# "Micromaster"-- Centerline twin using small industrial engines

### Help Support HomeBuiltAirplanes.com:

#### Vigilant1

##### Well-Known Member
I think it would be better to have enough extra HP so that the remaining engine(s) give a good climb rate without any extra mods or add on's. That way the climb would be pretty spectacular with all engines operational.
Agreed. Howver, as DonEstenan points out, the price (and even weight per HP) of these this little engines has a "knee" at about the 810cc level (about 28 stock HP). Above this the cost per HP jumps.
In this case, one way to keep using the "cheap" engines, not hop them up or do anything heroic in exotic fuels, and still get good performance with one engine out is to use 3 of the things ( I can feel the eyes rolling!). Now we have even >more< spectacular performance with all engines operating, we aren't asking them to do anything they aren't designed to do, and the cost is probably about the same as modifying two engines with custom cams, billet conrods, a special carb (and then the dang things overheat anyway). If using three engines and sticking with a single place plane, maybe we can get away with the horizontal shaft 23HP 627-670cc engines (and not need to turn 810cc 28HP vertical-shaft engines on their sides).

What about an industrial V-twin to modified to run @ say 4500rpm, and an electrical motor connected to the PSRU to help on take-off/initial climb and in case the engine dies?
How does the cost/weight of an approx 28 HP (21KW) electric motor and the batteries needed to run it for 10 minutes compare to just having another gasoline engine? I'm not trying to be a wet blanket--I really don't know. A second gasoline engine would give us longer running time. How doe we charge those batteries (sounds like we need the weight of a generator, too, or we plug into the hangar). With lots of chemical energy already aboard the plane (in the form of gasoline), it seems inefficient to haul around more chemical energy (batteries) just for this emergency case.
A little screaming, loud, 6000 RPM 28 HP "emergency" 2-stroke hooked to the PSRU with a centrifugal would probably be light, cheap, and would let us fly for hours with the fuel aboard if necessary. Meter some oil into the fuel on the way to the motor. If only we could count on it starting rapidly when we need it.
Of course, any emergency powerplant (gas or electric) that uses the same prop and PSRU bearings/shaft/etc as the main engine is still subject to some single points of failure.

View attachment 75986

Here is the Bronco arrangement. The landing gear is 20.4 inches behind the quarter-chord. Putting the MLG further aft will make it difficult to get the airplane to rotate for take-off. It looks like all three engines could be moved forward and the central pod could be moved forward to solve the cg situation. This is a good looking airplane but the wing mounted props are running right beside the cockpit. I abandoned this configuration without refining it.
Thanks. A side-by-side seating configuration might also work in solving the CG problem, possibly at the cost of putting the pilot's head behind the LE of the wing.
Thanks to the prop tips just outside the canopy, the interior noise of the OV-10 made it the loudest cockpit in the USAF inventory at the time (which is saying a lot, since the T-37 "Dog Whistle" with it's centrifugal-compressor engines were also in competition).

About performance: I used direct drive engines - no PSRU - and 54" props tuned for 3,600 RPM at 110 smph. These are my guesses, feel free to offer yours; at 23 hp, 3,300 rpm and with .75 prop efficiency at 112 fps climb out each working engine makes about 84 pounds of thrust. With one engine out our 960 lb. (Do I hear 1,000?) airplane seems to climb out at 600 fpm (Using a L/D of 12). This is at sea level on a standard day.
So you assumed the props are optimized for 110MPH (96 knots) and the plane climbs out at bout 68 knots. That's about the same climb airspeed I used. Does your sim/spreadsheet, etc take into account the drag from a stopped prop?
Here's where my assumptions took me (3 engine airplane climbing on 2 engines with drag from one stopped 46" prop, from Post 46 ). You got better 1 engine out climb performance than I did, and I assumed 2 additional HP (but 70% prop efficiency).

With our two remaining 25 HP engines and 70% efficient props, we will get 35 HP (effective). Subtracting the HP needed to stay airborne at 70 knots, we have:
Single seater (917 MTOW): 12.2 HP of excess power. This will give this plane a one-engine out climb capability of 440 FPM
Two seater (1127 lb MTOW): 9 HP of excess power for the 2-seater. That will give this plane a one-engine out climb capability of 266 FPM
I don't know if a 25 HP engine with approx 42 lb/ft of torque will be able to swing a 54" prop at 3600 RPM. Maybe it can, but they will be wispy blades!
Thanks again.

