Four Stoke Findings for CLPA


My 4-stroke development began in earnest in mid October 2001. Being that the winters in Dallas are pretty mild, I felt that this would be perfect time to do some experimentation with 4 cycle engines that Windy Urtonowski had recommended in several of his Typhoon stunt videos. I had just completed surgery to re-attach the lower tendon on my right biceps muscle so I was not able to fly for myself in the beginning.  Doug Moon volunteered to be the team “mule”, a few old airframes were retrofitted with the Saito 56 and 72, and we began our journey into “4 Stroke Land”. The system that was actually refined in the winter has remained unchanged and has performed exceptionally well through the hot summer, including a rock solid performance at the 2002 Nationals (6th in Advanced). 

To date, all of our experimentation and development have been centered on the Saito 72 and 56.  It is our plan to develop the OS 52, OS 26, and YS 63 in the winter of 2002.  Considering this fact, all of the recommendations contained herein are meant to serve as an overview of my introduction to ONE BRAND of 4-cycle engine (Saito).  

Recommendations for the Use of the Saito 4 Cycle Engine for Stunt 

1.      Which Engine?-  My preferred engine is the Saito 72.  Our 4-cycle experimentation actually began with the Saito 56.  The Saito 56 appeared to be a reasonably powerful engine, but yielded a stunt run that was somewhat “unconventional”.  The lap times were typically faster than the maneuver speeds.  In effect, level laps were faster than a typical 2-cycle engine and maneuver speeds were slower.  This would seem to bear out the stunt runs that were reported from friends in the Northwest who were utilizing the Saito 56 almost exclusively. Considering the size and weight of our test airplanes (650 Sq in-60 to 65 oz), the Saito 72, almost immediately, appeared to be superior to the Saito 56 in every way.  It was evident from the first run that the Saito 72 was more capable of producing a huge SURPLUS of power over a wider RPM range, and with a larger selection of props.  It was my initial goal to find a 4-cycle engine that would match the output of the mighty PA 65.  It took the added displacement of the Saito 72 to achieve this goal (this would correlate exactly to the manufacture’s recommendations).  The only detraction of the Saito 72 over the Saito 56 is an added 2 oz of engine weight. These two engines actually bolt into the same holes. The weight penalty, however, appears to have no detrimental effect at all (more on that later).

From personal observation I would say that the Saito 56 is most closely matched to a model in the 55 oz range.  The Saito 72 is more closely matched to the 65 to 75 oz ranges. 

2.      Venturi VS Carburetor- initial testing with both engines was started using the stock carburetor controlled by a pushrod contraption that I designed.  This setup did have the capability to change RPM via the throttle for each flight, this proved very handy for trying different props.  While this setup did work, and I would not hesitate recommending starting with the carburetor for the sake of experimentation, the setup that finally proved to be utterly reliable was the utilization of the Pat Johnson manifold with OS Max FP venturi.  I have broken-in a lot of these Saito engines on the bench in the last year (one gallon per engine).  Since Saito recommends that their engines be broken in at low RPM (utilizing the throttle), I have spent a lot of time to listening to these engines run with the carburetor in a fixed position (the one gallon of break-in takes about 4-5 hours run time).  I can now say with all assurance that the carburetor does not run as well as the venturi at full load.  This would seem to bear out what we have for in-flight testing also.  The carburetor, even when held rigidly, appears to sputter, pop and misfire, even though the effect IS VERY SUBTLE.  The carburetor never exhibited the DROP DEAD in-flight and run-to-run reliability of the venturi setup.  This appears to agree with opinions that have been shared with me by the flyers running some the 4-cycle engines on the West Coast. 

