# "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 16, 2018

### lr27

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From reading that post, it seems like the soft system is the way to go, if you can figure out how to build in some compliance without too much weight, space, or complication. I can imagine doing something with spring loaded idler pulleys, but I don't know if that would provide enough movement.
I'm wondering if there's some size below which these issues aren't as big of a problem. However, considering how small a BD-5 is, that's probably wishful thinking. So I also wonder why we don't hear about more trouble with things like the Ace redrive. Maybe I'm just not listening in the right place.

If a redrive is necessary, I can imagine using two engines with coaxial props. One could fail while leaving the other intact. Not sure what it would sound like, though. Hard to tell how loud or obnoxious this is from a video:
The props would probably have to be larger than with a normal system, though, in order to maintain a reasonable disc area for use at low speeds.

2. Nov 16, 2018

### pictsidhe

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MRS's TV post is right on the money.
After wasting numerous envelopes, I currently favour a soft system with sprung idlers and a poly-V belt. Belt slip to take the system safely through startup and shutdown.
I will be disappointed if I can't get well under $1/hr belt running cost. 3. Nov 16, 2018 ### lr27 ### lr27 #### Well-Known Member Joined: Nov 3, 2007 Messages: 3,216 Likes Received: 462 Are there existing examples of this type around? 4. Nov 16, 2018 ### blane.c ### blane.c #### Well-Known MemberHBA Supporter Joined: Jun 27, 2015 Messages: 2,827 Likes Received: 477 Location: capital district NY If engine powers a generator specifically designed to be efficient for aircraft, then most if not all of the crank load concerns go away. Engines can be mounted anywhere on aircraft. Electric motors can turn at any rpm you want, it doesn't matter what rpm the engines turn. Since the engines could be installed in banks down inside the fuselage just install as many as you need to power your motors. They could be designed to be swapped in and out with battery packs, you decide if you want x amount of battery packs and x amount of engines on any given day. 5. Nov 16, 2018 ### Vigilant1 ### Vigilant1 #### Well-Known Member Joined: Jan 24, 2011 Messages: 3,494 Likes Received: 1,490 Location: US As I may have mentioned, I think the discussion of PSRUs and direct drive prop hubs for these small industrial engines is important and deserves it's own thread. With the problematic search capabilities of this site, the best hope of finding useful info is to get it corralled at the time of posting, and no one will be looking for PSRU info in this plane-specific thread (just as no one will find the gobs of 2 stroke and small v-twin info now posted in the General 992 thread). Obviously, we should feel free to post and discuss anything you want in this thread, I learn from every post and will be yammering on myself. But nobody is likely to be able to find it later when they need it. Electric hybrid is a distraction until somebody shows the weight and (especially) cost advantage over the mechanical approach. With numbers. Weight and cost are critical. Also, delivered horsepower is important if we are scratching for every pound of thrust, so let's include all the (substantial) conversion losses. And, as above, I'd recommend it get it's own thread. Last edited: Nov 16, 2018 6. Nov 16, 2018 ### Vigilant1 ### Vigilant1 #### Well-Known Member Joined: Jan 24, 2011 Messages: 3,494 Likes Received: 1,490 Location: US I'm not sure that belt slip (or stretch) is very effective at getting through the typical low RPM startup/shutdown shocks. The Ace Aviation units apparently depend on it, but most other mfgrs seem to need at least a loaded idler and sometimes a sprag clutch (apparently very controversial) or a centrifugal clutch. If I'm understanding pictsidhe's approach, I think the Valley Engineering PSRU approach is an example. 7. Nov 16, 2018 ### jedi ### jedi #### Well-Known Member Joined: Aug 8, 2009 Messages: 1,678 Likes Received: 374 Location: Sahuarita Arizona, Renton Washington, USA IMHO it would be better to use two counter rotating props as in the old ParaPlane. If two engines are driving one prop, when one engine dies the other is driving too munch prop and can not develop full rpm. Instead of getting 50% power you get 30% of the two engine power. There is still some reduction even with the two prop system as the one still turning is not in the high speed flow of the other prop that has the failed engine. 