What criteria determine whether to use 3,4, or 5 blade propellers?

[Reluctant Poster]

It actually is. The number of blades was chosen to provide best efficiency within installation and regulatory constraints. Also, once again, induced losses decrease as number of blades increase, and profile drag is determined primarily by blade area. Root thickness is driven by aeroelastic loads at takeoff; much of the blade is stalled and vibratory loads are very high. The design process at Hamilton Standard would include that in the selection of number of blades.

There is no magically universal "best" number of blades. There is no universal number for best efficiency. If you diagree with that statement, well there are a lot of reports from NASA, NACA, ONERA, DLR, ARC, and journal articles from professional organizations, such as AIAA to refute.

Actually I agree. My argument is with the statement that the fewer blades are better

 

[MiTasol]

Perhaps the total thrust delivered matters most. So the question is "How much thrust will this (e.g. B-17) engine give us with 3 blades vs a (e.g.) P-47 4-paddle-blade propeller?"
Avro Lancasters used 3-blade, running Merlin engines, while the P-51 and Spitfire used 4 & 5 blade props.

Gotta compare prop diameters, which is not always given in net-available search-findable sources.

Agreed. The desired result of meeting the regulatory requirements and space (and rpm) requirements is the compromise that provides the best thrust and fuel efficiency.

For prop diameters for those US Army aircraft in service at the time see the manual 03-20-1 Propellers & Governors for Service Aircraft

I thought I got my copy from this forum but a search came up empty so here it is

 

[Builder 2010]

I've been told that a single blade is the most efficient, but needs a lot of counterbalancing to make it function, and it would be very long. Each blade added increases the disturbance of the air effecting the other blades. A one-bladed wind turbine is the most efficient since it is always rotating in "clean" air. The Typhoon I'm building went from 3 blades to 4 as the Saber's HP rose. The same happened on the Corsair which started with 3 blades, but ended up with 4 when the R-2800 gained power. As many have said, it's all a compromise. As it were, the Corsair's prop was the reason for the remarkable gull wing. They needed to have a lot of ground clearance for the prop, but didn't want landing gear that were too spindly, so they brought the wing down closer to the deck. The Bearcat had very long gear and didn't need the gull wing.

 

[GreenKnight121]

The gull wings were also to allow the wing-fuselage joint to be at 90 degrees - which is much cleaner, and creates less drag than one at a different angle.

 

[pbehn]

Discussed here

 

[pbehn]

Hi,

I've been digging around for a while but can't really pin down why only the British went to five bladed props, whereas the US went from 3-4 and the Axis generally stuck with 3 blades from what I can tell. I understand that in some of the cases the move to 5 bladed props was to absorb the extra power and cope with ground clearance issues, but surely the same issues affected the late Me109's ?

ANy observations?

 

[wells]

I've been told that a single blade is the most efficient, but needs a lot of counterbalancing to make it function, and it would be very long. Each blade added increases the disturbance of the air effecting the other blades. A one-bladed wind turbine is the most efficient since it is always rotating in "clean" air. The Typhoon I'm building went from 3 blades to 4 as the Saber's HP rose. The same happened on the Corsair which started with 3 blades, but ended up with 4 when the R-2800 gained power. As many have said, it's all a compromise. As it were, the Corsair's prop was the reason for the remarkable gull wing. They needed to have a lot of ground clearance for the prop, but didn't want landing gear that were too spindly, so they brought the wing down closer to the deck. The Bearcat had very long gear and didn't need the gull wing.

That's a myth that has perpetuated since I don't know when, before the 1920s, I think. I'll try and find a source. But Swampyankee is 100% correct that the losses occur in between blades and that more blades is more efficient, given equal blade area and diameter. Air moves through the propeller disc and so no blade is operating in "dirty air" as long as thrust is being created. The difference is small though, about 2% between a 2 blade and 4 blade propeller. There was a NACA report from 1939 on this.

https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19930081563.pdf

The Corsair's gull wing had more to do with hangar clearance and pilot visibility and really had nothing to do with propeller diameter. There are other planes with 13' propellers that don't have gull wings ( Hellcat, P-47 ) and the Typhoon even had a 14' propeller and no gull wing.

 

[pbehn]

That's a myth that has perpetuated since I don't know when, before the 1920s, I think. I'll try and find a source. But Swampyankee is 100% correct that the losses occur in between blades and that more blades is more efficient, given equal blade area and diameter. Air moves through the propeller disc and so no blade is operating in "dirty air" as long as thrust is being created. The difference is small though, about 2% between a 2 blade and 4 blade propeller. There was a NACA report from 1939 on this.

https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19930081563.pdf

The Corsair's gull wing had more to do with hangar clearance and pilot visibility and really had nothing to do with propeller diameter. There are other planes with 13' propellers that don't have gull wings ( Hellcat, P-47 ) and the Typhoon even had a 14' propeller and no gull wing.

I have read it before about R/C models, is it possible that its true for a small model but not a proper plane?

 

[swampyankee]

I have read it before about R/C models, is it possible that its true for a small model but not a proper plane?

Models may start to run into problems with poor airfoil performance at low Reynolds numbers, which can make smaller chords undesirable. This is not a problem for full sized aircraft until very high altitudes— the sort where three-stage supercharged engines are needed.

