Yorkman Posted October 7, 2009 Share Posted October 7, 2009 A new challenge, and a new conundrum. I inherited dad's half completed Bristol Bombay (high wing, twin radial, fixed u/c early WWll bomber/transport)-5.5 foot span, hopefully weigh about 4lbs, don't know yet! As usual, dad was resisting the forces of change and had fitted two speed 600s with in-line reduction gearboxes, I intend to go brushless. So-trying to do a bit of the thinking for myself (!), I have two criteria to start with-motors need to be inrunner 36mm dia to fit the nacelles/mountings, and prop dia will ideally be 9" to scale. Having read all the beginner info here, and retained about 5% of it...decided need a low KV motor for lower rpm, so looking at Giantcod inrunner brushless 900kv 36mm dia motors, there are two- a Black Mamba BM3650, and a Keda Brushless Inrunner KB36-50-38S. Using the link from GC to 'peakeff' I entered the relevant numbers, and got two graphs-which I would now add, but haven't looked at how to add pics to threads yet However,the answers I got are, for 9" APC props-identical- APC E 9.0x6.0 1032g,54 mph,90% efficiency at about 17-18 amps. There are others-9x9 for example-but don't think I need to do 78mph! So-am I close? A pair of either of those motors on 9x6s and a 3s lipo with 30amp ESCs? Do I get a gold star from teacher? Or just the dunces cap...again... Quote Link to comment Share on other sites More sharing options...
Andy Gates Posted October 7, 2009 Share Posted October 7, 2009 Question for you, what rpms were given for the 9 x 6 E prop? If the rpms are still quite low, you may do better with a slow fly prop to give a bit more thrust if it is your intention to fly slowly. Being 5.5ft and quite draggy it is likely to fly slow. Other than that the set up you have looks do-able. I do not see a mention of the capacity and rating of the lipo you are intending to use. That is sort of critical. 1 lipo or 2? I assume you have gone for inrunners to make the mounting easier? Quote Link to comment Share on other sites More sharing options...
Tim Mackey Posted October 7, 2009 Share Posted October 7, 2009 Yes we need a little more info yorky. I am leaning toward thinking that a larger diameter prop would seem the way to go, after all 5.5" wingspan is quite large, and its not really speed you are after. I think you mention a 3s lipo, but TBH you may find 2s vbetter, cos these will allow you to prop up, yet keep the RPM and the all important current draw under control. Dare I ask have you got a wattmeter yet?If not ....WHY NOT Ok forget a lot of that - I have just read your posting again and see that you actually want 9" props as they seem scale. Edited By Timbo - Moderator on 07/10/2009 21:26:43 Quote Link to comment Share on other sites More sharing options...
Yorkman Posted October 8, 2009 Author Share Posted October 8, 2009 Yep-9" props for scale. Ran 'peakeff' again with 8v (2s) and 9x6 props theoretically will pull only 12amps... What are your views on this peakeff program Timbo? Quote Link to comment Share on other sites More sharing options...
Tim Mackey Posted October 8, 2009 Share Posted October 8, 2009 I never use them so cant comment tbh.I much prefer to select the powertrain based on my usual methods, ( IMO Kv and maximum current or power capability being the most relevant ) and use my whattmeter t fine tune the final prop selection. I do prefer to take advantage of the electric motors ability to swing larger props than many IC engines, and never worry that much about perfect scale sizing....preferring the efficiency and quiter running of the slower turning larger prop. Quote Link to comment Share on other sites More sharing options...
Yorkman Posted October 8, 2009 Author Share Posted October 8, 2009 So- I'm on the right lines with my Kv and prop sizing as my starting point? And this peakeff program tells me that with my selected motor I can use a 9" prop with a 3s battery with a current draw of 18amps or a 2s pulling 12amps. I'm going to need 320-400watts....and I forget the little sum one does to work this out, sorry ....hmmm...watts=volts x amps? If that's correct then I need to pull 30amps on a 3s or 40 on a 2s? My brain hurts.... PS 9" is scale dia and also can't go larger cos fuz is in the way! Quote Link to comment Share on other sites More sharing options...
