Author Topic: 4 Stroke verses classic 2 stroke (Read 1569 times)

Bob Reeves · « **on:** August 27, 2007, 09:15:47 AM »

Lately I have been flying a couple 4 strokes that simply blow me away on run quality. In my mind both airplanes are just about perfect, no speed up or slow down anywhere in the pattern, tons of tension overhead and no surprises even in the wind. Just gives me that confident feeling when I'm flying that this airplane will do anything I ask it to and more if the need arises.

On the other hand I've been messing with my classic ship for over a year and still not really happy with it's performance. This is a 53 ounce Skylark 46 that presently has a ST 51 in it, started life with a ST 46 but really wasn't enough motor, at least I didn't think so. Enter the ST 51 and I'm still not getting the feeling of confidence the 4 strokes are giving me. Now I know people are flying 60+ ounce airplanes with ST 51's and are perfectly content and here I sit not feeling it has enough power for a 53 ounce Skylark.

Thinking it must be me and/or my setup I searched the forums and came up with the APC 12.25 X 3.75 setup several are raving about. This morning I went out and put 5 flights on the Skylark, 3 with the APC and two with a RevUp 12-5 W that actually measures about 4.5. Launched the APC at 10 grand and launched the RevUp at about 9600.. Didn't really see much difference in the performance between the two props, lap times were about the same and run times were close.

So OK that's the background now for my perceptions... The airplane actually slows down in maneuvers to the point that it effects line tension, the two top legs of the squares in the square eight are most noticeable. Also line tension over head simply isn't anything to write home about.. Please don't tell me to move my leadouts forward these issues are all engine.. If I didn't have the 4 stroke experience I would probably not be writing this and go about flying the Skylark skinny, dumb and happy.

Guess what I am trying to find out is, is this the way it is with a non piped two stroke, did we really put up with this and learn to live with it. I was flying a Genesis with a ST 46 when I quit back in the 70's but can't remember much about how it flew. If this is expected performance (and assuming a pipe run is more like the 4 stroke) I can really understand why pipes took over.

Any thought or views fell free to say what you think..

Bradley Walker · « **Reply #1 on:** August 27, 2007, 10:50:52 AM »

In my opinion.

Pitch is drive. Pitch is line tension.

You are feeling the pitch driving you through the pattern. When you switch to the low pitch is feels light up top.

Look at the rest of the world compared to the USA. You simply will not find low pitch setups prevalent outside of the USA. When Werwage won the World Champs in the 1980's he was using 7 pitch. His article in MA stated very clearly that pitch was needed to pull you through the pattern in bad conditions. Several years later he went to the pipe, and then that was all out of the window.

I saw Constantine fly in the Nats in 2003. He is a Russian guy from Canada (great modeler BTW). He stood flat footed and flew through some of the worst winds I have ever seen somebody fly in competively. He uses a 7 pitch prop on a low RPM motor of his own design. He was flying flat footed and not winding up, there were many top fliers that crashed in the same conditions, they would simply stop moving into the wind...

This is just my opinion. There are plenty of people that will say that everything I said here is wrong.

Maybe you have discovered something that a lot of people have not.

Bob Reeves · « **Reply #2 on:** August 27, 2007, 11:08:21 AM »

Hi Brad, glad you chimed in.. I for sure won't disagree with you but from what I have been able to tell so far the ST 51 doesn't like high pitch props. I have been trying to run it on 6 and 6 1/2 pitch props without sucess, the best runs so far were the flights this morning with the lower pitch props.

Yesterday I had a CF prop on it that had quite a noticeable underchamber and the poor ST wouldn't turn it over 8500.. It was obviously over propped. I have aquired quite a collection of 7 to 8 pitch props but am afraid the 51 will either bog down, wind up like crazy or run away in the wind.

Planning on comming to Dallas next weekend, hope to see you..
Bob

PatRobinson · « **Reply #3 on:** August 27, 2007, 06:45:04 PM »

Hi Bob,
It occurs to me you may already have the solution for your classic plane situation. You mentioned you had a 40 4-stroke, but not what brand, but I noticed that the OS-40 Surpass, that Bob Zambelli raved about in his reply to the OS-56A thread, weighed about the same as a Tigre 51 and an aftermarket muffler. You should use less fuel and benefit from that weight savings as well so you shouldn't change the planes trim too much with the engine change. I don't know what plane you have planned to use your 40 on but it might work well on your Skylark. Hey, it's just an idea and so I took a shot.

Best Wishes for a solution,
Pat Robinson

Bob Reeves · « **Reply #4 on:** August 28, 2007, 02:39:12 AM »

Hi Pat, believe me the thought has crossed my mind however would like to keep my classic ship somewhat "Classic"..

Looks like I will just have to live with what I'm getting out of the ST 51 but would like to see if I can't come up with a solution to it slowing down in the corners. Maybe I just haven't found the right prop yet, props are magic...

