
Dec 30, 2012, 08:08 AM  
Joined Jun 2012
810 Posts

Discussion
Lowering pitch vs lowering voltage  differences in power system behaviour?
Hello,
Quite simply: If you have a 12x8 prop running on 4s and you want to lower the max speed, then you have two ostensibly similar options: 12x6 on 4s OR: 12x8 on 3s Which would be more energy efficient from the battery at the same speed? Additional info: At 12x8 on 4s, the ESC is near its limit, the battery is at 80% of its C rate, and the motor is ~at its practical current limit. I have more accurate numbers if necessary but I want to keep this discussion general if possible so it can be more useful to different people and situations in future. If more info is needed even to give a decent generalised response, please let me know. For completeness of discussion it would be good to discuss the parameters of either system at full throttle steady state, partial throttle steady state, and during acceleration/low speed where pitch becomes interesting. It might also be interesting to compare to 12x8 on 4s at 75% throttle. My own understanding: A few months ago I would have said 12x6 on 4s, but now it's somewhat up in the air due to what I learned in a previous thread regarding pitch and prop efficiency. I don't feel my understanding is strong enough ATM, so I will wait till I can think about it more and maybe read some responses before I contribute too much personally . 
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Dec 30, 2012, 11:05 AM  
Joined Jun 2012
810 Posts

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This is interesting and worth exploring (but I think you have covered it in almost as much detail as you can with so little information already). But what about in a situation of same speed, or both at full throttle? 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. 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. What about both at 50% throttle? Would that be appreciably different to the relationship when both are at 100%? Etc. 

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Dec 30, 2012, 08:15 PM  
Joined Jun 2012
810 Posts

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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:
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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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Dec 31, 2012, 05:20 AM  

Considering the things you cannot change, the plane and motor, (or do not want to change, other than prop and battery), the only way to really determine the best 'efficiency', is to fly the model with different props and different Lipo packs, (number of cells).
Then determine from the flight results, duration, speed, rate of climb, etc., which was the best combination to meet your requirements. I don't think there is a way of coming up with an definitive answer here, other than try and see. The plane itself is a large part of the overall 'efficiency' of the system, choosing the right one to fit the flight parameters is only the start. Well actually defining the flight parameters is the start, then choosing all the components to meet those parameter. Even then, flight testing will always be the final proof of meeting or failing them, and of determining if any changes are required. Starting with what may be equipment that may not be suitable will always end in compromise. 
Dec 31, 2012, 06:26 AM  
Joined Jun 2012
810 Posts

Quote:
Even though my plane has an OSD and a bunch of measuring equipment on board that gives me a decent advantage for example, my GPS based ground speed measurement is neither accurate enough, nor can it account for wind, for me to get the same speed twice in a row with two different setups (full throttle will not lead to the same speed with both props almost certainly). There are other limitations as well. I have an accurate altimeter for flying level but cannot account for thermals helping my plane out. I won't bother continuing to list issues Meanwhile, Bruce Abbot for example has started to touch on some interesting stuff, I think for example that he was saying that aerofoil based lift from the props gives the 12x8 prop a significantly higher (zero?) thrust speed which is a very important issue. Going into more detail here could be fruitful. Profile drag from the prop at higher speed impacting efficiency and so on are also both interesting points to bring up. I really don't think it is worth throwing away theory in this case just because we cannot guarantee it's correctness in one particular case. If you cannot find the truth in a specific case due to lack of information, it is still worth it to cover what you can so that when more information can be found, or reasonable assumptions can be made, the theory can become useful. I think I annoy a few people on these forums with these threads they don't see the point in, but I have actually learned a lot from every thread I have made so far, and the last paragraph is just a strong opinion I hold that I would like to share so that people can understand where I am coming from in future 

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Dec 31, 2012, 08:15 AM  
Joined Jun 2012
810 Posts

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The planes requirements will likely be met with either option, I'm just trying to figure out which one will most likely meet it better, since I will only be purchasing either a lower cell count battery or a lower pitch prop for now, rather than purchasing both to be able to test. 

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Dec 31, 2012, 11:03 AM  
Toronto Canada
Joined Dec 2002
5,328 Posts

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Props are designed to have the best liftdrag ratios at Geometric Pitch Speed (RPM X Pitch). Props might develop their best efficiency at 70% of VIRTUAL PITCH SPEED or Zero Thrust Speed which is 20 to 40% higher than Geometric Pitch Speed depending on the P/D ratio. Quote:
Doesn't this imply that the power dissipation in both cases is the same and if the motor were being operated in the flat portion of its efficiency curve that the power requirement would be approximately the same? 

Dec 31, 2012, 11:15 AM  
Joined Jun 2012
810 Posts

Quote:
Also I feel like one or the other prop (not sure which) would get the plane going slightly faster at a given throttle setting due to being able to overcome the planes drag with slightly less difference between plane speed and zero thrust speed, or having a higher zero thrust speed. Mind you, 'same speed' is probably a more important comparison than "same throttle" for partial throttle cruise situations (which is a lot of what a lot of planes do). 

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Dec 31, 2012, 11:40 AM  

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Your choice. Larry 

Dec 31, 2012, 11:42 AM  
Joined Jun 2012
810 Posts

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Bruce seemed to be indicating that the 12x8 would have a higher zero thrust speed, does that sound right to you? They should both have more or less the same geometric pitch speed, I'm interested to hear more about differences in zero thrust speed if you're aware of them. 

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