Will a new prop increase airspeed?

Discussion in 'Maintenance Bay' started by nrimmer, May 21, 2019.

  1. nrimmer

    nrimmer Pre-Flight

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    Took in my fixed-pitch McCauley prop to consider having it re-pitched for cruise. I was told that one side is right at the spec limit and the other side is just a hair above. So time to plan on buying a new prop.

    The prop guy commented on how you can see how "skinny" the prop is towards the ends. I asked how much wider a new prop would be and he said about an inch (I'm guessing that is half an inch per side).

    Given that it is fixed-pitch ,my thinking is that the "skinny" prop isn't absorbing as much energy as a new "fat" prop for any given RPM, thus not transferring maximum forward thrust for that RPM. In practice it makes intuitive sense because the POH says to run WOT at 10K' and there is no way I can run WOT without overspeeding the engine - that skinny prop just can't absorb the energy.

    Should I expect to pick up airspeed just by installing a new prop?
     
    Last edited: May 21, 2019
  2. Checkout_my_Six

    Checkout_my_Six Touchdown! Greaser!

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    no....no more speed unless it's re-pitched or spins faster.
     
  3. Capt. Geoffrey Thorpe

    Capt. Geoffrey Thorpe Touchdown! Greaser! PoA Supporter

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    Why not? If a wider blade lets him put more power in for the same RPM (less slip) then it ought to go faster. But I ain't no A&P.
     
  4. FORANE

    FORANE En-Route

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    I bet it will be faster both from being able to run more power and from less drag with a clean prop.
     
  5. Checkout_my_Six

    Checkout_my_Six Touchdown! Greaser!

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    it may be a knot or two faster....but not noticable. He could see better climb performance. But doubtful on both accounts.
     
  6. 455 Bravo Uniform

    455 Bravo Uniform En-Route

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    Getting way out of the OP’s original request/question...but the cleaner the prop, let’s say it was 100% frictionless for theoretical purposes, would it propel the plane faster or slower? Is there some friction needed for the prop to work on the air molecules?

    My take is no. The propulsion of air molecules rearward has the equal and opposite reaction of those molecules propelling the prop forward. The less friction, the better.

    So to get closer to the topic, would the wider new prop (same pitch) have more propulsion?

    At what point would a wider and then even wider prop stop being efficient? Friction?
     
  7. Capt. Geoffrey Thorpe

    Capt. Geoffrey Thorpe Touchdown! Greaser! PoA Supporter

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    Lift = Coeff of Lift * density * velocity^2 * area /2.

    More area, more air displaced, more thrust (assuming you are spinning it at the same speed).
     
  8. kgruber

    kgruber En-Route

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    Could EASILY be slower.
     
  9. 455 Bravo Uniform

    455 Bravo Uniform En-Route

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    So to take it to the extreme, would a spinning auger (one blade, full 360 coverage, same pitch) as a prop have the most thrust, all else being equal? Legitimate question, no snark intended.
     
  10. mondtster

    mondtster En-Route

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    Yup. I've flown a few airplanes that owners thought were going to be faster with a "cruise" prop that ended up going the same speed or slower than an identical airplane with the standard prop. I've found little gain in changing pitch from the factory issue, they're a pretty good compromise.
     
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  11. Capt. Geoffrey Thorpe

    Capt. Geoffrey Thorpe Touchdown! Greaser! PoA Supporter

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    Interesting thought.
    It would seem that it would make more thrust - but it would take a lot of power - but given the power required, more thrust would come from a larger diameter prop with a much higher aspect ratio (traditional long skinny blades). There also problems with one part of the prop effecting the rest - and as you get around your auger, the effective angle of attack would change as portions of the blades moved into already accelerated air which would reduce the net gain in thrust.
     
  12. nrimmer

    nrimmer Pre-Flight

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    Thanks for all of the responses. To visualize, I was thinking of it like a boat propeller. A tiny little prop spinning at 2700 RPM wouldn't produce much forward motion, but a massive propeller moving the same size boat and spinning at the same 2700 RPM would move a lot more water and thus move the boat a lot faster. In cruise I always have available power, but I run out of RPM, so my thinking went that a bigger (new) prop will allow take more power to move at the same RPM because it "bites" more air, thus moving the airplane faster. But I am no physicist.
     
