Adrian Day Posted January 11, 2012 Author Share Posted January 11, 2012 Steve Lets see if i get this right or wrong so on your maths then if we work out the vintage 75-80 rule i would need to do this 80W / 7.5V = 10.6A so i could do with a 12A ESC ? ? think thats woking in the right direction?? so prop size is really the main option and to test what feels / flies best for such a model just have to remember your low - big - slow .......high - small - fast idea but yes woking it out now sometime you just need some one to lighten the gray area to get the idea the way you have put it has helped immensely.. thanks again and can i add as friend on here please Quote Link to comment Share on other sites More sharing options...
Steve Hargreaves - Moderator Posted January 11, 2012 Share Posted January 11, 2012 Of course you can Adrian.....I need all the friends I can get..... Yep, thats pretty much it on the maths.....remember its always better to err on the side of caution when sizing components.....anything working flat out will not reward you with a long life. Always allow a bit of "headroom" & frankly the difference in size/weight & cost between a 12A & an 18A ESC will be negligible so I would probably go for a 18A one....but thats just my personal feeling. Plus if you find that your mates Pix E is faster than yours you can get more power with a bigger prop without bothering to change the ESC (assuming the motor will take it of course!!)... So let us know which motor you get & we can decide on a prop & then run it all up on a Wattmeter & check that we have the correct power & that nothing is working beyond its limit......if we don't or it is we can change the prop..... The Pix E is a vintage type design & will waft around on very little throttle turning a big prop so slowly you can count the blades......think lazy summer evenings & you're about there.... Quote Link to comment Share on other sites More sharing options...
Pete B Posted January 11, 2012 Share Posted January 11, 2012 All very nicely put, Steve, if I may say so. You've explained the mysterious fundamentals in a way most folk should be able to get their heads around! You don't teach for a living, do you? Pete Quote Link to comment Share on other sites More sharing options...
Tim Hooper Posted January 11, 2012 Share Posted January 11, 2012 What Steve said! That's quite a tutorial you've posted here, Steve. Many thanks for taking the time to express it all so clearly.Adrian, I just want to affirm what Steve has said. My own Pix-E will waft about on a calm day on 1/3 throttle without any fear of it dropping out of the sky. At full whack it will climb at 45°, but that kind of defeats the object, yes?Even more surprising is shutting the throttle completely, and seeing how long it takes to glide down from height - just watch out for thermals!tim Quote Link to comment Share on other sites More sharing options...
Adrian Day Posted January 12, 2012 Author Share Posted January 12, 2012 well from what Steve put i have made a simple program in excel for further use hope that its a working out close to if you see anything in the picture that dont make sence please tell me, its just for a basic idea, I have to agree with Pete B on that Steve i have to say your way of explaning things is in easy terms that most can understand so thats perfect . thank you Tim for your input on the power and what it does on the setup your using so do you think you could power the model with something a little smaller?? and it would still do the job if there is much smaller out there question: if you know the watts needed but the motor seller only has the kv how do you work it out say the model is 3lb sports model so 300w on a 3s LiPo 12A - 18A esc how would you convert that to a kv on a motor???? RE: BUILD. Got a little done today cut parts for Tail surfaces and the 2 fig of 8's F2 F3 and glued all together and finished Will post pictures tomorrow as soon as i get the chance.. build going ok ish so far. fus to start next i think Adrian Quote Link to comment Share on other sites More sharing options...
