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Old Dec 26, 2012, 07:55 AM
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Just for sharing purpose after experimenting with many motor +CS10 or CS12 fan setup. i finally flew my ARC/Exceed 70mm F-18 with the leopard 3400KV + CS10. the results were impressive... it flew with plently of thrust. Flight time was around 3mins 30 sec and setup for landing another 10-15 sec. Using 4S 4000mah 35C lipo. All in all , around 4mins.

It is more than 1:1 thrust, it can do all the 3D stuff without any lack of thrust... and WOT is around 90A+ but i hv put in a 150A ESC... lol.. Anyway most of the time, i am flying around half throttle and occassionally few speed passes...

Although i hv been flying jets with HET motors, i think Leopard motor is one of the best motor around which not many RC flyers knows.
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Old Dec 26, 2012, 08:15 AM
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Originally Posted by green_flyer View Post
sorry for more sidetracking. I didnt beleive it myself (that Thrust ~ rpm^2), so after a little searching on the web the MIT page is helpful in showing that Thrust is (amongst other things) proportional to rpm^2



http://web.mit.edu/16.unified/www/SP...es/node86.html

if you look further down the Power coefficient equation also confirms Andy's statement that Power is proportional to rpm^3
If you combine the two equations (substituting out rpm) Thrust is propotional to Power^2/3.
Profuse thanks GreenFlyer - you've simply made my day Faced with seemingly quite a few non-believers here, i was begining to think my abilities to derive stuff from first principles must be waning... maybe the onset of retirement, becoming a grandad or other rinklytastic factors creeping in?... Might i have been loosing it?

The relationships i recently derived for Thrust, Power, and Speed appear identical to the ones you have quoted in the MIT paper. Including the one where:
Power required = fn(sqrt[Thrust**3])

Incidently - i never said it was 'worse than that'... What i did say was something like:
What makes matters worse is that when you've produced your thrust - you're then up against a little matter of aircraft drag being a fn([airspeed]**2). (or) put another way: If you want to be a v. fast jet-jocky you'll have to throw oodles of power at it!

I've since been playing around some more with these equations and they lead to some pretty exciting discoveries... i believe the coming year will yield lots more engineering fun! However, i'm just now off to see my little grandaughter - that's even more fun than playing with algorithms

Again, many thanks GreenFlyer. (and incidently your contribution was certainly NOT sidetracking )

Wrinklies take heart...

Happy Boxing Day to all,

Andy
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Old Dec 26, 2012, 09:14 AM
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Originally Posted by PeterVRC View Post
You didn't mention how many C the battery was.
But the results sound great!
Just a 25C pack, I need to get some better packs but hobby parts is always out of stock!
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Old Dec 26, 2012, 09:45 PM
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The more I think of it, my numbers seem kind of low on 5s (1269w and 69 amps with a 2550kv motor), I am thinking the two factors which will help will be 1.) installling the fan system in the jet (amps/watts will increase) and 2.) Using some higher C rated batts.

Thoughts?
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Old Dec 26, 2012, 10:06 PM
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1269w / 69 Amps = 18.8V
Those numbers look good and right, but that is a 'good' voltage being maintained, which suggests good batteries were used. But 25C would really struggle at that, so it seems amiss in those terms. I would expect you would need 40C or better to do that.
Maybe those numbers are OK if it was JUST for the initial; WOT startup short seconds. (eg Peak numbers). I would expect 25C, under such a load, to drop Volts down to 18.0v or lower.

In-plane the numbers will pretty certainly drop. Seeing the ducting restriction will reduce power.

It is all still a good result, but check over that ongoing power and numbers, and monitor the battery temp as I would expect that to be getting hot too. If a long run gets it too hot, then you do need to move up in C.
I would think anyone using 2550kv 5S (which a few have) would have found they need that 40C to 65C level of battery for it to really be up to that power, and cope acceptably (heat).

3300mAH 25C = 82.5A "Capable"
You should find that a battery will not maintain good volts unless you have at least 1.5x the Amps it is run at. eg Your 69Amps needs a battery spec of 105Amps. (there is some tolerance range, and some batteries do better than others).
But generally if you do not have the 1.5x then your volts will drop (as the battery cannot truly deliver the load due to IR), and it will run hotter. (Hotter than a battery that can do it adequately - plus possibly too hot to be good for it)
So that 5S 3300mAH 25C... run at 69Amps... does not look likely to be good.
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Old Dec 27, 2012, 06:51 PM
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Quote:
Originally Posted by PeterVRC View Post
1269w / 69 Amps = 18.8V
Those numbers look good and right, but that is a 'good' voltage being maintained, which suggests good batteries were used. But 25C would really struggle at that, so it seems amiss in those terms. I would expect you would need 40C or better to do that.
Maybe those numbers are OK if it was JUST for the initial; WOT startup short seconds. (eg Peak numbers). I would expect 25C, under such a load, to drop Volts down to 18.0v or lower.