Bill Little · « **Reply #5 on:** August 28, 2007, 07:54:01 AM »

Hi Bob,

On the wood props, I found the *surplus* Rev Up 12-5 with a touch of diameter removed to work really well on the ST G51 on a 64 oz. SV-11. On my 54 oz. USA-1, an Eather flat back 12-5 3 blade (trimmed to *about* 11 1/2) turned out to be the best overall prop. The G51 does like to turn up more than the ST 46. Just a touch different in timing, and it just doesn't seem to produce the power at lower revs.

Brad has a good theory, but it is all up to the individual, their plane and engine. Juri Sirotkin won the World's in 1964 using a 10-4 prop and running in a two stroke. Different engine/plane combinations in different conditions require different props. As you know.

Bradley Walker · « **Reply #6 on:** August 28, 2007, 11:37:31 AM »

Quote from: Bill Little on August 28, 2007, 07:54:01 AM

Brad has a good theory, but it is all up to the individual, their plane and engine. Juri Sirotkin won the World's in 1964 using a 10-4 prop and running in a two stroke.

No doubt this is true, Bill.

Obviously, the pitch is related to the sleeve timing, compression, type of porting ect.

Bill Wilson has the OPPOSITE opinion of what I said. He likes RC timed motors better than stunt timed engines. He says they make more revs and he can run even lower pitch.

To me, it is just louder...

RandySmith · « **Reply #7 on:** August 28, 2007, 11:55:55 AM »

Quote from: Bob Reeves on August 27, 2007, 09:15:47 AM

Lately I have been flying a couple 4 strokes that simply blow me away on run quality. In my mind both airplanes are just about perfect, no speed up or slow down anywhere in the pattern, tons of tension overhead and no surprises even in the wind. Just gives me that confident feeling when I'm flying that this airplane will do anything I ask it to and more if the need arises.

On the other hand I've been messing with my classic ship for over a year and still not really happy with it's performance. This is a 53 ounce Skylark 46 that presently has a ST 51 in it, started life with a ST 46 but really wasn't enough motor, at least I didn't think so. Enter the ST 51 and I'm still not getting the feeling of confidence the 4 strokes are giving me. Now I know people are flying 60+ ounce airplanes with ST 51's and are perfectly content and here I sit not feeling it has enough power for a 53 ounce Skylark.

Thinking it must be me and/or my setup I searched the forums and came up with the APC 12.25 X 3.75 setup several are raving about. This morning I went out and put 5 flights on the Skylark, 3 with the APC and two with a RevUp 12-5 W that actually measures about 4.5. Launched the APC at 10 grand and launched the RevUp at about 9600.. Didn't really see much difference in the performance between the two props, lap times were about the same and run times were close.

So OK that's the background now for my perceptions... The airplane actually slows down in maneuvers to the point that it effects line tension, the two top legs of the squares in the square eight are most noticeable. Also line tension over head simply isn't anything to write home about.. Please don't tell me to move my leadouts forward these issues are all engine.. If I didn't have the 4 stroke experience I would probably not be writing this and go about flying the Skylark skinny, dumb and happy.

Guess what I am trying to find out is, is this the way it is with a non piped two stroke, did we really put up with this and learn to live with it. I was flying a Genesis with a ST 46 when I quit back in the 70's but can't remember much about how it flew. If this is expected performance (and assuming a pipe run is more like the 4 stroke) I can really understand why pipes took over.

Any thought or views fell free to say what you think..

Hello Bob

I have ran the G51 in planes with Bolly 12x6.5n 3 blades and 12.5 x 5.5 2 blades with excellent results, The lower pitched props were OK but not nearly as good as the higher pitched ones, The G51 has a very small effective counterweight and really is not as happy in higher RPMs, it shakes a lot more in Higher RPM runs and In my opinion, It also will wear out much faster and vibrate more. The ring, and rod, will also wear much faster in a high RPM 3.5 to 4.25 pitch runs.
Bene Rodriguez from Brasil Ran a G51 on his Stuntship with a 11 x 7 4 Blade Bolly with very good results, The motor ran like mine in a nice 4\2 stroke run. This was a large 60 size ship Bene used too.
This is not saying it will not work with lower pitch, but I have found the G51 which has very mild timing, (like a ST 46) works best with a 5 to 7 pitch. If I were to run a high timed one I think I would use the RC sleeve.
Many other people have used the props I mentioned and 12x5 , 12x6 Rev Ups and other with good results, Info for thought, use it if it helps.

Regards
Randy

Bradley Walker · « **Reply #8 on:** August 28, 2007, 12:19:47 PM »

I would set one up like a Big Jim 60. I know Jim did this for Windy it worked fine.

For me, I would try the TF 13-6, and a .305 venturi, and 5%/25 oil fuel to start (I have all of these BTW Bob, including the venturi---I have been using these in my ST 60). I would shoot for a target RPM of 7800 to 8200. I would raise the head until it breaks like a ST 60. I know this works, I have seen it. Joe Reinhard had this working more than a decade ago...

I would not run it underloaded. I think this is the problem, most people are underloading these engines trying to get a gutteral 4 stroke through the entire run, and the engine has nothing to hold it back. Or they are running them on these little low pitch props and the engine is simply underloaded, with noithing to hold it back. If you want to run pitch you need to get the load up to match. This will hold the engine back and keep it from charging.

