Hello,
Following Brett Buck's suggestion, I have removed the exhaust system extension ( aluminum 3/8 pipe, 18 grams  and two zip ties: 2 grams) and shortened the large DuBro silicon exhaust deflector attached to the Windy carbon composite pipe. The exhaust timing of my Jett60 is ~140 degrees therefore, following Randy's CL Stunt Pipe Length Chart, I have set my pipe at the distance of ~18.75" from the glow plug to the baffle, assuming I will get around 10,000 RPM using 12.5x4 wooden, 2 blade Zinger ( cut from 13x4). This prop is narrower compared to my previous prop ( 12.5x4, 2 blade Zinger cut from 14x4 ) and also 3 grams lighter.

Ground test results: RPM max: 11,200 decreased to stable but 2 cycle 10,250 with the static thrust =2700 grams ( 95.2 oz). RTF Intrepid XL without fuel weights now 2014 grams ( 71.0 oz). Yes, I know the model is heavy but I am not competing with it. My goal at this moment is to understand the engine and pipe setup, props selection criteria and so on.

Thanks Brett. By following your advice I have added more than 600 RPM, 250 grams of static thrust and removed 20 grams ( 0.7 oz., aluminum pipe and two nylon zip ties).

I have noticed that, while the engine run at 10,250 RPM for about 5 minutes, the turning of the needle +-90 degrees did not result in noticeable change of the RPM.

Was it the pipe that took over and governed the RPM making the RPM v/s Torque graph more or less flat around 10,250 RPM?

The engine was not in 2-4-2 break at 10,250 RPM but perhaps it will be when the model is launched and I try wingover?

Perhaps the elusive 2-4-2 break is characteristic for the engines having expansion mufflers and engines with tuned pipe behave differently?

I would be grateful for comments.
Thank you.
Matt

Hello,
Following Brett Buck's suggestion, I have removed the exhaust system extension ( aluminum 3/8 pipe, 18 grams  and two zip ties: 2 grams) and shortened the large DuBro silicon exhaust deflector attached to the Windy carbon composite pipe. The exhaust timing of my Jett60 is ~140 degrees therefore, following Randy's CL Stunt Pipe Length Chart, I have set my pipe at the distance of ~18.75" from the glow plug to the baffle, assuming I will get around 10,000 RPM using 12.5x4 wooden, 2 blade Zinger ( cut from 13x4). This prop is narrower compared to my previous prop ( 12.5x4, 2 blade Zinger cut from 14x4 ) and also 3 grams lighter.

Ground test results: RPM max: 11,200 decreased to stable but 2 cycle 10,250 with the static thrust =2700 grams ( 95.2 oz). RTF Intrepid XL without fuel weights now 2014 grams ( 71.0 oz). Yes, I know the model is heavy but I am not competing with it. My goal at this moment is to understand the engine and pipe setup, props selection criteria and so on.

Thanks Brett. By following your advice I have added more than 600 RPM, 250 grams of static thrust and removed 20 grams ( 0.7 oz., aluminum pipe and two nylon zip ties).

I have noticed that, while the engine run at 10,250 RPM for about 5 minutes, the turning of the needle +-90 degrees did not result in noticeable change of the RPM.

Was it the pipe that took over and governed the RPM making the RPM v/s Torque graph more or less flat around 10,250 RPM?

The engine was not in 2-4-2 break at 10,250 RPM but perhaps it will be when the model is launched and I try wingover?

Perhaps the elusive 2-4-2 break is characteristic for the engines having expansion mufflers and engines with tuned pipe behave differently?

   No, this engine will do a nice 4-2 break if you need to. There's little reason to do it, the pipe should regulate it regardless of whether its 4-stroking or 2-stroking. Mine runs in a constant 4-stroke until it runs out of gas, for the most part, speeding up and down but not breaking. The RO-Jett is very good at not having excessive power change when it goes from 4 to 2, but a lot of others have vastly too much and people try to avoid breaks in general.

     Sounds like you are making progress, but I suspect that the airplane will be much too fast. The point of the changes was not really to increase the RPM, but to reduce the load so that the engine will respond to load in the air.

    As mentioned off-line, I launch mine at about 10000 or so and it can very easily reach 11500 or even 12000 on the ground if I lean it out by blocking the tank vent. That gives it very little load variation in flight, and good responsiveness to load and regulation.

    I think in your case, the load of the prop is astronomically higher than the props normally used, so it's just running out of power. From the other post, it looked like you were generating a whopping 2.3 hp, and now it's even more - around 2.8 hp. That's an astonishing amount of power and if it flies at a reasonable speed, you are burning something like 2 HP in prop inefficiency.

    I take it that when its 10250 and won't go higher, it's leaned all the way out, peaked in a 2-stroke, right?  That's progress because your launch RPM will be lower or much lower than that, meaning it will be at least a little backed-off in flight, but it should easily reach 11500 on the ground in that configuration. The diameter is OK and the pitch is potentially close, but it must be FAR FAR draggier than the props we normally use.

   I think thats where to look next - call Randy and see what he can do. Anything that works on the PA61 will work on the RO-Jett, you want something like a 12.5-3.75 3-blade with narrow blades and a flat-bottom airfoil (or at least, very close to flat). If push comes to shove, at least ground test it with something like an APC 12.25-3.75 2-blade, which is a decent prop and *very inexpensive*. It should rev that prop to 12000 with your current settings very easily, and be in a deep 4 at 10000 rom you will need at launch. If I get a chance, I will try it myself with the 2006 NATs motor but don't wait around for me do to anything - I am completely buried in other things that I need to do and months behind on most of them.

    Brett