2 blade to 3 blade prop

[Oliver Terschowetz]

Any wise words of wisdom would be much appreciated please.

 

I have an old model spitfire with a 2 stroke glow .91, currently running well on a 2 blade master air screw 16 x 8.

 

I was contemplating putting a 3 blade on to make it look a little more authentic. Before I commit and purchase a shiny new spinner (£25 - £30 !), I just wanted to check first....

 

IS IT REALLY WORTH IT? 

 

I have picked up two sizes of 3 bladed props, 16 x 10 and 14 x 9.

 

If there are no detrimental effects of going to the 3 blade prop, what are the general rules of thumb regarding prop dia and pitch?

 

Would these two sizes be appropriate?

 

Am I wasting my time 🙄

 

Thanks for any help.

[Don Fry]

If it does well on the existing prop, (although Master Airscrew are not the most efficient brand), the pitch is good. So keep the pitch and loose an inch of diameter  at that size prop. So 15 by 8. 
Rule of thumb only, you are aiming to get the same revs.

Three blade props are not as efficient as 2 larger blades.

[Allan Bennett]

Yes, one inch less pitch or one inch less diameter has worked for me in the past.

[Oliver Terschowetz]

It's actually a OS MAX .61.

 

O.k, so Don your saying keep the 2 blade and by dropping an inch on Dia will that be 'kinder' on the motor?

 

Allan, is your "yes" FOR the 3-blade? Or "yes" to Don's "Three blade props are not as efficient as 2 larger blades".

 

What is "efficiency" regarding model engines? Just generally a happy motor !?

 

Thank you both.

[Martin Harris - Moderator]

If you want a prop for display then it would be better to go for a scale diameter dummy prop and spinner - neither a 91 nor a 61 2 stroke glow will swing a scale size propeller. 
 

3 blade props are only available in a limited range of sizes so you might end up with a compromise and I would only use them for ground clearance or noise limitation - as soon as the engine starts all props look the same!

[J D 8]

 When it comes to prop blade efficiency the theoretical best is just a single blade. In fact some indoor rubber duration props do just this with a weight opposite to balance.

The more blades there are the greater disturbance in the air for the following blade reducing efficiency.

 Full size use multiple blades for clearance issues, ground, airframe clearance and to absorb the power of powerful engines.

 At model size a two blade is simplest and best but a 3 blade will not lose much. The issue with model IC engines is they will not drive a scale diameter prop in most cases. This is because they are not geared.

 The prop on my SNJ [Navy AT6] is pretty much scale diameter because engine is a geared RCV 60sp.

 

 

[Oliver Terschowetz]

I think you've hit the nail on the head there Martin. (I was thinking the same thing). I get my enjoyment out of flying the darn thing rather than it sitting in the garage looking pretty with 3 blades! 

J D 8 - my engine is also geared. But "if it ain't broke...." I'll stick with my 2 blade and eBay the three blades!! 

Thanks for your input. 👍

[Allan Bennett]

My "yes" was for the one-inch size change.  I'm not clued up about the efficiency angle.  In fact the only time I used 3-blade with an i.c. engine was to reduce the noise.  It was an OS 120FS, but so long ago that I don't remember the actual prop size.  By following the one-inch 'rule' the performance felt the same and the noise did drop a little due, presumably, to extra load on the engine.

[Oliver Terschowetz]

Thanks for that Allan. I suppose I'm lucky in that I don't need to worry about noise. It was purely for a 'better look'. But now come to the conclusion after reading replies to this post that, what's the point! It runs perfectly well with 2 blades! And as Martin pointed out, who's counting when running! 

Kind regards.

[Cuban8]

Personally, I think you'd be better spending the money on an APC two blader of the appropriate size. I ran a Master Airscrew prop on my Goldberg Chippy for years until it must have got a bit brittle and lost a chunk of its trailing edge on my chicken stick when the engine didn't start cleanly. The new APC 14x7 was noticeably better all round.

[Martin Harris - Moderator]

1 hour ago, Allan Bennett said:

By following the one-inch 'rule' the performance felt the same and the noise did drop a little due, presumably, to extra load on the engine.

Sort of - the noise is primarily reduced by the resulting lower propeller tip speed as the disc diameter is reduced. The load on the engine won’t be vastly different due to the increased pitch so the RPM will be similar. 

[Manish Chandrayan]

13 hours ago, Oliver Terschowetz said:

I think you've hit the nail on the head there Martin. (I was thinking the same thing). I get my enjoyment out of flying the darn thing rather than it sitting in the garage looking pretty with 3 blades! 

J D 8 - my engine is also geared. But "if it ain't broke...." I'll stick with my 2 blade and eBay the three blades!! 

Thanks for your input. 👍

Started reading your post and was wondering what kind of performance were you getting using a 16x8 on 91 two stroke? Got a bit more incredulous as I read further when you said it was 61.....till the time you clarified it was a geared 61😀

[Don Fry]

As an aside, for yer average .91 two stroke, 16x8 is a big prop, 14x8 strikes me as more normal to save overloading the poor thing.

forget above, just realised, geared motor.

Edited November 7, 2023 by Don Fry
Admission of not reading the whole thread.

[Martin Harris - Moderator]

Of course, you could always model an early MkI and use a 2 bladed prop legitimately!

