Azurak

I'm converting a pair of ryobi 31 cc for a p-38. The engines need to rotate both clockwise, and counter clockwise. To make the one rotate ccw I'm going to have to relocate the ignition coil. Does anyone know the amount of advance needed for a reasonable start/run compromise?

Azurak

av8tor1977

My Feedback: (6)

28 degrees total advance. However, many magnetos have a built in advance that you will either have to measure with a timing light on a a running engine and duplicate, or else carefully measure the coil and flywheel mounting positions and reverse them for the "backwards" running engine. Fabricating a new, sturdy mounting for the coil is not easy. I highly suggest that you just convert to CDI electronic ignition. Then it will be a breeze. Ignitions are only around 50 bucks nowadays, and make for an easier starting, nicer handling engine.

AV8TOR

Azurak

Thank you AV8TOR,

The point about a rigid coil mount has been on my mind for a few days. At present my inclination is to use a piece of 1/4" 7075 T-6 plate with a "strongback" on the outboard edge. It was also my intention to re-locate the original mounting a few degrees outboard to increase cylinder cooling. I've had pretty good luck ( so far ) pressing apart crankshafts and changing the keyway index.
However, I may not pursue that mod as many conversions seem to cool fine with the coil in its original mounting.

I've considered making a conversion thread but not sure it would be of much value to the readers as I have no expertise in these matters.

Azurak

av8tor1977

My Feedback: (6)

Once again, I really advise you to consider electronic CDI ignition, especially for what you want to do. There are SO many advantages, including lighter weight which is always an important consideration on any warbird, but even more so with a twin.

Also, you don't have to move the crank keyway. The key is only there to guide the assembler in getting the timing set without checking it. The key does not hold the flywheel once it is tightened down, and can actually be left out. Leave the key out and set the flywheel anywhere you want. If it will make you feel better, you can add a drop of Loctite to the crank taper, but it's not necessary.

Have a look here:

http://www.rcextremepower.net/rcxpignitions.html

www.ch-ignitions.com

AV8TOR

Azurak

Thanks again for the input.

I can see advantages to electronic ignition for a twin,... synchronizing the engines would be a snap by comparison and I've heard there is an available "fail safe" for such systems that kills the remaining engine should one drop out.

I'm unsure about the weight issue. What I mean is,...

A p-38 has a lot of weight behind 1/4 mac. I was thinking a magneto ignition would place more of the "usable" weight forward of that point. The extra batteries required to support electronic gear may not be substantial, I don't know. I do know they would have to be forward of 1/4 mac, which is where all other batteries will need to be. I like long flight times, 30, sometimes 40 minutes rather than multiple short trips.

My preference would be to do things old school and being inventive rather than increasing the budget (which is limited) to overcome issues.
Having the timing resolved leaves two (hopefully) issues of concern

Having an auto kill switch for the second motor should one fail,.. ( I have some Ideas )
And what seems the most difficult to me,... how to synchronize engines.

If these are beyond my ability, or if the cash outlay starts to look like 60-70 % of cdi and engine management, easier is definitely better.

So,...What do you know about kill switch systems and synchronization?


Azurak

Propworn

My Feedback: (3)

Most two stroke engines are designed to rotate in one direction by the location and heights of the ports. Two strokes that are designed to run in both directions have the locations and port heights designed to do so. You may find you cannot simply move the ignition and expect the engine to rotate in the opposite direction with the same power. Some success at getting an engine to counter rotate by rotating the cylinder 180 degrees. Some nitro motors had both front bearing/crankshaft housing and back plate bolted on to the crankcase with the same bolt pattern and you could end up with a counter rotating motor by switching these end around. The piston relative to the cylinder still rotated in the same direction but the crankshaft being on the back of the engine gave it the opposite direction of rotation. Enya 4 strokes were reversible by swapping the intake and exhaust cams then spinning the cylinder, head, piston and rod 180 degrees on the crankcase.

Dennis

Azurak

Thanks for the input Dennis,

I cannot truly say without experimentation how this will work. These particular engines are very basic in porting.
They have a single rear facing exit with twin, (left right) inlets feeding from the crankcase with an aft facing carburetor over a lackluster reed valve. Initial measurements revealed both intake ports to be dimentionally symmetrical. The piston is also symmetrical in left-right aspect. The counterweight on the crank is left/right neutral. In theory it should operate in both directions equally.

Azurak

w8ye

My Feedback: (16)

In the past the Ryobi 31's ran equally well in either direction with electronic ignition.

Azurak

Thanks!

This is good to know.

Azurak

Azurak

I have done a bit of online searching in regards to estimated cost of controlling magneto ignition engines, definitely cheaper than adding cdi,... however, a point raised earlier has been uncovered as an issue. The plane will not make the 20 lb limit with magneto ignition, It may actually be difficult to get it to 20 lbs. using electronic ignition.

I like to think I'm not stubborn to the point of foolish,.. or maybe I'm just plane foolish.
Now I'm wondering if the engines I have can even be made to do what I want. They may be too heavy, as well they may not develop enough power, I would like them to swing a prop that has a little bit of scale appearance. I was thinking an 18 inch 3 blade @ say 8500 rpm?


Going to start a thread to help develop this project.




Azurak

Truckracer

My Feedback: (19)

I doubt that any 30ish cc engine can pull a 18" 3 bladed prop that has acceptable pitch for a warbird anywhere near that RPM. What have other people used for engines in that airframe? I think I'd be copying known good setups rather than trying to reinvent the wheel.

Propworn

My Feedback: (3)

Why a counter rotating engine plenty if not most model multi engine air frames use common engines all rotating in the same direction. Finding suitable props for the counter rotating engine may be a bit of a hassle not something your local hobby shop will be carrying.


Dennis

av8tor1977

My Feedback: (6)

Quite true Truckracer, plus there is the fact that the Ryobi 31cc engines are only safe to run up to 7500 rpms static. Above that they are susceptible to connecting rod breakage. They are good engines though when used within their limitations. I have one in a 76" wingspan Tiger Moth biplane and it is a match made in heaven. Love it, and love the fact that it fits entirely within the cowl. But they are a bit heavy, especially when used with the mag, and in stock form are not exactly powerhouses....

AV8TOR