Prop pitch, rpm, and performance

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

Help Support HomeBuiltAirplanes Forum by donating:

  1. Dec 9, 2019 #61

    Armilite

    Armilite

    Armilite

    Well-Known Member

    Joined:
    Sep 6, 2011
    Messages:
    3,111
    Likes Received:
    251
    Location:
    AMES, IA USA
    =========================================
    Using this Prop Calc, and 77F Temp:
    http://godolloairport.hu/calc/strc_eng/index.htm

    125HP at 2600 rpm. Pitched for 2500rpm:
    (2) Blade 74"x 50 = 502.51 lbs Static Thrust, needs 159.158 hp
    (3) Blade 74"x 50 = 703.51 lbs Static Thrust, needs 222.822 hp
    (4) Blade 74"x 50 = 854.26 lbs Static Thrust, needs 270.570 hp

    (2) Blade 74"x 47 = 502.51 lbs Static Thrust, needs 149.609 hp
    (3) Blade 74"x 47 = 703.51 lbs Static Thrust, needs 209.453 hp
    (4) Blade 74"x 47 = 854.26 lbs Static Thrust, needs 254.336 hp
    ------------------------------------------------------------------

    Ideal 74" Prop seems to be for 125 hp Pitched for 2500rpm:
    (2) Blade 74"x 39 = 502.51 lbs Static Thrust, needs 124.144 hp
    (3) Blade 74"x 28 = 703.51 lbs Static Thrust, needs 124.780 hp
    (4) Blade 74"x 23 = 854.26 lbs Static Thrust, needs 124.462 hp


    =========================================

    130HP at 2800 rpm. Pitched for 2700rpm:

    (2) Blade 74"x 50 = 586.12 lbs Static Thrust, needs 200.494 hp
    (3) Blade 74"x 50
    (4) Blade 74"x 50
    ------------------------------------------------------------------
    (2) Blade 74"x 47
    (3) Blade 74"x 47
    (4) Blade 74"x 47

    Ideal 74" Prop seems to be for 130 hp Pitched for 2700 rpm:
    (2) Blade 74"x 32 = 586.12 lbs Static Thrust, needs 128.316 hp

    (3) Blade 74"x 23 = 820.57 lbs Static Thrust, needs 129.118 hp
    (4) Blade 74"x 19 = 996.41 lbs Static Thrust, needs 129.519 hp


     
  2. Dec 9, 2019 #62

    Dana

    Dana

    Dana

    Super Moderator Staff Member

    Joined:
    Apr 4, 2007
    Messages:
    8,777
    Likes Received:
    3,138
    Location:
    CT, USA
    Static thrust is meaningless when you're trying to optimize a propeller for climb at 60 knots.
     
  3. Dec 9, 2019 #63

    AdrianS

    AdrianS

    AdrianS

    Well-Known Member

    Joined:
    Jul 5, 2014
    Messages:
    423
    Likes Received:
    268
    Location:
    Australia
    But it was in colour and everything.
     
    mcrae0104, Topaz, Dana and 1 other person like this.
  4. Dec 9, 2019 #64

    TFF

    TFF

    TFF

    Well-Known Member

    Joined:
    Apr 28, 2010
    Messages:
    11,918
    Likes Received:
    3,413
    Location:
    Memphis, TN
    High static ground thrust means you will fly through max thrust at low speeds and the prop then becomes a brake. Unless STOL competition only, you want max load at climb or cruise or somewhere in between. Before you start moving means it’s always loosing.
     
  5. Dec 9, 2019 #65

    proppastie

    proppastie

    proppastie

    Well-Known Member

    Joined:
    Feb 19, 2012
    Messages:
    3,963
    Likes Received:
    1,165
    Location:
    NJ
  6. Dec 10, 2019 #66

    blane.c

    blane.c

    blane.c

    Well-Known Member HBA Supporter

    Joined:
    Jun 27, 2015
    Messages:
    3,348
    Likes Received:
    591
    Location:
    capital district NY
  7. Dec 11, 2019 #67

    Lendo

    Lendo

    Lendo

    Well-Known Member

    Joined:
    Feb 7, 2013
    Messages:
    375
    Likes Received:
    67
    Location:
    Brisbane
    Propeller design is an art. I look at everything I can, I managed to get a copy of Jan Carlsson's Prop program, while he was alive. I would love to know his background, as a lot of work went into this.
    However it wasn't the be all and end all as his new program was a step up with the latest Prop Theory. He only used this for his consulting work.

    I also love Paul Lipps work and spoke to him via e-mail only briefly - he felt I understood his basic philosophy and theory.

    Both men were on the right track to achieving the optimum prop design, I have developed my own theories on the subject, but that's all they are just theories.

    All I can say is best of luck with all this, the Big Boys have the Big computers and best programs money can buy.

    I say this tongue in cheek, if only the bloody things could morph in the best shape and pitch for the conditions :).

    George
     
    Pops likes this.
  8. Dec 11, 2019 #68

    proppastie

    proppastie

    proppastie

    Well-Known Member

    Joined:
    Feb 19, 2012
    Messages:
    3,963
    Likes Received:
    1,165
    Location:
    NJ
    Basic question:?

    85% prop....so thrust x density of air x volume of moving air = 85% of engine hp: torque x rpm?...
     
  9. Dec 12, 2019 #69

    BJC

    BJC

    BJC

    Well-Known Member HBA Supporter

    Joined:
    Oct 7, 2013
    Messages:
    10,047
    Likes Received:
    6,796
    Location:
    97FL, Florida, USA
    Example of 85% efficient propeller: 100 HP out of engine into the propeller produces 85 HP out of the propeller available for propulsion. Note that aircraft speed plays a big role in prop efficiency.


