2007 Scale &
NonScale
Events

Electrifying
Larger Models 

Do you want
to build a 25%, 30%, 40% or larger Park Flyer? Do you know how
large of a motor you're going to need or how/what to power it
with? If you're not sure read more ...Most modelers use the watts/pounds ratio
to ballpark the power requirements for their electric model
projects. The following power/weight ratios are fairly
appropriate for matching flying styles to your various flying
projects.
Using the guidelines below,
Scott Stoops used the following to electrify his 77 wing span
Carl Goldberg Edge 540; 3D style or 150 watts/pound was his
target performance. 
Slow Flyer
Trainer  50 watts per pound
Scale
Aerobatics  100 watts per pound
3D Aerobatics  150 watts
per pound 
So if you work backwards, the Edge is advertised at 12.5 lbs
13.5 lbs if you used a 1.10 size glow motor. Knowing that hed add some weight
using an electric power system, he estimated that that his final
project would be in the 15.0 lbs range. Using that weight as a
baseline he knew he would need at least 2250 watts/pound, so set
his goal to be 3000 watts. 

There
are many ways to achieve the 2200 3000 watts, so start
with this basic equation and build backwards.
Watts = Volts x Amps 


Knowing he
wanted roughly 3000 watts, he first looked for a battery pack
that could deliver that level of power. A Duralite 10S 5000mAh
LiPoly pack could deliver 100amps while still maintaining
around 38volts under load. Knowing the available Volts and
desired Watts, we solve the equation above to determine the
rough amp load required to deliver that power.
In this case we
can see the 79amps (3000W/38V) is well within the capability of
the Duralite cells. The final combinations was the Duralite 10S batteries, Castle Creations Phoenix 110HV brushless
speed controller and the Astro 120 which is advertised to be a
1.20 1.40 size replacement, most think its closer to a 50cc
gas motor with its ability to deliver up to 4000 watts. With a
Kv (RPM/Volts) of 275, this combination gave him a setup
turning a 20x10 APC E in the low 7000 rpm. 
Good luck with your large electric project! 

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