Also, is this a Chinese or Ity .51? I think the Chinese engines have lower sleeve timing, although I have yet to measure one.

Bob Reeves · « **Reply #9 on:** August 28, 2007, 03:29:47 PM »

Thanks guys, good info to ponder..

It's the China CL version with a Bowman ring and B. Buck inspired spigot venturi. Choke area is same as stock but can't remember what that is. Needle valve body has large hole but do need to order a few of Randy's long needle valve assys. Been running it on PowerMaster 10-22 50/50. This airplane won Advanced Classic at Brodaks with a Thunder Tiger 12.5 - 5.5 prop cut to 12 inches but I was having to fly soft corners and big to keep it from falling out of the sky.

I have tried a Rev-Up 11-6 EW, RevUp 12-5, Rev-Up 12-5 W, Thunder Tiger 12-5.5 and the APC 12.25-3.75. Almost had a Bolly 12-4.5 3 blade on for a test flight but did something stupid and destroyed the prop. Had a 12-4 CF 3 blade from the GMA estate that has a large undercamber but the engine wouldn't turn it over 8500, too much load. It would turn the Bolly 12-4 3B at over 11 grand WFO.

As you can see by the list most everything I have tried has been under a true 6 pitch. I will find a good 12-6 in my prop collection and see what happens... Anything over a 12" two blade will require either longer landing gear and/or a nose rework to make the spinner fit. Reason I haven't ordered needle valves is I might end up needing to order taller landing gear.

Bradley Walker · « **Reply #10 on:** August 28, 2007, 04:50:36 PM »

Quote from: Bob Reeves on August 28, 2007, 03:29:47 PM

It would turn the Bolly 12-4 3B at over 11 grand WFO.

As you can see by the list most everything I have tried has been under a true 6 pitch. I will find a good 12-6 in my prop collection and see what happens... Anything over a 12" two blade will require either longer landing gear and/or a nose rework to make the spinner fit.

1. Repitch the 12-4 to 6 and try that. Reduce the nitro to 5%. Start raising the head until it is right.
2. A 12-6 will be a rocket ship. Not enough load.
3. Get longer gears. Try the TF 13-6 like I said.

What size is the venturi?

Bob Reeves · « **Reply #11 on:** August 28, 2007, 07:20:44 PM »

Quote from: Bradley Walker on August 28, 2007, 04:50:36 PM

What size is the venturi?

Can't remember, Am pretty sure I calculated it out so it was the same as the stock venturi with the spray bar. Going to have to put the airplanes on hold for a bit.. Right now both our cars have AC problems, taking my van into the shop in the morning and going to try to fix Pat's myself.. What a mess just before a contest...

Bob Reeves · « **Reply #12 on:** August 30, 2007, 05:43:28 AM »

Took a little time this morning to see exactly what I was running for my venturi.. It's a spigot and the actual drill size is .193, If I calculate out the actual choke area (including deducting for the spigot) and then figure what the equivalent would be with a ST needle it would be .296. The stock 51 venturi is slightly smaller at .281.

Like I said several people I know are flying 60 ounce airplanes with the stock venturi running 12-5 RevUp surplus props bit I wasn't happy at all with a 12-5. I have had two ST 51's in this airplane, one completely stock and the one in it now that went through Bowman. Haven't seen allot of difference between the two except the Bowman engine has better compression when cold.

This will be a good candidate for the Dallas Stunt Clinic this fall, hoping I can make it.. It's going to say home this weekend, Elwin can't get Friday off so we won't be in Dallas till around noon on Saturday. Too late to enter anything on Saturday, need to get a couple flights on the Score if the circles are clear early enough.

Bill Little · « **Reply #13 on:** August 30, 2007, 11:08:47 AM »

I saw the Bolly 12.5 X 5.5, 2 blade, that Randy mentioned. I had forgot but I DID use that prop and really liked it. Worth a shot.

Author Topic: 4 Stroke verses classic 2 stroke (Read 1569 times)

Bob Reeves

4 Stroke verses classic 2 stroke

Bradley Walker

Re: 4 Stroke verses classic 2 stroke

Bob Reeves

Re: 4 Stroke verses classic 2 stroke

PatRobinson

Re: 4 Stroke verses classic 2 stroke

Bob Reeves

Re: 4 Stroke verses classic 2 stroke

Bill Little

Re: 4 Stroke verses classic 2 stroke

Bradley Walker

Re: 4 Stroke verses classic 2 stroke

RandySmith

Re: 4 Stroke verses classic 2 stroke

Bradley Walker

Re: 4 Stroke verses classic 2 stroke

Bob Reeves

Re: 4 Stroke verses classic 2 stroke

Bradley Walker

Re: 4 Stroke verses classic 2 stroke

Bob Reeves

Re: 4 Stroke verses classic 2 stroke

Bob Reeves

Re: 4 Stroke verses classic 2 stroke

Bill Little

Re: 4 Stroke verses classic 2 stroke