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Old Dec 31, 2012, 11:47 AM
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Originally Posted by Lnagel View Post
Apparently you missed the whole point of my post. Using the same battery and changing the pitch from 8" to 6" you will see a slight change in performance. Maybe better, maybe worse depending upon your desires and the airframe. Staying with the 8" pitch and dropping a cell in the battery, you will see a dramatic change in performance, most likely for the worst. You will have dramatically less speed, acceleration, and vertical.

Your choice.

Larry
Although the advantages to 12x6 4s in accel and vertical are are quite clear to me, those areas are actually quite irrelevant to this plane since it has more than enough performance in those areas regardless of prop/cell count combination.

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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Old Dec 31, 2012, 11:51 AM
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Originally Posted by Nereth View Post
I'm interested to hear more about differences in zero thrust speed if you're aware of them.
At Zero Thrust Speed the current drawn by the motor is limited to the no load current Io plus the load presented by the profile drag of the prop.

This means that in both cases that the Zero Thrust RPM would be approximately equal to but slightly less than the Applied Voltage X Kv
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Old Dec 31, 2012, 12:14 PM
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Originally Posted by Martyn McKinney View Post
At Zero Thrust Speed the current drawn by the motor is limited to the no load current Io plus the load presented by the profile drag of the prop.

This means that in both cases that the Zero Thrust RPM would be approximately equal to but slightly less than the Applied Voltage X Kv
I don't think I understand what you are saying here?

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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Old Dec 31, 2012, 12:46 PM
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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.
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Old Dec 31, 2012, 08:37 PM
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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 !!
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Old Dec 31, 2012, 09:08 PM
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Originally Posted by Martyn McKinney View Post
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.
Correct me if I'm wrong, but aren't you basically saying that if the plane is travelling at zero thrust speed (which is impossible due to drag, but that's another issue), then the props would be spinning just below Kv X V due to induced drag? I'm not sure if that tells us anything useful? But my question is, are the zero thrust speeds the same? Does the aerofoil situation perhaps make one of them eject air faster despite both being at 0 AOA, due to its higher speed?

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Originally Posted by yomgui View Post
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 !!
Hi Yomgui, happy to see you in one of my threads again

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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Old Dec 31, 2012, 09:09 PM
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Originally Posted by Nereth View Post
Although the advantages to 12x6 4s in accel and vertical are are quite clear to me, those areas are actually quite irrelevant to this plane since it has more than enough performance in those areas regardless of prop/cell count combination.

How big do you think the drop in speed will be though?
It's easy enough to find out. Just fly it at 75% throttle with the 4s battery. That's approximately what the maximum full throttle power level will be with the 3s battery. If you normally fly at 75% throttle or less and the airplane is capable of taking off at 75% throttle with the 4s battery then you will have no problem with a 3s battery. Efficiency wise it won't make much difference. It takes X amount of power to fly a plane at any given airspeed regardless of whether it is a 3s battery or a 4s battery that is supplying that power.

Larry
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Old Dec 31, 2012, 09:18 PM
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Originally Posted by Nereth View Post
But my question is, are the zero thrust speeds the same? Does the aerofoil situation perhaps make one of them eject air faster despite both being at 0 AOA, due to its higher speed?.
As I stated in a previous post, the Zero Thrust Speeds are approximately the same.

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.
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Old Jan 01, 2013, 02:57 AM
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Originally Posted by Lnagel View Post
It's easy enough to find out. Just fly it at 75% throttle with the 4s battery. That's approximately what the maximum full throttle power level will be with the 3s battery. If you normally fly at 75% throttle or less and the airplane is capable of taking off at 75% throttle with the 4s battery then you will have no problem with a 3s battery. Efficiency wise it won't make much difference. It takes X amount of power to fly a plane at any given airspeed regardless of whether it is a 3s battery or a 4s battery that is supplying that power.

Larry
That tells me what it will do on 12x8 3s but doesn't tell me how fast it will go on 12x6 3s, which is a possible interesting difference that some people have alluded to, although it is starting to sound like it is a difference that doesn't actually exist based on what I am understanding from Martyn McKinney's post..
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Old Jan 01, 2013, 04:41 AM
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Originally Posted by Nereth View Post
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?
First you say lower max speed ... then go to say same speed ? I assume you want us to consider 'cruise speed' rather than all out max ?

Quote:
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.
ESC and battery are hard at work ... but motor is in its upper regions .. electrics are more efficient, so my teachers told me when allowed to approach design speed .. load up too high and power demand gets ridiculous.

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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.
Yeh ... OK ... I'll leave that for the armchair experts !!

Quote:
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 .
mmmmmmmmm I live in a speed desire world ... a lot of my models I push to limits to extract as much speed as possible.

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
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Old Jan 01, 2013, 08:01 AM
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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 50C (and probably more) on 4S, and only around a steady 25C 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.
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Old Jan 01, 2013, 08:46 AM
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Originally Posted by yomgui View Post
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).
That is actually precisely what I was thinking when I made this thread, but ofcourse I wanted others opinions.

