Colin,
The OS 26 with pull ships like a strong FOX 35, 45 - 48" span, 40 oz ish weight. I had mine in a TF Tutor (original version) and ran 10 x6 prop, 10% nitro, 22% total oil (mostly synthetic). From my notes you want a wide blade prop (like an RSM or BBYO) around 9900 rpm. I needed 2 3/4oz of fuel for the PA pattern. Use a tank tall and skinny clunk works with the fuel pickup tube being 5/32" OD and drill out the cluck to 5/32" (this gives better fuel draw), use large fuel line and wire onto fuel nipple. Set the centerline of the tank inline with the center of the carb/venturi to start and adjust up/down to get equal lap times. Set up vent for standard vents (not uniflow) and muffler pressure.

Best,   DennisT

Colin,
The tall tank is just the regular cluck tank stood on its side. This minimizes change in fuel head being about 1 1/4” vs. 2 1/4” if set on its bottom. 4strokes don’t have as much pumping fuel draw as higher rpm 2 strokes that’s why they usually run better on conventional venting. One tip is to use 5/32” fuel pickup tube (drill out the cluck too) and larger fuel tube .

Best,  DennisT

Colin,
Change out the fuel pickup line to 5/32" (copper is best for nitro) and drill the cluck tip to same.

Best,    DennisT

Colin,
Due change out the fuel pickup line to 5/32" (copper is best for nitro) and drill the cluck tip to same.

Best,    DennisT

Thx Dennis, will do!

I used one of mine in a Sterling Mustang.
Flew great, took home many trophies in OTS and classic stunt.
I once beat a Super Tiger 60 powered Cardinal with the Mustang.

Bob Z.

Colin:
Don’t worry about competition - just go out and get as much enjoyment out of the sport as you can.
If you do decide to compete, you can take advantage of one of the four skill classes.
The important thing is to have fun!

And, keep us posted on your progress with the 4-stroke engines.
Keep in mind that the Sterling Mustang and Yak 9 are very close in performance.
Build them light and if you want, make the flaps non-moveable. I’ve done them both ways and couldn’t tell the difference.
Bob Z.

Colin.
For the short tanks the Sulllvan tubing is very thin walled and flexible but surgical tubing will be more flexible for the 2oz tank. Other option could be a round tank with a fixed copper tube and conventional vent pointed to the inside of the circle side of the tank, just make sure you mount the air side of the vent above the tank and on the inboard side of the fuse (this gives clean airflow to the vent).

Best,    DennisT

Colin,
Yes the Sullivan tubing is thinner walled and more flexible than the Dubro. Again use the 5/32" copper tube and drill out the cluck.

Best,   DennisT

I have a number of planes flying with the Surpass 26, hundreds of flights.
One plane flys with THREE of them.
I’ve had very good results with a plain VECO two-ounce metal tank, non-uniflow, over/under venting, no muffler pressure. The engines are profile mounted.
Up to now, I’ve been using Brodak 10/23, 50-50 castor/synthetic but I plan to mix up some 20/20, 18/2 synthetic/castor to try out.
The Surpass 26 is a very good four cycle engine, requiring minimal fussing and maintenance.
The OS four cycle plug works perfectly.
Best of luck!!
Bob Z.

I still have a question in the OS 26FS relating to fuel pressure (muffler pressure) versus standard over/under venting for a full speed CL application. A few have said they use no muffler pressure. However, the FS26 owner's manual shows the recommended fuel system set up and shows muffler pressure applied to the fuel tank. It seems muffler pressure just enhances fuel draw.

I plan to set up the carb to be about 1/3 open to start and allow for some brief warm up. Then ground adjust to desired full speed before takeoff.

The 26FS is basically designed as an R/C engine. Maybe fuel pressure not needed for a fixed speed CL application?

Thanks Dennis.

Concerning the speed adjustment, I got this idea from a flying buddy who has a few CL 4-stroke models. I installed a simple aluminum gear (a part from a hobby shop intended for an RC car) on the carb. The gear teeth are in contact w/ the choke rod and keep the carb where you set it. So I can set the carb partially open to start. Then rotate gear to full open before takeoff. My opinion only, and maybe not necessary as you say, but it is not a bad idea to start at a lower speed and allow for some brief warmup. With all the FS moving parts (compared to 2-stroke) a brief low speed warmup can't hurt.