Adrian Day Posted January 12, 2012 Author Share Posted January 12, 2012 Posted by Steve Hargreaves on 11/01/2012 16:08:09: Of course you can Adrian.....I need all the friends I can get..... Yep, thats pretty much it on the maths.....remember its always better to err on the side of caution when sizing components.....anything working flat out will not reward you with a long life. Always allow a bit of "headroom" & frankly the difference in size/weight & cost between a 12A & an 18A ESC will be negligible so I would probably go for a 18A one....but thats just my personal feeling. Plus if you find that your mates Pix E is faster than yours you can get more power with a bigger prop without bothering to change the ESC (assuming the motor will take it of course!!)... So let us know which motor you get & we can decide on a prop & then run it all up on a Wattmeter & check that we have the correct power & that nothing is working beyond its limit......if we don't or it is we can change the prop..... The Pix E is a vintage type design & will waft around on very little throttle turning a big prop so slowly you can count the blades......think lazy summer evenings & you're about there.... have sent a friend request dont know if it sent right first i have done TY Will have to invest in a wattmeter for sure got a little to get in between , pending on money if you know what i mean, some one may have to talk me round how do do the wattmeter test, oh now thats a good idea looking forward to summer now cant wait, summer eve flying is a distant memory sad to say something i havent done in over 10 years last time was with my dear departed dad, Adrian Quote Link to comment Share on other sites More sharing options...
Adrian Day Posted January 12, 2012 Author Share Posted January 12, 2012 Good Morning All day 3 of building the little pix-e went ok with just some cleaning up of joints to be done but structure is solid for its 1/8th sq parts, today will be to start the fus may only be the cutting of parts and a little buildingdepending on time, think its going ok ???? will say Tim, idea of constructing joints makes for stronger joining Adrian Quote Link to comment Share on other sites More sharing options...
Tim Hooper Posted January 12, 2012 Share Posted January 12, 2012 Looking good Adrian! Sure the Pix-E will definitely fly on less power than the set-up I used - the little bell motor just happened to be lying about doiung nothing at the time, that's all! Personally, I wouldn't be in too much of a hurry to spend money on a Wattmeter, if there are other, more urgent, things on your shopping list. A decade ago (when e-flight was still very experiemental), a meter was more or less essential. These days, we have guaranteed-to-fly set ups on the shelf of every model shop, and the online know-how to check the results. I have to confess that once I know how a particular set-up works in practice, then I just tend to copy it for any similar model! I'm not one for testing lots of props, and then logging all the results, so once I find a system that works, it seems a little excessive to try to fine tune to the nth degree. But, then, I'm just lazy that way.......tim Quote Link to comment Share on other sites More sharing options...
Adrian Day Posted January 12, 2012 Author Share Posted January 12, 2012 Tim thank you am really trying taking my time to make sure that it looks pretty close to your workmanship. An old mane once said to me if you find something that works for you stick to it, if only to get you by those dark evenings. i would say that as you said in your post if you know a particular setup you sick to , i think your doing it right from what i see and the models you build , i only dream to be that good, and your living that dream, A hobby and something your great at what more could you ask for that and you have a partner who has the hobby blood to ... now thats hard to come by cant wait to see what you come up with next time Regards Adrian Quote Link to comment Share on other sites More sharing options...
Adrian Day Posted January 13, 2012 Author Share Posted January 13, 2012 Well today got a little more done to the pix-e. Cut the fuselage side parts and built the first side looking ok on the board and laying perfectly flat. Fridays tasks side 2 of fuselage and cut the rest of the joining 1/8 sq parts from the plan. then fit F2 F3 and close the tail end together lets see how the day goes shall we? slow but productive little worried about making the landing gear as not sure by the design angles and fittings??? any one know how please let me know TY Adrian Quote Link to comment Share on other sites More sharing options...
Stephen Grigg Posted January 13, 2012 Share Posted January 13, 2012 This one is coming along well,well done AdrianEdited By Stephen Grigg on 13/01/2012 00:56:19 Quote Link to comment Share on other sites More sharing options...
Adrian Day Posted January 13, 2012 Author Share Posted January 13, 2012 TY Stephen for your nice comment ... tying my best .... welcome to my build and hope you'll come back Adrian Quote Link to comment Share on other sites More sharing options...
