Prop pitch, rpm, and performance

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

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  1. Nov 13, 2019 #1

    Dana

    Dana

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    As I’ve related elsewhere, I’ve been experimenting with a new prop on my Hatz. I don’t want to dilute that discussion, which is more about data collection, while here I want to talk more generally about rpm, pitch, and performance.

    First, the engine, which is a Lycoming O-290-D which has a “rated power” of 125HP at 2600 rpm. The manual also lists a “takeoff power” of 130HP at 2800 rpm, but nowhere in the manual does it say what “takeoff power” means. I’ve heard of “5 minute takeoff power” ratings for other engines, but there’s no mention of anything like that in the Lycoming manual. (Edit, it's in the TCDS Dan posted, strange it's not in the manual).

    So, the prop: The original prop on the plane is a 74x50 Sensenich metal prop. Static rpm is 2480, around 2500 in a 60kt climb, I cruise 70kts at 2250, and it maxes out at 2600 rpm WOT at 80kts in level flight. The new prop is a 74x47 Sterba wood prop. Static rpm is 2440, 2425 at 60kt, and it maxes out at 2565 at 85kt in level flight (the full throttle rpm averages 75 lower over most speeds). Even though the new prop has less pitch, rpm is down and speed is up… but takeoff and climb performance are worse.

    My goal with the new prop was to get a little close to the 2600 rated rpm for more power during climb, hopefully without losing too much airspeed in cruise. That the reduced pitch didn’t achieve this points to the other differences between the props. The wood prop has a different blade shape (it’s wider than the metal prop near mid blade but narrower closer to the hub) and a thicker airfoil section. I will be sending the prop back to Sterba for repitching, but how much? According to Ed an inch of pitch is good for about 50 rpm.

    I’ve seen references to pitching the prop so it’s at redline rpm at full throttle in level flight, but what is “redline”? The manual makes no mention of any limiting rpm. The old prop maxes out at the 2600 “rated rpm”, but of course I won’t be cruising at full throttle. I’m thinking reducing the pitch by 3” to pick up, say, 150 rpm (so it’d make 2575 during climb) would be about right. It would bring the pitch down to 44” which is what two other prop makers recommended (Sterba recommended 50 but I thought 47 would be a better place to start). That would (presumably) lower the cruise speed at the same rpm, or raise the rpm needed to get the same speed, which is OK if it’s not too drastic. It also means the rpm would go above 2600 (some might say “overspeed”) at full throttle in level flight, but the answer to that is “don’t do that” (or maybe it’s OK for 5 minutes?). I’m not looking to get up to the “takeoff power” rpm of 2800, I suspect that a fixed pitch prop that allowed that would perform very poorly in cruise.
     
    Last edited: Nov 13, 2019
  2. Nov 13, 2019 #2

    Dan Thomas

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    Wood props, being thicker, offer a little more drag. But the real queation I have has to do with how Sterba is measuring the pitch on his props. Is he using the bottom surface (the "face") of the blade as the datum for measuring the blade angle relative to the hub face, or is he using the blade's chord line? Using the flat side will give a higher pitch than measured. Metal props are thin enough that there isn't much difference between the face and chord.

    You could measure it yourself and see what you get. Angle at 70% from the hub centerline to the tip. Distance from centerline to 70% times pi for distance travelled, and do the trig to get the pitch.
     
  3. Nov 13, 2019 #3

    Dan Thomas

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  4. Nov 13, 2019 #4

    BJC

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    Dana:

    You decide what is a comfortable RPM for your engine and the type of flying that you do. I operate a FP at 3200 to 3250 RPM full power straight and level on a parallel valve Lycoming IO-360 for aerobatics vs. a factory red line of 2700. Life of the engine is reduced, but it’s worth it to me.

    I replaced the factory FP on a Cessna A152. It reached the red line of 2550 RPM at full power straight and level, with a FP that reached 2800 RPM under the same conditions. The STC specified a 2550 RPM red line. The engine and propeller were TC’ed in another airframe. The increase in performance was really noticeable. Fuel consumption went up, but so did top speed and max RoC. I had the option of matching the original performance and fuel flow, but chose to make more noise.

    Ed generally seems to be wiling to work with customers. I hope that you end up with something that you will be comfortable with.


    BJC
     
  5. Nov 13, 2019 #5

    Vigilant1

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    If I can tack on a related question:. When a prop is re-pitched, don't the blades necessarily lose a little camber and width? If so, you might want to be conservative with the re-pitching, because you'll not only be reducing the effective AoA of the blades, you'll also be slightly reducing their camber and area (compared to what things would be if you'd started fresh with that pitch). All of those things will increase the RPM your engine will produce, and you don't want to go too far.
     