My venturi setup for the Saito engine is as follows: 

    • Pat Johnston Manifold-a beautiful and simple little device that replaces the stock carburetor and allows a stock OS FP venturi to be inserted utilizing the standard “spray bar through the venturi” arrangement.  When I first started in the 4-stroke program, Pat was manufacturing the manifolds himself (he is quite good at it).  Since then, the manufacture of the manifolds has been passed to Ultra Hobby Products.  I helped develop the UHP version with Dallas toolmaker Dirk Tolendaar.  We took the original design and cleaned up some of the machining and added an o-ring seal to the carburetor downtube so that no silicone was needed for installation. The manifold is sold as a complete kit with everything needed to convert your Saito 56 or 72 into a stunt engine.  This includes the manifold with new mounting screws, three venturi’s (.265”, .275” and .285”), and an OS Max 40 FP style needle valve.  The cost for the package is $60.00.

FYI:  Pat Johnston is truly one the nicest guys I have ever met in stunt, and has been tremendously supportive of my puny efforts.  Please, don’t hesitate to contact him.  At present he is wringing out the Saito 56 quite nicely and has lots of interest in the findings of others. 

3.      Venturi– The venturi that I started with, in the winter of 2001, was the stock OS Max 40 PF venturi “large” (.285” opening).  This is would be the large #3 venturi included in the Ultra Hobby Products manifold set. (Note:  You might need to use the #2 venturi (.275”) with the UHP set, as the OS FP needle valve has a smaller cross section than my OS 35-S needle valve). I assumed at the time that the thinner summer air would most likely require a larger opening, but this has never proven to be true.  I tried several larger venturi sizes as the temperature began to change, but never liked the results.  Changing to a larger venturi only served to increase the speed of the airplane, but not necessarily the power (compensations for temperature are made using nitro content-discussed later under fuel). Comparing notes with Pat Johnston in Idaho, he has been running the same size venturi on the Saito 56.  Jim Aaron was also using the same size venturi (.285”-with ST 60 needle valve assembly) in his Saito 56 at the 2002 Nationals in Muncie.  Once his airplane was converted to the “Dallas setup” his Saito made great power all week.  This being said, it would seem obvious to me that the torque of the Saito engine works very effectively at lower RPM’s.  I stay between 8500 and 9200 RPM, depending on the propeller.  I have found 8900 to 9200 to be optimum. 

4.      Needle valve– I am currently using the old OS Max 35-S needle valve assembly.  This needle has the same cross section as the typical ST type needle valve, but is of the “click” variety.  A ST type needle could be readily substituted (look for the drop in replacements for the OS Max FP type engine available from Tom Dixon, RSM, etc).  The Ultra Hobby Products manifold kit includes the newer OS FP type needle which is identical to the needle I use, but has a smaller cross section and a finer thread on the needle.  I opted for the OS Max needle because I have always liked the “clicks” available with the OS Max type needle.  I feel that the “click” method of fine tuning the engine is the most consistent.  The advantages of this type of needle will be discussed in the tuning section. 

5.      Fuel– There are special considerations when selecting a fuel for a 4-cycle engine.  Several discussions with RC flyers, and my own experience, have shown me that the typical approach that we stunt flyers take to fuel could POSSIBLY have a detrimental effect on the 4 cycle run.  Typically, our 2-cycle stunt fuel has very high oil content, the effect being that it is advantageous to take heat out of the engine to maintain the classic “four stroking” run.  Since a 4-cycle engine only fires every other stroke, it can be detrimental to take heat out of the cylinder too rapidly.  Low cylinder heat has a tendency to “blow the plug out”, especially in high G maneuvers.  This again, correlates to the information gleaned from the RC flyers that I have spoken to that have been running 4-cycle engines for almost 15 years.  Basically, most of these RC flyers avoid castor oil in all of their 4-cycle engines.  To this end, I began utilizing fuel that was developed specifically for 4 stroke engines in RC Pattern, which is an all-synthetic oil blend with slightly higher nitro content than we would typically see used for 2-cycle stunt.   

