What determined the number of prop blades?

[grampi]

Looking at many of the WWII fighter planes you see many different styles of prop blades, but the one thing that's always puzzled me is how they determine the number of prop blades. The Germans seemed to believe 3 blades was the way to go, as hardly any of their fighters had more than 3 blades, the British seemed to be open to any number of blades, while the U.S. pretty much stuck with 3 or 4 blades. Some planes would start out with 3 blades, then end up with 4 (P-51 and F4U for example). How was the number of blades determined, through testing, or through calculations?

 

[Shortround6]

Some of both. You needed a certain amount of blade area to transmit the power ( a bit like wing area/wing loading.) Disc area enters into it too. But like wing loading/size the propellers ability to "bite" the air depends on the air density, same HP engine needs a smaller prop at sea level than at 20,000ft. A 400mph fighter can use a smaller prop than a 200mph bomber/transport at the same altitude. Variable pitch helps but many props are a compromise of different requirements.

 

[GregP]

The power determines the amount of blade area needed and the diameter possible determines the number of blades. The Germans stuck with 3-bladed props because they primarily used fuselage-mounted weapons and needed to fire through the propeller arc, Fewer blades means more ammunition can be fired in one revolution. Many of their props had narrow hbs and very wide blades.

The British and Americans mostly used wing-mounted guns and had no issues with firing through the props.

The Germans, easily the best fighter spilots the world had ever known, said that one gun in the fuselage was worth two in the wings. Fuselage-mounted weapons don't have to be harmonized to converge on a point some 300 yards in front of the plane. Wing guns do.

As planes approached 2,000 HP, they usually needed a lot of either blade area or blades. Look at the props on the Fw 190D series or the Ta 152 series. Narrow hubs and high-altitude wide blades.

 

[drgondog]

applying propeller design is all about a.) which performance feature you wish to optimize (speed, cruise, climb), which combinations of two at which altitudes, trade offs between weights, what HP you are trying to apply to the optimal performance, tip speed considerations, chord variation as a function of radius, activity factor, efficiency of prop to the Horsepower generated, etc, etc.

Prop design was nearly as much an art as a science and frequently what looked good in design caused unanticipated problems in the air.

 

[stona]

The power determines the amount of blade area needed and the diameter possible determines the number of blades. The Germans stuck with 3-bladed props because they primarily used fuselage-mounted weapons and needed to fire through the propeller arc, Fewer blades means more ammunition can be fired in one revolution.

Which is almost exactly what I would have written, had I replied first

Cheers

Steve

 

[pbehn]

The obvious conclusion was the contra rotation prop but these also had reliability issues, weighed a lot more and (I read on here somewhere) contra props had a lower "G" limit.

 

[GregP]

It did have a lower g-limit because the internal driveshaft was of a smaller diameter. That's EXACTLY why the Spitfire that was converted (upon request, please recall) to contra-props by Fighter Rebuilders was converted back to a single prop ... worries about the prop coming apart under fighter-type maneuvers. It was, after all, a unit from a Shackelton and was never designed for fighter type g-loading.

You CAN make a contra-prop for a fighter, but I don't believe anybody did it and got it into production very successfully.

 

[stona]

Some Spitfire Mk 21s were converted, post war, to contra rotating propellers and issued to squadrons for evaluation. Seafire 47s also featured contra rotating propellers.

There was a plan to fit the system to Spitfire XIVs. There is space for ballast in the fin to counterbalance the heavier propeller installation. It didn't happen.

Cheers

Steve

 

[GregP]

Good thing. When they really needed maneuverability, one prop, the other, or both would likely shear off ... if the unit wasn't designed for 10+gs. That would not be the preferred outcome ...

 

[nuuumannn]

Constant speed props are probably the best way of attempting to get around different thrust requirements for different flight profiles for given blade profiles. fighters received c/s props somewhat later than larger aircraft; transports, bombers etc, the Ham Std Hydromatic prop with the Woodward constant speed governor, later licence built by Ham Std and Hartzell, was a winning combination.

 

[Edgar Brooks]

Ground clearance was the main reason in the Spitfire; Taking advantage of the extra power of the later Merlins/Griffons could have been done with a bigger diameter prop, but that would have hit the ground when the tail was raised during take off. A longer undercarriage was not an option, and cantilever operation would have introduced another unwanted complication. Extra blade = same diameter = problem solved.

 

[Gixxerman]

I remember asking a senior BAe aerodynamist about this some years ago. It was when civil prop planes with multiple 'cutlass' style props started appearing whether this was appearing now just because of new materials or was it unknown aerodynamics back then.
He told me the most aerodynamically 'pure' solution was as few blades as possible but that new materials meant the compromise with more bigger blades was less.
In short a 6 (or more) blade prop could have been made with similar profiles but the materials back then meant no advantage ( a lot of disadvantage - ie weight both of the unit itself it's controlling mechanisms) could be made from it.
Maybe if jets had not appeared for several more years we might have seen the need drive the tech that way?

 

[pbehn]

I just read the other day on the net that when variable pitch props (coarse fine) were first introduced the biggest change was in take off length. A fixed pitch prop is very very inefficient at low speed, take off lengths (and presumably times) were almost halved which was a big thing to the RAF looking to build new airfields all over UK. Maybe that is why constant speed props first appeared on transports.

