Why are air plane propellers small?

[Pau Hernandez]

Why are the blades of air plane propellers thin compared to ship propellers? Why shouldn't air planes use the same blade design?

 

[billy_joule]

Put simply, water is different to air and aircraft are different to boats.
If you want more specific information you'll need to do some of your own research and ask more specific questions.

 

[Pau Hernandez]

I did. All I found is that the design of the blade has an impact on the power the engine has to output and tension within the blades. Comon design parameters are pitch, size, count. I know that the viscosity of air and water differ. Is that the reason? Thicker, bigger blades to ensure higher stability?

 

[SteamKing]

I did. All I found is that the design of the blade has an impact on the power the engine has to output and tension within the blades. Comon design parameters are pitch, size, count. I know that the viscosity of air and water differ. Is that the reason? Thicker, bigger blades to ensure higher stability?

For one thing, ship propellers and aircraft propellers operate at vastly different speeds. If you spun a ship propeller at the same speed as an aircraft propeller, the ship propeller would start to cavitate, which means that bubbles of water vapor would form on the low pressure side of the blade. These bubbles disrupt the flow of the water over the blade, and the propeller cannot absorb the power it is designed to do. These bubbles also collapse quite violently, leaving the blades eroded and damaged where this occurs.

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Ship propellers also tend to be big. Here is a propeller which is 11 m in diameter:

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It would be tough to fit such a beast on a Cessna.

 

[jack action]

I'm no expert in either type of propeller designs, but air and water differ in more way than just viscosity. Densities are also different. Air is compressible, so the speed of sound is of concern. Water is incompressible, but cavitation is a problem.

HYDRODYNAMICS VS. AERODYNAMICS OF A PROPELLER
In spite of the fundamental similarities of lift on an air and a water propeller, why do airplane propellers and boat propellers look so different? Besides the fact that they operate in different substances, there are practical limitations on boats that limit the diameter of a propeller, and boat propellers are, for a number of reasons, placed at the stern, which means they operate in an unstable flow, the wake. In addition, in water, unlike air, there is a limit to how much low pressure, lift per unit blade area, can be created before capitation occurs. Boat propellers therefore have a smaller diameter to RPM, effectively slow the blade tips down and therefore have far more blade area to make up for it. They also tend to have more blades to compensate for limited diameter and perform more satisfactorily in this unstable flow by reducing vibration.

 

[russ_watters]

I'm no expert in either type of propeller designs, but air and water differ in more way than just viscosity. Densities are also different.

Density is probably the biggest difference.

 

[Ranger Mike]

there is a very practical reason aircraft limit propeller size. Look at the Vought F4U Corsair. It was designed from the onset as a carrier-based fighter, not only had the largest propeller of any U.S. fighter, but was also expected to face rough landings aboard a pitching carrier deck. The inverted gull wing allowed the landing gear to be short and strong, and to retract straight back, improving internal wing space. An additional aerodynamic advantage was that the wing/fuselage connection is perpendicular and has inherently lower drag than any other connection. This plane had the largest prop of any plane in the hay day of propeller aircraft at 13 feet 4 inch diameter. You get too big a propeller and the plane will rotate about the air screw in flight!

 

[NTW]

In general, it is cheaper to deliver a thrust through a large mass of air launched backwards at a small speed than with a small mass launched at high speed. The momentum of that mass, and the corresponding reaction of the aircraft, may be the same, but remember that the energy has the v term squared... Propellers have a size limited by the tip speed, that should not be too high, lest drag rises too much. But that tip speed is a vector sum of the rotational velocity and the airspeed of the aircraft; thus, that airspeed shouldn't bee too high, either. Besides, there are problems with the undercarriage (that should not be too tall) and with the torque, but this last problem can be solved with contra-rotating props.

But, even taking into account all the limitations mentioned above, the larger a prop, the better... That's the reason helicopters have very large rotors.

 

[OCR]

Propellers have a size limited by the tip speed...

Maybe not completely limited by tip speed, but it does seem to cause big problems... as in the case of the "Thunderscreech".

You aren't big enough and there aren't enough of you to get me in that thing again.

Noise ...

The XF-84H was quite possibly the loudest aircraft ever built.

On the ground "run ups", the prototypes could reportedly be heard 25 miles away. Unlike standard propellers that turn at subsonic speeds, the outer 24–30 inches of the blades on the XF-84H's propeller traveled faster than the speed of sound even at idle thrust, producing a continuous visible sonic boom that radiated laterally from the propellers for hundreds of yards. The shock wave was actually powerful enough to knock a man down; an unfortunate crew chief who was inside a nearby C-47 was severely incapacitated during a 30-minute ground run. Coupled with the already considerable noise from the subsonic aspect of the propeller and the dual turbines, the aircraft was notorious for inducing severe nausea and headaches among ground crews. In one report, a Republic engineer suffered a seizure after close range exposure to the shock waves emanating from a powered-up XF-84H.

YouTube ... "Thunderscreech"

 

[Baluncore]

Why are air plane propellers small?
Air plane propellers are not small, they are big compared with a boat propeller for a similar mass vehicle.

Why are the blades of air plane propellers thin compared to ship propellers?

Blades are airfoils. A propeller blade in low density air is traveling relatively fast compared to the speed of sound. If it was not thin it would cavitate.

 

[NTW]

Cavitation takes place only in liquids, where a gas bubble may form and then collapse destructively. That never happens in air...

 

[Baluncore]

Cavitation takes place only in liquids, where a gas bubble may form and then collapse destructively. That never happens in air...

OK, so replace my “cavitation” with “cavity formation”. Cavities can form in air once speeds approach supersonic.

Cavitation is where a liquid cannot move fast enough to fill the space behind a moving propeller blade. A cavity forms that contains vaporised liquid.

When air cannot move fast enough to fill the space behind a moving propeller blade, the airflow separates and a partial vacuum is formed. That depression may then contain a mist of condensed water vapour.

Both situations can be resolved by reducing the angle of attack, or by reducing the thickness of the propeller blade.