# "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 22, 2018

### cluttonfred

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I think folks are missing the point. The attractiveness of converted industrial engines is not power or efficiency, it's availability and low cost. Once you get into modifying or hopping up the industrial engines to any degree, you've lost a whole lot of people that wouldn't be comfortable doing much more than bolting on a redrive and propeller.

This tandem twin idea is fun to explore, but the real market would be a relatively simple, easy-to-build design that could carry a large adult on just 25 hp or so. That would mean a motorglider-like design in the single-seat microlight (300 kg gross weight) category, perhaps along the lines of the Airsport Song but simpler. If you really want a twin then it's not a big jump from that to a baby Schweizer Twin Condor.

2. Nov 22, 2018

### Victor Bravo

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I love that Twin Condor. Those tailbooms and vertical fins and outer wings look reeeeal familiar to this old sailplane pilot.

How many different secret military airplanes can you design using recycled components from training gliders?

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3. Nov 22, 2018

### cluttonfred

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With about 30 or 35 hp engines, then a two-seat baby Twin Condor starts to look practical.... ;-)

4. Nov 22, 2018

### Sockmonkey

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You rang?

I think a tandem wing/lifting tail design would also work well with Skyrocket type engine layout.

5. Nov 22, 2018

### cluttonfred

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In terms of tandem-wings, Claude Piel designed a couple of push-me-pull-you tandem-wing designs, though to my knowledge none were built.

The smaller, VW-powered design appeared to use Mignet-style pitch control via the front wing incidence with a large anti-servo tab, ailerons on the rear wing, and endplate rudders.

The larger design was reported to use elevons on the rear wing but I don't know if that's really correct though it does appear complicated. See https://en.wikipedia.org/wiki/Piel_CP.500

6. Nov 22, 2018

### DonEstenan

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Even as tiny aircraft as SD-1 is really struggling with just 25hp or so. Maybe if you are 60-70kg, but with 90-100kg pilot 25hp is simply not enough for safe takeoff/climb out.

Yes, you can give it more wing, at the cost of flying _really_ slowly, but then the utility decreases quite a bit, and the climb is still abysmal.

The cheapest way to get to 75-100hp in engines is really to get 3 of them, but then the system's complexity skyrockets, and the gain is questionable (the drag of the three engines is significant, weight and fuel burn also, fuel subsystem, all kinds of failure modes....).

The annoying thing is that 25hp is sufficient for level flight, even at interesting speeds (100mph-100kt?) if you have clean airplane.

I checked batteries for 10min boost of 20kW - not too bad, 400 of those https://eu.nkon.nl/rechargeable/18650-size/samsung-inr-18650-30q-3000mah.html can be had for the price of one industrial engine, but reliable lightweight motors and controllers are insanely priced. :-(

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7. Nov 22, 2018

### Vigilant1

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Yes, guilty. I contributed to that, sorry. It's just that it appears that we only need a few more HP from each engine to make a big difference. Still, the twin engine, single seat idea looks do-able with two 30 HP engines. Our stock Vanguard 810cc gives us 28HP at 3600 RPM, so if our homebuilder is comfortable with bolting on the redrive and defeating the mechanical governor (easy) maybe our goal is in sight.

Isn't Topaz designing one for us?
Clean, long wings, 660 lbs gross (including 70 lb engine) , cheap, and easy to build--that's a tall order. It would be great, but the designs I'm aware of have had to give up on some of these criteria. Often it is the easy-to-build idea that gets excessively compromised and we are back to "1000 sticks".

I've never flown one, but they are claiming 515 FPM with 24 HP at 508 lbs gross weight (304 useful load--so plenty for a 100 KG= 220 lb pilot and fuel). 515 fpm isn't a rocket, but IMO it is very safe. Are the published numbers "optimistic"? It wouldn't be the first time.

All true. For this exercise (practicality and options for using multiple cheap industrial engines) the question, harkening back to the OP, is: Is the complexity, cost and weight of adding another engine (or 2) worth it in terms of reducing the chances of a forced landing, reduced need for complexity in the structure (to save every pound--" a thousand sticks"), and to get really >good< performance (1000 FPM climb, high available cruise speed, etc) for the 99% of the time that all the engines are running? Maybe, maybe not.

