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

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#### pictsidhe

##### Well-Known Member
Drop variable pitch and cost goes down a lot. Consider folding instead:
View attachment 75957
How about a two pitch prop. One that auto feathers when not being driven? The complexity of a folding prop, but works with fat cowls in conventional installations.

#### lr27

##### Well-Known Member
Somewhere I've seen a photo of a prop that folds forward. That should be possible with a spring. For a fat cowl and more conventional folding, one could make the folding axis further out from the driveshaft. However, I would guess that 3 engine installations would have more drag than one. I wonder if the redrive gets rid of enough pounding that a folding prop would survive.
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I wonder if the one of two engines out scenario might be improved by a small tank of nitrous for emergency use only. I'll admit I don't know how much of a hassle that is to set up. Probably there are all sorts of reasons not to.
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picsidhe:
How do you define +/- 30 degrees of compliance? Presumably that's at a certain amount of torque?

#### Vigilant1

##### Well-Known Member
Somewhere I've seen a photo of a prop that folds forward.
Maybe the picture was in autoreply's post #79?

I wonder if the one of two engines out scenario might be improved by a small tank of nitrous for emergency use only.
I'lll confess to thinking of the same thing during the Beetlemaster discussions. It is tempting--you just need the power for a minute or two. The setup isn't very complex, you just need a way to get additional fuel added when you trigger the nitrous. It's simple with EFI, only slightly more complicated with a carb. Still, it's "another thing," and it's just a whole lot simpler and safer to design the plane so that, when an engine fails, the pilot can keep flying the plane with the controls he is familiar with. And, of course, boosting the one good engine does put it at greater risk of failing (just when you need it most).

The twin engine configuration doesn't work out because the engine-out performance is so poor.
So we are on the same sheet: How are you analyzing performance, what HP engines did you use, what prop efficiency did you use, and what was your criteria for inadequate engine out performance?

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.
I'm surprised. Where did the main landing gear end up? With two engines at the front of the wing, a passenger (if any) under the wing, fuel (if any) in the wing center section, and the pilot out front I would have thought it would work. Well, we always have the old standby: move the very heavy battery to the nose.

Drop variable pitch and cost goes down a lot. Consider folding instead:
View attachment 75957
Howdy, stranger! It's good to see you here.

If a true variable pitch prop could be had at a reasonable price it would be great for this project (or the Beetlemaster): Both props pulling well at high cruise, both props pulling well at climb, and in single-engine mode we can feather one and get optimum performance from the good engine. The folding prop is a good idea, I'll see if I can find some prices. As sailplane items, they may cost more than both engines of this project.

#### BBerson

##### Light Plane Philosopher
HBA Supporter
Two RC electric motors could provide energency boost power for say 4 minutes with minimal cost and weight.
In an emergency, running the small batteries hard or to destruction isn't a big concern.
I think that was Burt’s plan with the Bipod. (wherever that went)

#### Sockmonkey

##### Well-Known Member
At that point we've come back around to making an electric plane with a petrol-driven range extender.
I don't have a problem with that myself, but design creep is an insidious thing.

