"Micromaster"-- Centerline twin using small industrial engines

Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by Vigilant1, Nov 13, 2018.

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  1. Nov 14, 2018 #41

    Vigilant1

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    Lots of things would need to be checked. But here's my math:
    - A 22' span wing with 70 sq feet of area lifting a 7171lb airplane at 70 knots generates about 24.5 lbs of induced drag.
    - A stopped 46" long two-blade prop with a AR of 8 has an equivalent flat plate area of .77 sft, and at 70 knots (SL) generates 13 lbs of drag.
    - Profile drag: Roundabout method: The SD-1 requires about 12.84 HP for level flight at 70 knots, which is equal to about 64 lbs of thrust (at 70 knots). Subtracting the induced drag (19.4 lbs with this wing and weight) leaves 44.6 lbs of profile drag for the SD-1 at 70 knots. As you point out, the bluff tail end of our MicroMaster is likely to be draggier than empennage of the SD-1, so let's add 25% (a total WAG) to the profile drag, so we can expect about 56.5 lbs of profile drag.
    Total drag of the Micromaster in level 70 knot flight with one stopped prop: 24.5 + 13 + 56= the 94 lbs total. That requires 18.8 effective HP at this speed, and if our prop is 75% efficient, it will require an engine output of 25 HP to maintain level flight. Each 100 FPM of climb at this weight requires 2.15 effective HP, or about 2.6 HP with our assumed 75% prop efficiency. So, if we've got a 28 HP engine (e.g. the 810 cc stock Vanguard as used by the Luciole in direct drive), we can generate about 100 FPM of climb. If the hopped-up Predator can give us 32 HP, then we'll see about 270 FPM of climb.

    Lots of assumptions here that could work either way. The biggest ones are the prop efficiency, the profile drag of the MiniMaster, the real practical engine outputs. I've also assumed that climb occurs at 70 knots, if the wing performs better at 60 knots then all our profile drag numbers (incl the stopped prop drag) go down by 27% and our thrust-per-HP goes up by about 8%, both of which translate into improved climb rates.

    I share your skepticism. But the racing carts, racing mowers, mud-boat skippers, etc are undoubtedly producing a lot more HP than stock, so there's that.
     
    Last edited: Nov 14, 2018
  2. Nov 14, 2018 #42

    Vigilant1

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    300px-Dornier_Pfeil2.jpg

    If we don't want a long driveshaft, then it's going to be relatively blunt back where the rear prop attaches. Without long (thin, light) tailboom(s), we're having to build structure and acft skin to cowl that big engine as far back as the tailfeathers go. The tailfeathers will need to be larger and heavier if they are aren't on a long arm. And then there's the issue with spin dynamics/recovery with engines at both ends. I think for efficiency it would be hard to beat a smooth pod and generous boom
     
  3. Nov 14, 2018 #43

    Riggerrob

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  4. Nov 14, 2018 #44

    Doggzilla

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    Could definitely work. I was not even aware of the Defiant until you mentioned it. I thought I knew all of his designs.
     
  5. Nov 14, 2018 #45

    cluttonfred

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  6. Nov 14, 2018 #46

    Vigilant1

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    Last edited: Nov 14, 2018
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  7. Nov 14, 2018 #47

    cluttonfred

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    As much as I enjoy where this is going in terms of imagining fun multiengine options, by the time you've spent the money and time on converting three fairly heavy 25 hp engines, possibly with redrives and definitely with props, wouldn't you be better off with one proven engine like a 60 hp 1835 cc VW and a parachute if you're nervous?
     
  8. Nov 14, 2018 #48

    blane.c

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    What if you pull the chute and the wind pushes you into high tension lines? Or something equally disastrous. With three engines you lose one and "you" choose were to go.
     
  9. Nov 14, 2018 #49

    blane.c

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    Less drama, it's just a pain in the * to land or crash off the airport environment. Embarrassing and expensive too, also no guarantee you will not be forced to make hasty preparations to remove your craft and further damage may occur.
     
  10. Nov 14, 2018 #50

    blane.c

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    I think glider is fine, certified engine is fine, 3 maypops.
     
