Hi Tim, friends, and other readers,
This note is mostly for those ECL interested parties that read these posts for the information they provide to "help them fly ECL", but are not necessarily interested in all the science that is behind everything.
To find fancy formulas to be used in system selection is admirable, but "basic" rules of thumb and basic simple formulas are very useful to those who are new to ECL and those that just want a "turn key" system to go fly stunt. The Watt #s the engineers are using on this thread are the watts required FOR THE ENTIRE FLIGHT so they can see the details of what size battery and motor meet the overall requirement.
MAKE YOUR LIFE EASY:
When selecting your system components for ECL you can use the mfg. recommendations for electric RC 3D aircraft. Almost all the mfg. (Hobby Lobby AXI, Hacker, E-Flite, etc.) web sites give us the 3D aircraft weight/power recommendations. We can use this ERC-3D recommendation for our ECL planes. This is very close to our E-CLPA mission. The following rules of thumb are based on HUNDREDS OF THOUSANDS of electric powered flights. They are very reliable #s. They have been tweaked to fit our ECL mission as close as possible.
PLEASE don't confuse some of the computations on the other parts of this thread with the most important # we can use and that is the:
"MAX WATTS AVAILABLE THAT THE SYSTEM CAN DELIVER AT ANY MOMENT"!
This means you need a system for competitive CLPA that can deliver between 175 to 200 WATTS per pound of A/C (10.9 to 12.5 watts/oz.) This # makes it very easy to select a system just using the published info. from the mfg./distributors web sites. Most of these sites give you the MAX Amps the motor can safely use. This is usually given as two #s: normal steady Amps, and MAX momentary Amps (usually <15 sec at a time). In computing our system we can use this momentary Max # because in ECL we only need our HIGH Watts # for very short bursts. For most of the motors we use in ECL these #s are in the 40 Amp to 50 amp range.
The other published # is the recommended voltage range for the motor. In our case it is mostly 14.8V
This gives us: 14.8 Volts * 50 Amps = 740 Watts 65 oz plane = 4 lb. 740 Watts / 4 lb. = 185 Watts / Pound (right where we want it :-)
I (and others, Dean, etc.) came up with this rule of thumb years ago to make it very easy for new ECL flyer's to select their components. This Watts/pound # is already corrected for the fact that your system will have lower power available toward the end of the ECL flight (reduced V, and Amps, etc.), that is why we need to start out so high.
The other important EASY rule of thumb for Battery mAh size is approximately: "60 mAh for each ounce of plane weight" In our example: 55 oz plane * 60 mAh/oz. = 3300 mAh size battery ....... 70 oz plane * 60 mAh/oz = 4200 mAh size battery.
Since we are constrained in our battery size selection by what is available we sometimes have to compromise a little on the above rule of thumb.
Dean's excellent post below helps show those interested in the "backstory" how to get to these #s. Again, all the above rules of thumb are based on hundreds of thousands of electric flights and they have proven to be solid #s at the highest levels of competition. They will certainly get a newcomer to ECL in the ball park, ..... and even to the correct row and seat.
Regards,