Rubber model propellers from 1/2" balsa sheet

Text by Mike Woodhouse, drawings and layouts by Terry Rose.

This is a transcription of an original article in BMFA News, October 2002. It is reproduced here in slightly edited form with the permission of the authors.

It would appear that the activity that stops many from making a rubber model is the carving of a propeller from a square chunk of balsa. The purpose of this web page is to allay that fear. Basically if you can build the rest of the model then you can carve a propeller.

The secret

The answer is quite simple, the "secret" lies in the careful planning of said construction. What I will explain is how to lay out the propeller blank through to final finishing. I will not deviate into propeller theory, as I for one will soon get lost! The propeller is constructed using 12mm (1/2") thick sheet rather than a large block of balsa. The use of sheet is cheaper and allows for a more careful selection of material.

I will base this description on the construction of a 560mm x 660mm (22" x 26") propeller with helical pitch washed out 2 degrees at the tip. I said no theory, however I do need to explain what this translates to in English.

Laying out the propeller

The pitch diagram is drawn for the propeller. This process can be adapted for any diameter/pitch combination. Note the 2 degree washout adjustment has been applied.

Propeller definitions

"R" = radius of the blade from the centre of the shaft: 560 / 2 = 280 mm (11")

"P" = pitch, the distance travelled forward in one revolution: 660mm (26")

"C" = circumference of the propeller disk (2 pi R) = 2 * 3.142 * R = 1760 mm (69") The next stage is to apply the propeller blade outline and fit this to the pitch angles that have been drawn.

Graphical construction for pitch angles

This stage is the most critical and is probably the hardest bit of laying out the blank. As the process uses 12mm thick sheet, the sheet has to be set at an angle to match the twist of the blade. The best way is to take the angle near the centre and widest points of the blade. This may take a little bit of fiddling to get the best position. A bit of practise will soon resolve any dilemmas.

Once you have decided the angle the sheet must be set at apply this angle to each pitch station that you have drawn. Place the "sheet" in such a way that it allows the blade to twist evenly through the 12mm blank. A good way is to allow the tip template leading edge to be at the bottom of the blank and that nearest the hub to be close to the top after allowing for the thickness of the blade. Ideally the sections should rotate about the centre line of the blade.

Fit the 12mm blank to all other sections along the blade. Once you have done this you are ready to transfer the details to the propeller material and to start working in three dimensions.


Select a piece of 12 mm sheet of about 0.25 kg/m^3 (6 lbs/cu.ft.) density and of even texture. If you are able, try to have a block that will allow quarter grain to show in the majority of the blade – this will stiffen the blade. Cut the blanks out to the general shape of the blade. Cut them from the sheet as shown below. Cut them facing the same way to ensure that the grain is the same in both blades; this will ensure that they will flex in the same manner.

Layout for blade blanks

Mark out the leading and trailing edges of the blades using the distances from the lower surface of the blank that were measured from the pitch template. Connect the pitch positions. This should present a smooth line round the blank. If the line is not smooth check your drawing. You will soon see what needs to be adjusted.

Temporarily attach the blades to your building board at the angle that you have decided for the blank. Fix them in position with templates made from a piece of scrap wood.

Using a set square mark a rectangle on the hub end of the blade. This will be the hub of the blade.

Blade root layout

Carefully pare the block away for about 25mm (1") or so. Note the slight outwards taper. Check and double check that you have cut the area away correctly. Remember to measure twice and cut once. When you are happy face the hub with 0.8mm (1/32") plywood. Again keep checking, making sure that both blades match each other and the template.

Blade root construction

You are now ready to carve. Work on each blade alternatively and do the bottom first. A few saw cuts positioned at 25mm (1") intervals will help in the removal of the surplus wood. Carve and carefully sand in the appropriate undercamber. Now trim the blade to its final outline and again match both blades. Carefully carve and sand the top surface to section. Check with the blade template that you have both the section and the thickness correct.

Fit a brass bush and stop into holes drilled through the hub. Make sure using a template that the bush is set square and that the pitch is correct. Make sure both blades are the same. Many plans show an angled hinge to improve the way the propeller folds against the fuselage sides. However, if this is your first propeller I suggest that you concentrate on getting everything dead square. I'll do a note at the end about angled hinges. The stop and bush are fixed into the blade using a small quantity of 24 hour epoxy.

Check that the propeller balances (for this you will need a wire hub – see below). If you have selected a homogenous piece of timber and worked consistently then there should not be too much difference in blade weights. Correct the balance by comparing the blades, spotting where there is extra bulk on the heavier blade and removing it. The blades can now be finished by covering with light tissue and applying a couple of coats or so of thin dope. Lightly sand between coats of dope and recheck the balance.

Traditional nose block and wire hub

Having made the blades you need to make the nose block and wire hub. The shaft is bent and soldered as shown.

Wire hub

The nose block is cross- laminated pieces of scrap sheet that is then carved and sanded to a conical shape. The plug for the nose block that fits into the fuselage is a piece of hard balsa faced with ply. A brass bush is inserted for the shaft: this bush must be fitted square in all directions.

Nose block assembly

All the joints must be well made because a solder joint failure will be catastrophic for the model. A small compression spring is slid onto the shaft followed by a ball race; a biro spring cut to size is ideal. The shaft is inserted into the bush and bent over on the other side, taking care not to bend the shaft itself. Keep the clearance to about 3mm (1/8"), sufficient for the compression spring to pull the bent shaft against the stop pin. The end of the shaft is then finished to accommodate whatever rubber fixing you prefer. I've shown the fixing to be used with a wire loop, this is my preferred method.

Angled hinges

To make a neater propeller fold an angled hinge can be added. The idea is that as the propeller folds it twists so it fits flatter to the fuselage sides. The hinge has to be a compound angle that is mirrored in the hub and propeller blade. 15 degrees is a typical angle. Care has to be taken when making angled hinges in order that the wire hub and blade are compatible.