scratch built extra 300s

[rotccapt]

hi all i decided to start working on my future project an extra 300s that will be flown with a ryobi 31 gas engine. i decided to design it with a 70" wing span. i dont plan on doing 3d with it just sport flying. here is a pic of what i have so far what do you think

[JeremyHolt]

Looks pretty good but I think you should move the Canopy forward. It should start about mid wing or near the tube. You have about the same distance of turtle deck as you do top hatch area which looks a little odd. Moving the canopy up should eliminate that.

[JeremyHolt]

By the way.. how do you like that program and werd ya get it?

[rotccapt]

i am building this off of a 3 view of the s series extra which is the single seater version which kept the pilot at the same spot and just removed the front seat and adjusted the canopy accordingly. do you think the size is about right for the ryobi?

i dont know where the program came from i got it as a gift. it is not too bad not the best but not the worst. it is only 2d and you can not import pics. i can export and import dxf files with it along with a couple other file types. it is a very straight forward program and has an excellent tutorial to help you learn the program. it is defiantly designed for the novice user it takes a bit of getting used to but once you get the hang of it it is not too bad.

here is a pic of an extra 300s that i have been using as a guide line

[grumman-cats]

Interested to see how all this comes out with your choice of power plant. I would like to make a twin kingair 200 with the ryobi powerplants. There clean inline shape would work well in those cowlings.

I'm also glad to see someone designing there own plane. great job. can't wait to see the finished product.

[Chris]

Just be sure and give it plenty of wing area. The Ryobi is heavy--even with an electronic ignition system. They weigh about 4.5lbs.

Consider stretching to 72" and make sure you have about 1100--1200 square inches of wing. Build light. Keep the plywood to a minimum. Use balsa for formers. Balsa is more work to build, but it's lighter than ply.

The problem with stretching out to a bigger plane:
Yes, you get more wing area but your also increasing wood, so weight goes up. It's a compromise.

Contact Frank Bowman in New Mexico and get one of his rings for the Ryobi. Try and get a twin ring Ryobi. They make more power. Consider having a friend mill the bottom of the cylinder off a bit to increase the compression. You can take the squish band down to about .020" or even .018", but I'd stop there.

When you drop the cylinder, your messing up the exhaust duration, so take some off the top of the exhaust port. If you drop the cylinder .150" then you should also raise the top of the exhaust port back up that much so that you aren't just chasing your tail. Higher compression does nothing if you can't get the exhaust out.

Bend the reed stop out so it only clears the crank pin by about 1/8". This will allow the reed valve to open up and pump more gas in.

Carbs:
You can get a bigger carb. The stock carb venturi isn't as big as the hole in the backplate. Bigger carb = more fuel = more power. But, only to a point. You can put too much carb on it and then it won't run worth a . If you just get a carb thats the same size as the hole in the backplate--you'll be fine. I used to know the Walbro carb number, but I've forgotten. WT438? maybe?

Carb theory:
Small carb is for low RPM and a big prop. At 6800-7000RPM, the stock carb will work fine and the engine should swing a 16" or 17" prop. The RPM is high enough to pull the right amount of vacuum through the venturi and atomize the fuel correctly.

Big carbs are for high RPM and little props. The engine shouldn't be run over about 7500RPM. MAYBE 8000RPM, but I can tell you that they will explode at 8800RPM after about 40 minutes of running. Spectacular--but deadsticks aren't any fun. You can use a little 15" prop and a big carb. Let it spin 8000RPM and go like nuts. But, an Extra isn't designed to go fast, so I'd avoid this setup.

It's possible to slap a GREAT BIG carb on a ryobi and a tiny 14" prop. You have to dremel out the backplate hole to match up with the venturi size on the carb-- otherwise it's wasted effort. BUT, they don't last long at high RPMs. Trust me, I know. Seen one explode in the air and I can tell you without a shadow of a doubt--I wouldn't have wanted to be wityhin 200' of that thing if it had exploded on the bench. Big bang--lots of hot metal parts flying around.

Build the plane light. Hop the motor up a bit. Should be fun. Keep it under 8000RPM on the bench.

[rotccapt]

thanks for the info that should help a lot i do have to warn you it may be a while before i start building still finishing my stearman and my priorities have changed from me to a family you know how that goes

[rotccapt]

if i were to get my hands on a lathe would i be able to add a ring the engine. the one i have has only one ring

[Mitsu1]

	Quote: Originally Posted by JeremyHolt
	Looks pretty good but I think you should move the Canopy forward. It should start about mid wing or near the tube. You have about the same distance of turtle deck as you do top hatch area which looks a little odd. Moving the canopy up should eliminate that.

This is interesting. I wrote a while back about how model designs are getting so far from the real plane, that it makes me wonder if some are drawn off somebody elses rendition of a model instead of the actual 3 views of the real plane. So now, when a model is done right from 3 views, we have posts that think it doesn't look right! ... What??? The plans shown here look close to the real 300S ... and that's a good thing.

[Chris]

	Quote: Originally Posted by rotccapt
	if i were to get my hands on a lathe would i be able to add a ring the engine. the one i have has only one ring

One ring is okay. Maybe you can find somebody who has a 2-ring piston? It would be a lot simpler to just swap pistons. One ring is okay though.

Call Frank Bowman in Farmington New Mexico. Do some google searches and his name and phone # will pop up. He does rings for anything and probably already has 50 rings for your Ryobi. All you gotta do is send him a paypal payment and he'll ship it. Worth the money.

There isn't much you can do to lighten a Ryobi. If you study the crankcase, you will find a few spots where you can grind off a bolt hole or some extra material here and there. They are still heavy though. But they can be made to run like a raped ape with minimal $$ and just a little time. 31cc of hopped up Ryobi on a 72" plane is gonna be sick, if you can get it built light enough.