Prop pitch, rpm, and performance

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

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  1. Nov 16, 2019 #41

    blane.c

    blane.c

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    Also it is lighter than a metal one so different inertia.
     
  2. Nov 16, 2019 #42

    Vigilant1

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    I wish I could understand better what goes on inside that black box. Like most folks, I wind up guessing/estimating a lot.
     
  3. Nov 16, 2019 #43

    TFF

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    This is not to down Sterba, but they are the budget prop makers. Value for money is their market. If you want performance you get a prop from Performance. It will cost you three times as much as a Sterba.
    I know Dana has read the prop comparisons on the Biplane Forum but for everyone else not there, the metal Sensenitch ,performance wise, beat out all comers except in weight. That was the main reason to change props on aerobatics if the other losses were not to great. Now if you got the bucks Sensenitch has a composite version for homebuilts. To get like performance from a wood prop will require something like a Cato or Performance.
    I can only afford the metal prop that came on my plane or a Sterba. Dealing in low budget or lower performance aircraft can be a little hard because there is going to be very little change for the amount available to spend.
     
  4. Nov 16, 2019 #44

    Vigilant1

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    That sounds likely. Also, there could be an apples-to-oranges situation if the metal prop's pitch is measured along the chordline and the wood prop is measured along the flat at the bottom.
     
  5. Nov 16, 2019 #45

    Dana

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    Thanks Blane, that's one of the best prop articles I've seen. Back when Sport Aviation had real content...

    One thing the article mentions, which of course should be obvious, is to look at similar performing certificated planes such the same engine and what they use. For me, that'd be the L-14 (Piper J-5D)... which uses a 74-44. But interestingly, one version of the PA-18 with the O-290 used an 80x32!

    Sensenich uses the flat back to measure the pitch of their metal props, and my own measurement agreed. Sterba said the same, but the back of his prop isn't flat. But measuring 1/8" in from the leading and trailing edges is pretty close to the actual blade chord line, and agrees with what it's supposed to be.
     
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  6. Nov 16, 2019 #46

    mcrae0104

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    Blane, what's the source for this graph? I'd like to read up on it.
     
  7. Nov 16, 2019 #47

    blane.c

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    This is article, but he did the math to the wrong conclusion … he interpreted 5.36 feet to 54 inches instead of 64 inches. But otherwise good.
     

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  8. Nov 16, 2019 #48

    blane.c

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    My O-290-D2 had a McCauley 82" by 42" pitch. I could of gone with a 39" pitch but at those speeds would of amounted to maybe 20 feet less take-off roll and I was already using less ground for take-off than landing so seemed moot and I chose a slightly better cruise speed. Cruise was an optimistic 90mph but I flight planned 75mph. That dia. McCauley for Cubs is called a "borer prop". http://store.cubcrafters.com/McCauley-Borer-Propeller_p_1586.html

    The "borer" was originally developed for the O-290 125HP
     
  9. Nov 16, 2019 #49

    plncraze

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    Blane's reading list looks like he got bit hard by the propeller bug. Quentin Wald's article "Aerodynamics of the Propeller" and Neal Willford's spreadsheet that accompanied his propeller article in Sport Aviation are also good.
    There are a lot of assumptions in designing props they seem to feed off each other.
     
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  10. Nov 16, 2019 #50

    Dana

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    On what plane, a Cub?
     
  11. Nov 16, 2019 #51

    BBerson

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    What is that 1 over the square root symbol mean? Can't make sense of that formula.
     
  12. Nov 16, 2019 #52

    Dana

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    Interesting.... just did some reading. I had heard of Borer props but always assumed it was a brand name. So the bush plane guys are going for large diameter flat pitch props for best takeoff and climb at the expense of cruise. Makes sense, if it's an acceptable compromise. But it might also make sense for a draggy biplane that doesn't go fast anyway...
     
  13. Nov 16, 2019 #53

    Vigilant1

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    That's a numeral "4", not a numeral "1". It denotes the fourth root (the square root of the square root)
     
  14. Nov 16, 2019 #54

    TFF

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    A Cub is a race plane compared to the Hatz. I believe someone on the biplane forum has an 80” prop on their Hatz.
     
  15. Nov 17, 2019 #55

    blane.c

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    Yes. I forget the mans name who worked with McCauley originally to develop the propeller for the Super Cub, but they did it back when the Super Cub had 125hp if memory serves at all. It is considered necessary for off airport. Light my Cub would take off at 1700RPM if I didn't hold it on.
     
  16. Nov 17, 2019 #56

    blane.c

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    It seems that Sensenich uses the "Rose E" profile which does not have a flat bottom like other wood propellers. It is supposed to be more efficient nearly as efficient as metal without the stress of a metal prop.
     
  17. Nov 17, 2019 #57

    Rockiedog2

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    Also makes sense for a hi power UL we’re trying to keep legal. Like 58-18 on my LEU
     
  18. Nov 17, 2019 #58

    BBerson

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    An Anchorage mechanic named Roger Borer got the original STC for large props made by McCauley for Alaska Cubs.
     
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  19. Nov 19, 2019 #59

    blane.c

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    One of the things that really slows down the Cubs is the large tires, you will lose 10mph or more with those 30" air streaks. So if your airspeed is already compromised you may as well go all in. 82"x2700RPM = 658.66mph which ain't bad. 82"x3400RPM = 829.42mph. Still under for a wood prop. Most say 90mph cruise for the Cub, I always planned 75mph and no issues with that, The actual cruise speed is more than 75mph but quite a bit less than 90mph that is for the bush outfitted O-290's and O-320 Cubs.
     
  20. Nov 19, 2019 #60

    BJC

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    Even with rounding down to 658 and 829 MPH, that is impressive.


    BJC
     

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