green river rc

My Feedback: (2)

I was wondering if there is any info out there on such modifications? I see them a lot at swap meets and on e-bay but since I like to tinker anyway I would like to try it myself. I have an OS LA .40 with a missing carb that I would be willing to use as a guinny pig.

downunder

There doesn't appear to be any hard and fast rules as to what gives a nice 4-2-4 break but there's lots of theories. Some say that a very short venturi like on the Fox 35 is what causes it. Others say a slight rise and fall in head temps caused by changing load on the prop is the trigger. Others say very mild timing is necessary.

From an experience I had with an ST G51 I'd suggest running the engine stock standard with a suitable size venturi and if the break into a 2 stroke is too strong and doesn't want to come back to a 4 stroke then try a few percent more oil. This suggests that the temp change is the controlling influence but it worked with my ST after I'd tried everything else to tame it.

My son flies a 3D type RC model with an Enya 80X in it. At part throttle it's got more than enough power to do huge loops and at part throttle it runs in a 4 stroke. But do a loop or even square loops as he likes to do and it's got a magnificent 4-2-4 break .

gcb

green river rc,

Many attribute "blow down" timing, the difference between intake and exhaust timing in the cylinder as the major factor in a 4-2 break. The best explanation I've found is in Bob Hunt's "Stunt Flyer Video Magazine". Vol1, No1 has an explanation by Dean Pappas (including a paper to be made into a timing wheel). He shows how to check YOUR engine timing. In Vol1, No3 he shows how to modify an engine for the correct blow down. The videos (or DVDs) can be ordered from Bob Hunt.

Caution: If you make a mistake, you will probably ruin your cylinder.

You might try adjusting head shims before drastic measures, though.


George

Lou Crane

A few thoughts inspired by some other sites...

The OS 46 LA seems to need less modification to run a nice 4-2-4 mode than the LA 40. A head gasket or two might be helpful. The LA engines run nicely on 10-11-11 fuel - which translates to 10% nitro, 11% each synthetic and castor oil.

Almost all mid-price-range OS glow engines are designed for the RC Sport user, so they have a broad RPM range in 2-cycling, and peak power well above RPM used in stunt. They run simply and well in the lower part of their 2-cycling range, again, at higher RPM than 'classic' (read iron piston in steel cylinder) stunt engines did. Lap times are regulated by prop load: pitch relates pretty well to airspeed, diameter to 'burden.'

As the LAs, and several other AB* engines, are based on running in their upper 2-cycling RPM range, too much diameter can cause them to overheat, and that can cause erratic in-flight quirks. So, a fairly flat pitch prop, with diameter that allows the engine to unload comfortably, may be more consistent - and kinder to the engine - than the traditional lugging a high-pitch prop at around torque peak RPM. (Most of our engines peak torque between 9.000 and 11,000 RPM. Power - horsepower - figures from torque times RPM with several other factors, of course.

With the upper-mid RPM range, RC Sport-intended engines, just riching them to 4-cycle takes them too far down their RPM range to give useful and consistent flight performance, unless the correct mods are done. That can be expensive, if ordered up from a specialist, or if parts turn out damaged by less expert work. Unless you really MUST have the 4-2-4 sound, you can get a lot of good flying in with an essentially stock 'modern' engine running 'wet-2' mode. (That is, a low, clean 2-cycle all the way through the flight, but with plenty more RPM available if you lean the needle further.) The power is amazing, as you're working the engine nearer to what it was designed to do.

Just some ideas to consider...

green river rc

My Feedback: (2)

The whole reason I got into this is because I have a pretty nice Brodak Nobler with a LA .40 on it already and it is very powerful and fast... to me anyway. I have only 3 flights on it and another by fellow RCU member Vaughn Sharp. To be honest I'm scared to death of it! I want to slow it down, maybe 4-2 break is the wrong approch? I have a Shark .15 that I can beat around without sweating a drop and it's just as fast, just not as intimidating.

Jim Thomerson

I would try any or all of the following: longer lines, smaller venturi, a head shim or two. Milder fuel. Prop with lower pitch.

green river rc

My Feedback: (2)

Where can I get head shims? Can I make them from steel shim stock, how thick? I have access to .001" and up.

uliner

My Feedback: (2)

Extra head gaskets, or trace out out the one you have in the shim sotck.

First what prop are you using? Use a prop with 4 inches pitch or less. A 6in pitch prop+ lean setting = sore arms....

I use the older FP 40's and my setup is large size venturi, APC 11x4 or 11x 3 prop. Stock muffler pressure and a rich 2 cycle setting. Fuel with a little castor in it.

Try that first. Some guys grind on the head a little to reduce the compression, but I have never had to, an extra head gasket should do it.

There is a lot of mystery and lore about the OS 40 and what setup to use, most of the mods are attempting to slow the RPM down. I have been able to get it to run well in most cases with a flat props.

green river rc

My Feedback: (2)

How thick should the shim be for a starting point? Maybe .005"?

Jim Thomerson

You are supposed to cut them out of aluminum beer cans. However thick that is.

green river rc

My Feedback: (2)

Then I can't do it then...I don't drink beer, taste like to horse pi$$ to me. How about Dr. Pepper?

Jim Thomerson

That will work. I have a vague idea that Pepsi cans are thicker than the usual beer can. But surely you can find a beer can beside the road somewhere.

gcb

Well, I like beer...never tried the other.

Dr. Pepper cans can be used as an alternate.

In desperation, you can order extra head shims from online retailers (like Tower), or perhaps through your LHS (PN23364000 - head gasket + backplate gasket).

You can also get a CL venturi (PN23312000), nozzle for remote needle (PN21381980).