#### BBerson

HBA Supporter
No, a small $1000 electric boost is an alternative to nitrous. Nothing like an electric aircraft. Most twins are not able to climb out after engine failure on takeoff. The actual purpose of the two engines needs to be specified. Is it safety on takeoff or safety on long overwater crossings. Which is it? #### Vigilant1 ##### Well-Known Member Lifetime Supporter Most twins are not able to climb out after engine failure on takeoff. I'm pretty sure most twins (if we look at all registrations) can climb out on one engine (though admittedly I didn't go through the registry and count by type certificate ). Every certified recip twin in the US weighing more than 6000 lbs must be able to climb on one engine at MTOW (climb gradient of 1.5% if type certified 4 Feb 1991 or later, or if before that "climb in feet per minute at 5,000 MSL must be equal to at least .027 Vso squared." If we add in all the turbines, airlines, etc, I think we can say that most twins can climb out on one engine. It's only twins weighing less than 6000 lbs that the FAA doesn't require a single-engine climb capability. Every new recip twin for sale today in the US (there are only six models) can climb at MTOW on a standard day (though it might be anemic). But a Piper Aztec loaded to gross on a hot day--it's almost surely not going to climb. In the context of a small E-AB twin/triple: I wouldn't have any use for one that couldn't climb at least 200 FPM (standard day) after losing an engine. If losing any engine on inital climbout is going to put me into the trees before I can get back to the runway or will force me down within, say, 20 miles from cruise altitude, I'd rather just have a single-engine airplane and cut the chances of an engine failure in half. Last edited: #### BBerson ##### Light Plane Philosopher HBA Supporter I meant something like a Homebuilt twin with fixed props and heavy engines, like a primitive old Apache. Modern twins are much different with high powered engines. This article says the Apache can barely climb on one engine, if at all, in other than standard conditions. https://disciplesofflight.com/the-piper-apache-and-piper-aztec/ #### rotax618 ##### Well-Known Member Why stop at only 3 engines, 1/2 dozen should give you more redundancy, an old relation of mine used to sy that he would never fly in an aeroplae with less than 10 engines and a certificate to say that it could fly on one. BJC #### rotax618 ##### Well-Known Member Sad news yesterday, the chap who invented predictive text has pissed away. His funfair is next monkey. #### Aesquire ##### Well-Known Member I would not recommend Nitrous oxide as a power booster outside of race applications. The sudden spike in heat can cause piston failure both by melting, and by expanding the rings quickly so the ends bind, often breaking the piston. Yes, many hot rod guys use Nitrous on a daily basis for drag racing or street fun. If the engine has forged pistons and appropriate ring gap, ( to the point of smoking a bit, an efficiency and pollution issue ) a momentary, 5-9 seconds, boost is practical. And, I admit, dollar per horsepower, economically great performance, compared to turbocharger or cubic inches.... Both, I must point out, give you power over a much longer period of time. And both, especially size, have higher costs in fuel consumption almost all the time. So why am I disinclined to recommend Nitrous for takeoff on light planes? Because the typical air cooled engine, even "properly" warmed up, usually gains heat rapidly during the full throttle takeoff and climb phase of flight, while cooling airflow is lower than cruise. Adding the sudden heat spike from Nitrous seems foolish. Also, few car racers use Nitrous for an entire minute, much less the time needed to climb high enough ( on a marginal climbing plane ) to fly the pattern & land. ( land speed record and standing mile racers..... For times measured In seconds, not minutes ) In the case of emergency operation in the event of engine failure on a twin, you are doubling down on low speed cooling flow and full throttle plus power needs. A typical scenario of one engine having a fuel system fail, then the other being pushed hard, raises the specter of a second fuel feed failure, massive super-lean condition, and sudden, unplanned engine disassembly in flight. Nitrous can work wonders. It's great stuff. But I don't like it in this application. YMMV. IMHO. ETC. #### Aesquire ##### Well-Known Member For your Sport category Reno racer, the idea of the addition of a separate, pressurized fuel system, and Nitrous, for passing on the straights, with already high cooling air flow, is attractive. On a sub liter industrial engine? ? Don't get me wrong, I'm following these threads because I'd love a light four stroke engine in the 40-50 hp range for a pt103 thingee. Keep talking! I'm learning! #### Sockmonkey ##### Well-Known Member No, a small$1000 electric boost is an alternative to nitrous. Nothing like an electric aircraft.
I mean in terms of complexity and parts count.
I would not recommend Nitrous oxide as a power booster outside of race applications.

The sudden spike in heat can cause piston failure both by melting, and by expanding the rings quickly so the ends bind, often breaking the piston.
What about water/methanol injection? IIRC that has a cooling effect, or could you not carry enough of it to matter?

