Carve Your Own Propellers

Wooden propeller in a workshop, held in hand against a workbench.

Free Flight Sport

As seen in the December 2024 issue of Model Aviation.

HAND CARVING YOUR OWN PROPELLERS is one of the most satisfying aspects of Free Flight, and carved propellers offer many benefits that are unobtainable with the manufactured variety. The biggest reason to carve your own propellers is that most manufactured propellers are underpitched for the best performance. Also consider that by carving, you’ll have complete control over every aspect of the creation process, from diameter to pitch distribution to blade width, shape, and thickness.

Diagram of a carved prop block layout with measurements.

Basic Propeller Geometry

What is pitch? The answer is simple. Pitch is the distance that a propeller will theoretically travel forward in one revolution. Pitch is easily calculated at any point along the radius of a propeller using the blade angle at that point. Instead of analyzing the math, you should instead internalize some basic concepts:

Rubber-powered models give the best duration with propellers that have pitch-to-diameter (P/D) ratios between 1.1 and 2.0. The low end is optimal for Outdoor models; the high end, between 1.5 and 2.0, is only appropriate for the very lightest Indoor rubber-powered models.
For Outdoor models, aim for P/D ratios between 1.1 and 1.4.
Faster-flying models and those with less drag, such as streamlined monoplanes, can generally handle a bit more pitch than slow-flying, draggy Scale subjects such as multiplanes, radials, and seaplanes.
Measuring pitch near the blade root or near the tips is misleading. A better average measurement of pitch can be found near the midpoint of the diameter.
Regarding blade shape, the maximum chord should occur at approximately the 60% radius point. The blade chord at the tips should taper to approximately 50% to 75% of the maximum chord.
Models with larger cross-section motors can handle more pitch than those with motors of a lower cross-section.
Wider propeller blades mimic the effect of greater pitch; more rubber cross-section will be required to effectively produce thrust from these propellers, but stubbier blades are also less efficient.
Longer-diameter, skinnier propellers are more aerodynamically efficient. The lesson here is to strive for a greater diameter instead of a greater blade chord where practical.

The process of carving a propeller is straightforward. It just involves a series of interrelated steps. Certain steps do require special tools (a drill press, a scroll saw, or a band saw), but all that’s required for carving and shaping is a sharp pocketknife and three or four grades of sandpaper. I use a $20 folding pocketknife with a non-serrated, slightly curved 3-inch blade. Some experts argue that a proper woodcarving knife is better, but I think it’s overkill. Balsa is, after all, a very soft wood, so almost any sharp knife will do.

Eight Steps to a Perfect Propeller

Step 1: Calculating the Block Geometry

The following formula is the only one that you need for calculating block geometry.

P = Π × D × T ÷ W

Where:

P = Pitch

Π = 3.14

D = Diameter at the pitch measurement point∗

T = Block thickness at the pitch measurement point

W = Block width at the pitch measurement point

∗Typically, the diameter at the pitch measurement point will be 50% to 65% of the finished propeller diameter. You can calculate pitch at infinite other points along the diameter as well.

You should settle on a finished diameter and blade chord before calculating your block dimensions. For smaller rubber model propellers in the diameter range of 6 inches to 9 inches, a good value for W is .20D to .14D. For larger-diameter propellers, 9 inches and above, use .15D to .12D.

Step 2: Marking the Blank

First and most importantly, choose a straight-grained block of balsa of uniform density. I have found medium-density balsa (approximately 8 to 12 pounds per cu. ft.) to be excellent for Outdoor propellers.

You might want to choose lighter wood for pusher/canard propellers or for Indoor models. Use a fine marker, a straightedge, and a tri-square to mark your blank. Make certain to achieve a mirror image on both blades. The slightest mistake here can result in a wobbly, unbalanced mess when power is applied to your propeller for the first time.

Table of recommended blade thickness by diameter.

Step 3: Drilling the Blank

Be sure to drill the blank before you begin carving. Use a drill press to get a perfect right angle.

Step 4: Sawing the Blank

A band saw is ideal for this operation, but a scroll saw will also work for propeller blanks up to an approximate 12-inch diameter. It’s usually best to cut the top view first, followed by the side profile.

Step 5: Hand Carving

Hand carving balsa is not at all difficult. If anything, it is too easy. That is, mistakes are easy to make because balsa is so soft and easy to cut. The best advice that I can give a beginner carver is to work slowly. Peel off very thin shavings at first. It is more time consuming to carve a propeller this way, but it will give you more practice in controlling the blade. A sharp knife makes this infinitely easier. If your knife is dull, it’ll be much harder to control.

Rough-carve the backs (undersides) of both blades first. Strive to carve them with a flat bottom; undercamber can be added later if it’s desired. It is much easier to add undercamber by sanding anyway.

Wooden propeller on a perforated workbench.

The back side of each blade is sanded first after rough-carving it with a knife.

Hand holding a wooden propeller in a workshop.

The front sides are finished and the final blade shape is added. The blade shape is up to you!

Next, carve the blade fronts. While carving, take care to control the blade thickness (refer to Table 1 on the previous page). Try to carve the blades to a thickness slightly greater than the numbers quoted; you can finetune the thickness with sandpaper.

Step 6: Marking and Cutting the Final Blade Shape

Sketch and cut from cardstock a template of your final blade shape. Using this template, mark the backside of the propeller blank and cut or sand the blades to their final shape.

Step 7: Sanding

You’ll need at least three grades of sandpaper to do this right: 100-grit, 150-grit, and 220-grit. Work the back of the blades first with the 100-grit wrapped around a cylindrical object such as a scrap piece of 1- to 2-inch diameter piping. With a cylinder sander, you can easily add undercamber to the blades if you wish. Experts disagree whether undercamber gives a thrust advantage or not (the difference is probably negligible). What is important is that you at least flatten the undersides of the blades. When you’ve fine-sanded with 150-grit then 220-grit, you’re finished with the backs.

Next, work on the blade fronts. Here, a flat, narrow sanding block is best. I like one of an approximate 1-inch width for best precision and control, but use what you’re most comfortable with. Check the blade thicknesses periodically with a dial caliper or micrometer (eyeballing is not nearly as accurate). Make sure the propeller is statically balanced, and then finish-sand with 150-grit and 220-grit sandpaper.

Step 8: Finishing

Depending on the weight of your model and its flying speed, you might need to strengthen the blades further with an application of fiberglass cloth. For smaller propellers (less than 8 inches) and for Indoor models, you won’t need any extra strengthening.

For larger outdoor propellers, I like to cover them with 3/4- to 1-ounce fiberglass cloth at a 45° bias, front and back. I do this by first doping the cloth to the blades then trimming the outline slightly oversize (about 1/16 inch) with scissors. You could add several more coats of dope at this point and finish-sand with 400-grit sandpaper and you’d be finished, but I prefer an extra step that I believe produces a stronger propeller that is also more attractive.

Here’s what I do: After the first coat of dope is well dried, I smear on a very thin coat of high-quality, slow-cure epoxy resin, such as West System 105 or 205. This epoxy increases the weight by a gram or so but adds strength and fills the pores of the fiberglass cloth nicely.

When the epoxy has cured (overnight), lightly sand with 220-grit sandpaper, and then 320-grit or 400-grit. Make sure that the blades are still in static balance. Brush on several coats of thinned 50/50 nitrate dope, sanding between coats with 400-grit. After approximately four to five coats, your propeller will begin to shine like a fine piece of furniture.