What would modern propellors add, if anything?

[mumbojumbo]

I saw an advert the other day for a turboprop propellor which seems to have a more complex shape than an old one, and it got me wondering what changes in performance, if any, à modern propellor would give to an old warbird. Is it as simple as min/max pitch angles and surface area, or is there more we know now that would help to improve performance. 

 

There's a bit of a gap in my knowledge when it comes to propellors. 

[JtD]

Just like the aircraft and the engines, props saw a lot of development in the WW2 era. If you look at pre-war and early war designs, you'll still find simple designs - fixed pitch, two stage pitch, variable manual pitch controls and prop sizes, shapes and layouts that left a lot to desired.

 

At the same time propeller underwent systematic research in several facilities, NACA for instance being one of them. The results of research are quick to find their way into practice in wartime, and by the end of WW2 props were pretty well understood and well designed. With planes then being as fast as they were, trans-sonic behaviour popped up and was still being researched. Other than that, hardly any breakthroughs since.

 

Times were new prop technology/invention could add 20% to an aircrafts climb rate were long past by 1945. Prop experiments were mostly about finding the proper prop for a new aircraft or engine, no longer about including new technology. What we have today might be some better materials, lighter and stronger, such as carbon fibre. This may add a bit of performance, given that props and other can be made lighter. But in terms of aerodynamics, the advantages would probably be minimal.

[PainGod85]

The issue with equipping WW2 era planes with modern props would generally be that their engines' reduction gear was meant to spin fewer long and thin blades more quickly, while today's props generally have no less than six paddle or even scimitar blades rotating at lower revs to keep wingtip drag in check.

 

In essence, for any large increase in net power you'd have to run a given engine at lower RPM as to not lose the excess in base prop efficiency to wingtip drag again.

Edited November 21, 2018 by PainGod85

[AndyJWest]

Modern turboprop propellers have to take into consideration noise regulations that weren't relevant in WW2, and I suspect this may be a major reason why they have changed. This 1995 NASA report seems to indicate that there is a direct benefit in using a slower-turning multiblade design, and it also discusses other shape refinements for noise reduction.

https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19960008053.pdf

[mumbojumbo]

Thanks all for the info, will read the report. 

[ZachariasX]

You can also see that the Reno Air Racers use propellers very much like they were installed toward the end of the war. Although they might be thinner and lighter, the concept of the layout is unchanged.

 

The Spitfire got a 5 bladed propeller due ground clearance issues, not because 4 blades would be less efficient. 5 blades came at a performance penalty.

 

I remember some of the towplanes (Remo DR400) in our flying club, they came out unmuffled and with a twin blade propeller. in the 1970 people didn‘t care so much about noise, but over time, they git a three blade, then a four blade propeller plus a huge muffler. Performance remained largely the same, but the plane got much quiter. New propellers did very little to it. It just became apparent when constant speed propellers were introduced, that you can do with 80 hp almost as much as you do with 180 hp on a fixed pitch propeller. That was a leap in performance gain.

 

Gearing a prop to liw rpm also means you are running your prop with a lot of torque. If you have a lot of power, it is not trivial to make a reduction gear that can take these forces. The Airbus A400M gives a lot of headache because of that.