  6. Nov 13, 2019 #6

    Dana

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    Ed said he uses the flat back face... but the back face isn't flat. I measured it using points about 1/8" in from the leading and trailing edges, and got 47.25", pretty close to what I ordered.
     
  7. Nov 13, 2019 #7

    Dana

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    Right. Ed says 50 rpm per inch of pitch, presumably he's taking the chord reduction into account.
     
  8. Nov 13, 2019 #8

    Pops

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    I believe this is getting into the black arts :) Way over my head.

    When I built the Lyc 290 for the Falconar F-12. I used a 320 crank ( thicker prop flange) and a 290 case. 320 accessory case and oil sump, 320 carb, 290-D-2 higher compression cylinders and pistons. 290 GPU cam and solid lifters, smoothed and matched all the intakes. Ran a Sen metal 74"x 56" pitch prop. ( at least that was what was stamped on it ) 2 into one , match length exhaust with no mufflers. WOT at 3k level flight at 70 degs was 2900 rpm in the Falconar F-12. Pretty clean airframe. I cruised at 2650-2700 rpm at 150 mph and a 1700' ROC at GW and just me at 2200 FPM. EW was 937 lbs. My Cherokee 140 with the Lyc 320, 150 hp had a Sen 74"x 56" pitch prop.
     
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  9. Nov 13, 2019 #9

    plncraze

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  10. Nov 13, 2019 #10

    plncraze

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  11. Nov 13, 2019 #11

    Dana

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    Oh yes, prop design definitely gets into the black arts...:eek:

    I saw that quite some time and several airplanes ago but thanks, I had forgotten about it. His rough formula definitely gets me into the ballpark, says 42.66" pitch, but I'm already in the ballpark, I don't think I want to go that low. But he does give the same pitch vs rpm relationship as Ed is using.

    I'm still thinking 44 is where I want to be.
     
  12. Nov 14, 2019 #12

    Rockiedog2

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    What BJC said.
    On my biplanes with parallel valve Lycs I routinely turned those 3300. Most of the acro guys I know ran the Lycs that high.
    Even if it shortens the life the motor will still outlive most of us.
     
  13. Nov 14, 2019 #13

    Dana

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    Well, I'm not doing hardcore acro in the Hatz, but it's reassuring to know that I don't have to worry about overspeeding the engine. But I'll still pitch it to keep rpm reasonable, if only to maintain a reasonable cruise speed at a reasonable fuel consumption. Though I suppose that with a finer pitch, less torque (and thus less power, and thus less fuel consumption, up to a point) is required to turn it at a given rpm.
     
  14. Nov 14, 2019 #14

    BBerson

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    A thick wood prop can't compete with a metal prop for tip speed (1000fps allows more rpm)
    A stiff and thin composite tip might get close to metal thinness.
     
  15. Nov 14, 2019 #15

    Dana

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    A 74" prop at 2800 rpm will be at 900 fps or 0.8 Mach, right where you'd start with classic propeller design. I don't expect a 44" pitch will let it turn that fast, except maybe in a power dive.
     
  16. Nov 14, 2019 #16

    TFF

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    The problem with props can be you get the RPM you are looking for and loose the performance you were trying to get at the same time.

    Max horsepower is when the engine can’t spin the prop faster than max rated RPM. It has to be loaded. RPM and max manifold pressure is at that point. Of course at sea level.

    Fixed pitch aerobatic airplane you would want 2700 rpm in vertical and you don’t care where it runs in level; you are not supposed to be level in aerobatics.
    Top speed you would want 2700 in level.
    Everything else is in between on how you load it.

    My guess in the black magic is don’t go all the way to 3”. 1 1/2 or 2”. You don’t want to step over and turn it into wall art. It might be short on what you want but you also don’t want to go backwards. Different brand props do act different.

    It’s good to see the data points with your findings.
     
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  17. Nov 14, 2019 #17

    BBerson

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    A thick wood prop has more tip drag so why use a wood prop if you have a suitable pitch metal prop?
     
  18. Nov 14, 2019 #18

    Dana

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    The metal prop also has a bit too much pitch, it's subject to an AD, and it weighs about 15# more; a wood prop runs smoother, puts less stress on the crankshaft during acro, and just looks right on a biplane.
     
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  19. Nov 14, 2019 #19

    Rockiedog2

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    Have you thought about trimming a half inch off of the length? (Each end?)Sounds like you got nothing much to lose?
    I’ve trimmed a number of losers and sometimes the results are surprising.
    Yeh I rather have the wood for the reasons you mentioned .
    Interesting experiment?
     
    Last edited: Nov 14, 2019
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  20. Nov 14, 2019 #20

    BBerson

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    With that, then I think I would try to make the tips thinner while doing the pitch change and apply composite tips for strength, if that works with your classic wood look. That might require a prop designed for the thinner composite tips from the start, however.
     

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