The spinner on the front of your airplane is there to do more than just look pointy—it’s also important for drag reduction and cooling airflow. A spinner is also typically made of fairly lightweight material, so it has to be mounted carefully to avoid crack-inducing stress and vibration. Therefore, it’s important to mount the spinner to the propeller as accurately as possible.
Ideally the spinner should run perfectly true with absolutely no wobble, or as machinists say, “zero runout.” In practice, you just want to minimize the amount of runout by being as careful as possible when you are measuring and drilling. For the spinner on my RV project, I decided to shoot for ±0.030 inch or better, which is an ambitious goal.
My RV, like many airplanes, has an aluminum spinner backplate installed behind the propeller, plus an additional aluminum bulkhead in front of the prop hub. The spinner itself is then attached to the backplate and front bulkhead with screws and nut plates. Getting the spinner aligned accurately and locating the holes precisely presents a problem to be solved. To accomplish this, I devised my own approach to fitting a spinner. My application is for a Hartzell constant-speed prop mounted on a Lycoming engine, but the technique should work for any airplane project.
Building a Prop Rotisserie
What’s needed is a way to mount the prop to the workbench that allows it to rotate, just like it will when attached to the engine. The first step is to make a circular plate out of wood that you can mount the prop to. First I marked a center point on a piece of 3/4-inch plywood and laid out six holes spaced every 60° around a 4.75-inch-diameter circle, matching the Hartzell K/R prop hub mounting bolt pattern. Then I turned it into a 13-inch-diameter circle using a router and circle jig. This part doesn’t have to be perfectly circular, so you could use a jigsaw or a band saw; I just happened to have this tool on hand. Last, using a drill bit through the center as a sort of axle, I used my drill press to make six 5/8-inch holes for the prop bolts. The actual bolts are ½ inch, so this gives some wiggle room, which becomes important later.
I then mounted the round plate with short wood screws to a swivel bearing that I sourced from my local hardware emporium. This is the same sort of bearing you’d use to install a Lazy Susan in your kitchen. I did my best to align the center of the plate with the bearing’s axis of rotation, which I achieved by lining up the four screw holes with reference lines that I’d previously marked through the center point.
Next I grabbed another piece of plywood from the scrap pile. This was around 18 inches by 5 feet, but the exact size isn’t strictly important. In this I cut a hole whose diameter was roughly the same as the open part of the swivel bearing, then I attached the swivel bearing to it with more wood screws. I clamped the resulting contraption to the bench with the bottom hole halfway hanging over the edge and then bolted the propeller to the top plate. Voilà, a prop rotisserie.
The prop is secured to the rotating plate from underneath using hardware-store nuts and washers. You don’t get full thread engagement through the nuts, but it doesn’t matter for this application.
Important note: Obviously with your expensive propeller hanging partway off the edge of the table, you want to be sure the jig is firmly secured in place! I wouldn’t suggest using it as a chin-up bar either.
I know what you’re thinking—a pile of wood and a Lazy Susan bearing from the hardware store—how accurate can this really be? Well, as it turns out, it’s plenty accurate enough for this application. The bearing I bought is rated for a much heavier load than what the propeller weighs, so it turns very smoothly. There’s also no side loading on the bearing, so I found that it turns without a noticeable amount of slop.
Using the dial indicator in a magnetic holder to center the prop in the jig (right). The machined prop dome provides a known reference surface, and the mounting of the prop to the jig is tweaked until it runs as true as possible.
Anyway, we’re not totally relying on the accuracy of the jig. Instead, what I did was to iteratively tap the prop this way and that until I had it running as true as I could get it. It may or may not be perfectly centered on the plate, but what’s important is that it’s centered on the bearing’s axis of rotation. To do this I used a dial indicator on a long arm, which happens to also be my engine hoist.
Again, I know what you’re thinking, especially if you’re a real machinist—a wobbly Harbor Freight engine hoist is a terrible indicator stand! Well, yes, I agree with you, but for my purposes here it’s good enough. If you look closely at the photo, you’ll see that I used a couple of bungee cords to take out most of the slop, which helps quite a bit. If you’re careful and you avoid bumping it while you’re measuring, it does a good enough job. The steel engine hoist also provides a convenient place to attach a magnetic indicator holder.
Checking for Runout
I used the base of the constant-speed prop dome as my reference surface, on the assumption that it should be concentric with the prop flange and also smooth on account of the way it’s manufactured. If your prop doesn’t have a feature like this, you can use any circular part of the hub, such as the inner diameter of the center hole.
By carefully rotating the prop without applying any side load—use one hand on each blade—you can find the highest spot, which will be the farthest from center. Then you give the base of the prop a couple gentle taps with a mallet and a wood block, shifting the high spot toward the center. Repeat until you have as little runout as possible and tighten the bolts. In my case I got it to less than ±0.010 inch, which is basically within the measurement error of this Rube Goldberg setup.
