Month: May 2009

Note to nubes about thrust ‘tests:
Two stroke engines are about managing very rapidly oscillating pressure pulses. The pulses are managed by intake tract configuration, intake valving(usually by flexible reeds), shape, size, angle, and timing (position and angles of edges) of ports, ignition timing, and timing of exhaust pressure pulses reflected back by expansion chambers.

When intake and exhaust pulses are timed to push the most fuel/air mix into the cylinder as the ports close, you get the most power. The speed of the pulses is affected by rpm; it’s not possible to make the engine charge at maximum efficiency over its whole rpm range.

Tuners are left with some choices. Motors can be tuned to produce a high power number across a narrow rpm range, or lower number across a much wider range. Engine makers who tune for peak power, like a motocross bike, like to compare their peak numbers to FBs, because FBs are tuned to produce a lower number across a wide range, like a street motorcycle or a conventional airplane engine.

The engines tuned for peak power work best for flying styles that resemble motocross racing; if you do a lot of acro such that your throttle is always either wide open or idling, you’ll be happiest with something tuned for a peak power number. The engines tuned for useful power across a wide range of rpm are better for level cruising. They easily hold constant rpm over a wide range, which means you don’t have to pay close attention in order to cruise along in level flight with your engine running smoothly and happily.

Engines tuned for peak power very much want to run in their narrow zone of efficiency, which is often at a power level too high for level flight. If you run them where they are happy, they climb. If you insist they cruise level, they are harder to hold constant, and because they are out of their zone of harmonized intake pulsation, port timing, and exhaust pulsation, they run raggedly and blow a lot of unburned fuel right through.

A while ago, a friend who had only flown motors tuned for peak power asked to fly my FB Solo 210. He ran it up on the ground, and was concerned that it might not make enough power. When he launched it, he ran 20-25% farther than he did on his peak-tuned motor. His climb rate was not as good as it was on his peak-tuned machine. Not good so far.

His attitude dramatically changed once he leveled off and went for a cruise. His radio chatter was all about how incredibly smooth, smooth, smooth the 750-hour-old FB was. He went on and on about how easy and relaxing it was to fly. It appeared to him to be more powerful flying level than his peak motor, and at those low rpm levels, it probably was. He landed smiling, and ordered a FB Simo the following Monday.

No matter what your salesman tells you, your two-stroke can’t have both killer power and Cadillac cruise. If you want a motor that runs like a car over a wide range, cruising comfortably and economically over long distances, the peak power guys will out-climb you. If you want the thrust king, it won’t be as pleasant to fly on long cross-country trips. Those are two-stroke facts of life.

Fly high,
Mike Nowland