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Old Dec 30, 2012, 08:15 PM
Nereth is offline
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Quote:
Originally Posted by Bruce Abbott View Post
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.
It's difficult to adequately describe the airframe unfortunately, all I can really say is the plane can easily take off on both props at less than full throttle, and can double if not triple stall speed at full throttle.

Quote:
Originally Posted by Bruce Abbott View Post
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.
I am probably wrong, but I thought thrust speed was the term for the speed at which air is ejected from the spinning prop, i.e. it's the pitch speed plus some more to account for the prop blades aerofoil. Perhaps it is called the zero thrust speed, at which the prop can no longer make thrust?

The idea with these props and batteries is they both theoretically have the same pitch speed/thrust speed. Why would the 12x8 get it slightly faster in a slick airframe? Does it actually produce slightly more 'thrust speed' than the 12x6 due to aerofoil considerations?

Quote:
Originally Posted by Bruce Abbott View Post
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).
see above question, "Why would the 12x8 get it slightly faster in a slick airframe? Does it actually produce slightly more 'thrust speed' than the 12x6 due to aerofoil considerations?"

Quote:
Originally Posted by Bruce Abbott View Post
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.
I think this comes back once again to the afforementioned question?

Quote:
Originally Posted by Bruce Abbott View Post
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).
It was my understanding that props are most efficient at precisely their pitch speed, which is something like... 80% in turn of their thrust speed (a term I have no choice but to keep using until someone lets me know what the correct one is :P )

Quote:
Originally Posted by Bruce Abbott View Post
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!
Both props should be able to pull the plane at ~3x stall speed at full belt so that isn't an issue =D.

The note about the 12x6 having more blade drag is interesting, that's probably quite a significant effect, isn't it?
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