[Shawn 507]

OK so why is it the Army's P-51, P-47 came with 4 bladed props and the Navy's F4F, F6F, and F4U all had three bladed props during WW2?
- Shawn

[flyXwire]

That's a very curious question, and this would be my response:
More powerful engines would need more propeller blades. To understand this problem, we need to realize that a propeller must be tailored to the specific needs of an engine. The job of the propeller is to "absorb" the power produced by the engine and transmit that power to the air flow passing through the propeller disk. Thus, energy is added to the air to speed it up and generate a thrust force on the propeller blades. If the propeller and engine are not properly matched depending on the power of the engine, both become inefficient and performance suffers.

[Shawn 507]

Um ok, but the Corsair later got a 4 bladed prop. Was that due to a new engine? And was a Hellcat that far off from say the P-47? BTW, I have no flippin' clue on this matter.
- Shawn

[timbo]

Ok here are the engines for each plane (gleaned from a web site and not double checked for accuracy)

P47 - Powerplant: 1× Pratt & Whitney R-2800-59 twin-row radial engine, 2,535 hp (1,890 kW)
Corsair - Powerplant: 1× Pratt & Whitney R-2800-18W radial engine, 2,100 hp (1,565 kW)
Hellcat - Powerplant: 1× Pratt & Whitney R-2800-10W "Double Wasp" two-row radial engine with a two-speed two-stage supercharger, 2,000 hp (1,500 kW)
Mustang - Powerplant: 1× Packard Merlin V-1650-7 liquid-cooled supercharged V-12, 1,695 hp (1,265 kW)

I would have though if it was just a power thing then both the Corsair and the Hellcat should have had 4 blade props. Was there some other reason?

[jim]

Originally Posted by timbo

Ok here are the engines for each plane (gleaned from a web site and not double checked for accuracy)

P47 - Powerplant: 1× Pratt & Whitney R-2800-59 twin-row radial engine, 2,535 hp (1,890 kW)
Corsair - Powerplant: 1× Pratt & Whitney R-2800-18W radial engine, 2,100 hp (1,565 kW)
Hellcat - Powerplant: 1× Pratt & Whitney R-2800-10W "Double Wasp" two-row radial engine with a two-speed two-stage supercharger, 2,000 hp (1,500 kW)
Mustang - Powerplant: 1× Packard Merlin V-1650-7 liquid-cooled supercharged V-12, 1,695 hp (1,265 kW)

I would have though if it was just a power thing then both the Corsair and the Hellcat should have had 4 blade props. Was there some other reason?

I have just been rummaging around trying to find a technical book I have about this topic.
I cannot find it (it will be in the loft somewhere).
What I can say is that more power usually means more props.
Factors include engine speed and torque, length angle and width of blades. All this must be worked out in relation to fuselage length tail surface size.

If the props are too long the danger is that the speed of the prop tips will go faster than the speed of sound thus creating extra drag and a major loss of efficiency.
Also long props have the tendency to cause a lot of torque ( twisting the aircraft over) both during take off and also in the dive.
The P40s suffered from excessive torque in the dive but for different reasons.( short fuselage being an early problem).
Another factor is revs, the ideal is slower revs but lots of torque through the engine shaft as in the case of the Corsairs.
An engine that reaches full power at high revs usually needs a short prop with wide blades to absorb the power and for want of a better description bites more air. plus it transfers less torque through the airframe.

As said before if you can build an engine that gives out max power at lower revs then you can make life a lot easier for designers.

Keep in mind that the best brains in aviation have battled with this problem and only achieved partial success, its always a compromise. every new engine has spent at least some of its development being tested with different props before going into production.
Supermarine worked out that the longer the prop the lighter it needed to be towards the tips, so up to the MKV, Spits had Balsa wood prop tips!
All this and the use of variable pitch, constant speed etc.
I am sorry that my post is a little obscure but its such a complicated subject with so many variables it would be impossible to go into in detail on the forum.

[Captain Eddie]

Very interesting Jim, thanks for the brief education! I really had no idea. I do, however, remember reading how the P 47 became a much better aircraft when it was matched to a "paddle" blade.

[jim]

Then again we havnt touched on the subject of contra props, no torque= very good, transmission failure in one of the props gearbox = hello ground.

[aeroeng]

Here's a pretty good discussion on this subject.
http://www.ww2aircraft.net/forum/avi...prop-3669.html

[planenutsfrog]

Could it simply be that naval fighters ldg gears have longer legs, than thoose of their land based counterparts, allowing larger diameter props to be fitted (instead of increasing the number of blades)...?

[jim]

Originally Posted by planenutsfrog

Could it simply be that naval fighters ldg gears have longer legs, than thoose of their land based counterparts, allowing larger diameter props to be fitted (instead of increasing the number of blades)...?

AH but longer legs give more travel , ideal for deck landings.