Three-bladed vs Four-bladed prop

[Marshall_Stack]

Just wondering....

The P-47, F4U, and F6F all had the same radial engine. The P-47 had a four-bladed prop, the Corsair initialy had a large three bladed prop (then a 4 bladed), and the Hellcat had the "standard" three-bladed prop. The Hellcat was substantially slower than the P-47 and F4U. If the Hellcat was given a four-bladed prop would it been faster? I know that the F8F Bearcat was faster with a four blade prop, but it was also lighter (and I think it had an uprated engine).


What do you think?

[FLYBOYJ]

Originally Posted by Marshall_Stack

Just wondering....

The P-47, F4U, and F6F all had the same radial engine. The P-47 had a four-bladed prop, the Corsair initialy had a large three bladed prop (then a 4 bladed), and the Hellcat had the "standard" three-bladed prop. The Hellcat was substantially slower than the P-47 and F4U. If the Hellcat was given a four-bladed prop would it been faster? I know that the F8F Bearcat was faster with a four blade prop, but it was also lighter (and I think it had an uprated engine).


What do you think?

3 or 4 bladed propellers are determined by engine efficiency (determined by engineers) - operating propeller pitch and length are also considered. If you go from a 3 blade to a 4 blade propeller, it doesn't mean increased performance and efficiency.

[davparlr]

Originally Posted by FLYBOYJ

3 or 4 bladed propellers are determined by engine efficiency (determined by engineers) - operating propeller pitch and length are also considered. If you go from a 3 blade to a 4 blade propeller, it doesn't mean increased performance and efficiency.

The original question is a good question. What were the engineering or programmatical design goals that caused the selection of a three bladed prop on F6F and the four bladed props for the F4U, et. al.? Did the F6F have a different optimum operating envelope? What caused the change to the F4U that would not help the F6F? Etc.

I don't know much theory behind propellers. I do note that most of the high performance German planes use three bladed props. These must not have been optimum since after the war, most high performance propellers were four bladed (A-1, B-50, most commercial liners, etc. although the B-36 had three bladed). The C-130 has gone to even more blades. Gotta be several graphs that must be optimized for the desired characteristics.

[loomaluftwaffe]

and the Spit had Five

[FLYBOYJ]

Originally Posted by davparlr

The original question is a good question. What were the engineering or programmatical design goals that caused the selection of a three bladed prop on F6F and the four bladed props for the F4U, et. al.? Did the F6F have a different optimum operating envelope? What caused the change to the F4U that would not help the F6F? Etc.

I don't know much theory behind propellers. I do note that most of the high performance German planes use three bladed props. These must not have been optimum since after the war, most high performance propellers were four bladed (A-1, B-50, most commercial liners, etc. although the B-36 had three bladed). The C-130 has gone to even more blades. Gotta be several graphs that must be optimized for the desired characteristics.

It boils down to what you want to do - climb fast, go fast. cruise efficiently, take off from a high altitude airport or fly quietly - there are a few of the determining factors...

[wmaxt]

Flyboy is right, there is a lot to do with it. One of the considerations is that each blade causes turbulance as it goes through the air. I used to have a site on the net that explained a lot about this, Iwill try to find it and post it.

wmaxt

[syscom3]

The availability of four bladed props in the early war years was an issue. The early Curtiss-Wright 4 bladers had electrically controlled props that had some serious reliability issues (B26's were crashing quite frequently because of this).

I dont think things really improved untill Hamilton-Standard perfected their four blade designs.

[FLYBOYJ]

Originally Posted by syscom3

The availability of four bladed props in the early war years was an issue. The early Curtiss-Wright 4 bladers had electrically controlled props that had some serious reliability issues (B26's were crashing quite frequently because of this).

I dont think things really improved untill Hamilton-Standard perfected their four blade designs.

Electric props were at first very unreliable - electric current to the pitch changing mechanism's was run through brushes and a slinger disk - these used to fail and when that happened the prop would go to flat pitch - not good, especially on take off....

[syscom3]

Ive heard that the 11th wonder of the world is to listen to an R2800 at full throttle and load, go into flat pitch with zero load.

[FLYBOYJ]

Originally Posted by syscom3

Ive heard that the 11th wonder of the world is to listen to an R2800 at full throttle and load, go into flat pitch with zero load.

heheheh....

