Why two-bladed props?

[Mart61]

Hi Guys

 

Here's another probably dumb question.  My boss, who is big time into warbirds (but not RC) asked me:

 

"Why is it that, whilst great effort has been put into a scale warbird's construction, just about every one is fitted with a non-scale two bladed prop?  Why do they not have the three, four or even five bladed props that many warbirds had?"

 

What should I tell him?

Edited By Mart61 on 26/09/2010 12:09:10

[winchweight]

Two bladed props are more efficent in models. They leave cleaner airflow for the next blade to pass through, they allow the use of larger diameter props without excessively loading the engine / /motor...

 

There'll be many more reasons.

[Myron Beaumont]

Easier to flick for one !

[Steve W-O]

It seems that serious scale have the scale props, but as an aircraft flies, and when it flies that is when it looks best, it is somewhat difficult to count the blades as 5kRPM plus, the better scale flying appearance  that the two bladed prop gives, opposed to the maybe not so scale flying with say a five bladed prop outweighs the static appearance.

 

Many scale planes are not the easiest to fly, and none look good in small pieces, so any compromise in control is best avoided.

 

Then there is availability, to look scale, there are limitations on engines that will fit, and may be hard to find a suitable multi bladed prop..

 

 

There's a couple more possible reasons !

[flytilbroke]

Many scale competitors fit scale props for the static judging then re-fit with two blade props for the flying part. Some of the scale props are a true work of art too.

[Mart61]

All makes perfect sense.  Thanks guys - I'll pass it on.

[winchweight]

They're cheaper if you break them

[Biggles' Elder Brother - Moderator]

Mart - to answer your question I suggest you begin by turning the question round.

 

Why did warbirds need 3, 4 or even 5 bladed props in the first place?

 

Answer - as piston engine power went up (about late 1930's and the start of WWII) the size of the two bladed props needed to convert that power to useful thrust became unacceptably large - they wouldn't clear the ground!

 

So designers moved first to three bladed, then four bladed and ultimately 5 and even 6 bladed props. You can track this development from, say, the prototype Spitfire (two bladed), through the early Hurricane (3 bladed) to the early production Spitfire (four bladed) to late Spitfires (5 bladed). As the engine power went up so the props had to acquire more blades in order to stay the same diameter but use the extra power.

 

But,...what Shaun said is spot on. Nothing beats a two bladed prop for efficiency. In, say, a 3 bladed prop each blade is much more "in the wake" of its precedesor than is the case for a two bladed prop. The situation is even worse when you move to 4 and 5 blades. With these each blade is having to do the best it can in the disturbed air of the blade before it. But the full size designers of the time had no choice. It was the only way they could handle the increasing engine power and yet keep the prop diameter sensible.

 

So, that's the full size story - what about our models? Our engines are very different from the full size. Ours produce most of their power at very high revs - typically 10,000rpm+. This compares with full size warbird engine revs of 2-3000 rpm - much lower. Both engines produce comparable (scaled) power but in a different way. The full size produces high torque at relatively low speed, ours produce low torque at high speed. As I say "same" power - but different running regime.

 

Because of the low torque offered by most model engines (in comparison to the full size on a scale basis) they struggle to swing a scale diameter prop even when it is only 2-bladed. The original plane might call, at scale, for say a 14 inch 4 bladed prop. In practice we would be lucky if we could swing a 13 inch 2-blade - but we'd swing it very fast - thus getting the scale thrust from our power output but in a different way from the full size! If you were to fit a 3 bladed prop to such a model, because of the engine's need to rev out high to deliver its power, such a prop would probably have to be limited to 10 inches diameter. Maybe even less. Alternatively, if you wanted a 4-blade prop it would be about 8-9 inches diameter - rememeber the original requirement for scale was 14 inches so we are now well off.

 

You could solve this by using a step down gear box. A 4:1 reduction gearbox would put you in about the right ball park. Your output shaft speed nd torque would then be more in line with the full size - then you could run inefficient 4 bladed scale size props. But it would be strange to take all that trouble to run a less efficient prop - especially when you can't even see the diffence in the air most of the time! The full size engineers would have killed to have been in our position - which they would have much prefered!

 

BEB

Edited By Biggles' Elder Brother on 27/09/2010 01:18:46

[Stephen Grigg]

And what did a variable pitch bladed prop do,was it like changing gear and how did it work?

[Martin Harris - Moderator]

Pretty much sums it up.  Fine pitch gives less slippage to enable more efficient acceleration and climb and coarse pitch allows better fuel economy and less engine stress in the cruise.

