3 vs 2 Blades

[Paul Taylor]

Can we start a list of 3 bladed props vs 2 blades?

1. I know one is ground clearance for 3 blades.
2. A disadvantage might be weight.

[Brian Massey]

Weight is a problem. I have a beater Magician with a B40 that is always to fast. A friend suggested using an 11x4 vs the 10x5 I was using (both 2 blade).  The first launch with the heavier 11 incher had the plane turn in at me during take off; I had to backpeddle a few until in the air with sufficent speed. I flew that prop several flights and found the tendency to yaw inward "annoying" during outside squares too. So it was back to the 10x5 and the problem went away. Still to fast, so I've fitted an old Fox 35. When the weather clears up I'll see how that works.

I use a 3 blade on my TT, but that's for ground clearance issues . . . I built the landing gear "that much" to short.

Brian

[Jim Treace]

Can we start a list of 3 bladed props vs 2 blades?

1. I know one is ground clearance for 3 blades.
2. A disadvantage might be weight.

3 blade: maybe a little quieter, but they look cool!

[Randy Cuberly]

Weight is a problem. I have a beater Magician with a B40 that is always to fast. A friend suggested using an 11x4 vs the 10x5 I was using (both 2 blade).  The first launch with the heavier 11 incher had the plane turn in at me during take off; I had to backpeddle a few until in the air with sufficent speed. I flew that prop several flights and found the tendency to yaw inward "annoying" during outside squares too. So it was back to the 10x5 and the problem went away. Still to fast, so I've fitted an old Fox 35. When the weather clears up I'll see how that works.

I use a 3 blade on my TT, but that's for ground clearance issues . . . I built the landing gear "that much" to short.

Brian

Brian,
If changing from a 10X5 to 11X4 caused that much of a yaw problem, I would suggest that there are other trim problems that are simply being masked by the extra speed provided by the smaller prop.  The actual precession difference between those two props shouldn't be that great.
I would suggest checking tip weight line rake etc before condeming the larger prop.  It simply shouldn't make that much difference.

Randy C.

[Brian Massey]

Hi Randy; yes, this plane has many trim issues. I built it in the mid to late 60's, flew it until the early 70's and since then it has hung in various garages and shops. When I started flying again last year I built another wing, sprayed just enough blue on a yellow plane to fuel proof it and took off. For all that, when all is right, it will fly an "ok" pattern. Making a good trainer for me.

Brian

[Brian Massey]

Brian, try a 10 x 4 two blade. Or longer lines. One foot equals .2 sec. Roughly. 

I've been flying the Magician on 58' (eye to eye) lines, but have 60 & 62 foot lines in the box. Not sure why I haven't tried them. For now the Fox is mounted and the sun is shinning!    Still 5 hours to go before we can fly at our field and a storm is coming in . . . I'm hoping to get in a few flights. Depending on how the old Fox performsm, I may reinstall the B40 and switch to longer lines. Thanks for the .2 sec info, I've always wondered, but never known.

Brian

[Derek Barry]

More blades = less wind up and more consistant maneuver speeds.
With a 2 blade you will notice that the plane is easier to turn.

[Randy Powell]

I'm with Derek. In addition, more blades in smaller diameter can help with spiral airflow and GP/P-Factor issues. But with a two blade it's easier to turn.

[Paul Taylor]

OK, I have to ask, how does 2 blades make it easier to turn.

[Derek Barry]

OK, I have to ask, how does 2 blades make it easier to turn.

Don't know why it does; it just does. I am sure Randy, Brett or , Howard could explain.

I am sure some of it has to do with it being lighter.

[Jim Pollock]

I use 3 blades mostly for their ability to pull vertically better than a two blade.  I'm not sure how I know that they do, but that's what I feel happening at the handle anyway.....

Jim Pollock 

[Brian Massey]

Don't know why it does; it just does. I am sure Randy, Brett or , Howard could explain.

I am sure some of it has to do with it being lighter.

Could it have something to do the smaller/lighter spinning mass creating less tendancy to yaw?

Brian

[Alan Hahn]

OK, I have to ask, how does 2 blades make it easier to turn.

I think it just has to do with the 3 blade being a bigger flywheel, assuming it is turning about the same rpm as the 2 blade prop. So it tends to not want to turn upwards.

With regards to it climbing better, that has to do, I believe, with the 3 blade prop having a steeper thrust vs airspeed slope than the 2 blade prop.

Remember in level flight, they are both making the same thrust (otherwise the lap speeds would be different--and I think most would tune that out by adjusting pitch, or with an electric motor, simply adjusting the target rpm  ). When you pull the nose up, the airspeed begins to drop off. The 3 blade prop then makes more thrust than the 2 blade prop (I am assuming the rpm stays constant, but most likely it is falling off if it is a glow engine). The extra thrust (="pulling") helps preserve what airspeed you can keep, and so you have more tension in the overheads.

I have some calculations using "Prop Calc" that show this in the engineering section (http://stunthanger.com/smf/index.php?topic=14862.0). I believe the better pulling is basically just more blade area for the 3 blade prop. A wider (or even larger diameter) 2 blade prop would work better too--if you have the horsepower to turn it.

[Randy Powell]

More blades in a similar fan area create more of a gyroscopic effect so it's a bit harder to move it. You have to balance things like anything else on a plane. With a three blade prop, you have to balance the additional thrust and stability you get with the greater amount of control deflection it takes to move the plane. I try to get a two blade to work first. If I get sufficient thrust and drive from the two blade, I stick with that. If the plane struggles in high "G" situations or just doesn't have the poop to pull it high horsepower areas, I usually go to a 3 blade.

[Derek Barry]

 "The 3 blade prop then makes more thrust than the 2 blade prop (I am assuming the rpm stays constant, but most likely it is falling off if it is a glow engine). "

All of my motors break as I turn the nose up to give me the extra rpm needed to provide the line tention up top. So as I climb my rpm increases instead of "falling off".

[Alan Hahn]

"The 3 blade prop then makes more thrust than the 2 blade prop (I am assuming the rpm stays constant, but most likely it is falling off if it is a glow engine). "

All of my motors break as I turn the nose up to give me the extra rpm needed to provide the line tention up top. So as I climb my rpm increases instead of "falling off".

Derek,

Are you sure about that?

The reason they break" is that the load goes up. The load goes up when the airspeed drops.  I know there is a mixture change but from what I have seen it mainly looks like a load effect.

I've measured a few planes and they all lose rpm as they begin their climb. Some may finally speed up past the original level lap rpm but that is after the load has begun to drop (above 45 degrees).

I'd love to measure a piped plane, but haven't had a chance to do that yet. I've posted some of my stuff in the engineering section (under Fourier Measurement of rpm).

I'll also note that when an engine breaks into a 2 cycle, it sounds like the rpm increases because the engine exhaust doubles its frequency.

So that's why I ask.

[john e. holliday]

Think of putting a gyroscope on the front of your engine.