Prop pitch, rpm, and performance

Discussion in 'Firewall Forward / Props / Fuel system' started by Dana, Nov 13, 2019.

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  1. Nov 14, 2019 #21

    Pops

    Pops

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    That is the problem== Different brand props do act different. Darn.
     
  2. Nov 14, 2019 #22

    Rockiedog2

    Rockiedog2

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    It’s like trying on shoes these days
    The numbers are just a starting place
    It’s experimental aviation especially with props, always has been
    If it’s got an AD on it trim the tips that might fix it. Or make it worse. You do have a cable on the motor?
    3/16 is better than 1/8
     
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  3. Nov 14, 2019 #23

    Pops

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    IF you ever broke a crank or throw part of a prop blade, you will wish you had a cable on the motor. Been there and done that.
     
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  4. Nov 14, 2019 #24

    Rockiedog2

    Rockiedog2

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    Pops, you have used up way more than your 9 lives!
     
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  5. Nov 14, 2019 #25

    Dana

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    Trimming the tips is an option, but I think I'd rather reduce the pitch and/or chord first, especially since three other prop makers suggested 44 or 45. Gonna talk with Ed Sterba first, see what his thoughts are.
     
  6. Nov 14, 2019 #26

    BBerson

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    Reducing both the pitch and chord is easy. Just sand off the trailing edge to some calculated thickness, such as 1/8". Then sand the flat bottom till the trailing edge is back to the original 1/16" or whatever it was.
     
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  7. Nov 14, 2019 #27

    Dan Thomas

    Dan Thomas

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    I did that to my Jodel's wood prop in an effort to get some more RPM and therfore HP, and LOST performance when I got the few extra revs. That long prop (76-44) was so efficient that I was getting no indicated slip in cruise. The geometric pitch x RPM gave a speed that was the same as my TAS. Shortening it ruined that. I took an inch off each tip, then another inch, ending with a 72" prop. Should have left it alone.

    When it was at 76" it had the strange habit of the RPM falling about 30 or 40 revs as the airplane accelerated on the runway before liftoff, and one could feel it "bite" more. A little bit more acceleration. After shortening, it didn't do that anymore.
     
  8. Nov 14, 2019 #28

    Dan Thomas

    Dan Thomas

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    And it saves the engine if you have a prop strike.
     
  9. Nov 15, 2019 #29

    delta

    delta

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    I had a friend get caught in a rainstorm with a culver prop on his Q2, and he picked up some performance out of the deal.
     
  10. Nov 15, 2019 #30

    kent Ashton

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  11. Nov 16, 2019 #31

    Marc Zeitlin

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    While that certainly reduces the chord and prop area, won't that INCREASE the pitch, not DECREASE it? Take your procedure to a limit - if you sanded off 1/2 of the prop chord and then sanded the flat bottom to match the much larger TE thickness, that would substantially INCREASE the pitch, no?
     
  12. Nov 16, 2019 #32

    BBerson

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    Uh, no.
    You only sand off the bottom at the trailing edge to lessen the pitch and the actual angle.
    If you want more pitch then sand the top off the trailing edge instead, and a bit off the bottom of the leading edge. Doesn't take much, try just the outer 8" first. Make a new prop if you need a bunch of change.

    Hope that makes sense.
     

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  13. Nov 16, 2019 #33

    Marc Zeitlin

    Marc Zeitlin

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    Duh. Yes. Once I see the picture, it's obvious :). Thanks.
     
  14. Nov 16, 2019 #34

    blane.c

    blane.c

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    It sure would be helpful if you could borrow a couple of props or three before modifying. Or maybe you could at least rent one or something.

    Dia. = K (either 318 for metal or 285 for wood) multiplied by the [ fourth root of the { horsepower divided by ( RPM squared x the airspeed in mph ) } ]. This is in feet so multiply by 12 to get inches. Sorry I can't type a proper formula on this keyboard it lacks things like fourth root.

    For pitch.
    upload_2019-11-16_8-36-18.png

    upload_2019-11-16_8-35-37.png

    Having the propeller balanced while on the engine has proved worthwhile to me.
     
  15. Nov 16, 2019 #35

    Vigilant1

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    Is there more explanation of that graph? Are the x-axis and y-axis both labelled "propeller efficiency"?
     
  16. Nov 16, 2019 #36

    blane.c

    blane.c

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    yes, no.

    After doing formula you will end up with a (usually) decimal fraction like .562 for instance. From the bottom using your number go up till it goes into an arc nearest the apex, so for .562 you would pass the .6 arc and go up to the .7 arc as it fits nearer the apex. Avoid being on the backside of an apex. Then .7 time the propeller diameter will = pitch … so for a 5.8 foot dia. then 4.06 foot pitch or 48.72 inches and that is at the 0.75 diameter. You can look to the left of the curve for propeller efficiency were your number intersects the arc so for .562 it will be around 78% or 79% efficient.
     
  17. Nov 16, 2019 #37

    Vigilant1

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    Got it. You enter the horiz axis with the result from working the formula.

    The "black art" comes with figuring/estimating "N" (RPM). It is pretty important to this whole enterprise, but is a function of engine torque curves and the torque required curve for each prop. And at the end of this, you'd get a prop optimized for one point.
     
  18. Nov 16, 2019 #38

    blane.c

    blane.c

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    This Sport Aviation Article.
     

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  19. Nov 16, 2019 #39

    Dana

    Dana

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    There are a lot of techniques and formulas for propeller design, in fact I did my aerospace engineering senior design project on a propeller design using blade element analysis, but then there's the real world. The calculated design is just a starting point for testing.

    The fact that my new propeller has less pitch than the old one, but turns slower, pulls worse at low speed, but goes faster in level flight (in short, acts like a higher pitch prop) is interesting. I suspect that the thicker section and higher camber of the wood prop gives it a higher effective angle of attack, I.e. more effective pitch.
     
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  20. Nov 16, 2019 #40

    blane.c

    blane.c

    blane.c

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    Yes, but I don't have a copy of Jan Carlson's like you do.
     

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