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

Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by Vigilant1, Nov 13, 2018.

### Help Support HomeBuiltAirplanes Forum by donating:

1. Nov 13, 2018

### blane.c

#### Well-Known Member

Joined:
Jun 27, 2015
Messages:
2,819
476
Location:
capital district NY
What if you used the two engines to drive two generators instead of a psru? There is no propeller load on the crankshafts not designed for propeller load. The two generators each drive two electric motor driven propellers (4 electric motor driven propellers total). The electric motor driven propellers are symmetrically driven that is, one generator drives an electric motor driven propeller on each the starboard side and the port side, one generator drives two tractor propellers and one generator drives two pusher propellers or other symmetrical scheme. That way the loss of either generator still equals symmetrical power and thrust and you are just out the drag losses.

A bank of battery's could supply supplemental and emergency power to the motors.

2. Nov 13, 2018

### Vigilant1

#### Well-Known Member

Joined:
Jan 24, 2011
Messages:
3,467
1,480
Location:
US
When an engine was inop, did the prop windmilll in flight?

I was (mostly) joking about this, but the design problems when using three of these engines are a lot less demanding. Even at the higher empty weight (three of these heavy engines), when you lose one you still have 50-60 HP available. That makes overcoming the drag from the dead prop much easier, and climb rates, even at much higher gross weights, would be considerably better.

You are right about the drag at the back of the pod. One thing I've thought about: These engines are designed to run at full chat all day mowing lawns when it is 100 degrees outside and they sit on the ZTR mower right there in the sun. Those fans they have must do a darn good job. Given that, if we just leave the fan on the engine for the rear powerplant in the Micromaster, all we'd have to do is bring in enough undercowling air to keep the ambient temps below 100 F (i.e. we don't need a sophisticated scoop and pressure recovery plenum to force air through the fins of that engine). This isn't "free" cooling, that fan is robbing us of HP (just as the fan on the Cessna 337 robbed some HP), but if that's the price we are willing to pay for good cooling of the rear engine in ground ops and at low airspeed, we don't need to pay it again with a draggy scoop. A generous exhaust air route and a simple low-drag NACA duct might be all that is needed at the back.

With the Skymaster, single-engine the performance was better on the rear engine than the front engine primarily due to the difference in drag at the back of the pod. When the rear engine was running, the air stayed attached to the rear cowling better and there was less turbulence/drag.

Last edited: Nov 13, 2018
3. Nov 13, 2018

### BBerson

#### Well-Known Member

Joined:
Dec 16, 2007
Messages:
11,192
2,027
Location:
Port Townsend WA
I never tried it with one engine off. It could barely fly with both engines. I doubt a small prop would windmill on any one or two cylinder engine (with a bunch of compression compared with four cylinder).
The fan would be a smart way to go on the rear engine.

Pops likes this.
4. Nov 13, 2018

### Vigilant1

#### Well-Known Member

Joined:
Jan 24, 2011
Messages:
3,467
1,480
Location:
US
We'd have to look at the weight. I suspect it would be prohibitive. Each PSRU only weighs about 10 lbs, and that includes the prop hub. The generators and electric motors would weigh a lot more, plus (thick) wires, controllers, etc. Cost would also be higher. Does the speed/torque flexibility of electric motors do anything to help us with the lack of a variable-pitch prop?
On the bright side: You could certainly run your TV and stereo with the juice when you got where you were going!

5. Nov 13, 2018

### blane.c

#### Well-Known Member

Joined:
Jun 27, 2015
Messages:
2,819
476
Location:
capital district NY
I strongly advocate three engines in a low power to weight unproven engine design. If Ford had turbine engines available, it would have been the "Ford single" you can bet your sweet * on that.

Think 30% power each wing engine and 40% power center engine for generalities. That mitigates any asymmetrical thrust scenario to realistically a non event.

I believe that generators driving motors will supplant psru in the near future as more people want electric and efficiency vs reliability between psru and electric generator/motor will mitigate. Also options to supplement power in various ways with batteries will be desirable.

6. Nov 13, 2018

### blane.c

#### Well-Known Member

Joined:
Jun 27, 2015
Messages:
2,819
476
Location:
capital district NY
People want what people want. They want green power.

Remember "if you want an economy car you are going to have to pay for it".

7. Nov 13, 2018

### DonEstenan

#### Member

Joined:
Feb 14, 2016
Messages:
17
3
Location:
Slovakia
Hmm, what about a generator at the back of the (mostly laminar) central pod, powering fixed cruise propeller (and hopefully reducing the pod's drag by sucking the boundary layer) + 2 electric motors (+ some batteries) on the wings powering simple (but foldable) climb propellers. This way all the propellers are simple, light & cheap, optimized for their role.