Last edited:

#### BJC

##### Well-Known Member
HBA Supporter
Someone already mentioned it, but I will ask again. What would it be like flying a twin or triple without propeller synchrnozation?

BJC

#### Vigilant1

##### Well-Known Member
Someone already mentioned it, but I will ask again. What would it be like flying a twin or triple without propeller synchrnozation?
BJC
Yep, it was BBerson. He had tried it with two single cylinder engines, didn't like it.

I don't remember reading any adverse reports about vibration/noise/buzzing from those who flew the Rutan Defiant, but that's a small sample size.

Cessna did put a synchrophaser in their 337.

The Powers-Bashforth MiniMaster (push-pull with 2 stroke Rotaxes) reportedly had some bothersome vibration. They ended up changing to a two blade prop on one end and a three blade on the other, which they said helped a lot. That would be one fix.

Today, it would be fairly easy and cheap (at least in theory) to synchronize the props in a direct-drive setup >if< we are going to use a purpose-built electronic ignition system for these engines anyway.

#### BBerson

##### Light Plane Philosopher
HBA Supporter
View attachment 75986

Here is the Bronco arrangement. The landing gear is 20.4 inches behind the quarter-chord. Putting the MLG further aft will make it difficult to get the airplane to rotate for take-off. It looks like all three engines could be moved forward and the central pod could be moved forward to solve the cg situation. This is a good looking airplane but the wing mounted props are running right beside the cockpit. I abandoned this configuration without refining it.

About performance: I used direct drive engines - no PSRU - and 54" props tuned for 3,600 RPM at 110 smph. These are my guesses, feel free to offer yours; at 23 hp, 3,300 rpm and with .75 prop efficiency at 112 fps climb out each working engine makes about 84 pounds of thrust. With one engine out our 960 lb. (Do I hear 1,000?) airplane seems to climb out at 600 fpm (Using a L/D of 12). This is at sea level on a standard day.

54" props are reasonable. I have a 58" prop on my GX 670. Depends on cruise speed or climb angle desired. My design desire is highest climb angle.
"The Light Airplane"* by Ivan Driggs (1924) recommends up to 66" for low power, low speed.

*reprinted as NACA TM #311 and #326

Last edited:

#### syclone

##### Member
The comment about "wispy props" is appreciated. If I understand the ignition sequence on these V twin engines, this is like driving a propeller with an air-hammer. Evidently wooden props can put up with it, but a folding prop might not. Propeller tip speeds, I believe, are not a problem at about M.79.

#### Vigilant1

##### Well-Known Member
Bill, thanks much for the info and citations.

54" props are reasonable. I have a 58" prop on my GX 670. Depends on cruise speed or climb angle desired. My design desire is highest climb angle.
"The Light Airplane"* by Ivan Driggs (1924) recommends up to 66" for low power, low speed.
The 66" diameter works for the 3000 RPM, 25HP engine in his example. Just to be clear, a 66" dia prop wouldn't work at 3600 RPM direct drive speeds for these little engines (rotational tip speeds (disregarding acft fwd speed) would be Mach .92, or 1037 FPS. Driggs recommends we stay below 820 FPS). At 3600 RPM, his 820 fPS tip limitation puts us at a max prop length of 52". Others give different max speeds for wood props, Raymer says up 850 FPM.

*reprinted as NACA TM #321 and #326
I think there may be a typo there and it should be NACA TM #311 rather than 321.

Again, thanks.

#### BBerson

##### Light Plane Philosopher
HBA Supporter
Yeah thanks, it is TM 311, I corrected the typo.
My 58" wood prop has metal mesh and fiberglass tips, so it can handle the higher tip speed and still be very thin. One of the many details needed to make good thrust with a bigger prop at low ultralight speed.
I get 3000 rpm static. I think 3200-3400 rpm is max desired in flight for me. I just don't like high rpm.

#### lr27

##### Well-Known Member
Not so sure those tip speeds wouldn't be a problem. Especially with wooden prop. Maybe not on a composite prop with a little sweep at the tip. Check out what happens to a thinned (to 10 percent) Clark Y at transonic Mach numbers:
https://history.nasa.gov/SP-4219/Chapter3.html
Do search on Clark Y within the page and you'll find a chart showing a big rise in drag as the Mach number goes from 0.6 to 0.8. Just how thin can you make the tip of a wood prop? I suppose one could make a wood prop with composite tips that had some of the advantages of both.

