What about helicopters? You don’t have to worry about the rotors scraping the ground since they are horizontal. Yet I noticed the number of rotors still vary- from 2 to 6 or more.
I’m curious the advantages and disadvantages in these cases. Since they’re horizontal, they can make them longer (until they are so long they droop too much). So why have different configurations or even pairs?
That’s a highly complex question. To start with, helicopters with a single rotor blade have the same efficiency as would be achieved by a single blade propeller. Unlike a propeller a helicopters rotor provides directional thrust and has unusual problems with a single rotor blade and so far only experimental craft have attempted this. Helicopter rotor blades are much longer than propeller blades for an airplane, and this makes the material properties more important, they need to be strong enough at longer lengths to withstand the greatly varying loads on the rotor. The directional thrust needed from a cyclic control which aerodynamically ‘tilts’ the rotor complicates this even more. Adding more complication is the problem of retreating blade stall in forward flight, as the helicopter moves forward at higher speeds the effective airspeed of the rotor moving in the direction of the flight is much greater than the airspeed of the same rotor as it rotates around and is moving in the opposite direction of the helicopter.
You might as well open a new thread on this subject, helicopter rotors are far more complex than the simple propellers used on airplanes and boats.
Again, the droop thing - or the whole excessive flex thing, including I suppose an anti-droop when creating lift - would be a non-trivial nuisance, and also the blade tip will go supersonic at a much lower RPM given that the blade is already far longer than a propeller’s. So once more, when you’re trying to convert a lot of BHP to lift you will end up using more blades rather than longer ones.
Related question: why aren’t paddle wheels more efficient than (ship) propellers? AIUI the propeller being in the water creates drag as well as thrust, but only the “important” bit of the paddle wheel is in the water at any given time.
In a nutshell: Lift.
Paddlewheels work by directly pushing water backwards. Net of what leaks around the edges of the paddle, creating turbulence & drag. It’s stone age physics, but it works.
Propellers work by lift. They pull themselves forward. This is using more modern, subtle physics. The net turbulence and drag produced is less.
Which is another way of saying that more of the available shaft-turning engine power is converted to useful outputs and less is lost to useless waste.
IANA aeronautical engineer, etc. But I think we might be missing something here when comparing airplane propellers and boat propellers.
Air is a compressible medium. Water is not (for most practical purposes). I would think, therefore, that the design of the 2 different propellers would be very different.
J.
p.s. I also thought about mentioning cavitation in regards to boat propellers, but realized I don’t know if that only affects boat propellers, or if a form of it also affects airplane propellers.
I believe air is treated the same as water when using Bernoulli’s principles regarding lift; that is, as a fluid medium.
The shape of the propeller blades will be very different because of the different characteristics of water and air, but the underlying principles are the same, the propeller blades are foils which generate lift.
Cavitation is not a problem for aircraft propellers that I know of, but they have a similar problem mentioned here already, if the tip speed of the propeller approaches the speed of sound they’ll throw off supersonic vortices which can damage the propeller. Besides reducing efficiency and producing noise cavitation can produce the same result as the vacuum bubbles it creates collapse.
So:
The fewer propellers, the smaller the losses between the power the engine produces and the work accomplished?
The more propellers, the greater the power-to-weight ratio of the engine+propellers?
It would be the power-to-thrust ratio or something like that, I’m not sure of the proper term. That would be based on number of blades where the blades were of the same size and shape, but it’s never that simple.
Thanks. That makes perfect sense.
For something closely related, consider the difference between an old-fashioned square-rigged ship and a modern sailboat. The former essentially presents its sails as flat barn doors for the wind to push against. The latter presents its sails as airfoils to generate lift.
The difference in potential performance is ginormous. e.g. High-performance sailing - Wikipedia
(You are quite correct that they typically do this more efficiently than a paddlewheel does.)
A paddle wheel also creates some amount of aero drag/turbulence for the time it is not in the water, so that will compromise its relative efficiency over a prop. Consider the wind turbine trade-off: a prop-style turbine has to be proactively aimed into to wind, but once it is, its blades are always working; a vertical axis turbine, like an anemometer, does not have to be aimed into the wind, and the wires from the generator never have to flex, but its blades are only capturing the wind effectively for about a third of their travel and going against the wind for a similar angle on the back side.
… or worse.
One often neglected consideration on aircraft propellers is that one or two bladed propellers load the crankshaft differently than those with 3 or more blades.
When the aircraft yaws a three (or more) bladed prop acts like a disk, producing a steady precession load on the crankshaft. A two bladed prop produces a pulsing load as the engine rotates, with a peak load of about twice the multibladed prop. Aircraft engine crankshafts are designed to cope with this, but it has been a known problem on, for example, converted VW engines.
A propshaft is not infinitely stiff. If it flexes or vibrates in the wrong way, it can alter the pitch on the two blades of a two-bladed prop, and at some critical speed the prop-shaft system becomes unstable. A multi-bladed prop is inherently stable, so the shaft can be lighter, or safer for a given weight. The teetering single blade prop approach is another way of decoupling the spinning prop dynamics from the springiness of the shaft. It is very common on helicopters, even though they have two blades, and there are more reasons there as well.
A two bladed prop could be made stable by putting the blades at, e.g., 120 degrees, balanced by a counter-weight. While I have never seen such on an aircraft, this approach is used in the usual configuration of a boomerang, where the heavy “elbow” of the 'rang serves as a counterweight for the two blades.