8. Nov 16, 2018 ### Vigilant1 ### Vigilant1 #### Well-Known Member Joined: Jan 24, 2011 Messages: 3,494 Likes Received: 1,490 Location: US Coaxial or single prop does allow centerline thrust, which is good. Compared to two independent props (Skymaster, etc) it reduces redundancy and safety (one thrown belt or seized bearing can leave zero power). The dead prop on a coaxial/counterrotating has more drag than if it we're independent (due to much higher slipstream velocity over the dead prop compared to ambient airspeed). Since single engine ops are the most critical flight mode, this is a big deal. And, as Jedi mentioned, in normal ops, the slipstream from the front coaxial prop requires the rear prop to have a steeper pitch for optimum thrust at design airspeed and engine(s) RPM. With fixed pitch props, if the front prop stops then we have too much pitch on the rear prop to let it get to max engine HP RPM (the same problem, but for a different reason, that we have if both engines are swinging a common prop). Plus, with two-driving-one or coaxial props, we get more weight, cost, and complexity than one prop per engine. Aside from the novelty, in single engine mode (our most challenging problem), what's the advantage of counterrotating and coaxial props or two-driving-one one prop? Last edited: Nov 16, 2018 9. Nov 16, 2018 ### blane.c ### blane.c #### Well-Known MemberHBA Supporter Joined: Jun 27, 2015 Messages: 2,827 Likes Received: 477 Location: capital district NY Typically aircraft engines spend the majority of their time at approximately 85% to 90% of max power RPM, that is 60% power to 75% power is around 85% to 90% of the max rpm (more or less). So if a industrial engine is designed to spend it's life … say cutting grass … at 3,600 rpm then couldn't that be used as say the 85% rpm load for aircraft use? Then un-governing the engine we could spool it up to 4,200 rpm for take off power. This should take a 25hp engine up to about 33hp for takeoff without much expense for other modification. Unless you want to use a twig for a propeller once you've gotten to this rpm the gyroscopic and other loads generated by a propeller at this rpm strongly suggest a reduction drive. From what I can deduce from the internet the simplest and most popular for engines in this power range is the grooved flat belts. If quality parts are used "good service" is the norm, but here is were we need a definition of "good service" and "I think" 2,000 hrs ain't happening. Also the engines are not designed for 2,000 hrs either. So you want a lot less initial outlay of cash you get 15% to 25% of that 2,000 hrs or in other words 300 hrs to 500 hrs is a more reasonable expectation of good service. Considering this is for most private pilot type operations in and about ten years worth of accumulation of time it is a good deal. But it also begs another question if you are only expecting 300 hrs to 500 hrs out of a four stroke engine and you can get the same dependability from a two stroke engine for less weight at the same power are you flogging a dead horse? Another thing is engine location, I don't like for safety reasons related to ground operations a front of the airplane engine and a rear of the airplane engine on the same airplane. It is a recipe for someone getting hurt or killed. All the engines in the back or all of them up front is safer for the people you care about and for strangers too. The exception to this is both engines on a pod above the aircraft. As it has been pointed out on this forum by wiser minds, 2 engines is the conventional wisdom for private multi-engine operation and side by side twins are going to have about 37% of there actual power available for forward thrust in single engine operation. However I am bucking conventional wisdom hard, but with a caveat that seems to altogether go missing when argument opposes. I believe that with a computer controlling engine functions whether you call it efi or eem or whatever, then it is safe to run three or four engines instead of two. Removing engine adjustment (not monitoring) from the pilot duty's is the equalizer (actually it more than compensates) and a logical amount of computer engine control is what makes multiple engines a safe option. Three engines, one center and one on each wing and you lose one engine the control input forces are not nearly as exaggerated as in a twin, also the strain on the remaining engines is not as great increasing their dependability. If you are using cheap dependable engines, you can expect cheap dependability, and having a spare ain't a bad idea. Some day you are going to like that extra engine I guarantee it. 10. Nov 16, 2018 ### blane.c ### blane.c #### Well-Known MemberHBA Supporter Joined: Jun 27, 2015 Messages: 2,827 Likes Received: 477 Location: capital district NY Another thing is if you go to three or more engines single ignition is good enough. Your second ignition system on each engine is replaced by two or more extra engines instead. Considering the reliability of modern ignition systems, dual ignition systems is redundant in a triple or four or more engine arrangement. A caveat to this is we are of course talking small engines that can make full rated hp on a single ignition. 11. Nov 16, 2018 ### blane.c ### blane.c #### Well-Known MemberHBA Supporter Joined: Jun 27, 2015 Messages: 2,827 Likes Received: 477 Location: capital district NY I wonder if in a pusher engine arrangement, props could be put into a yoke a short distance from the hub. If supported with mechanical stops. The props would swing out under centrifugal force to the first mechanical stop, and the slipstream would pin them against the second if rotation slowed enough. 12. Nov 16, 2018 ### pictsidhe ### pictsidhe #### Well-Known Member Joined: Jul 15, 2014 Messages: 5,642 Likes Received: 1,436 Location: North Carolina I would be quite happy to change a$40 redrive belt in 20 minutes every 100 hrs. Fuel costs will dwarf that. I'd like a 500hr, reliable engine. I don't want to spend $4k on an engine. Around$2k looks doable with an industrial conversion.