[lesofprimus]

Originally Posted by syscom3

Ive heard that the 11th wonder of the world is to listen to an R2800 at full throttle and load, go into flat pitch with zero load.

I can vouch for that.... I've heard that same saying from the mouths of guys who heard it IRL....

[Twitch]

If we look at unlimited air racing in post-war times we'll see planes mounted with 4 blade props that never used them in combat. The reason is that racing has no compromise of efficiencey, fuel economy, torque excesses, maneuverability, or pilot farigue. Racing is just balls out full power.

[wmaxt]

As I understand it prop efficiency follows these rules:

1. Diameter - Clearance requirements
2. Horsepower

Clearance dictates the max diameter of the Prop.
Horsepower and prop drive reduction gear dictates the blade area required to transmit that power.
The reduction gear is dependant on the prop diameter (to keep prop tip speed sub-sonic.
The number of blades is then calculated to transfer the HP(torque) to thrust.
The number of blades is also dependant on blade area and desired/available blade width.

The blade area is critical, more blades = thinner blades. High activity constant speed prop controls can affect this as well.
Here is a good site that goes into this :
http://www.mh-areotools.de/airfoils/...ler_design.htm

wmaxt

[GregP]

The most efficient propeller is one blade with a counterweight on the other side. Next most efficient is two blades, followed by three, then four, etc.

The real reasons to go to more blades are two:

1) There comes a point where the diameter of the propeller at the RPM in question approaches the sound barrier. You thenm opt for more blades at a lesser diameter to slow the prop tips.

2) There comes a point where the power to be absorbed by the propeller requires more blades or an increase in diameter that causes a) supersonic or transonic tips or b) unacceptable loss of ground clearance. (It is WAY easier to design a shorter prop than to fit longer landing gear)

The F6F and F4U make a really good comparison, and are the source of long-standing errors in performance estimates. They both had the same engine, same propeller at first and almost the same frontal area and, although the two planes had different performance numbers, they flew almost identically if flown side-by-side.

Grumman figured the Corsair had airspeed pitot installation errors causing it to be thought of as faster than it was. Naturally, Vought disagrees.

Famed Grumman test pilot Corky Meyer related that he flew an F6F-5 and an F4U1 side by side in 1943, alternating between the two, and there was no performance difference except in the main stage where Grumman did not use ram air and Vought did. Grumman did this to help prevent carb icing in cold weather, and MANY fewer F6Fs were lost to carb icing than F4Us. Both used ram air in high and low blower.

Anyway, the F4U was developed into the F4U-4 and had a much more powerful (2,600+ hp) P&W R-2800 than the F6F-5 (2,000 hp). Because of that, it needed more blades to absorb the power. Increased diameter was not an option since the landing gear was fixed.

Interestingly, the F8F Bearcat also had an R-2800. Later ... MUCH later ... one Bearcat used for racing owned by Lyle Shelton had (and Rare Bear still has) an R-3350 fitted. It was fitted with a propeller that requires the pilot to keep the plane in a 3-point attitude at all times. Lowering the noce will cause the prop to intersect the ground. So ... it works, but is quite a handful on the ground or when approaching a landing. Then extra power is also destabilizing.

More power without an attendant airframe modification is ALWAYS destabilizing, and results in a plane taht doesn't fly as well as one designed for the installed power. this is, of course, true for PISTON planes.

More powerful jets don't entail the addition of destabilizing blade area in front of the aerodynamic center of lift. Then again, jets can usually hit the airframe's critical Mach number more easily than can a propeller-driven plane. That is another story entirely, one that several early owners of Lear 25s discovered as they dived into the ground while pulling back futilely on the stick with all their strength. Geoffrey de Havilland also found that out when he died in his D.H 108 Swallow, I'm sure much to his regret. Therer were many more, both before and after these examples, some in WWII.

The Lockheed P-38 Lightning, among others, was known to get into Mach tuck at high speeds, though it wasn't called Mach tuck at the time. It was called "Compressability."

[davparlr]

Great discussion and very informative. Also, interesting info on F4U performance. Does anyone know if any F4U flew in unlimited air races?

I understand that he Russian "Bear" had supersonic props, or at least props that went supersonic, which made it a lot of fun to fly next to. Also, it must have been fun inside.