Dec 31, 2012, 11:47 AM  
Joined Jun 2012
811 Posts

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How big do you think the drop in speed will be though? This is interesting. I know this is also very airframe dependant, if it helps, the plane right now reaches about 76% of pitch speed on 12x8 4s, if that helps. So how much higher would the % of pitch speed be on 12x6 I wander? Is it a lot? Or is it just a few %. Anyone have any ideas? 

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Dec 31, 2012, 12:14 PM  
Joined Jun 2012
811 Posts

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I'm asking what the diff is in zero thrust speed between these two setups at same throttle settings (and therefore the same geometric pitch speeds). I don't know if that's the question you answered, sorry if I'm a bit dim at the moment 

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Dec 31, 2012, 12:46 PM  
Toronto Canada
Joined Dec 2002
5,369 Posts

At max throttle on 3 cells at Zero Thrust Speed the motor RPM would be a little less than 11.1 X Kv
At max throttle on 4 cells at Zero Thrust Speed the motor RPM would be a little less than 14.8 X Kv Zero Thrust Speed with the 12X8 on 3S would be proportional to 8 x 11.1 X Kv. = 88.8Kv Zero Thrust Speed with the 12X6 on 4S would be proportional to 6 x 14.8 X Kv. = 88.8Kv In both cases the Zero Thrust Speed would be approximately the same. 
Dec 31, 2012, 08:37 PM  

What propellers (brand and type), motor (weight and kv) and batteries (mAh) are you looking at ?
What's your plane looking like ? AUW ? wing area ? wing span ? does it has landing gears, FPV camera, antennas... ? is it a classical plane or a bit stranger one ? picture ? Can you describe its mission ? Do you have an external BEC ? sorry, that's a lot of questions ! but for good answers, we need good inputs !! 
Dec 31, 2012, 09:08 PM  
Joined Jun 2012
811 Posts

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I know it's perhaps boring but once again I am talking about a configuration of FPV easyglider (although I was hoping the discussion could extend to other similar planes and setups, for the sake of future use and other people who might be interested). Props are aeronaut cam carbons, on 62mm hub, batts are 4000 mah, possibly 5000 if I go 3s, motor is 840kv 174grams (sk3 35 48 840). It carries FPV gear and its all up weight is around 1.5kg, one dipole antenna recessed in tail with about 5cm exposed, and one 2.4ghz cloverleaf up the front. No landing gear. Not sure why it matters, but It has 2 external BECs, one internal, one BEC converted into a pi filter, and one dual PSU (lol electronics ), all running various stuff. Mission is semi long range fairly stable FPV system with strong ability to cut through wind via decent weight and above average top speed and thrust. The wingspan is 1.8m with a 645 sq inch wing area according to the mpx manual. 

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Dec 31, 2012, 09:09 PM  

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Larry 

Dec 31, 2012, 09:18 PM  
Toronto Canada
Joined Dec 2002
5,369 Posts

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No air is being ejected, they are both at zero thrust. They are not at 0 degrees AoA. Zero degrees occurs at Pitch Speed. At Zero Thrust Speed the average AoA is about 5 degrees. Flat bottomed airfoils (propellers) still produce lift at negative angles of attack. 

Jan 01, 2013, 02:57 AM  
Joined Jun 2012
811 Posts

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Jan 01, 2013, 04:41 AM  

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Increase of motor KV or LiPo cell count  makes for significant AMPS increase for what I determine less efficient speed change. Change of PITCH has a benefit when reducing  to allow you to run a higher KV or cell count pack and keep AMPS down ... Change of DIAMETER also helps  BUT can result in mushy model due to lack of thrust from the now reduced prop 'disc'. Personallly a combination of change of LiPo cell count allied to pitch change can produce very good results ... you could drop from a 3S and up 1 in pitch, that gives you still good max, but lower power draw... you could stay with 4S and significantly reduce pitch, but then you get all scream and rpm if speed reduction is desired. Lets take the 540 Racer I have ... stock is 7x6 and 4S ... she is quick and responsive, drawing 32A. I also fly her on 3S with same prop drawing 22A .... she is still quick, matginally less but response is still there ... I could up to a 7x7 .. if I could find one ! and keep the lower AMPS draw .. probably 25A It's far more than just what prop ... there's a lot of factors come into play ... Just my tuppence worth ... Nigel 

