Accessory Stanchions


This is a totally custom part which will have use in the final turboprop assembly. Much earlier, I had described my goals for the turboprop; among them was a contained, modular design, with as many accessories as possible mounted on the turbine rather than being plumbed "loose". I know the latter is more useful for actual aircraft mounting, but if carefully done, a lot of the dangling goodies needed for turboprop operation can be secured. One of the better places to do so is in a screened intake area near the gas gennie compressor. Mike Murphy did a superb job of this on one of his prototypes, and I want to replicate it. Another aspect of mounting is dealing with the tremendous forces this engine will generate at high throttle settings... I need to carefully analyze the plans and engineer a mount which will suffice. My preferance is in a set of detacheable side rails, which need to be secured to both the gas gen section and the gearbox section, in such a manner as to distribute the thrust loads at suitable points. These side rails can also carry accessories such as external oil sumps, pumps, and lube lines.

For the screened area near the intake, a means of cleanly attaching both a screened intake "manifold", and any other accessories, was devised with the results shown here. The MW54 front cover bolt circle used is the same one which carries the service ports. There are at least 4 sections of this circle which can accept this type of stanchion, and onto these posts can be attached tach sensors, starter motor supports, and eventually the screened manifold.

 

  There is no easy way to make a solid, elegant shape. I decided early to do a good job of it, and create stanchions which are very solid, and which stand the scrutiny of other model engineers. Please remember that I am a model engineer first, and an RC pilot second, and that while this may seem to be overkill, it gladdens the heart of any engineer to see well machined aluminum rather than folded sheet steel!

To start, a 1" long tube of aluminum was turned. The rim you see here will rest perfectly in the space on the front of the Wren engine, between the delrin intake cone, and the sloping, 45 degree cut in the front cover. The last picture will show this concept perfectly.

  Once the tube was turned and bored, it was clamped in the vertical mill, and 11 holes were drilled which perfectly match the 11 in the front cover of the MW54. Like all bolt-circle exercises, I derive the X, Y coordinates of the holes with a simple PC program, which prints a diagram with coordinates to 0.0001". These are rounded to 0.0005", the limits of the DRO on my mill. The DRO (Digital Read Out) of course, is what makes this all possible.

The holes were drilled to clear the M2.5mm SHCS used to secure the front cover to the diffuser.

Back in the lathe, the ring is parted, and deburred.
Stanchion number one will be used to carry one of my tachometer transducers, which is a potted magnetoresistive sensor in a 1"/25.4mm long by 0.25"/6.35mm dia. aluminum tube. The perpendicular hole for the tube can be seen here. With the ring mounted on a 3-jaw chuck in the mill, hogging cuts were taken to remove the aluminum down to a 0.125" thick base. Two of the mounting holes will be used to secure this stanchion.
The segment of the ring is cut from the remainder, deburred, and generally ceaned up. Just visible on top is a 6-32 tapped hole for a set screw to secure the tach probe. Be sure to loctite any fasteners which have even the remotest chance of vibrating free and being sucked into the turbine!
I'll bet you were wondering how it fits... here it is, snuggled nicely onto the front of the gas gen. All that remains is to bore a 1/4" hole in the cone. The tach sensor needs to be ~6mm closer to the nut for reliable sensing. Other portions of the big ring can be similarly shaped for supports (as needed) along the service port bolt circle.

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