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Old Jan 01, 2013, 04:19 PM
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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 ?).
I am curious how the 12X8 on 4S and the 12X6 on 4 cells can have almost the same static thrust.

Is it the efficiency of the motor at static current levels?
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Old Jan 02, 2013, 11:25 AM
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In fact it's because the rpm are a bit higher with the 12x6 that the calc finds a little more thrust with it.

Static thrust coeff are almost the same for both props.

The 12x8 being from direct measurements.

The 12x6 being from direct measurements of a 11x6.5, scaled to 12 inch diameter and twisted to 6 inch pitch, so static (and dynamic, but static is the worst case) thrust is certainly not very very accurate.

Anyway, the 12x6 doesn't exist...
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Old Jan 02, 2013, 03:41 PM
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Originally Posted by yomgui View Post
Anyway, the 12x6 doesn't exist...
Hi
There is one APC 12x6E (ref LP12060E)
There is also an "APC like" PEE-1206
and maybe other references
Louis
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Old Jan 02, 2013, 03:51 PM
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Hi
There is one APC 12x6E (ref LP12060E)
There is also an "APC like" PEE-1206
and maybe other references
Louis
I think he means an Aeronaut CAM Carbon 12x6 doesn't exist.
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Old Jan 04, 2013, 11:30 AM
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I got the 12x6.5 for a flight and just tried it out. Main reasons for choosing 12x6.5 was cost, and a mistake in interpretting the sizes of the 3s batteries available, and a desire to keep using a 4s system since it gives me more options in future without overheating the motor as a 3s might at similar power levels.

Went out to a farm for a day trip of flying (picture attached! =D although it comes from a faulty antenna, not the main flight one), and tested 12x6.5 4s in flight, and 12x6.5 3s ~static thrust.

The static thrust on 12x6.5 (not 12x8) 3s was still around 1.2:1 according to my friend holding the plane vertically, which puts it at around 1.7-1.8kg.

On 12x6.5 4s it was more like 2:1 apparently, which means a touch over 3kg. Static current draw on 12x6.6 on fresh battery out of the box was 60.0 amps (it's about 75 amps or so on 12x8 4s).

Both of these are obviously really damned easy to take off with.

on 12x6 4s, I feel like I did notice the loss of efficiency when trying to match previous current draws with new, *but* I think this was due to extremely innacurate seat-of-the-pants-dynamometering, since the wind speed was really very high, and the camera was pointed too far up to see much of the horizon, so I couldn't develop much AOA without losing the horizon (which I don't like doing), nor could I tell how much excess air speed I had because it was damned windy, so I think most of my attempts at climbing just involved scooting around in near-level flight at high speed. I will have to try again on a calm day with the camera pointing slightly further downwards.

Meanwhile, Yomgui, you helped me with some simmed out stuff before in this thread, I was hoping you could help again =D, I know you simmed out a 12x6.5 prop at cruise speed for me before, if it is only a matter of changing some quick numbers, I would be very thankful if you could do the same sim with a 11x6, so I can see what the change in efficiency is for lowering the blade diameter from where it is now. I know you made a 'virtual' 12x6 prop for me before since it doesn't exist in your database (although I hear AN has recently released one!), could you make either a virtual 12x6 or 11x6.5 as well so I can compare two same pitch, different diameter props around this operating point at cruise? I hope this isn't asking too much!

I am wandering about this because, although I was flying pretty docilely, the motor and ESC came down a little warm (I could hold my finger on them, but they were probably in the 45-50 range I guess as there was a little sting to it). It's probably due to poor cooling airflow (which I will be dealing with soon), but I want to be able to comfortably fly a bit more aggressively, so I'm thinking about depropping slightly more (not sure it will help though since I think the plane will go almost the same speed and thus pull just as much current anyway). As an alternative to depropping a bit, I am also considering trying my hand at rewinding to drop the Kv of the motor by 5-10%. Could be a fun experience.

Thanks for everyones help!
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Old Jan 04, 2013, 02:45 PM
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Hi Nereth,

Well, I guess you find that the 11x6.5 will simply produce less thrust at any speed. About efficiency you can't be sure of anything, the best is to try ! (how much power at given speed...).

It's not that I don't want to do it, it's just that I think I can't have significant results here, so I'd rather not give you wrong ideas...

Too many parameters are unknown, and this is probably because of the props and the hub. I've understood that you are using a large hub (62mm?), larger than the one used for the testings from which I have the data (42 mm... I guess !). This can be seen by the bigger amps drawn you have. Moreover, the data I have for aeronaut folding props suggests that the nominal pitch isn't correct, so it's impossible to accuratly estimate the performance of a prop that isn't in the database. Even for APC thin electric it's not very very very accurate though they should have a correct twist distribution.

Also I don't have direct data of your motor...

