New engine... a little help

[MikeB]

I’m the new owner of a TRM engine. Even though, I don’t know anyone personally that owns a TRM engine; I do know people that own a Hanson modified engine and a stock 231, so we get to compare engines and setups, which is a good thing in my book.

In the instruction sheet it says to keep the engine rpm low during break-in, but what does low mean, 10,000 to 11,000? Please let me know.

As far as engine rpm, gear ratio, head speed and blade size goes, I would like theory explained of these relationships. I know this is asking a lot but it doesn’t hurt to ask. With a solid understanding of said relationships, I can make an informed decision on what gear ratio to run for my particular flying style and skill level.

So…if anyone is up to speed on the relationships between engine rpm, gear ratio, head speed, blade size and any other factors that contributed to overall performance, I would like to hear your thoughts.

Thanks

[bigrcr]

The "low RPM" requirement is not so much of a specification as a it a warning not to let the RPM get out of hand and allow over-speed.

The gear ratio and blades will spell the load, RPM and head speed for the model for the most part. If the load is high, the RPM will be lower. A high RPM is achievable if the load is low enough or the horsepower output overcomes the load. Blade size/weight, friction and the airfoil of the blade will determine the load for the most part. The relationship between these is of give and take. More load=less RPM and more heat; Less load=more potential for RPM and less heat.

The 7.15 Spectra-G gear ratio was selected for 3D and high-end sport flying since it allows the correct amount of load to be applied to the engine so that it's horsepower/torque is properly used. The ratio works well in the RPM range that the G-231 motors put out (12,500-13,500) to deliver a nominal head speed of 1750-1850 RPM without applying so much load that the engine cannot quickly recover from increased load. A G-26 engine is not happy at the RPM range that the 7.15 ratio allows it to operate. The G-26 is much happier at a ratio allowing a maximum RPM of around (12,000-12,500 and possibly more load) due to issues with vibrations.

Larger (or smaller for that matter) disc loads from the rotor will change the load demand on the engine. This will determine what ratio to use to optimize the power of the engine within its power RPM band.

I could be more specific, but must go to work now...Later :wink:

[MikeB]

John, first thank you for your response.

Don’t over-speed the engine during break-in… got it.

I do believe I understand what you have written, so let be recap a bit. This engine has sweet spot of 12,500 – 13,500 rpm and to achieve this sweet spot one will need 7.15 ratio with a head speed of 1750 – 1850. This head speed is needed for sport + flying and will load the engine well depending on what blade size one is running.

The rotor disc load depends on blade length/weight and the relationship of collective and cyclic pitch. Too much pitch and to big of blade will bog the engine, which equals more heat and stress to the engine. Running 6.77 gear ratio with a head speed of 1750 would put the engine rpm at 11,847. One would still have same disk load of the 7.15 ratio, but would not have enough engine rpm to recover in extreme maneuvers, but enough power for simple sport flying.

John, again, thank you for your response and I hope I got some of what you wrote correct.