2 Blade or 3 Blade .. is one better?

[Vev]

What is the advantage / disadvantage of a 2 or 3 blade prop .... which is better and why?

 

I'm looking to buy a new prop to stick on the front of a Rotax 914.

 

Cheers

 

Vev

 

 

[Derby]

Hi Vev.

 

2 blades are more efficient Than 3 but as you get more power you need more blades to be able to use the power that is why all or most planes with lots of power have 3 or more blades ww2 fighters had 3 or more if the blade length could be increased you could get away with 2, the most efficient number is 1 but it's very hard to balance.

 

 

[djpacro]

and a couple more general comments - the larger the diameter the greater the prop efficiency until tip speed gets to the drag rise Mach number. Higher tip speed also results in more noise.

 

Not all props are created equal so over to people with experience of actual props with that engine - be good to know the aircraft type to focus the discussion.

 

 

[Gnarly Gnu]

Generalising but I understand three blades will be smoother and typically quieter and a well designed version will often still be more efficient than a poorer-designed two blade.

 

 

[Deskpilot]

Now here's a thing I've been puzzling. Take a pusher plane with the prop close to the trailing edge of the wing. Will you (or rather the prop, 2 bladed) experience pulses at it passes behind the wing. If so, how detrimental is it to the prop and or engine and reduction unit?

 

My instinct tells me yes, better to go with a 3 bladed prop.

 

Sorry if this is a slight 'detour'.

 

 

[facthunter]

You only use more blades when you have a lot of power to absorb. 2 blades is enough for most engines under 200 HP and stronger and simpler to make if they are fixed pitch wood or metal.

 

CS hubs are not particularly more difficult with 3 blades. (In principle). Large dia props can only be used on geared or slow reving engines as tip speeds get too high (as mentioned). Undercarriages have to be longer.

 

Pusher props are in disturbed air and lose something from that, but tractors cause high velocity airflow over the fuselage which is an efficiency loss too., but the engine is cooled better. Nev

 

 

[Guernsey]

This could be helpfull.

 

Alan.

 

PROPELLERS - how many blades.doc

 

PROPELLERS - how many blades.doc

 

PROPELLERS - how many blades.doc

[Guernsey]

Now here's a thing I've been puzzling. Take a pusher plane with the prop close to the trailing edge of the wing. Will you (or rather the prop, 2 bladed) experience pulses at it passes behind the wing. If so, how detrimental is it to the prop and or engine and reduction unit?My instinct tells me yes, better to go with a 3 bladed prop.

Sorry if this is a slight 'detour'.

My Swing Wing aircraft (pusher prop) used to experience pulses but they were from the prop blades passing through the air from the exhaust pipe.

 

Alan.

 

 

[Yenn]

Pushers are not necessarily less efficient. The C337 had a problem if it lost its rear engine, much more so than losing the forward one.

 

 

[Vev]

Thanks for all of the responses guys... lots of wisdom!

 

I spoke with Sensenich in the US on Friday and asked them for their thoughts. They say there isn't much between their two props in terms of performance but they did say the 3 blade will feel smother and will be quieter.

 

Another point they made was about weight, stating that the 3 blade is 1lb lighter than the two blade owing to a new lay up technique and hub design ... this was not quite what I expected to hear!

 

Cheers

 

Vev

 

 

[eightyknots]

This could be helpfull.Alan.

Thanks for posting the article. The conclusion is:

 

‑ three for show but two for go.

 

 

Seriously though, I think that is a really good summary of prop blade numbers.

 

Thanks for all of the responses guys... lots of wisdom!I spoke with Sensenich in the US on Friday and asked them for their thoughts. They say there isn't much between their two props in terms of performance but they did say the 3 blade will feel smother and will be quieter.

 

Another point they made was about weight, stating that the 3 blade is 1lb lighter than the two blade owing to a new lay up technique and hub design ... this was not quite what I expected to hear!

 

Cheers

 

Vev

I'm quite surprised to hear that a three bladed prop is lighter than a two bladed one!

 

 

[facthunter]

The three bladed one will be a smaller diameter for the same application. It is quieter because the tip speed is lower. Obviously a three bladed one with the same or similar sized blades would have to be heavier, but would also shift more air and consume more power.

 

Lots of blades could cause interference. ( Like bi and tri wings) They work OK in ducted situations or shrouded blades. I am still an adherent of the two blade wooden article, perhaps skinned and with good leading edge protection. If it gets to strike something hard, you get less engine damage too. You can make your own if you are inclined and the engine torsional harmonics are handled better. They are about the safest thing out there. Nev

 

 

[Guernsey]

Thanks for posting the article. The conclusion is:‑ three for show but two for go.

 

Seriously though, I think that is a really good summary of prop blade numbers.

 

I'm quite surprised to hear that a three bladed prop is lighter than a two bladed one!

You'll probably find that the two blade prop is black whereas the three blade is white and therefore much ' lighter '.

 

Colourfull Alan.

 

 

[eightyknots]

You'll probably find that the two blade prop is black whereas the three blade is white and therefore much ' lighter '. Colourfull Alan.

Your kind of engineering advice is in what I would call a grey area.

 

 

[eightyknots]

You'll probably find that the two blade prop is black whereas the three blade is white and therefore much ' lighter '. Colourfull Alan.

Your kind of engineering advice is in what I would consider to be a grey area.

 

 

[facthunter]

Paint your 3 bladed prop RED, BLUE and YELLOW. When it is turning it will go white. Then you can tell if your engine is running without looking at the tacho. Nev

 

 

[Guest Wigg]

We went from a wooden 2 blade which was damaged at the air show, to a 3 blade brolly prop & found it was quiter than the original one, more economical, a lot quicker on take off & much smoother flying.

 

 

[eightyknots]

We went from a wooden 2 blade which was damaged at the air show, to a 3 blade brolly prop & found it was quiter than the original one, more economical, a lot quicker on take off & much smoother flying.

The more I read about this, the more I come to the conclusion that it is not just the number of blades that determines efficiency. There are a lot of other factors.

 

 

[facthunter]

You're not wrong Narelle... Nev

 

 

[Guernsey]

The two blade Bolly prop was much more efficient than the three blade Bolly on the 3300 Jab in the Morgan Sierra.

 

The two blade wooden Morgan Aeroworks prop gave the same performance as the two blade Bolly.

 

We only changed to the Bolly because of stone damage caused by gravel strips at Gawler airfield.

 

Alan.

 

 

[Mick]

The more I read about this, the more I come to the conclusion that it is not just the number of blades that determines efficiency. There are a lot of other factors.

Even different materials suit different shapes. I did alot of experimenting with props on aerobatic models back in the day when we had maximum engine capacities for competition models, hence we were trying to maximise prop performance. A prop that was molded in fibre reinforced nylon which performed really well, we flopped a mould of our own and copied it in carbon fibre, the carbon one was useless. We worked out that the nylon one was designed to flex & de-pitch under load but the carbon one was too stiff to do this.

 

As we also had very strict noise limits we also worked out that generally an efficient prop is a quiet prop.

 

We also found that the ideal prop on a particular engine type would not necessarily be that best prop on the same engine in an only slightly different design or different weight model.

 

You don't want to know how many model props that I have hanging on my workshop wall, but thankfully back then they were only between $15-$30 each.