3 and 4 blades

[Mongoose]

Going by an airport near my house I saw a twin engine airplane.  One propeller had three blades, the other propeller had four blades.  I'm thinking they are in the process of replacing one of the props.  But it got me to thinking;  What would be the effect of this on the airplane?  I'm guessing an imbalance of thrust, and a lot of vibration.  I'd like to hear from some of the real airplane experts around here.  What would happen if you flew that plane that way?

[Vulcan]

The real question would be what happens if you put said plane on a conveyor belt.

[Arlo]

The real question would be what happens if you put said plane on a conveyor belt.

Pancakes.

[Shuffler]

The real question would be what happens if you put said plane on a conveyor belt.

It would be conveyed. 

To the OP, I figure it would fly fine with tuning the engines to equal thrust. Should be no vibration if props are not damaged. Might be some sound/harmonics.

[Arlo]

Might be some sound/harmonics.

In that case, install a harmonica.

[Shuffler]

In that case, install a harmonica.

(Image removed from quote.)

Bet you had to fiddle to get that one to twang.

[save]

I replaced a 2-blade wood propeller for a 3-blade Arplast composite propeller on my ultralight.

The result was most notably in it's climb rate, +100 feet per minute more with the Arplast.
.

[hitech]

It would fly fine, you could simple adjust the manifold pressure to = the thrust between the engines. And there would not be very much difference between the thrust of the props except at slow speeds depending on the diameters could also change the thrust curve at slow speeds.

At higher/cruise speeds there would be only a minor difference in thrust with the 4 blade most likely producing less then the 3 blade.

And I can not think of any reason that it would increase vibration.

HiTech

[Arlo]

Bet you had to fiddle to get that one to twang.

[Mongoose]

It would fly fine, you could simple adjust the manifold pressure to = the thrust between the engines. And there would not be very much difference between the thrust of the props except at slow speeds depending on the diameters could also change the thrust curve at slow speeds.

At higher/cruise speeds there would be only a minor difference in thrust with the 4 blade most likely producing less then the 3 blade.

And I can not think of any reason that it would increase vibration.

HiTech

Ok, so it wouldn't necessarily be hard to fly.  Except when it's on the ground the other planes will laugh at it.

[Vraciu]

I was told by one of my early instructors, an old school piston guy who flew every round engine plane you can think of that generally speaking you’ll get better climb performance out of four blades and better cruise out of three.    He also said three bladed props are smoother because of the geometry required to balance them compared to four-bladed props. 

For the same RPM you’ll have more noise/vibration because the blades pass the cabin more frequently with four blades vs three.   OTOH, to have symmetric thrust my guess would be the engines might not be at the same RPM, thus you’ll get the annoying “Washing Machine Charlie” thrum.

Just a guess.

[Vraciu]

Going by an airport near my house I saw a twin engine airplane.  One propeller had three blades, the other propeller had four blades.  I'm thinking they are in the process of replacing one of the props.  But it got me to thinking;  What would be the effect of this on the airplane?  I'm guessing an imbalance of thrust, and a lot of vibration.  I'd like to hear from some of the real airplane experts around here.  What would happen if you flew that plane that way?

Here is Hartzell’s take. 

http://hartzellprop.com/are-more-propeller-blades-better/

They don’t get into the frequency of the pulses and so forth.    Higher frequencies are easier to block with a headset or ANR system than low frequencies are.    The number of blades will affect this for a given thrust output.