"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 13, 2018 #1

    Vigilant1

    Vigilant1

    Vigilant1

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    I’ve been thinking about this aircraft idea for awhile, so I thought I’d just put it out here for public ridicule, er, comment.

    In a nutshell: A small, single-seat, twin engine, centerline thrust airplane powered by small, inexpensive 4-stroke industrial engines. I’ve been thinking of it as the “Micromaster.” (For those who remember the Beetlemaster discussion, this may look a little familiar--two inexpensive engines in a centerline thrust plane, Cessna Skymaster style. In reality, the planes would be very different. The Beetlemaster is an idea I still I think about all the time . . .)

    The HP-per-buck of these little engines is attractive, and of course there have been several single-engine designs that use them successfully, including the Colomban MC-30 Luciole and the Minisport SD-1. These airplanes get good performance and reports indicate they are fun to fly.

    So, why make a twin? The objective of adding a second engine would be:
    1) To safely remain airborne in the event one engine experiences a loss of power
    2) Improved climb and cruise performance in normal operation (i.e. with both engines running)
    3) Enable the use of heavier but simpler, easier, and more robust construction (e.g. maybe solid-foam core wings and a welded-tube cabin rather than “a thousand sticks”)

    These sturdy (but heavy) little engines are quite a bargain at prices ranging from $750-$1500 each. Even if a bolt-on additional bearing or a simple redrive is used to take prop loads, it’s possible the hardware for the whole twin-engine setup could cost less than $5000. “Hop-up” parts and shops who will do enhancements are out there. Increasing the output of the stock engines by 30+ percent is not expensive, and the reliability issues/concerns of doing these "enhancements" might be reduced in a twin engine installation if it has safe single-engine performance.

    The numbers below are a quick, rough estimate of the two-engine and single-engine performance we might expect from a Micromaster. I made a lot of assumptions and did not do a complete drag buildup on the Micromaster (I just based things off the SD-1, increasing the induced drag due to the amount of extra lift we'd be needing). Despite the low HP/weight of these engines, it does look like it may be possible to use two of them to get good “normal” (twin engine) climb rates, and also still have acceptable single engine performance. There’s no doubt that the single-engine situation would require some careful attention during the design phase, and this first look indicates 25 HP engines might not be enough.

    There's nothing here about configuration (high vs low wing), construction material type, etc. Just some back-of-the-envelope figuring.
    Supplied as grist for the discussion mill.

    The logical next step would be-- 3 engines, two seats. We could call it the "Tri-Mow-tor."


    [TABLE="width: 500"]


    SD-1 Minisport
    ..........“MicroMaster”.........
    ...............................................

    Length (ft)
    14.27
    ~ 16’


    Wing span (ft)
    19.62
    ~22


    Wing area (sqft)
    65.66
    ~ 70


    Aspect Ratio
    5.85
    6.9


    Empty Weight (lbs)
    276
    401 ( Note 7)



    Useful load (lbs)
    304
    316 (Note 8)



    MTOW (lbs)
    580
    717


    Horsepower (gross)
    24 hp (Note 4)
    Estimates below for 25, 28, and 30 HP


    Effective HP (80% eff)
    19.2 hp



    Prop diameter (in)
    46”
    46”


    Span loading
    1.51
    1.48







    PERFORMANCE

    TWO ENGINES
    SINGLE ENGINE (one stopped prop)

    Cruise Speed (mph)
    81
    Unk


    Level Top Speed (mph)
    112
    Unk (Note 12)


    Clean Stall Speed (mph)
    50
    52 (Note 11)
    52 (Note 11)

    Climb rate at MTOW (fpm)
    515
    2 x 25HP (80% eff=20hp): 920 FPM

    2 x 28 HP (80% eff) : 1130

    2x 30 HP (80% eff) : 1280
    25HP (20 eff@80%)= 0 FPM

    28 HP (22.4 eff) = 100 FPM

    30 HP (24 eff) = 175 FPM

    L/D

    13:1
    13 : 1 (estimated)
    10.5 : 1 (estimated, incl 13 lb of dead prop drag at 60 mph)

    HP reqd for level flight at 60 kts (Note 1)

