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Thread: HP to Weight Ratio

  1. #1

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    HP to Weight Ratio

    All other things being equal is HP/Weight a good indicator of performance?

    As an example- would a 1200 lbs pa-18 w a 160 hp perform the same as a 1012 lbs 135 as they both have around 0.1333 hp/lb.

    160/1200=0.133
    135/1012= 0.133

    Im talking primarily about climb performance.

  2. #2
    Gordon Misch's Avatar
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    Power in excess of that required for level flight at a given speed is what would be available for climb rate at that same speed. So excess horsepower to weight for an aerodynamically identical airframe would work out like you're thinking.
    Gordon

    N4328M KTDO
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  3. #3
    cubdriver2's Avatar
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    Quote Originally Posted by Cardiff Kook View Post
    All other things being equal is HP/Weight a good indicator of performance?

    As an example- would a 1200 lbs pa-18 w a 160 hp perform the same as a 1012 lbs 135 as they both have around 0.1333 hp/lb.

    160/1200=0.133
    135/1012= 0.133

    Im talking primarily about climb performance.
    Just remember, the basic original design ( E2,J2) flew on 37 hp. But weighted only 555 lbs. And I have flown both and they do very well

    Glenn
    "Optimism is going after Moby Dick in a rowboat and taking the tartar sauce with you!"

  4. #4
    BC12D-4-85's Avatar
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    Also wing loading, as in all up lbs/sq ft of wing area, comes into play during takeoff, climb, cruise, and landing. Fewer lbs/sq ft is better. Cub is 178.5 sq ft. Weight is yours to determine.

    Gary
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  5. #5

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    Nothing better than a light cub to play in. Load that cub up and it’s a different animal. There’s a reason why guys hauling loads go to big engines.
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  6. #6
    stewartb's Avatar
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    I’ll take power every time. More power is an airplane’s best friend.

    Wing area lifts the mass. Power (thrust) makes acceleration. More power opens the door to better props/more thrust.
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  7. #7
    stewartb's Avatar
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    FWIW, in the original question? Add a reasonable load number for pilot and fuel. Let’s say 300#. Power loading advantage goes to the 160hp airplane. The more load you add, the bigger the advantage.
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    BC12D-4-85's Avatar
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    Sure, power is fun, but it can be a hungry beast to feed fuel over time and distance. And that can create weight issues that make the basic wing work harder than intended beyond its original design size or shape. Keep the flight radius short - like within an hour's flight one way even in climbing/descending terrain - and it'll create smiles with short takeoffs and high rates of climb. But go carry a load over distance and typically it becomes a choice of either throttling back and/or extreme leaning to stretch MPG's.

    Someone once told me there's no point in flying if you can't takeoff and then out climb terrain. Some wisdom there.

    Gary

  9. #9
    stewartb's Avatar
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    It takes less fuel to fly a 160hp Cub 100 miles than a 150hp Cub. Same for stock 180 vs Pponk.
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  10. #10
    BC12D-4-85's Avatar
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    Quote Originally Posted by stewartb View Post
    It takes less fuel to fly a 160hp Cub 100 miles than a 150hp Cub. Same for stock 180 vs Pponk.
    Yes well proven and due primarily to compression ratio being more efficient on fuel burn at the same airspeed. Fish Cops (and others) discovered that when they converted their fleet. I guess I'm talking larger aircraft when noting fuel hungry power with a large load over distance. Edit: And typically 160 props can be pitched higher which promotes less fuel burn at the same airspeed.

    Gary
    Last edited by BC12D-4-85; 08-13-2022 at 12:54 AM.

  11. #11
    stewartb's Avatar
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    From an old thread. Maybe it helps Kardiff digest the power variable.

    I ran lots of PL and WL numbers when I was deciding what plane to build and why. These are derived from manufacturer's specs. The only honest empty weight report is by BCSC.

    Just SS XL wing loading empty- 6.8
    gross- 11.75
    power loading empty- 5.0
    gross- 8.16


    Carbon Cub FX3 WL empty- 5.23
    gross- 10.84
    power loading empty- 4.83
    gross- 10.02

    BCSC Rev 2 WL Empty- 6.47
    gross- 10.7
    power loading empty- 6.17
    gross- 10.2

    Stock Supercub 1100#/1750# @150hp
    WL empty- 6.16
    gross- 9.8
    PL empty- 7.66
    gross- 11.66

    For grins, how does my own 180 stack up?
    WL- 10.34 empty
    18.33 gross
    PL 6.54 empty
    11.6 gross.
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  12. #12

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    @stewart- where can i find this thread?