The part numbers are from the OS exploded view in the owner's manual. Additionally, you can probably use the FP needle assembly if you do not like the remote needle. Someone else can verify that. The only LA I have is the .15 LA-S.

Good luck with your modifications.

George

rjbranchii

Green River

How long are the lines you are flying the La .40/nobler on? You should be about 63 ft. Try an APC 11X4 prop and set up with the needle 4 cycling with occaisional 2 cycle breaks on the ground. If you are launching in a screaming 2 cycle it will fly way to fast. You should see laps of about 5 to 5.5 seconds. 5 will be a tad fast and 5.5 a tad slow. This range should give you decent line tension. Be sure you are using a fuel with at least 22% oil at least half castor and a hot (like an OS A-3 or any 4 stroke) plug. A fox RC long works well too. Its a good flying bird.

Bob Branch

green river rc

My Feedback: (2)

I am flying on 60" lines but have some new 70s" I plan to cut down a bit. Line tension is not a problem, it pulls like hell! I am using Wildcat 20-20 synthetic, no castor. What will castor do for me? I also have a F and A3 plug ready to try.

Yesterday I crashed my Shark .15 (Norvel .15 with the carb wired open) with minimal damage, in front of about 15 people, it flys way to fast too, all that is in another post somewhere. I will have to learn with the Shark .15 before I will try the Nobler again but I'm going to have it ready.

rjbranchii

Green River

Sorry to hear about the Shark man. That always hurts. I seem to have the same issue, crashing in front of crowds. Must be Murphy just keeping us humble. The nobler should be a very gentle flier, much easier than the smaller plane. Larger planes are usually more stable and easier to fly. My guess is you are running too many rpm's. All synthetic oil fuels are not very good things for control line engines. The oils burn off at too low a temp. Higher temps cause the runs to lean out and fry the pistons of lapped engines like your OS .35. Castor doesn't burn off at such a low temp and stays in the motor and carries away alot of heat. A very good thing in stunt motors. When I was a kid we always flew everything at a screaming 2 cycle. Planes went fast and we didn't know any better. When I returned to stunt hearing the motors running in a 4 cycle just breaking on verticle uplines was a whole new thing to me. When the plane returns to level it should come back to a 4 cycle. If it doesn't you have a lean run going. The castor in keeping the motor cooler lets it doe that. Most engines will have difficulty doing that with a synthetic. There is a downside (isn't there always?). The castor really makes a mess on the plane. You will notice it builds up alot more cause it doesn't burn off. The low pitch props keep the motors from getting too loaded and help them return to the 4 cycle mode also. Alot of folks like the OS .35 stunt motor. I flew with it alot in the mid 80's and its a real challenge to get to run consistantly. But back then I didn't know the castor deal and can't remember what fuel I used. Some newer motors that work well are the Brodak .40 and the Thunder Tiger .36. The absolute killer motor in the class is Randy Smith's (Aeroproducts) rework (like complete guts replacement and retiming) of the Thunder Tiger into the Aerotiger .36. That motor runs like it has an electric switch to turn the 2 cycle on and off. It what I fly today. Nothing out there compares to it in its size range. But the LA is a very common and popular motor. Getting a venturi and control line needle valve will help you tame it alot too. Randy's web site (Aeroproducts. com) can be a help. He is very active on the stunt forums and is an engine expert in control line stunt.

Good luck and have fun. I still enjoy CL more than RC though 3D RC is really being a ball too. After the demise of my Saito .72 powered Brodak Strega (the reason I wound up flying the electric smoothie in modern stunt) the Saito is on its way into an OMP Fusion 3D profile. Guess you'd have to say I'm AC/DC when it comes to my modeling.

Bob Branch

green river rc

My Feedback: (2)

Thanks for your reply Bob. The only thing is that I don't have an OS .35, I have an OS LA .40 in my Nobler and Norvel .15 on my Shark .15 (repairable). Should I try castor in these engines also?

gcb

A little history on fuels: In addition to the things mentioned above, you need to consider the metalurgy in the engines. Old iron/steel engines would suffer "thermal runaway" if ran too lean. That is, the piston would heat and expand faster than the cylinder which would create more heat. At a certain temperature, most synthetic lubes flash off, or burn, removing their lubricating properties. When castor heats up it thickens until it eventually becomes a sticky "varnish". As the castor thickens, it slows down the engine until it eventually stops. With insufficient lube (even castor) you can ruin an engine in as little as one extremely lean run. With proper castor, it can lessen the wear. Of course, a little over lean will have less impact and you will just get less wear with castor.

ABC, ABN, etc. metalurgy usually does not suffer thermal runaway, at least not as much. The theory is that the plated brass sleeve will expand more than the high silicon aluminum piston. In this case, with an over lean run the castor will still lubricate even when the synthetic flashes.

Bottom line is that castor is insurance against ruining your engine when you have tweeked the needle a little too lean.

Also it is important to maintain sufficient lube in the fuel. Many consider 20% a minimum, and 22%-25% ideal. For some engines like the Fox .35, McCoy .35, etc. 25%-28% is ideal.

For the two engines you mentioned, the fuel is OK unless too lean, then you do not have the castor protection. Your choice.

Oh yes, you may need longer lines on that Shark 15, or you may close the carb a bit with the limit screw.

George

Jim Thomerson

I've had two Shark 15's with TD 09's. 55 ft 012 lines worked fine. Never timed a lap, but I could stunt them OK.

rjbranchii

Green river

Yea, everything I said for the OS .35 S for the LA. It will not be as susseptable to heat overspeed as the .35S. Its a more user friendly engine. Sr. Moment brainfaid. Been flying too much 3D lately. Gets me more dizzy than stunt though, trying to keep up with the plane.

Bob