3 blade props, 4 blade props, 5 blade props Why all the props?

[post76]

Well you evidently know what your talking about. Do you know why the Hellcat, Corsair and P47 would all have such different props when they are all large, heavy fighters, all 3 have the same basic engine and all 3 started out with 2000 hp? I can't see why all 3 wouldn't have the same prop.

You might also notice that all three types have different super/turbocharging ratios and different FTH altitudes despite having the same basic power plant.
A different prop is required to meet design needs..
I know that the same exact fighter types also used different props. Some props favor top speed, while others favor climb, and others favor acceleration, particularly in a certain speed range. It just depends on the use of the fighter, long range escort vs interceptor.
The chord of the blade and width will maximize performance where its needed.
Tip speed was also an increasing concern as fighters developed. They began squaring off the tips of the blades to help control mach effects.
You might also notice German designs have a thicker chord toward the center of the blade, almost triangular in shape this was also said to help control mach effects.
I think understanding the propeller type is as important as understanding the power chart of the engine.
It gives a better picture of how engine power actually translates into thrust.

 

[Shortround6]

There are two different big factors that affect propeller performance. Propeller diameter (disc area) and propeller blade area. Adding blades (or using broader cord blades) adds blade area but does nothing for disc area.


The size of the prop, even for the same power, varies with the speed of the aircraft and the altitude. The faster the expected speed of the aircraft the smaller diameter the propeller that is needed. The lower the altitude the plane operates at the smaller diameter that is needed. for instance a plane that does 400mph can use a prop that is about 6% smaller in diameter than a plane that does 300mph with the same power. A plane flying at 30,000ft needs a prop about 6-7% larger than a plane flying at 20,000ft. (and 22% bigger than at sea level) A plane with a 4 bladed propeller needs a prop about 6% smaller than a 3 bladed prop.
A four bladed prop is heavier than a 3 bladed prop. The Prop on a F6F was about about 60lbs lighter than the prop on an early P-47D and about 180lbs lighter than the paddle blade prop.

Other factors are the speed of the propeller (gear reduction of the engine), airfoil of the blade and shape of the blade. Every propeller is a compromise and while a different propeller can improve performance in some areas it can also reduce performance in other areas.

 

[davparlr]

If I remember correctly the fewer the blades the more efficient the propeller. I think I read that the most efficient propeller was a single bladed counterbalanced propeller, which has never been used for obvious problems.

 

[Edgar Brooks]

During WWII research, it was found that increasing from 4 to 5-bladed props only increased the propeller's efficiency by 1%, which is probably why the blade shape changed, as well.
Edgar

 

[Shortround6]

If I remember correctly the fewer the blades the more efficient the propeller. I think I read that the most efficient propeller was a single bladed counterbalanced propeller, which has never been used for obvious problems.

Actually it has been used, but mostly (if not only? Aeronca?) on Taylor Cubs or Piper Cubs with 37-40hp engines.

 

[Old Wizard]

The new Northrop Grumman E-2D Hawkeye has an eight bladed scimitar shaped propeller!!8)

 

[Gixxerman]

If I remember correctly the fewer the blades the more efficient the propeller.

This is what my ex-BAe aerodynamicist ex-father-in-law told me on the subject once.
I was wondering why modern prop planes all seem to be going for the multi-bladed cutlass shaped props and how come if this was the best solution it wasn't seen before, esp in WW2.

Advances in materials is a part of the answer I gather too but from his answer I took it that those who worked to more 'pure' aerodynamic teaching would have a natural inclination to prefer as few blades as possible.

Interference is something I am sure I recall reading about as something that put designers off contra-rotation pusher-puller types for a long while (the flow around these designs must be quite something to see in the wind tunnel!) .

 

[davparlr]

This is what my ex-BAe aerodynamicist ex-father-in-law told me on the subject once.
I was wondering why modern prop planes all seem to be going for the multi-bladed cutlass shaped props and how come if this was the best solution it wasn't seen before, esp in WW2.

The problem didn't really arise until towards the end of the war when very high powered piston engines were becoming available. There was an effort to put these powerful engines on fighter aircraft and best solution at the time was counter rotating props, which were problematic. However, jet engines were also coming along and had much better promise and little effort was spent on fighter size prop aircraft. However bombers and cargo aircraft could carry the larger propellers (B-29 propeller diameter was about 16').

Advances in materials is a part of the answer I gather too but from his answer I took it that those who worked to more 'pure' aerodynamic teaching would have a natural inclination to prefer as few blades as possible.

I suspect that technology to produce scimitar type props did not come about until the very strong carbon composite made its debut.

 

[swampyankee]

Well you evidently know what your talking about. Do you know why the Hellcat, Corsair and P47 would all have such different props when they are all large, heavy fighters, all 3 have the same basic engine and all 3 started out with 2000 hp? I can't see why all 3 wouldn't have the same prop.

Why different props? Different constraints on diameter.

I did prop aero for a few years. There are a lot of reasons why somebody would chose 3 vs 4 blades, including available diameter. Here's some tradeoffs:

Advantages of more blades:
> With more blades, you reduce the induced drag losses
> The individual blades are smaller and lighter
> Since the blade chords are smaller, the forces that the pitch change system needs to provide are smaller.