  To add to what Bob is saying, an R/C carb that is wide open is way bigger than what a restrictor venturi with a spray bar through it would be and add to that all the clearance around the throttle barrel passing air. Closing down the throttle decreases the opening and adds some choke to help with fuel draw, and 4 strokes are all about fuel draw. I would test it at about 1/3 closed, and run muffler pressure. In my limited experience with both OS and Saito four strokes, this has always worked the best for me. It's more or less the "Bob Reeves" philosophy or running a 4 stroke engine and has worked pretty well for me. I applied what I learned running and flying my Saito .56 to an OS.70 and it responded the same as the Saito did. I had to use the carb closed off, as no one made any bolt on venturi for C'L stunt for it. Flying four strokes has been fun and is a viable power plant, and I hope to get my Top Flite Score recovered soon and get back to it, and maybe a couple of other models in the future, but I haven't sold off any of my 2 stroke engines just yet!
  Type at you later,
   Dan McEntee
 
   

Dan, not sure I fully understand. Are you saying you have had good luck with 4-strokes (on a CL application) with the throttle maybe 66% open (to simulate a venturi restriction), then adjust the needle for desired full speed?

Or if the engine is not running well at full throttle (due to air leakage), test and start to close the throttle to see if the engine performance improves?

Thanks in a advance.

   That is exactly correct. Search out threads on here by Bob Reeves. He and some others did a lot of research and experiments into this. You could also go to the Tulsa Gluedobbers web site and a lot of it is posted there. Bob has since passed away but he left a lot of information behind. Bob advocated for the low RPM/high pitch prop method of running 4 strokes. If you happen to have a C/L venturi on an engine, he came up with the idea t add a screw to the throat of the venturi that goes in on one side only, and by running the screw in and out, you effectively change the venturi choke area. Running it in makes it smaller and draws more fuel, out makes it bigger and draws more air. Not really a new idea, the speed limit combat guys have been doing the same thing for a long time to get their engines to run a constant speed.. As you turn the screw in, you turn the needle in next to get a max RPM setting just above 8000 RPM. That is what he said to do for a Saito 56/62. props are in the 6 and 7 inch pitch range. If the weather gets warmer, you can screw the blocking screw out just a bit to let more air in and reset the needle. When it gets cold out,  you do the opposite, screw in in to let in less air and reset the needle. It will kind of go against everything you already know about engines and props for 2 strokes, but it works. With an R/C carb, the barrel takes the place of the screw. Just play with it and get familiar with it. For a 4 stroke that small, i don't know if you need a new RPM range or not, but just experiment, go by lap times and line tension. before you try any maneuvers. The engine might not sound like it will pull your hat off your head, but just ignore that and go by lap time and feel. This set up with muffler pressure will make the engine run very consistently where ever it is flying. You will have to get used to that and if you have been flying 2 strokes for any length of time it will sound foreign to you.   The 4 stroke is a different animal, and resist all temptation to run it like a two stroke. This low RPM set up lets the engine get plenty of fuel, and thus it runs cooler. I have run several gallons of fuel through my Score and Saito .56 and all of the chrome is still shiny. I ran all of this by Gilbert Berringer from France. He is one of the top stunt pilots in Europe and helped bring the 4 stroke engine along. He has a large manufacturing company that makes aircraft wheels and brake systems, and comes to Oshkosh every year, along with Sun and Fun in Florida during the winter. He flew my Score several times and was impressed by it. He sets his engines up with a very small venturi to start with, no bigger that 6mm, he told me, and when you put a spray bar through that, the choke area is pretty small. I run Powermaster YS-20/20 for the most part, and it's getting pricey now, but i only use about 3.5 ounces per flight, so it lasts a while. Try some of the Powermaster synthetic blends in the 15% range. It would not hurt to add some extra synthetic oil to get the total oil content up closer to 20% like the YS blend. I have been trying not to add any castor, as there are a LOT more and smaller moving parts in these engines, and even smaller yet for your .26. If it sets up and runs the lower ROM ok, it might not be necessary. It will take a lot of attention on your part as you go through the motions. Everything from starting them to picking props and setting the needle will be different and again, the big thing is DO NOT run it like a two stroke!! The closest thing I have to your engine is a Saito Black Knight .30 but I have yet to pick a model for it.
  Type at you later,
   Dan McEntee

Colin,
Dan gave some very good information. Here is an engine review of the OS 26FS Surpass (http://sceptreflight.com/Model%20Engine%20Tests/OS%20FS-26%20Surpass.html). I like to run mine in the 9500rpm range and use a higher pitch (like 7") with a wide blade, diameter around 10". The point Dan made about the sound is important. With the muffler it is like flying electric, but the note is a little flat compared to 2 stroke (think Harley vs, Yamaha). You will notice that you may be flying faster lap times with the 4 stroke but it will seem like it is slower because of the sound.

