Making a 4 blade wood propeller

[Pete Twissell]

While it is of course possible to assemble two 2 blade props as you describe, the problem is that both are now half of their original thickness in the centre, at the most highly stressed location.
That is why I researched construction of 4 blade props and found how they were made with overlapping laminations.

If you are replacing a 2 blade prop with a 4 blade prop, you will need a smaller diameter but the same pitch.

Broadly...
The pitch determines the theoretical forward motion of the prop through the air for each revolution. This is determined by the desired engine speed, airspeed and thrust.
The diameter determines the effectiveness of the prop at turning rotary motion into forward thrust, or its 'grip' on the air.
High effeciency props are large diameter and turn slowly, such as those seen on human powered flying machines and rubber powered models.
High power props are smaller diameter and turn faster, such as those seen on control line speed models - some being single bladed!

My 4 blade prop project has been stalled for some time, while other projects have taken priority. I'm hoping to get back onto it soon.

[gocap]

May i Ask: is it possible to make a 4 blade prop from 2 regelar 2 blade props? just cutting the harts in two slices? And normally the engine needs a 26x10 prop, which regelar blads do i can use for the 4 blades?

[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".

[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.

[Dbahnson]

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

[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.

[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]

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]

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.

[powerandpassion]

Thank you Pete

[Pete Twissell]

Attached photos of 4 blade prop hubs from Shuttleworth.

[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]

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.

[Rayra]

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

[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!