    12.84 hp
    15 HP (Note 9)

    15 HP (note 9)+ 5.2 HP (13 lb drag from stopped prop) = 20.2 HP

    Addtl HP reqd for claimed ROC (Note 2)

    9.05 hp
    (see climb rates above)
    (see climb rates above)

    Total HP reqd for flight + climb (Note 5)

    21.89 hp



    Induced drag at 70 kts (SL, level flight, lbs) (Note3)

    19.45 lbs

    (~7.8 HP)
    22.5 lbs
    22.5 lbs (~9.0 HP) (Note 10)

    Power Loading (lb/HP)
    (ref: Cessna 152: 15.2)

    24.2
    2 x 25 HP engines: 14.35

    2 x 28HP engines: 12.8

    2 x 30HP engines: 12
    25 HP engine: 28.7

    28 HP engine: 25.6

    30 HP engine: 23.9





    [/TABLE]

    Notes:
    1: Computed from stated L/D, MTOW, and assumed 60 kt airspeed for best glide distance
    2: At stated climb rate and MTOW assumed
    3: Estimated lbs of drag based on MTOW, wing span and area, and resultant required Cl. Corresponding HP estimates are rough, depend on propeller efficiency at that airspeed and other factors.
    4: The SD-1 has several engine options. Specs here are for the 28 HP 4-stroke B&S engine
    5: Sum of the two cells above.
    6. (deleted)
    7. Empty weight buildup: SD-1 weight of 276 lbs + another engine (75 lbs), + prop and hub (12 lbs) + 38 lbs additional structure (engine mount, larger&stronger wing, etc) = 401 lbs
    8. Useful load: 250 lb (pilot + baggage) + 66 lbs fuel
    9. Required HP derived from sink rate at assumed 13:1 L/D at 60 kts and acft at MTOW.
    10. With 22’ span, 70 sq ft wing, MTOW of 717 lbs.
    11. Assumed “clean” Max 2D Cl of 1.4
    12. Max and cruise speeds will be highly dependent on propeller efficiency at higher airspeeds. Fixed pitch props will likely have to be chosen to produce acceptable single-engine performance rather than top speed. If we simply apply the “speed increase = power increase ^ 0.33” equation and use the SD-1 as our “base case,” then the Micromaster with two 25 HP engines would have a top speed of 141 MPH. In reality, it will be appreciably less than that.
     
    Last edited: Nov 13, 2018
  2. Nov 13, 2018 #2

    BBerson

    BBerson

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    Unless you hand prop them, the weight of dual starters and flywheels, mufflers, etc. is an excessive percentage of useless engine weight.
    I built a twin with two one cylinder lawn mower engines in the '80’S ( hand propped). The noise and vibration of the two non-sycronized engines is annoying. I would lean toward bolting the two engines together for smooth running with a single prop for less drag. The engine out performance likely isn't that great since these engines are heavy and one alone can't do it with the drag of the dead prop, I think.
     
  3. Nov 13, 2018 #3

    lr27

    lr27

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    What's your "mission" for this airplane?

    I think your estimates of prop efficiency are on the high side, especially when flying slowly and with only 46 inch prop diameter. Single engine climb performance might be improved with a bit longer wing. But, from my limited expertise (or lack thereof), I don't think you're too far off.

    BTW, how about more, smaller engines and no redrive? I hear these are capable of 10 hp if allowed to breath and only weigh 25 lbs or so when stripped:
    https://www.harborfreight.com/engin...hv-horizontal-shaft-gas-engine-epa-69730.html
    Can you imagine the sound of 4 of these? The price is right. I suppose it might be safer to use the Honda equivalent, but now we're talking money. Now if you could just find a place to put them all... ;-)
     
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  4. Nov 13, 2018 #4

    lr27

    lr27

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    Maybe it should look like a small one of these:
    Monsted-Vincent_photo2.jpg

    or a push me-pull you version of the Alcor:
    alcor-duo-6-lockheed.jpg
    ;-)
     
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  5. Nov 13, 2018 #5

    cluttonfred

    cluttonfred

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    I like the concept, though the need for a multiengine rating would turn some people off. I'd definitively go go with redrives for more efficient propellers, the Ace Aviation ones from India are quite affordable. I would suggest looking at modest VW-powered aircraft with big wing areas as a good place to start, and perhaps the Grumman Skyrocket layout. How about a twin-engine, twin-tail conversion of a Jodel D.9 Bébé or Druine Turbulent? In 1930s US Navy colors, of course!
     