The first fuel that produced exceptionally good results was Powermaster YS 20/20.  This fuel is 20% synthetic oil and 20% nitro.  Of all the fuels I have tried of various blends, no other fuel really came close to the Powermaster YS 20/20. In particular, the Saito 72 seems extremely happy on this blend and exhibits all of the traits that are beneficial for stunt.  It appears to be a drop-in improvement to any Saito 4 stroke setup.  I switched Jim Aaron’s Saito 56 to the Powermaster YS 20/20 at the Nationals and there was an immediate improvement in the running characteristics of the engine.  The representative from Powermaster that I spoke with at the Nationals said that Horizon Hobby (the Saito distributor for the Americas) highly recommends their Powermaster YS 20/20 fuel.  Also, Powermaster has been very consistent fuel and I have been extremely impressed with Powermaster’s factory support for CLPA.  So, all that being said, I now use the Powermaster YS 20/20 for normal conditions and 30/20 for extremely hot conditions in Dallas (I required no 30/20 at the Nationals). 

My Saito 72 uses approximately 4 oz of fuel to complete the pattern using Powermaster YS 20/20.  4 ¼ oz are required for 30/20.  The Saito 56 will use closer to 3 ¼ oz of fuel.  This type of excellent fuel economy overrides the higher cost of high nitro fuel. 

Second to the Powermaster YS 20/20, I would recommend the Rich’s Brew YS 20/20.  At the recommendation of Bill Bischoff, Dallas’ nationally renowned Carrier guru and local hobby shop guy, I tried the Rich’s Brew version of YS 20/20.  This has proven to be good fuel.  The engine does not produce quite the RPM that the Powermaster equivalent does, but the capability to needle the engine with this fuel appears to be very, very good (this is MUCHO important-this will be discussed further in the tuning section) and the price is about $5.00 a gallon cheaper.  Another possible advantage to the Rich’s Brew line is that it is available in 25/20 and 30/20, with the oil mix left the same and the nitro increased.  I use Rich’s 50/20 Magnum fuel mixed 2 to 1 with the Powermaster YS 20/20 to make my hot weather 30/20. 

Sig Champion 15% fuel also appears to needle quite nicely and produce smooth runs but never really had the superior characteristics of the Pattern based synthetic oil fuels.   

Long story short, I feel there are several fuels that may work exceptionally well in the 4-cycle for stunt use, the most important factor is how they run in the engine (DUH).  I will attempt to explain how to determine if your fuel is correct in the “tuning” section.  Also, nitro is good.  Higher nitro in the 4 cycle has none of the ill effects demonstrated by 2 cycle stunt engines.  Nitro just makes the engine run BETTER.  It even stunts better.  I much prefer adding nitro to opening the venturi for more power.  One interesting effect is that I rarely touch the needle valve switching between 20/20 and 30/20 in subsequent runs. 

6.      Glow plugs– at the recommendation of Windy and his 4-cycle videos, and Bob Zambelli (and just about every 4-cycle RC flyer I spoke to) I began running the Saito on the OS Max “F” plug.  I recommend this plug highly.  If all else fails use this plug.  One of my favorite plugs for 2-cycle engines has always been the Sonictronic RC-300.  Randy Smith of Aero Products turned me on to Sonictronic more than ten years ago and I ran a WHOLE bunch of these plugs over the years and NEVER had a bad one.  To this end, once I learned that Sonictronic was producing two new 4-cycle plugs, a call was made to Mr. Hankinson (President of Sonictronic) who was nice enough to give me four plugs for testing.  Being an old control line flyer, he was interested enough in our hobby, and what we thought of his product, to “COMP” me the plugs.  I have tested their two different plugs.  There is one version made specifically for the Saito and Enya, and another that is designed specifically for the OS Max (it appears to be an exact copy of the OS Max “F” plug).  The Saito version ran very well and never gave me any trouble, I would recommend it highly.  The Sonictronic copy of the OS Max type “F” is now my favorite plug.  It appears to be superior to the OS Max for retaining heat (and that is saying something) and equal to the OS Max for reliability.  Considering that the Sonictronic version of the “F” plug appears to be equal to or superior to the OS Max, it is an exceptional value at $5.00 and some change (half of the Hobby Shop price of the OS Max at about $10.00 and change).  

Sonictronic plugs are available directly from the manufacturer:

It is my hope that our CLPA “cottage industry” dealers will take the opportunity to carry these plugs.  I find it refreshing that Sonictronics is interested in the needs of the CLPA community. 