8. Nov 22, 2018

### cluttonfred

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Vigilant1, I think you might be off a little on the SD-1 weights. Per their site, the lightest version is the taildragger with Hirth F-33 at 116 kg/258 lbs empty weight, and the heaviest is the steerable trigear with the SE31 (industrial V-twin conversion) at 135 kg/ 302 lbs empty weight. Gross weight is 240 kg/533 lbs, so that leaves a payload of 105-124 kg/231-273 lbs. With 24 kg/54 lb of fuel, the pilot weight range is 81-100 kg/179-220 lbs.

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9. Nov 22, 2018

### syclone

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Multi-engine instructors in light twins have an old saying: “If one engine quits, the other one will take you diretly to the scene of the accident”. My point is, twin engine airplanes must be OVER powered to be safe if one engine fails. Using heavy industrial engines does not put us in that category. If we insist on flying cross country over rugged terrain or at night, we should be looking at three engines.

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10. Nov 22, 2018

### Victor Bravo

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If you are talking about actually using stock 25 or 28HP engines (to keep the cost below $X), then my gut tells me we are talking about something limited to the size of a Jeanie's Teenie morphed into the shape of the Do-335. Or something the size and shape of a Monnett Moni morphed into the Do-335. If you go towards a more efficient motorglider-ish size and span, the structure weight is going to render those engines insufficient to accelerate the mass to takeoff speed. Having two industrial engines mounted in tandem, with a common driveshaft running above the engines (essentially creating a V-4 engine with a redrive) would overcome some of the aircraft configuration issues, but then you start paying a weight penalty for carrying around two engine crankcases, crankshafts, etc. 11. Nov 22, 2018 ### Bill-Higdon ### Bill-Higdon #### Well-Known Member Joined: Feb 7, 2011 Messages: 387 Likes Received: 121 Location: Salem, Oregon, USA There was a proposed VW Twin called "Libella" in an old issue of Sport Aviation 12. Nov 22, 2018 ### DonEstenan ### DonEstenan #### Member Joined: Feb 14, 2016 Messages: 17 Likes Received: 3 Location: Slovakia I can't find it now, but I read an investigation report into the crash of G-CIMA (SD-1 with V-twin engine). The conclusion was: inadequate takeoff performance (good in January, but by May it progressively declined; combined with no headwind and inability to raise the tail quickly). Anyway, with Hirth-F23 the weights and performance is good, but 2-stroke is not everybody's ideal solution. 13. Nov 22, 2018 ### BJC ### BJC #### Well-Known Member Joined: Oct 7, 2013 Messages: 8,880 Likes Received: 5,732 Location: 97FL, Florida, USA The Zenair CH 620 was a twin VW. BJC 14. Nov 22, 2018 ### lr27 ### lr27 #### Well-Known Member Joined: Nov 3, 2007 Messages: 3,216 Likes Received: 463 Actually, since some of these industrial engines are commonly used in kart racing, etc., souped up versions aren't terribly expensive. I wouldn't be surprised if some are even competitive on$/hp, especially considering the significant reduction in weight. Obviously, one would have to use a redrive. The real questions is how much power could be reliable, and adequately cooled.

I think a single place aircraft with a long wing and a redrive could do just fine on 25 hp. Particularly with the strategic use of carbon fiber to keep the weight down. The wing AREA wouldn't have to be much larger, so it might not have to be all THAT slow.

15. Nov 22, 2018

### Vigilant1

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I was using the trigear (non-steerable) with the SE24 (24HP B&S), so an empty weight of 276lbs (here). As you noted, MTOW for all versions should be 533 lbs (I'm not sure where I got 580 lbs). So, by their math, it can safely climb with a 220 lb pilot and 37 lbs (6 gal) of fuel.
Thanks for catching that.

