How many kv`s?

[Nick Dainton]

Hi, I am currently about half way through a build of the 66" Lysander plan from RCME. Originaly it was to have a four stroke 48-52 engine. But the useability, less mess of electic power is causing a rethink. Could you advise on motor power, suitable prop size, and battery power for this aircraft. All up weight is to be around 6lbs,

 

thanks , Nick 

[Romeo Whisky]

I'm surprised you're still waiting for an answer on this one Nick - but perhaps there aren't that many "leccies" online just now who fly bigger stuff like this - and to be frank I don't either.

 

For what it's worth, I think you'll need something pretty hefty with a low Kv - say 850-950Kv and a big prop, and probably 4S LiPo with a separate Receiver pack .  You'll need a big ESC too so be prepared for quite a big bill for this setup.

 

You really need to get some advice from someone who's into the bigger electric models.  You might also try ringing BRC or Overlander as they both sell a big range of electric motors and have years of technical expertise in electric flight.

[Tim Mackey]

At AUW of 6lbs, you should aim for around 500 -600 Watts of power.

Low Kv as RW said, but a lot depends on your proposed prop size.  If you want a nice slow truning but large diameter prop ( and why wouldnt you ) then something around 500 Kv is best. Use  maybe 4 cells ( if going Lipo ) or perhaps 6 if using A123s

4s Lipo is around 14 V and therefore for 500 Watts you will be pulling a nice manageable 35A at wide open throttle. A 50A ESC will be plenty big enough.

 

The "bill" need not be big at all, suitable motors are to be had for around £30 or less, ( like this one here ) and simple style ESCs at around £10.  A 4s LIpo of decent capacity .... perhaps 35 - 40 squid.

Total powertrain cost  £80

[Pete Rieden]

I think you'd struggle with a 4s setup, mainly because it's a big, draggy model but also possibly because getting it down to 6lbs might be a bit optimistic. I'd go for a bigger power reserve (which will mean longer durations if you find that it does fly on less power), and I'd look at something equivalent to the Axi 4120/18 on 5s and a 15-16" prop, or 6s on a 14" prop. 

Although I personally prefer the "quality" motors, there are several "budget" 500rpm/volt, 50-60A motors from the usual suppliers that will serve just as well. You'll need a speed controller that will handle 50A for short periods, which means a 50A "blue chip" ESC or a minimum of 70-80A in the budget brands. On a 5-6s setup you'd normally use an "opto" controller with an external BEC or separate receiver battery, although some of the more recent ESCs have a switched-mode BEC that would also work (depending on how many servos you're using).

6s would be easier simply because you could get a pair of relatively cheap 3s3000ish packs (like the Loong Max ones) and connect them in series. these packs could then be used individually in 300-400w sports models, which takes the sting off the investment. The packs could be charged individually or charged as a pair using one of the many balance lead adaptors which plug into a 6s balance socket and split it into two 3s balancer plugs.

£0.02 supplied,

PDR

[Steve Hargreaves - Moderator]

Tony Nijhuis asked George Worley at 4-Max to work out an electric set up for the Lysander following the amount of interest in such a set up.....details are here 

 

PS it looks a little bit OTT to me...not least on the price of the suggested items...but should provide a starting point.....

Edited By Steve Hargreaves on 09/11/2009 10:09:45

[Doug Ireland]

Sorry if this is a bit off topic but do all brushless motors have the same operating voltage range?

[Steve Hargreaves - Moderator]

Not an expert here Doug but no they don't.....

 

Generally though even if a motor is only rated to 3S it would probably be OK on 4S....it's the current that kills them so, as long as you stay within the max current, you should be OK. Higher voltages tend to cause problems with the insulation but at the voltages we're talking about (ie sub 40 volts generally) the insulation is unlikely to break down.

 

One issue however is the speed of the motor....put more voltage on it & it will turn faster which may cause (centrifugal) problems for the magnets or the windings....or exceed the rated speed of the bearings......to take a stupid example to illustrate the point......put a 10S pack (37 volts) across a 4000 kv motor & you would get a theoretical 150,000 rpm out of the motor which would tear it apart!!!!

[Doug Ireland]

Thanks for that Steve. Could be interesting while it lasted though!

[Nick Dainton]

.

Well the build of the Lysander is progressing along quite well. I have been keeping a close eye on weight as I go along, and at the stage I am now, all wings and fuselage covered, radio gear fitted and working, undercarriage in place.............it comes in at exactly 4lbs, which should put me in a better position to choose the power pack. it would seem that it is going to be difficult to get a definitive answer, I have spoken with many people, and their answers just `muddy the water` a little more. Based on what I now know the aircrafts true weight is (and will be) can anyone `input` their thoughts before I take the plunge?

[Bruce Richards]

So by the time you have fitted the motor and battery you will be between 4. and 5 lbs. Which suggests a power set up giving 450-500W. This is a bit much for 3S Lipo (50A) so it would be better to go for 4s or higher. A motor of 800kv on 4s will theoretically be turning at about 12000 rpm flat out which is about what I usually aim for. In reality it will be less than this. To get 500W on 4s you will need to pull 33A so you need a motor of 800Kv with a current rating of about 40A and a 50-60A ESC. The only question then is what size prop?