    BJC
     
  10. Dec 12, 2019 #70

    proppastie

    proppastie

    proppastie

    Well-Known Member

    Joined:
    Feb 19, 2012
    Messages:
    3,963
    Likes Received:
    1,165
    Location:
    NJ
    how is it measured
     
  11. Dec 12, 2019 #71

    BBerson

    BBerson

    BBerson

    Well-Known Member HBA Supporter

    Joined:
    Dec 16, 2007
    Messages:
    12,494
    Likes Received:
    2,525
    Location:
    Port Townsend WA
    85 thrust hp used by the prop. Measured in pound of force over time.
    One hp is 550 pounds per foot per second.
     
  12. Dec 12, 2019 #72

    BJC

    BJC

    BJC

    Well-Known Member HBA Supporter

    Joined:
    Oct 7, 2013
    Messages:
    10,047
    Likes Received:
    6,796
    Location:
    97FL, Florida, USA
    Efficiency is output divided by input, i.e., Thrust X aircraft velocity / HP into the prop.


    BJC
     
  13. Dec 12, 2019 #73

    Vigilant1

    Vigilant1

    Vigilant1

    Well-Known Member Lifetime Supporter

    Joined:
    Jan 24, 2011
    Messages:
    4,247
    Likes Received:
    1,975
    Location:
    US
    FWIW:
    If your speed is in MPH and you want thrust in lbs, then:
    Thrust = HP x prop efficiency x 375 / airspeed

    So, if we want to know the thrust at our 67 MPH climb speed and we have a 22 HP engine, and the prop that allows it to turn at max HP RPM at 67 MPH is 65% efficient at that RPM and airspeed, then:

    22 x .65 x 375 /67 = 80lbs of thrust

    Observations:
    1) Propellers generally get more efficient at higher airspeeds (until their design speed is reached, if it is fixed pitch)
    2) Despite the above rule, thrust generally decreases with airspeed (as would be expected due to that airspeed term in the denominator of the formula above)
     
    proppastie likes this.
  14. Dec 12, 2019 #74

    Dana

    Dana

    Dana

    Super Moderator Staff Member

    Joined:
    Apr 4, 2007
    Messages:
    8,777
    Likes Received:
    3,138
    Location:
    CT, USA
    Power is thrust times velocity, one HP is 550 lb-ft/sec, which is lbs times ft per sec, not pounds per ft per second.

    That's thrust HP (THP), as opposed to shaft HP (SHP), which is what the engine actually produces (often called brake HP (BHP) in the automotive world).

    Prop efficiency is THP divided by SHP. Note that by definition, THP (and thus prop efficiency) equals zero if the aircraft isn't moving.
     
  15. Dec 12, 2019 #75

    BJC

    BJC

    BJC

    Well-Known Member HBA Supporter

    Joined:
    Oct 7, 2013
    Messages:
    10,047
    Likes Received:
    6,796
    Location:
    97FL, Florida, USA
    Which is entirely logical, since the purpose of the propeller is to propel.


    BJC
     
  16. Dec 12, 2019 #76

    BBerson

    BBerson

    BBerson

    Well-Known Member HBA Supporter

    Joined:
    Dec 16, 2007
    Messages:
    12,494
    Likes Received:
    2,525
    Location:
    Port Townsend WA
    Hmmm. I just know that 550 pounds lifted in one second is one hp. (/ is read as per, I thought)

    Anyway, it's interesting that the 375 (in the post 73 formula) is derived from the hp formula. (550x60=33,000 and 33,000 divided by 88fps is 375)
    So I just remember that at 375 mph, one hp is equal to one pound thrust at 100% efficiency. Can't do better than that.
     
    Last edited: Dec 12, 2019
  17. Dec 12, 2019 #77

    Pops

    Pops

    Pops

    Well-Known Member

    Joined:
    Jan 1, 2013
    Messages:
    7,383
    Likes Received:
    6,343
    Location:
    USA.
    I just know enough to know that I would be wasting my time designing and building a prop. Local friend of mine designed and built and sold props for several years for different hp 1/2 VW engines.
     
  18. Dec 12, 2019 #78

    AdrianS

    AdrianS

    AdrianS

    Well-Known Member

    Joined:
    Jul 5, 2014
    Messages:
    423
    Likes Received:
    268
    Location:
    Australia
    All those odd magic numbers based on the length of a king's foot...
    ...would disappear if you used SI units.
     
    pictsidhe likes this.
  19. Dec 12, 2019 #79

    TFF

    TFF

    TFF

    Well-Known Member

    Joined:
    Apr 28, 2010
    Messages:
    11,918
    Likes Received:
    3,413
    Location:
    Memphis, TN
    What is the fun in that?
     
  20. Dec 12, 2019 #80

    BJC

    BJC

    BJC

    Well-Known Member HBA Supporter

    Joined:
    Oct 7, 2013
    Messages:
    10,047
    Likes Received:
    6,796
    Location:
    97FL, Florida, USA
    Anyone can calculate in SI; it takes a mastery of obtuse units to work in Btu’s, feet, inches, miles, knots, water that turns solid at 32 degrees, slugs, pounds force, inches of mercury, etc. We do, however, share time units with SI.


    BJC
     
    akwrencher, Pops, dino and 1 other person like this.

Share This Page



arrow_white