The generator power/propeller is set to provide sufficient cruise power + a little bit extra for recharging the batteries. The batteries just good enough for 2x(takeoff, climb to say 2000ft). 2x as a reserve if you need to go around when landing.

The extra electric power is sized by how much batteries you can afford ($and/or weight) - but imho needs to be at least equal to the central pod engine (in total, but preferably on each wing). Place the batteries next to the motors for shorter/lighter wires and less bending moments on the wings. 8. Nov 14, 2018 ### Vigilant1 ### Vigilant1 #### Well-Known Member Joined: Jan 24, 2011 Messages: 3,467 Likes Received: 1,480 Location: US The attraction of these little industrial engines is, primarily, that they are reliable (in their intended applications) and cheap. The introduction of generators, controllers, and electric motors suitable for flight use--ultimately powered by these same small heavy engines, takes us in the opposite direction in cost, reliability, and HP/lb. 9. Nov 14, 2018 ### FritzW ### FritzW #### Well-Known Member Joined: Jan 31, 2011 Messages: 3,169 Likes Received: 2,808 Location: Las Cruces, NM I think it could make a neat little stand off scale O-2 in the spirit of the current flock of WW1 fighters (cheap, easy, simple, fast build ...at the cost of exact scale looks). As a fun flyer it wouldn't need to be optimized to milk every last bit of performance. It would just have to fly safe and handle well. If single engine performance only stretched the glide that would be fine. The good engine only has to have enough power to get you to the crash site.:gig: ... + ... ... + = 0.625 x ... 10. Nov 14, 2018 ### cluttonfred ### cluttonfred #### Well-Known Member Joined: Feb 13, 2010 Messages: 6,350 Likes Received: 2,204 Location: World traveler I'm just sayin'.... A stand-off scale Grumman Skyrocket gets you almost the same benefits as a centerline twin in terms of single-engine handling, avoids engine cooling issues, and gains infinitely more style points. Even at simple balsa model levels of fidelity to the original with small diameter "radial" cowls it would still look great. Basic ultralight construction, flat control surfaces, Cub airfoil, fixed gear, move the canopy back a little.... Last edited: Nov 14, 2018 11. Nov 14, 2018 ### Vigilant1 ### Vigilant1 #### Well-Known Member Joined: Jan 24, 2011 Messages: 3,467 Likes Received: 1,480 Location: US >If< my rough calculations are close, then a single 25 HP engine will keep the plane flying level (standard day= 59 deg F) at MTOW while dragging a dead prop. I'm reasonably confident 30 HP would allow fair climb performance even at more typical temperatures. The >cheapest< way to get there using new engines, as far as these little 4-stroke engines go, is probably to hop-up the Predator 670 engines. I spoke with Andy of "Performance670" and one ready-to-run version they sell for$1800 will provide 32HP at 3600 RPM. He thinks it can go 500 hours at that rate without a problem, a customer with a mud boat had hundreds of hours on his. According to Andy, when it crumped (the prop hit a stump), the inside still looked great. Obviously, no guarantees. The major work they do on these engines is 1) install a new cam (cost of part: $250), install new billet aluminum conrods that have bearing inserts (parts cost:$200), and they port the heads and modify the stock muffler (zero parts cost). They don't need to change the carburetors, or do anything else to make 32 HP at 3600 RPM. So, if the engine is bought on sale from Harbor Freight and the builder can do the work himself/herself, each engine might have $1100 in parts. Then, we need a prop hub and some way to support it. One ready-made answer is the Ace Aviation belt PSRU (about$820, including freight). It has the advantage of raising the prop from the low crank location on these engines, which is a plus. I gotta think, though, that something cheaper can be designed, especially as we don't need the reduction feature.

These Predator engines aren't perfect. The charger is anemic (3-6 amps), sleeve bearings, and the motor is not as light as some others.

The economy option, which requires more grease under the fingernails, scrounging, and research, is to buy a good run-out used engine (B&S Vanguard series, etc) and overhaul it. It would allow purchase of a higher-quality engine (strong alternator, good case, good parts availability, etc) and if you did the work to find/overhaul it, you'd probably feel comfortable fixing things when they break. These engines are common--on tons of pro-grade mowing equipment. If we make friends with the folks who fix them, we'll find out about good used ones. The Vanguard 810 engine (which is the main engine that is modified and used on the Luciole--maybe for a good reason) puts out 28 HP (stock) and weighs 91 lbs (stock). It's very, very common in the professional zero turn radius (ZTR) mower market. These guys run them many hours per day in hot, dusty conditions, it probably doesn't take long to
burn through the wear-parts on even a good engine. A 1000 "TBO" engine will last 4 months at 8 hours per day, so there should be a lot of them out there.
A standoff scale O-2 would be cool. It would have to be really light and clean to make this work, though.
75% would give a wingspan of 28.5' and a length of 22', probably too large for one of these engines to lift unless you go to ultralight construction techniques (AL tube, Dacron sails). A 60% scale O-2 would give you a span of 22' 10", wing area of 72sqft, and a length of about 19', which might make it possible to use "conventional" construction and still stay airborne on one noisemaker. The cabin of the O-2/Cessna 337 is fairly tall, so even at 60% I think there would still be room to sit.
Rockets..Willie Pete or flechette?