Jan 01, 2013, 08:01 AM  

Oh, I was asking about the BEC just because I haven't an external BEC and I've just noticed few days ago that, with the data logger and an extra cam, my ESC was heating up to 50°C (and probably more) on 4S, and only around a steady 25°C on 3S, though the amps were about the same.
I remember we already talk about your easyglider, and your props with this large hub that makes my prop data (well, not really mine in fact !) a bit useless (it changes the twist distribution of the blades along the radius). Anyway, I guess it's true that the 12x8 on 3S and the 12x6 on 4S should have nearly the same zero thrust speed. But, since the plane's drag isn't zero, the 12x6 should lead to a faster plane because the slope of the thrust vs airspeed is steeper, the 12x6 giving more thrust at lower speeds. Something like 3 or 4 mph for a top speed is the 60 mph range (just a picture, can't be sure about the real values). This also mean a better acceleration and climb rate for the 12x6. On the other hand, the 12x6 should have a significantly lower peak efficiency, this can lead to a better autonomy with the 3S and the 12x8 though this setup has less Wh (11.1*5 = 55.5 Wh ; 14.8*4 = 59.2 Wh), and so less battery weight. For example, I can compute those values for a 35 mph cruise speed: 4S4000mAh  12x8  54% throttle : Power In = 93W  autonomy = 36 min (optimistic) 4S4000mAh  12x6  62% throttle : Power In = 109W  autonomy = 31 min (optimistic) 3S5000mAh  12x8  67% throttle : Power In = 86W  autonomy = 37 min (optimistic) You may have noticed that the available thrust at launch is already huge with the 12x8 on 3S (around 1700g ?). Also I compute a climb rate around 9 m/s, so probably 7 or 8 m/s in real conditions, that's huge for a FPV plane ! This is just theory of course, but should be representative of what one can see when lowering speed by reducing pitch or applied voltage. ImagesView all Images in thread

Jan 01, 2013, 08:46 AM  
Joined Jun 2012
811 Posts

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This comment in post #5 by Bruce Abbot is the one thing making me wander right now though "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'm wandering if he is saying that the 12x8 actually has a slightly higher zero thrust speed which means, on a very low drag plane, it could go a bit faster. My plane isn't really a very low drag plane, but it's still something I am interested in. Quote:
One interesting thing is that the 4s 12x8 consumed more power than the 3s 12x8 at the same speed  why is this? Is this simulation including efficiency in the ESC or similar? I'd love to hear more about this as that was the kind of thing I made the thread for. Quote:
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Thanks for your post! 

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Jan 01, 2013, 08:59 AM  

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Pitch speed is the difference between actual airspeed and the speed that the prop would pull the model through the air if it was 'solid' and the prop was 'screwing' through it. As Martyn McKinney pointed out, most props have a profile with positive camber (eg. Clark Y), so they still produce some thrust even at their nominal 'pitch speed'. However the situation is a bit more complicated than that, because most props do not have a truly helical blade shape right to the root. The results is that the middle of the prop runs into 'zero thrust' speed sooner, and becomes ineffective at high speed. Another confounding factor is that different props have different blade profiles, and actual pitch may not match the designation. For example, most GWS props have significantly lower measured pitch than APC props (eg, the GWS EP1060 is about the same pitch as an APC 10x5E). What this means in practice is that small theoretical differences between props of different pitch may be swamped by undocumented differences in blade shape and actual pitch (an infamous example was the EMP 6x4 vs 6x5, which I measured at 4.6" and 3.6" pitch respectively!) Quote:
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According to my calculations, both setups should produce identical top speed. The 12x8 on 50003S should net a slightly longer cruise time, but the 12x6.5 on 40004S has a 50% better climb rate. Both options should provide excellent performance with good prop efficiency, but at cruise the motor is underloaded and loosing efficiency. However this is all theoretical. To find out which option is really the best, you may actually have to test both setups! It might also be worthwhile trying other combinations, eg. 11x6 on 4S. 

Jan 01, 2013, 09:53 AM  

The exact amount of thrust each setup will produce depends also of the rpm, of course since things are never ideal there can be an offset. So Bruce can be right if the 12x8 thrust curve crosses the 12x6.5 (oh yes !!) curve above the drag curve. Difficult to real know about this point.
My calcs do include some part throttle effects, I can't really explain the theoretical background, just that it works fine with my own setups when predicting part throttle amps. As far as I know, the APC thin electric series are the only ones (that I know) that has a twist distribution matching almost exactly the ideal twist according to the pitch = the pitch annouced is the right one ! Bruce, the graphs from the UIUC database are not corrected for the mounting fixture drag, see here: http://www.ae.illinois.edu/mselig/p...correction.pdf J at peak eff is then a bit higher. Also others, in other wind tunnel, has a slightly different result for this prop (APC 11x7E), which shows peak eff at around J=0.57 ! Prop efficiency is very sensitive to measurement offsets (torque, thrust and speed). 
Jan 01, 2013, 10:03 AM  
Joined Jun 2012
811 Posts

Hi Bruce,
Thanks for telling me about the term "Efflux velocity", I will try to keep using it in future where it is appropriate . Am I right in saying it is simply the difference between the free speed velocity and the speed of the ejected air? I.e. "the amount by which the prop is accelerating the air"? Quote:
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I really like what this thread has taught me  the 12x8 has actually got significantly higher efficiency than I thought it would, enough to potentially have a longer cruise despite slightly less energy in the battery (and therefore a lighter plane too, which can be nice), it would be better still if I found an equal weight/equal energy battery. Yet the 12x6 has significantly more pulling power (although I already knew this)  although I don't know which I will choose (cost and convenience certainly come into it), I definitely feel like this thread has armed me well to make the decision  so I want to thank everyone that contributed Which isn't to say the thread is over, I'm sure there is more that can be discussed, and would be happy to hear it 

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