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 5, 2019 #21

    BBerson

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    It would be more efficient to decrease the prop rpm (and tip speed) as the aircraft approaches cruise speed.
    At cruise the tip speed is high. Some helicopters do this. But of course pitch change is needed also.
     
  2. Apr 8, 2019 #22

    Armilite

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    Do you have some reference to these 670cc Cranks Breaking? I have seen no evidence of that happening. I don't follow the Air Boat crowd much these days.
     
  3. Apr 8, 2019 #23

    Vigilant1

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    Only the quoted portion in the first post on this thread. A person said he broke the shaft on a HF 22HP engine when turning a prop direct drive on his airboat. He liked the idea of the belt PSRU. His entire statement:
    FWIW, if crankshaft breakage is an issue for direct-drive V-twins at 22 HP, then crankshaft breakages can be expected to be even more likely with the larger torque pulses of a large single-cylinder engine of similar (or smaller) shaft size.
     
    Last edited: Apr 8, 2019
  4. Apr 8, 2019 #24

    BBerson

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    1" shaft is small. The flywheel end is more like 1.375" or something.
     
  5. Apr 9, 2019 #25

    Armilite

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    OK, in Direct Drive mode. Yes, I think that would be a problem with most of these 2 Strokes & 4 Strokes, especially if used on an Air Boat. A Gear or Belt Drive is always better for these Small PTO Shaft Engines.
     
  6. Apr 9, 2019 #26

    Armilite

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    For Direct Drive I would use a Forged Crank, they make them for these Honda/Briggs.
     
  7. Apr 9, 2019 #27

    BBerson

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    The Honda 690 has a forged crank. The Briggs not likely.
    But shaft diameter is more important than forged or not.
     
  8. Apr 9, 2019 #28

    wsimpso1

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    In these engines, designed for full torque to be taken off of the crank pulley, I bet they are really pretty stout to torsional inputs from things like blower rotors and mower blades, both of which have big rotational inertia. But hang a prop on the crankshaft and then fly an airplane with it and you have big gyroscopic reactions, and that can be huge - you can get significant yaw and pitch rotation giving big gyroscopic reactions that are fully reversed with every revolution of the crankshaft. These engine designers most likely never intended anything like that... A PSRU takes that load set off of the crank and puts it on the output pulley shaft, which MAY be designed for that by PSRU designer.

    I would ask the PSRU seller what the maximum gyroscopic reaction the PSRU is rated for, then get your prop inertia from the prop maker. They might give maximum prop inertia and prop rpm, and leave yaw and pitch rates to what airboats are capable of. Airboat units may or may not be designed for the level of pitch/yaw rates we have.

    Math: I*omegashaft*omegayawpitch - the easy way is to have I in kg*m*m and omegas in radian/sec, then convert the result back to lb-ft then lb-in. Ask for the ratings and inertia in metric units if you can get it. These calcs can tangle you up in your undershorts in imperial units.

    Billski
     
    Last edited: Apr 9, 2019
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  9. Apr 9, 2019 #29

    Armilite

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    Yes, Bigger OD is better for Direct Drive, but that isn't an Option here unless you have a Custom Crank made. There is a Big Difference between a Cast Crank and a Forged Crank. Big difference in Price also! Since these Engines were not Designed for this Application, You have to do the R&D. Look at the PTO problems they had with the Rotax 185 used in Direct Drive on the Lazair Ultralight. They had to use a Small 32 x 10 Prop at 5000rpm I believe. They upgraded the PTO to 7/8" I think for only 9.4hp@5000rpm! Kick Back can also cause a Crank to break also. As I said, I would use a Forged Crank, but you still are going to be Limited to what Size Prop you can use in Direct Drive. It all depends on How many Cranks do you want to Break to find that Info out? A Belt or Gear Drive would be your best Option.
     
  10. Apr 9, 2019 #30

    blane.c

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  11. Apr 9, 2019 #31

    Armilite

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    O-200 100HP@2750rpm Forged Crank and they can Break in Direct Drive!

    BRIGGS 22HP@3600rpm.

    O-200 CRANK 100HP.jpg

    BRIGGS V TWIN 22HP CRANK.jpg
     
  12. Apr 9, 2019 #32

    blane.c

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    Or ?

     
  13. Apr 9, 2019 #33

    Vigilant1

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    FWIW, the crankshaft (and connecting rods) of the B&S Vanguard engines are forged. I don't know about their other engines.
     
  14. Apr 9, 2019 #34

    Vigilant1

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    As always with these electric dreams: let's see the weight and the price. That's where the good ideas fizzle out, esp on a project where cost is an issue. If someone needs a 22-30 HP engine and is considering a B&S or Harbor Freight instead of a Verner, Hirth, or Rotax, then cost is likely the reason.
     