Last edited: Nov 14, 2018
12. Nov 14, 2018

### BBerson

#### Well-Known Member

Joined:
Dec 16, 2007
Messages:
11,192
2,027
Location:
Port Townsend WA
Are you saying the Honda engineers install the wrong cam in the GX670-GX690 engines for 3600 rpm?

13. Nov 14, 2018

### Vigilant1

#### Well-Known Member

Joined:
Jan 24, 2011
Messages:
3,467
1,480
Location:
US
I didn't mention Honda engines.
But, I do think it is clear that folks make changes to many of these engines to produce higher HP at a specific RPMs, and that these changes are effective at increasing engine output. That doesn't mean the original cam (or carb, or the way the ports are finished, or the construction of the connecting rods, etc) was "wrong". Maybe the stock configuration was cheaper to make, gave better low-speed idle or better power at lower RPMs, gave better long term reliability, etc. The Honda engineers made compromises/choices, and they might be different compromises/choices than we'd make for an airplane powerplant. And, maybe the choices for a single-engine plane with a stall speed of 70 knots will be different from the choices we'd make for an ultralight, or a plane with 2 engines that can fly safely on just one.

Last edited: Nov 14, 2018
14. Nov 14, 2018

### BBerson

#### Well-Known Member

Joined:
Dec 16, 2007
Messages:
11,192
2,027
Location:
Port Townsend WA
Sorry, I figured the Predator was a clone of Honda.
So are you saying the Predator 670 engineers are installing the wrong cam for 3600 rpm?

15. Nov 14, 2018

### BBerson

#### Well-Known Member

Joined:
Dec 16, 2007
Messages:
11,192
2,027
Location:
Port Townsend WA
For direct drive, the optimal prop rpm is very likely around 3200 rpm, not 3600. So the hp is less than rated hp.
The Honda 690 is listed at 22.1hp at 3600 rpm. Figure around 20hp or less at 3200.
https://engines.honda.com/models/model-detail/v-twin-horizontal#PTO

Sure you could try 3600 in climb, that would get you 22.1hp not 32hp.
I don't believe claims of hop up sellers. :ermm:

16. Nov 14, 2018

### Vigilant1

#### Well-Known Member

Joined:
Jan 24, 2011
Messages:
3,467
1,480
Location:
US
Sorry, I'm out of sync with you. Please see what I added to Post 33. The decisions made by Predator, Honda, etc engineers etc aren't "wrong", but might reflect a different set of priorities than we'd have with an airplane engine. Of course, those engineers presumably then did a lot of testing to assure everything played well together. When we make a change, it could have lots of unpleasant results in addition to the intended result. I do think this is easier to accept in a twin engine aircraft that can be flown successfully on one engine.

17. Nov 14, 2018

### BBerson

#### Well-Known Member

Joined:
Dec 16, 2007
Messages:
11,192
2,027
Location:
Port Townsend WA
Well, I am saying that for direct drive at below 3600 rpm, no internal changes are needed or desired or even possible to increase power by any significant amount.
If you run a redrive at say 5000 rpm then you likely need special parts.

Vigilant1 likes this.
18. Nov 14, 2018

### Victor Bravo

#### Well-Known Member

Joined:
Jul 30, 2014
Messages:
5,639
4,526
Location:
KWHP, Los Angeles CA, USA
I believe you could make a tiny little stand-off scale Do-335 and come out with a smaller and lighter aircraft than the other options mentioned. The Do-335 style aircraft (standoff scale or non-scale outlines) can be the size of the Davis DA-5 or a Jeanie's Teenie, but with a better "fineness ratio". The pilot would be in a very reclined position, like the old Diamant racing sailplane.

This layout would also reduce drag considerably, which might make a big difference when you are dealing with minimal horsepower. Some versions of the Do-335 had longer (high aspect) wings if I am remembering right. That will also play a large part in getting any performance out of this.

The F5F Skyrocket is nice, but I thought this was about centerline thrust.

Vigilant1 likes this.
19. Nov 14, 2018

### Doggzilla

#### Well-Known Member

Joined:
Jun 2, 2013
Messages:
1,574
321
Location:
Everywhere USA
Burt Rutan originally powered the Quickie with a small engine very similar to a 15hp lawnmower engine. Its rate of climb was very disappointing.

But two of them... might actually work acceptably.

20. Nov 14, 2018

Joined:
Feb 23, 2016
Messages:
627