All of the other plugs mentioned are available at your local hobby shop. 

7.      Tank- Testing originally started on the Saito 56 utilizing a standard 4” GRW uniflow stunt tank.  This tank really did not function well enough for pattern flying until the rear of the tank was cocked at the maximum angle inside the fuselage, as the performance of the engine at the latter portion of the pattern was non-existent.  After this mod, we were actually able to put in patterns, but performance at the latter portion of the flight really was suffering.  Shutoffs and fuel consumption were erratic at best.  Overhead performance was generally pretty poor.  Muffler pressure seemed to not help or hurt the run.  Tank shimming for inside and outside speeds was very critical.  Upon switching to the Saito 72, the same tank was utilized for the first few sessions.  It became quickly apparent the 4” tank would not be large enough for the Saito 72.  Considering the space available for the tank in the nose of the test airplane, it was decided to try a 6 oz clunk tank.  I bought a DuBro 6 oz tank at the local hobby shop for $3.50 and plumbed it upside down (with the bulbous part toward the motor beams).  The clunk and overflow was fashioned in the standard manner and the vent line was bent to the most inboard front of the tank, as per the standard setup utilized for piped engines in Dallas.  This vent arrangement creates a true non-uniflow effect.  I only have one way to describe the clunk tank’s effect, WOW!  I can honestly say that before the clunk tank I was ready to give up on the 4-cycle.  Everything was fixed in one fail swoop.  The engine now ran through the entire pattern and usually only changes speed about .1 seconds of lap time through the entire run (depending on the prop, sometimes there is no real change).  The engine ran the same straight overhead as it did in level flight, and performance at the end of the pattern was as good as at the beginning.  The shutoff is downright spooky!  The engine just shuts off.  Most of the time there are no beeps at all.  Also, one more thing, the lap times slowed down .2 of a second (with prop #1).  No lie!  Before the clunk and suction vent arrangement, we were seeing the “fast level lap” phenomenon.  Fast level laps, slow maneuvers.  After the addition of our clunk tank the run became very “conventional”. 

I have since switched to the Sullivan “square” 6 oz clunk.  The performance on inside vs. outside maneuvers proved to be superior with the Sullivan.  The vent line is plumbed to the center front inboard corner of the tank.  The tank shim is approximately 1/8” off the beams if using the Ultra Hobby Products manifold (for both the Saito 72 and 56).  I use UHP “Moon Pads” under my engine, these pads are slightly thicker than 1/8” and my tank shim is ¼”.

I have come to the conclusion that the 4 cycle just does not work well, or REQUIRE uniflow venting.  There are no advantages to uniflow venting, because the engine does not speed up excessively through the pattern with a suction arrangement.  The only thing I see uniflow doing is making the engine go IMMEDIATELY over-lean the second the nose is pointed up.  I observed this in Jim Aaron’s setup while we were practicing on the grass early in the week in Muncie.  I really felt bad for Jim, as I noticed that he was really struggling and was definitely not pleased with his choice to show up to the Nationals with a Saito 4 cycle.  Well, that was completely unacceptable!  Jim was using the metal uniflow tank that he had used with his PA pipe engine.  I explained that the engine would never perform on that tank.  It was evident from takeoff that the engine was unloading to over-lean the second it took off.  It got even worse when he pointed the nose up or took the plane overhead.  Finally, Ted Fancher convinced Jim to try my tank.  A quick trip to Muncie Hobby Shop, with a little counter-top fabrication yielded an exact duplicate of my tank.  The tank was set at the same shim as mine, the tank was filled with some complimentary Powermaster YS 20/20, the needle set to max RPM and BOOM!  Stunt heaven.  Jim was smiling all week. 

If I have one thing to offer from all this to the other 4-cycle people, try my  clunk tank. 