##### 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.

#### BJC

##### Well-Known Member
HBA Supporter
Most twins are not able to climb out after engine failure on takeoff. The actual purpose of the two engines needs to be specified. Is it safety on takeoff or safety on long overwater crossings.
Most twins have two engines because that was the only cost effective way to have enough installed HP to drag the airplane into the air.

BJC

#### DonEstenan

##### 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.
The problem is that engine prices go up very steeply as the power increases.
Weight & fuel burn is also a problem.

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?

#### syclone

##### Member

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.

#### cluttonfred

##### Well-Known Member
HBA Supporter
Personally, I'd go with a name brand engine if I was going to have to bet my life on it. Honda GX690 V-twins cost under $1,100 each from a place like Northern Tool and make about 21 hp right out of the box. Take of the cooling fan and shroud to save a little weight and gain a pony or two if tractor mounted (leave the fan on for pusher), add an Ace Aviation 1.8:1 redrive (about$800 each delivered) and Ultra-Prop ($350 each). So now you've got twin engines and 42-44 hp of quiet, four-stroke power complete with props for about$100/hp. If the dead engine windmills with one off you could install some sort of manual, cable, operated prop brake on each. You need a light, clean design, but it doesn't have to be a sailplane. Easiest solution might be to take a modest, two-seat pusher like a Kolb Firestar and set it up as a single-seater with one engine in the standard location and one on the nose.

#### Vigilant1

##### Well-Known Member
Personally, I'd go with a name brand engine if I was going to have to bet my life on it. Honda GX690 V-twins cost under $1,100 each from a place like Northern Tool and make about 21 hp right out of the box. Take of the cooling fan and shroud to save a little weight and gain a pony or two if tractor mounted (leave the fan on for pusher), add an Ace Aviation 1.8:1 redrive (about$800 each delivered) and Ultra-Prop ($350 each). So now you've got twin engines and 42-44 hp of quiet, four-stroke power complete with props for about$100/hp. If the dead engine windmills with one off you could install some sort of manual, cable, operated prop brake on each. You need a light, clean design, but it doesn't have to be a sailplane. Easiest solution might be to take a modest, two-seat pusher like a Kolb Firestar and set it up as a single-seater with one engine in the standard location and one on the nose.

View attachment 75992
The Hondas are undoubtedly nice engines, but IIRC (working from my hazy memory) they haven't been favored for projects like this due to some peculiarity of the spark, governor, or induction system. Id be pleased to find out I'm wrong.
The B&S Vanguard engines really are quite good. I had to dump my B&S preconceptions. The Vanguard line was designed and built for B&S by Daihatsu in Japan. In terms of quality, they are about like the (now discontinued) Subaru/Robin engines--top notch, and available throughout the US and Europe. Production is now shifting to new/rebuilt US assembly lines (I think that is scheduled to be complete mid-2019?), but B&S seems committed to preserving the reputation they have built.
In the OP, I made a few assumptions that were optimistic (esp the 80% prop efficiency) and I wasn't able to generate single-engine climb with a 25 HP engine at 717 lbs, a clean airframe, and 22' wingspan. Maybe a plane, like the Kolb, using ultralight design and accepting very modest cruise performance could climb a bit on one of these small engines.

#### cluttonfred

##### Well-Known Member
HBA Supporter
I am not married to the Honda brand, my comment about "name brand" was meant as a swipe at the Chinese knock-offs. I guess it comes down to the mission parameters and budget. Direct drive, the smaller engines seem to put out more power for the money, but with the cost of the redrives added in the advantage is less.

I still think that two tractor-mounted engines as close as the props would allow (Grumman Skyrocket style or a twin pusher) could make for a very interesting design that would avoid some of the cooling and interference issues of the tandem-engine layout. Maybe a slim single-seater inspired by the Praga E.210 with your choice of taildragger or tricycle gear? The engines were splayed out slightly to improve single-engine handling. You could even go with a "profile" rear fuselage to keep the engines as close the centerline as possible.