Steve Hargreaves - Moderator Posted January 13, 2012 Share Posted January 13, 2012 Ooooh...lots to cover there....lets see if we can do it all in one post!!! Thanks for the kind comments regarding the explanations....no I'm not a teacher but if my inane ramblings have helped in some way then I'm very pleased!!! I've responded to your Friend request Adrian.... Your excel sheet looks good but a couple of comments if I may.... a LiPo cell is nominally 3.7 volts so a 2S battery will be 7.4 volts not 7.2....also these are nominal voltages....fully charged they will be higher at 4.2volts per cell & under load they will be lower....around 3.4 - 3.5 volts per cell. Personally I use 10 volts for a 3S battery because its easier to do the maths. Also 200watts is on the high side for an aerobat....150watts/lb will be quite exciting...200watts will be unlimited vertical...... question: if you know the watts needed but the motor seller only has the kv how do you work it out say the model is 3lb sports model so 300w on a 3s LiPo 12A - 18A esc how would you convert that to a kv on a motor???? Simple answer...you can't... kv & power have no correlation....you can get 300watts out of a motor with a high or low kv.....all that would change would be the prop size. the kv is fixed when the motor is designed by the number of turns, star or delta winding etc....its a factor of the motor in the same way that the shaft size is. Frankly if the seller can only offer you the kv value & not the power then I'd find another supplier....... Its not really the power (watts) of a motor thats important but the maximum current it will take. if we imagine a motor that takes 20A max then it will produce 140watts from a 2S battery (7 volts x 20Amps = 140watts) & 200watts from a 3S battery (10 volts x 20 amps = 200watts) However it will spin much faster on a 3S battery so we would need to change the prop for a smaller one..... Look for the maximum current & be aware that several suppliers often state the absolute maximum the motor will take for 30 or 60 seconds....this is not the value you should be looking for if you want a long & trouble free life out of your motor (see earlier comments!!) An example....lets say our 20A motor has a kv of 1000 so for every volt applied it will spin at 1000rpm.....we'll ignore losses & put a 9 x 6 prop on it & a 2S battery......so it spins the 9 x 6 at 7000 rpm (7 volts from a 2S) & consumes 20A to do it.....everything is fine & we fly until dusk. Now we use a 3S battery (10 volts) & our model is off like a rocket but after a few seconds it all goes quiet & there is lots of smoke.....whats happened? Well we have applied 10 volts to the motor & it has tried to spin the 9x6 at 10000 rpm. Spinning a prop 3000 rpm faster takes a lot more power so the motor consumes a lot more amps....maybe 30 or more...& it can't take the current. the wires are too thin....so they melt & destroy the motor.....what we need to find is a prop that will cause the motor to consume 20A but at 10,000 rpm......you guessed it...we need a smaller prop....so we fit an 8x4 or something & the motor spins this at 10000 rpm & still consumes 20A.... Wattmeters.....much as it pains me I'm going to disagree with my learned & honourable friend Mr Hooper here. I would consider a wattmeter essential & at £14 they are quite reasonably priced these days....indeed you can save this money the first time you use it if you find the motor/esc/battery are working beyond their limits.....blow one ESC & that will cost you the equivalent of a wattmeter......Agreed if you are transferring an established set up then it will probably be OK but I ALWAYS check a set up with my wattmeter..... A wattmeter simply plugs between your battery & ESC & shows the on load voltage of your battery plus the current consumed & resultant power in watts... If you'd like me to run through an example of how we determine a set up then I will be more than happy to do so although Tim Mackey covered this in great detail in his article in the mag a few months ago.....maybe dig that out & have a read... Quote Link to comment Share on other sites More sharing options...