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  6. Nov 13, 2018 #6

    Toobuilder

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    There are plenty of aircraft powerplant options available for your $5k budget that will deliver much more power at higher reliability. From time to time, zero to low time O-290's show up for sale - typically for 3500 or less. Grab one of these and your engine search is over.
     
  7. Nov 13, 2018 #7

    Pops

    Pops

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    This is going to harder to design than the Bettlemaster because of the smaller size and having to have light weight ,small HP, efficient engines. Somehow if you could live with the weight of re-drives with the higher thrust it would help.
    Once in a while I still get all of my notes on the Beetlemaster out and look at them. I still think it would be a great little airplane. At my age, I just don't think I should start building one. If I built any thing else, it will be a smaller project. Heck, I spent 2 years just designing the JMR before starting construction. If I were to get back on the design of the Bettlemaster, with the time to finish the design and then the time for the construction, I would be to old to ever fly it. But then, 6 in my family lived to be between 100 and 106. Would be something to do the first flight of the Bettlemaster on my 100 birthday wouldn't it ?
    O-well, we can dream can't we.
     
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  8. Nov 13, 2018 #8

    Dan Thomas

    Dan Thomas

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    A non-feathering prop on a dead engine is a lot of drag you cant afford with low power. Even the traditional twin has serious performance degradation (climb, especially) when an engine quits and the prop is feathered.
     
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  9. Nov 13, 2018 #9

    Vigilant1

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    I included the estimated drag of the dead prop in the table below, and it looks like it could fly and climb (a little--probably no worse than a heavy Aztec) on one engine. If the design can be made to climb on one engine, then I'd think it best to go that way rather than re-introduce a single source of power failure with two-engines driving a single prop. If neither small engine can keep the plane aloft by itself, then one larger engine probably makes a lot more sense if such an engine can be had.

    Ah, that's one of the great things: no multiengine rating is required for a single seat experimental with 2 engines. There, we just saved a few thousand dollars!

    I haven't run Jan's amazing prop program on these, so I can't speak with authority on the prop efficiency. But, are these props small? I don't think so. From a disk-loading perspective, they are big. A C-152 (110 HP, 72" prop) has a disk loading of 3.9 HP/sqft. A Micromaster with a 30HP engine and a 46" prop would have a power loading of 2.6 HP/sqft. And the tip speeds are lower: Cessna 152 at 2550 RPM = 801 fps, Micromaster at 3600 RPM = 722 FPS. So, it's not clear that prop speed reduction buys us much improved efficiency. Still, a PSRU might be useful for getting the prop loads off the engine's bearings and crank (improving engine longevity and >maybe< system reliability). A belt-drive PSRU might also let us keep the engine lower nd get the prop higher which could improve the propulsive efficiency by getting the rear prop higher and in "cleaner" air, and maybe allowing a larger prop/shorter nose gear and a more streamlined cowling in front than is possible with the typical low crank location of these V-twins. It looks to me like we don't really need the prop speed reduction, but a separate prop support displaced from the engine crank could be useful. FWIW, neither the MC-30 Luciole or the SD-1 (with 4-stroke engines) use a PSRU.

    As sketched out here, it could be a local area "fun flyer" for somebody in an area with mountainous or heavily forested terrain where an off-airport landing would be a "significant emotional event." The wing loading and potential cruise speeds could also make it a pretty good traveling airplane for one person--a 200 lb pilot could carry 50 lbs of gear, which ain't bad, and with 11 gallons of fuel it can probably fly for 3.5 hours if using 40 HP (about 3 GPH). A bigger tank would probably be a good idea--IMO the pilot/baggage size I started with is pretty generous, and having the option to take more gas would be good. If we wanted a plane for short fields, we'd need to add more wing (especially span) to get the landing and takeoff speeds lower. It would need beefier (draggier) landing gear, etc. I think it would not be a good choice to design/operate the plane so that loss of one engine results in an unsafe situation (descent into unlandable terrain, etc)--you'd be safer with one engine than two (critical to flight engnes) in such situations.
    It's a tradeoff for sure. I saved about 5 lbs of induced drag when I increased the Micromaster's span to 22' from the SD-1's span of 19.6. But, a longer wing (even of the same area) costs more weight than a shorter wing, it also decreases roll response. Most important, I was very impressed by seeing the wing of the SD-1 being de-rigged and rigged: It's amazingly simple, and the wing stored easily in the plane's little trailer. A longer wing makes that more cumbersome. The Luciole wing panels (about 10.5' long each) can be lifted off by one person, so I figured these panels (about 10' long, but wider and heavier) would be okay.

