Aircraft Thimbles and Tubes
In this part we continue with cable rigging and push-pull tubes. Many aircraft control systems in experimental and certified aircraft use cables or push-pull tubes to move the flight surfaces. These provide a simple and reliable way of operating the controls.
Push-pull rods also have the advantage of not having any slack or play in the controls, this gives the pilot a better 'feel' of what the aircraft is doing.
And the big advantage is that these rods will not get thinner and break as flexible cabling might do after several thousands of movements day in day out. So the primary choice for the aircraft designers should be rods for the major control surfaces like aileron and elevators and cabling for trim and rudder surfaces.
Thimbles and tubes
Strength is determined by the number of wires and their arrangement. The cable cross section can be seen above. The cables are classified by the number of wires and arrangement:
1x7 cable strand, 1x19
1x19 cable strand, 7x7 of even 7x19
7x19 cable strand.
|Flexible, carbon||Flexible, corrosion resisting|
|Dia, inch||Weight, lbs per 100 ft||Breaking strength, lbs||Weight, lbs per 100 ft||Breaking strength, lbs|
The following are instructions for installing thimbles on the ends of the cables for rigging Elevator, etc. The instructions show a standard installation procedure for a 5/32" cable. The procedure is the same for the different size cables also for attaching cables to turnbuckles.
Put a 5/32" nico on the end of a 5/32" cable. Pass the end of the 5/32" cable through the WT-06 Tang and over the 5/32" thimble. (Figure 1.35.2). Thread the cable back through the nico (slide nico tight against thimble) and `press' the nico, equally spaced, twice.
Cut a length of heat shrink tubing and place it over the two cable ends close to the nico. Cut off excess cable and shrink the heat shrink tubing.
The heat shrink tube is to protect you from cutting your hands on the sharp edges of the cable.
Push Pull Tube Fabrication
The following is a standard practice for manufacturing the push pull tubes used throughout the aircraft. The example shown is for the 1/4" (HM-4M) Rod End Bearing on 3/4" x .035 tubing. Other combinations are:
- 1/4" HM-4M Rod End Bearing, AN316-4 Jam Nut and CC-29 End Plug for 3/4" x .035 tubing
- 1/4" HM-4M Rod End Bearing, AN316-4 Jam Nut and CC-28 End Plug for 1" x .058 tubing
- 5/16" HM-5M Rod End Bearing, AN316-5 Jam Nut and CC-30 End Plug for 1" x .058 tubing
The length of the tubing will be determined by the distance between the two Rod End Bearings. Figure the total length between centers and subtract the head of the end plugs from both ends and the length of the End Bearings from the center of the hole back to the jam nut with three to five threads showing at the top of the bearing. Cut the tube.
Debur the ends of the tubing and insert the appropriate CC End Plug. On the tube draw a line 3/8" in from the ends and layout an equally spaced rivet pattern for four (4) rivets. Drill #30 holes. Remove the End Plugs. Debur, zinc chromate and re-install the parts and rivet together using some 1/8" x 1/4" SS rivets.
Shown above is a possible push pull tube construction found in many types of kit aircraft, you will find different sizes of push pull tubes depending on aircraft manufacturer and application it is used for.