Adrian Day Posted January 13, 2012 Author Share Posted January 13, 2012 Steve Wow so much to take in in one go think i will have to read that a few times to get in the brain... firstly thank you for the acceptance of the friend request next lets see read a little and write a little so much to take in have made some changes now to my excel list as didn't realise I had put wrong digits now it might have the right workings thanks you again... on the kv/watts well i didn't think there was going to be a simple exchange there just the luck i have been on a number of sites looking for motors and got only the kv with not much other information.. i was kind of wondering if there was another way that you can work things out like working backwards or something i may have missed. but that's good that there isn't something and its just not pos. so on the motor front the idea is if you up the volts /amps in doing so changes the kv so adjust the prop size in accordance e.g: motor speed changing faster then you would put smaller prop ... if you lower the volt/amp then you would put larger.. ..would that sound about right?? and as you say about companies putting the burst amps making you think you can push it but in the most part and for safety you can only use 50% of what they are putting i hope what i have put sounds about right but think this is going to take some getting round.. will work on that one thanks again for everything Adrian Quote Link to comment Share on other sites More sharing options...
Adrian Day Posted January 13, 2012 Author Share Posted January 13, 2012 Tim / Steve Do you Think you could do the same thing a standard multi meter or would it not get the same results?? if not may be an investment that may come in handy at some stage and cost effective for the future Adrian Quote Link to comment Share on other sites More sharing options...
Steve Hargreaves - Moderator Posted January 13, 2012 Share Posted January 13, 2012 Posted by Adrian Day on 13/01/2012 14:20:06: so on the motor front the idea is if you up the volts /amps in doing so changes the kv so adjust the prop size in accordance e.g: motor speed changing faster then you would put smaller prop ... if you lower the volt/amp then you would put larger.. ..would that sound about right?? and as you say about companies putting the burst amps making you think you can push it but in the most part and for safety you can only use 50% of what they are putting H'mmm Kind of....but the way you have phrased it makes me think you haven't quite fully understood it....... First off don't worry too much about the kv value...its fixed & doesn't change & its a decision you make when selecting the motor.....a bit like the mounting dimensions or the shaft size.....you can't change it but the rpm of the motor will change according to how many volts you apply (volts x kv = rpm). The kv figure is important in the inital selection process but unless you are wildly out it can usually be worked around. If you bought a 3500kv motor for your Pix E that might cause us a problem but as long as you stay in the 1000 to 1500 range that will be fine. The kv figure often gives us a feel for what sort of motor we have...fast & buzzy (high kv) or slow & lazy (low kv). The voltage is the main thing you have control of via the battery....this is inversely proportional to the prop size. ...for a given motor if one goes up the other must come down to try & keep the current (amps) constant..or at least below the max of the motor. Or if you look at it another way for a given battery we change the prop to alter the current & this will change the power produced. Let's set you some homework for the weekend..... I have a 3S 2200mAh battery that I want to use to power my 2.5lb sport model.....I want good but not silly performance..... Tell me....... The max current of the motor I should use? The sort of kv I will need & expected rpm in use The current capacity of the ESC How long will the motor run at full throttle before the battery is flat (bit of a difficult one this as we've not covered capacity particularly but Hey! who said it would be easy) Extra marks can be gained by showing your working & assumptions.... Have a go & see what you come up with....let me help you a little bit with the formula Power equals voltage x current (P=VxI) Quote Link to comment Share on other sites More sharing options...
Steve Hargreaves - Moderator Posted January 13, 2012 Share Posted January 13, 2012 Didn't spot your last post.... Yes you can use a multi meter provided it goes high enough.....most don't go beyond 10A so you might struggle here..... Plus its a bit of a pain because you have to set it up & run it to show the current. Then set it up again to show the volts & then multiply the two to get the watts.....a wattmeter will do it all in one hit. Maybe borrow one initially but if you're going to get into electric flight then I would advise getting one ASAP.... Quote Link to comment Share on other sites More sharing options...