Well... here's the link to my calc if you wish to play with it:

http://aerotrash.over-blog.com/pages...r-6330077.html

I know it's a bit uneasy to use... but very accurate with good inputs !
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Old Jan 04, 2013, 08:16 PM
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Which would be more energy efficient from the battery at the same speed?

a 10x8 prop.

its a glider. you won't need acceleration provided by a 12x8 prop. probably just add more drag and higher amps if you kept the 12x8 prop.
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Old Jan 05, 2013, 01:02 AM
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Originally Posted by yomgui View Post
Hi Nereth,

Well, I guess you find that the 11x6.5 will simply produce less thrust at any speed. About efficiency you can't be sure of anything, the best is to try ! (how much power at given speed...).

It's not that I don't want to do it, it's just that I think I can't have significant results here, so I'd rather not give you wrong ideas...

Too many parameters are unknown, and this is probably because of the props and the hub. I've understood that you are using a large hub (62mm?), larger than the one used for the testings from which I have the data (42 mm... I guess !). This can be seen by the bigger amps drawn you have. Moreover, the data I have for aeronaut folding props suggests that the nominal pitch isn't correct, so it's impossible to accuratly estimate the performance of a prop that isn't in the database. Even for APC thin electric it's not very very very accurate though they should have a correct twist distribution.

Also I don't have direct data of your motor...

Well... here's the link to my calc if you wish to play with it:

http://aerotrash.over-blog.com/pages...r-6330077.html

I know it's a bit uneasy to use... but very accurate with good inputs !
Fair enough, I myself am not certain about the idea of lowering the diameter - it only really effects current draw during acceleration and heavy climbs, which are two things I don't do much of. I really think my heat issue is related to lack of cooling due to the amount of electronics inside, since I am within spec of the parts (both hobbyking spec, and a more 'common sense' spec), especially with the way I fly.

It's unfortunate that the easyglider has such a fat nose, and that AN doesn't make a 57 mm hub, so their 55mm hub requires a 62mm yoke.

Quote:
Originally Posted by paulatgis View Post
Which would be more energy efficient from the battery at the same speed?

a 10x8 prop.

its a glider. you won't need acceleration provided by a 12x8 prop. probably just add more drag and higher amps if you kept the 12x8 prop.
I was under the impression that all other things being the same, larger props were equally or more efficient. It's a folding prop so drag isn't an issue.
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Old Jan 05, 2013, 04:46 AM
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Well, that's not so simple, as said before higher P/D props are more efficient (up to a point) than lower P/D props of the same D. But also prop efficiency is efficiency at a given advance ratio, meaning that efficiency depends of the rpm and incoming airflow speed. Basically the efficiency of the prop changes from cruise to climb, etc.

I agree with paulatgis that your easyglider looks like a rocket with this setup (and I'm probably underestimating it's power !).

Vertical climb with the 12x8 and fresh batteries no ? 15 to 20 m/s of climb rate maybe. My FPV plane climbs gently at around 4 m/s ! and I have absolutly no trouble with hand launch though the wing loading is much higher than yours (46 g/dmē vs 36 g/dmē).

All of the props you are looking at should be ok, pehaps not the best available, also the big hub is giving an offset in the twist distribution. But you may be cruising at so very low throttle level (you should be able to fly with less than 40 % of WOT ?) that it's probably the main noticeable source of losses. At 40 % throttle the motor+ESC efficiency may drop, let say, down to 50 % ! and maybe 30 ~ 40 % of those 50 % are lost by the prop... ouch !
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Old Jan 05, 2013, 06:05 AM
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Originally Posted by yomgui View Post
Well, that's not so simple, as said before higher P/D props are more efficient (up to a point) than lower P/D props of the same D. But also prop efficiency is efficiency at a given advance ratio, meaning that efficiency depends of the rpm and incoming airflow speed. Basically the efficiency of the prop changes from cruise to climb, etc.

I agree with paulatgis that your easyglider looks like a rocket with this setup (and I'm probably underestimating it's power !).

Vertical climb with the 12x8 and fresh batteries no ? 15 to 20 m/s of climb rate maybe. My FPV plane climbs gently at around 4 m/s ! and I have absolutly no trouble with hand launch though the wing loading is much higher than yours (46 g/dmē vs 36 g/dmē).

All of the props you are looking at should be ok, pehaps not the best available, also the big hub is giving an offset in the twist distribution. But you may be cruising at so very low throttle level (you should be able to fly with less than 40 % of WOT ?) that it's probably the main noticeable source of losses. At 40 % throttle the motor+ESC efficiency may drop, let say, down to 50 % ! and maybe 30 ~ 40 % of those 50 % are lost by the prop... ouch !
on the original (12x8 4s) setup it could gently climb on 4.5 amps, and yes it launches on about 45% throttle.

