Variable Pitch Propeller Mechanism

Updates description at the bottom

Overview

A model of a variable pitch propeller mechanism based on a 1935 patent by Robert Stanley, adapted for 3D-printing.

Variable pitch propellers allow the pilot of an airplane to adapt the angle of the blades according to the speed of the airplane for optimal performance. While most variable pitch propeller systems use hydraulic or electric power, this purely mechanical system uses two planetary gears to control the pitch of the blades on the rotating propeller from a driving gear on the stationary housing.

Using 3D-printing allows for these planetary gear assemblies to be made with print-in-place herringbone gears (impossible to disassemble), which simplifies tremendously the assembly process.

See the mechanism in action :

I included pages of the original patent if you want to read the details.

This system was used on the Curtiss Turboelectric propellers that equipped the Douglas C-133 Cargomaster.

Parts

All parts are 3D-printed. No additional hardware is necessary. The whole assembly is held by three 3D-printed nuts.

You will need to print :

Propeller assembly

• 1 x front-hub (or front_hub_alt, see update Oct 11th)
• 1 x rear-hub (or rear_hub_alt, see update Oct 11th)
• 1 x hub-nut
• 5 x blade
• 5 x spur-worm-gear
• 5 x thust-washer

Housing and shaft assembly

• 1 x front-housing
• 1 x rear-housing
• 2 x bearing-1 (see note 1)
• 1 x shaft
• 1 x spacer-1
• 1 x rear-nut

Final assembly

• 1 x rear-planetary-assy
• 3 x planet-axis
• 1 x spider
• 1 x front-planetary-assy
• 1 x spacer-2
• 1 x bearing-2 (see note 1)
• 1 x spacer-3
• 1 x control-gear (or control-gear-alt, see note 2 below)
• 1 x blade-setting-crank
• 1 x front-shaft-nut
• 1 x motor-crank

And the (optional) tools:

• 1 x front-shaft-nut-socket-wrench
• 1 x hub-nut-spanner
• 1 x rear-nut-spanner

I built all parts from PLA with 0.2mm layers using the default Prusa Mk3S profile. Only the rear housing requires supports.

Notes :

1. I did not manage to get the bearings to run for real, both for bearing 1 and bearing 2 the rollers are either stuck to the cage or they have too much friction so the rollers don't really roll, the best I could do in the limited time before the contest deadline was to have the inner and outer rings move freely. This does not prevent the mechanism from functioning properly.
2. The mechanism was designed for a 20-tooth control gear, but because of freeplay in the assembly, on my prototype it tends to slip easily on the external gear of the front planetary assembly. So on my assembly a 21-tooth control gear which was theoretically too large gave the best results and is therefore provided as an alternative (control-gear-alt) if you encounter the same problem.

Instructions

See the build video :

Exploded views

Propeller assembly

Housing and shaft assembly

Final assembly

Reminders :

• Don't tighten the nuts too much, it's only plastic !
• Some gears and bearings will require a bit of breaking in before they run smoothly so be sure to do this before final assembly in order for things to run properly without overstressing the parts.

Design notes

Information that may be useful for remixes :

All the gears have a module of 1 and a contact angle of 20°.

The planet gears are scaled down to 97% in lateral and vertical to give enough clearance to print the planetary assemblies "in-place".

Updates

October 11th, 2022: New propeller hub

I designed an alternative propeller hub (file names front_hub_alt and rear_hub_alt), that works exactly like the original but looks much better.

The new parts are however a bit harder to print and you will need to use supports.

The only difference in the assembly is that now the thrust washers have to be inserted in the small side slots on the rear hub rather than from its front face.