13. Nov 16, 2018

### lr27

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The first mechanical stop might even be necessary in operation, though it might be handy when the aircraft is parked with a tailwind.

14. Nov 16, 2018

### blane.c

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Pratt and Whitney chose the words "Dependable Engines" carefully.

15. Nov 16, 2018

### blane.c

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I like stops on the props with quality rubber bumpers. Say "Rubber Baby Buggy Bumpers" three times real fast.

16. Nov 17, 2018

### Vigilant1

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I think that's very reasonable and probably do-able.
On the question of PSRU vs direct drive (with additional bolt-on support bearing/hub): Have you had the chance to do the calculations to determine the difference in thrust (PSRU with bigger prop vs direct drive with smaller prop) for your anticipated HP and airspeed of interest? I ask because the SD-1s and Lucioles are using the 25-28 HP 4-strokes in direct drive mode and it seems to be working well for them. It would be lighter, less expensive, and possibly more reliable. As noted previously, the disk loading with a 46" prop is not high--it is less than we see with a typical Lycoming on a C-152.
For a plane like the MicroMaster, from a layout standpoint either a belt drive or direct can be made to work. Using a belt drive to raise the prop higher than the low-ish crank on these V-Twins would be a plus, however.
I will try to get Jan's prop program up and running soon (I need a real copy of Excel to run it), that may give info of use if you don't have it already.

17. Nov 17, 2018

### lr27

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If you're going for a soft redrive, how about using a very long belt to make it soft? You could put the prop for the front engine on the back and vice versa. Not entirely joking. A long belt should last much longer too. The belts could run through a tunnel down the middle, so if something snaps it won't slap your leg. I'll admit I don't know what the spring rate of belts is

18. Nov 17, 2018

### pictsidhe

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I'd like a scale-ish prop for my Hurricane. Scale would be 86". That would mean halving the engine RPM. I could go to the bigger 821 engine, but I'd still only have ~2/3 the power of a redriven 627. That would be adequate, until a Hun in a redriven 109 got on my tail...
The toothed belts are very stiff. The poly-V belts have polyester tensile members, so some spring. You need a surprising amount of angular compliance for a soft system. I think I was looking at +/- 30 degrees for my twin to get resonance well below idle. I havce failed to get any belt stiffness specs from Gates so far. I even 'forgot' to merntion that my high MOI load was a propellor.

Something that might obviate the clutch/slipping belt in a soft system is to have a decompressor on one cylinder activate near resonance. The engine would have to power through the resonant frequency on one cylinder, then both could activate. If you lose one cylinder, you need to avoid the new resonant frequency. I read about a redriven Mazda that worked great, until the time it fired up on one cylinder only. It instantly stripped its gears... A clutch or belt slip seems good insurance against the unknown. I like the slipping belt as it can use inexpensive poly-V belts. Slip will be for a few revs on startup and shutdown, so it shouldn't wear the belt too horribly. I'd want the single cylinder mode to at least be away from any continous RPM, too. If it develops a misfore on startup, don't fly. If it misfires in the air, stay away from the critical RPM till you are back on the ground.

19. Nov 19, 2018

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Drop variable pitch and cost goes down a lot. Consider folding instead:

20. Nov 19, 2018

### syclone

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I can't tell if I've been able to post a picture or not. The twin engine configuration doesn't work out because the engine-out performance is so poor. Three engines might work if we can get the firewall forward (and aft) weight down to 75 pounds. I tried the OV-10 configuration with an aft centerline engine on the back of the central pod. The (empty) weight and balance didn't work so well - it kept falling over on its tail when the pilot climbed out.

The best configuration seems to be a high wing T-tail with an engine on the nose and and a pusher engine on each wing. Engine out performance is marginal and may limit the airplane to a single occupant during the middle of the day in the summertime, but this configuration seems to have two advantages: 1. It can use folding props on the two wing mounted engines which will improve performance and limit the amount of cross control required with a wing engine failed. 2. Crashworthiness is improved.

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