Which Motor?

[Scubajon]

I am building an 8ft wingspan Lancaster and, since it's never going to sound like the real thing whatever engine I put in it, I have decided to"go electric".  Can anyone recommend which engines I should use.  I have read many of the threads in the forumsm and it seems most of you have your own favs. All and any experience and advice would be welcome.  I have come back into flying after a long lay off and things are very different from the last time I flew!  Jon

[iawnski]

that would be lovely as a slope soarer go on youd love it ( but i am biased) lol

[Former Member]

[This posting has been removed]

[Scubajon]

.

It's from plans by Chris Golds (which I bought many years ago as a retirement project)and is estimated to be between 4.0-4.5k with IC engines. The original spec for these was 4 X HGK21's, an old derivation but the modern equivalent is avaiable.    Obviously the elecs are lighter but then there is the added weight of the batteries etc.    So until it's finished an exact weight cannot be determined.  I am intending to use  3 bladed props as per the real thing although the plans show 2 bladed ones.  Have done some research on this and it should be ok. I have been advised to use 4" props X 3 blades.  If any one has a better idea on this, I am open to advice.  Have read some of Timbo's replies to other peoples questions and he seems very knowlegable.  If you know how to get hold of him it would be appreciated.-,  unless it's just through the forums and then there's no guarantee he would answer.     

[birdy]

Timbo is the man to ask! If you can find the weight wihtout all the ic stuff then you can use the 100 watts per lb rule.

[Scubajon]

cheers.  will work it out when it's built.  try to get hold of timbo then.  ta.. scuba

[birdy]

Scuba?

[Scubajon]

short for my nickname  scubajon

[Tim Mackey]

Hello scubajon, and welcome aboard. Perhaps you have also read my 2 part article "electrikery" which is now available online here in the forum - that may also help. Your lanc is a pretty big model@8ft wingspan, and perhaps surprisingly you may well find that the AUW comes out not a lot different to the IC version....but a lot depends on how you decide to power it. If you know the airframe weight ( without motors and fuel tanks ) then you can usually reckon on adding circa 25 -30% of that for the powertrain. At around 9lb for the IC version, lets be optimistic and say you get her in at 8.5lb. 100 watts per pound is average for most sport type models,but a large scale model will often be fine on less ...say 80 or so. If you reckon on 80wplb then you need a combined power on tap of 680 watts...lets say 700. Allowing for some lower efficiency of the 3 bladers you fancy, lets nudge it up to 800. 4 motors each of around 200 watts should do it then. Now I like using large props, especially on scale warbirds - they are more efficient and generally spin at more scale like speed - but you are a bit limited with ground clearance I guess, so a little faster than ideal will probably be required. If the original was using IC 21s then I expect the prop size ( 2 blader ) would be around the 9 x 4-6 size, so going to 3 bladers I usually stick with the same pitch and drop an inch off the diameter. This leaves us with 8" props as a starting point.

So we want a motor that will swing an 8" prop and consume around 200 watts. Maybe something like these?

Or possibly these.... complete with ESC for around £35 - more powerful too

I would have normally recommended something a little lower Kv, and swinging larger props, but as already stated we need to compromise a little here. This particular Kv version of the motor will be quite powerful and swing the smaller props quite fast, yet still have sufficient watts for the wplb guide. You could even use an inrunner variant, but these do tend to be more costly, and have even higher Kv. You other decision needs to be batteries and ESCs . I would strongly recommend a seperate ESC for each motor, and if using 3S lipo and wanting around 200 watts per motor the current needs to be around the 20A mark X 4 = 80A. Depending on layout, 4 seperate batteries might be best, one for each motor and ESC - as asking for 1 x 3s pack to supply 80A is a bit too much !

In these cases we usually go up in cells and down in current but that needs different props and Kv again

As ever, there is more than one way to skin the cat

[Scubajon]

Wow, there's a lot of info there for me to digest!   Some if it I don't understand.  Will go through it again...............and again!  and get back to you.  This is great info and i can't thank you enough.  I'm sure it will help considerably,.......when I understand it all!   Will get back to you tomorrow or so when I have digested it with questions.  cheers.