The Electric Starter and Housing, Part 1


I remember well from my days as a T-37 student the thrill of cranking up the Continental J-69-T-25 engine for the first time. The J-69 was an ancient engine which bears a remarkeable similarity to our model turbojets... it was definitely equipped with a radial compressor. I'm not too sure of the turbine section, but I'd like to believe it was axial-flow. Any T-37 IP's want to correct me? The engine was equiped with an electric start motor, and with the exception of the lack of propane, startup was again like a model turbojet. The engine was motored, and at a certain percentage, the fuel flow was started, ignited, and the starter left engaged to another, higher percentage to keep the start cool. Sadly, due to the radial compressor, spoolup of the J-69 was agonizingly slow, and many a student and IP were killed by stall accidents in the pattern, with the engine unable to spool up in time to provide saving thrust.

On THAT unhappy note, here is the serialization of the electric starter for the MW-54...

The motor is a Speed 300. The Bendix-style actuator plans are from the GTBA members web site, courtesy of Mike Murphy. I modified the mechanism only very slightly to accomodate my Imperial tooling. Getting the spindle to extend and retract properly was tricky, and the internal spring must be "just right". Don't make the fits too tight!

 

Rather than having the motor hanging out exposed, I wanted it concealed in a bullet nose like the commercial engine starters. The diameter chosen was 1.125", leaving a wall of .084". This was just adequate to allow a 32 TPI thread for the nose itself to seal the Speed-300 inside.

The bar is turned to size, and the interior bored to accept a nice sliding fit of the motor and spindle assembly.

 

The front of the housing is relieved and screw-cut 32 TPI.

 

Another scrap of aluminum is bored for the hemispherical nose, leaving a shoulder in place in the interior to press on the motor case to secure, and relieved for the motor wires. The interior is also screwcut to fit to the housing.

 

Threaded in place, I elected to apply a very fine coin knurl to the end cap. Using a simple computer program, I generated a series of diameters to cut so as to create a rough hemisphere with a parting tool. This is then smoothed with a file and wet-dry paper.

Layed out, the case, with its 3 milled support slots, the end cap, and the motor/spindle assembly.
All we need now are the support arms, see Part 2!

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