8.      Propellers– To date, I have been using mostly large wooden propellers that are substantially reworked Zinger 14-5’s.  I never had much use for Zingers in the past (for use on the ST 60).  I  typically opted for the Rev-Up or the BY&O propellers, but since my supply of both of these other brands of props dried up at my local hobby shop, I opted for the Zinger.  The Zinger is a pretty nice HUNK of wood (Bill Bischoff calls them “prop kits”).  Initial runs with the Saito 72 were actually made with stock 13” props of various makes.  While these props proved to be less than optimal, I would have no hesitation to recommend these props to the average stunt flyer that might not be interested in reworking props.  A Saito 72 with a stock 13-5 or 13-6 BY & O, Zinger, Rev-UP, APC, or Master Airscrew is still a pretty nice rig.  (The APC 12-6 appeared to work nicely on the Saito 56, as well as the stock Bolly 13-6).  Basically, the seven props I have produced so far are variations on a central theme.  I have found that the real key to the Saito 72 performing in various conditions and producing very steady speed is not overdo the pitch in the last 1” of the prop (station 14 on a 13” to 14” prop).  Tip pitch is like nitrous oxide to the 4-cycle, the corner acceleration (and arm numbing line tension) that can be had with just a ¼ more pitch in the tip is INCREDIBLE. This, I believe, shows the real power of the big 4-cycles over the 2-cycle.  The 4-cycle does not easily become bogged down, it just powers through.  In short, HOLD ON TIGHT if you plan to run a lot of pitch at the tips outside of my recommendations.  Following the lead of Al Rabe and Bill Rutherford, I started clearly marking my props at every-other-station starting at station #4.  This makes it very easy to change props at the field and know immediately the desired effect, as the pitches are clearly written on the back of the prop.  As I said, my props are pretty well worked down.  After the prop has been pitched to whatever variation I choose (I try to buy props that are pretty darn close to begin with) the airfoil is reduced about 30% and all of the machine-made inadequacies are removed from the back of the blade.  II use a Black and Decker Mouse Sander-this is also quite good for re-pitching wood props because of the pointy tipped nose.  The object here is to thin and shape the prop to a true airfoil of some kind.  I use a set of calipers to measure thickness at each marked station, thereby assuring some semblance of consistency.  The rest is just feel.  The hubs are not reworked as I have found that the 2 ½” spinner that I use with the Saito just covers the hub anyway.  The tips are rounded in a conventional way and blade widths are kept at stock, for the most part. 

Target thickness the prop blade is as follows:

    • Station 4-.20”
    • Station 6-.20”
    • Station 8-.183”
    • Station 10-.168”
    • Station 12-.14”
    • Station 14-.10”


These, again, are targets.  Al Rabe told me he goes slightly thinner than I do.  Some variation on this theme might not be a bad idea, but these numbers will get you close to what I am CURRENTLY using, which works pretty well.  Interestingly, these dimensions are nearly identical to the stock Rev-Up. 

Prop pitches as I stated before are variations on theme that began with prop #1.  I have found that pitches in the stations #4 and #6 have no real effect, so I usually do not touch them.  The model speed and LOAD on the engine are found in station #8, #10 and #12.  Acceleration, good or bad is found in station #14.  All prop pitches will be stated in the form Station #4, #6, #8, #10, #12, #14. 

The pitches of my props thus far are as follows: 

·        Prop #1 (5.5, 5.5, 5.5, 5.5, 5.25, 5.25)- This prop measures 13.75” and was, as you can see, the original prop that I made.  The diameter was modified after a bad takeoff.  This prop has functioned as a good benchmark, but has limited use.  This prop is EXCEPTIONALLY good in dead air on a heavy model.  The tip pitch of 5.25 has proven to be slightly excessive, and will accelerate wildly in high winds.  It does however; produce much NEEDED acceleration in excessively calm conditions with a portly model.  Ground RPM with my fuels is about 8400-8500 RPM.  Lap times are a slightly quick 5.1 to 5.2 seconds on 70’ lines.  Corner acceleration can lead to a somewhat heavy feel in hard corners.  Line tension is actually somewhat excessive for scribing perfect patterns, except in dead air.  Still a good prop.  If you feel you need this application, leave the diameter at a full 14”.