 

[pbehn]

Models may start to run into problems with poor airfoil performance at low Reynolds numbers, which can make smaller chords undesirable. This is not a problem for full sized aircraft until very high altitudes— the sort where three-stage supercharged engines are needed.

I was just thinking it was like people saying if an ant was the size of a human it could lift a grand piano in its teeth, things don't work that way, you need an ant with hardened steel jaws etc. The length of a model prop blade is much less than the tip or the shaft of a real aeroplane and the tip is just a few inches from the centre of rotation.

 

[Simon Thomas]

With all else being equal, a V tail Bonanza is faster with a 2 blade prop, but take-off distance, climb rate and noise are better with a 3 blade prop.
There is not much of a difference though, a few % at best. Similar to the way 2 tail surfaces vs 3 tail surfaces doesn't really make much of a difference...

 

[wuzak]

Avro Lancasters used 3-blade, running Merlin engines, while the P-51 and Spitfire used 4 & 5 blade props.

Lancasters, mostly, used single stage Merlins, while the Merlin Spitfires and Mustangs with 4 blades had the two stage engines, which retained power to higher altitudes. The props had thinner air to work with.

From memory, no Merlin Spitfire had a 5 blade prop - that was reserved for the Griffon powered versions from the XIV onward. The XII used a 4 blade prop.

The Spitfire XII had a smaller prop than the IX due to the output shaft being lower, reducing the diameter which could be used without compromising ground clearance. The XIV had a smaller 5 blade unit for the same reason, plus the engine was tilted down at the front for better pilot view.

 

[Reluctant Poster]

Perhaps the total thrust delivered matters most. So the question is "How much thrust will this (e.g. B-17) engine give us with 3 blades vs a (e.g.) P-47 4-paddle-blade propeller?"
Avro Lancasters used 3-blade, running Merlin engines, while the P-51 and Spitfire used 4 & 5 blade props.

Gotta compare prop diameters, which is not always given in net-available search-findable sources.

Halifax IIs and Vs were refitted with 4 blade props to reduce the severe vibrations induced by the poor nacelle design. Preference was given to the outer engines where the problems were more pronounced This was one of the many improvements suggested by Rolls Royce to solve the many Halifax issues.

 

[pbehn]

Perhaps the total thrust delivered matters most. So the question is "How much thrust will this (e.g. B-17) engine give us with 3 blades vs a (e.g.) P-47 4-paddle-blade propeller?"
Avro Lancasters used 3-blade, running Merlin engines, while the P-51 and Spitfire used 4 & 5 blade props.

Gotta compare prop diameters, which is not always given in net-available search-findable sources.

I think the P-47 with paddle blade props looks remarkably like a B-29. I don't know what diameter a lancasters prop was, they didn't have ground clearance issues, but they also didn't go at the altitude and speed of a P-51 or Spitfire .

 

[Michael Hope]

I think the P-47 with paddle blade props looks remarkably like a B-29. I don't know what diameter a lancasters prop was, they didn't have ground clearance issues, but they also didn't go at the altitude and speed of a P-51 or Spitfire .

The Lancaster Propeller, Canadian build, with HS prop had a 13 foot basic diameter, using the 6519A-0 blade, 23EX Propeller.
The B-29 had a 16 foot 7 inch basic propeller diameter, 24F60 propeller.
The P-47 with the HS paddle blades, basic blade model 6507, basic 13 foot diameter, 24E50.
The P-51 with HS prop, 24D50, basic diameter 11 foot 2 inches.
The 6507 and 6519 are very similar in dimensions, differences is the inboard half of the blade with a slight difference in widths. The 6507 was the same blade used on the F4U and DC-4.

 

[jmcalli2]

It actually is. The number of blades was chosen to provide best efficiency within installation and regulatory constraints. Also, once again, induced losses decrease as number of blades increase, and profile drag is determined primarily by blade area. Root thickness is driven by aeroelastic loads at takeoff; much of the blade is stalled and vibratory loads are very high. The design process at Hamilton Standard would include that in the selection of number of blades.

There is no magically universal "best" number of blades. There is no universal number for best efficiency. If you diagree with that statement, well there are a lot of reports from NASA, NACA, ONERA, DLR, ARC, and journal articles from professional organizations, such as AIAA to refute.

Interesting discussion; it made me wonder, where does the prop on the XF-84H fit into it?
Thanks from a curious layperson.

 

[GrauGeist]

I keep seeing the reference to the P-47's paddle-bladed prop.
It wasn't until the P-47-D-22, that it received the Hamilton Standard "Paddle Blade" prop.

 

[swampyankee]

Interesting discussion; it made me wonder, where does the prop on the XF-84H fit into it?
Thanks from a curious layperson.View attachment 602206

This was an airplane which was so loud, it could be used as a weapon

 

[MiTasol]

This was an airplane which was so loud, it could be used as a weapon

Off topic that reminds me that the British Viper engine was often claimed to convert fuel to noise with thrust as a byproduct. It was also referred to as a variable thrust constant noise engine.

 

[mad_max]

I keep seeing the reference to the P-47's paddle-bladed prop.
It wasn't until the P-47-D-22, that it received the Hamilton Standard "Paddle Blade" prop.

The High Activity Prop as it was really called was retro-fitted to the D-11 model at the same time water injection was added in the field . You are correct that the D-22 was the first to have it installed from the factory.