  13. #13
    Utah-Jay's Avatar
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    What is the equation?

    Can anyone calculate my numbers?

    185 sq.ft. Wing
    200 hp
    1200 pounds empty weight
    2200 max gross

  14. #14
    Gordon Misch's Avatar
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    Wing loading (Stewart abbreviated as WL) weight / wing sq ft. Power loading (PL) weight / HP. Or were you looking for something different?
    Gordon

    N4328M KTDO

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    Quote Originally Posted by Utah-Jay View Post
    What is the equation?

    Can anyone calculate my numbers?

    185 sq.ft. Wing
    200 hp
    1200 pounds empty weight
    2200 max gross
    WL gross: 11.89#
    Empty: 6.48#

    PL @ gross: 11#/hp
    Empty: 6#/hp
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  16. #16
    frequent_flyer's Avatar
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    I suggest that anyone flying a fixed pitch prop and using rated power to estimate climb performance is deluding themselves. An engine only develops rated power under standard conditions and at the rpm for which rated power was specified.

    If you really expect to get rated power for takeoff or climb you need a constant speed prop or to accept a very, very, low cruise speed.

    I love my trailblazer CS prop but I've still never seen rated power for takeoff.
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  17. #17
    stewartb's Avatar
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    I don’t think they’re estimating climb performance on anything more than 160hp will out-climb 135hp. Take a 160hp Cub and a 180hp Cub and load them to equal weight. Takeoff distance is very similar. That’s where similarity ends. The 180hp plane will climb far better. And that’s with two planes using fixed pitch props. Cub takeoff performance at different weights defies common logic.

    Why haven’t you seen rated power? Density altitude? Why’d you pay the money for that 363 instead of a 340? More power, right? Not full rated but proportionally more. Very predictable.
    Last edited by stewartb; 08-14-2022 at 07:59 AM.
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  18. #18

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    Quote Originally Posted by Gordon Misch View Post
    Wing loading (Stewart abbreviated as WL) weight / wing sq ft. Power loading (PL) weight / HP. Or were you looking for something different?
    And lower numbers are better

  19. #19

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    Whats the weight difference between a 0290d2 and a 160hp 0320?

  20. #20
    Charlie Longley's Avatar
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    Quote Originally Posted by Cardiff Kook View Post
    Whats the weight difference between a 0290d2 and a 160hp 0320?
    Their weight is within a few pounds of each other according to the TCDS.
    Last edited by Charlie Longley; 08-14-2022 at 11:02 AM.

  21. #21
    frequent_flyer's Avatar
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    Quote Originally Posted by stewartb View Post
    Why haven’t you seen rated power? Density altitude? Why’d you pay the money for that 363 instead of a 340?
    Yes, my FX-3 has never seen sea level density altitude. 6 am takeoff yesterday for RNAV/LPV instrument approaches showed density of 3,100 ft for an airport elevation of 1,500 ft. Garmin G3X estimated power at 89% of rated. Safety pilot still impressed by the short takeoff roll.

    (Engine is not a customer choice on FX-3 and CC aircraft with the 340 don't have CS prop option.)
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  22. #22
    stewartb's Avatar
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    And they call them experimental?
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  23. #23

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    Quote Originally Posted by Charlie Longley View Post
    Their weight is within a few pounds of each other according to the TCDS.
    I dont see engine weights in TCDS. Where is it located? I see “use actual weight” listed.

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    BC12D-4-85's Avatar
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  25. #25

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    Just another data point for more HP on the same airplane: Replaced an O290-D2 135 hp (Macauley 7841) on a 7GC Champ with O320-B2B 160 hp (same prop at 44 pitch). Takeoff roll about the same, climb rate 50% better at gross weight. Fuel consumption seemed minimal increase at similar power settings. Only disadvantage was less "prop braking action" with coarser pitch.
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  26. #26
    stewartb's Avatar
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    What was your weight increase?

  27. #27

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    I think it was not over 5 pounds after I replaced the generator with an alternator.
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