Greetings all,

I am in the process of tinkering up a Ryobi 31cc motor for a Dynaflight Decathlon I have the hots for. A couple of questions...

1) What motor mount is the shortest/best fit for the Decathlon? I see on the several websites that offer Ryobi conversion parts that there is a range of finished lengths... The Dec calls for about 6 7/8 firewall to spinner backplate.

2) What muffler of the several available makes the most power and would work with a smoke system? Noise is not really an issue at our field.

3) I plan on putting the Bowman ring in the mill, other than that no mods planned. Will probably run Amsoil after the ring gets seated to pick up some rpms. Any other tips? I need to get sparkplugs, too. What works best?

4) How big a prop will the Decathlon handle? I would like to go with an 18-incher for more thrust if possible. What do you think, 18-8? What brand? I mostly use APC in the smaller sizes (up to 14 inches) in my other planes.

5) Any mods to the airframe necessary for strength or balance? Servos in tail or under wing?

Thanks in advance. I got a lot of answers searching this forum already, these were just some specifics I needed clarificaiton on.

Tim

I recently finished a mod. job on a ryobi using the RS kit. If I remember correctly it is 7 1/2", firewall to spinner hub. Havent fired it up yet, too busy with other projects that need finishing before starting another. I too have the hots to do the Dyna-Decathalon with this engine.
Edwin

I have a Ryobi with the Carr Precision parts on it and it measures 8 - 1/4" to the back of the prop.

It was a short shaft engine and the hub assembles a little longer on them than a long shaft engine.

A Jag mount and hub stands a good chance of assembling at 6 - 7/8" with a long shaft engine.

With the Jag mount, the carb is right up against the firwall and you will need a slide type choke on the carb.

With the Carr mount, you can stick you finger over the venturi.

On mine, the hub is 1 - 1/2" long and the mount is 2 - 1/2" long.

The main body of the engine makes up to be 4 - 1/4".

With a short shaft engine it is possible to make a special prop nut by taking the original nut and threading it 5/16" - 24 all the way through and then turn it around to hold the prop on. The prop will have to be bored out to 1/2"??? so the prop nut can go inside it kind of like Super Tiger style. You will need to mill the fins off the flywheel flush with the counter weight. To clear the coil, you will have to space the prop out 1/4" from the flwheel hub to clear. On the long shaft engine you can mount the prop up close to the flywheel easy by boring the prop out to 3/8" and mounting it up against your 1/4 " spacer. You use a regular 3/8" fine nut ands washer on the front of the prop.

With the Carr mount it would be 6 - 7/8" from firewall to back of spinner. With the Jag mount, it would be 6 - 1/4 ".

Jim

Jag ofers a VERY short hub for the CH conversion for long shaft Ryobi's.

My Ryobi 31 is a 2-ring, short shaft engine with Paragon motor mount, prop shaft, prop hub & Jag muffler. From firewall to the rear prop face is 7-inches. Don't know how Jag muffler works with smoke but outlets can be rotated 360 degrees to position as desired. Has Frank Bowman rings, about one hour run-time & turns 7400-7500 with 18x8 MA classic prop. Has manual choke but use standard electric starter without choking.

I'm 'tinkering' as well. Application will be into a SPAD. I know nothing about the plane you are building.

I used the front starter housing on the Ryobi for a mount. It measures 1.220" in fore/aft dimension. The top mount holes are 3.60" c/l and allows the engine to center port/starboard in a 110 mm pipe flange. The carb protudes past the 'mount' but for me I can drill some access holes for adjustment screws and will use the modified choke plate and a bike spoke (2-56 ) to choke from outside the fuse

I like the idea of moving the prop aft on the shaft, W8ye.

All I have seen and read about for prop hubs are Alum.
Why do you fellas not use either UHMW or VHMW for the prop hubs? What temperature does the output / prop shaft have on a gas engine? The material is tough and machinable and it floats!! I'm sure the Alum hubs would sink! Alum Spec. Gravity 2.64 UHMW .928-.940 Tensile strength @ yeild 2900 psi ; Flexural Modulus of Elasticity 100,000 + psi. If it is OK for engine mouints, why not for prop hubs?

mikenlapaz

The UHMW would be fine in the applications where you are actually just using it for a spacer when there's just a hole drilled through it.

But UHMW would never be able to hold the 5/16" - 24 threads required for a regular hub.

Jim

Agreed on thread holding ability. I was referring only to the prop hub.

The S.G. figures indicate a UHNW hub would weigh only 35% of an Alum prop hub. For a 65% weight reduction on one former metal part seems logical.

The temp of the output shaft seems to be my major concern. If it melted the UHMW at shaft contact you would be have to pry it off just like a pressed fitting! Maybe some silicone spray or lube would be good insurance on the first test stand run.

What type of weights are the 'typical' prop hub used on planes?

mikenlapaz

I doubt if the shaft normally gets above the threshold of pain? The tightness requirement of the prop may possiblky cause it to squash out a bit? The UHMW (white nylon to us old people) is somewhat softer than aluminum.

Jim

You will totally destroy a wooden prop and probably most of the composite ones before you hurt or deform this stuff!

Glad to hear about temp of shaft.

I don't have any experience with the Ryobi but to answer questions 4 & 5, I am using a Pro Zinger 18x10 with a G-38 and have good prop clearance. I have the servos mounted per the instructions with a 1800 mah battery attached to the servo hatch. This provided correct CG. The only change I would make is to put blind nuts in at both ends of the wing struts instead of using the self tapping screws.

John