Single blade prop effeciency myth

[ReddyWatts]

Looks like another myth is busted.

Single-Blade Myth
One of the myths that has been propagated in the aviation community, to the point that it’s almost become gospel, is that the most efficient prop is a single-blade and that all props with higher numbers of blades fall further and further short of this paragon. Did you ever consider that a single-blade prop, developing thrust on only one side of the plane as it revolves, would cause the engine to cone violently in its mounts as it is twisted by the prop?

Airbus Military’s latest turboprop transport, the A400M, has eight-blade props! The Boeing MD-900 helicopter has a five-blade rotor. A popular regional turboprop airliner has a five-blade prop. Hasn’t anybody filled these aircraft manufacturers in on the errors of their ways? In a past issue of a popular aviation magazine, the author of an article on props uttered the same fallacy. He maintained that multiple blades interfere with each other.

When I pointed out to him that at 200 mph and 2800 rpm the blades on my three-blade prop follow three distinct helical paths through the air, and each blade is 25 inches ahead of the previous blade at the same point of rotation, he rather lamely explained that in static conditions interference occurs. Static? Who uses static thrust? Airplanes are meant to fly, not pull tree stumps!

[ReddyWatts]

Even the fastest full-scale boats usually use 3-bladed props. The two fastest FE boats in the world ran 3-bladed props to set their records.

We often have more power than we can get to the water, we need blade area to transfer that power. With two-bladed props you get more blade area by increasing diameter...but that can cause terrible prop walk which hurts top speed. Three-bladed props can have enough blade area without an excessive diameter.

Now two-bladed props can certainly perform very well, but they hold no real advantage because of some imagined "efficiency".

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[m4a1usr]

I dont see how you state that its a myth busted? The math alone states clearly that a one bladed prop operates with the best coefficients as well as has the least amount of friction into the media with which its imparting energy. The US Navy has done decades worth of research and the answers are still the same today as it was back in the 1940's.

While multi bladed props can put more energy into the water its only due to surface area and laws of physics. A single bladed prop has issues transfering energy because of imbalance and structure.

Efficiency -- propeller efficiency = (J × Kt) / (2πKq)

While I dont purport to know lots about props I do read enough to begin to understand them and the issues with mechanical energy transfer. The efficiency issue is related to the lowest numbers of that energy transfer. The more blades, the higher the numbers in th equations. Here is a good source for read if you dont want the wade thru the technical papers (of which there are a ton) to educate yourself.

http://atljsoft.com/default.aspx

John

[ReddyWatts]

I dont want users to think a two bladed prop in boats is necessarily better than three when trying to achieve top speeds, due to this theory.

[Rumdog]

Well, since single blade props don't work, I consider it a moot point. Math, or not, it cannot be more efficient because it doesn't work!

[AndyKunz]

Unfortunately, the basis for "debunking" the myth is also a myth - that the blades "screw" through the air. In fact, in a proper design, the prop moves air MORE than just the amount of the pitch - even at "0 pitch" a prop is providing an measureable thrust due to the airfoil section of the blade. The "downwash" normally associated with a wing is also present, pushing a larger mass of air down (back from the prop) than would have occurred with a simple screw action.

Thus, that 25" is actually more likely quite a bit less, and the efficiency losses are caused by inward-moving air the lowers the angle of attack of the blade pitch.

Rare Bear is such a clean design that the top speed is about 25 MPH more than the pitch speed of the prop. That's a huge amount, and certainly not the norm, especially when you consider that the aircraft still have induced and profile drag.

This is part of what causes a prop to "unload" as the vessel speeds up. The decreased effective angle of attack of the prop puts less load on the motor, allowing it to speed up. We recently demonstrated this playing around with the telemetry on one of my planes. We tached the motor at 9300 RPM full throttle on the bench. On the runway, as the plane sped up the RPM also increased, so that by the time it was at rotation the prop was at 11K!

An more-important reason than hydrodynamics for the use of 3-bladed props is that the rules limit the diameter. All Unlimiteds are allowed a 16" dia wheel. To get enough power out, they increase the blade area and count.

Same thing for aircraft and helis. The engineers are usually given a maximum rotor diameter, or other factors (landing gear length/strength/etc.) will dictate a maximum diameter. The Corsair, for instance, has its famous inverted gull wing not because of aerodynamics, but because the engine needed to spin a large diameter 3-bladed prop and the gull shape allowed them to use shorter, stronger struts for the undercarriage.

With modern computer modeling, engineers now have more tools at their disposal to meet size, power, and aerodynamic constraints as well as verify efficiency. Sometimes you have to trade off efficiency against other constraints. An airplane has been called, "A collection of tradeoffs flying in formation."

Andy

[Grimracer]

One also has to consider the altitude that a full scale plane fly’s at. Altitude plays a large role in the size, speed and number of blades.

Grim

[properchopper]

My Amp goes to Eleven ....

[AndyKunz]

Good point, Mike. Altitude <--> viscosity. I'm sure Jay can add even more here.

Andy

[Doby]

Well, since single blade props don't work, I consider it a moot point. Math, or not, it cannot be more efficient because it doesn't work!

Agreed, not even worth a discussion

[ReddyWatts]

you missed the point.

is that the most efficient prop is a single-blade and that all props with higher numbers of blades fall further and further short of this paragon.

I dont want users to think a two bladed prop in boats is necessarily better than three when trying to achieve top speeds, due to this theory.

[NativePaul]

single blade props do work well for planes, a pylon racer at our field got fed up with breaking props and made a balanced spinner to take the one good blade of his large supply of broken props, he rarely breaks a prop now and is going faster than he was with 2 blades using less power.

I dont see it working for FE because our hubs are too small to get the balance weight which would need to be considerable given our heavy bronze blades, the bearing issue is a red herring though in a surface drive however many blades you have its only the bottom one thrusting so the thrust is offset anyway, we are allready coping with those extra bearing loads just fine.

[ReddyWatts]

Airplane props are using the full rotation in compressable air.

Our boats are using only half of the rotation in a non compressable fluid.

Could we be comparing apples and oranges?

[vasy]

There is also a difference between a boat and a plane.
In plane the air has air all around it so a single bladed prop always pushes air.
In the water with surface drive one blade is out of the water 1/2 the time so with one blade you would be not pushing anything 1/2 the time.

[AndyKunz]

C/L pylon planes have flown on one blade since the 1940's or 50's, but they can do so only because the RPM is VERY constant and they run a counterweight.

Andy