Predator (Harbor Freight) 670cc: Airboats, PSRU, Crank breaks, etc

Discussion in 'Firewall Forward / Props / Fuel system' started by Vigilant1, Apr 1, 2019.

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  1. Apr 9, 2019 #41

    pictsidhe

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    Actually, probably less likely for a single than a twin. Lower excitation frequency. The shaft will have snapped due to TV. A crank between two flywheels is a stiff system. Making it stiffer OR reducing the excitation frequency will both reduce the likelihood of failure.

    If you want a direct drive prop on the PTO end, you really should consider removing the flywheel completely. They are around 20lb for the v-twins, so there's a lot of of incentive to do that. Yes, you'll need a new ignition trigger. But it's just a magnet.
     
    Last edited: Apr 9, 2019
  2. Apr 9, 2019 #42

    pictsidhe

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    It's not possible to get rid of the chaff, but you can make it invisible with the ignore feature. Few people notice it now...
     
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  3. Apr 9, 2019 #43

    Topaz

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    That's not even something you need to build. There's a bolt-on system here: https://www.performance670.com/product-detail/electronic-trigger-ignition
     
  4. Apr 10, 2019 #44

    Armilite

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    They have a Predator 670, 43hp@4700rpm. Taking .040" off the Head probably makes it around 10.0cr. Predator 670 43hp at 4700rpm.jpg
     
  5. Apr 10, 2019 #45

    blane.c

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  6. Apr 11, 2019 #46

    rv7charlie

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    The 'flywheel end vs PTO end' discussion never seems to die on the interwebs. I think it's worth asking, 'Why do they call it the PTO (as in, Power Take Off) end?'

    Armalite beat me to posting that pair of pics in post #31. Look at the aircraft crank's 'PTO end'. What's similar to the PTO end of the industrial engine crank in the next pic? I think it's likely that the a/c engine designer and the industrial engine designer both knew the best way to handle bending loads on the PTO end of a crankshaft. :)

    A big ATTABOY to the 1st right answer.

    (Cutting a keyway in a shaft is another issue, but one that can be designed around.)
     
  7. Apr 11, 2019 #47

    akwrencher

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    Basically it comes down to the way you extract power from the engine. Those PTO ends are designed typically for belt drive or direct, but the direct drive situations on industrial application don't normally have the large gyro loads of a propeller. Using a psru i would think the pto would be just fine. Direct drive prop, I would probably do some research, maybe find someone to help me do the math to determine which end was best.
    Just my thoughts on the matter.....
     
  8. Apr 11, 2019 #48

    rv7charlie

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    Nope. :)
    Next?
     
  9. Apr 11, 2019 #49

    blane.c

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    The stiffer the part of the crankshaft is that the prop is bolted to the better? And if stiffer is better why isn't the end with twice the bearing area the stiffer end?
     
  10. Apr 11, 2019 #50

    Vigilant1

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    The bearing area, type, and material is an issue, and it looks like there's more crank >length< on the PTO end for a bearing. This might decrease bending loads.
    But, in the case of the industrial engine with the direct drive prop, the reported break was outboard of the bearing.
    And, if that's the/a weak point, then that key channel is a concern (as the VW guys know well).
     
  11. Apr 11, 2019 #51

    Hot Wings

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    Neither end on the typical industrial engine is particularly good for attaching a prop. The designated PTO end has more bearing area but the abrupt reduction in diameter just outboard is a stress riser. Couple that with a prop flange that doesn't have enough clamping force inline with the crank and you get a tension load, in addition to the bending.

    The flywheel end has a much better tapered attachment but less bearing area. There are also no standard mounting bosses on the flywheel end making PSRU attachments one of a kind.

    The light but expensive solution is to build a taper fit prop hub for the PTO end, similar to the VW prop hubs, where the taper is cut under the PTO bearings and the prop flange then becomes the bearing surface. The requires lathe work on the crank and good surface on the prop flange. edit: or conversion to ball bearings.
     
  12. Apr 11, 2019 #52

    rv7charlie

    rv7charlie

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    The bearing area, type, and material is an issue, and it looks like there's more crank >length< on the PTO end for a bearing. Ding! This might decrease bending loads. Not so much ding. :)

    It increases the ability of the crank to handle the bending loads. Look at the aft end of the a/c crank. Visualize putting a bending load on that end. (Gyro loads are bending loads.) Because the bearing is so short, the bending load goes into the relatively thin web of the 1st crank 'throw'. That's the reason a/c cranks always have that ridiculously *long* main bearing on the 'PTO' end of the crank; it keeps bending loads away from the crank throws. Same goes for the industrial engine; a belt or chain puts high bending load on the crank, just like gyro loading from a prop. Neither designer would waste all that extra material without a reason.