The Yak is a very good ship for this size engine.

Best,    DennisT

Colin: Actually, I've had very good results with a Master Airscrew 9-6 plastic as well as a 10-6 cut down to 9 1/2.

Also, if you can find some, the old Top Flite 9-6 wide blade wooden props work very well.

However, it's best to experiment - try some of the ones you already have and pick the one that best suits your flying style.
Generally, I prefer to use props with at least a 6 pitch.

Bob Z.

I used one of mine in a Sterling Mustang.
Flew great, took home many trophies in OTS and classic stunt.
I once beat a Super Tiger 60 powered Cardinal with the Mustang.

Bob Z.

Bob, just curious. I am thinking ahead on my future Brodak Yak 9 build w/ OS 26FS.

On your Sterling Mustang w/ OS 26FS, what trick(s) did you use to manage the CG with the heavier four stroke engine? The kit is probably designed around a 25LA or FP. With stock muffler, the 25LA is around 8 oz and the 26FS around 10 oz.

Did you move the engine back a bit, or just add lead in the tail as needed?

Thanks in advance.

Thanks Bob

Have not purchased the Brodak Yak-9 kit as yet but am thinking and planning ahead.

Your Mustang and the Yak are nearly identical size models. Your Mustang is built pretty light. My build skills are somewhat limiting, and I probably can't build as light as you. My Yak will be pretty much stock-built. If I were to guess, the Yak will come in somewhere in the 30-32 oz range with corresponding wing loading around 13.4-14.3.

One question I have is whether or not to build the model with flaps. You had mentioned the Mustang flies great either way. Considering the heavier model with OS 26FS (compared to yours), would you suggest flaps or not? Flaps will probably improve flight performance, but also increases drag with corresponding increased power needs.

Thoughts welcome.

Colin,
Regarding the fixed vs coupled flaps for the Mustang or Yak 9, it depends on what you are building the model for. If you are planning on flying it in Old Time Stunt you can use either, but with the coupled flaps you lose 5 bonus points. If you build it with fixed flaps leave the flaps flat and square at the ends. This is an old Lew Andrews tip, he used on his Barnstomer's. What this is supposed to do is give you the effect of having around 10 degrees of flap as the ship turns through maneuvers. If you decide to go with the coupled flaps set them up for 2:1 elevator to flap. These ships have short tail moments and to much flap deflection will make it turn very slow. Also, you only need +- 20 degrees elevator, much more and the stall like a Ringmaster.

Best,    DennisT

Reading this thread earlier this week inspired me.

I decided to pull the .26FS I had in my collection but had never run (its used and had definitely run before). I broke it most of the way down and cleaned out all of the dried castor and then lubed it back up.

I put it on the test stand, and to my big surprise it actually started on the first flip. I'm running it on some very old 15%, but after warming up and settling in, it ran beautifully. I'm extremely happy with it.

I was running it on a wide blade 10-4 that I happened to have bolted to it and that seemed to be a good prop to run on the stand. Lighting was not good for getting a good reading with the tach, but it is very strong.

I definitely have to build a bird for this engine finally!!!

My new OS 26FS Surpass is planned for an OTS Brodak Yak-9. My Yak-9 build is early in the build process.

In preparation I have run my 26FS in my test stand to start the initial break-in process. I have bench-run it on 10 nitro/20% oil. It has started easily and run very well so far. I am excited to finally get some experience with a FS engine on a CL model.

Coln,
The OS 26 Surpass FS is a great little engine for the YAK 9. One thing for the tank setup, the FS like conventional tank vents NOT uniflow, some have used muffler pressure also. The RC plastic tanks work well, set them to be tall and skinny with the vent line to the top inside corner of the tank and the open end up in the free airstream above the fuse profile (see photo). This gives very consistent runs. Mine likes a 9 1/2" x 6 at about 10K.