  13. mondtster

    mondtster En-Route

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    What do you mean "run out of RPM"? Do you hit redline before full throttle in level flight or do you mean that in level flight you can have full throttle and not reach redline?

    EDIT: Do you know what the prop is right now compared to what is supposed to be on it?
     
  14. nrimmer

    nrimmer Pre-Flight

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    It is the factory prop, it has just hit the service limit for minimum thickness. Every prop wears down from years of filing out nicks, and as the prop shop described to me, even just hitting rain and air will wear it down over time. This prop has nearly 50 years and 2800 hours on it. I will hit the 2700 RPM redline before hitting full throttle. This is most apparent at high altitudes where, according to the POH, I should be able to run WOT. There is a good 1/4" of throttle left when I hit 2700 RPM at the 12,500' service ceiling. That's why I though that a new prop with another 1" of thickness on it would absorb the remaining power power without hitting redline, and that extra power is converted to forward motion.
     
  15. bnt83

    bnt83 Final Approach PoA Supporter

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    Fixed pitch props -- The fact is that low airspeed creates a high angle of attack on the propeller blades. High airspeed creates a low angle of attack and reduces drag (and thrust).

    Climb props should require a bigger throttle reduction to prevent engine overspeeding when approaching cruise airspeeds, airplane will go slower in cruise

    Cruise prop --it will turn slower on takeoff and during climb, reducing engine power output and the airplane will seem more lethargic during those phases of flight. Airplane will cruise a tiny bit faster with higher power settings. It will be more difficult to make the engine overspeed in cruise.
     
    Last edited: May 21, 2019
  16. Tantalum

    Tantalum Final Approach

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    The original designs for helicopters and what not had one giant auger type as referenced above.. and when I say original, I mean Leonardo DaVinci and Wright brothers

    But remember that the propeller is actually just a wing, except it's pulling the plane forward instead of up. If you look at the most modern designs these tend to favor long skinny thin shapes.. look at any sailplane or glider.. U2, wind turbine, etc. Long skinny and thin appears to be the design that maximizes aerodynamic efficiency
     
  17. Tantalum

    Tantalum Final Approach

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    You want to be able to absorb that extra power *and* turn it into forward motion. If the old blade is worn and already at or past its design spec then I would say yes you should be able to go a knot or three faster with a new prop. I would not say it's thickness you're after though, just a good efficient blade design
     
  18. Cap'n Jack

    Cap'n Jack Final Approach PoA Supporter

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    Wright brothers? AFAIK, the only thing they had to do with helicopters was a toy they made as children. But a lot of people at that time got their propeller designs from ships. Wright props top image (closer to modern props), Langley aerodrome propeller bottom image (closer to an auger shape).
    [​IMG]

    [​IMG]
     
  19. Tantalum

    Tantalum Final Approach

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    Didn't mean quite as literally, but I remember seeing helicopter designs from "way back when" that had those large auger designs
     
  20. mondtster

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    What does it do down low? What is the static rpm and is it within spec for your aircraft?

    You didn't answer my other question. Do you actually know what the pitch of the prop is that is on there? Is it the standard pitch prop for your application or did someone repitch it sometime in the past and it is now a climb prop? Perhaps someone put a seaplane prop on a wheel plane or something? I'd expect a repitch would take more material off the prop than 2800 hours worth of flying and dressing will.

    My opinion, you really need to know what you're starting with before you can make an accurate assessment on if a new prop and/or a different pitch is going to make a difference. It sounds to me like it could use a little more pitch but you'd better make sure you're going to be satisfied with the results before proceeding. As I posted earlier, I've flown a few airplanes with poorly thought out pitch changes that were attempts to get more speed out of an airplane and every one of them resulted in poorer performance than the standard prop.
     
