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

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#### Vigilant1

##### Well-Known Member
Sure you could try 3600 in climb, that would get you 22.1hp not 32hp.
Lots of things would need to be checked. But here's my math:
- A 22' span wing with 70 sq feet of area lifting a 7171lb airplane at 70 knots generates about 24.5 lbs of induced drag.
- A stopped 46" long two-blade prop with a AR of 8 has an equivalent flat plate area of .77 sft, and at 70 knots (SL) generates 13 lbs of drag.
- Profile drag: Roundabout method: The SD-1 requires about 12.84 HP for level flight at 70 knots, which is equal to about 64 lbs of thrust (at 70 knots). Subtracting the induced drag (19.4 lbs with this wing and weight) leaves 44.6 lbs of profile drag for the SD-1 at 70 knots. As you point out, the bluff tail end of our MicroMaster is likely to be draggier than empennage of the SD-1, so let's add 25% (a total WAG) to the profile drag, so we can expect about 56.5 lbs of profile drag.
Total drag of the Micromaster in level 70 knot flight with one stopped prop: 24.5 + 13 + 56= the 94 lbs total. That requires 18.8 effective HP at this speed, and if our prop is 75% efficient, it will require an engine output of 25 HP to maintain level flight. Each 100 FPM of climb at this weight requires 2.15 effective HP, or about 2.6 HP with our assumed 75% prop efficiency. So, if we've got a 28 HP engine (e.g. the 810 cc stock Vanguard as used by the Luciole in direct drive), we can generate about 100 FPM of climb. If the hopped-up Predator can give us 32 HP, then we'll see about 270 FPM of climb.

Lots of assumptions here that could work either way. The biggest ones are the prop efficiency, the profile drag of the MiniMaster, the real practical engine outputs. I've also assumed that climb occurs at 70 knots, if the wing performs better at 60 knots then all our profile drag numbers (incl the stopped prop drag) go down by 27% and our thrust-per-HP goes up by about 8%, both of which translate into improved climb rates.

I don't believe claims of hop up sellers. :ermm:
I share your skepticism. But the racing carts, racing mowers, mud-boat skippers, etc are undoubtedly producing a lot more HP than stock, so there's that.

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#### Vigilant1

##### Well-Known Member
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.

If we don't want a long driveshaft, then it's going to be relatively blunt back where the rear prop attaches. Without long (thin, light) tailboom(s), we're having to build structure and acft skin to cowl that big engine as far back as the tailfeathers go. The tailfeathers will need to be larger and heavier if they are aren't on a long arm. And then there's the issue with spin dynamics/recovery with engines at both ends. I think for efficiency it would be hard to beat a smooth pod and generous boom

#### Riggerrob

##### Well-Known Member
I strongly advocate three engines in a low power to weight unproven engine design. .........
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Agreed!
Large helicopters (Sikorsky CH-53 and Eurocopter EH-101 Merlin/Cormorant) have three engines to allow them to still climb away if one engine quits. ...... that still leaves them with 66% power.

OTOH Losing one engine - on a twin - means you only have 50% power remaining.

#### Doggzilla

##### Well-Known Member
HBA Supporter
A mini-Defiant?
Could definitely work. I was not even aware of the Defiant until you mentioned it. I thought I knew all of his designs.

#### Vigilant1

##### Well-Known Member
I strongly advocate three engines in a low power to weight unproven engine design. .........
————————————————-

Agreed!
Large helicopters (Sikorsky CH-53 and Eurocopter EH-101 Merlin/Cormorant) have three engines to allow them to still climb away if one engine quits. ...... that still leaves them with 66% power.

OTOH Losing one engine - on a twin - means you only have 50% power remaining.
I understand the general sentiment, so here's what I hinted at in the OP. On to the specifics.