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.

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Originally Posted by yomgui View Post
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)
These numbers are great! It is actually a bigger difference in efficiency than I thought - I know they are only indicative, but they indicate a bigger difference than expected

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.

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Originally Posted by yomgui View Post
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 !
The plane is truly insane in real life I can hand launch it while wearing goggles wired to a ground station (so I can't allow my head or lower body to move for fear of goggles falling off, so the hand launch is quite weak) on less than half throttle (~11 amps) and it climbs nicely out. At about 4.5-5 amps it can still slowly climb. I didn't check the minimum current draw it can achieve without losing altitude, but it is probably impressively low These figures are the reason I am not worried about having too little power on 12x8 3s. 75% throttle on 4s is more than I need for anything, so 3s isn't an issue

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Originally Posted by yomgui View Post
This is just theory of course, but should be representative of what one can see when lowering speed by reducing pitch or applied voltage.
Precisely We both know these numbers are not exact to the real world, but give excellent indications of how changes effect the system when they are compared to other numbers computed in the same way, and that is why they are so useful.

Thanks for your post!
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Old Jan 01, 2013, 08:59 AM
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Originally Posted by Nereth View Post
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 speed at which air is ejected is called the Efflux Velocity. It is similar to (but not necessarily the same as) the difference between actual airspeed and 'zero thrust' speed.

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:
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?
Higher pitched props generally have higher efficiency and produce a bit more thrust in the mid to upper range, and a slick airframe with low drag will get into this region. However a draggy airframe will not permit the aircraft to go as fast, so the prop's performance close to pitch speed is less relevant.

Quote:
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 )
Unfortunately there is no 'precisely' about it, as even different props purporting to be of the same type may have divergent characteristics. I don't have any wind tunnel data for 12x8 props, but the APC 11x7E (which has a similar pitch/diameter ratio) achieves peak efficiency at an advance ratio of about 0.45, corresponding to 71% of (nominal) pitch/diameter.

Quote:
I am talking about a configuration of FPV easyglider...

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,

No landing gear.

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.
First issue is that Aeronaut don't make a 12x6 CAM carbon folder, only a 12x6.5. That 0.5" might make difference.

According to my calculations, both setups should produce identical top speed. The 12x8 on 5000-3S should net a slightly longer cruise time, but the 12x6.5 on 4000-4S has a 50% better climb rate. Both options should provide excellent performance with good prop efficiency, but at cruise the motor is under-loaded 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.
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Old Jan 01, 2013, 09:53 AM
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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/m-selig/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).
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Old Jan 01, 2013, 10:03 AM
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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"?

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Originally Posted by Bruce Abbott View Post
Higher pitched props generally have higher efficiency and produce a bit more thrust in the mid to upper range, and a slick airframe with low drag will get into this region. However a draggy airframe will not permit the aircraft to go as fast, so the prop's performance close to pitch speed is less relevant.
Oh I see! So rather than saying the Efflux velocity (correct usage? ) of the 12x8 system is higher, you were saying that it is more efficient, and if it didn't need to pull a heavy plane around (which is what the high speed, low pitch 12x6 is for), it would potentially capitalize on that. I wander how slick a plane would have to be to allow for that advantage to shine!

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Originally Posted by Bruce Abbott View Post
Unfortunately there is no 'precisely' about it, as even different props purporting to be of the same type may have divergent characteristics. I don't have any wind tunnel data for 12x8 props, but the APC 11x7E (which has a similar pitch/diameter ratio) achieves peak efficiency at an advance ratio of about 0.45, corresponding to 71% of (nominal) pitch/diameter.
Hmm, it seems what I accepted as true based on what I percieved to be common consensus (never a good reason to assume truth) may not be so true at all - I'll have to look into this further.

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Originally Posted by Bruce Abbott View Post
First issue is that Aeronaut don't make a 12x6 CAM carbon folder, only a 12x6.5. That 0.5" might make difference.
Hahaha, yes touche', I didn't think of that. Still the discussion remains interesting as it applies to any other combination of props and so on where this could come up.

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Originally Posted by Bruce Abbott View Post
According to my calculations, both setups should produce identical top speed. The 12x8 on 5000-3S should net a slightly longer cruise time, but the 12x6.5 on 4000-4S has a 50% better climb rate. Both options should provide excellent performance with good prop efficiency, but at cruise the motor is under-loaded 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.
Yes motor being underloaded at cruise is not news to me Max throttle and steep climb into a slow dive and repeat is the most efficient way to fly this plane IMO (unfortunately it is structurally lethal to the plane on 12x8 4s as it can't take the stresses generated) - alternately simply a higher speed cruise works nicely as well (doesn't cover as much distance but it sure feels satisfying to peel away during formation FPV flight ), or some mix of the two.

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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