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

Discussion in 'Engines' started by pinsog, Aug 31, 2011.

  1. pinsog

    pinsog Member

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    I'll keep this thread to fighter planes. Why did many fighters of the same approximate power have so many different props? The Germans only fielded 3 blade props, England went all the way to 5 blades on the late Spitfires.

    The US had 3 and 4 blade props. THe Hellcat had a skinny 3 blade prop, the Corsair started off with a skinny 3 blade and went to a skinny 4 blade, the P47 started off with a skinny 4 blade prop and went to a fat 4 blade prop. They all had the same basic engine with the same basic horsepower, why wouldn't they all have the same prop? Would a Hellcat have benefitted from a 4 bladed paddle prop? Would the Corsair? And we all know the story of the P38K that was never produced. WHY WHY WHY????
     
  2. TheMustangRider

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    It is my belief that having armament and a gun synchronizer in the nose dictated to a certain degree the number of blades the propeller assigned to that fighter aircraft should have.
    Most if not all German fighters had armament in the nose and all kept the 3 blade propeller, the P-51A had at some point 2 .30 cal MG and a 3 blade propeller that was changed for a 4 blade propeller once the Packard Merlin engine was installed and the machine guns were deleted from the B variant onwards.
    That's as much as I know, let's wait for the knowledgeable people to pass by and share their knowledge :D
     
  3. pinsog

    pinsog Member

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    Never thought about synchronized guns, make sense though.

    I'm sure one of the engineer guys will straighten us both out here in a little while.
     
  4. Piper106

    Piper106 Member

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    #4 Piper106, Aug 31, 2011
    Last edited: Aug 31, 2011
    When engines get more powerful there are only a few ways for a propeller to convert the increased horsepower into more thrust; get bigger in diameter, wider blades or more blades.

    Getting bigger in diameter is usually a problem since diameter is limited by the landing gear length.
    The Spitfire had to have been a propeller problem since HP more than doubled from the Mk.I to the Mk.22/24, but the landing gears legs stayed short.

    Another issue is that the propeller has to be big enough to absorb the engine power at high altitude where the air is thin (low density). A fighter that flies at high altitude has to have 'more propeller' than the same HP fighter that is limited to a lower altitude. Again the Spitfire is an example of a fighter where the 'working' altitude the propeller had to be designed for also increased significantly over its production life.

    On the other hand, you want to stay with just a few blades if you could because (think of the propeller's blade path) more blades are more likely to get in the wake turbulence (or even sonic disturbance) of the proceding blade which reduces the ability to convert horsepower into thrust.

    That is all I think I know.

    Piper106
     
  5. pinsog

    pinsog Member

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    I can see your point 100% and agree with that, BUT, why would 3 fighters, Hellcat, Corsair and P47, that have the same engine and at the start they had about the same horsepower, all have dramatically different props? All 3 of them were rather big aircraft, especially the Hellcat and P47, very similiar in size and weight.
     
  6. Piper106

    Piper106 Member

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    On the Mustang, the P-51B/C/D had both more power and the Packard Merlin two stage supercharger could deliver that power at higher altitude than the Allison in the P-51A, so the P-51B/C/D needed 'more propeller' than the P-51A. Again. landing gear length is already set, so the only choices are more blades or wider blades (or a little of both).
     
  7. Piper106

    Piper106 Member

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    I can see your point 100% and agree with that, BUT, why would 3 fighters, Hellcat, Corsair and P47, that have the same engine and at the start they had about the same horsepower, all have dramatically different props? That is a head scratcher for me too.

    My guess is that the P-47 had a higher 'design altitude' from the get go (more prop needed) than the Hellcat or Corsair, so it started with a four blade rather than a three blades like the Hellcat and Corsair. After that I am dazed and confused.
     
  8. Piper106

    Piper106 Member

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    #8 Piper106, Aug 31, 2011
    Last edited: Aug 31, 2011
    What I think I know is that the later Hellcats should have switched to the same four blade prop used on the late F4U-1.

    Logic indicates that there could be two reasons this did not happen;
    1. Very scientific analysis pointed toward using the always short supply of good stuff on the better of the two, the Corsair.
    2. A emotional personality driven decision was made by someone in the Navy to send the good stuff to the Corsair program.

    There was a prototype XF6F-6 with the C series R-2800-18W and a four blade prop, but developement was dropped. The F4U-4 with the same engine went into production (yes, it was 30 mph faster than the Hellcat with the same engine).

    I will leave why we did not get both the F4U-4 and the F6F-6 and also why the Hellcat was phased out almost immediately at the end of the war, while the Corsair continued on into the Korean war to better historians than I.

    Piper106
     
  9. mcoffee

    mcoffee New Member

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    With the exception of a couple of test aircraft, no F4U prior to the F4U-4 used a four bladed prop. The higher horsepower of the -4 dictated the change of props.

    The P-47 went with a smaller diameter, four-bladed prop for ground clearance consideration, versus the larger diameter three-bladed props used on the F6F and F4U.

    The XF6F-6 was abandoned in favor of the F8F.
     
  10. pinsog

    pinsog Member

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    What was the size of the 3 blade F6F prop vs the 4 blade P47 prop? Do you think speed or climb would have been improved on the F6F or F4U with a 4 blade or 4 blade paddle prop?
     
  11. davparlr

    davparlr Well-Known Member

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    This thread has already been discussed on the forum. However, I am unable to find it as I am unable to find anything on the search function.
     
  12. pinsog

    pinsog Member

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    I didn't mean to repeat a posting, but I searched for it and couldn't find anything. Would you by any chance have the answer?
     
