By Chris Brainard • Originally published in Stunt News, July/August 2009
Editor’s note: This article was originally published in Stunt News and has been reformatted for digital readability. Original terminology, measurements, and technical details have been preserved where possible.
I love building model airplanes—at least most of the time. Although there are certain portions of the building process which have enhanced my reputation for procrastination. One of these that used to drive me crazy was installing spinner rings. I tried a bunch of different methods, but could never really find one which consistently gave excellent results. Perhaps you have faced this same frustration. If so, maybe the solution that worked for me will work for you too.
The difficulty in locating a spinner ring properly is that we are trying to align it in a plane defined by three different axis. 1. Aligning the radial centerlines of the spinner ring with the radial centerlines of the crankshaft prop mounting stud, 2. Aligning the face of the spinner ring perpendicular to the longitudinal centerline of the prop mounting stud, and 3. Locating it the correct distance from the spinner back plate for proper clearance.
The solution which worked for me was to design a jig (affectionately referred to as Chris’s Spinner Ring Jig) which bolts to the engine and automatically aligns it in all three axes. To use it, the engine is mounted in the airplane to the crutch in what will be its final location (complete with whatever down/right thrust is desired) and the jig (with the spinner ring temporarily mounted) bolted onto the prop mounting stud.
This will place the spinner ring in its proper position, making it easy to trim the fuse sides or fit a filler block up to the ring. Then it is simply glued in place and once the glue has cured, the prop nut removed, the ring “popped” loose from the alignment jig and the jig slipped off the end of the prop mounting stud.
My preferred method is to fit the sides leaving a gap of a few thousandths of an inch, remove the jig/spinner ring, mix a batch of epoxy/micro balloons, apply it and then bolt the jig/spinner ring back on. I leave the slight gap because if the initial fit is too tight, chances are that it will spring back slightly and lose the alignment accuracy once the prop nut is loosened.
There are a several things to note about this jig. 1. The normal thicknesses of plywood chosen for spinner rings have no effect on them working properly. 2. The inside and outside diameters of the spinner ring must be concentric. 3. The inside diameter of the spinner ring (1 inch) must be accurate and a snug fit to the raised alignment lip on the jig. 4. This jig is set up for spinners with flat back plates. If you use a back plate which is recessed for the prop drive washer, the recess that the prop drive washer contacts will have to be recessed by whatever the offset is plus 1/32 inch for proper clearance between the spinner ring and spinner back plate. 5. Through experimentation, I have settled on 1/32 inch initial gap between the spinner ring and back plate. This gap will become smaller with the application of a finish. I prefer to err on the side of too small a gap, since it is easy to shim the back plate away from the drive washer. 6. I believe a realistic range of spinner rings this jig will accommodate to be from 1 1/2 inches to 2 1/2 inches. Smaller diameters won’t have enough “meat” left in the ring. Larger diameters may not have enough support at the outside to keep them flat.
My first spinner ring jig was carefully built from multiple pieces of plywood and worked very well. The biggest problem was that over time it became damaged when inadvertent drops of glue would stick it to the spinner ring, and when pried apart, chunks of the jig would end up stuck to the spinner ring (or chunks of the ring stuck to the jig). Later I purchased a small metal lathe from Harbor Freight and made one out of aluminum, eliminating that problem. Although this jig is set up for a 1/4-inch-diameter prop mounting stud, it can be drilled for whatever size is required or, better yet, machined to accommodate various bushings for different prop stud diameters. If the recess for the prop drive washer is machined deeper, these bushings could be made with a head of various heights to allow spinners with recessed back plates to be mounted.
After trying various thicknesses of ply for making spinner rings, I’ve settled on using 1/8-inch aircraft (five layers) ply. I use a 1-inch-diameter Forstner drill bit for the 1-inch-I.D. hole. I drill this hole first and then rough cut the outside diameter. A stack of eight to 10 of these is mounted on a mandrel and the whole bunch clamped tightly together and machined to the final outside diameter. The 1/8-inch aircraft ply may seem like overkill, but after the fuselage is faired into the spinner ring, I use a Dremel hand tool fitted with a 1/2-inch-diameter sanding drum and sand out the inside of the spinner ring—usually until there is only about 3/16 inch left, except at the bottom where I leave it full depth. I leave the full depth at the bottom because lately I’ve been mounting my cowls with magnets and by using the 1/8-inch ply and leaving extra material at the bottom, I have room for a pin to key the cowl to the spinner ring. Once I’ve finished sanding it to shape, the inside of the cowl ring represents a large “D” with the flat part of the “D” on the bottom. I’ve included a picture of how this looks on my latest Caprice. If this isn’t a feature you need, the bottom can be sanded out also to remove extra weight.
This article first appeared in the July/August 2009 issue of Stunt News and has been preserved as part of the PAMPA digital archive project.