Layout: Seems it could be a "Tri-Mow-tor" or (even more fun):
-- The love child of an OV-10 (front end) and an O-2 back end (i.e. an engine at the back of the OV-10's fuselage pod). The visibility out the front would be tremendous, and with the engines in the wing at the front of each boom plus one at the back of the pod, the wing will be far enough aft that the pilot will get great visibility to the sides, up--everywhere. Plus, there will be great slipstream flow over the tail no matter which engine is out to help reduce Vmc and improve safety.

Numbers:
- Weight: 2 engine Micromaster MTOW (717 lbs, see first post) + 130 for third engine, prop, hub and mount + 30 lbs (5 gal addtl fuel)+ 40 lbs additional wing/structure = 917 lbs MTOW
For grins: Let's add another seat. (Make it a true Bronco-style tandem). 30 lbs of structure, 180 lbs of passenger = 1127 lbs MTOW
- Drag (70 knots, assumed to be best climb speed) :
-- Form drag: We'll use the numbers we just made up . . er .. estimated for the MicroMaster and add another 25%: 70 lbs of drag
-- Induced drag: With a 26 foot wingspan and 75 feet of wing area, induced drag would be: at 917 lb weight--31 lbs of induced drag
With 2-seat (1127 lbs total): 47 lbs of induced drag (more wingspan would help here, but let's keep it simple)
-- Drag from stopped prop: 13 lbs
So, total thrust needed for level flight with on stopped propeller:
-- Single seat (917 lbs): 114 lbs (22.8 HP at 70 knots)
-- Two seat (1127 lbs): 130 lbs (26 HP at 70 knots)

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

Not too shabby, and probably a fun plane to fly when all three engines are turning (1000 FPM climb for the 2 seater at max gross). With about \$3300 invested in engines (plus props, hubs, support bearings, etc).

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#### cluttonfred

##### Well-Known Member
HBA Supporter
As much as I enjoy where this is going in terms of imagining fun multiengine options, by the time you've spent the money and time on converting three fairly heavy 25 hp engines, possibly with redrives and definitely with props, wouldn't you be better off with one proven engine like a 60 hp 1835 cc VW and a parachute if you're nervous?

#### blane.c

##### Well-Known Member
HBA Supporter
As much as I enjoy where this is going in terms of imagining fun multiengine options, by the time you've spent the money and time on converting three fairly heavy 25 hp engines, possibly with redrives and definitely with props, wouldn't you be better off with one proven engine like a 60 hp 1835 cc VW and a parachute if you're nervous?
What if you pull the chute and the wind pushes you into high tension lines? Or something equally disastrous. With three engines you lose one and "you" choose were to go.

#### blane.c

##### Well-Known Member
HBA Supporter
Less drama, it's just a pain in the * to land or crash off the airport environment. Embarrassing and expensive too, also no guarantee you will not be forced to make hasty preparations to remove your craft and further damage may occur.

#### blane.c

##### Well-Known Member
HBA Supporter
I think glider is fine, certified engine is fine, 3 maypops.

#### narfi

##### Well-Known Member
Could definitely work. I was not even aware of the Defiant until you mentioned it. I thought I knew all of his designs.
If I lived on pavement I would probably be building one right now. For some reason I have always really liked that plane.

I did find there is a smaller plane like that using 40hp engines (planed to use herth 40s) but its two seat and only 5 ever made.
https://en.wikipedia.org/wiki/Aeronix_Airelle

#### Vigilant1

##### Well-Known Member
As much as I enjoy where this is going in terms of imagining fun multiengine options, by the time you've spent the money and time on converting three fairly heavy 25 hp engines, possibly with redrives and definitely with props, wouldn't you be better off with one proven engine like a 60 hp 1835 cc VW and a parachute if you're nervous?
Maybe. It is definitely more effort to design an entire new airframe and convert three engines for airplane use. But by that logic, virtually everyone would be better off with a used C-152 and skipping the building entirely.

There's no doubt that these little motors weigh a lot. I wish I knew more about the options for getting a prop mounted on one.

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#### Vigilant1

##### Well-Known Member
If I lived on pavement I would probably be building one right now. For some reason I have always really liked that plane.