Tim Mackey Posted October 8, 2009 Share Posted October 8, 2009 Watts = Amps x Volts yes. 3s Lipo will be around 10.5V under use, so 400 / 10.5 =38AEdited By Timbo - Moderator on 08/10/2009 16:24:48 Quote Link to comment Share on other sites More sharing options...
Yorkman Posted October 9, 2009 Author Share Posted October 9, 2009 Sorry if it's like banging your head against a brick wall Timbo... So-again-how do I work out which motor I want if I want to swing a 9" prop and develop 400 watts of energy? Using the peakeff program again,the 1500Kv Black Mantis on a 3s with an APC E 9x4.5 theoretically draws 48 amps, while the 8x4 draws 35.... If you don't use such a program, how do you arrive at which motors to put in a new project? Quote Link to comment Share on other sites More sharing options...
Tim Mackey Posted October 10, 2009 Share Posted October 10, 2009 No problem Yorky... ask away my friend .Have you looked at the beginners guide to electrickery on the forum - we have sections dedicated to motors etc. If you want 400 watts, and a smallish prop, then look for a motor which can handle the watts required, and has fairly high Kv. The one you chose seems pretty right, but the amps are very high....on 10.5V ( 3s lipoo ) 48A is 504 Watts, so you are OTT. What you need to do is either prop down ( a la the 8 X 4 ) or use a lower Kv motor.Either of these moves will drop the current down to more sensible levels, and the power gets closer to your target. This for example, on a 9" prop would probably get it about right.Its really a 500 Watt capable motor, and can happily run on 4s lipo and slightly larger prop, but if you use a 3s, and the 9" prop, it will be running at less than maximum capability, so will be cooler, and less strained all round. Try running that combo through motorcalc -and lets see what you get...or even ring BRC and see if they have figures for that combo.One difficulty for your set up is the desire for an inrunner..... and most inrunners are high Kv for smaller props and EDFs. Is there no way you could fit outrunners?? Quote Link to comment Share on other sites More sharing options...
John Cole Posted October 10, 2009 Share Posted October 10, 2009 I'm not sure but I THINK you mean you want 400 W total, for your twin (4 lbs @ 100 Watts per lb) : that is 200 W per motor ? Quote Link to comment Share on other sites More sharing options...
Yorkman Posted October 10, 2009 Author Share Posted October 10, 2009 Posted by John Cole on 10/10/2009 11:37:50: I'm not sure but I THINK you mean you want 400 W total, for your twin (4 lbs @ 100 Watts per lb) : that is 200 W per motor ? that's both Timbo and I missing the obvious-well spotted John! Quote Link to comment Share on other sites More sharing options...
Yorkman Posted October 10, 2009 Author Share Posted October 10, 2009 Having gathered myself again....thanks John for pointing out that I'm looking for 200w per motor-if I run this motor http://www.giantcod.co.uk/black-mantis-bm3650-900kv-inrunner-brushless-motor-p-403926.html through the peakeff program, I'm getting a theoretical 20amp draw @ 10.5v with an APC E 9x7.5-that's got to be close, surely? BTW I hope you're out flying in this glorious sunshine, not stuck inside like me! Quote Link to comment Share on other sites More sharing options...
Tim Mackey Posted October 10, 2009 Share Posted October 10, 2009 Yep...been soaring, rehearsal for tomorrows forumite fly in OK so back to the maths ... 20 A X 10.5V = 210 Watts. Bingo Quote Link to comment Share on other sites More sharing options...
Yorkman Posted October 10, 2009 Author Share Posted October 10, 2009 So I'm going to award myself a gold star as my opening post actually was correct! Even though I had failed completely to do the wattage calculation....... Quote Link to comment Share on other sites More sharing options...
Tim Mackey Posted October 11, 2009 Share Posted October 11, 2009 here ya go............ First off one of these Then this Quote Link to comment Share on other sites More sharing options...
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