Prop tip speed in cruise vs. hp & torque. Is bigger better?

Discussion in 'Firewall Forward / Props / Fuel system' started by TXFlyGuy, Mar 1, 2016.

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  1. Mar 1, 2016 #1

    TXFlyGuy

    TXFlyGuy

    TXFlyGuy

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    Here is the question...

    With the LS3 V8 and the 96" 4 blade paddle prop:

    3100 engine rpm = 210 hp / 375 lbs torque - tip speed is 466 mph (prop rpm 1630).


    With the LS3 V8 and the current 84" 4 blade prop:

    3600 engine rpm = 260 hp / 385 lbs torque - tip speed is 473 mph (prop rpm 1890).


    With the above data, it appears that we could go considerably faster in cruise with the smaller propeller. Is this correct?

    The above power settings would represent max cruise.
     
  2. Mar 1, 2016 #2

    KeithO

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    The prop blade tip is travelling at close to 90 degrees to the flight path. So no, prop tip speed is not connected numerically to cruise speed.

    However, prop drag will be connected to the tip speed and will be higher for the smaller diameter prop turning at higher RPM. For greater thrust, you need a longer and higher aspect ratio prop. Think about the rotor on a helicopter. Very long and slim, can lift the aircraft vertically, even when not in ground effect. Of course you need not go that far, its safe to assume you are not building an Osprey...

    There is an excellent writeup of the compromises in propeller design here Propeller Design and seems to indicate software for doing propeller design studies too.
     
  3. Mar 2, 2016 #3

    djschwartz

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    The helix angle at any given point on the propellor blade is the arc tangent of the forward speed of the aircraft divided by the angular velocity at that point on the blade. For the example above that cites a roughly 470 MPH tip speed, if we assume a 200 MPH cruise speed the helix angle at the tip would be tan-1(200/470) or about 23 degrees. That is to say, the blade advances from the plane of rotation at that angle at that point.

    As has been stated, determining which of the proposed propellor designs would be most efficient, as defined by the highest cruise speed for a given amount of horsepower all else about the design of the aircraft being equal, is a MUCH, MUCH more complex problem than the simple calculation of helix angle.

    Dave
     
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  4. Mar 2, 2016 #4

    TXFlyGuy

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    We are looking at max cruise speeds in the 220 - 230 mph range.

    Plus, the new 96" prop is a complete new design. While we desire the larger (3/4 scale) prop, the concern was the potential loss of speed in cruise.
     
  5. Mar 2, 2016 #5

    KeithO

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    Assuming this is a variable pitch prop, the larger prop should be better at anything the smaller prop can do. If it is not a variable pitch prop, then one would have to be careful that the prop is not "overpitched" relative to the engine output curve. A high speed fixed pitch prop would usually be partially stalled at low speed and engine RPM could be expected to drop as speed increased. Very much like the prop that was fitted to the AR-5 - which was designed for a speed record attempt. It might be more practical to fly with a little less pitch on a prop that one would expect to put any amount of hours on. Takeoff performance would be better for sure.

    (scroll ahead to 11 min 16 sec to avoid a bunch of unrelated material)
    https://www.youtube.com/watch?v=FMvzzhLZtNg
     
  6. Mar 2, 2016 #6

    Toobuilder

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    Really?

    Can you calculate the tip speed of a feathered propeller on a multi engined aircraft traveling at .5 mach?
     
  7. Mar 2, 2016 #7

    TXFlyGuy

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    Yes, constant speed propeller.
     
  8. Mar 2, 2016 #8

    KeithO

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    A feathered prop is related to cruise speed how ? How was feathering even part of the discussion ?

    When the OP knows the prop diameter and the RPM he is going to turn it at, the tip speed of the prop is one item that can be readily calculated. (He ran the numbers himself) But what speed the airframe will be doing is a rather different matter. Assuming equal quality of design, it is well known that a larger diameter propeller will produce more thrust than a smaller one spun at higher rpm, thus the larger prop (all else being equal) should produce a higher cruise/maximum speed.

