2 stroke alternative ?

Discussion in '2-Stroke Aircaft Engines' started by MadProfessor8138, Jun 6, 2019.

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  1. Jun 25, 2019 #41

    Sockmonkey

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    How much would performance suffer if you used a smaller four bladed prop directly driven instead of a big two blade with redrive?
     
  2. Jun 26, 2019 #42

    mm4440

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    Hi, efficient, light aircraft like the MC 30 and the SD 1 can fly with the direct drive v twins. If the direct dive prop has to run at lower rpm you take a hit on power on top of lower prop efficiency. On a low speed aircraft it can make a difference between a dog and a plane that flies ok.
     
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  3. Jun 26, 2019 #43

    Armilite

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    ===================================

    Use this Calc, it will tell you what you want to know and how much HP is required. The more Blades the more expensive it gets.
    http://godolloairport.hu/calc/strc_eng/index.htm
     
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  4. Jun 26, 2019 #44

    Sockmonkey

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    That's what I'm getting at. A smaller diameter prop can be run at higher RPMs without the tips going transonic. So you wouldn't need the reduction gear, but you would want to add more blades to properly load the engine. Less efficient, but is that loss greater than what the friction and mass of the redrive costs you? See what I mean?
     
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  5. Jun 26, 2019 #45

    pictsidhe

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    Efficiency loss from using a smaller than optimum prop depends on how much smaller.

    Generally, the friction of a redrive is going to be much less than the gain from using a bigger prop.

    Have a play with javaprop. Or work through NACA TN 212. I keep meaning to spreadsheet 212, but haven't got round to it, yet.
     
  6. Jun 26, 2019 #46

    henryk

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    =seems prospective ? (27 HP / 15 kg ) !
     
  7. Jun 26, 2019 #47

    Vigilant1

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    As with most of these questions, "it depends." If we are talking about very modest HP, then even a prop that may appear to be "short" still results in low disk loading compared to what we may be used to with larger engines, and the efficiency can be pretty good. And, of course, the payoff from a longer prop is bigger at low airspeeds. Cessna 152 shown for comparison (numbers from Jan Carlsson's program).

    C-152 (but pitch optimized for shown airspeed): 110 HP, 72"D prop, 2700 RPM. HP/sqft: 3.9 Effic at 60 MPH: 55% Eff at 100 MPH: 73%
    Generic V-Twin: 30 HP, 47" D, 3600 RPM. HP/sqft: 2.5 Efficiency at 60 MPH: 60% Efficiency at 100 MPH: 77%
    Generic V-Twin with PSRU: 30 HP, Engine RPM 3600, prop RPM 1800 (68" prop). HP/sqft: 1.2 Eff at 60 MPH: 70% Eff at 100 MPH: 82%

    The numbers above do not include PSRU losses. Flat/multi-V belts have efficiencies of about 96% to 99%, and there will also be additional (small) losses in the additional pulley. In the example above with the 30 HP engines, we'd expect that the PSRU would give us an efficiency gain of about 6% at 60 MPH, but gains might be only a couple of percent at 100 MPH. If we add in the greater weight of the PSRU and the longer prop, performance at 100 MPH would be expected to be about the same with or without a PSRU.
     
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  8. Jun 26, 2019 #48

    Vigilant1

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    "It depends".:)
    The blades on a 4 blade prop for a 30 HP engine start to get very narrow, and even at diameters as low as 47" they would be too narrow to be practical/safe. We could go as low as 42" dia:

    Generic V-Twin: 30 HP, 42" D 4-blade prop, 3600 RPM. HP/sqft: 3.2 Efficiency at 60 MPH: 65% Efficiency at 100 MPH: 80%

    So, in this case it about splits the difference in efficiency between a 2-blade direct drive and a longer 2-blade with a PSRU. The 4 blade will be more expensive than a 2 blade, and will be heavier than a solid 2-blade. If a solid wood prop is desired (for damping purposes), a 2 blade has some practical fabrication advantages.
     