I did find there is a smaller plane like that using 40hp engines (planed to use herth 40s) but its two seat and only 5 ever made.
https://en.wikipedia.org/wiki/Aeronix_Airelle
There was another one-off "baby Defiant" style plane that used two VW engines. It was based in Calif, and we mentioned it in the Beetlemaster thread. I'll see if I can find it.

#### pictsidhe

##### Well-Known Member
I'd be tempted to have a single prop and two engines coupled to it via centrifugal clutches and their own redrives. If one engine dies, throttle back to disengage both clutches, then reapply throttle. You could also shut one engine down for economy cruise.
A freewheeling prop can have more drag than a stopped prop. Feathered is lowest drag, but more hassle.

#### blane.c

##### Well-Known Member
HBA Supporter
Maybe. It is definitely more effort to design an entire new airframe and convert three engines for airplane use. But by that logic, virtually everyone would be better off with a used C-152 and skipping the building entirely.

There's no doubt that these little motors weigh a lot. I wish I knew more about the options for getting a prop mounted on one.
https://www.bing.com/videos/search?q=kohler+ultralight+engine&view=detail&mid=8105D62E7A4FD57A05E98105D62E7A4FD57A05E9&FORM=VIRE

https://www.bing.com/videos/search?q=u-tube+industrial+motor+tike+aircraft&&view=detail&mid=066D19E7D3D3596670A8066D19E7D3D3596670A8&&FORM=VDRVRV

https://www.bing.com/videos/search?q=u-tube+industrial+motor+tike+aircraft&view=detail&mid=B2C67373A573A6BE23EEB2C67373A573A6BE23EE&FORM=VIRE

http://www.walbro.com/walbro-eem-around-ideal-fuel-injection-solution/

#### Vigilant1

##### Well-Known Member
Thanks. Chains (with and without idler), belt with idler (Ace Aviation uses belt without idler). Interesting that Bob in video one says we don't need to worry about cowling/baffling on air cooled engines, incl large VWs--"everything is in the carb jetting. Get the carb jetting right and you won't have any heat problems." I think many people would disagree,

And in addition to those approaches, we have the direct-drive hub used by Luciole builders who use the B&S 810cc engine, plus direct-drive hubs on B&S engines in SD-1s. There are a lot of ways to get the prop to turn, I think we'd all lie to know the details on some of them (weight, price, problems/track record, limits on engine HP, prop size, weight, and RPM, etc. And we have the thread here where Topaz and Hot Wings were chatting about a prop support bearing that Hot Wings had drawn up. Anyway, these PSRU and direct-drive prop bearing discussions deserve their own thread.

#### lr27

##### Well-Known Member
I expect that, with lousy cowling and baffling, one has to be much more conservative with the carb jetting. That, I think, would mean higher fuel consumption and more pollution.

#### Vigilant1

##### Well-Known Member
I expect that, with lousy cowling and baffling, one has to be much more conservative with the carb jetting. That, I think, would mean higher fuel consumption and more pollution.
Right, that's what I thought when I heard it. Sure, you can keep an engine cooler by running things very rich, but that's not a great answer.
"Don't listen to what folks say about insulating your house so you can keep it comfortable in the winter. If you properly burn your furniture, you can keep things toasty inside with no insulation. "

#### pictsidhe

##### Well-Known Member
That chain drive video is a good example of why you shouldn't just wing it. He has moved to chains as his improvised belt drives were coming apart. His chain drive 'rentensioned' itself on it's first run. When you see it run, there is some nasty snatch on shutdown. Not good for a long life. I wouldn't pay too much heed to videos of people had a successful few hours running. You want to find something that runs for hundreds of hours, or always have a place to land... TV can break the cranks of redriven and direct drive cranks. It can play hell with belt life too. Poly-V belts will slip when torque ges too high. That will save your crank, but is why belt drives so often eat belts in hours.

Proper baffling will mean lower drag cooling, that means more useable power.
Or, you can just burn your furniture...