on the upper side of the shot. The micro-switch is again visible at the bottom and is positioned so the middle of the wheel hits the end of the micro-switch arm.
This shows the wheel in the retracted position, before the doors have moved. The hinge line of the doors is the outer edge on the surface of the lower fuselage skin. I have used Diamond tape to hinge them and it seems last well.
This view is taken looking forward and shows the cables that operate the doors. The control horns are cut from a sheet of glass-fibre tissue made on a piece of glass. The approximate horn thickness is 0.5mm (0.02") and the cable holes are drilled equidistant from the hinge line. For simplicity, the cables have had a knot tied in the end and a drop of cyanoacrylate adhesive applied to stop it working undone. The cables are the pulled through the horns from the rear.
The two cables (rigging cord), shown at the top of the photograph and red on the sketch, run through a 12mm (") wide wire guide and then forward to one servo arm. The other cables, shown green, run through a narrow (1mm (1/32")) wire guide and forward via the wide guide. The wire guides act as pulleys changing the direction of the cables.
Now animated

This shows the Hitec HS-77 retract servo (top left) with connections to the tailwheel running aft (right) and main wheel valve further forward. Also shown is the Union Micro UM-9G (red-orange colour) tailwheel door servo and the cables running aft. The servo is sited just in front of the main wing trailing edge. This allows some stretch in the cables to take up any alignment problems in the system. The servo is driven by a simple switched servo driver that I built, which moves the servo between end points (adjustable) when the micro-switch is operated / released.

I tied a loop, at about the correct length, into the cables at the front and fixed them with a drop of cyanoacrylate. I then bent up some brass hooks and attached them to the servo using control clamps as shown. At each end point, the working cable (i.e. the one holding the door in position) should be reasonably taut, but the other should be slightly loose if possible (as can be seen in the picture - left is slack, right is taut).
An alternative method of attaching the cables is shown in the sketch to the left. A piece of wire is bent in a shape like that shown in grey in the sketch, and the Z bend fed through the servo arm. The 'kink 'in the middle can be opened or closed to adjust the tension.

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