Number of propeller blades

Discussion in 'Aviation' started by Jenisch, Aug 15, 2013.

  1. Jenisch

    Jenisch Active Member

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    What is the advantage of a plane having more propeller blades?
     
  2. Shortround6

    Shortround6 Well-Known Member

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    None if you are using a low powered engine.

    Blade area and disk area have to be balanced against the desired performance of the plane and at what altitudes. A fighter can use a smaller propeller for the same power than a transport or bomber can. High altitude planes need bigger propellers more blade area to deal with the thinner air.
     
  3. Jenisch

    Jenisch Active Member

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    The Bf 109 had considerable hp. What is the justification for three and not four blades like the P-51?
     
  4. silence

    silence Active Member

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    I know that the number of blades on a ship's propeller can have an effect on vibration. For example, North Carolina experienced severe vibration problems above 25 knots right after her commissioning and during her working up period that it wasn't until her props were replaced by props with a different number of blades that the vibration problem was (mostly but never totally) solved.

    Shortround6:
    Is it the total surface area that matters? Like, can a three-blade prop with a wide chord (like the TA 152H) and a four-blade with a slimmer chord (like the Mustang) provide the same performance?
     
  5. silence

    silence Active Member

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    Gun synchronization concerns, perhaps?
     
  6. GregP

    GregP Well-Known Member

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    A certain power is best addresed by a known blade area and diameter.

    The diameter is dictated by the landing gear height when the tail is up. If the plane has fuselage-mounted armament, the number of blades can limit the weight and rate of fire through the propeller arc. The Germans no doubt might have gone to 4-bladed propellers had they been using wing armament, but elected to go with wider blades on a 3-bladed prop to keep up rate and weight of fire with their cannons and MG that fired through the propeller arc. Wing root cannons would have to be synchronized along with the fuselage MG.

    Considering the performance of the German planes, particularly the fighters, I'm not sure I'd disagree with their choices in general in the performance department. The Bf109 and Fw 190 were both good-performing aircraft relative to their worldwide opponents.
     
  7. Greyman

    Greyman Active Member

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    An elementary way of looking at it is to compare it to rowing. More muscle (rowers) = more oars.
     
  8. gumbyk

    gumbyk Well-Known Member

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    Vibration characteristics of engine/gearbox/propeller may have had a part in the decision.

    Its not simply a case of bolting on a different propeller and going flying. Sometimes a propeller just won't work on an engine.
     
  9. silence

    silence Active Member

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    Depends on the size of the rowers!! Do you want four Arnold Schwartzeneggers or six Danny DeVitos? (after all, they are twins!)
     
  10. Shortround6

    Shortround6 Well-Known Member

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    prop size.jpg

    Chart form old book, It was a poor scan and I tried to fill it in a bit.
     
  11. swampyankee

    swampyankee Active Member

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    Overall -- I did prop aero for a living -- you've got a couple of competing issues going on, but, if there is a diameter constraint (there always is) and a tip speed constraint (ditto), induced losses decrease as the number of blades increases. So, the real question is why don't propellers have like, oh, 20 blades? The reason for that is that there is an optimum blade area, which is just enough so that the blade doesn't stall. The actual value of the area is set by the propeller's operating conditions and constraints: too much area, and the efficiency drops because of excessive skin friction; too little, and the efficiency drops because the blades are stalled. The minimum area for each blade is set by structural considerations, usually vibratory loads during take-off, and may be mildly constrained by Reynolds' Number effects, where airfoil performance tends to worsen as Reynolds' Number decreases. Blades with larger chords will tend to be more damage tolerant, which could impact serviceability. Manufacturing costs for individual blades are not strong functions of blade area, so a blade for a three-bladed propeller will tend to cost about the same as the blade for a five bladed propeller designed for the same conditions (this may be one of the very few cases where German military engineers of the WW2 era moved from the "optimal" solution to a less expensive one ;)). On the other hand, manufacturing constraints will come into play: it may be easier to design a new hub and modify the pitch change mechanism for an extra blade than it is to bring a new blade into production. And this doesn't get into the control system, as the blade pitching moment is proportional to the cube of the chord, so the forces that a pitch change mechanism needs to produce for a three bladed propeller are greater than those for a five bladed propeller designed for the same conditions.

    Here's a place to start: 11.7 Performance of Propellers
     
  12. davparlr

    davparlr Well-Known Member

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    If I remember my aero the fewer props the more efficient. I think someone actually tried one prop with a counter balance.
     
  13. gumbyk

    gumbyk Well-Known Member

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    Yeah, Bolkow built a helicopter with a single blade, the Bo-103
    histbo103g.jpg
     
  14. swampyankee

    swampyankee Active Member

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    Yes, an early constant speed prop for general aviation aircraft. Propeller drag is a combination of profile drag and induced drag. If you spread a given blade area out among more blades, the lift coefficient remains constant, but the aspect ratio increases, so the induced drag coefficient drops. Lower C[sub]d[sub]i[/sub][/sub], same area, same dynamic pressure, less total induced drag.
     
  15. GregP

    GregP Well-Known Member

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    #15 GregP, Aug 15, 2013
    Last edited: Aug 15, 2013
    Hi SHortround!

    Thanks for posting that chart.

    And thanks for the link Swampyankee.

    I flew a control line speed plane with a 1-blage prop. Very fast and smooth. I used an ALuminum strap with melted lead inside the spinner as a counterweight. Flew GREAT.
     
  16. Shortround6

    Shortround6 Well-Known Member

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    #16 Shortround6, Aug 15, 2013
    Last edited: Aug 16, 2013
    Such a prop was marketed and used on some 1930s Piper, Taylorcraft and Aeronca aircraft. Usually those with 37-40hp engines. I believe it was called the "Everal".

    One (or more?) Pictures can be found in Juptner's series on U.S. Civil Aircraft.

    Found picture on the net:

    EVEREL-Prop-Corp-E88-1_sm.jpg
     
  17. davparlr

    davparlr Well-Known Member

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    I knew it had to be somewhere.
     
  18. Tante Ju

    Tante Ju Banned

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    I doubt syncing would be a great issue - an engine revving 2800, such as typical for German engines and roughly geared down to 1:2 means that the prop still makes 1400 revolutions per minute - and a three blade prop theoretically offers 3x times, a four blade prop 4 x times of that opportunity, even if the latter is probably more delicate to tune. In other words, if there is just one firing opportunity between the blades in a full turn, you still should be able to achieve max firing rpm.

    Weight of an extra blade might be a serious issue though, even as little as an extra 20 kg can be quite a serious weight gain that can be used for other things, and needs to be counterbalanced. More blades also mean more drag and, lessened propeller effiency due to turbulance between the blades, so you might as well end up worse with more blades. As other said, matching blades to power and requirements is a delicate tuning task.
     
  19. drgondog

    drgondog Well-Known Member

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    This is one of the best overall texts linking Thermo to Aero/aerospace studies that I have seen. Is there a follow on course in his syllabus? What he presents here are all of the fundamentals to link aircraft/missile performance to 'efficiencies' - then extend to propulsion systems design in a comprehensive way.

    I like the way Spakovsky puts it all together.

    Thanks for posting this.
     
  20. Jenisch

    Jenisch Active Member

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    Just remember that initially the Ki-43 had a two-blade prop, which was moved to a three blade one in the Ki-43 II IIRC.
     
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