    Last edited: Jun 27, 2019
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  9. Jun 27, 2019 #49

    mm4440

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    With reduction you could run at 4800 rpm and have about 40 hp and a larger, more efficient prop with a small increase in weight and cost and clearly higher performance. If the direct drive, small prop gives adequate performance for a particular airframe it is a reasonable choice. It might not look so reasonable when you are staring at the trees at the end of the runway on a hot summer day.
     
  10. Jun 27, 2019 #50

    henryk

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  11. Jun 27, 2019 #51

    Vigilant1

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    Opinions about " reasonable" may differ from person to person. I'd choose another engine before asking a sub-1 liter air cooled industrial engine to give 40 HP in airplane use on a continuous basis. And I'd choose not to use that specific power level even for TO/climb. Exception would be for racing (with the concomitant degradation in reliability, and use of all the needed tricks).
     
    Last edited: Jun 27, 2019
  12. Jun 27, 2019 #52

    Sockmonkey

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    Thank you. I was just wondering if there were cases where it would be a viable alternative.
     
  13. Jun 28, 2019 #53

    sotaro

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    On the snowmobile four stroke conversion front Duke Larsen is developing the Yamaha liquid cooled two cylinder DOHC 500cc engine with a prototype adapter to a centrifugal clutch and a 3.47:1 Rotax C gearbox. This has been run on a test stand with a 3 bladed prop of 66" producing 365 lbs of thrust. At 11,000 rpm about 80 hp. 3.7 gpm at 9500rpm. Estimated weight is 155 with prop, radiator, liquids, exhaust (including a muffler), engine, gearbox etc ie FWF. Larsen estimates within a few lbs of the Rotax 582.

    https://www.facebook.com/groups/164864990778457/search/?query=phazer&epa=SEARCH_BOX
     
  14. Jun 29, 2019 #54

    Victor Bravo

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    To address the OP's comment about Japanese motorcycle technology being used for aircraft engines, this exists now and you can buy one today. The HKS-700E is a 4 stroke high-tech / high performance using modern Japanese motorcycle technology and metallurgy. But it was designed from a clean sheet to be an aircraft powerplant, making 58HP while turning something like HALF of the RPM of the screaming "rice rocket" bike engines. I have about 5-6 hours flying one, and I have to admit it was reliable and behaved well. My problem with it was that the combination of engine/gearbox ratio/propeller diameter/airframe did not work as well as I wanted it to. But that was NOT the engine's fault.
     
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  15. Jun 29, 2019 #55

    henryk

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    =static thrust /propeller diameter (gear type) ?
     
  16. Jun 29, 2019 #56

    mm4440

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    http://aeromomentum.com/am10.html Would be a less of a screamer at similar weight to the snowmobile engine conversion with all new parts and compleat in a box so much less effort at a reasonable price.
    VB, I have a tiny Chinese turbo on the way if we want to try it on a V-2 conversion.
     
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  17. Jun 29, 2019 #57

    Victor Bravo

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    It made about 250 pounds of thrust (measured on a spring scale), using a 65 inch diameter 3 blade Arplast propeller, at 6200 RPM (crankshaft). The engine had a 3.47-1 gearbox.
     
  18. Jun 29, 2019 #58

    sotaro

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    The Phazer, dry, weighs 88 lbs, 109 with adapter and gearbox. I believe the latter is without exhaust or radiator. That latter number is about 30 lbs less than the Suzuki AM. Any ideas on the FWF weigh of the Suzuki AM?
     
  19. Jun 29, 2019 #59

    mm4440

    mm4440

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    Should be a bit lighter with one less cylinder and 1/2 the displacement. A good example of what I have been saying for a long time, "RPMs do not weigh anything." 500cc/1000cc but twice the RPMs = the same power but not 1/2 the weight so I guess my statement is not true. RPMs weigh somewhat less than displacement.
     
  20. Jun 29, 2019 #60

    henryk

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    250 pounds=115 kG ?...

    115 kG/ 58 HP=circa 2 kG/ HP =not to big !

    230 kG/ 70 HP=3.3 kG/HP (differential gear with 170 cm conterrotating propellers)

    180 kG/ 70 HP (SUZUKI G13)=2.6 (single 170 )
     

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