    Last edited: Apr 9, 2019
  15. Apr 9, 2019 #35

    Vigilant1

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    As noted previously, the crank on many of these engines has a greater diameter (and often no keyway cut into it) on the flywheel end than on the PTO end. And if the flywheel (with the ignition coil) is retained, it is also on the end opposite the PTO, another reason to put the prop there (if it can be done).
    We don't know for certain that direct drive off either end is a problem, but it is an issue. Another approach is to add a hub and bearing outboard of the engine case to take the prop loads. Direct drive RPMs (3600-4000, depending on mfgr) don't pose a significant tip speed problem at the prop diameters appropriate for these small engines. Yes, slightly longer props (turned slower) using a PSRU do give higher thrust (esp at lower airspeeds). Whether that is worth the weight and cost of a PSRU-- I don't know.
     
    Last edited: Apr 9, 2019
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  16. Apr 9, 2019 #36

    BBerson

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    The Harbor Freight likely isn't forged. (This thread)
    My point to Armilite is that forged is nice but not absolutely required if the shaft is large enough.
    Using the 1" PTO shaft and breaking it as the airboat guy did proves nothing. He should have used the flywheel end which is much larger diameter.
     
  17. Apr 9, 2019 #37

    Armilite

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    Your link didn't take me to the stuff in the photo. Most people here are looking for a Cheap Light Weight Engine. That 582UL with Cooling is around $7500+ New, plus the add-on the Front ????? Cost. It says 40kw = 53.64088hp Continious at 6000rpm. A 503UL with just an R&D Tuned Pipe made 62.3hp@6500rpm, 57.6hp@5750rpm, 54.1@5500rpm, 52hp@5250rpm and it can still be Improved a little, 497cc/7cc = 71hp!

    These Big V Twins 993cc - 1000cc depending on Brand of Engine used could also make around 77hp@5500rpm. If the Honda/Briggs Singles can be Big Bored to 100mm, I'm sure the Twins could be also. A Big V Twin Big Bore (100mm x 86.5mm) 1359.2cc.

    A 380HO made 57.26hp@7000rpm with 30mm Carbs and 11.2cr, about 52hp@6500rpm. Around 42hp@5750rpm.
    Add Bigger 34mm/36mm Carbs +3-4hp. Just say 3hp now 45hp@5750rpm.
    Now do a Cylinder & Case Porting Job, now 45hp +10% = 49.5hp@5750rpm.
    Stock 380HO was rated 48hp@7000rpm. 48hp to 57.26hp = 20% Increase for the Tuned Pipe they used. Could you build a better 25%, 30%, 35% Tuned Pipe, Yes?

    Stock 380HO with 30mm Carbs, 11.2cr, 48hp@7000rpm. If you use the 7cc to make 1hp Rule at 6500rpm, (377UL/380) 368cc/7cc = 52hp@6500rpm! Your (440F/447UL) 436cc/7cc = 62.2hp@6500rpm! Your 503F/503UL 497cc/7cc = 71hp@6500rpm!

    380HO
    48hp +20% = 57.6hp@7000rpm.
    380HO 48hp +25% = 60.0hp@7000rpm.
    380HO 48hp +30% = 62.4hp@7000rpm.
    380HO 48hp +35% = 64.8hp@7000rpm.
    380HO 48hp +40% = 67.2hp@7000rpm.

    You can see in this Dyno Sheet at 6500rpm it was making more like 53-54hp!
    380HO 57.26hp@7000rpms.jpg
     
  18. Apr 9, 2019 #38

    Vigilant1

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    Just because someone keeps posting chaff: Air cooled 4-stroke engines have proven to be limited to 1 HP (continuous) for every 30-33 cc of displacement. At outputs above this level, the heads have not proven capable of shedding heat fast enough to keep local temps around the exhaust valves within the range allowed by the typical alloys, and various problems result. In addition, operating an engine at high compression ratios or RPMs can be expected to reduce the reliability of cranks, bearings, rods, etc. Problems that are acceptable in a racing cart or lawnmower may be a bigger issue in an airplane.
     
    Last edited: Apr 9, 2019
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  19. Apr 9, 2019 #39

    Topaz

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    Moderator Note: Let's everyone please stay on-topic in this thread. Which, if I'm reading the original post correctly, means that particular PSRU and the prospect of crankshaft breakage in general. Radically increasing the power is off-topic for this thread. It might be a great discussion for another thread to be started independently, but not here. Thanks.
     
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  20. Apr 9, 2019 #40

    Topaz

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    Posting now as little 'ole me, I think one thing to very strongly consider in terms of that reported airboat crankshaft breakage is that the airboat guys - and especially the "homebuilt" airboat guys - really LOVE to toss their airboats around rather violently, subjecting the output shaft to prop loads one would only see in an aerobatic airplane. For conventional "cruising" airplane use, the gyroscopic loads developed would be quite a bit smaller than those developed by these little airboats, often spun hard within their own length.

    I agree with others that a flywheel-end prop attachment is probably the safer of the two options, but even a PTO-end mount seems to be working for other airplane installations, without the breakages the airboat guys are seeing. A "third-bearing" installation on the PTO shaft, such as HotWings has advocated for in the past, would be lighter than a PSRU, and would off-load the crank quite a lot for any application.
     
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