Adrian Day Posted January 13, 2012 Author Share Posted January 13, 2012 Steve this is going to be a hard one for me don’t think im good enough for this test. Though your a good teacher maybe im just not a great student Are you quoting a little of ohms law if my memory serves me correctly?? p=v x i Ok here goes..MOTORworking on the assumptions of the sports model 100w per lb as you said earlier in post.2.5lb x 100w = 250w / (3s) 11.1 = 22.52amps so on that i would say go with this one motor with the workings looking something like thisMAX CURRENT TO USE 22AMP OR 244WATTS ---- 22A x 11.1v = 244wKV YOU NEED 2200 KV x 11.1V = 24420 rpm AS HIGH RPM I WOULD SAY I WOULD USE WITH A SMALL PROP ESC Im going with this one ESC CURRENT CAPACITY 30AMP WITH 40 AMP FOR 10 SEC BURST reason for using this 30amps is it 2amps over the 60sec burst rate of the max amp the motor can handle and can run on either 2s or 3s LiPo as you asked.. As for how long would the battery last my working out may be wrong but Im going to say 10 mins max Working out like this … motor takes a constant 22 amp draw so im working it out like this 2200mah bat = 2.2amps so 22A draw from motor / 2.2A bat =10min or 2.2 x 10min = 22A don’t know if this is right or not never worked on this before. Maybe a little backwards though JUST A GUESS.. SORRY IF this is wrong maybe I should stick with building and let others tell me what to put in..Adrian dont think i got it though Quote Link to comment Share on other sites More sharing options...
Adrian Day Posted January 15, 2012 Author Share Posted January 15, 2012 Afternoon all well been building over the last 2 days, been very productive, now got the fuselage half built up got all internal formers in and some of the main turtle decks stringers in place with rear window bars in too, i have made the tail up with hinges so now control surfaces moving. have covered the tail plane .. have chosen the colour scheme going for a blue and white colouring. i want to get the pix-e stickers made up with white writing but i cant do printing as printers gone pop will have to have a think will post some more pictures soon showing the progress Adrian Quote Link to comment Share on other sites More sharing options...
Steve Hargreaves - Moderator Posted January 16, 2012 Share Posted January 16, 2012 Posted by Adrian Day on 13/01/2012 22:17:47: Stevethis is going to be a hard one for me don’t think im good enough for this test. Though your a good teacher maybe im just not a great student Well I think you did pretty well actually.....its not easy to get your head around so many variables & assumptions so lets just go through it.......Are you quoting a little of ohms law if my memory serves me correctly?? p=v x i Yes its kind of part of Ohms law.....Ohms law is actually V=IR but P=VI is derived from it.... Ok here goes..MOTORworking on the assumptions of the sports model 100w per lb as you said earlier in post.2.5lb x 100w = 250w / (3s) 11.1 = 22.52amps so on that i would say go with this one motor with the workings looking something like thisMAX CURRENT TO USE 22AMP OR 244WATTS ---- 22A x 11.1v = 244wThis is fine although personally I use 10 volts for my calculations on a 3S battery. The nominal voltage is indeed 11.1volts & hot off the charger it will show 12.6volts but battery voltage always drops under load.....10 volts makes the maths easier too...so 250watts divided by 10 volts equals 25AKV YOU NEED 2200 KV x 11.1V = 24420 rpm AS HIGH RPM I WOULD SAY I WOULD USE WITH A SMALL PROP You've come slightly unstuck here......2200kv is a very high reving motor.....again if we use 10volts this would give us a theoretical 22,000 rpm probably close to 20k under load which is very high....a quick look at the motor spec tells us the suggested prop is very small...5" or so...which won't fly oiur 2.5lb model very well. I've found that a kv of around 1000 to 1200 works best with 3S batteries for general sport models ESC Im going with this one ESC CURRENT CAPACITY 30AMP WITH 40 AMP FOR 10 SEC BURST reason for using this 30amps is it 2amps over the 60sec burst rate of the max amp the motor can handle and can run on either 2s or 3s LiPo as you asked.. Excellent....spot on with maths & thinking..... As for how long would the battery last my working out may be wrong but Im going to say 10 mins max Working out like this … motor takes a constant 22 amp draw so im working it out like this 2200mah bat = 2.2amps so 22A draw from motor / 2.2A bat =10min or 2.2 x 10min = 22A don’t know if this is right or not never worked on this before. Maybe a little backwards though JUST A GUESS.. SORRY Close .....you've got the theory but slipped up slightly in your maths.....if we divide the battery capacity (2.2Ah) by the max current (22A) we get 1/10th......of an hour....which is 6 minutes.....so wrong answer but marks awarded for your working.... So overall pretty good Adrian......I'm afraid you're still going to have to go on that Cross Country run on Thursday though...... Quote Link to comment Share on other sites More sharing options...
