PSRU Gear Ratio vs. Prop Tip Speed

Discussion in 'Chevy' started by TXFlyGuy, Jan 18, 2017.

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  1. Jan 18, 2017 #1

    TXFlyGuy

    TXFlyGuy

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    Which option will produce the most thrust, and be efficient?

    LS3 480 V8 + 96" Prop.

    Gear ratio 1.9 - 1
    This would allow a max cruise engine rpm of 3100. This equates to 195hp (10,000' AGL), and the prop spins at 1630 rpm.

    Or

    Gear ratio 2.21 - 1
    This allows a max cruise engine rpm of 3600. That's 232hp (10,000' AGL), prop turns at 1630 rpm.

    In both examples, the tip speed is 465mph, or .60 Mach.

    Which one will give the best cruise speed in IAS?
     
  2. Jan 18, 2017 #2

    Winginit

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    If your engine can cruise at the lower rpm, better mileage,longer engine life,less noise. It's like go you want to drive your car on the expressway at sixty mph in second gear or third gear. The question I would be asking is which will be best for takeoff. If the 1.9 does a good job, then it's fine.
     
  3. Jan 18, 2017 #3

    TXFlyGuy

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    We always have the option of throttling back to a slower rpm. And 3600 rpm for the LS3 is only half of what the engine will do, so even at that it is not being overworked.

    The question is, at 1630 prop rpm, does 232 hp get you more thrust than 195 hp? It seems obvious to me, but I am not an engineer.

    edit: The designer at Whirlwind told me the obvious - more hp always equals more thrust.
     
    Last edited: Jan 18, 2017
  4. Jan 18, 2017 #4

    Dan Thomas

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    True, but sometimes not much more. There are guys with Cessna 185s (300 HP @2850 RPM, 86" prop) that say they get equivalent performance on takeoff of they reduce the RPM 100 or so (with the prop control); the prop makes less noise, indicating less power lost to drag. High tip speeds suck a lot of HP and give little extra thrust.

    The O-200 is supposedly 100 hp@2750 RPM. I have flown O-200s and C-90s and found that the 90 pulls as well as, or better than, the O-200; it runs at 2475 RPM. Less power lost to prop drag. Jan Carlsson once said here that they use 85 HP as a baseline for designing props for the O-200.

    The OP's performance will depend a lot on whether he's using a constant-speed or fixed-pitch prop.
     
  5. Jan 18, 2017 #5

    TXFlyGuy

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    I'm sorry. Should have mentioned constant speed. And the above is why I want a slower turning prop, with a faster turning engine. Just like those big turbo-prop engines. Really slow turning blades, but fast spinning engines.
     
  6. Jan 18, 2017 #6

    Dana

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    Where are you getting the HP numbers? If from the engine's published hp data curves, those are valid only at full throttle, not reduced throttle cruise.

    If the prop rpm is the same, then the configuration using more shaft HP is going to give you more thrust, and you'll have a higher prop pitch to absorb the increased power, or wider / more blades, or a combination of the two.

    Mach 0.6 is a bit on the low side. Are you limited to 96"? And why are you assuming the same prop rpm for both gear ratios?

    You say, "3600 rpm for the LS3 is only half of what the engine will do". Is that based on the max HP and rpm in an automotive application? Or the max possible sustained power/rpm (I doubt that).

    Dana
     
  7. Jan 18, 2017 #7

    TXFlyGuy

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    See above.
     
  8. Jan 19, 2017 #8

    Dana

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    OK, a couple of things, then. The rev limiter may be set to 6600 rpm, but that doesn't mean mean you can do that for any length of time without overheating or other bad things happening. What that time is I have no idea; could be a minute, could be 5 or 10 minutes. Next, the dyno data is, as I said above, for WOT at the rpm shown. In cruise you'll be at a much lower throttle setting so it won't be developing the HP the chart shows at that rpm.

    OK. Looking at the chart from the chevy website, it looks like peak HP is at about 6000 rpm. Say you're using the 2.21 reduction, that puts the prop rpm at 2714. Even at zero airspeed, that puts your tip speed over Mach 1. Thus you can't use the full rpm or 480HP, you need to limit it to 4800rpm (2172 prop rpm), or you need to reduce your prop diameter to something like 75", either of which will give you a Mach 0.8 tip speed.

    But consider that a 72" two blade prop is what you'd expect to see on a 65HP engine at 2300 rpm. You might end up with a wide 4 blader that looks like it belongs on a T-28.

    Are others using this engine and if so, what parameters are they using? Where do the reduction ratios come from?

    Classical propeller design is as follows:

    1. Choose the propeller diameter for Mach 0.8 at the max HP rpm.
    2. Pitch the propeller so the blade airfoil is at max L/D at the design airspeed (full throttle climb, or reduced power cruise, or a compromise).
    3. Size the blade chord and/or choose the number of blades so the prop can absorb all the engine's power.
    4. Analyze and/or test and iterate.

    With a variable pitch prop and/or a redrive where you can choose the ratio, it gets more complicated, you need to do it at all the possible configurations to see which works the best.

    Dana
     
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  9. Jan 19, 2017 #9

    TXFlyGuy

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    The rpm range will be limited, for sure. It (ECU) is being programmed for peak torque from 2500 to 4500 rpm. Doubt if we ever go above that.
    Similar engines are in use, a V8 & V12. They employ a 1.9-1 reduction. We will go with the 2.21-1 ratio. I have no idea why the 1.9 ratio was ever decided on.

