2 or 3 Blade Prop Thrust

[Terry Plumridge 1]

Does a 3 blade prop produce more thrust than a 2 blade ? Same Dia. same pitch, same R.P.M.

[Engine Doctor]

As far as i know it doesnt. In laymans terms for our model size a single bladed prop is the most efficient as used on speed models. The air in the arc has time to recover before the blade takes the next bite. The single blade is counter balanced by a wieght. Two bladed props are easier to balance and give good all round performance. I think in simple terms , air is a constant and on our sized models the more blades spining in an arc will disturb the air flow for the following blade causing poor thrust. With bigger props as per full size the air isnt disturbed as much due to bigger distance between blades and so a gain in thrust can be achieved. The huge multi bladed props as used on later spitfires and on Corsiars were used to tame and load the massiive engines.

There will no doubt be someone on the forum who can explain the theory and in better detail .

In practice three blade props dont produce the same thrust like a two blade prop on our models. So multi bladed props for display and two bladed for flying.

Last year i was given a Durafly (or similar ) spitfire with a three bladed prop . While landing it nosed over breaking a blade off . No prps availabl locally so i fitted a two nlade and plane flew much better with more power. Then found a three bladed prop and fitted it to move it on. One last fly and it was , as before very tame.

[Shaun Walsh]

From personal experience with electrically powered models a 3 blade prop will give more thrust than a 2 blade prop of the same diameter and pitch but certainly not 50% more. The caveat being that the prop does not cause the motor to exceed its maximum current rating.
I have only resorted to a 3 blade prop when it was impossible to increase the prop diameter to increase thrust and I did not want to install a higher KV motor or a higher cell count battery.

Edited December 2, 2023 by Shaun Walsh

[PatMc]

1 hour ago, Terry Plumridge 1 said:

Does a 3 blade prop produce more thrust than a 2 blade ? Same Dia. same pitch, same R.P.M.

Yes it does, so long as the blades are also near identical.

 

ED is describing a different situation.

Edited December 2, 2023 by PatMc

[Martin Harris - Moderator]

There will be losses due to the extra blades working in disturbed air but the ability to turn horsepower into thrust was the reason the blade count increased from 2 to 5 over the life of the Spitfire, for example.  Larger diameters would have been more efficient if ground clearance wasn’t an issue. 

[Dickw]

Slightly off topic, but it is interesting to note that the electric RC speed guys always seem to use multi blade props to convert the power available into required combination of high speed and high thrust. The pic attached shows an example of a 4 blade 12 x 29 prop used to reach 600 km/h.

 

Dick

 

 

 

[J D 8]

Reason for the above is likely that at the speeds those prop blades are going a normal 2 blade would have to be made a lot stiffer and so heavy to withstand the strains

[Futura57]

12 hours ago, Engine Doctor said:

As far as i know it doesnt. In laymans terms for our model size a single bladed prop is the most efficient as used on speed models. The air in the arc has time to recover before the blade takes the next bite. The single blade is counter balanced by a wieght. Two bladed props are easier to balance and give good all round performance. I think in simple terms , air is a constant and on our sized models the more blades spining in an arc will disturb the air flow for the following blade causing poor thrust. With bigger props as per full size the air isnt disturbed as much due to bigger distance between blades and so a gain in thrust can be achieved. The huge multi bladed props as used on later spitfires and on Corsiars were used to tame and load the massiive engines.

There will no doubt be someone on the forum who can explain the theory and in better detail .

In practice three blade props dont produce the same thrust like a two blade prop on our models. So multi bladed props for display and two bladed for flying.

Last year i was given a Durafly (or similar ) spitfire with a three bladed prop . While landing it nosed over breaking a blade off . No prps availabl locally so i fitted a two nlade and plane flew much better with more power. Then found a three bladed prop and fitted it to move it on. One last fly and it was , as before very tame.

Efficiency isn't the same as thrust. 3 blades will give more thrust than 2 at the same diameter, pitch and rpm, but not 50% more because the configuration is less efficient. 3 blades will, however, require significantly more power to match the rpm of 2 blades because of the extra drag. I blew an ESC last year after replacing a 2 bladed prop with a 3 bladed of 1 inch smaller diameter. Current draw went from 35A to nearly 55A. Ducted fans tend to give more thrust with more blades (and sound better) but eat batteries. Afterburners give more thrust but are less fuel efficient.

And DITTO the Spitfire comment.

Edited December 2, 2023 by Futura57

[SIMON CRAGG]

I recently purchased a second hand FMS 55" Mustang, which came with a very nice looking 4 blade prop matched to an FMS predator motor.

 

After a bit of refurbishment it was time to test fly.

 

I had read reports that they were underpowered.

 

It flew ok, but not as sprightly as I hoped, so back to balancing blades, re setting throttle esc and even adjusting the esc timing.

 

Result?...........Still fairly gutless.

 

Put all the spec. into e.calc which advised me on a couple of two bladed options.

 

Furnished with new blades and spinner, I ran the motor up and checked everything was ok with the watt meter.

 

In the air? A complete transformation, shorter take off, better throttle response, and generally more "Grunt".

 

Definitely worth the effort!.

[Nigel R]

23 hours ago, Terry Plumridge 1 said:

Same Dia. same pitch, same R.P.M.

 

That makes it a completely different prop with far higher loading on the motor.

 

Apples and oranges.

 

You can of course convert from a given two blade to a three blade with equivalent load, and it will produce (give or take) the same thrust.

 

Full size aircraft use high blade count props and have done for years.

 

[PatMc]

The OP doesn't say that the props are necesarily on the same motor/engine or even on the same model. 

Edited December 3, 2023 by PatMc

[Nigel Heather]

On 02/12/2023 at 10:00, Terry Plumridge 1 said:

Does a 3 blade prop produce more thrust than a 2 blade ? Same Dia. same pitch, same R.P.M.

 

Just answering your specific exam question, yes, I believe it will generate more thrust.

 

Of course as others have said, if you just remove a 2 blade and replace it with a 3 blade the results are undefined because there will be extra drag, extra current, so there is no guarantee that the powertrain could turn the 3 blade at the same RPM.

 

But if you can turn a 3 blade with same diameter, same pitch, same RPM then it will generate more thrust.

[Futura57]

7 hours ago, Nigel Heather said:

 

Just answering your specific exam question, yes, I believe it will generate more thrust.

 

Of course as others have said, if you just remove a 2 blade and replace it with a 3 blade the results are undefined because there will be extra drag, extra current, so there is no guarantee that the powertrain could turn the 3 blade at the same RPM.

 

But if you can turn a 3 blade with same diameter, same pitch, same RPM then it will generate more thrust.

I concur entirely

[Terry Plumridge 1]

Alles Klar! Thanks for all replies.

Terry.