     
  9. Mar 2, 2016 #9

    Topaz

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    The tip speed of the propeller is a combination of the rotational speed of the prop and the forward motion of the aircraft. The latter is not trivial, and must be taken into account.
     
  10. Mar 2, 2016 #10

    Dan Thomas

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    A squared plus B squared equals C squared, where A is rotational tip speed, B is forward speed, and C is net tip speed. The forward speed is a small factor until the cruise speeds get higher. The OP's 466 MPH tip speed, at 230 MPH cruise, becomes 520 MPH. Not an insignificant increase.
     
  11. Mar 2, 2016 #11

    KeithO

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    I got the impression that the OP does not yet know how fast his plane is going to fly with either prop ? Which means that any calculation of "true" tip speed is a matter of speculation.

    My understanding of the question was whether a larger diameter prop turned at a lower RPM would be better than a smaller diameter prop turned at a higher RPM ?

    To this question I believe the answer is : The bigger prop should always win - all else being equal. It should be quieter, more efficient and produce more thrust. One is usually limited by ground clearance.
     
  12. Mar 2, 2016 #12

    Topaz

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    Right up until the tip velocity starts exceeding about 80% of the local speed of sound, whereupon the efficiency starts to drop. It's often the limiting factor on prop diameter.
     
  13. Mar 2, 2016 #13

    KeithO

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    Did you bother doing the math ? I ran numbers from 200-350mph for the scenarios given by the OP and in every instance, the tip speed of the smaller diameter higher RPM prop was higher ? Its the outcome I was expecting, so there is a flaw in your reasoning.

    Are you assuming both props turn at the same RPM ? Since that is clearly not the case (1630 vs 1890 rpm respectively) ?

    The local speed of sound governs how small in diameter and how fast you can spin a prop while absorbing X hp. The maximum diameter will be governed by structural limitations and flutter for a high aspect ratio prop - not the local speed of sound. Otherwise helicopters would be out the realm of feasibility.

     
  14. Mar 2, 2016 #14

    StarJar

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    I've heard that large diameter props will usually have more static thrust, but sometimes smaller diameters are better for high speed. I'm not much of an expert on that though. Just questioning the helicopter logic, when there's not a lot of speed there (upward).
     
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  15. Mar 2, 2016 #15

    TXFlyGuy

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    We do not have speed data yet for our airplane as it is the first to fly with this engine / wing / prop combination. Should be flying by July.

    I was wondering how much the engine performance would effect the speed. Lower hp and torque at the slower rpm.
     
  16. Mar 2, 2016 #16

    rv6ejguy

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    Several people in the Subaru RV world have trimmed composite VP prop diameter several inches and gained speed. Lots of Sport Class Reno racers have pretty small prop diameters for the installed hp and do over well over 400mph. Diameter is good for low speed static thrust, makes way less difference at high speeds.
     
  17. Mar 2, 2016 #17

    Toobuilder

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    Read a story about a guy going somewhere in his Glassair III at altitude and a cruise power setting (RPM). He was late for a meeting and decided to run the prop up to max RPM and eat the fuel to make up the time. The airplane actually slowed down a few knots. Despite the increase in HP, the combination of the big 84 inch prop, high TAS and altitude pushed the mach number and tip speed close enough to destroy the efficiency of the prop. So yes, forward speed and density altitude is a big influence on tip mach number.
     
    Last edited: Mar 2, 2016
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  18. Mar 2, 2016 #18

    TXFlyGuy

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    This begs the question, what is the definition of high speed? In my case, we are talking about cruise speeds in the range of 225 to 240 mph.
     
  19. Mar 2, 2016 #19

    rv6ejguy

    rv6ejguy

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    The point being that bigger is not always better for speed as some others have put forth here. Many factors involved and the best people to ask that question of is the prop maker. They know more about matching the prop than most folks here. If you must have the scale diameter, you might want to consider lower redrive ratios to keep the tip speeds down.
     
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  20. Mar 2, 2016 #20

    TXFlyGuy

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    Yes, good point. And I thought the same thing. But the ratio is 1.9 - 1. I would re-gear the PSRU.

    The prop manufacturer is Whirlwind (Titan Aircraft).
     

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