Adrian Day Posted January 16, 2012 Author Share Posted January 16, 2012 Steve thank you Worth the try ..didnt think i got it ...need to work on my electrical skills . there was a 10 in there some where .. didnt know how to work it out ..so its battery / motor =0.1 ........ rather than what i put being motor / battery i guess that the run would more likely be to get my model as its would run out 4 mins earlier than i would have thought couldn't remember ohms law so just put you equation think maybe i should just leave it to the pro's dont think i have the brain for it Adrian Quote Link to comment Share on other sites More sharing options...
Adrian Day Posted January 17, 2012 Author Share Posted January 17, 2012 well the last few days the build has slowed a little due to being busy. so only done the small things that i can before bed .. adding formers and wing rods so on. i should put some pictures on really so you can see the progress.. will do that soon as i can. one thing i have been doing is thinking about the power side of things and working out the best that i can from what Tim and Steve have been teaching me .and PM'ing Tim my info just to make sure im getting this model the way he intended it.. i have now chosen the following electrical components MOTORESC LiPo2 SERVO'SJust ordered them from G.C so will be keeping eye out for the postie in the next few days Then once that gets here the build will come together pretty fast .. i think. hope this is keeing you up to date Adrian Edited By Adrian Day on 17/01/2012 10:41:17 Quote Link to comment Share on other sites More sharing options...
Steve Hargreaves - Moderator Posted January 17, 2012 Share Posted January 17, 2012 A good choice of hardware there Adrian.... I see you've plumped for a 1300 battery too....does this mean you are keeping the servos at the back? A couple of things you might have missed.....a prop adaptor & ..er...a propellor!!! Maybe the 9x3.8 of 9x4.7 off this page might be worth a try.....although maybe an 8" prop might be better..... I have the very same motor so if you'll bear with me I stick a few props on it tonight & see what power I get......it will give us a starting point at least..... Quote Link to comment Share on other sites More sharing options...
Adrian Day Posted January 17, 2012 Author Share Posted January 17, 2012 good morning Steve ty for your comments there i was looking on there last night trying to gain some extra knowledge on motors n esc so i opted for what i could work out.. on the prop / adpt . I just had the same comment there from Tim in his pm didnt remember about that till i had ordered .. 3mm prop adaptor and a 8x4 slow fly prop. is Tims suggestion... Thank you for the link. and yes will hold five for your testing .. will make life a little easier having the same motor and much appreciated .. Yes I'm thinking about keeping the servos at the back but also considered putting the inside the rear of the plane with just a small push rod.. before covering maybe a little hatch. i don't like seeing the servo on air craft just looked at your link there isnt a 8x4 only a 8x6 would that be to much ?? but have found this one 8x3.8 Adrian Edited By Adrian Day on 17/01/2012 11:38:57Edited By Adrian Day on 17/01/2012 11:44:26 Quote Link to comment Share on other sites More sharing options...
Steve Hargreaves - Moderator Posted January 17, 2012 Share Posted January 17, 2012 Yep a 8 x 3.8 would be good....whats 0.2 of an inch between friends.....I would try & keep the pitch around the 4" mark.....pitch affects the flying speed & what we want is lots of thrust & not much speed.....which you get from a large diameter, fine pitch prop. This moves lots of air....slowly which is exactly what we want...... I'll strap my motor to a bit of wood & try out a few props tonight.....I don't think I have any slow fly props in that size but we'll see how we get on..... Quote Link to comment Share on other sites More sharing options...
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