  13. davparlr

    davparlr Well-Known Member

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    If I remember correctly, most of the reasons have been covered. One that has not is that as horsepower increases, there is a tendency to increase the length of the blades, however the length of the blade is also limited by mach. Propeller efficiency degenerates quickly at the speed of sound. And then there are shock waves. So to harness the increasing power and not have long landing gear and not have to slow the rpm down, blades are added. I am sure the engineers designed propellers based on the design performance envelope desired. In the olden days, an auto manufacturer would have several transmissions and rear ends options for the same engine, all selected for various performance desires.
     
  14. mikewint

    mikewint Well-Known Member

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    Propellers are similar in aerofoil section to a low-drag wing and thus do not operate very well when at other than their optimum angle of attack. Therefore a method is needed to alter the blades' pitch angle as engine speed and aircraft velocity are changed.
    A further consideration is the number and the shape of the blades used. Increasing the aspect ratio of the blades reduces drag but the amount of thrust produced depends on blade area, so using high-aspect blades can result in an excessive propeller diameter. Using a smaller number of blades reduces interference effects between the blades, but to have sufficient blade area to transmit the available power within a set diameter means a compromise is needed. Increasing the number of blades also decreases the amount of work each blade is required to perform which limits the local Mach number. This establishes a significant performance limit on propellers.
    A propeller's performance suffers as the blade speed nears the transonic. As the relative air speed at any section of a propeller is a vector sum of the aircraft speed and the tangential speed due to rotation consequently a propeller blade tip will reach transonic speed well before the aircraft does. When the airflow over the tip of the blade reaches its critical speed, drag and torque resistance increase rapidly and shock waves form creating a sharp increase in noise. As a result aircraft with conventional propellers do not usually fly faster than Mach 0.6. There have been propeller aircraft which have attained speeds up to the Mach 0.8 range, but the low propeller efficiency at this speed makes such applications rare.
    There have been efforts to develop propellers for aircraft at high subsonic speeds. The 'fix' is similar to that of transonic wing design. The maximum relative velocity is kept as low as possible by careful control of pitch to allow the blades to have large helix angles; thin blade sections are used and the blades are swept back in a scimitar shape. In addition a large number of blades are used to reduce the work per blade and to maintain circulation strength contra-rotation is used. Such propellers are more efficient than turbo-fans and their cruising speed, in the range of Mach 0.7–0.85, is suitable for airliners, but the noise generated is tremendous.
     
  15. pinsog

    pinsog Member

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    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.
     
  16. Readie

    Readie Well-Known Member

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    The British produced hugely powerfull engines for fighters.
    The Americans used them in Mustangs as well as their own engines which were needed for different applications (durability range especially).
    If you want to study propeller design look no further than the Spitfire which progessed from a two blade, through 3 blade to 5 and then contra rotating propellors.
    Basically the propellor has to absorb the engine power and as power output climbed so the props got bigger or more bladed.
    The performance of the Spitfire increased through its service life only finally giving ways to the Jet in the 1950's.
    Cheers
    John
     
  17. Kryten

    Kryten Member

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    Interesting to note that the modern solution is multiple "scimitar" profiled blades, as seen on new prop aircraft as the A400M, so maybe the direction taken by the Mkxiv Spitfire for example was the best route to go for coping with high power outputs!
     
  18. mikewint

    mikewint Well-Known Member

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    I’m not knowledgeable about actual aircraft I’m more a “I know some of the tech stuff” guy. For example while I can tell you what the specifics of “inertial coupling” are I really have no idea what a real aircraft would do as it moves through the air. So I can tell you that any aircraft propeller in motion has five forces acting upon it:
    Thrust bending force
    Thrust loads on the blades act to bend them forward.
    Centrifugal twisting force
    Acts to twist the blades to a low or fine pitch angle.
    Aerodynamic twisting force
    As the centre of pressure of a propeller blade is forward of its centerline the blade is twisted towards a coarse pitch position.
    Centrifugal force
    The force felt by the blades acting to pull them away from the hub when turning.
    Torque bending force
    Air resistance acting against the blades, combined with inertial effects causes propeller blades to bend away from the direction of rotation.
    Why these particular aircraft have different style/type of props will have to be answered by someone more familiar with their particular flight parameters. I also know that any design is always a compromise involving gains in some area and losses in another. Part of that compromise is also dependant upon the structural materials available at the time.
    As to the curved or scimitar props they have been studied since the 1940s. The prop blade is curved away from the direction of rotation so as to delay the onset of shockwaves. This delay in shockwave formation (prop blades are airfoils) is similar to wing sweepback, where the blade tips approach the speed of sound.
     
  19. davparlr

    davparlr Well-Known Member

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    #19 davparlr, Sep 2, 2011
    Last edited: Sep 2, 2011
    I would guess that, for the F6F and F4U, there was really no difference other than a slight diameter difference. Both used three bladed props until each changed to the PW-2800-18W engine (F6F-6, F4U-4), when they both switched to the four bladed propeller. The -18W engine produced more power and and at a higher RPM. I suspect both of these drove the need for four blades (I don't know if the -18W prop had a smaller diameter). As for the P-47, first, it was an AAF aircraft and had a different manufacturer for the propeller. It was also always designed as a high altitude fighter where mach number is more critical (mach number gets lower as temperature goes down). My two cents worth.

    Apparently, the F4U-4 prop was 2" less diameter than the F4U-1.
     
  20. pinsog

    pinsog Member

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