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This is a thought I asked myself often, but never did real efforts to get an answer. Barsmonsters question about number of fans of a wind turbine made me think of it again

Why do blades of aircraft propellers or wind turbines cover only a small fraction of the area they circle? Propellers of ships or Kaplan turbine propellers cover almost the entire circle. (Independent of number of blades). Same for steam turbines and jet turbines.

I am shure that the forms of those propellers are very close to optimum, due to decades of experience.

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And then there are the jet turbines, which I think covers most of the circle, and the fan leaves in a turbomolecular vacuum pump, which covers considerably more than a circle... I wonder if we should start an Area 51 petition for an engineering stack exchange? – Willie Wong Jun 18 '11 at 17:11
And there is of course the single-blade propeller aircrafts. Reading the wiki for that I think there must be multiple situations where design decisions are made from more than just aero-/hydro-dynamical efficiency. – Willie Wong Jun 18 '11 at 17:19
Maybe someone with access the the article "Design of optimum propellers" by Adkins and Liebeck can provide a summary of what is known? – Willie Wong Jun 18 '11 at 17:21
@Willie: There are a number of proposals for engineering SE's (one of them mine). – Colin K Jun 18 '11 at 20:44
@Willie, right I fixed the question. – Georg Jun 19 '11 at 16:06
up vote 4 down vote accepted

This question of the week article describes the tradeoffs of aircraft propeller blades - number, angle, shape, length etc.

The final paragraph gives a good summary and relate directly to your question about why the blades do not cover the entire circle. It turns out that they do cover as much as is needed for a given engine to transfer the available energy to the air, which is I guess why you see everything from simple 2-blade props to jet-turbines.

We are now left with the final two options, increasing the blade chord or the number of blades. Both have the effect of increasing the solidity of the propeller disk. Solidity simply refers to the area of the propeller disk occupied by solid componenets (the blades) versus that area open to the air flow. As solidity increases, a propeller can transfer more power to the air.

While increasing the blade chord is the easier option, it is less efficient because the aspect ratio of the blades is decreased resulting in some loss of aerodynamic efficiency. Thus, increasing the number of blades is the most attractive approach. As the power of engines increased over the years, aircraft designers adopted increasingly more propeller blades. Once they ran out of room on the propeller hub, designers adopted twin contra-rotating propellers on the same engine. Two good examples are the Tu-95 bomber and Tu-114 airliner. These Russian aircraft were equipped with the most powerful turboprop engines ever built, and both designs feature a total of eight propeller blades per engine.

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An engineering answer:

Note that it's not just about water vs air. It depends on a lot of things: the density & compressibility of the fluid, and tradeoffs between torque, efficiency, cost, materials, maintenance needs, fouling hazards, and so on. Below is a water turbine blade, the SeaGen, that's not much different from a wind turbine blade, because it's doing a similar job, with similar constraints, but in water rather than air. But bear in mind that this is (AFAIK) the first tidal turbine to achieve commercial grid operation, so later designs could diverge.

Some blades are indeed close to optimised given the materials available at the time, thanks to decades of experience. However, new materials can enable further optimisations to blade design.

SeaGen Turbine

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From the point of view of aerodynamics, a smaller number of longer blades is preferred. From the point of view of engineering strength, a larger number of shorter blades is better. What is actually chosen is a trade-off.

It should be obvious why shorter blades are stronger. Longer and thinner blades are more efficient aerodynamically for the same reason that longer thinner wings are more efficient -- the longer thinner wings have less drag compared to the amount of lift produced. Hence high efficiency gliders have long thin wings. The wikipedia photo:
Glider picture

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Apart from fluid dynamics there are also practical considerations.
Ships propellers have to fit under ships - it would be inconvenient if they were 100m in diameter, however efficent.

Model aeroplane propellers, were size is pretty much a free choice, ideally have single blade propellers (just one side with a counter weight) which are much to large to let the aeroplane take off/land on wheels

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I simple answer for a simple question. The main difference between Hydrodynamic and Aerodynamic is that a gas “Air” is spongy in other words it compresses. This is the main reason a propeller on a plain gets propulsion threw the air by having a high speed, narrow and long blade. Where the prop in the water gets it from a much lower speed but a wider and shorter blade

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Ahh, You think that the air behind a planes propeller is compressed? I doubt that. In that case the air would flow back to the front side via the area between blades easily. – Georg Jun 18 '11 at 20:44
@Georg No there is actually compressed air in front of the blade. that is way the blade moves easiest threw a gas Vs a liquid. Now the compression is relative to the speed of the blade and the motion of the plane. – Fortunato Jun 18 '11 at 22:30
Low speed (with respect to the speed of sound) flow is reasonably modeled by the incompressible fliud equations. I don't think wind turbines are fast enough for this to be much of an issue. However airplane props are definitely fast enough for compressible flow. Water has cavitaion issues, and cavitation can cause severe erosion of turbine blades. I suspect you can't push a prop in shallow water very fast without incurring cavitation, and in order to cover the swept area the blades must be wider. – Omega Centauri Jun 19 '11 at 14:51

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