fuzzybabybunny Posted August 26, 2016 Share Posted August 26, 2016 Say that you've got a choice between a 130cc motor that's only 1kg lighter than a 200cc motor. The prices are both similar. As I understand it, thrust is governed by how fast the prop is spun. A 125cm prop spinning at X RPM will produce less thrust than the same prop spinning at X + Y RPM. A more powerful motor allows a larger gear ratio to be used so that a big motor at 5,000 RPM can spin a prop at X + Y RPM while a smaller motor is only powerful enough at 5,000 RPM to spin the prop at X RPM. Horizontal thrust from the prop translates into airspeed across the wing which translates into lift. So you climb faster with more thrust, which may be useful for quickly getting to a safe height after takeoff. But this is where I stop seeing the advantages. A bigger motor means worse fuel economy, so to maintain level flight a 130cc motor will use less petrol (like 33% less) than the 200cc motor when you have the wing, chassis, and pilot constant. I guess a different gear ratio can mean the larger motor can spin slower than the smaller one to maintain the same propeller RPMs for level flight, but it seems that overall the bigger engines simply use more petrol than the smaller ones no matter what. Maximum fuel economy is getting the smallest engine you can find and driving the biggest propeller you can get, right? The rest of the flight characteristics are governed by the wing, not the motor. A more powerful motor wouldn't necessarily even translate into a longer lifespan, right? So unless you're flying tandem, are heavy, or fly at high altitudes, is there really a reason to go for a 200cc motor? Seems like compared to a Top 80 or something that motor will simply use more petrol. Quote Link to comment Share on other sites More sharing options...
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