[Oliver Terschowetz]

So forgetting the 'geared' spec of my .61 for the moment, should I stick with the 16 X 8? In anyone's experience what changes could I make to either improve longevity of engine and or increase in power? As I mentioned she plods along quite happily but would be nice to have a little more thrust in reserve.

 

[Oliver Terschowetz]

1 hour ago, Cuban8 said:

Personally, I think you'd be better spending the money on an APC two blader of the appropriate size. I ran a Master Airscrew prop on my Goldberg Chippy for years until it must have got a bit brittle and lost a chunk of its trailing edge on my chicken stick when the engine didn't start cleanly. The new APC 14x7 was noticeably better all round.

 

Noted, thanks. Would that 14 X 7 be suitable to replace the 16 X 8?

[Peter Jenkins]

If your geared 61 is happy swinging the 16x 8 replace it with an APC of the same dimensions.  It will be quieter and give more thrust for the same rpm.  APC props are the best in terms of blade shape and give the highest rpm from most engines.  If you want to spend on carbon fibre props, then some of them might be better but they are not a tolerant to damage as APCs.

[Oliver Terschowetz]

8 minutes ago, Peter Jenkins said:

If your geared 61 is happy swinging the 16x 8 replace it with an APC of the same dimensions.  It will be quieter and give more thrust for the same rpm.  APC props are the best in terms of blade shape and give the highest rpm from most engines.  If you want to spend on carbon fibre props, then some of them might be better but they are not a tolerant to damage as APCs.

 

Thank you Peter. 👍

[Cuban8]

2 hours ago, Martin Harris - Moderator said:

Of course, you could always model an early MkI and use a 2 bladed prop legitimately!

I believe that there was much consternation at the time in that the first fixed pitch, two bladed Spits were performing no better than the already in service Hawker Hurricane!

Only really sorted when various two pitch and constant speed versions were employed, I recall reading.
i

[Oliver Terschowetz]

32 minutes ago, Cuban8 said:

I believe that there was much consternation at the time in that the first fixed pitch, two bladed Spits were performing no better than the already in service Hawker Hurricane!

Only really sorted when various two pitch and constant speed versions were employed, I recall reading.
i

 

Would love a Hurricane...

[John Stainforth]

18 hours ago, Oliver Terschowetz said:

Any wise words of wisdom would be much appreciated please.

 

I have an old model spitfire with a 2 stroke glow .91, currently running well on a 2 blade master air screw 16 x 8.

 

I was contemplating putting a 3 blade on to make it look a little more authentic. Before I commit and purchase a shiny new spinner (£25 - £30 !), I just wanted to check first....

 

IS IT REALLY WORTH IT? 

 

I have picked up two sizes of 3 bladed props, 16 x 10 and 14 x 9.

 

If there are no detrimental effects of going to the 3 blade prop, what are the general rules of thumb regarding prop dia and pitch?

 

Would these two sizes be appropriate?

 

Am I wasting my time 🙄

 

Thanks for any help.

I have a Hanger 9 Spitfire with an Saito 100 4s, originally with a 14 x 8 prop. I changed this to a 3-blade 13 x 8 - I can't remember why! It certainly wasn't for scale looks because the 3-blade looks even more obviously too small than the 2-blade. Maybe it was for better ground clearance. Anyway, the interesting thing was that I didn't notice any drop in performance at all; I even got the impression the plane had more oomph with the 3-blade than the 2, but these things are very subjective. The main cost was the aluminium spinner.

[Oliver Terschowetz]

7 minutes ago, John Stainforth said:

I have a Hanger 9 Spitfire with an Saito 100 4s, originally with a 14 x 8 prop. I changed this to a 3-blade 13 x 8 - I can't remember why! It certainly wasn't for scale looks because the 3-blade looks even more obviously too small than the 2-blade. Maybe it was for better ground clearance. Anyway, the interesting thing was that I didn't notice any drop in performance at all; I even got the impression the plane had more oomph with the 3-blade than the 2, but these things are very subjective. The main cost was the aluminium spinner.

 

Yes that's a good point actually. You go to the 3 blade for scale look and it ends up being shorter in blade length and looks prob worse than the 2 !! I just wouldn't want to upset how it fly's at the moment. Your looking at £30 odd quid for the spinner. 😵

[Peter Jenkins]

Of course, if you were flying electric, you could pick a motor with a suitable kv for a scale sized propellor.  Not so easy with IC without having a physical gear box that the electric motor with a low kv represents.  I know, it doesn't sound the same!  But look at that scale size prop!

[Oliver Terschowetz]

20 minutes ago, Peter Jenkins said:

Of course, if you were flying electric, you could pick a motor with a suitable kv for a scale sized propellor.  Not so easy with IC without having a physical gear box that the electric motor with a low kv represents.  I know, it doesn't sound the same!  But look at that scale size prop!

 

Call me old fashioned but give me the noise, smell and oily fingers every day of the week. An aircraft with 'soul' and a thumping heart at its core! 😁 ♥️I.c.

[Peter Jenkins]

I know, just not the same but then again, even radial engines don't sound like full size and as for V12 noise, forget it.  Most of us make do with a single cylinder 4 stroke.  Not the same as the sound of a Merlin though is it?