I agree that, based on the rough number crunching similar to that in Post 46, even using 3 cheap 22HP Harbor Freight Predator engines (approx $2100 total in engines for 66 total HP) will allow a single-seat plane to climb comfortably (225 FPM) if one stops running, and even a 2 seat aircraft could stay airborne and maybe eek out a meager climb rate (90 fpm-- it ain't much, but it's up!) Another alternative that may be acceptable (depending on the results of testing and individual risk tolerance) is using two "hopped up" engines (higher RPM, through a redrive). If the plane can safely climb on just one engine this might be okay. With a centerline thrust twin, the challenges of single engine flight are a whole different kettle of fish compared to a "conventional" twin with an engine on each wing. The biggest differences, of course, are the lack of a "minimum single engine controllable speed" and the lack of the (considerable) aerodynamic drag of rudder (and ailerons) needed to keep the plane flying straight. So, if we are going to try to use just one 25 HP industrial engine, and if we accept (for argument's sake, I am not asserting it here) that the SD-1 has unacceptable performance using an industrial engine of approx this size (per the info forwarded by DonEsetman here), we should ask how we could improve on the performance of the SD-1. - Weight? The SD-1 is pretty darn light (empty weight of 276 lbs, and that includes the relatively heavy 24HP 4-stroke industrial engine). Lots of wood sticks, it already has CF spars (each wing weighs only 27 lbs total). So, I don't think there's much ground to gained in reducing weight unless we go very exotic ($\$ or lots of time)
- Increased wingspan for lower induced drag that can be turned into climb? At max gross weight and 70 KTS (assumed climb speed), the SD-1s wing (19.6' span, 65.66sq ft area) produces just 16.4 lbs of induced drag. If we make the wingspan 26' and keep the wing area the same, the induced drag will be 10.9 lbs, a reduction of just 5.5 lbs, with takes us approx 2 HP to produce. If we apply the 2 HP to the plane's 533 lb MTOW, we get a climb rate that is improved by 120 FPM. That will be helpful. It won't transform the plane, though. Roll responsiveness/rate will be diminished. Wing panels that are each 3' longer will make it more cumbersome to rig/de-rig, especially for one person. We also haven't accounted for the increase in structural weight due to the longer span--it won't be much, but it won't be zero.
- Decrease profile drag? There's potential for this, but at a cost. The most obvious significant reduction in form drag would come from using monowheel gear, and some folks would cheer that. I think it comes with a big "cost" in utility (taxiing in a crosswind) and market acceptance (not important in a one-off). The prop hub would probably need to be very high, so a direct drive setup with these industrial engines is precluded. Prop length might still be reduced compared to a TD/NG setup, and that will reduce propulsive efficiency.

Maybe a 25 HP single place with performance significantly better than the SD-1 can be designed and built, but I don't see any low-hanging fruit that would make it easy. And if we want building one to be simple and cheap--it gets even harder.

But, if we add an engine or two . . .

Last edited: Nov 23, 2018
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16. Nov 23, 2018

### lr27

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You'll note that I recommended a redrive, which improves the prop efficiency at lower speeds. The longer wing keeps the induced drag down at lower speeds. So the best rate of climb should be at a significantly lower speed, requiring much less horsepower for flight alone. The airplane might be slightly slower. Similar arguments would apply for a two engine, two place version.

17. Nov 23, 2018

### lr27

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For an example of what might be done, let's make a motorglider out of a 1-26, which was actually a popular kit aircraft at one time. The original aircraft had a 575 lb gross weight. A later version had a 700 lb gross weight. We'll assume a motor and different landing gear could be added with that weight increase, and we'll use the 3 fps rate of sink from the heavy version. Let's assume that, at minimum sink speed, our powered version will have 15 percent more drag. If the redrive can allow the prop to have 65 percent efficiency at this speed, then we should be able to get 540 fpm climb at 44 mph. If we assume that, at a higher speed, the prop can be 75 percent efficient, and, at this higher speed, drag is 25 percent greater than for the original, then we can work out that the speed will be off the graph, which tops out at 90 mph. Very rough extrapolation suggests a top speed around 105 to 110 mph. Obviously, this involves some questionable assumptions, but I don't think it's unreasonable to expect a newer design could be a little more efficient than a 1-26 in the first place.

A three piece wing could make the panels easier to handle, or perhaps some kind of winglet could make the panels a little shorter with similar performance, but if you just don't like long wings, then you don't like long wings and you should get a more expensive engine.

Anyway, I expect similar considerations would apply for a two engine, two place version.

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18. Nov 23, 2018

### BBerson

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19. Nov 23, 2018

### lr27

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I forgot about that one. Of course it was much heavier than a 1-26 and had far more power than we're talking about. However, from those specs it seems to have had a MUCH higher rate of climb than a Taylorcraft BC-12D using the same engine. 1,000 fpm vs 500. The Taylorcraft's gross weight, however, was 100 lbs more. Top and cruise speed for the 1-30, according to what I've read, were somewhat higher. Struts are draggy, but that doesn't explain the 1-30's better climb. However, I don't know how much we can rely on any of these numbers, since I've seen quite a few for other airplanes that are hard to believe.

20. Nov 23, 2018

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