Making a 4 blade wood propeller

[Pete Twissell]

Hello all,

I am currently building a model aircraft engine, based on the AirDisco V8, as used in DH51 etc.
I would like to make a 4 bladed propeller, if only for demonstration purposes.
I have made small 2 bladed wood props in the past, both carved from solid and laminated.

I am having difficulty finding any detail of the design of 4 bladed wood props, specifically around the hub arrangement and how laminations are arranged to avoid the obvious weak spots at the roots.

Does anyone have any links, hints, direct experience etc. to point me in the right direction?

My prop will be around 12" diameter, 4" pitch. The engine should be able to turn it at around 7,000 RPM.

Thanks in advance,

Pete.

[Dbahnson]

Here's one link which might be of interest. I'll look through Bob Gardner's books to see if there's some useful information in his extensive documentation.

[Pete Twissell]

Thanks for your prompt response and the link.
The photo shows the laminations having only a small overlap (at least where it is visible) but also a large crack exactly where I would expect such a construction to fail!

[Rayra]

Check my propeller out I'm trying to sell. It's an antique. Thx

[Pete Twissell]

A visit to the Shuttleworth Collection yesterday was very informative. Among the many exhibits of aircraft and vehicles, there are a few 4 bladed props and hubs from damaged props on display.
A couple of the hubs had begun to delaminate, allowing me to identify the method of construction.
Each lamination is essentially as it would be for a 2 blade prop, except that in the area of the hub, a section of the lamination is reduced to half thickness and a second lamination, similarly cut, is laid into it at 90 degrees to form the intermediate blades.
Each such pair of laminations is arranged at an angle of a few degrees to the pair below. This means that there is sufficient crossing of the grain of the laminations to provide strength in the assembly and also means that the joints at the reduced thickness areas do not coincide with one another.
If anyone is interested, I will post photos when I come to make the prop.

[powerandpassion]

Pete, would love to see detail photos of the Shuttleworth 4 bladed hubs and your scale prop being made to understand the method, especially the shifting ‘by degrees’ of the laminations.

[Pete Twissell]

Attached photos of 4 blade prop hubs from Shuttleworth.

[powerandpassion]

Thank you Pete

[Pete Twissell]

Much time has passed and I've finished building the engine for which I need to make a prop. The prop fitted below is a Nylon item for test runs.
IMG_20240101_125631_HDR.jpg
The first step in making my wooden prop is to model it in CAD. This was done by creating a series of cross sections for one blade, then joining them together by 'lofting', which is a CAD process for creating a set of surfaces or solid body which is defined by the set of cross sections.
This creates a single blade, which is then copied three times and all four are mated to a cylindrical hub.

[Pete Twissell]

The next part of the process is to slice the CAD model into what will become the laminations.
Propeller.jpg
I will be using 0.040" mahogany sheet, so I sliced the model into 0.040" layers.
Each slice is created as a 180 degree pair of blades, with the other blades (at 90 and 270 degrees) cut away with straight edges which will run parallel to the grain of that lamination.

[Pete Twissell]

I then created CAD drawings of the slices:
Propeller.jpg
Theimage shows 7 of the 12 slices. Note how the edges where the second pair of blades will join are at different angles relative to the hub bolt features.
The grain in each slice will run parallel to those edges, so each lamination will have its grain at an angle to the adjacent laminations.

[Pete Twissell]

I will cut two of each lamination. One will have the centre (square) portion cut to 1/2 thickness on the top side, the other will be similarly cut away on the bottom side.
The pair will be glued together at this central joint to make a 4 bladed lamination.
I will make a fixture for cutting away these centre portion, in order to ensure that the joint is accurate and the four blades of each lamination are at 90 degrees.

[Dbahnson]

That's a VERY impressive amount of work and detail!

[Pete Twissell]

Thanks!
I will endeavour to share my entire process of making this propeller, including any issues which may arise.
Please note that this propeller is for a model. It is 12" diameter and I will be testing it under controlled conditions to verify its integrity before it is fitted to the engine.
I would not recommend this process for manufacture of full size propellers.

[Pete Twissell]

Some time has passed and I have finally had a few hours to spend on the prop.
I cut two laminations to each pattern from 0.040" thick mahogany sheet.
Each lamination has the centre section, where it will overlap with the othe one of the pair, thinned to 0.020".