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Simply supported propeller blades

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timberwolf8199

Well-Known Member
Joined
Sep 3, 2011
Messages
281
Location
Grand Rapids area, MI, USA
Hopefully this is still clear the second time I write it. I was about to hit submit when my computer decided to reboot...Grrrrr.

I was tossing an idea around in my head last night and thought I would get some feedback. This is somewhat hypothetical, but like all questions of the type it has some real world implications. I have a design in mind that this would fit into beautifully and (assuming a reasonable solution exists) I would love to build.

A shrouded propeller could easily have its tips connected by a ring which is hidden inside of the shroud. Doing this could possibly provide many benefits:
  • By connecting the tips you reduce/eliminate any angular deflection of the blades caused by things like wake, boundary layers, and turbulent air.
  • If the ring is supported axially you also reduce/eliminate the deflection of the blades resulting from the thrust they produce.
    • if the blades are cantilevered radially inward from the ring a true hubless design could be achieved but most of the functional advantages are lost.
    • if the blades continue through the center, you have a well supported structure. The extra support would permit the construction of the blades and ring to be very light.
  • The airfoil profiles of your blade will eventually blend into one another as the blades converge on the single center point of the circle, so some sort of hub will obviously be necessary. This hub could be purely aerodynamic and therefore as small as possible...essentially just a cross between a spinner and a wheel pant.
  • If the prop is driven from the ring you would also have reduced TV issues.
    • driving from the OD provides a long moment arm to counter torque pulses resulting in lower forces that need to be absorbed.
    • increased stiffness of the prop will raise resonant frequencies significantly. Admittedly, I'm not sure yet if this is good or bad.
    • repositioning of mass to the outside (ring) will increase the rotational inertia relative to total mass and increase the flywheel effect of the prop.
  • If driven at the ring you also have a built in PSRU resulting from the large OD and (presumably) much smaller diameter of the driving component.

The ring would need to be supported radially and axially:
  • A set of three bearings positioned and sized appropriately could easily handle the radial containment.
  • The axial support becomes a little tricky because the ideal method would be to provide continuous support. With periodic support you will see bending forces and will have to either increase it's strength (and weight) or you'll lose some/all of the structural benefits listed above.

Since it makes structural sense to continue the blades through the center, and aerodynamics require a hub of some kind anyway; it is intuitive to support the axial prop forces at the center. However, as soon as you go down that road (support structure behind the prop, bearings, shaft, etc) you very quickly end up back at a traditional propeller design due to weight and complexity. So in the interest of keeping the hub purely aerodynamic, I was hoping that you all might be able to provide some ideas/direction on how the axial forces could be best managed at the ring. - Dan
 
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