·        Prop #2 (5.5, 5.5, 5.25, 5.25, 5.0, 5.0)- This prop measure 14” and is the best prop I have made if you chose to fly on lines that are 67’ to 68’.  At this line length the lap times are about 5.4, at 70’ the lap times are 5.6 to 5.7 which is too slow for my tastes (you might like that speed).  Line tension with this prop is lighter, but on a well-trimmed model in the 60 oz range, this prop is “stunt heaven” in just about every condition imaginable.  Speed in the corner is very conventional and windup is almost non-existent.  Ground RPM is 8700 to almost 8900.

·        Prop #3 (6.0, 6.0, 5.5, 5.25, 5.0, 5.0)- this prop measures 13.5”.  The jury is till out on this prop on 70’ lines.  This prop was originally intended as a smaller diameter version of prop #2.  The idea was to allow the engine to turn more RPM with the lower pitch on 70’ lines.  The load is quite mild, for that reason I could suggest this prop as a starting place for a new motor.

·        Prop #4 (6.0, 5.5, 5.25, 5.25, 5.25, 5.0)- this prop measures 14”.  This was my first really good prop for all conditions on 70’ lines.  Lap times were 5.4 to 5.3 (right in my pocket).  Line tension was just about right everywhere, all the time.  The original prop #4 and was replaced with an identical #7.  I would recommend this prop for a 60 oz to 65 oz model on 70’ lines in just about any conditions.  Very good wind prop. Very powerful in the corners, without too much power.  Ground RPM was around 8700 RPM.  This is the prop I ran all week at the 2002 Nationals. As of right now, this is the prop I would first recommend for competition.

·        Prop #5 (6.0, 6.0, 5.5, 5.5, 5.5, 4.75)- this prop measures 14”. As you can see, I took my assertions on the pitch to a little bit of an extreme compared to #4.  This did appear to pay off.  Lap speeds are 5.5 on 70’ lines.  Line tension is very even everywhere in the hemisphere and windup is vastly improved.  The motor is loaded more with this prop than you might think, and ground RPM is slightly below 8500 RPM.  The high load creates a very clean, hot burn in engine, and as result the engines barely varies in speed anywhere in the pattern.  The low pitch at the tips controls the windup and the forces in the corner.  This prop is pretty good on my 68 oz model on 70’ lines (don’t ask about the excessive weight, the poor guy is doing the best it can) but I guarantee this prop will be AWESOME on my new 60 oz model.  The only advantage to this propeller is the extremely powerful run required to run it, due to the excessive pitch load.  Try some 30% nitro on a 65 oz model.

·        Prop #6- 14” Rev-up with the same pitch as #1 (stock Rev-Up 14-6).  See explanation of prop #1.  This was an attempt to try a Rev-Up.  Good bolt on solution to start.

·        Bolly 14-6 2 blade– This prop actually comes 14.25” long and was a pleasant surprise. Trimmed to 14” it performs equally well with less load on the engine.  I would buy one of these, take it out of the package, balance it, and bolt it on.  It actually outperforms my best wood props.  I would have been running this prop in Muncie, but alas…

By the way, I tried my PA 65 prop ONCE, in an attempt to test the “high RPM-low pitch-super efficient prop” approach.  Let’s just say, it was a miserable failure.  The valves floated horribly and the engine shut off after two laps.  I do, however, hear through the grapevine that Brian Eather is working on four stroke carbon props right now (Jim Aaron had one on his Saito 56 at the Nats, it worked quite well).  My guess is that they may have a vastly different appearance than the current stock of piped engine props.  I can’t wait to see (that boy makes some COOL props-so does Bolly for that matter).