Once I had the prop running true in the jig, the next step was to use the indicator to get the front spinner bulkhead and backplate running true as well. (Obviously you have to have the backplate mounted before you attach the prop to the jig or else you’ll have to take it apart and align it all over again!) I did this by indicating off the backplate, with the indicator affixed to the workbench. I stuck my magnetic stand to a steel clamp that I ran through a hole in my table, but you can mount the dial indicator any way you like as long as it’s solidly attached and allows the prop to be rotated on the rotisserie.
My backplate in particular had some surface variation that made it tedious to get centered, but I just did my best. I tapped it here and there until it was as centered as I could get it, then I torqued the mounting bolts. For the front plate I found the most effective approach was to try different orientations until I found the one that made it run the truest, then I marked that so I’d know how it should be clocked when removing and reinstalling it.
During this process I also did a test fit of the spinner so I’d know where to position the front bulkhead. In my case I needed two regular AN960-416 washers between the bulkhead and the prop to make the spinner fit correctly, plus another regular washer and a -416L under the bolt heads. It’s all iterative; I just took my time and measured a lot.
The astute reader might be asking at this point: Couldn’t you do all this with the prop mounted on the airplane? Why bother with this rotating jig thing?
Well, there are a couple of good reasons. First, a rigid jig clamped to a sturdy workbench provides a more solid base for accurate measurement. And second, with the prop pointing upward you can use gravity to your advantage rather than having it fight you.
Locating the Mounting Screws
Once I was sure I had the front bulkhead and backplate aligned as accurately as possible, I needed to mark the spinner for the mounting screws. I drew a circle the diameter of the spinner on a piece of butcher paper and marked a center reference line. I then marked the locations of the seven screw holes on each side of the base of the spinner, with the end hole on each side being 3/8 inch in from the flanged blade cutouts. I used a pair of dividers to make sure I had identical spacing between every hole, and I also made sure that the center hole on each side hit the reference line exactly. Then, without moving the spinner, I also marked the six forward holes. The result is that that the middle screws on each side of the spinner are exactly lined up back-to-front, which is not strictly necessary but does look nice. At this point I was only marking the hole locations in terms of their angular position, not their position fore-and-aft.
To mark the forward hole locations, I used a large plastic drafting triangle. It’s nice to have a second pair of hands to hold the bottom of the triangle against the spinner, but if you are working solo you can clamp a board (I used a long level) to the table instead.
The trickiest thing about marking the spinner hole locations is finding the correct distance to the forward screws. With some fiberglass spinners you can predrill the holes in the bulkhead and then shine a bright light through from the back, but you can’t do that if your spinner is opaque. If your fiberglass spinner is gel-coated, or aluminum like mine, you’re out of luck unless you have X-ray vision. Fortunately, everything is better with lasers!
With the prop still in the jig, and verified as being level to the floor, I set up my laser level and projected a horizontal line onto the middle of the front bulkhead. Then I carefully put the spinner in place on the prop and marked the spot where the horizontal line crossed one of the previously marked locating lines. The result should be the correct location of one screw hole.
The laser setup was a bit too wobbly for me to trust it for marking multiple holes precisely, so instead I transferred the established distance between the laser mark and the base onto a scrap piece of aluminum. Into the edge I filed a notch to provide an accurate place for the tip of the marker to land, then I used it to mark the locations for all six forward screws. The rear holes were much easier—I just used a combination square set to the appropriate distance.
After taking a deep breath, I drilled #40 pilot holes through the spinner for all 20 screws. Then I clamped the base of the spinner to the prop backplate and fiddled with the fit until I had it running as true as I could possibly measure. Last, I match drilled into the front bulkhead and backplate, thus fixing the fit of the spinner to the prop.
The Final Fitting
The next step was to mount the prop on the engine for final fitting. I unbolted the prop from the jig, hoisted it up to the crankshaft and threaded in the six bolts. A prop sling is helpful if you’re working by yourself—a propeller is heavy!
With the prop bolted to the engine and the spark plugs removed, I turned it through a few revolutions and used the dial indicator to verify that the prop dome runs as straight when mounted on the engine as it did when it was mounted in the jig. The measurements become more sloppy at this point—it’s hard to spin the prop without inducing error when the airplane is sitting on rubber tires and the engine is on rubber mounts—but I did satisfy myself that the jig had been an excellent substitute for having the prop mounted on the actual engine. I Clecoed the spinner in place and checked the runout again, with good results. Everything was just as I had measured it in the jig, just with noisier measurements due to the more flexible setup.
From there, the process of enlarging the holes to final size and installing nut plates follows the usual pattern. Finally, with the spinner completely fitted and affixed with the proper screws, I made one more measurement of the final runout figure. I ended up with a runout of approximately ±0.015 inch, which is just about at the limit of my ability to measure anything at all. So given the inherent inaccuracy involved in this measurement, I’m going to call that basically dead nuts on. Hopefully this will result in a long-lived spinner with no cracks.