Disadvantages:
> The dimensions of the blade roots are dictated by vibratory loads during take off (much of the blade is stalled); this means that there is the possibility of choking in the blade roots (this is, iirc, not a problem with fewer than seven or eight blades, and only at flight speeds of about M=0.7 and up.
> More blades are more expensive, as the cost of the individual blade is more a function of blade span than of area.
> More blades means that more parts are needed.

 

[N4521U]

I love this forum.
Subjects get brought up that elude most of us.
The responses are most insightful and knowledgeable and we understand aircraft design even more.

Thanks!

 

[vikingBerserker]

I have to agree with you Bill.

 

[FLYBOYJ]

Why different props? Different constraints on diameter.

I did prop aero for a few years. There are a lot of reasons why somebody would chose 3 vs 4 blades, including available diameter. Here's some tradeoffs:

Advantages of more blades:
> With more blades, you reduce the induced drag losses
> The individual blades are smaller and lighter
> Since the blade chords are smaller, the forces that the pitch change system needs to provide are smaller.

Disadvantages:
> The dimensions of the blade roots are dictated by vibratory loads during take off (much of the blade is stalled); this means that there is the possibility of choking in the blade roots (this is, iirc, not a problem with fewer than seven or eight blades, and only at flight speeds of about M=0.7 and up.
> More blades are more expensive, as the cost of the individual blade is more a function of blade span than of area.
> More blades means that more parts are needed.

Also factor in engine RPM, torque and weight (the weight of each extra blade)....

 

[Ivan1GFP]

Does anyone here have information on moments of inertia for these large propellers?

Yes, I know how to calculate MOIs, but that is with very crude assumptions about the weight distribution of the propeller. What I need is the actual MOI values.

Thanks.
- Ivan.

 

[swampyankee]

Ivan, likely the only place you could find that information would be in the manuals for the propellers. It would be information supplied to either the airframer or the engine maker, especially the latter, as it would be critical for the vibration analysis, especially torsional vibrations, but also for airframe issues, like in the early L188s.

 

[john sexton]

Didn't the early Corsair and Hellcat have to power to spin a 4 blade prop? Wouldn't it have added performance?

Didn't the early Corsair and Hellcat have to power to spin a 4 blade prop? Wouldn't it have added performance?

I see a lot of High tech and Historival facts on the F4U-4 on when and why the 3 blade or 4 blade prop, Having served with both I won't try and give all the High Tech on the reason why other than the term of Power loading, Also some facts are off on here, the F4U-4 in Korean war had 3 blades but the F4U-5NL had 4 blades, I was Fight deck AB and was close and personal with them both.

 

[tomo pauk]

I see a lot of High tech and Historival facts on the F4U-4 on when and why the 3 blade or 4 blade prop, Having served with both I won't try and give all the High Tech on the reason why other than the term of Power loading, Also some facts are off on here, the F4U-4 in Korean war had 3 blades but the F4U-5NL had 4 blades, I was Fight deck AB and was close and personal with them both.

Welcome to the board.
A question - would you be so kind to post the Korean war-era photos of the F4U-4 that might show the prop nicely, since I have hard time to pinpoint the F4U-4s with 3-bladed prop? Thanks

 

[john sexton]

Welcome to the board.
A question - would you be so kind to post the Korean war-era photos of the F4U-4 that might show the prop nicely, since I have hard time to pinpoint the F4U-4s with 3-bladed prop? Thanks

Click o any of the Carriers of Korean war, Uss Princeton, Uss Boxer, Uss valley Forge ect. most had 4 blades but at very beginning in 1950 and 51 there are 3 baled as we used ww2 corsairs, The Marines had some 3 bladed ones also,,

 

[tomo pauk]

...
Click o any of the Carriers of Korean war, Uss Princeton, Uss Boxer, Uss valley Forge ect. most had 4 blades but at very beginning in 1950 and 51 there are 3 baled as we used ww2 corsairs, The Marines had some 3 bladed ones also,,

'Most had 4 blades' statement is in collision with 'Also some facts are off on here, the F4U-4 in Korean war had 3 blades but the F4U-5NL had 4 blades' statement.
The pictures show predominantly 4-bladed props on the -4, on Boxer and Walley Forge.

 

[john sexton]

'Most had 4 blades' statement is in collision with 'Also some facts are off on here, the F4U-4 in Korean war had 3 blades but the F4U-5NL had 4 blades' statement.
The pictures show predominantly 4-bladed props on the -4, on Boxer and Walley Forge.

As i said the 3 Blade was beginning of war and the Marine on their carrier like the CVL and CVE, I know the F4U 5 NL was four Blade props I cranked a few and directed and fueled a few, Look up Lt Guy Bordelon Only Korean war Navy ace , he was flying a night fighter F4 U 5 NL,, I was there to help him out of cockpit and at his award ceremony. The replica of his plane is in museum, but the original is in scrap heap in Korea, That too can be Googles

 

[john sexton]

As i said the 3 Blade was beginning of war and the Marine on their carrier like the CVL and CVE, I know the F4U 5 NL was four Blade props I cranked a few and directed and fueled a few, Look up Lt Guy Bordelon Only Korean war Navy ace , he was flying a night fighter F4 U 5 NL,, I was there to help him out of cockpit and at his award ceremony. The replica of his plane is in museum, but the original is in scrap heap in Korea, That too can be Googles

I am not sure your motive here for question,, Maybe you don't accept my word,, if so like I always told my sailors, don't ask the dam question if you don't like the answer.lol