  1.  Tuning- When I first started with the 4-cycle I set a lot of bad needles, and watched a lot of bad runs (sorry Doug).  My only bad runs to this day are when I cannot find the needle (this will be explained).  I will attempt to sum up my philosophy on how to set the needle for my setup (YOU MUST USE MY TANK).  Make the prop go as fast as it will go.  Always start with PEAK RPM.  The tank richens the motor just the right amount, there is no reason to richen the engine on the ground.  This is one of the reasons that the Saito 72 has been such a pleasure to practice with.  The peak is always the peak.  Usually the needle is set on the first flight, and if I choose to run the same prop all day, with the same fuel, the needle is seldom touched again.  I seldom TACH twice, unless I am fiddling.  To set the needle, start the engine and use a tach.  Richen the needle until the engine starts to slow down, only 5-6 clicks are needed.  Now that the engine is running slow, turn the needle in, one click at a time and watch the tach.  Go slow; let the engine change speed slowly.  Pay close attention with your ear, it doesn’t take too long to learn to “hear” a 4-cycle.  Watch for a peak RPM.  You will know when you have gone too far because on the other side of the peak the RPM will drop off slowly.  Now you are couple clicks overlean.  Back out the needle one click at a time, resting between clicks and watching the tach.  The RPM should go right back up, another ten seconds of fiddling in this couple click range and you will find the peak.  Go slow.  If your engine will not show you a predictable 1-2 click peak and you see the tach bouncing around a lot in a 5-6 click range, YOU HAVE THE WRONG FUEL.  When the engine likes the fuel and plug you are using, it will show you this peak, plain as day.  Do this on the bench at home.  If you miss the needle by 5-6 clicks, you will have a long day at the field.  From peak RPM I vary either one click in, or one click out depending on the prop, on subsequent runs.  One click rich will slow the lap times about .1 seconds, again, depending the on the prop, and will allow more acceleration in the wind.  One click overlean can actually kill some of the acceleration in the wind.  All of this is basically true, if you run pressure to the overflow.  With pressure to the tank, the ground RPM (peak) will appear to be at peak in the air.  My experiments with non-pressure proved it to work just fine, but setting the needle was more difficult, engine would go slightly rich after takeoff due the ram air effect at the overflow.  This forced me to run the needle leaner 2-3 clicks on the ground to get peak RPM in the air.   If no pressure is desired, I suggest venting the tank to the inside of the fuselage.  This may stop the richening in the air.  For me, it is just so easy to vent the tank to pressure. 

10. Starter-You will need an electric starter if you choose to pursue the use of the 4-cycle.  Hand flipping a 4-cycle is tricky at best, especially if you like muffler pressure (or the nose of your airplane to remain attached to the nose of your airplane).  You see, if the pressure line is attached to the muffler and the engine starts backwards, it will continue to run very nicely backwards.  The only problem is that the exhaust pipe becomes a venturi and your new venturi  (on the inside of your beautiful airplane) becomes a smoking hot exhaust stack.  Paint burns good!  I know this from experience.  That explains my ugly, fat test airplane somewhat, poor little guy. 

11. Maintenance-  It is imperative that the Saito valve lash be adjusted per the instructional manual included with the engine.  If the valves are not checked for proper clearance, the output of the engine will suffer!  I would suggest checking the valve lash after break-in (I bench run 2/3 gallon), and every 10 or 15 flights for the first three sessions.  After that I would check the valves after every 20 flights until there appears to be no change.  Most likely, no further adjustment will be necessary after break in and the first few flying sessions, but it does not hurt to check.  Checking the valves takes about 5 minutes and can be done with the engine in the airplane, so do it!  As far as my tips for adjusting the valves, tighter is not better, a fraction loose is safer.  Be aware that the valve springs on the Saito are EXTREMELY light.  If the gauge is inserted with the lash too tight there is a chance that he valve will actually move a little and give you the impression that the gauge fits in the gap. That being said, the gauge should fit a touch loose.  I would say that there is almost no friction when the gauge is inserted.  This ensures that you have at least the gauge width (.005”).  .001” gap extra does not seem to hurt performance at all (that I can tell).  .001” to .002” too tight WILL effect performance!  In other words, the performance fall off is much more drastic toward the tight side (much more drastic).  So in summation, adjust the valves to “loose” on the supplied gauge.  “Loose” is defined as the gauge is touching both the valve and rocker at the same time, but barely.

Why a Four Stroke? 

Long Story short, I think there is something to it.