 

Most warbirds and modern VP props are constant speed units which (assuming sufficient power) maintain the prop rpm selected thus avoiding overspeeding. They are usually operated hydraulically with centrifugal weights sensing the rpm and controlling valves.

Edited By Martin Harris on 27/09/2010 01:41:47

[Stephen Grigg]

Thankyou Martin

[Klippy]

The early Spitfires had two pitches, coarse and fine, with a 'push pull' knob on the instrument panel. Many pilots crashed on take off because they left it in coarse! Douglas Bader included!

[Mart61]

Once again, many thanks for your comprehensive replies.

 

Having now explained the reasons why to my guv, he still says;

 

"Well that's as maybe.  They still don't like right though!" 

 

[Biggles' Elder Brother - Moderator]

He is absolutely correct of course - on the ground they don't look right (or even "like right" ) But "needs must" and all that....

 

BEB

[Mike Rolls]

Electricity is the answer. My ST51 powered Acrowot went best on an 11x6. My current electric version gives equal performance on a 13x8. Suitable drive train choices could use a bigger prop still if I wished (but that would run into the same ground clearance problems the full size designers had).

Mike 

[Myron Beaumont]

It's been discussed before I know but why only three blades on a wind turbine .The wind is all around and free .Old windmills were efficient weren't they !

[Danny Fenton]

Of course when you do use a scale sized prop it does make the model leap out as being more authentic. This is Phil Clarks (Fighteraces) Spitfire MkI

 

Cheers

Danny

Edited By Danny Fenton on 28/09/2010 08:05:15

[Ultymate]

BEB wrote " Nothing beats a two bladed prop for efficiency" well actually a single blader does but usually only found in very specialist applications.

[Peter Miller]

Er..Danny says.

 

   Of course when you do use a scale sized prop it does make the model leap out as being more authentic. This is Phil Clarks (Fighteraces) Spitfire MkI.

 

IT would look a heck of a lot more authentic with someone in the cockpit or was he bending down doing up a shoelace.

 

Edited By Peter Miller on 28/09/2010 08:26:14

[Danny Fenton]

Hi Peter, lol yes you are absolutely right. Kind of a spooky shot that one....... I think it was flying the "missing man" formation

 

 

[David perry 1]

Ref three blades on a modern wind turbine, I wonder if its to do with balance? Easier today to ensure bang on 120 degrees than in days of yore?  Easier to balance four blades ?

 

D 

[Biggles' Elder Brother - Moderator]

Posted by Ultymate on 28/09/2010 08:20:12:
BEB wrote " Nothing beats a two bladed prop for efficiency" well actually a single blader does but usually only found in very specialist applications.

 

 

Ha! Ha! Spot on Ultimate. You are of course correct! But I must admit I've never seen a warbird witha one bladed prop! Just to prove me wrong I bet someone now posts a picture of the little known Russian WW1 fighter the Illlychevskichov Mk14 with its famous one bladed prop!

 

BEB

[Myron Beaumont]

Although it's still strictly classified information I can tell you that the fastest one-bladed prop was/is used by aeroplanes that just go round in a circle. Well -They call them aeroplanes .

[Biggles' Elder Brother - Moderator]

Re Myron's question - which is a very interesting one - I don't know (not having done much work in that area) but I would guess the following might contribute to the reasons:

 

1. There maybe certain advantages in three blades.

A two bladed system delivering the same power would be of a significantly larger diameter - it would also have to rotate faster. These two facts might lead to unacceptable extra loading on the bearings and very likley higher blade noise. I do know that they do have "life issues" over the mechanics so anything that lowers the loading or makes its less "fluctuating" is viewed very positively so that could well be a factor. We all of course know that wind farms have faced a lot of opposition on grounds of noise - so that could also be a reason.

 

2. The penalities may be less.

Given the scale of many of these devices the penalty for going to three blades rather than two may not be as great as in an aircraft propellor. Both rotate in air with the same physical properties and so the rate of decay of turbulent eddies etc would be the same for both. But in the case of the windmill the speed of rotation is very much lower so there is s a much greater elasped time before one blade is moving in the "same space" as was occupied by the previous blade. This would mean the disturbance caused by the previous blade would have longer to die away. Also of course the blades in a windmill are physically much further apart - especially from say the mid way point to the tip

 

So given the possible benefits, and the relatively low penalties, three blades may well be optimium for windmills.

 

BEB

[Steve Hargreaves - Moderator]

Stephen G. part of the reason for a variable pitch prop is because the air gets thinner (less dense) as you go up so at a given pitch & rpm you will shift less mass of air at say 15,000ft than you will at sea level.

 

The mass of air you shift equates to the thrust generated....Make the pitch coarser & you shift more (less dense) air & maintain the thrust.