    I'm just talking about design principles; as to a particular engine, its crank (or a particular part of the crank, ex: the reduced diameter (& possible keyway) in the pictured example) may be inadequate for the task.
     
    Last edited: Apr 11, 2019
  13. Apr 11, 2019 #53

    Vigilant1

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    I hate to say it, but at some point a PSRU starts to look good. The prop bearing can be designed for the job (to include thrust loads, while we are at it). Getting the prop hub up higher would be a benefit on most designs. Turning a longer, slower prop does help efficiency somewhat (resulting in a gain of a few pounds of thrust at higher airspeeds and at maybe 10 pounds at 60 knots at these HP levels. For one case I have figured, gaining 3 lbs of thrust increases climb rate as much as decreasing weight by 50 lbs, so the PSRU's weight could "pay for itself." And it is "free" HP--no additional fuel burn.) Yes, the PSRU will cost some money, but a direct drive will also come with costs (as Hot Wings intimates, maybe including an external bearing, mount, prop hub, etc.) And, the PSRU might last through several engines.
    As a bonus: Some of these engines have a 3 yr manufacturer's warranty. If I've carved up the crankshaft to fit a prop hub and bearing, I might get laughs if I try to make a claim. If I just need to unbolt the PSRU and re-fit the shroud and stock carbs, maybe all is well.
     
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  14. Apr 11, 2019 #54

    blane.c

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    If on the long bearing end of the crankshaft the propeller adapter was a press fit on both the small diameter of the crankshaft from the spot were the keyway ends to the face were the crankshaft increases in diameter and on the large diameter of the crankshaft for the short distance allowed without interfering with the crankshaft oil seal and when drawn up with the end bolt was a tight fit against the face of the crankshaft were it changes diameters, wouldn't that transfer the stresses to the large diameter of the crankshaft at the spot with the most bearing area and rigidity?
     
  15. Apr 11, 2019 #55

    blane.c

    blane.c

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    It seems to me it would be easiest. And greater propeller length is not a good option which goes hand in hand with the PSRU.
     
  16. Apr 11, 2019 #56

    Vigilant1

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    ?? A longer prop is an option if a PSRU is used, but not a requirement. But for some situations it would be useful, especially low airspeed. Examples:

    Prop design speed of 60 MPH, 22 HP engine, 2-blade 47" dia (19.27" pitch) direct drive prop at 3600 RPM: 62% prop efficiency, 85 lbs of thrust
    Prop design speed of 60 MPH, 22 HP engine, 2-blade 58" dia (29.88" pitch) 1.6:1 PSRU (engine 3600, prop 2250 RPM): 68% efficiency, 93 lbs of thrust (same as getting 2.5 additional HP from the direct-drive 47" prop)

    Prop design speed of 100 MPH, 22 HP engine, 2 blade 46" dia (30.52" pitch) direct drive prop at 3600 RPM: 78% efficiency, 64 lbs of thrust
    Prop design speed of 100 MPH, 22 HP engine, 2 blade 58" dia (48.45" pitch) 1.6:1 PSRU (engine 3600, prop 2250 RPM): 81% prop efficiency, 67 lbs of thrust

    (Actual PSRU thrust might be slightly lower than shown due to gear/belt losses?)
     
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  17. Apr 11, 2019 #57

    blane.c

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    15lbs per PSRU. More moving parts. What about a four blade propeller? Less dia. = less gyro loads? Would using a flexible propeller design like Prince's but in a four blade get you more take-off power and better cruise than a two blade rigid?
     
  18. Apr 11, 2019 #58

    Vigilant1

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    Weight: Every case is different, of course. In the case of the MicroMaster, 15 lbs of additional PSRU weight costs about 30 FPM in single-engine climb. But an additional 8 lbs of thrust at climb speed (due to the PSRU) would make up for it in spades (over150 FPM additional climb, IIRC)
     
    Last edited: Apr 11, 2019
  19. Apr 11, 2019 #59

    rtfm

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    Jan's dead? Bugger me...
     
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  20. Apr 11, 2019 #60

    Vigilant1

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    Yep, very sad. A great help and always with a good sense of humor. I would have liked to meet him, like so many other folks here.
     
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