2 blade to 3 or 4 blade prop

[Steve Cornell]

Hi all,

 

This is something I was pondering the other evening whilst the wife was engrossed in yet another over the top story-line in Eastenders. I've read the thread about selecting the correct size prop and what to avoid in order to prevent burning the motor out; but what if we were to change a two bladed prop for a three or four bladed prop?

 

I know there is little to be gained by going to three blade as the pitch will be the same though the diameter would be less. But with the four blade, the pitch would change as well as the diameter and in doing so thrust is gained but some speed is lost; would this be of benefit though for electric motors, could it reduce the draw sufficiently on the motor to increase flight time? The other benefit of course, though I doubt it makes much difference with electric, is the reduction in noise as well.

 

Of course, on scale aircraft this would not fit with the overall appearance, but for fun flight and non-scale aircraft, would this be of benefit?

Edited By Steve Cornell on 06/01/2012 10:07:57

[Keith Simmons]

You can check using MotorCalc for the different prop set ups. 3 blade uses more watts to drive the prop as compared to a 2 blade. I have one 4 blade, (2 blades added together for a walrus as done on a full size) but it's not tested yet.

[Steve Cornell]

Good call, thanks Keith.

[Ben B]

The main reason for using multi-blade props on warbirds was to absorb the crazy amount of power the planes were using. If they had just gone on using bigger and bigger props they would have started hitting the runway!

 

If you go to a three blade prop it's traditional to reduce the diameter to compensate for the extra load of the extra blade(s). The pitch would stay the same so the pitch speed remains constant.

 

Try experimenting on Ecalc or motocalc it's quite interesting what happens to the static thrust and pitch speeds and motor current.

[Steve Cornell]

Electric flight is new to me, I'm moving over to it now but wanted to know what effect the change would have on the motor and battery - so I'm going to have a little play with Ecalc and see what happens! I'm not contemplating changing the prop at present, just interested in the options.

[Vecchio Austriaco]

Advantage of a 3-blade prop is that it looks good...

If I am modeling a plane which has a 3 blade prop on the original I try to do the same on the model, even I know that the best efficiency is on the 2-blade.

A 3 blade prop normally is one size smaller than the recommended 2-blade prop to have approximately the load for the engine.

The only technical advantage is that because of the weight it has a certain flywheel effect which could help with the idle of an IC engine.

cheers VA

[Martin Harris - Moderator]

Thing is, they all look the same when they're running. I always think it's best to stay with 2 blades for flying (more efficient) unless there's a technical requirement such as using a smaller diameter for ground clearance or noise reduction and make a proper scale prop if you want to look at it on the ground!

 

An added benefit of using a 2 blader for a scale model that used a multi-blade prop is that you're more likely to get a flying prop approaching scale diameter which will look better if the original had painted tips (you can see the disc more clearly in this case).

Edited By Martin Harris on 06/01/2012 12:57:02

[Ian Jones]

I accept all that has been said however I do not dwell too much on losing the efficiency of a two blade prop.

 

I'm more inclined to look at why I would use a three blade prop and if I do decide to do so, will it deliver the performance I need.

 

I used three blades on a Galaxy Twin Scorpion which I think should have had two 25s fitted but I used 32s, it was pretty quick!

I also have a three blader on my PC9 fitted with a 180 4st which seems to be fine but due to a number of other reasons I haven't flown many times. The indications are good though and once again I've gone for an engine at the top end of the recomended range.

In both cases the engine size was dicated by what was available at the time at a price I could afford and that in turn dictaed the use of three bladed props for ground clearance. Unfortunately I can't remember how I decided what size to use but it certainly involved a few good chats down at Barnstormers model shop.

 

[Ian Jones]

I've just been trawling the web for something else and came across the Master Airscrew advice on this very subject, so here it is.

 

(1000-5 and counting)

[Steve Cornell]

That's excellent!! Thanks for that Ian, printing it off now for future reference.