LF Thoughts on Propeller design

[Rick Lowe]

Hi, all.

So I am in the midst of looking at various propeller ideas for various re-engined builds and was wondering what would usually be used in the RW.
Disclaimer - I'm not that knowledgeable about this sort of stuff, this is more of a beginner's guesstimation.

UK - seems not to have any preference, as they used 3-, 4-, 5-, and 6-bladed props, though the 6s were usually counter-rotating 3s.
US - seem to prefer 3- and 4-bladed; I don't recall anything with 5, and the current trend is for 6 sickle-shaped ones.

The engines I am using are loosely based on an Allison-style turboprop, one of which is on a US aircraft and the other on a UK one.

I was thinking the UK one would suit a 6-bladed contra-rotating and the US design a 4-bladed one.

What do others (particularly the more knowledgeable ones - i.e. everyone other than me...) think?

TIA

[Wardukw]

Mate depends on when in away..you mentioned the new sickle style props ..those  produce more thrust but also reduce fuel consumption as there carbon composite blades so their very light.
For a turboprop you can use a 4 or more prop ..even tho counter rotating always looks great .
It's just a matter really of what you want to use.
It's a thing about thrust to weight ratios and other super techy stuff but it works out kinda like this..to small a prop ..not alot of thrust and the engine will rev its nuts off and go nowhere fast..to big and then the engine would struggle to turn it fast enough for the thrust needed..so slow speeds with stressed engine...the trick is finding the right size blade for the right application.
Also when you up the power of the plane you up the number of blades to make better use of it.
I've been lucky to have gotten very close to some WW2 fighters and when you stand beside a P47 you see a stonking big plane with a monster of a propeller yet stand beside a (my absolute favorite single seat fighter) a MK24 Spitfire and the blade isn't near as big yet the engine is producing kinda close horsepower..P47D is what I stood beside so I'll use it's HP which is like 2500hp + where as the MK24 I stood beside had a Griffon 67 in it..was told that by the head mechanic ,which is 2340hp.
So to keep the blades working the Brits just added more blades..5 if they had used 4 it wouldn't have been as efficient.
For the cool factor mate..counter rotating goodness 😍

[Dizzyfugu]

The number of blades is dictated by the engine power that has to be converted into thrust. You can distribute the propeller disc area into a large 4-blade prop, or onto two discs in the form of a contraprop. A smaller prop might also achive higher speeds, and might need deeper blades to create the same active area as a bigger prop. The number of blades might also reduce prop speed and therefore noise. And the contraprop has the side benefit - despite being technically more complex - that it balances torque forces, highly helpful on carrier-borne aircraft, and IIRC the Griffon-poweer Spitfires with the five-blade prop were very prone to suffer from these torque effects during starting and landing, and the single P-51 that was trialed on a carrier also had severe torque problems that made naval opertions quite hazardous.

[PR19_Kit]

Adding  a larger, more powerful engine to an existing design will need more blades or a larger prop diameter to absorb the extra power. But you can't always go the larger blade route unless you lengthen the main landing gear, or the prop will hit the ground!

So a lengthened main gear will probably need re-engineering the wing or fuselage and that's EXPENSIVE.

Thus the 4, 5, and 6 bladed contra props on the later Spitfires, the P-47 and Corsair were designed with BIG power in mind, but Mitchell probably never thought his masterpiece would more than double its power during its life.

[zenrat]

From a purely aesthetic PoV I like the square tipped four bladed designs seen in the fifties.

[PR19_Kit]

I'm with you on that Fred. 

[Pellson]

There's another limiting factor to keep in mind - tip speed. If the propeller tips break the sound barrier, overall efficiency will suffer heavily. Hence, a larger diameter prop will have to turn at a lower rpm than a smaller one. Some aircraft designs however missed out on this, and apart from not being very fast, they make a LOT of noise, the Harvard trainer being a prime example.

Reversing this - repropellering (I LOVE English where you can make up words on your own like this  ) an existing design with a smaller diameter prop but keeping the rpm will make it quieter as the tips will move slower through the air. This works even if the original prop tips were below the speed of sound. However, to keep thrust, you will have to increase the no of blades to compensate for loss of blade area. Prime example - glider tug Piper Pawnees.

[Old Wombat]

Now I could be completely off the reservation here but, if my ageing memory serves me rightly, the 6-to-8 curved-bladed propellers on modern turbo-prop aircraft are designed to enable the maximum power at the highest rpm possible & the blade tips are actually trans-sonic but the curve of the blades prevents a standing shock wave forming along the blade, & the forming shock wave "slides" off the end of the blade tips, giving them that weird sound they have.

[jcf]

As engine power increases the options are more blades, longer blades or blades with increased chord. The last is the option that
became most common in Germany, the three blade layout retained with fat blades.
The three-bladed propellers of the B-36 remain the largest ever of that type.
The US also adopted the so-called paddle-blades, square-tipped blades and cuffed blades. Work on contra-props in the US
dated back to the 1930s and they were used on a handful of designs, but they weren't considered to be of much importance by
the services and as the age of the turbojet was dawning they were of low priority. The repeated claim that the US couldn't get
contra-props to work is incorrect, for the most part they couldn't be bothered to pursue development and production due to the
lack of interest.

It's often stated that US was lagging in supersonic research at the end of the war, but this is an incorrect assumption, the US
actually had the largest body of research into tran-sonic and super-sonic aerofoils because of long term extensive propeller
research throughout the 1930s by the NACA, USN BuAer, the USAAC Engineering Division at Wright Field and the manufacturers.
The NACA research and blade aerofoil work was the longest running and most intensive of the studies.

[Rheged]

I'm not a scientist of any kind, so the technicalities here are all new to me,  but I do agree with Zenrat and Kit about the superior aesthetics of certain 1950's square tipped blades.

[PR19_Kit]

The three-bladed propellers of the B-36 remain the largest ever of that type.

While I agree about that for aircraft, the largest props used on other craft were those of the SRN4 hovercraft, IIRC anyway.

[jcf]

The three-bladed propellers of the B-36 remain the largest ever of that type.

While I agree about that for aircraft, the largest props used on other craft were those of the SRN4 hovercraft, IIRC anyway.

Ah, but those are four-bladed.

[Rick Lowe]

Thanks for all the input, I guess it really comes down to what looks cool enough to me.
I agree the square-tipped and chunky look is good, so the Corsair will probably get that, while the Sea Fury is likely to sprout a contra 6-bladed.

Thanks for the info! 

*To throw a moggie in amongst the feathered rats, what size were the Soviet props on their hovercraft? 

[Wardukw]

Well the British hovercraft were  19 ft (5.8 m)  ..so if ya want bif Rick..there ya go 

[PR19_Kit]

*To throw a moggie in amongst the feathered rats, what size were the Soviet props on their hovercraft? 

Knowing the way they think, I expect they would be 19 ft 1" dia. but they missed out on that bit, they were actually smaller. The hovercraft themselves were just INCHES longer than an SRN4 Mk III though.

'Ner, ner, ner, our are bigger than yours'....

Just kids really.