I know that the P/D ratio reduction reduction reduces efficiency, but I thought adding diameter increased it - It's really a matter of which one is the bigger effect, increase in P/D as you drop diameter, or reduction of diameter as you... drop diameter.

Ultimately I think the best plan is to get a lower KV motor, probably through a rewind (which I have to learn to do first). I don't think HK sells a motor of similar cost/performance and size (It's very large for an easyglider, but due to the added gear, it actually balances very nicely, it would be difficult to balance with a smaller motor), at a slightly lower kv.

Edit:

Actually, if I don't rewind, this might be the best solution to my problem:

http://www.hobbyking.com/hobbyking/s...ner_Motor.html

But I wander how lossy it will be.

3542 has some reasonable motors but I think even in the full forward battery position, the plane will be tail heavy with one.
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Old Jan 05, 2013, 09:38 AM
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I know that the P/D ratio reduction reduction reduces efficiency, but I thought adding diameter increased it - It's really a matter of which one is the bigger effect, increase in P/D as you drop diameter, or reduction of diameter as you... drop diameter.

.
PROPELLER efficency is determined primarily by the P/D ratio.

The airframe SYSTEM efficiency might be improved by using a large diameter prop, but increasing the prop diameter does not increase PROPELLER efficiency.

Imagine how efficient the propellers on a TWA Super Constellation would be if this were true.
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Old Jan 05, 2013, 12:48 PM
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PROPELLER efficency is determined primarily by the P/D ratio.

The airframe SYSTEM efficiency might be improved by using a large diameter prop, but increasing the prop diameter does not increase PROPELLER efficiency.

Imagine how efficient the propellers on a TWA Super Constellation would be if this were true.
Oh, I didn't realise this :O, I guess I didn't correctly interpret the graphs I saw in a previous thread. But then, why is the general advice when choosing a prop (before the choice of motor and cell count) to go for 'the largest prop that will fit on your plane' (barring undesireable torque effects and clearance issues). Shouldn't it be something closer to "the smallest prop that will fit your plane and give you enough thrust to take off and fly", since such a prop could have an impressive P/D ratio?
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Old Jan 05, 2013, 02:09 PM
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A propeller should fit the airframe and flight envelope well, it's not a matter of the biggest pitch or the biggest diameter.

Efficiency is indeed a tricky term. One should make the difference between the efficiency of a propeller, which is not a constant, and it's peak efficiency, which can slightly vary with rpm (in the same way that reynolds number affects airfoils performance) but basically happens at a given ratio of incoming airspeed and rpm (for a given prop).

Up to square props (more or less), and with diameter remaining the same, the peak efficiency increases with P/D ratio (talking about RC props). But a low P/D ratio propeller can be more efficient than a larger P/D ratio one, just ask the 3D guys if a 10x10 is efficient... It's just because 3D planes are flying at low speed with high rpm, so advance ratio is low (J=v/nD), and at low advance ratio low P/D ratio props are more efficient than square props.

Large diameter props usually has a higher peak efficiency than smaller one of the same geometry (same P/D), but for only a small amount, rpm for example also can have a big impact on efficiency. A 5 inch prop as a lower peak efficiency than a Super Constellation propeller, I don't think Martyn will disagree on this ! But in the usual range of choice when propping a RC plane, diameter is not really the only factor that will give you an efficient propeller. Looking for the larger diameter that will not burn your motor can lead you to a none adapted propeller (not enough pitch).
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Old Jan 05, 2013, 10:56 PM
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So, my thoughts compiled while lying awake last night :

Since thrust increases by I think the square of RPM and cube (I believe) of diameter and efflux velocity increases linearly with RPM and pitch, then when lowering the Kv of a motor, you have to increase the pitch more than you have to increase the diameter to maintain the level of thrust and efflux velocity, which means the lower kv motor by necessity runs at a higher P/D, which for *most* cases with reasonable P/D props (e.g. between about 1:2 and around 1:1) should be an efficiency increase, at reasonable speeds, and with reasonable flying styles (i.e. not prop hangs).

Sound about right? This is similar to lowering voltage except it doesn't absolutely ruin your power level like going from 4s to 3s does.

And in my case I don't actually need a new prop to run at a lower kv because I'm happy to lose some thrust, and I was purposefully trying to dump speed.
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Old Jan 06, 2013, 05:14 AM
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For propellers of the exact same shape (8x4, 9x4.5, 10x5,...) idealy thrust increases with rpm at 2nd power and with diameter at 4th power, like in the formula for thrust :

T = rho.Ct.n^2.D^4

Indeed, if you wish to match the same slope of thrust vs airspeed when lowering kv, you'll need to increase both diameter and pitch. Probably the propeller will have a better efficiency. But the motor may suffer more... so in the end... who knows ? might be worst ! might be just the same... I guess in most of the case you'll only know after some flights.
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