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Pau Hernandez
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Why are the blades of air plane propellers thin compared to ship propellers? Why shouldn't air planes use the same blade design?
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.Pau Hernandez said: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?
http://www.psychosnail.com/boatingarticles/boatpropellers said: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.
Density is probably the biggest difference.jack action said: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.
Maybe not completely limited by tip speed, but it does seem to cause big problems... as in the case of the "Thunderscreech".NTW said:Propellers have a size limited by the tip speed...
Lin Hendrix Republic test pilot said:You aren't big enough and there aren't enough of you to get me in that thing again.
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.
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.Pau Hernandez said:Why are the blades of air plane propellers thin compared to ship propellers?
OK, so replace my “cavitation” with “cavity formation”. Cavities can form in air once speeds approach supersonic.NTW said:Cavitation takes place only in liquids, where a gas bubble may form and then collapse destructively. That never happens in air...
There are several reasons why air plane propellers are small. One of the main reasons is to reduce drag. A smaller propeller creates less resistance to the air, allowing the plane to fly more efficiently. Additionally, smaller propellers are lighter, which reduces the overall weight of the plane and helps with fuel efficiency.
Yes, the size of a propeller can affect the speed of an airplane. Larger propellers can generate more thrust, which can increase the speed of the plane. However, this also means they create more drag, which can decrease the speed. Smaller propellers, on the other hand, create less drag but also provide less thrust, so they may not be suitable for larger planes or those that need to travel at high speeds.
In general, smaller propellers are more efficient than larger ones. This is because they create less drag and require less power to turn. However, this also depends on the type of plane and its intended use. Some planes, such as cargo planes, may require larger propellers to carry heavy loads and maintain a certain speed.
Yes, the size of a propeller can affect the noise level of a plane. Smaller propellers create less noise because they rotate at a higher speed, which produces a higher-pitched sound that is less noticeable to the human ear. Larger propellers, on the other hand, rotate at a slower speed and produce a lower-pitched sound, which can be louder and more noticeable.
Yes, there are other factors that determine the effectiveness of a propeller. These include the shape and angle of the propeller blades, the materials used to make the propeller, and the engine power. The design of the propeller is also important, as it needs to be balanced and properly matched to the plane's engine and speed requirements in order to function effectively.