 

[nuuumannn]

Yep, Pbehn, the c/s prop also gave greather thrust during manoeuvring as well over two-pitch props. The Bf 109E, although fitted with variable pitch rather than a constant speed unit - this didn't come in to the VDM unit on the Bf 109 until the Friedrich, could perform better in the climb than the Spitfire and Hurricane as a result of the British types initially being fitted with watts fixed pitch, then de Havilland two position props. The Brits were rather late in the game regarding props, despite the Hele-Shaw Beecham prop being fitted to a Gauntlet - this was the very first c/s prop and was pateneted in 1924. https://en.wikipedia.org/wiki/Henry_Selby_Hele-Shaw

Another thing that aided transport aircraft was the fitting of fully feathering reverseable props.

I remember asking a senior BAe aerodynamist about this some years ago. It was when civil prop planes with multiple 'cutlass' style props started appearing whether this was appearing now just because of new materials or was it unknown aerodynamics back then. He told me the most aerodynamically 'pure' solution was as few blades as possible but that new materials meant the compromise with more bigger blades was less. In short a 6 (or more) blade prop could have been made with similar profiles but the materials back then meant no advantage ( a lot of disadvantage - ie weight both of the unit itself it's controlling mechanisms) could be made from it. Maybe if jets had not appeared for several more years we might have seen the need drive the tech that way?

Interesting thoughts, but its also worth remembering that axial flow compressors were being conceived for driving propellers before they were fitted as a sole means of propulsion.

The turbine propeller obviously has advantages in economy that the pure jet doesn't and that's the real benefit of them, although from a technical prespective the turbo-prop is a complex beast, particularly the gearbox and propeller pitch controls; PCUs etc. Here are a couple of images to illustrate your post Gixxerman, the ATR and the Dash 8 are both powered by PW-100 series engines that produce roughly the same power output and despite the number of blades the props are both Ham Sundstrand 14SF props; identical in design inside the hub. The most obvious thing here that the ATR carries more pax than the Dash 8 has little to do with its engines and propellers and more to do with low airframe weight, thus providing considerable fuel savings, although performance wise, the Dash 8 is better than the ATR.

These photos were taken with my phone, so they're not too clear, not to mention the lighting in our hangars is crap at night.

 

[GregP]

I can tell you this. In control line speed models, the fastest planes always have a one-blade prop that is counterweighted with lead on the side the second blade was removed from. What they do is get a 2-blade prop that is the correct length for the single blade, cut off the other blade, secure a metal strap around it with lead inside it, and balance the prop statically as perfectly as possible.

Here is a pic:

There aren't very many public pics of them, but they hold all the records. The lead is inside the spinner.

Thing is, they aren't very practical for full-scale aircraft since the single blade would be longer than the landing gear ... but it IS the fastest you can get with a prop. Landing gear length determines the longest prop you can operate with. The 1980's version of Rare Bear was an extreme example.

Here is a pic:

This is when Lyle Shelton was running custom blades cut down from P-3 Orion prop blades. There was NO ground clearance to speak of and the aircrft had to take off and land in the 3-point attitude. If they raised the tail, they would strike a prop blade! I'd call that a decidely specilaized application that demanded very careful flying!

 

[nuuumannn]

This is when Lyle Shelton was running custom blades cut down from P-3 Orion prop blades.

The HS 54H60 - the blades are heavy muthers! Although we never had to polish them like that!

 

[GregP]

Lyle didn't either ... but DID for speed. They are very significantly reshaped. The airfoil is nowhere NEAR a P-3 blade shape, and the tips are almost round instead of square as on the P-3. The diameter is what it took to avoid a strike in the 3-point attitude ... according to Lyle, who definitely KNEW.

The Electra airframe had significant power and could fly even through a stall just by adding power!

Good plane after the "fix" that never really WAS a fix ... but they DID learn to stay away from resonance in the props.

 

[nuuumannn]

Yep, looking at the size of Rare Bear's blades, they've been cropped alright. The P-3 and Electra were real powerful aircraft, significantly overpowered, in fact.

Good plane after the "fix" that never really WAS a fix ... but they DID learn to stay away from resonance in the props.

Wasn't so much resonance in the props, but Whirl Mode, gyroscopic effect the propeller had on the engine mounts; the prop's aerodynamic forces were literally ripping the engines from the wing. I worked on P-3 ESUs when doing my apprenticeship and looked into this. The majority of the P-3 drawings are still marked L-188 Electra, including the engine mounts. I've got a data plate from one somewhere.

P-3 ESU with T-56 fitted. You can see the main structural load bearing members that the engines attached to as struts, that go from the firewall aft to where the side cowl stay attaches to the body of the ESU. This entire section (the ESU) was ripped from the wing and these engine bearers were strengthened as a result of the Electra LEAP (Lockheed Electra Action Programe):

 

[grampi]

What was the diameter of the 3 blader on Rare Bear compared to today's 4 blader?

 

[alejandro_]

Hello GregP, very interesting message.

The Germans stuck with 3-bladed props because they primarily used fuselage-mounted weapons and needed to fire through the propeller arc, Fewer blades means more ammunition can be fired in one revolution. Many of their props had narrow hbs and very wide blades.

I can see the point, but then these blades were wider. This needs to be taken into account in the synchronization. Is it easier to synchronize a 3 wider blade propeller than a 4 blade one?

Another issue is that as you get into higher speeds tips become supersonic. A 3 bladed propeller should have less problems.