In-plane the numbers will pretty certainly drop. Seeing the ducting restriction will reduce power.

It is all still a good result, but check over that ongoing power and numbers, and monitor the battery temp as I would expect that to be getting hot too. If a long run gets it too hot, then you do need to move up in C.
I would think anyone using 2550kv 5S (which a few have) would have found they need that 40C to 65C level of battery for it to really be up to that power, and cope acceptably (heat).

3300mAH 25C = 82.5A "Capable"
You should find that a battery will not maintain good volts unless you have at least 1.5x the Amps it is run at. eg Your 69Amps needs a battery spec of 105Amps. (there is some tolerance range, and some batteries do better than others).
But generally if you do not have the 1.5x then your volts will drop (as the battery cannot truly deliver the load due to IR), and it will run hotter. (Hotter than a battery that can do it adequately - plus possibly too hot to be good for it)
So that 5S 3300mAH 25C... run at 69Amps... does not look likely to be good.
Great analysis Pete! I am happy with the Gens Ace, I think they are ample rated for my setup, so I am going to drop it in the jet and see how the numbers turn out once installed and go from there. In general, it seems the Gens Ace packs are under rated, and a 25C actually performs like a 40C. If the numbers go over the 3300 x 25C rating then, I might consider some increased C batts.
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Old Dec 27, 2012, 07:25 PM
EDF rules... :)
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Quote:
Originally Posted by PeterVRC View Post
You should find that a battery will not maintain good volts unless you have at least 1.5x the Amps it is run at. eg Your 69Amps needs a battery spec of 105Amps. (there is some tolerance range, and some batteries do better than others).
But generally if you do not have the 1.5x then your volts will drop (as the battery cannot truly deliver the load due to IR), and it will run hotter. (Hotter than a battery that can do it adequately - plus possibly too hot to be good for it)
So that 5S 3300mAH 25C... run at 69Amps... does not look likely to be good.
+1, I agree that you need to size the battery capacity so that the amp draw will not kill the battery due to its internal resistance. I too size the capacity so amp draw is less than 75% of battery capacity and it is better at less than 60% the battery will last a lot longer.

Eric B.
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Old Dec 27, 2012, 07:46 PM
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+1, I agree that you need to size the battery capacity so that the amp draw will not kill the battery due to its internal resistance. I too size the capacity so amp draw is less than 75% of battery capacity and it is better at less than 60% the battery will last a lot longer.

Eric B.
Awesome, thank you.
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Old Dec 28, 2012, 01:36 AM
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Originally Posted by AirX View Post
+1, I agree that you need to size the battery capacity so that the amp draw will not kill the battery due to its internal resistance. I too size the capacity so amp draw is less than 75% of battery capacity and it is better at less than 60% the battery will last a lot longer.

Eric B.
Another +1 on that.
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Old Dec 28, 2012, 02:35 AM
It should fly at least once
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I always look for less than 50% of battery current available, this makes flying in the hotter months doable with out destroying your batteries. I also like my ESC and motor to run cool so I use ones that can cope with the current draw easily.
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Old Dec 28, 2012, 08:27 AM
we can take off without that
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I finally understand why when comparing different motors in the same fan, there is a Power^2/3 term, and this is better to compare than comparing raw Thrust/Power ratios. Its simply a re-arrangement of the above eqations to derive the constant. Motors with a higher constant are more efficient and produce more thrust for the same input watts.
As Don told me K = Thrust / Power^2/3
now it makes sense!
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Old Dec 28, 2012, 08:59 AM
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Originally Posted by green_flyer View Post
I finally understand why when comparing different motors in the same fan, there is a Power^2/3 term, and this is better to compare than comparing raw Thrust/Power ratios. Its simply a re-arrangement of the above eqations to derive the constant. Motors with a higher constant are more efficient and produce more thrust for the same input watts.
As Don told me K = Thrust / Power^2/3
now it makes sense!
Yes, these equations are really useful and it does all make sense . However don't forget that they have been derived for/and apply to just a fan unit on it's own. When working out, for example, your empirical constant K this will not be totally accurate - because your experimental value of power (watts) will always include the losses in the motor/drive system. What we ideally need is way to isolate the two powers - 1. That required to turn the fan (shaft power), and 2. The losses in the drive system (heat, friction etc). I'm still working/thinking on that one... but I think there may be a way That would be really cool because then we could really start evaluating the efficiencies of different motors and stuff.
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Old Dec 28, 2012, 11:11 AM
we can take off without that
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yes, I noticed this with some fans I tested. The constant varies slightly for the same motor at different power levels. (The constant value will automatically include losses at a given power level when measured empirically). I'm guessing thats because the motor efficiency itself varies as the load on it increases/decreases. I only noticed very slight variation in the order of one decimal place, but I guess that may be because I didnt over-drive the motor. For example, when a manufacturer quotes a motor to deliver 4000rpm/volt under (optimal) load at 90% efficiency, then I'm guessing our constant K will remain the same as long as the motor isnt over loaded. However from what I have read, we seem to usually over-drive our motors in EDF applications since they can survive it.
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Old Dec 28, 2012, 12:10 PM
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thx - and what good is common sense

