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Engine basics => Four strokes only => Topic started by: Allan Perret on August 28, 2011, 10:41:27 AM
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I know of two expert level flyers that had tried 4-stroke power for stunt, but noticed they went back to 2-stroke.
When I asked why they both said the vibration level was "shaking their planes apart".
Do you build more beef into a plane that will use a 4-stroke. Also interested in general comments about vibration levels of 4-stroke versus 2-stroke.
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All I can tell you is I see little difference between my 4 strokes and m Skylark with a ST 51. I don't do anything special other than all my 4 strokes in built up fuselage ships are mounted on plastic RC engine mounts. I did notice that when I tried an electric prop on the 62 today the vibration was significantly worse. Came to the conclusion it was the blades flexing.
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All I can tell you is I see little difference between my 4 strokes and m Skylark with a ST 51. I don't do anything special other than all my 4 strokes in built up fuselage ships are mounted on plastic RC engine mounts. I did notice that when I tried an electric prop on the 62 today the vibration was significantly worse. Came to the conclusion it was the blades flexing.
Yikes! I am glad you are going back to regular props. I took Scott Bair's cylinder pressure data from an ST46 and was alarmed at the peak torque during a rev even for a wimpy ST. I would expect it to be significantly higher with a 4-stroke, particularly a big one. That's almost certainly why it vibrates - the hub gets so far ahead of the tips that it sets up all sorts of deflection in the blades. The only thing you have going for you is that the exhaust stroke does not require any reverse torque (unlike the dead stroke on a 4-stroking ST46), but that could help or hurt depending on the blade stiffness.
I would love to put a strobe on an engine running one of the flimsy electric props, I bet you would see all sorts of crazy motion.
Brett
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I know of two expert level flyers that had tried 4-stroke power for stunt, but noticed they went back to 2-stroke.
When I asked why they both said the vibration level was "shaking their planes apart".
Do you build more beef into a plane that will use a 4-stroke. Also interested in general comments about vibration levels of 4-stroke versus 2-stroke.
In R/C Pattern, the vibration is mitigated with "Hyde" type motor mounts.
The larger the piston mass, the more vibration whether its 2 or 4 stroke. Obviously, the vibration level is higher for a 4S vs a 2S of the same displacement.
The main advantage with a 4S in Pattern was the constant speed character of these motors. The downlines are controllable as the motor acts like a brake. A 2S accelerates on the downlines and the electric motors require a brake function to prevent speeding up as the plane dives.
I flew a Saito 56 in my Pathfinder for my first contest. I could have used my LA 46, but the 4S gives me more time to see the plane and think about my next input. Its more than a matter of timing, its adjusting the transmitter inputs to accommodate the position of the plane. My saito is set up to run about the same lap times, but it comes down at a constant speed which, as a beginner, I appreciate.
Mike
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I know of two expert level flyers that had tried 4-stroke power for stunt, but noticed they went back to 2-stroke.
When I asked why they both said the vibration level was "shaking their planes apart".
Do you build more beef into a plane that will use a 4-stroke. Also interested in general comments about vibration levels of 4-stroke versus 2-stroke.
At least two master modelers had airplane fly apart on 4-strokes. There's some chance that running a fiberglass radial mount would isolate it, and those few soldiering on with 4-strokes are running them at exceptionally low revs so not a lot of energy available whether it vibrates or not.
I expect the real reason that almost no one uses them any more is that electric can do the same thing but with far more flexibility and repeatability. You have to run a 4-stroke the one way it wants to run, which included grossly excessive pitch. You can run an electric that way, or with 3.5" of pitch, or anything in between, with one twist of the knob or a couple of keystrokes.
Brett
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I expect the real reason that almost no one uses them any more is that electric can do the same thing but with far more flexibility and repeatability. You have to run a 4-stroke the one way it wants to run, which included grossly excessive pitch. You can run an electric that way, or with 3.5" of pitch, or anything in between, with one twist of the knob or a couple of keystrokes.
Brett
Brett,
Would you say the same can be said for 2S vs electric?
Mike
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All I can tell you is I see little difference between my 4 strokes and m Skylark with a ST 51. I don't do anything special other than all my 4 strokes in built up fuselage ships are mounted on plastic RC engine mounts. I did notice that when I tried an electric prop on the 62 today the vibration was significantly worse. Came to the conclusion it was the blades flexing.
Bob,
What electric prop did you try? I have been using the JXF electric 14x6 and 14x7 and it works much better then anything else I have tried on my OS 70 Ultimate. The JXF regular 14x6 was too heavy for my engine. My engine works best with light props, it also works very well with the Rainbow props from Ukraine, which are light and have very thin blades. I was the first one to start using the JXF electric down here and then everybody switched to use them with 4S and 2S.
Martin
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I doubt any one ever made a model that a 4S actually shake it apart. I built a P-51 (clone of Al Rabe´s) in 2006 with a Saito 72 and let me tell you it was very light construction in the nose. Notice that the formers are lightened 1/8 ply and the engine mount ends on F1. Used 1/64ply doublers.
This P-51 is still flying today and has survived quite a few prop hits which made the engine vibrates pretty strong.
I am not suggesting this is a good way to build the nose for a 4S. But I guess it shows you that there is no need to change anything if building for 2S or 4S.
