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How to choose an electric power system |
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Mustang Sally 
Platinum (AE) USERID: 52654 Hobbyking Addict 
Joined: 19/Feb/2009 Online Status: Offline Posts: 1956 |
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 Topic: How to choose an electric power systemPosted: 02/May/2012 at 2:00am |
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Free online RC calculator:
http://rcadvisor.com/ |
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A minimal technical knowledge is required for this hobby; please ensure you know the differences between up and down.
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pacoaguila
Platinum (AR) USERID: 203346 Newbie 
Joined: 20/Jan/2011 Online Status: Offline Posts: 9 |
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Posted: 03/Apr/2011 at 1:33pm |
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I need a >1500 kv motor for cessna 182(560) grs. But 2408-21 and 2409-12 not in stock.
Anyone help me to search in stock the best option?. Thanks. |
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what_the?!
Platinum (AU) USERID: 178022 Bargain Addict! 
Joined: 20/Mar/2009 Online Status: Offline Posts: 50 |
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Posted: 18/May/2010 at 11:28am |
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well kind of out to lunch... you can get the maximum current draw on the bench then convert that to expected minimum flight time. set that time on your Tx so it beeps when its time to land. you can also estimate avg current draw. |
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what_the?!
Platinum (AU) USERID: 178022 Bargain Addict!
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Posted: 18/May/2010 at 11:26am |
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actually this isnt correct. i can see why you wrote this, but its misleading. essentially whjat you are saying is that: change in rpm = change in thrust squared = change in power cubed. simply think in terms of dimensions. "RPM" is a linear dimension. i.e. a magnitude only with no spatial dimensions. The thrust is determined by the area that that propeller sweeps..so is a 2Dimensional equation. (just like area is for a circle- pi*R^2)- affected by the magnitude of the speed of the prop. Finally, Power- this is the energy required to drive that area of thrust in a certain direction. So it's a volume. We all know that volumes are 3Dimensional, right?...so now we have the speed of the prop, and the area that the prop spins across, and finally how much area the prop moves. Thats why its related to the change in rpm cubed. To go from thrust to power: thrust = power ^1.5 its to the power of 1.5 because that equals "3/2"...which is the respective relationship to the common denominator- rpm of each respectively. so if you want 10% more rpm from your 100% prop, youll get 110%^2 = approx 120% (or 20%) more thrust, but you'll require 110%^3 = approx 130 (or 30%) more power to get there. now, power is not proportional to V cubed. since power P= IV, or I^2R, then its proportional to the change in both Voltage and Current- or Current and Resistance. |
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Rikku007
Platinum (CA) USERID: 409046 Newbie
Joined: 28/Jan/2010 Online Status: Offline Posts: 18 |
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Posted: 18/Mar/2010 at 1:07am |
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o yea guys dont forget that if your talking about lipos, you cannot actually use 100% of its capacitly since that will damage the lipo beyond repair, lipos can only be discharged to 3 volts which menas theres around 10% or more capacity thats still left in the battery that u cannot use. So the mAh rating is really only a guideline and doesnt actually tell you how long you can fly for, the only way is to calculate flight time using mAh, then take off 30% of that time and then set that as your timer, its always good to land your plane with some power left than to land it with no power. Also i usually set my cutoff to soft cutoff at 3.2 volts because this way i can still use a bit of throtle and remember that servoes also draw current so if you cutoff at 3v, while ur in the air, by the time u land, you would have already gone below the 3 volts due to servo amp draw.
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-rikku007
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rewolff
Platinum (NL) USERID: 340356 HobbyKing Constituent 
Joined: 03/Dec/2009 Online Status: Offline Posts: 119 |
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Posted: 01/Feb/2010 at 8:04am |
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Although you asked this a long time ago, I think it is asked so often that it pays to still answer this question.
Yes, exactly that's the calculation! (I changed the numbers a bit: dividing by 1 or multiplying by one is not different...)
The C-rate determines the maximum discharge rate. So a 1200mAh battery rated at 10C would only just be able to discharge at 12A. But a 20C or more would easily cope with a discharge that fast. Most people recommend staying away from the exact limit and over-dimension your batteries a bit. So if your plane uses up to 12A, go for at least 15C 1200mAh. (or 600mAh and at least 30C. Or 400mAh and at least 40C) A 1C battery can only discharge at 1.0 times the capacity. So a 2200mAh 1C battery can only be discharged at 2.2A. (e.g. the "transmitter batteries" have very low C-rating. http://www.hobbyking.com/hobbycity/store/uh_viewItem.asp?idProduct=9787&aff=170178 and http://www.hobbyking.com/hobbycity/store/uh_viewItem.asp?idProduct=6955&aff=170178 |
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W3FJW Ron
Platinum (US) USERID: 63218 Forum Admin 
Joined: 19/Feb/2009 Location: Tacoma WA Online Status: Offline Posts: 9067 |
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Posted: 03/Dec/2009 at 10:50am |
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A good tutorial about electric power is
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I can only remember half of what I say.... I used to walk on water 'til I found out it was more fun sitting in the boat drinking beer Ron |
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snuffy9278
Retail (US) USERID: 358882 Newbie
Joined: 10/Nov/2009 Online Status: Offline Posts: 1 |
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Posted: 03/Dec/2009 at 10:21am |
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Im in the same boat,could easily fly my Mini Telemaster on an .09 glow,but dont know how to compare this to electric power.The plane is 215 sq.inches about 20 to 28 oz.
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vasy1
Platinum (US) USERID: 15488 HobbyKing aficionado 
Joined: 10/May/2009 Location: Canada Online Status: Offline Posts: 1064 |
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Posted: 22/Sep/2009 at 9:09am |
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You can only guess the time as per your formula, just have to take into account about 20% or so (just a guess) of loss into heat and wire/connector resistance. You can only estimate true times at a steady speed, after that, all bets are off.
One person can fly one plane for 15 minutes and another for 3. Same plane, same battery same power system. You are correct in your post as per time and as you noted, battery does not discharge at the same rate so you can't correctly estimate time., only guess Also some will be lost to heat and efficiency losses. The only true way, it to fly it and see how long it lasts. You can purchase some on-board log equipment that will show you how much you draw during flight, so after a few flights with say 2500 battery you can estimate your flight time with a 2000 or 300 battery, but the data will only be a starting point to your friend as he has a different idea as to how far forward he likes to push his throttle stick. Flight times very greatly with flight the type of flight you want. I can fly one my plane for 15 minutes in a nice relaxing way, cutting the throttle and gliding and the next battery, just leave it in WOT and be done in just a few short minutes. Am i wrong or out to lunch here? Edited by vasy1 - 22/Sep/2009 at 9:13am |
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mirterra
Retail (BR) USERID: 277948 Newbie
Joined: 29/Aug/2009 Online Status: Offline Posts: 6 |
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Posted: 22/Sep/2009 at 6:02am |
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How do you calculate the battery time?
Is it something like this? 1000mAh -> means that if it the battery is discharged at 1A, it will last 1 hour. So, if you discharge at 10A it will last 6min... However, it doesn,t in consideration the C rate... Should I multiply it by the C rate? Maybe divide? How do I? |
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power is proportional to voltage cubed