"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. May 29, 2019 #781

    BBerson

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    That might work with the soft coupled PSRU. The prop is unlikely to windmill and turn the engine twice rpm.
    Normally RC glider props don't always fold because of centrifugal force and they need an electric brake.
     
  2. May 29, 2019 #782

    Blackhawk

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    I think the tractor prop has spring tension to keep it folded when engine is stopped; still very simple.
     
  3. May 29, 2019 #783

    Blackhawk

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    I am sure these plans for a twin blade folding prop are open source by the designer.

    They're designed to use Ivoprop blades.

    They include the spring version as well.
     

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  4. May 29, 2019 #784

    Vigilant1

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    A thrown blade makes a simple engine failure look like a picnic.

    A direct-drive system (stiff) looks best for this plane. Cost, weight, reliability. What a PSRU offers (ability to turn the engine at well above 3600 rpm, or turn a much larger prop) isn't very useful to us.
     
    Last edited: May 29, 2019
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  5. May 29, 2019 #785

    Blackhawk

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  6. May 29, 2019 #786

    Blackhawk

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    This folding prop hub could be used on direct drive or reduction drive
     
  7. May 29, 2019 #787

    blane.c

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    Solid wood prop is likely to be easier on the crank.
     
  8. May 29, 2019 #788

    BBerson

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    That three blade folded prop with massive hubs sticking out doesn't look very low drag. A fixed prop could be fitted with inner "cuffs" for minimal stopped air drag if the tips are narrow.
     
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  9. May 29, 2019 #789

    Blackhawk

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    I have always liked this design from 1952 only using pusher props between the wings.

    It could also go electric because of the CG

    SS Boxwing.jpg
     
  10. May 29, 2019 #790

    Vigilant1

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    That's not known. If it has worked on a 4 cyl run at 3000 RPM does not mean we won't get into trouble (harmonics that rattle that hinge joint to atoms??) with a V-Twin at 3600 RPM. It would take a lot of testing to know.
    On this plane, a folding prop, in theory, would have one use: Reducing drag on a dead engine's prop. That's a critical and important flight condition, but we anticipate it will occur very rarely. We can estimate the drag of a stopped prop, and we've done it already: a stopped 46" long prop with an aspect ratio of 8 has an equivalent flat plate area of .77 sq ft and will generate about 13 lbs of drag at our (postulated--maybe too high) SE climb speed of 70 knots. After accounting for prop efficiency, it takes us 3.1 engine HP to overcome this drag from the stopped prop. So, lets consider three options:
    a) Invest about $5K-$10K (200% to 400% of the price of the engines.:eek:) to buy two factory built folding props and hope they work with this engine. A failure (thrown blade) produces a possibly catastrophic event (structural damage from intense vibration, engine torn from mounts with resultant out of CG issue, tail boom severed by the departing blade, etc).
    b) Invest lots of hours to develop a homebuilt folding prop. Same risks as above.
    c) Use engines that are 3 HP larger.

    We went with option C long ago (this project started by considering 22 and 25 HP engines, we've determined we need 28 HP and the dead prop is one reason.) It makes a lot of sense. Why?
    1) A folding prop doesn't eliminate all drag from a dead prop, it just reduces it. The 3 additional HP from a bigger engine entirely eliminates the performance deficit caused by the stopped prop (at 70 kts)
    2) The additional 6 total HP is available all the time--it makes the plane a better aircraft (safer, more agile) the 99.9% of the time that both engines are running. We can run bigger engines at a lower % of their max rating in normal ops, that improves their reliability and lowers maintenance costs. Conversely, folding props would >reduce< reliability and increase maintenance costs for the 99.9% of the time when they aren't helping. We are asking a lot from a lawnmower engine crank and bearings, and adding the additional metal-on-metal lash from a prop hinge won't make that engine more reliable.
    3) The folding prop helps with only one situation: an entirely dead engine. Additional available HP helps with >lots< of other emergency/operational situations.

    So, that's why the folding props don't look very appealing--we addressed the problem in a different way. The figures indicate we don't have an operational deficit that needs fixing--the plane will climb acceptably at MTOW on one engine as presently envisioned (if our numbers and estimates are right).

    If we wanted to do a prop development project, what this plane could >really< use is a fully in-flight adjustable prop. We'd get the ability to feather a prop, but even better we could optimize thrust at both climb speed and cruise (providing better single engine climb and some spiffy cruise numbers when both engines are singing). Again, this is a benefit that would be present every moment we are flying, not just during an engine failure. But it would be expensive and risky. I think $300 fixed-pitch props driven by slightly bigger engines provide the answer we need.
     
    Last edited: May 29, 2019
  11. May 29, 2019 #791

    Vigilant1

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    Since every pound of thrust is important in the single-engine climb mode, we'll probably need to let aerodynamics and prop "black magic" determine the prop configuration. The stopped prop produces 13 lbs of drag (at 70 kts) and the good prop is producing about 96 lbs of thrust, so if reducing the blade width to reduce prop drag costs even a little lost thrust from the good engine, then we're losing. As it turns out, a 46" dia prop for a 28 HP engine at 3600 rpm and optimized for 75 MPH will have blades about 3" wide at the 75% radius point, so they aren't paddles. I couldn't find the blade dimensions on the Warp Drive web page, but they look to be about that width.
     
    Last edited: May 29, 2019
  12. May 29, 2019 #792

    Blackhawk

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    Vigilant1

    I only put the idea of a simple folding propeller units in this thread as an alternative option for engine malfunction, instead of a feathering propeller system which could create a minefield of problems.

    Looking forward to see what you come up with.
     
  13. May 29, 2019 #793

    Vigilant1

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    Blackhawk.
    No worries. If you had this question/idea, I'm sure others did, too. And since you did research on it and thought it worthwhile, I just wanted to re-trace the logic (?) we'd used to get here. I thought it would be rude to not provide a rationale for not going with the folding props.
     
  14. May 29, 2019 #794

    revkev6

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    I know a lot of things on this forum tend to be academic and "how would we go about solving this impractical problem" scenarios but... this seems like a really impractical one. we are talking about an already fairly heavy industrial engine that makes low hp.... we are then trying to make two of them work and accounting for failure modes... sounds like a perfect spot for a single VW instead.... not sure I get it.
     
  15. May 29, 2019 #795

    Vigilant1

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    You aren't alone, others felt the same way and expressed it in earlier posts.
    A single VW puts out about 60-75 HP, two of these industrial engines set up for acft use weigh about the same and will be good for about 58-60 HP continuous. So we can expect the VW to have same/better performance in a similar airframe. Two of these engines cost about $3k, and will need mods to run in an airplane.
    Compared to a single engine plane, we'd expect better performance from the Micromaster after an engine failure. If someone flies in an area where a forced landing would be especially problematic or the just don't want that excitement, then they might see some value in this idea. Or not.

    It is hard to make a practical case for anything that involves designing a new airplane or even building one from an established design. There are just various degrees and flavors of crazy here.
     
    Last edited: May 29, 2019
  16. May 29, 2019 #796

    mcrae0104

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    What is your performance goal? ___ ft/min, single-engine, @ ___ density altitude? (Or stated differently, what single-engine service ceiling do you require?)
     
  17. May 29, 2019 #797

    blane.c

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    Inner cuffs for minimal drag?
     
  18. May 29, 2019 #798

    BBerson

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  19. May 29, 2019 #799

    BJC

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    You really summed it up there, Vigilant1.


    BJC
     
  20. May 29, 2019 #800

    blane.c

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    You can see some fattened portions at the root of the props, that are cuffs? How would they help a fixed pitch prop have less drag?


    [​IMG]
     

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