Monday, March 2, 2009

How to Carve a Propeller -- Part 1

A search of the internet will turn up half a dozen How-To articles for making a propeller. What follows is a series of articles explaining how I make my props. My method isn't any better or worse than most of the others; you'll end up with about the same propeller. I'm offering my method because I've found it works a bit better when it comes to teaching folks how to build a prop. I'm not sure why that is so. Perhaps it gives you more confidence or something. In either case, take advantage of those other How-To articles; read them carefully. But keep your money in your jeans until you've given my method a fair trial.

This propeller is for the Chugger's engine. Chugger is a draggy, single-place, all-wood, strut-braced parasol. Chugger's engine is a big-bore stroker based on after-market VW components. The crankshaft is a Chinese import having a stroke of 84mm (stock is 69mm), fitted with 94mm pistons. The stock camshaft is retained but is retarded between -4 and -7 degrees, moving the engine's torque curve to a point well below 3000 rpm. (The stock VW engine had to be able to get a vehicle weighing more than 3,000 lbs under-way from a dead stop. Even with a box of gears that takes a fair amount of torque.)

A high-torque, low-rpm engine requires a prop with a lot of blade area and a diameter of up to 68". This particular propeller has a diameter of 62" with an aerodynamic pitch of 34". One blade of this prop is shown in the illustration above. (Double-click for a larger image.) The illustration is also in the File archive of the Chuggers-Alt Group where it is in DeltaCAD's native formate. If you have DeltaCAD -- they offer a 30-day trial copy for free -- you will be able to print-out the pattern shown above then glue it to a piece of plywood or poster-board.

In the illustration (above) you can see that the propeller blade has been divided into ten segments or Stations, each 3.1" wide. The propeller will use a Clark-Y airfoil, which happens to be flat on the bottom. After laying-out your glued-up blank, the first step in making the prop is to carve that lower surface. Protractors for each Station have been provided in the Files Archive. These are used to verify that the bottom surface of the prop has the proper angle.

In the lower part of the illustration to the RIGHT the WIDTH of the propeller at each Station has been measured. In the upper part of the illustration the geometric pitch for each station has been laid out by dividing the Aerodynamic Pitch in inches (ie, 34) by 6.28 (ie two pi ).

If you haven't obtained a copy of DeltaCAD, which would allow you to use the templates in the Files Archive, by using the dimensions from the two illustration you have enough information to manually lay-out & draw the blade pattern onto 1/4" ply or similar. Take your time finishing the pattern since it will be reproduced in the blades of the propeller. When you are satisfied with your workmanship, mark it in the hub area: 62x34 and give it a coat of varnish.

The horizontal lines in the upper part of the illustration represent the laminations (ie, 3/4"). For this first prop I want you to use full-length laminations. That is, four pieces of 1x6 birch, which is available from most Home Depot stores. (But if this is your first-ever attempt to carve a prop, use 1x6 pine shelving instead of hardwood. Don't worry; it won't go to waste.) In later props you will see that the upper-most three laminations do not need to be run full-length. By using shorter pieces for the upper-most three laminations you can reduce the cost of the wood by almost half. However, when you have a blank that uses partial-length laminations it can be very difficult to carve the lower surface. Here's why:

The lower surface is flat but twisted. You can see that from the different angle at each Station. Normally, we carve the lower surface by using a back-saw to make a reference cut at each Station. The reference cut starts out even with the trailing edge but comes out on the leading edge at some point above the trailing edge. That means the cut is at an angle to the horizontal. If you'll look at the upper part of the illustration above you will see a dotted red line that represents where the lower surface comes out on the upper surface edge of the prop blank. Study that for a minute and you will see that in full-span blank -- one in which all four of the laminations are a full 62" in length, to carve the lower surface we must remove about half of the wood in the prop-blank.

Removing wood isn't difficult; we cut down into the prop-blank with our back-saw not just at every station, but about every inch (!). Then we chunk it out with a mallet and chisel. When compared to a prop-blank that uses laminations of different lengths, this method adds to the work -- although that isn't much of a consideration since we aren't punching a clock... but it also adds to the cost, which can be a major consideration for the lo-buck builder. That's why I'll be showing you how to make-up a blank in which only one lamination is full-length. The other three laminations are only partial-length. I think this is worth mentioning now because a prop-blank that uses full-length laminations is so easy to work with that it may give you the wrong idea about making props. When using a prop-blank made up of partial-length laminations you're going to find that just holding the thing down can be a major problem.

Among the patterns and templates in the Propeller File in the archives of the Chuggers-Alt Group, I have included Airfoil Patterns, Lower Surface Protractors, and 'Chunking' Templates. By applying the templates to your prop-blank, most of the work will be taken care of when you chunk-out the blank.

Remember now, I'm uploading all this stuff to the Files Archive of the Chuggers_Alt Group. DeltaCAD will print them out full-scale. You then cut them out and spray-glue them to a suitable substrait. For the protractors you want something fairly substantial, like 3/4" stock. For the airfoil patterns you want something thin but rigid. Back in the Day we used shim brass, typically between .006" and .010" but nowadays beer-can stock is generally used.

The third illustration in the group above shows the lines needed to chunk-out the upper surface of the propeller blade. As with the lower surface, the upper camber is twisted. You want to keep that in mind as you do your chunking.

When you get done with chunking out the upper surface you'll be left with a lot of corners. That's when you reach for your block plane, draw-knife or an angle-head grinder an angle-head grinder fitted with an #80 grit sanding disk.

The tricky bit here is that any of the above tools leaves a stripe. That is, a flat spot. The more you work at it, the more stripes you'll have and the skinnier they'll be. Rub a piece of chalk on the cambered surface of your gauge, fit it to the matching Station an the chalk should transfer to the 'corners' of the strips. What you want to do is get your stripes down to a width of about 1/8". When you do that, trial-testing with the template leaves a kind of dotted line that makes it easy to see where some strips are a bit too wide, meaning they need a bit more work.

The final finishing involves little more than working your way down through three progressively finer grits of sandpaper, using an orbital-type block sander. For the leading edge, you want a perfectly faired surface. I've found the handiest way to get it is to simply fold some #120 paper in the palm of my hand and work it down until my eye tells me to stop.

The Stations nearest the hub DO NOT follow the rules which apply to the Stations from #3 through #10. These inner-most Stations do not contribute thrust nor cooling, since they are obscured by the spinner. In fact, about 50% of your thrust is produced by the blade of the prop beyond about Station 6. But the inner-most Stations are vital with regard to the structural strength of your prop.

Stations 1 and 2 are simply nice, fat streamlines, with as much camber on the lower surface as on the top. We can do a bit of chunking here but it's mostly an eye-ball job.


3 March 2009

I haven't uploaded any of these drawings to the Chuggers_Alt Group. The truth is, I've still got about two dozen drawings to do and the airfoils are especially pesky. I also want to include some photos but I've got a pretty full plate. I'm going to go ahead and post this to my Blog but keep in mind that it isn't complete as yet.

-Bob Hoover


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Anonymous said...

Kinda weird to see only one comment, and that's by a non human entity, apparently.

Prop stuff is kind of interesting. Is there a reason to use a chisel instead of a drawknife? I suppose the chisel is cheaper, but when I carved an oar using a drawknife I was really impressed. I made a decent looking oar for the first time in a couple of hours. Too bad it warped; next time I should cut the blank down the middle, turn over one side and glue. OTOH, I couldn't get a spokeshave to do anything useful.