BTW, it's my understanding that props which are swept a bit can be made to have higher pitch when going fast and lower pitch when going slow. However, I understand this makes matching the elastic modulus of the wood in each blade more important. A further thought is that inclining the pivot axes for folding blades might introduce a favorable variation in pitch with load and rpm.

#### Vigilant1

##### Well-Known Member
BTW, it's my understanding that props which are swept a bit can be made to have higher pitch when going fast and lower pitch when going slow. However, I understand this makes matching the elastic modulus of the wood in each blade more important.
The Prince "P-Tip" props are advertised to change pitch under load (approx 4" change in effective pitch overall is reportedly typical). They are popular in the Sonex community, and many who fly them have seen significant improvements in climb rate with the same top speeds they had with their old props. It won't help the MicroMaster's stopped prop drag issue, but it might allow improved cruise speeds when both props are turning and also provide better thrust/climb at lower speeds (higher prop load) when one engine is out.

I have not yet done any in-depth prop calculations, but I think we'd find that, with 25-30 HP engines at 3600 RPM, an approx 70 knot climb speed and a cruise speed of 100 kts, that going with a prop longer than 46" may not have much of a payoff. It's certainly less significant than it would be at Part 103 airspeeds or if we were loading up the disk with a higher HP engine.

Last edited:

#### pictsidhe

##### Well-Known Member
Prop critical mach number is very dependent on the tip airfoil used. The NACA 65A and 66A series do better than most.
I'm hoping to extract 40hp NA from a vanguard 627. I'm expecting to have to pay a lot of attention to cooling. One idea I like a lot is water or MW50 'injection'. Both work as anti-detonants, allowing higher CR and as coolants. They tend to get used as antidetonants at up to double the fuel flow. If it is only needed above, say, 3/4 throttle, for 1/10 the flight by fuel consumed, the water volume would bet 20% of the fuel used. Not too hideous. A higher CR will lower part throttle BSFC, so the range hit isn't as bad as appears. 4 gallons of fuel and 1 gallon of water/MW50 appeals to me. MW50 (stronger screenwashes are 40%...) would need to mixture messing with so is the complex option. Straight water could be 'injected' via an auxillary carb jet. I have a couple of plastic mower ones stashed already.
I did come across a report on an internally water cooled diesel project. I think the water flow was something like 4 times the fuel flow. That was with NO other cooling. It definitely looks like it could help with borderline cooling cases.

#### Aesquire

##### Well-Known Member
What about water/methanol injection? IIRC that has a cooling effect, or could you not carry enough of it to matter?
Sure, proven effective on two thousand eight hundred cubic inch engines in Hellcats and transports. Good on Five hundred fifty cubes at Reno. Should work on a sixty cubic inch engine too.

But "bang for the buck" in a weight limited airplane with 44 straining ponies is probably spent on a belt drive PSRU.

The only real downsides of water injection is the weight of the unit itself and the electric system to run the pump.... And a 12 volt windshield washer pump is good enough. ( the unit for the R2800 weighs more than your engine ) Oh! Yeah, the tuning and fuel control so you don't just kill the engine when you push the button. But you'd perfect that before flight, just as you'd test the prop.

However, as some smarter guy here points out, if it's not there it can't break & weighs nothing.

#### jedi

##### Well-Known Member
Anyone interested in a single seat version of "The Beast"?

Push me pull you R 912s.

#### Vigilant1

##### Well-Known Member
But "bang for the buck" in a weight limited airplane with 44 straining ponies is probably spent on a belt drive PSRU.
I could make the case for or against a PSRU in this application (and probably have done both!) At 3600 engine RPM and 70-100 KTS, I don't think we'll get a whole lot more thrust from a prop, a PSRU adds weight, complexity, cost, and failure modes (incl resonance issues) compared to a direct drive (with or without an exterior bearing to accept gyroscopic and thrust loads from the prop). On the "plus" side, there would be some improvement in thrust, it would let us clean up the crummy aerodynamics (and looks, IMO) typical of these V-twin installations by getting the prop higher and further away from the engine "blob."