thanks for digging up the MIT source , green_flyer - I should have done myself before posting something "I think I remember". That the load of a prop rises with the third power of rpm seems so "exaggerated" that my memory gave me "squared" because that already sounded like a lot. And the explanation for squared was right, but not for power but rather for thrust (I argued with the amound of air being hit by a blade, and the change in momentum of air molecules risining when hit by a faster blade concerns the force, not the power), but that's how it works: we use our (educated) common sense to explain things after we know or at least think we know how they work. The astonishing fact is, common sense hardly EVER works to predict how things should be. I always thought that's what happens when some shrink "explains" why someone acts this way or other, e.g., saying he is just like his father, doomed to make the same errors. But in other circumstances they could equally well say "he acted just because he wanted to rebel against his parents, "be different than his father" etc. If an assertion, and its opposite, can equally be taken as "explanation" of facts, there is sometfhing wrong. I really liked Duncan Watt's TED presentation on common sense, really recommend to watch it
TEDxMidAtlantic 2011 - Duncan Watts - The Myth of Common Sense (15 min 15 sec)

I don't think one has to read the whole book, with the telling title "everything is obvious - once you know the answer".
I would NEVER have guessed before seeing the derivation that the power to achieve a certrain airspeed rises with the third power of "power". So twice as fast means 8 times the power. Many think that it's sqared because the air resistance force risies with the suqare of the airspeed, but since power = force x velocity i's actually the third power. And that explains why, when making your model faster, you don't have to change the prop since for the prop, pitch speed rises with rpm linearly, but load rises with the third power of rpm. But you might have to change the motor :-)
As to the efficiency curves of electric motors, I find tools like www.drivecalc.de extremely useful. They also show that bigger, high quality motors, have higher efficiency, and the efficiency curve is much flatter towards higher amps.
cheers
Clemens
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Old Dec 28, 2012, 01:10 PM
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Quote:
Originally Posted by green_flyer View Post
yes, I noticed this with some fans I tested. The constant varies slightly for the same motor at different power levels. (The constant value will automatically include losses at a given power level when measured empirically). I'm guessing thats because the motor efficiency itself varies as the load on it increases/decreases. I only noticed very slight variation in the order of one decimal place, but I guess that may be because I didnt over-drive the motor. For example, when a manufacturer quotes a motor to deliver 4000rpm/volt under (optimal) load at 90% efficiency, then I'm guessing our constant K will remain the same as long as the motor isnt over loaded. However from what I have read, we seem to usually over-drive our motors in EDF applications since they can survive it.
Yes, we're not quite there yet with a methodology to derive these constants robustly, but I think we'll (eventually) find a way to isolate the motor drive losses from the raw characteristics of the stand alone fan. It may require a bit of mathematical jiggery-pokery I want to develop this so we can then use it to evaluate just how much total losses occur when driving these edf motors hard (often well beyond their so-called/stated limits). Even though a 450W rated motor may survive/not destroy itself at (say) 1100W - it may well be that it's actually operating very inefficiently when operating in the hi-stress/higher power range; but at the moment we just don't have a method to find out. When we DO have a method - it will be most interesting to properly evaluate how these new HK Dr Mad Thrust motors perform in comparison, since they are claimed to be good for much higher Wattages and are supposed to be much better quality jobbies. My hope is that they'll prove to be more energy efficient too... we shall see.

In the mean time i still don't have any edf kit of any description yet - come on HK, please get some Vampire kits into the UK warehouse soon! (Thanks,
Andy )
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