Simplified Monosoupape

[Peter Twissell]

Among my various projects, I would like to build engines for flying models.
Being keen on early aviation, it would be nice to build a proper rotary, but all the designs I've seen are quite large and I don't want to build a huge model to house it.
So I looked at the possibility of building a smaller Monosoupape, at about 1/8 scale.
It quickly became clear that the valve in piston arrangement would be too much of a challenge for my skills, so the later piston ported version would be preferable.
The next issue would be the power output and control, which would likely be marginal at small scale.
So - I have started drawing an engine based on the Detroit diesel principal, with a vane type blower of equal displacement to the cylinders.
The blower will charge the crankcase and the engine will run as a 2 stroke, but using a poppet exhaust valve in the head, allowing asymmetric timing and uniflow.
I have no immediate plans to start building this engine, but I would welcome any comments and opinions.

 

[Tim Wescott]

I think that a 1/8 scale 'real' Monosupape engine should be no more difficult to build than what you're considering, and if anything is going to overpower a 1/8 scale model of the prototype it flew in. Keep in mind that the LeRhone 9C has a displacement of 664 cubic inches -- that works out to 1.3 cubic inches for a 36 inch wingspan Bristol Scout model.

So you're going to take a model that could fly just fine on a .20 modern 2-stroke if it were built light, and a .45 if you're crazy, and you're going to stick a somewhat inefficient 1.3 cubic inch engine in it. You'll have a tiger by the tail, for sure.

 

[Peter Twissell]

Tim, you're right on all counts, with the possible exception of the difficulty of building a 1/8 scale Monosoupape and getting it running.
Certainly a 1/8 scale model could fly with a smaller, lighter modern engine, but that's not the point.
At the small scale, spark ignition is not practical. I can just about fit a modified glow plug in the side of the cylinder.
Running on methanol, I foresee significant issues in trying to get enough fuel to draw in to the cylinder to mix with the air that is drawn in through the valve. The mixture in the crankcase would have to be so rich that I can't imagine the fuel would remain in any kind of reasonably distributed suspension.
I'm not planning to try to build a true scale engine or aircraft. If and when I build the engine, it's performance will dictate the design and size of the aircraft.

 

[Tim Wescott]

I wasn't trying to say you should just fly something with a modern engine -- just that if you compare cubic inches, the scale engine is going to be monstrously big which should more than make up for any power losses in the design.

I hadn't considered your concern about the mixture in the crankcase -- I think my intuition disagrees with your intuition. There's going to be a lot of whirling going on in the crankcase, which should stir things up well. I'm not sure how to resolve whose intuition is right other than building a monosupape experimental engine, and that would be a lot of work.

I've thought about doing something like that -- the smallest reasonable sized engine I can imagine is a 2-cylinder, so the crankcase volume stays more or less constant. That still leaves you building cam, valves, etc..

Another counter-argument to my suggestion is that if you do build it with glow plugs then you'd have no ability to blip throttle. But a counter-counter argument to that is that a 1/4-32 spark plug isn't that much bigger than a 1/4-32 glow plug -- and a sparker gives you a lot more leeway in compression ratio, where a glow engine has to have enough compression to fire off the mixture.

 

[Tim Wescott]

All in all, the easiest way to deal with your desire for a 1/8 scale model of a real plane, with a scale model of the real engine, may be to build a 128 cubic inch, 8x scale model of an OS FP 25, along with a 32-foot wingspan Ugly Stik with a real cockpit & everything. It has the WW-1 vibe, and once you've flown the thing, a 48-inch Ugly Stik with an FP 25 will be a scale model! (Innovative solutions to vexing problems -- it's what I do best).

 

[Peter Twissell]

My intuition about crankcase mixture comes from my experiences with supercharged drag racing engines. At high boost levels, there is a lot of liquid fuel in the mix. It is only possible to go so far with carburetors before the liquid fuel starts to pool in the manifolds, leading to inconsistent mixture in the cylinders and the risk of hydraulic lock. This is why high boost engines have to use injectors.
With the Detroit diesel arrangement, a conventional carb will give proper throttle control.
Glow plugs are thick walled and the thread size can be reduced. They also don't require a complex ignition system.
The other attraction for me is the rotary engine itself, with all its issues of gyroscopic forces.
Excess power is not a problem. If it makes too much power, I can just restrict or with a smaller carb.
This is, after all, a hobby. If the result is an engine that is not practical for flying, I'll still have a pretty, running engine.