Martin
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Martin, It was a JFX 13-6, my 62 didn't like it at all, the standard JFX 13-6 was much better at a lower RPM. The two I bought will be given to another club member that is flying electrics.
I still believe most that tried a four stroke and gave up didn't get the results they were lookin for simply because they tried to apply what they knew worked with 2 strokes. Wanna see just how inconsistent and lousy a 4 stroke can run, stick a 4 pitch prop on and run it with the RC carb wired wide open at 9 grand on 10% nitro 1/2 castor fuel.
One reason I run Saito 4 strokes is simply economics. I can compete head to head against $400.00+ PA's and anybody's electric with a $100.00 used Saito (bought from an RC'er) a couple hours in the shop and a $5.00 prop.
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At least two master modelers had airplane fly apart on 4-strokes. There's some chance that running a fiberglass radial mount would isolate it, and those few soldiering on with 4-strokes are running them at exceptionally low revs so not a lot of energy available whether it vibrates or not.
I expect the real reason that almost no one uses them any more is that electric can do the same thing but with far more flexibility and repeatability. You have to run a 4-stroke the one way it wants to run, which included grossly excessive pitch. You can run an electric that way, or with 3.5" of pitch, or anything in between, with one twist of the knob or a couple of keystrokes.
Brett
(P1) Why not these mounts? I really don' think they would flex as much as the rubber/ stud types.
(P2) I noticed the current trend of a Stalker .81 w/ pipe, that's huge! Aren't you approaching the RPM's and props that the 4S guys already have? Mid range grunt w/ higher pitch prop.
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Martin, It was a JFX 13-6, my 62 didn't like it at all, the standard JFX 13-6 was much better at a lower RPM. The two I bought will be given to another club member that is flying electrics.
I still believe most that tried a four stroke and gave up didn't get the results they were lookin for simply because they tried to apply what they knew worked with 2 strokes. Wanna see just how inconsistent and lousy a 4 stroke can run, stick a 4 pitch prop on and run it with the RC carb wired wide open at 9 grand on 10% nitro 1/2 castor fuel.
One reason I run Saito 4 strokes is simply economics. I can compete head to head against $400.00+ PA's and anybody's electric with a $100.00 used Saito (bought from an RC'er) a couple hours in the shop and a $5.00 prop.
Hi Bob,
Perhaps you got a bad prop. I would not give up on them so fast. Here most felt a good difference using the JXF electric. Perhaps the JXF 13x6 did not load your engine enough? I felt that the glow JXF prop loads a lot more then electric.
I have seen first hand a fellow down here using a OS 48 Surpass with the RC carb wide open, APC 13x4 and regular 10% nitro flying a Score....man just perfect engine run! The amazing thing is that he is a beginner and flys very very tight figures, so tight it feels it will melt the engine, but the engine doe not miss a beat and keeps the speed perfectly, no slowing down no matter what he does. He then got an SV-11 with ST 51, well that ST 51 did not last long in there, he quickly switched to the OS 48 4S. His engine starts first flip everytime and always repeat the results, never seen him using a starter or tachometer, it just works everytime.
Martin
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(P1) Why not these mounts? I really don' think they would flex as much as the rubber/ stud types.
(P2) I noticed the current trend of a Stalker .81 w/ pipe, that's huge! Aren't you approaching the RPM's and props that the 4S guys already have? Mid range grunt w/ higher pitch prop.
I don't feel any special mounts are necessary for 4S engines. I may be wrong, but I feel that using this soft mounts it will rob some power, just like if you are running in sand as opposed to asphalt.
Martin
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I don't feel any special mounts are necessary for 4S engines. I may be wrong, but I feel that using this soft mounts it will rob some power, just like if you are running in sand as opposed to asphalt.
Martin
They absorb vibration, nothing more or less.
My only concern would be how much they flex in a corner maneuver. Since they are a better design then most I'd give them a try on my Saito .40/ Brodak Corsair.
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They absorb vibration, nothing more or less.
My only concern would be how much they flex in a corner maneuver. Since they are a better design then most I'd give them a try on my Saito .40/ Brodak Corsair.
I think that some soft mounts do indeed absorb some power but only if the resonant range of the mount is the same as the engines, if you doubt this then go quiz the pylon racer boys.
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I think that some soft mounts do indeed absorb some power but only if the resonant range of the mount is the same as the engines, if you doubt this then go quiz the pylon racer boys.
"only if" That's a very narrow window. 4-stroke C/L engines run at a fairly constant speed.
What do "plylon racer boys" running high-nitro piped 2-strokes know about 4-strokes at part throttle in a control line model?
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I use the SIG SIGEM001
http://www.sigmfg.com/cgi-bin/dpsmart.exe/MainMenuFV5.html?E+Sig
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"only if" That's a very narrow window. 4-stroke C/L engines run at a fairly constant speed.
What do "plylon racer boys" running high-nitro piped 2-strokes know about 4-strokes at part throttle in a control line model?
Hi Doug,
Some of Pylon boys know a heck of a lot about the science of transmitting every once of thrust into forward motion and would not entertain any system of mount that denies that, and they got that way examining all types of power plants.
Whats the question again? Oh yeah, shaking a model apart by using a 4 stroke single - personally I would use soft mounts if the vibration was the only issue and to hell we the lost power but what about the weight of those isolating mounts? That makes for a lot of metal in the snout of any stunt model.
Cheers.