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Old Dec 30, 2012, 05:53 PM
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Hastings, New Zealand
Joined Jan 2001
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Originally Posted by Nereth View Post
It's efficiency of the whole system, battery, wires, ESC, motor, prop - which should roughly equate to efficiency of the flight I assume?
You forgot the airframe.

Without knowing which motor, props and airframe are being considered, we cannot say for sure which combination will be best. However I have found that in general, a lower pitch prop at higher voltage produces more thrust and acceleration for takeoff and aerobatics, but a higher pitched prop at lower voltage cruises more efficiently.

Well, the added power of the 12x6 will mostly be relevant during acceleration, and it should slightly reduce the difference in speed between the thrust speed and the plane speed.
What is 'thrust speed'?

The 12x6 on 4S will produce more thrust at low speed, but the 12x8 on 3S will produce more thrust at high speed. If the airframe is draggy then the 12x6 will win, but if it is slick then the 12x8 will win.

If both were at full throttle the 12x6 I believe would go slightly, slightly faster in steady state. It would also get to that speed faster.
Not necessarily. The 12x8 has slightly higher pitch speed and its in-flight thrust drops away slower as airspeed increases, so it should go a bit faster (assuming the airframe permits it).

But what about at the same speed? Which would be drawing more current to go at X km/h in steady state? I think that is an interesting question.
Again, this depends on the airframe. If the model is heavy with a small wing area then it will need to fly fast, and the 12x8 will be more efficient (the 12x6 will be working closer to pitch speed and therefore losing more power to blade drag), but if it is light with a large wing area then it can fly slower so the 12x6 may work better.

What about both at 50% throttle? Would that be appreciably different to the relationship when both are at 100%? Etc.
At 50% throttle the pitch speed is halved. Most props achieve best propulsive efficiency at about 70% of pitch speed. If you cut the throttle back to 50% then you should be cruising at 35% of full throttle (static) pitch speed, but you also want to be at the bottom of the model's 'drag bucket', ie. not too fast or too slow (induced drag rises dramatically as you approach stall speed).

At 50% throttle the theoretical pitch speed of both props is similar (the 12x8 having a slight edge), but the 12x6 is spinning about 30% faster so it will suffer more blade drag and waste more power as it approaches its (lower) pitch speed. If the model has a high stall speed then the 12x6 may not even be able to keep it in the air!
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