Folding prop design

[pedwards2932]

I am designing a folding prop hub for 3D printing.  I am getting ready to do the center hub section.  Does it need to be a free wheel hub?  I am thinking that since it folds it does not need to be free wheel.

[Jmk89]

No freewheel required if you are using a tension stop system to trigger the fold.

[pedwards2932]

I'm hoping to get away with the wind as the rubber winds down will force it to fold.  I am designing the hub to be expandable so you can vary how wide the closure is when it folds based on fuselage width.  It will use bamboo skewers and cottage cheese cup blades so you should be able to vary the length of the prop and pitch. The picture shows the hinge point and the long part is the receiver for the bamboo skewer. When I get the hub designed then I can attach it to the shorter hinge point.  I will use fusion to make the hub all one piece

[Jmk89]

I think that you will find that the prop blades will fold back using just the drag on the blades arising from the forward motion of the whole plane.  However, the blades will flop around destroying the glide, unless you have a positive mechanism to stop the hub turning. 

The tension system puts a spring on the propshaft between the hub and the nose block which moves the hub and propshaft forward as the tension reduces at the end of the motor run so that the front hook hits a stop (typically a screw) on the back of the noseblock.  The remaining turns on the motor has sufficient rotary motion to keep the motor hook pressed against the stop so the hub stays still and the blades don't flop about.

[pedwards2932]

I could design a rubber band retraction system for it if needed.  shouldn't be too hard

[lincoln]

The stop screw should stop the prop in position where the axes of the hinges are vertical, so the blades food only back, without any up or down. Usually, that may be close to a horizontal position for the whole prop.

[pedwards2932]

The design only allows prop to fold back and stops when fully extended

[Jmk89]

I could design a rubber band retraction system for it if needed.  shouldn't be too hard

I think we are a bit at cross-purposes here.

The spring isn't to pull the blades back, it's to stop the hub turning so that the blades trail back neatly and don't flop about.  As lincoln says, you want to stop the hub turning at the same point of its rotation so the blades fold back into the same spot and stay there during the glide. 

The attached picture from AeroModeller June 1975 shows the setup for a single-bladed prop. The spring is the tensioner shown on the diagram and the screw stop is shown as a dowel stop.