On a 2-2-2 run (my XLS .36 powered Teddified Twister), if it sags in the OH8's, it's too much prop load. The prop of choice (TT Cyclone 11-4.5) stays, but the venturi size changes via Bruline air filter (none, fine, coarse). The key is how much below peak the launch rpm is. I don't know exactly how much "rich margin" that is, but about 500 rpm seems ballpark.

My 2Star .60bb runs a pretty nice 4-2-4, but thinking back about when I was flying it (plane crash, P.E.), the prop was maybe a little large. It would kick into a "2" about 3 times per loop. I found it annoying, and would prefer a 4-4-4, which would require a smaller prop, I think. Increased load makes the engine run leaner, and break sooner. To the credit of the 2Star, it returned to 4 very quickly.  Nice engine!  Steve

"What would be indications of an engine that is over or under loaded"

Allan, when you have the motor running on the ground, put a tach on it.  Say it is running 9000 RPM, then pick the nose up, if the motor increases RPM by about 500-600 RPM then the prop is just about right for prop load.  If you pick the nose up and it starts screaming at you, then the prop is too small.  If you pick the nose and the motor doesn't pick any RPM, then the prop is too big.

On a 2-2-2 run (my XLS .36 powered Teddified Twister), if it sags in the OH8's, it's too much prop load. The prop of choice (TT Cyclone 11-4.5) stays, but the venturi size changes via Bruline air filter (none, fine, coarse). The key is how much below peak the launch rpm is. I don't know exactly how much "rich margin" that is, but about 500 rpm seems ballpark.

My 2Star .60bb runs a pretty nice 4-2-4, but thinking back about when I was flying it (plane crash, P.E.), the prop was maybe a little large. It would kick into a "2" about 3 times per loop. I found it annoying, and would prefer a 4-4-4, which would require a smaller prop, I think. Increased load makes the engine run leaner, and break sooner. To the credit of the 2Star, it returned to 4 very quickly.  Nice engine!  Steve

Steve, I believe increased load will load the motor down more, causing it to run rich.  A smaller prop dowsn't load the motor as much which will have it lean out.

Steve,

I agree on air-load having a lot to do with it. At launch, with no forward speed, the prop is effectively stalled. That's a lot of drag, so a lot of load. On takeoff, the airflow over the prop cleans up, and that drag load eases off.

With a 'traditional' 4/2 engine, you might hear the engine sound 2-cycle at release, and then drop to 4-cycle after a half- to 3/4-lap. Think! That 4-stroking you hear is at more RPM than you had at launch. Has to be since the only change for the engine has been the prop drag unloading for level flight. Figures and line angle heights are a bit different...

Larger props are more load than smaller props at the same pitch: higher pitch props are more load than lower pitch props at the same diameter - any argument? Load drags RPM down, AND it can trigger a 4/2 break at an RPM lower than the free-running 4-cycle mode. I don't recommend trying this, but I did check that a break to 2-cycle CAN be induced from an engine 4-cycling on a test stand. I mounted a spinner on the engine, a glove on my hand, then CAREFULLY squeezed the front of the spinner cone a bit... (Still have all 10 fingers, though they ain't all pretty, anymore, and no damage occured from this type of test.)

What were the symptoms caused by what I did? RPM decreased, of course. Firing mode switched to 2-cycle AT THE LOWERED RPM FROM THE ARTIFICIAL DRAG LOAD (glove friction.) So, I heard almost twice as many combustions as before the friction was applied (firing every revolution instead of every other) while RPM remained less.

And, when I turned loose of the spinner, RPM increased immediately, and firing mode dropped right back to 4-cycling.

Relate this to maneuvering loads? I once figured out that low round loops need about 10g lift (average, plus/minus direction of gravity for the model's weight.) Wings make drag as they make lift, and if full-size aerodynamics can be applied, that "induced drag" increases as the square of the change in lift. Now, Induced Drag is very small for wings like our stunters at level flight - 1 g - conditions. But, 100 times something small isn't quite as small anymore. Think sea anchor? Drag chute? Anyway, it loads the prop by trying to slow the model.