  11. Nov 14, 2018 #51

    narfi

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    If I lived on pavement I would probably be building one right now. For some reason I have always really liked that plane.

    I did find there is a smaller plane like that using 40hp engines (planed to use herth 40s) but its two seat and only 5 ever made.
    https://en.wikipedia.org/wiki/Aeronix_Airelle
     
  12. Nov 14, 2018 #52

    Vigilant1

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    Maybe. It is definitely more effort to design an entire new airframe and convert three engines for airplane use. But by that logic, virtually everyone would be better off with a used C-152 and skipping the building entirely.

    There's no doubt that these little motors weigh a lot. I wish I knew more about the options for getting a prop mounted on one.
     
    Last edited: Nov 14, 2018
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  13. Nov 14, 2018 #53

    Vigilant1

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    There was another one-off "baby Defiant" style plane that used two VW engines. It was based in Calif, and we mentioned it in the Beetlemaster thread. I'll see if I can find it.
     
  14. Nov 15, 2018 #54

    pictsidhe

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    I'd be tempted to have a single prop and two engines coupled to it via centrifugal clutches and their own redrives. If one engine dies, throttle back to disengage both clutches, then reapply throttle. You could also shut one engine down for economy cruise.
    A freewheeling prop can have more drag than a stopped prop. Feathered is lowest drag, but more hassle.
     
  15. Nov 15, 2018 #55

    blane.c

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    https://www.bing.com/videos/search?...62E7A4FD57A05E98105D62E7A4FD57A05E9&FORM=VIRE

    https://www.bing.com/videos/search?...D3D3596670A8066D19E7D3D3596670A8&&FORM=VDRVRV

    https://www.bing.com/videos/search?...373A573A6BE23EEB2C67373A573A6BE23EE&FORM=VIRE

    http://www.walbro.com/walbro-eem-around-ideal-fuel-injection-solution/
     
  16. Nov 16, 2018 #56

    Vigilant1

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    Thanks. Chains (with and without idler), belt with idler (Ace Aviation uses belt without idler). Interesting that Bob in video one says we don't need to worry about cowling/baffling on air cooled engines, incl large VWs--"everything is in the carb jetting. Get the carb jetting right and you won't have any heat problems." I think many people would disagree,

    And in addition to those approaches, we have the direct-drive hub used by Luciole builders who use the B&S 810cc engine, plus direct-drive hubs on B&S engines in SD-1s. There are a lot of ways to get the prop to turn, I think we'd all lie to know the details on some of them (weight, price, problems/track record, limits on engine HP, prop size, weight, and RPM, etc. And we have the thread here where Topaz and Hot Wings were chatting about a prop support bearing that Hot Wings had drawn up. Anyway, these PSRU and direct-drive prop bearing discussions deserve their own thread.
     
  17. Nov 16, 2018 #57

    lr27

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    I expect that, with lousy cowling and baffling, one has to be much more conservative with the carb jetting. That, I think, would mean higher fuel consumption and more pollution.
     
  18. Nov 16, 2018 #58

    Vigilant1

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    Right, that's what I thought when I heard it. Sure, you can keep an engine cooler by running things very rich, but that's not a great answer.
    "Don't listen to what folks say about insulating your house so you can keep it comfortable in the winter. If you properly burn your furniture, you can keep things toasty inside with no insulation. "
     
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  19. Nov 16, 2018 #59

    pictsidhe

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    That chain drive video is a good example of why you shouldn't just wing it. He has moved to chains as his improvised belt drives were coming apart. His chain drive 'rentensioned' itself on it's first run. When you see it run, there is some nasty snatch on shutdown. Not good for a long life. I wouldn't pay too much heed to videos of people had a successful few hours running. You want to find something that runs for hundreds of hours, or always have a place to land... TV can break the cranks of redriven and direct drive cranks. It can play hell with belt life too. Poly-V belts will slip when torque ges too high. That will save your crank, but is why belt drives so often eat belts in hours.

    Proper baffling will mean lower drag cooling, that means more useable power.
    Or, you can just burn your furniture...
     
  20. Nov 16, 2018 #60

    MadRocketScientist

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