Wouldn't three props achieve the same result as four?

 

Wouldn't three props achieve the same result as four?

No.
Two props are rotating in one direction while the other two are rotating in opposing direction. This to balance the rotational forces and this results in a net zero rotational force.

 

Yes , of course, silly me.

 

That was actually a very good question ... and a very good answer. Not silly at all. One could argue one of the three rotors being larger and at a different rotational speed all ... but yes, it's a lot simpler to have four.

 

No.
Two props are rotating in one direction while the other two are rotating in opposing direction. This to balance the rotational forces and this results in a net zero rotational force.

Having four also makes the control algorithms for pitch and roll much simpler due to the symmetry. Though four isn't engraved in stone. Some have six. Other configurations also exist.

 

Though four isn't engraved in stone. Some have six. Other configurations also exist.

The general rule being that it's usually an even number of rotors ... right?

 

The general rule being that it's usually an even number of rotors ... right?

From a practical standpoint, that is probably the case. Not an absolute requirement, but the advantage is enough in most cases that it would be hard to justify a different choice.

As multi-rotor vehicles get larger and become more specialized, there may well be situations where other factors become sufficiently dominant to make that different choice defensible or perhaps even unavoidable.

 

Here's a thing I wonder ... is using an even or odd (or prime) number of blades per rotor relevant? ... I mean, I'm sure that performance can vary according to parameters such as blade profile, diameter or rpm's or Reynolds number or whatever. But is the choice of using an even number of blades per rotor of any advantage now that CAD/CAM/CAE have become commonplace?

 

Here's a thing I wonder ... is using an even or odd (or prime) number of blades per rotor relevant? ... I mean, I'm sure that performance can vary according to parameters such as blade profile, diameter or rpm's or Reynolds number or whatever. But is the choice of using an even number of blades per rotor of any advantage now that CAD/CAM/CAE have become commonplace?

With modern control software an odd number of blades is certainly possible. But why "fight physics"? Choosing an even number simplifies things, being naturally more stable.

 

With modern control software an odd number of blades is certainly possible. But why "fight physics"? Choosing an even number simplifies things, being naturally more stable.

RIght ... why fight physics, good question. My point is that nowadays it's easier to come up with an unorthodox solution that would've previously been considered highly impractical.

The toroidal propeller is shingling example of what can be accomplished with a little imagination and modern tools..

 

RIght ... why fight physics, good question. My point is that nowadays it's easier to come up with an unorthodox solution that would've previously been considered highly impractical.

Fair enough. But are there any advantages of using an odd number of blades?

The toroidal propeller is shingling example of what can be accomplished with a little imagination and modern tools..

I saw some of those on a drone and it was pretty impressive. They are slightly less efficient, but much quieter and less strain on the blades. I imagine for marine applications it might cut down on cavitation damage too.

 

When I just read the first few posts I thought "what are these guys talking about! how can the number of blades be related to forces balancing?".

Then I read it again and realized I was a dufus who failed to read the post>

 

With modern control software an odd number of blades is certainly possible. But why "fight physics"? Choosing an even number simplifies things, being naturally more stable.

Are you talking about the number of blades on a propeller, or the number of propellers?

I think two-bladed propellors are the most common, but there are lots of three- and four-bladed propellors out there. Higher rotational speed favors fewer blades because it gives the air more time to recover from the prior blade passage (which is why propellors are more efficient at slower speeds). But more blades have the advantage of lowering the thrust loading on each blade. Everything's a compromise.

On real aircraft, there's everything from two- to eight-bladed propellors (including five and seven).

 

Are you talking about the number of blades on a propeller, or the number of propellers?


I think two-bladed propellors are the most common, but there are lots of three- and four-bladed propellors out there. Higher rotational speed favors fewer blades because it gives the air more time to recover from the prior blade passage (which is why propellors are more efficient at slower speeds). But more blades have the advantage of lowering the thrust loading on each blade. Everything's a compromise.


On real aircraft, there's everything from two- to eight-bladed propellors (including five and seven).

Sorry, I meant propellers. But yeah as far as the number of blades it really just depends on what kind of performance you're aiming for. Three seems like a good choice in general. Any given configuration is going to have its trade-offs though.