Best,   DennisT

Dennis, maybe it is just one's terminology, but am a bit confused.

When I think about 'conventional' venting, to me it means simple over/under venting with both vents open during flight. (i.e.:, A tank similar to the old Perfect tanks.) But if muffler pressure is used, one of the vents would be plugged to maintain the pressure. Correct?

And would not a uniflow design tank keep the engine run more consistent? A uniflow tank can be run with or without muffler pressure. (I actually run an oval profile uniflow tank on a Fox 35 with just ram air to the UF vent with the overflow plugged. It runs better with ram air pressure vs muffler pressure.)

Also, others have mentioned that for some reason, a clunk type tank works best for a small FS.

I have all of the above tank designs mentioned. But I am thinking about trying a small Sullivan clunk tank, set up for uniflow, or a metal oval profile uniflow tank, but just using ram air pressure to the UF vent, with OF plugged.

Thoughts welcome

Thanks in advance.

Colin,
Sounds good, one thing you might want to experiment with is using an conventionally vented plastic tank. I was able to do the pattern on 2oz but that was 10% nitro fuel. For some reason the 4 strokes like the CV setup better. Tall and skinny works well, could try muffler pressure to the CV vent line. If you do the open CV vent make sure it is up in the free airflow above the fuse side (photo in prior posts in this thread).

I know it is a little hard to go to the CV venting after we have been told how great the Uniflow is all these years but the top guys that use the 4 strokes have proven it out so worth a try.

Best,   DennisT

This question is for Dan or Dennis or Robert. This post has been about using the four stroke on a profile plane. I recall reading somewhere that Bob Reeves suggested to mount the four stroke motor in the horizontal position (like a profile plane). What about in a full fuselage stunt plane - have you seen a big difference between mounting the four stroke engine inverted versus horizontal? Just wondering what you think, if it's a big difference or small difference. Thanks.

Colin,
Sounds good, one thing you might want to experiment with is using an conventionally vented plastic tank. I was able to do the pattern on 2oz but that was 10% nitro fuel. For some reason the 4 strokes like the CV setup better. Tall and skinny works well, could try muffler pressure to the CV vent line. If you do the open CV vent make sure it is up in the free airflow above the fuse side (photo in prior posts in this thread).

I know it is a little hard to go to the CV venting after we have been told how great the Uniflow is all these years but the top guys that use the 4 strokes have proven it out so worth a try.

Best,   DennisT

I did try to put together a small Dubro 2 oz clunk tank for possible use on a 40" wing smaller profile model w/ the OS 26 FS. One of my struggles with the small plastic tank setup is that the clunk tube within the tank is so short that the tubing is not flexible enough such that the clunk does not move freely within the tank. The other issue is that on a smaller profile model there is not enough room to install the tank behind the FS engine. I would have to move the tank to the inboard side of the fuselage. But this makes the fuel pickup tubing longer so is somewhat counterproductive (fuel-draw wise).

So, I guess my question for others is what is the best way to set up a small 2 oz clunk tank for a smaller profile model w/ small 4-stroke? I guess I could try a larger 3 oz clunk tank, but just fill it w/ 2 oz. But that also seems counterproductive all things considered.

 

Colin,
I agree about the short clunk tube. Sullivan had a very thin wall silicon tubing just for clunk lines I looked on Brodak's site and don't see it. You might be able to use a heavier clunk, maybe Jim Lee and help on this (Lee's Machine Shop in vendor section). The second choice is surgical tubing, this will work but needs to be change twice a year as it gets soft and may leak but in a plastic tank it is not a problem.

Last option is to use a conventionally vented 2 ish oz tank. Again, make sure the vents are in the free air stream.

Best,    DennisT

Thanks Dennis

I'm still going to initially try a Brodak 2 oz wide wedge uniflow tank w/ muffler (restrictor) pressure on my Brodak Shark 402. I actually did get in a couple of flights with this arrangement and the setup seemed to run well. But I still need to home in on the best prop, needle setting, throttle opening position, ground rpm, tank height, and line length to use. A bit of trial and error. I'll also eventually try different tank setups for comparison.

Colin,
Any update on the OS 26SF? What did you finally settle on for fuel, prop and rpm?

Best,   DennisT