A Rubber Power Folding Prop Hub Using Robart Hinge Points
The original prototype hub was made from wood and demonstrated the effectiveness of the design. The concept lends itself to components that are 3D printed. There are three sizes of the Robart hinge points available. The prototype used the 1/8" hinge points.
When setting up a folding prop, the angle of the fold line is important in terms of having the prop blade fold properly against the fuselage. The fold line is a function of the prop blade angle at the 70 percent radius point. For a given pitch to diameter (P/D) ratio, the blade angle at the 70 percent radius point is the same regardless of the prop diameter. A common P/D value is 1.3. For that P/D value the fold line is tilted 15 degrees to the left from vertical in the plane of rotation when viewed from the front. Tilting the prop fold line by 15 degrees will have the blades fold along the fuselage with the blades vertical. The blades will not, however, be straight back. One will be tilted up and the other tilted down a small amount. To have the blades fold so the tips are aligned with the prop shaft line and also vertical, the fold line needs to be tilted forward relative to the plane of rotation by the same angle as was used to have the blades vertical when folded.
The prototype prop hub that was fabricated to demonstrate the viability of using Robart hinge points was set up with a 15 degree tilt angle in the rotation plane so the 1.3 P/D blades would be vertical when folded. To create the needed compound angle with this set up so the blade tips will be aligned, it is simply a matter of rotating the hinge point forward 15 degrees relative to the hinge point axis along its length. The fold can be checked before any adhesive is added to the hinge point.
To see how this setup works, a 3D print file has been provided. This 3D printed part set follows the layout of the prototype hub made from wood by John and Larry and also uses 1/8" Robart hinge points. The fold line has been set to 15 degrees to accommodate props with a P/D of 1.3. For props with different P/D values the table below has been provided for reference.
P/D = 1.1 13 degrees
P/D = 1.2 14 degrees
P/D = 1.3 15 degrees
P/D = 1.4 16 degrees
P/D = 1.5 17 degrees