Un-equal blade spacing?

Has anyone tried a propeller with unequal blade spacing? Balance with unequal blade spacing is possible with 4 or more blades.

It's been common practice on automotive cooling fans for several decades to reduce noise. The noise from a fan with equal blade spacing is predominately at the blade frequency and its harmonics. With unequal spacing the noise is spread across a number of frequencies and their harmonics. The result is closer to a "white" noise and less obnoxious.

Unequal blade spacing on a propeller likewise would spread out the frequency of the pressure pulsations on the hull, strut and rudder.

 

Haven't seen it in propellers, but we did some experimenting in the early 1970-ies with uneven blade spacing in axial turbopump inducers in order to control asymmetric "rotating" cavitation. The outcome was not dramatically improved; other factors were more important.

 

Blade spacing can be varied, but the wheel must still be thrust balanced or it will quickly have a fatigue failure. Generally, the problems with wake defects lead to symetric wheels to minimze thrust line problems.

 

Same situation with fans. The unequal blade spacing needs to be selected such that the thrust is balanced though this generally occurs when the fan/prop is mass balanced. The unequal spaced blades leads to a spread in frequency of the interaction with the wake velocity defect. Same number of interactions per revolution, but at unequal time intervals.

Some pure speculation on my part - unequal spacing may work better with lower area ratios.

Do you know of any tests with unequally spaced prop blades?

 

Not sure that I made myself clear, the thrust balance is required across the plane of the wheel, not a radial balance that can be corrected with mass. Lack of thrust balance causes excessive bending in the shaft at the shaft/hub junction that is on top of the thrust column load on the shaft.

Could be, fans have low thrust loads compared to their mass and centrifrigal loads. And maybe slow turning small wheels could also get away with it, but a large wheel would quickly fail at the shaft/hub joint from thrust unbalance accross the disk plane so I can't recall anyone actually building a wheel that was not axisymetric about any blade.

 

There are unequal blade spacing configurations which would have balanced thrust loading as well as centrifugal loads without a bending moment on the shaft provided the blade interaction is small enough. No mass correction needed. It may be that props with 4 or more blades generally have blades which are sufficiently large and heavily loaded that the blade interaction would be too large. Or perhaps its been tried and the advantages are not sufficient to justify the added manufacturing complications.

4 blade example:
Blade 1 - 0 degrees
Blade 2 - 90 + x degrees
Blade 3 - 180 degrees
Blade 4 - 270 + x degrees

5 blade example:
Blade 1 - 0 deg
Blade 2 - x deg
Blade 3 - 120 deg
Blade 4 - 180 + x deg
Blade 5 - 240 deg

 

The 4 bladed example is balanced, the 5 bladed example is not. Also, all wakes will have at least one signularity which causes bending in the shaft. Having two blades close to each other will increase that bending moment, as well as being less efficient due to induced velocity issues (the Goldstien Corrections). See Chapter 6, Section 4 of PNA.

 

The 5 blade example is also balanced. Think of it as the superposition of a 2 blade prop and a 3 blade prop. Both are individually balanced.

It is not the only example of a 5 blade prop which is balanced.

 

The "2+3"-blade rotor is in mechanical balance ok, but as jehardiman points, it will NOT be hydraulically balanced, since the blade solidity is varying around the perimeter. This will generate an asymmetric pressure distribution around same perimeter, producing a scewed thrust, rotating with the propeller. This is where the shaft bending comes from, and it will also result in a rotating pressure field, causing low frequency vibrations.

 

The thrust given by the two closer blades is different than the thrust given by the solo blade so the wheel is not thrust balanced. The whole reason for equal spacing is to keep the induced velocities into each blade the same. While it could be possible to design the twin blades to balance the thrust as a single blade, the wake singularity would still screw this up and there is the added issues of decreased efficiency and mass balance. Trust me, Occam's Razor works in hydrodynamics.

Edit: Baeckmo beat me to it.