    That could be done. As we decrease 4-stroke engine displacement, engines generally produce less (stock) HP per lb. Yes, we can hop them up, but as we've learned well in the VW world (and the kart racers have learned in the small engine world), that comes at a cost in reliability. As you've identified, where the even tinier engines shine is in HP-per-dollar. A V-twin is going to be considerably smoother than a single. And, they are all >really< smooth when they stop running.:)
     
    Last edited: Nov 13, 2018
  10. Nov 13, 2018 #10

    WBNH

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    Sounds like a plausible update to the CanAero Toucan...which flew on two Rotax 277s.

    PROMO.jpg
     
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  11. Nov 13, 2018 #11

    Vigilant1

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    True. A dead, non-feathering prop on this plane at 70 knots results in about 13 lbs of drag, and it "costs" us about 4.8 HP to overcome that, which is a really big deal. I'm assuming that it isn't windmilling, which would make things worse. Explosive bolts?:) But, even with that, it looks like this plane may still be able to climb (if my assumptions are right, and on a standard day, etc).

    A few years ago when reading up on things related to the Beetlemaster, I was surprised to learn that for twins weighing less than 6000 lbs, the FAA does >not< require that they be able to climb on one engine. Wow.

    Both this design and the Beetlemaster would benefit a lot from inexpensive controllable pitch props. It would reduce drag from a dead engine (a feathered prop has about 1/8th the drag of an unfeathered one), will let us optimize pitch for best thrust from the remaining engine, and (in the 99% of the time both engines are turning) let us get some pretty impressive cruise performance. Two things in the "plus" columns: There's no Vmc for this centerline thrust design. That's a big safety factor--it's unlikely that trying to stay airborne on one engine will result in the plane flipping over. Also, Rutan's "Defiant" design had fixed-pitch props and could climb well when one engine stopped running.
     
  12. Nov 13, 2018 #12

    Vigilant1

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    Or the MiniMaster (but as a single seat):
    View attachment 75868

    These designs are probably too draggy at this HP. I've been thinking of a slick pod-and-boom design with a prop on both ends of the pod.

    Like this, but with another engine in front:
    779.jpg

    It could be high or low wing, twin boom or a single boom. Detaching/re-attaching the wing is easier without external struts, and spars don't weigh much anyway if we use pultrusions for the caps and keep the wing span reasonable (the main wing spar for the SD-1 weighs about 5 lbs, each removable wing panel weighs about 25 lbs). I think a low wing with a high-mounted rear prop might offer some advantage for this plane (ease of attaching/detaching wings when solo, getting the rear prop in cleaner air, maybe a reduction in fuselage weight), but I'm not sure.
     
    Last edited: Nov 13, 2018
  13. Nov 13, 2018 #13

    Tiger Tim

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    I wonder if skinny blades (like you see on a Warp Drive prop) would have about the same drag in fixed pitch as they would in feather. Provided the prop was stopped, that is.


    ...and because industrial engines are typically mounted from below, it seems to me like this configuration wants to be a low-wing with either end of the fuselage pod running under the engines sort of like a MiniMax.
    [​IMG]
     
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  14. Nov 13, 2018 #14

    BBerson

    BBerson

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    Not just the dead prop drag. Also that rear engine cowl with the huge pusher cooling ducts and the poor fuselage shape and bluff end.
    Much more overall fuselage drag than a SD-1. Both ends of the fuselage are high drag instead of one end.
    Anything can work if light enough. Usually things are not as light as projected.
     