Corners are much worse. We may pull around 30 g corners (needing 30 times as much lift as level flight.) 30 x 30 is 900. Induced Drag may become 900 times what it is in level flight. However small the level flight value is, 900 times that can't still be anything small.

The wonder of the "all-wet-2" run is that we don't hear a firing mode change. We do have to listen carefully for signs of straining through the overhead eights - it is a less distracting figure than the clover. Modern engines, except those specifically designed to run 4/2, have a very wide RPM range in 2-cycle. Almost all 2-cycle running sounds the same, but I'm convinced that the engine drops and regains RPM due to maneuvering loads. Since there's none of the drama of the 4-2 break, we don't really notice. Straining CAN be heard, if you remember to listen for it.

Cure? Rich a click? Lighter load prop? Hotter or colder plug? More or less nitro fuel? Different launch RPM? Somewhere among these, and several other trimming factors, you should find a consistent combiation that keeps both you and the model happy.

Lou...You and I agree...convince Matt. I recall seeing some sort of report by GMA about trying this test: George put an engine on the test stand and peaked it up with something reasonable (10-4?), then changed to an 11-4 or 12-4...it ran leaner. Re-needled for the large prop, changed to the small blade, and it 4-cycled. I don't recall where I saw this report, but was quite likely in Stunt News.  Steve

You have me convinced Steve and Lou.  Lou's post covinced me.  Thanks for correcting me

Not sure about all this.  How about this theory.
Prop load increases after launch and pulls engine back into a 4-stroke at a lower RPM.  Before launch the prop as you say is semi stalled,  running around in a somewhat localized partial vacuum, not moving as much air, which puts a lesser load on it than in flight conditions.  Kind of like what happens when you block the inlet port to your shop vac, you can hear it speed up, not pumping any air, lower load.  When you unblock it loads down as it starts pumping again.
 
But.. My theory here doesn't jive with you glove experiment.   Is your experiment valid?  You are inducing load to the prop in a static condition,  not its normal work mode of dynamic flight.
 
Think about this.  When is prop/motor performing more work.  Static on the ground, the work performed is represented only by the mass of air that its moving/pumping.  In flight the work is represented by the mass of the plane being pulled thru the air and its associated drag.  Which of those two are greater?  I would think the latter.  More work performed has to represent more load, right?

Hi Alan

The motors  do not unload and slow RPMs once airborne , even though you typically can launch in a 2 cycle and the motor drops to a 4 cycle once in the air, The RPMs almost alway increases, even though it sounds like it may from listening to it going into a 4 or deeper 4 cycle. We have checked this many times.

Regards
Randy

Well for what it is worth, when we fly electric with the rpm's governed to stay constant, you see the power input drop about 30% from takeoff to level flight.

Here's a plot from my data recorder on a electrified Nobler.  Purple is the rpm trace (left vertical axis, orange is the power input from the battery in watts (right vertical axis). The horizontal axis is in seconds.

Motor begins to spool up at the 60 second mark, the peak power is about 400 watts, the plane is released at about that point (70 second mark). You can see the takeoff and level laps, and I go into the wingover at the 5th lap (~92 seconds). The reason for the power variations during the level laps is just the wind. Going downwind causes the motor to work harder to accelerate the plane, while flying into the wind allows it to loaf! Average level lap power is ~260 watts.

Prop was an APC 12-6 electric turning at ~8000 rpm.

There isn't any reason to expect a glow engine to see anything different about the power loading, so if the prop load goes down, you wold expect the glow engine rpm to rise--unless you have a governor on it (like a pipe setup).

Just curious, how do you check RPM's once airborne ?
Onboard sensors.

Hi Allan

You can take your videos of your flight and  plug the sound into an oscilloscope and see the  engine RPMs in every maneuver and all through the flight

Randy