Why are airplane propeller blades so narrow, whereas something like a boat propeller (or the fan in my room) has very fat blades?
And what does the number of blades determine? How come I’ve seen airplane propellors with 2, 3, and 4 blades? What about those jet engine fans that seem to have dozens of blades?
The speed of a prop will determine the shape. Or what is being moved.
The number of blades will determine the volume of air moved or the speed of a boat.
Well, to begin with, water propellers have to deal with cavitation, where the water pressure drops so much that it turns into steam forming tiny bubbles that actually eat away the propeller… not good on itself but also it distorts the flow of water around the blades. So for efficient water propellers a large surface area paired with low revolutions is more desirable. The wide blades work better because there’s more surface area along which the fluid can be accelerated without abrupt changes in pressure that would lead to cavitation.
Airplane propellers of course don’t have to deal with cavitation, and stress efficiency in other design parameters.
A long, slender wing is more efficient than a short thick one, you may have noticed how sailplane wings are very long and thin. This basically reduces induced drag that to keep things simple is a vortex that develops at the tip of the wing/prop blade that produces a very significant amount of drag. So the blades are made as thin and long as possible restrained by the maximum length the airframe allows (clearance to the runway or some other part of the plane) and the structural limits imposed by the stresses the blades have to endure.
Another design limit for air props is that the propeller tips should not break the sound barrier, not only that would be a seriously loud prop, it also impairs efficiency something fierce and puts a lot of stress on the propeller.
Let’s see an example, the WWII fighter Supermarine Spitfire first flew with a two blade wooden propeller and a 1000 something HP engine, shortly after it was equipped with two blade, variable pitch propeller (think of a variable prop as having gears on a car, low gear/low prop pitch to get moving and hi gear/hi pitch to go faster).
As the plane evolved along the war years more powerful engines where fitted to it, so a prop had to absorb all that extra power while maintaining the same diameter and staying bellow high RPMs to avoid breaking the sound barrier at the tips. With those constrains the only way to go was to add more propeller blades to transfer the increasing engine power to accelerating a larger air mass. So the Spitfire was subsequently fitted with three bladed props, 3 wider blades, 4 bladed props, 5 bladed props and ultimately with a 6 bladed contra rotating propeller assembly being driven by a 2000+ HP engine.
More recently there had been high efficiency prop designs aimed at reducing drag and or noise, for example* scimitar* blades, blades that curve along the length giving a slightly swirly appearance.
As for jet engine turbofan blades, again, the idea is to move as much volume of air as possible within a limited fan disc area, the way to do that is to have many blades close together. A normal propeller wouldn’t work very well in that configuration, but jet engine fans are encased or ducted, this improves their efficiency at the flight regimes they operate normally, high speed and thin air at high altitude.
Correct me if I’m wrong, but IIRC the drag coefficient is maximum around Mach .85. Wouldn’t that be a number the designers wanted to limit to, rather than the actual speed of sound?
The correct spelling is “propeller”.
Propeller - Wikipedia
Well, I see “propellor” is an accepted variant. Never mind then.
I’d think designers would limit to a maximum of well bellow the speed of sound, even before reaching Mach 1 the transonic region is very nasty, some parts go supersonic, some subsonic. It’s a mess. 
The AT-6 trainer makes a loud snapping pop as it pass in front of you on takeoff because the propeller tips are going supersonic at takeoff power. Reduced RPM’s in flight bring them back subsonic.
The most efficient air screw is a single blade, of which some were made. problems with length and counter balancing did not allow it to gain enough to make it worth the trouble compared to the normal 2 blade.
The adverse effects aren’t just to do with C[sub]D[/sub]; the Aerodynamic Centre of the transonic section will move towards the 1/2 chord position, the flow around the aerofoil will change significantly (lose a lot of nice laminar flow) and a host of other complicated things that you don’t want to be happening across your blades.
Is there an equivalent phenomenon to cavitation for propeller blades operating in a gas, or is it a special property of liquids?
I’ve never taken compressible fluids; can you expand on this a bit? I fooled around on Wiki for awhile, but they don’t mention it in their transonic article.
It can only happen in liquids, because it happens when the pressure gradient is larger than the vapor pressure, causing gas bubbles to form. When these bubbles collapse, they cause a tiny shock wave, which is what causes the loss of energy and the damage to propellers and impellers. Since you can’t really form gas bubbles in the same fluid, there isn’t really an analogous situations. Eddy currents are about as close as you can come, but that’s really just a special case of turbulent flow.
Thanks!
Any aerofoil moving through the transonic reason will experience compressible effects to the extent that its AC will move aftward to about the 50% chord position (normal postion is about 25% chord). If you look at the flow diagrams for subsonic/transonic/supersonic and understand that the AC is the position on the aerofoil about which Cm is independent of AoA it becomes clear(ish, I still just take it a little as verbatim) why this happens. I was taught why last year; it’s a good idea to understand *what *compressibility effects are, but you may not often have to recall *why *they fully occur unless you’re specifically investigating compressibility itself. (I haven’t needed it in my exams thus far).
The closest I can find to a source online is this from Google Books.
I’ll look in my notes when I get home and come back with something more if I can find it.
Any discussion of supersonic propellers needs to include the XF-84H Thunderscreech. Early jet engines weren’t very responsive to changes in the power setting. The XF-84H had a turbine engine turning the propeller at a constant 3,000 rpm, and power changes were accomplished quickly by changing the pitch of the blades.
That was the theory, anyway. Read the link to see what happened in practice.