4 Blade Propeller configuration options.

Discussion in 'Firewall Forward / Props / Fuel system' started by blane.c, May 17, 2019.

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  1. May 17, 2019 #1

    blane.c

    blane.c

    blane.c

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    A four blade propeller can be configured a blade every 90*, a blade alternately at different angles like 60*/120*, or stacked like a bi-plane. What is the reasoning behind choosing one configuration over another?
    In a 60*/120* type configuration which blade should lead the forward blade or the rearward blade? 4 BLADE PROPELLER.png 4 BLADE PROPELLER 90.png 4 BLADE PROPELLERS STACKED.png
     
  2. May 17, 2019 #2

    Swampyankee

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    That last one is weird. Biplane blades would seem to have very little to recommend them; I’d like to see the reasoning of the maker.

    For the 60/120 blade set up, it probably doesn’t matter which blade is “first.”

    Performance wise, there’s not likely to be a significant difference between four blades equally spaced and four at 0,60,180,300 degrees. There may be a different acoustic signature. (The AH-64 Apache uses that setup on its tail rotor, but the reasons are constructional, not aerodynamic)
     
  3. May 17, 2019 #3

    blane.c

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    Biplane blades were factory on many Lazairs because they had a bunch of those propellers in stock when they increased the engine's hp, why they arranged them that way?

    I was wondering about engine harmonics on the X props?
     
  4. May 18, 2019 #4

    Vigilant1

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    What HP is the engine you'll be using, what RPM do you envision running at, what is the max prop diameter you are trying to achieve, and for what airspeed would you like to optimize the prop ?
     
  5. May 18, 2019 #5

    Vigilant1

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    I don't know the answers, but luckily it would be pretty easy to try out all the possibilities and see how things work out. My guess would be:
    1) I cant see any advantage the the "biplane blade" setup, and would expect the same loss of lift that biplanes experience from inter-wing effects,
    2) If put in a 60/120 cross, I'd expect it might do better with the rear blade leading (so the second blade is in cleaner air, rather than being in the turbulent downwash of leading blade). But I'd still expect blades at 90 degrees to be more efficient.
     
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  6. May 18, 2019 #6

    pwood66889

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    I wondered about that while watching over the `64's on Ft. Rucker. Should you come up with a better reason, please inform. Thanx.
     
  7. May 18, 2019 #7

    blane.c

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    28hp @ 3600RPM Max / 21hp @ 3000-3100RPM cruise, 42" (4 blade), 75mph at least / 90mph maybe.
     
  8. May 18, 2019 #8

    TFF

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    Noise is a main reason for not keeping the blades at 90 to each other. But usually in that instance noise is more important than thrust. 90 def should give the best thrust. The question is do you need four blades for the diameter you plan to run? Most of the time more blades is used as a space saver for the amount of horsepower. You can’t have a Corsair with a 25’ two bladed prop. Too much rpm on smaller diameter props and the following blade can’t bite clean air. It’s why most RC planes run two blades
     
  9. May 18, 2019 #9

    blane.c

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    I balanced a lot of tires when I was young. I was taught to put the weights (1/2 inside rim and 1/2 outside rim) in pairs about 60* apart opposite the heavy side, it was considered bad workmanship to put one weight on each side. After putting about 1/2 the weight on the first side turning the tire over and spreading the weights on the opposite side you could move the remaining weights closer together or farther apart to dial in the balance dead on.

    Being able to move the prop tips further apart or closer together would have to effect positively and negatively the various conditions of balance about the crankshaft, getting it right would undoubtedly result in a smoother running engine.
     
  10. May 19, 2019 #10

    Tiger Tim

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    I asked a former Lazair pilot about the biplane props once, he said it had to do with reducing drag when stopped. With such a narrow gap between them I struggle to see two stacked props having much more thrust than one, but they must have had a reason for two. Of course, the reason could well have been marketing magic.
     
  11. May 19, 2019 #11

    blane.c

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    Drag reduction is a good reason.
     
  12. May 19, 2019 #12

    Vigilant1

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    Yes, at least it makes sense. With one engine stopped and one at full power, reducing the drag on the dead side would be pretty important even if level flight/climb wasn't possible on one engine.
     

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