    As has been pointed out on this forum, the manufacturers test their engines at WOT, for many (hundreds?) hours. Running at 3600 rpm all day will not hurt the engine.

    The new 96" prop is being designed by Whirlwind Propellers in El Cajon. It will be a very efficient design. This is from the designer:
    Objective is to design the most efficient propeller with constraints of 4 blades and 96 in dia-using power train and flt conditions, which boils down to proper chord length to achieve max L/D for propeller.

    In the end, does the propeller care what the engine rpm is? As long as it is turning in it's designed sweet spot, should make zero difference.

    Dyno at WOT? Sure. But the data taken at 3,000, 3600, 4000 rpm is still valid. Those are points that you pass as you get to peak hp/torque and rpm.
    300 hp / 300 lbs torque at 3400 rpm is valid performance number, even if you stop the acceleration at that value.
     
  10. Jan 19, 2017 #10

    BBerson

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    There is an optimal propeller rpm for high speed cruise. Called Advance ratio. In other words, pick the optimal advance ratio.
    With low rpm the blades might have excessive pitch.
    I can't advise, but the prop maker might.
    Or get Fred Wieck's prop book.
    On tip speed, it depends on the material. Wood is low speed. Metal can handle 1000 fps. or more.
    Look at similar Cessna 185 for diameter, hp, and tip speed.
     
  11. Jan 19, 2017 #11

    TFF

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    2.1 to 1 with the 230 HP will be faster. The prop RPM is the same so with all else being the same, you will be able to crank in more pitch on the prop. More pitch; more faster.
     
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  12. Jan 19, 2017 #12

    lr27

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  13. Jan 19, 2017 #13

    Dana

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    No, probably not at 3600. But I wonder, when they do their endurance test, is it with the stock cooling system? I kinda doubt it.

    Correct.

    My point was that those HP/torque/rpm points are still at WOT while the load on the engine is limiting the rpm, not the throttle or ECU. Cruise is not at WOT, so the HP at that rpm point in reduced throttle cruise will be less than the curve shows.

    What you're saying makes more sense now. If you're reprogramming the ECU for max power at, say, 4800, that's a lot more reasonable, and you have a good place to begin your propeller design. But for the best prop design you should dyno the engine with the new ECU so you get a new set of HP / torque curves.

    Dana
     
  14. Jan 19, 2017 #14

    larr

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    HWGA

    This is from McCauley
    "How do you control the RPM
    We do it by varying the pitch of the propeller blades. In the sense that we're talking about it the pitch is the angle of the blades with relation to the plane of the rotation. As the blade angle is reduced, the torque required to spin the propeller is reduced and for any given power setting, the airspeed and RPM of the engine will tend to increase. Conversely, if the blade angle increases, the required torque increases. Then the engine and the propeller will tend to slow down. Thus, by varying the blade angle or pitch of the propeller we can control the RPM."

    So, torque is what you're concerned with

    TXFG.jpg
    The 2.21 is by far the better choice. The greater the reduction, the greater the increase in torque.
    The pilot always has control of the target speed of the CS prop, see what works best for you.

    Are you going with a 3 bladed prop?
     
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  15. Jan 19, 2017 #15

    BJC

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    That explains a lot about McCauley propellers.


    BJC
     
  16. Jan 19, 2017 #16

    TFF

    TFF

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    It's 4 blades
     
  17. Jan 20, 2017 #17

    TXFlyGuy

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    Dana - Yes, a dyno company in Houston is doing a run in on a friend's engine (LS3 480), plus it will be tuned for max torque from 2500 to 4500 rpm. The base numbers will be taken with a extractor exhaust system. Then, the short stack (P-51 style) exhaust will be bolted on, and another run made to determine what, if any, power loss occurs.
    We are using a Link ECU. Highly programmable. This is already flying in a couple of the T-51's with great results.

    We don't have a stock cooling system either! The problem is keeping the engine warm enough as the radiator is so efficient with all of that airflow. The radiator is custom built by an aftermarket racing company.
     
  18. Jan 20, 2017 #18

    Swampyankee

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    ...and proper selection of airfoil and proper twist distribution.

    No. Why would it matter? The only think that matters is the rpm (and direction of rotation) and power delivered to the prop. A turboprop's power turbine can be spinning at anything from about 8,000 rpm to 20,000 or more; it's reduced to an rpm the propeller needs, and that's all the prop cares about.
     
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  19. Jan 20, 2017 #19

    Swampyankee

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    Metal may be able to handle 1,000 fps, but the propeller may be extremely noisy; my experience in the propeller industry (I did prop aero at a little company that made propellers in Windsor Locks, Connecticut) is that most commercial operators (ATR-42, ATR-72, etc) keep tip speeds down to about 700 fps, with a couple of outliers to about 800 fps. We didn't do propellers small enough to be swung by an auto-derived V-8 when I was there, so maybe small props have different limits.

    Advance ratio is simply airspeed (in ft/sec) divided by the product of prop rotation rate (in rev/sec) times diameter (J = v/(nD)); it's one of the four important non-dimensional prop parameters (the others are thrust coefficient, power coefficient, and activity factor)
     
  20. Jan 20, 2017 #20

    TXFlyGuy

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    Our max cruise speed will have the tips turning at 718 fps. Yes, the designer left out the airfoil and twist info in his reply (above). But he does fully understand the nature of the beast!
     

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