(Philosophical rambling follows)
The PSRU opens the door to the temptation of turning these industrial engines at much higher RPMS (and HP output) than the engineers envisioned. The improved HP/LB is very tempting, but new camshafts, valve springs, billet conrods, new carburetor(s) is how a highly reliable $1000 3600 RPM engine might become temperamental, overheating, short-lived$2000 engine. And at some point we have to look at the sow's ear and wonder if it is the best starting point for the silk purse we want. It >could be< that a 600 cc engine designed to be light (HP/lb) and to run (and keep cool ) at higher continuous RPMs (a motorcyle engine?) is a better starting point, especially if we are going to use a PSRU anyway. I don't have an answer or even a suggestion on that score. I do know that a BMW opposed-twin motorcycle engine (complete with PSRU, alternator, prop hub, etc) reliably gives gives about 1.47 HP/cu inch, provides .58 HP/lb, and will reportedly do that for hours on end. If we could find a 500cc motorcyle engine (rather than the 1100cc BMW) with matching specs it would weigh (with PSRU, carbs, etc) 77 lbs and provide 45 HP, which would be great. But, wishing isn't the same as having. . . and maybe the hopped-up industrial engine, addressing each problem as it comes up, is the best approach after all. After all, that's the road we took over many years to eventually get reliable VW-based aero engines. The early VWs in the US (1300cc, 36 HP, about 150 lbs) had HP/lb (.24) that was even lower than today's stock industrial engines. Today, the current VW-based airplane engines (after decades of refinement, trial, error) provide about .47 HP/lb, or almost twice the specs of the first ones, and do it at 3600 RPM.

With unchanged RPM, engines can keep the same HP/cc as they get smaller, but they almost always get heavier per cc and HP.

Last edited:

#### BBerson

##### Light Plane Philosopher
HBA Supporter
And after decades of aviation VW experimenting, some 95% are direct drive.
The VW, with four cylinders is essentially the minimum for near perfection in balanced rotation.
A 1200cc. four cylinder direct drive that weighs about 80 pounds is the best end goal. (40 hp, 1/2hp per pound)

#### Vigilant1

##### Well-Known Member
And after decades of aviation VW experimenting, some 95% are direct drive.
The VW, with four cylinders is essentially the minimum for near perfection in balanced rotation.
A 1200cc. four cylinder direct drive that weighs about 80 pounds is the best end goal. (40 hp, 1/2hp per pound)
Ideally, a 4 cylinder opposed (i.e. flat/boxer) for dynamic smoothness--and produced in industrial engine quantity to get the price down. Oh, and with the exhaust ports somewhere that is easy to cool.

#### BBerson

##### Light Plane Philosopher
HBA Supporter
Ideally, a 4 cylinder opposed (i.e. flat/boxer) for dynamic smoothness--and produced in industrial engine quantity to get the price down. Oh, and with the exhaust ports somewhere that is easy to cool.
A cheap industrial four cylinder doesn't exist. And they don't make opposed two cylinders anymore either. So we are stuck with V-2 industrial engines.
So make a V-4 from two cheap V-twins.
Not so easy. Would require cutting off the extra ends and rewelded together the cranks and cases and cams, etc. Or bolting two together with a weight penalty.

Last edited:

#### BJC

##### Well-Known Member
HBA Supporter
And after decades of aviation VW experimenting, some 95% are direct drive.
The VW, with four cylinders is essentially the minimum for near perfection in balanced rotation.
A 1200cc. four cylinder direct drive that weighs about 80 pounds is the best end goal. (40 hp, 1/2hp per pound)
Ideally, a 4 cylinder opposed (i.e. flat/boxer) for dynamic smoothness--and produced in industrial engine quantity to get the price down. Oh, and with the exhaust ports somewhere that is easy to cool.
I’m thinking that someone should make a two cylinder, opposed, air cooled, relatively low RPM, 45 to 55 HP engine from aviation quality components. That would require the development of a new crankshaft, but it could be done by the right person. Sell it as either a kit or fully assembled.

BJC

#### Vigilant1

##### Well-Known Member
I’m thinking that someone should make a two cylinder, opposed, air cooled, relatively low RPM, 45 to 55 HP engine from aviation quality components. That would require the development of a new crankshaft, but it could be done by the right person. Sell it as either a kit or fully assembled.

BJC
Yes, maybe Scott Casler could put them together. Use OTS automotive components were possible. I predict a weight of 84-102 lbs for a 45 HP version (an "O-73"), and a price of $4700 to$6425 (depending on doo-dads, and if we want everything to be brand new)..

Okay, maybe Pete's O-100.

Last edited:
BJC

#### BBerson

##### Light Plane Philosopher
HBA Supporter
Using off the shelf industrial mower engine components would require four cylinders for the 40-50hp direct drive engine at low rpm. The GX690 cylinder/heads are one piece (like an airplane cylinder) but are only about 340cc each.