Single-blade folding propellers

[John Brandon]

For those who are interested in the physics and characteristics of single-blade, folding propellers I suggest you view Dafydd LLewellyn's research project at http://flysafe.raa.asn.au/groundschool/blanik.html

 

 

[djpacro]

An 8,000 flight hour fatigue life extension too - that should suffice for a long and happy retirement.

 

 

[kaz3g]

I did more chemistry and biology in earlier years than physics or maths, and this is really beyond me, but thank you for making an enjoyable read an almost understandable one for an old fart like me.

 

 

[Gnarly Gnu]

OK I'll bite, I get the efficient aspect.... but why am I not seeing single blade propellers in use?

 

 

[Dafydd Llewellyn]

OK I'll bite, I get the efficient aspect.... but why am I not seeing single blade propellers in use?

Because they look "wrong", I think. They're a bit heavier than an equivalent 2-blade, also. However, if you need to get rid of the propeller drag, as in a motor-glider, the alternative is a featherable Hoffman - and that costs about $ 15K AND you have to ship it back the Germany for overhaul every two years or so, AND it's not that light. I was after a vastly less costly solution.

 

 

[Gnarly Gnu]

Ah, OK Dafyyd... must admit I just skimmed the article, must read it through. So it can be feathered. And the need for it to be folding...?

 

 

[Dafydd Llewellyn]

Ah, OK Dafyyd... must admit I just skimmed the article, must read it through. So it can be feathered. And the need for it to be folding...?

No, it's fixed-pitch. it folds instead of being feathered, not as well as.

 

 

[Guernsey]

If it is a wooden single blade prop then moisture can affect the balance unless it is well coated with a waterproof top coat.

 

Alan.

 

 

[Dafydd Llewellyn]

If it is a wooden single blade prop then moisture can affect the balance unless it is well coated with a waterproof top coat.Alan.

Yes. Polyurethane is best.

 

 

[Keenaviator]

I had a nice folding prop on a power harness for hang gliding. Made by Bolly. 15 hp. Laurie

 

Ps the harness was an 'Explorer' made by Bob Bauer in QLD.

 

 

[Guest ozzie]

The single blade is fitted to a Solo engine on a Silent Glider in Italy. Doesn't fold but lines itself up and the pylon retracts into the fuselage.

 

The folding two blade system could be used on a single blade. just a big spring to fold the blade back when it stops.

 

 

 

 

[Dafydd Llewellyn]

Yes, there are a number of variations on the theme around. Single-bladers seem to be used mostly on retracting engine types, to minimise the size of the cut-out needed in the fuselage. Two-bladers are of course common on rubber-band powered model aircraft; I built one as a child. The novelty lies not so much in the single-blade propeller, as in the form of engine mounting necessary to allow it to self-balance for the blade drag that acts in the plane of rotation.

 

 

[facthunter]

Gotta be a crook idea in principle. You have a bending moment on the crank and also the blade drag you mentioned with will offset the crankshaft axis as it rotates. You engine mounts will be working overtime. You wouldn't get much sediment in the carb bowl(s) Nev

 

 

[Dafydd Llewellyn]

Gotta be a crook idea in principle. You have a bending moment on the crank and also the blade drag you mentioned with will offset the crankshaft axis as it rotates. You engine mounts will be working overtime. You wouldn't get much sediment in the carb bowl(s) Nev

No, Nev - If you read the description to which John Brandon provided the link, there's no bending moment on the crank - or more precisely, the gearbox output shaft - because the blade/counterweight assembly is hinged there. A hinge won't tranfer a bending moment; that's what it's for, after all. There is neither a bending moment due to the thrust being from a single blade, nor from propeller "P" effect, nor from gyroscopic loads. The gearbox output shaft has a holiday compared to the normal setup.

The blade drag force causes a run-out of about 4 mm at the propeller hub - i.e. the propeller runs about 2 mm offset, because that amount of offset generates a centrifugal load that balances the in-plane force on the blade; and yes, the engine mount has to be designed for that, which means the normal method of four rubber blocks on the four bolts at the bottom of the crankcase is quite unsuitable. It won't trouble the carburettor because it won't be mounted on the engine; I'm going to use a CD carbie (Bing type 64) mounted on the pylon structure, underneath the engine and feeding the rotary valve via a Y-branch that is flexible connected at both ends; the motion on the rubbers is no worse than for the usual cantilever mounting. The Rotax 2-cylinder engines throw their propellers around in a circle anyway, due to the residual out-of-balance of the two cylinders; but because the engine RPM is so high, the amplitude is small. I learned that back in the -80s with a redrive I designed for a Blanik 532 installation that had a heavy MT electric-feathering prop. on it (26 lbs). It showed up as heavy fretting wear on the redrive output shaft under the outboard bearing.