 

[Tim Wescott]

This is, after all, a hobby. If the result is an engine that is not practical for flying, I'll still have a pretty, running engine.

Copy that!

 

[petertha]

Go for it Peter!
What would the equivalent displacement be?
I might have this wrong from foggy memory, but I thought the Eric Whittle designs 'modified' conventional RC glow plugs to conform to the small cylinder/head size. Sounds like you are already pondering that. For reference
The EW v8 was 10.6cc according to this, so 1.325cc/cyl.
http://www.hemingwaykits.com/acatalog/The_V8_Aero_Engine___Eric_Whittle.htmlThe EW Peeweit flat-4 was 5.5cc so similar 1.35 cc/cyl
http://www.modelenginenews.org/ed.2011.04.htmlI'm not sure about comparable breathing to Monosoupe. Those engines still mystify me LOL. But the EW manifold length from carb to inlet might be some crude proxy?

 

[Peter Twissell]

I'm currently working with 15mm bore and stroke. I know the original engines were long stroke at about 4.5" bore and 6" stroke.
Keeping the central valve arrangement and working with sensible sizes, 15mm bore is about as small as I am willing to go. The reduced stroke allows clearance for rods while maintaining reasonably chunky wall thicknesses around the crankcase. The result is about 18cc in 7 cylinders.

 

[Cogsy]

Just an FYI on the sizing, Bob Shores' Peewee V4 has a 5/8" bore (15.875mm) and I had no problem fitting 2 poppet valves and a spark plug in the head. I believe Steve Huck's Demon V8 is also the same bore and plenty of those are running with spark ignition. So I don't think the size is too small for spark ignition, however I have no idea how the ignition works on a true rotary with the cylinders all swinging about the place so that may well be the complexity issue you mentioned.

 

[Peter Twissell]

I have set myself a problem by deciding to stick with the central valve.
I've no doubt I could get two valves and a 1/4" plug in, but with one valve in the centre, there's not much space in the annulus to fit a plug in.

 

[petertha]

You probably know, but just in case, Ron Colonna built a 1/8 scale BR2 as below.
https://www.craftsmanshipmuseum.com/Colonna.htm
Ron’s 1/8th scale Bentley BR2. He scaled down the ¼ scale version of Lew Blackmore’s engine. It is the only 1/8th scale version we’re aware of. 5/8" bore, 7/8" stroke, 43.2 cc displacement.

 

[Bentwings]

My intuition about crankcase mixture comes from my experiences with supercharged drag racing engines. At high boost levels, there is a lot of liquid fuel in the mix. It is only possible to go so far with carburetors before the liquid fuel starts to pool in the manifolds, leading to inconsistent mixture in the cylinders and the risk of hydraulic lock. This is why high boost engines have to use injectors.
With the Detroit diesel arrangement, a conventional carb will give proper throttle control.
Glow plugs are thick walled and the thread size can be reduced. They also don't require a complex ignition system.
The other attraction for me is the rotary engine itself, with all its issues of gyroscopic forces.
Excess power is not a problem. If it makes too much power, I can just restrict or with a smaller carb.
This is, after all, a hobby. If the result is an engine that is not practical for flying, I'll still have a pretty, running engine.

Being a life long scale model builder and flyer, I found early on that many things don’t scale ver well. Motor weight horsepower torque, prop size and volumes. For a few. Speed is another there is a calculation for this but I no longer have it. For the most part if it looks scale it appears best.a really big 1/4 scale WW2 fighter might weigh 50 pounds and only need 10-12 hp to fly it 100 mph but the full size weighed 12,000 pounds and flew close to 400 mph. And had 28,00 hp. So the bottom line is don’t get too fixated on exact scale. My 1/5 Corsair had just about the awuful gun sight but it looked like a real heads up display on the windshield. It got lots of good comments. The sliding canopy got good reviews too but the rails were far from scale. They just had a look about them.