Two Blades vs. Three Blades

[NCPilot]

So I've seen aircrafts with the same engine (Rotax 912) and yet some of them have two blade props and others have three blade props. What is the difference between the two, and how do I decide which one to get for my Kitfox?

[jnmeade]

The more blades you have, the more it looks like a fighter plane.
Seriously. A lot of decisions start and end with that.
Start first with props recommended by the airplane manufacturer or users group for that engine in the conditions you expect to fly.
Two blade props are more efficient (less drag). In fact, a one-blade prop (don't laugh) is most efficient and has been used. So, two blades are better for cruise. They're usually cheaper.
Three blades have better ground clearance. In some cases, that might mean they pick up less debris and consdquent knicks, but this is a nit. Because they're smaller in diameter, the tip speed is lower and thus it is a little quieter. They have better climb properties.
This is a topic subject to strong opinions, not all of which are substantiated with solid references. Caveat emptor.

Here is one discussion about Kitfox.
http://www.teamkitfox.com/Forums/showthread.php?t=1827

[drseti]

My opinion (and it's only that):
Three blades are better for flight training. Better climb performance, which is nice for pattern work. Better ground clearance, so less chance of prop damage.
Three blades also better for a bush or STOL plane, for same reasons
Two blades would be better for a personal XC machine. Faster, and cheaper.
YMMV

[Jim Stewart]

3 blades look cooler than 2 blades.

[drseti]

3 blades look cooler than 2 blades.

Yes, but I challenge you to tell the difference when they're spinning.

[roger lee]

Hi Paul,

I can tell and so can you. Put a 2 blade on your plane then try a 3 blade. You can tell with your eyes closed. Put you hand against the airframe or window. relax and feel the harmonic vibration transmitted by both. The frequency harmonics in the 2 blade is wide and courser and the 3 blade frequency harmonics are closer together and finer. The tone in the headset is different too. It tends to be the nature of the beast. I finished testing 5 Sensenich props against 4 other Mfg props, both 2 and 3 blade props several months ago. No astonishing revelations between 2 and 3 blade props.

[roger lee]

Hi NCpilot,

Personally I would stay away from wood props and even wood composite. Wood still absorbs the humidity and they do get out of balance. The newer Mfg composites of today tend to be better fine tuned for the length of the prop, the weight is easier to balance right from the factory, they don't get out of balance unless you do something to it, they don't absorb water, you don't need to do a static balance like a wood prop and can usually just do a dynamic balance. They tend to last longer. and there is no torquing of the prop bolts every 25 hrs like a wood prop.
2 verses 3 blade, well that's up to you, but I prefer the 3 blade for its sound and vibration qualities over a 2 blade. As far as one making your plane perform better, you couldn't tell the difference without instruments. A person doesn't have that quality providing all other factors are equal. You'll hear some make claims, but after doing some serious testing myself I really doubt it. Remember you would need to make everything equal, same plane or at least the same type and year, same time of flight, side by side, same rpm, same weight, ect... I would absolutely pick a ground adjustable over a fixed pitch prop any day. Too many things can change and you need to be able to change with them and a fixed pitch is just that, fixed and not flexible to your flying needs and or different flight characteristics that you may need from that plane some day.
The light composites are easier on engines than the heavier metal, woods or solid carbon fiber props. easier to start and easier on the engine to stop.

Everyone out in the world I'm sure is going to tell you why they like something else or that their choice is the best thing since sliced bread. After all my testing many claims were all just plain bread.

2 verses 3 is all up to you.

[FastEddieB]

3 blades look cooler than 2 blades.

Yes, but I challenge you to tell the difference when they're spinning.

Challenge accepted:


Untitled by fasteddieb, on Flickr

THREE!

What do I win?


BTW, taken just yesterday on the way to an EAA "Fly-Out" to Winder, GA for lunch.

[N918KT]

I prefer two blade props because of the asthetic looks of it on an airplane. It asthetically looks more simpler than a three blade. Three blade props makes the airplane look more advanced and fast.

[drseti]

You win, Eddie!