  21. Gary Austin

    Gary Austin Pre-takeoff checklist

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    Also, what are you using to see RPM, electronic tach is one I like
     
  22. Cap'n Jack

    Cap'n Jack Final Approach PoA Supporter

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    Gotcha, and thanks for the clarification.
     
  23. nrimmer

    nrimmer Pre-Flight

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    I was wondering the same thing when I brought it in - thought maybe it had been re-pitched for climb. He confirmed that it was the standard factory pitch for the prop/aircraft. The logbooks don’t indicate that it has ever been pitched for anything else.

    Static RPM is min 2400. I can get 2440 or so, checked using the EDM900 digital tach.
     
  24. Clip4

    Clip4 Final Approach

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    Where you are likely to notice the difference is on climb.
     
  25. Finnelly

    Finnelly Pre-Flight

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    If you add prop area and slow down the engine you will go slower. Look at the engine power charts.
     
  26. nrimmer

    nrimmer Pre-Flight

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    Makes sense. So if you add prop area and maintain the same engine speed you should go faster.
     
  27. mondtster

    mondtster En-Route

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    Not necessarily. Engine speed is only one component of setting power. As I posted earlier in this thread, every airplane I've been around that someone thought they were going to get more cruise out of it with a different prop failed. The problem was that it needed more manifold pressure to get the same engine speed, thus it was actually requiring more power to run at the same rpm. The overall airspeed of the airplane didn't change either; the prop choice was poor.
     
  28. Dan Thomas

    Dan Thomas Final Approach

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    On a 172M in the flight school I replaced a prop that was at wear limits with a brand-new prop. Same prop make, model and pitch. I expected a performance increase and got nothing at all.

    The OEM's normally prop their NA fixed-pitch airplanes so that the engine will reach redline RPM at full throttle at sea level. I've found that to hold true up to six or eight thousand feet. The decrease in air density affects the engine's power about the same as it reduces the drag on the prop. Therefore, I would suspect a tach out of calibration. They usually start under-reading, but I've found a couple that were over-reading. In Canada we have to check the accuracy of magnetic-drag tachs annually. That covers almost all the steam-gauge tachs out there. Rather than replace it with a new tach of the same sort, one should buy an STC'd digital tach and be done with it.
     
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  29. chemgeek

    chemgeek Pattern Altitude

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    Increasing propeller pitch is what will get you a cruise speed increase--typically at the expense of climb. Changing the prop profile may change prop efficiency, but I would suspect those changes would be minimal if the pitch remains the same. When I installed an STC propeller on my AA-5, the speed gains closely matched the theoretical increase in speed calculated from the pitch increase. Interestingly, the STC propeller is slightly shorter than the original prop, so much less area overall, but is still a higher pitch. The airspeed change corresponds to the pitch change. The change in prop efficiency with the STC prop means I don't lose as much climb capability as I would have by simply repitching the original prop.
     
  30. bluerooster

    bluerooster Pattern Altitude

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    Fluid dynamics. Water screw, or Air screw, principles are the same. Only the viscosity of the fluids change.
     
  31. Checkout_my_Six

    Checkout_my_Six Touchdown! Greaser!

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    Reynolds number....is key.
     
  32. SloRoam

    SloRoam Pre-Flight

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    Good thinking in this thread, so let's also add a factor: fuel consumption or where you run your engine.

    Maybe you will fly faster at some power setting if your current prop isnt able to "put down the power.". That makes lots of sense.

    But at lower cruise power settings I'm not sold that it will make a noticeable difference, because there are too many factors in the real world.

    For instance, what if your new prop is pitched slightly differently? That alone could have a big effect.

    That said, you're going to have to do this anyways right?

    I say you get 1 kt faster in cruise at your typical cruise setting.

    What if you don't go faster: are you thinking of any alternatives? Have you checked your rigging?
     
  33. Kyrpto

    Kyrpto Ejection Handle Pulled

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    I heard a new/faster plane will increase speed.
     
  34. Tarheelpilot

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    There was a Cherokee 180 in the family that had a new prop installed. We were all pretty confident takeoff distance and time to climb were reduced. Cruise was not noticeably different. Anecdotal at best but that what we walked away thinking.