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  15. Nov 13, 2018 #15

    lr27

    lr27

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    Vigilant1:
    I may be off about the prop diameter. My mind is on ultralights, and your plane is faster than that. I still think 80 percent efficiency is kind of high if you're using an ordinary wood prop, isn't it?
     
  16. Nov 13, 2018 #16

    lr27

    lr27

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    P.S. If it's for mountainous terrain, doesn't that require more single engine performance?
     
  17. Nov 13, 2018 #17

    Victor Bravo

    Victor Bravo

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    The Alco mentioned previously, or the Dornier Do-28B configuration, seems like it would be worth looking into. Engines closer to centerline than typical twins, might de-complicate the structure a little.
     
    Last edited: Nov 14, 2018
  18. Nov 13, 2018 #18

    Vigilant1

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    Yep, this one would be tougher than the Beetlemaster. The engines don't have the favorable HP/weight of a large VW (about 3.2 lbs/hp (stock, as delivered) for these little 4-strokes, while a 2180cc VW with starter/electrics etc is about 2 lbs/hp). And wings, tires, etc don't scale down well in weight, which makes the weight/single engine problem even worse for the MicroMaster. For example, for the Beetlemaster weight (1500 lbs max gross) and wing (140 sg ft, NACA 2415) you propose, it looks like the plane would climb single-engine (dragging a stopped prop) at about 390 FPM. That ain't shabby. And with both engines turning, she would climb at over 1200 fpm.
    It looks like you've got good genes working for you. That grandson of yours is gonna need something fun to fly when he's older and done with the F-5. After getting used to the safety of that (nearly) centerline-twin airplane, he won't want to risk his butt on anything else, and he sure won't want to put his own kids in anything less safe. Unless he hits the lottery, he won't be able to feed an F-5, but he could fly the Beetlemaster all day on the money spent to taxi an F-5 to the runway. And, this plane is even >more< unique. Safe, fun, economical--c'mon Pops, don't be selfish--your kin need a Beetlemaster :)!
     
    Last edited: Nov 13, 2018
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  19. Nov 13, 2018 #19

    Vigilant1

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    I'm not too sure about the efficiency. The Luciole uses a thin composite Airplast prop, so perhaps thinner than a wood/wood cored one. Looking back at my Beetlemaster spreadsheet charts (derived from a laughably small, unreadable corner of a chart in Raymer, and my not-getting-any-better eyesight), it looks like a wood prop optimized for thrust at 70 knots can give about 74% efficiency, but if we optimize it for higher speed flight it gets worse (e.g. optimize prop for 100 knots, the off-design efficiency at 70 knots is about 61 percent). But this includes the "wood factor", a decrease of 10% efficiency across the board for wood vs metal, so perhaps a true composite prop would let us recover most of that 10%. And the original chart was built in 1963, there may have been improvements in airfoils since then.:)
    Nothing would help more than an inexpensive, light, reliable variable-pitch prop. But "light, cheap, and reliable" is normally a "you can have any two of those, not all three" proposition.
    Ivoprop sells an electric variable pitch prop that weighs 7 lbs and can accept blades that give a prop diameter of 48" to 72." They are good up to 100HP. Their pitch can be adjusted from 18" to 32" or 35" to 70" (they apparently can't be feathered). They cost $1700 each. I have no idea if they have proven to be reliable. Given their cost and weight, they may be more appropriate for a Beetlemaster than a Micromaster, though Revmaster is the only VW-based engine manufacturer in the US that I know of that has blessed anything other than wood props (the VW-based certified Limbach engines can use two-position props, but I don't know anything about their bearings/etc that facilitate that. It apparently works).
     
  20. Nov 13, 2018 #20

    Pops

    Pops

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    Hard, but can be done. If it was easy everyone would be designing one.
    I have 6 grandsons and I designed the JMR Special so even the 2 largest at 6'5" x 260 lbs and 6'4" x 250 lbs can fly it. Another grandson received a share in a Luscombe when he was 16 years old but he is busy with life at this time and not flying. He is very busy with school and work. Never know, maybe I will change my mind and get busy on the Beetlemaster some day. It would be fun.
     
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