wot 4 classic conversion

[rcaddict]

thanks steve, thats good info, i would have put a G46 in it but they are out of stock !!

[Geoff S]

I've just this evening done a quick run up of the motor for my Ballerina. It's a Turnigy SK3 Aerodrive 740 rpm/volt also on 4S LiPo. On a 12x6 APCE it draws around 30 amps with power consumption of 480 watts. So you see a higher kv will draw quite a bit more current and hence greater power for a given prop. As the motor maximum current is 50 amps IIRC and I have a 60 amp esc upping the power by fitting a bigger prop will be easy to do. I think BEB was the one who recommended this combination back in January and it seems spot on.

One nice thing about electric propulsion is the ease of checking the drive train in the comfort of the workshop

Geoff

[Biggles' Elder Brother - Moderator]

Posted by Steve Hargreaves - Moderator on 02/08/2016 20:41:41:

Posted by Peter Christy on 14/07/2016 23:08:32:

BEB: That is a perfect analysis of the situation!

Now why hasn't that appeared as a magazine article? Or did I miss it?

--

Pete

Er....if I could point the honourable gentleman at page 80 in the Autumn 2013 Special......(Gawd is it really that long ago!!!)

OG the motor you have fitted has a kv of 410 compared with 675 for the G46...this means it will spin 265 rpm less PER VOLT APPLIED.....if we consider a 4S battery will be at about 13 Volts under load that's...er...er...3,445 rpm LESS than expected.....that equates to an awful lot of power. the way round this is to fit a larger prop to make the motor work harder by shifting more air....as indeed you have found out by fitting the 15" prop.

The relationship between the motor kv, the prop & the voltage applied is THE key to understanding electric flight.....after this everything else is just Ohms law......

Wow! That's spooky Steve! I honestly don't remember the article - that's the fault of my memory not your excellent article. But then I suppose both of us being engineers leads to similar thought patterns! And logic is logic afterall!

BEB

[Geoff S]

Posted by Biggles' Elder Brother - Moderator on 02/08/2016 21:52:36:

Wow! That's spooky Steve! I honestly don't remember the article - that's the fault of my memory not your excellent article. But then I suppose both of us being engineers leads to similar thought patterns! And logic is logic after all!

BEB

Memories are funny things. I returned to work for a company I'd left 2 years before and the first job I was given was to do some modifications on a project they told me I'd designed first time around. The only reason I knew they weren't kidding me was because my name was on all the drawings and documentation. I had zero recollection of ever having seen it before. Frightening!

Geoff

[Steve Hargreaves - Moderator]

Posted by Biggles' Elder Brother - Moderator on 02/08/2016 21:52:36

Wow! That's spooky Steve! I honestly don't remember the article - that's the fault of my memory not your excellent article. But then I suppose both of us being engineers leads to similar thought patterns! And logic is logic afterall!

BEB

I don't know......you spend all that time slaving over a hot keyboard.......

[Jonathan M]

Posted by Biggles' Elder Brother - Moderator on 14/07/2016 20:47:31:

OK lets do this the right way round! With IC you start with the engine - but you don't with electric power. Paradoxically the motor is almost the last thing you select.

We start with the weight..

Hope this helps not just in answering your direct question but in terms of equiping you to be able to work it out for yourself in future.

Thank you so much for explaining all this!

I'm looking to transplant the whole power-train from an E-Flite foam Hurricane into a Wot4 balsa ARTF (I'm happy with electric but really dislike foam, and have little budget left for new power-plant components, batts, etc). So I entered the method explained onto a spreadsheet and ran through the main inputs (excl prop sizes) to find out if it was feasible, then fiddled with some variables to see how these effected performance and flight times (each time leaving everything else the same):

E-FLTE foam HURRICANE, as existing:

• 4.6lbs AUW, 460sqin wing area (roughly similar to the WOT4?).
• 60A BEC ESC, 25BL outrunner 1000Kv (don't know what the 25BL stands for), 11.75x7 prop.
• Assume 75W/lb for scale flying, 65% average power, and a safe 35% capacity left in battery.
• 3s 11.4v 3300mAh 40C Purple Power pack produces a calculated result of 7mins flying time, which tallies exactly with my experience at the patch.

WOT4 balsa ARTF, with transplanted electrics:

• 4.8lbs AUW
• Same electrical equipment and battery as above (assume the 60A ESC and 1000Kv motor are right?)
• Same 65% average power and 35% remaining capacity after flight.
• Assume 100W/lb for full sports flying.
• Using same existing 3s 11.4v 3300mAh pack indicates a max of only 5 mins flying which is pants.

THEN, EITHER - reducing power-to-weight requirements:

• 5.7 mins at 90W/lb (moderate sports)
• 6.4 mins at 80W/lb (gentle sports)
• 7.3 mins at 70W/lb (quiet pottering), which all remains pants!

OR - increasing cells/voltage and capacity (allowing for marginal weight increases at each step):

• 6.4 mins - 14.8v 3300mAh
• 7.0 mins - 14.8v 3700mAh
• 8.3 mins - 14.8v 4500mAh, which still isn't great and is beginning to get very expensive!

Maybe, instead of pushing this any further, is there a slightly smaller/lighter alternative to the WOT4 that will suit my existing electrics (not foam, ARTF preferably but a kit if need be)? A robust tail-wheel intermediate sports flyer with similar wing-loading and good manners, and capable of the same wide performance window?

Thanks,

Jon

[Jonathan M]

PS

Of course I could add roughly a minute to each of the above proposed WOT4 flight times by running batts down further, say to to 25% capacity (thus the existing 3s 3300mAh 4-Max Lipos could give a max of about 7mins with non-extreme sports flying), but past experience of being cavalier with this resulted in shorter battery lives and also a couple of puffed-up units despite never charging at more than 1C.

[Richard Wood]

Posted by Jonathan M on 10/09/2016 13:53:04:

Posted by Biggles' Elder Brother - Moderator on 14/07/2016 20:47:31:

Maybe, instead of pushing this any further, is there a slightly smaller/lighter alternative to the WOT4 that will suit my existing electrics (not foam, ARTF preferably but a kit if need be)? A robust tail-wheel intermediate sports flyer with similar wing-loading and good manners, and capable of the same wide performance window?

Thanks,

Jon

Hi Jon,
Your 3S, 3300mAH battery is really too small to get good performance & duration
from a Wot4 IMHO.
You could consider the Uno-Wot. Not as aerobatic as the Wot4 but flies well with
the reduced dihedral as detailed in the instructions.
It should suit your electric set up very well.

[Jonathan M]

Posted by Richard Wood on 12/09/2016 08:11:36:

Hi Jon,
Your 3S, 3300mAH battery is really too small to get good performance & duration
from a Wot4 IMHO.
You could consider the Uno-Wot. Not as aerobatic as the Wot4 but flies well with
the reduced dihedral as detailed in the instructions.
It should suit your electric set up very well.

 

Thanks Richard, the Uno-Wot actually seems like a very good option. As befits a trainer design, it'll likely be more floaty (wing loading 15oz/sqft) than the Wot 4 (20oz.sqft), but the idea is for a gentler flying experience on calmer days. Plus it'll make a good trainer for when my son is old enough to concentrate for long enough!

Edited By Jonathan M on 12/09/2016 09:29:19