Not only have I done nothing but fly patterns with little or no fiddling, there is definitely something to the ability of the 4-cycle engine to improve the characteristics of an airframe.  Seriously.  It appears that the wings work better, and the flaps work better, you can carry more weight, etc. than you can with a 2-cycle.  Don’t over read this as nitro induced madness, but there is definitely something there. 

In a post on the Stuka Stunt Works Internet forum, I touched on a theory that has been muddling about in my head about the performance of 4-cycle engine vs. a 2- cycle, and how it behaves in hard corner.  Howard Rush challenged me to explain my theory and propose a physical solution.   

First of all, let me say that I do not feel I have the technical expertise required to explain my theory in aeronautical terms, as that is not my field of expertise.  I will attempt to explain in terms of simple physics, and I admit the factors may not be that simple.  So go easy… 

The hypothesis:

I have witnessed airframes set up with a 4-cycle engine that are capable of a harder corner, with a larger propeller, and further forward CG than an equivalent 2-cycle engine.  Windy spoke of this in his development, and marveled at the fact that airframes that were definitely nose heavy (compared to the same airframe that was previously utilizing a ST 60) with a 4-cycle engine appeared to require no real amount of tail weight to achieve the previous amount of hard corner.  I have also personally observed other 4-cycle planes that were running excessively large propellers (up to 16”) that appeared to lose nothing in the corner.  The usual effect with the 2-cycle being that a larger prop on a constant nose moment gets progressively harder to turn in a sharp corner in our small hemisphere.  I think we can all agree to this, being that the effect is created by several factors (precession for example). 

Why would a 4 cycle have less resistance to a corner than a 2-cycle?  Is it possible that a 4 cycle engine translates power to the propeller in such a way as to reduce a general resistance to a change in orientation (or precession) over a 2-cycle engine?  

My theory is as follows:

An engine is not applying torque the to blades of the propeller in a constant fashion.  Actually, torque is applied in pulses of very short duration.  The prop is spinning due to inertia for the majority of the period of one cycle.  This is the effect just like a bullet traveling a long distance due to an initial acceleration.  The bullet retains some of its energy over the length of its travel due to the engine imparted in the barrel.  The bullet is actually losing energy as it goes along. 

A spinning propeller is actually resisting a change in orientation due to simple mass (Precession) and is also resisting due to effective drag on the blades as they attempt to proceed at constant pitch through a corner.  In effect, for an inside corner the top of the propeller disc is traveling backwards and the bottom of the disc is trying to travel forward (I know this doesn’t happen, but I am at the limitation of my vocabulary) while all the time the blade is constant pitch.

In a 2-cycle engine, the blades are affected by imparted force twice as often as a 4 cycle.  The frequency of force applied to the blades is twice the frequency of the 4-cycle, even though the resulting average amplitude is equal.  This means that the 4-cycle propeller is “pin wheeling” twice as much as with a 2-cycle engine.  If we were to observe the actual speed of the aircraft in microscopic terms, the speed of the aircraft is not constant; it is actually “pulsing” along.  With a 2-cycle engine the pulses are twice as fast (one combustion for each 360 degrees of rotation), allowing a shorter distance of travel between each moment of imparted force.   

The drag of the prop disc against the corner is elevated at the moment of combustion.  At that point, the engine’s force is working against making a corner. 

In summary:

The 4-cycle plane would travel twice as far between each input of torque, allowing less resistance to a change in orientation over the same distance.  The average torque imparted to air would be the same in that distance, but at a much shorter period. 

This also shows how powerful the combustion cycle is in the 4-cycle.

It’s a theory… 

I actually discussed this theory with a very old friend of mine who is a motorcycle road racer, as he was fascinated with the fact that I was running a 4-cycle engine in a model airplane!  Without hesitation and with a very straight face, he said that type of theory is common knowledge in the world of motorcycles.  He explained that there is a drastic difference in handling characteristics between the hot 4-cycle and 2-cycle motorcycles in a hard corner.  For you see spinning mass plays a MAJOR role in the way a motorcycle handles.  He explained that similar “freewheeling 4-cycle” theories have actually been presented in the motorcycle world.  Whether it is true or not I thought the correlation was fascinating. 


It has been a gas, really.