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Feb 17, 2014, 09:21 AM
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Using eCalc to optimize motor selection.

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* Note: This article is based on an older version of eCalc and there are new features available.
* The free version of eCalc does not make available all the motors in the eCalc database, some will be greyed out until you buy a basic subscription for a few dollars.
* Most of the motor database information comes direct from manufacturers and some exaggerate or just plain don't tell the truth about their products.
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In the first tutorial we discussed using eCalc to get some basic answers
about a multirotor design. How long will it fly and can I lift that
camera and gimbal I want to use?

But eCalc can tell you much more. Let's talk about the limits and
warnings when looking at eCalc results. The mistake I made when I first
started using eCalc was to think the warnings were something I had to
stay way away from. I wanted to make sure I did not come close to those
warning limits in a design. But Markus pointed out that you want to use
your motor fully. You don't want such a powerful, and heavy, motor that
you never really use the full capacity. If you are selecting a 480 watt
motor, but the eCalc results show that, for the combination of parts you are
using, the maximum electrical power the motor is using is
only 150 watts, you are not using much of the potential of the motor. It
means that even at the highest throttle setting you can not get the full 480
watts of power.

Your design always needs to be tempered by the type of flying you do.
Are you an acrobatic (acro) flyer who really pushes your motors to the
limit with flips and fast climbing? Do you do Aerial Photography (AP)
and hover most of the time? Are you doing videography and need smooth
even passes? Each type of flying puts a different load on the motors.
ECalc tells you something about the performance for each type of flying.

For a given motor and payload combination, eCalc estimates the "flight
time," "mixed flight time," and "hover flight time." If you are an
aggressive flier, you want to maximize your "flight time," if you are
a videographer you want to maximize the "mixed flight time" and, lastly,
if you are a still photographer and hover most of the time you want the
highest "hover flight time." If you change props and check the results,
you can see that, in general, bigger diameter, lower pitch props provide more
power and lift needed for heavy camera-gimbal combinations. Yet for acrobatic
flight you may want smaller, more aggressive pitched props that are
faster, more agile and can change speed quickly. You can see the @ hover
percentage throttle going down as you increase the prop size. You will see the
flight times change in each of the categories as you experiment with different props.

You also need to watch your maximums. How much maximum current is the
motor pulling? How hot is it getting? What is the maximum power it is
pulling? The limits that eCalc warns you about are just guides. They
tell you that under maximum flight conditions the motor is pulling more
than it is designed for. So you need to take into consideration your flight
style once again. If you are a still photographer and are hovering most
of the time you can approach the limits more closely. You may seldom
drive your motors to the maximum. Let's say you have a 200 watt motor
and eCalc warns you that in maximum flight conditions you will peak at
210 watts. If you seldom fly at maximum this may not be a problem for
you. But if you fly aggressively this maximum limit may be more risk
for your motors. If you are using 125w motors and, for the given frame
and camera you have chosen, the maximum limit warns that the power used
is 250 watts you probably need to choose a more powerful motor for your
design. For average flight you want some headroom in your design but you
also don't want to under utilize your motors where the motors are rated
at, say, 480 watts and your maximum is below 200 watts.

In the example above we have a quad with 1000g of payload. The motors
selected are Sunnysky A2212-800kv and they have a maximum rating of 115
watts. In the first attempt we try 10" x 4.7" pitch props and the
throttle is at 92%. The maximum electrical power is 88.6 watts so we are
not using all of the power of the motors. We are under-propped.

Let's try a bigger diameter prop. Using 11" x4.7" props we have lowered our
hover throttle to 76%, not great but better, and our "hover flight times"
have increased and our hover current has gone down but our "flight time"
for the acro flier has gone down.

Using 12" props we are getting a warning and it says we are about 17% over the rating of the motor,
((135.1/115)-1)*100=17%. So this is probably too high for most situations and even though our "hover flight time"
is higher our "flight time" is lower.

Let's keep the 12" prop but lower the pitch to 3.8". Now we are down to only 5% over. For a non-aggressive
flier this may be acceptable. For the aggressive flier, who needs a smaller diameter prop, changing
the prop to a larger pitch can help lower the "hover throttle" and still keep the smaller, faster prop diameter.

You can also have the situation where the motors are too powerful and
you get, say, a 15% throttle at hover. This means if you just touch the
sticks you are going to shoot up. Finding the right balance between
weight, power and efficiency is the challenge.

We have not talked about the graphs in eCalc but they contain valuable
information. The motor efficiency is not linear. It means that motors
have a certain beginning and ending point range where they are most
efficient. Too low a current and the motor doesn't run well. Too high a
current and even though you add current it doesn't go much faster or
lift much more due to waste, usually in the form of heat. So you want to
find the best performance region: somewhere after the efficiency levels
off on the left part of the graph and before the waste rises too high on
the right side of the graph. For instance, although we want low current
draw during hover, we don't want to be down in the vertical part of the
graph of efficiency where the motor is not efficient at hover.

If we look at the efficiency curve, in light blue for the example above,
we can see the motor is not very efficient until it reaches 2 amps and
it peaks at about 4 amps and is pretty steady until 7 amps where it
starts to be less efficient. We can also see that there is more wasted
power, the brown curve on the bottom, as the current increases.

Each motor-load-prop combination has a sweet spot, the best balance for
your flying style. That is why you want to use eCalc for your specific
frame, payload, motors and flying style because what works for someone
else may not work best for you.

Don't forget to drop a donation in the paypal box to keep this excellent
tool updated.

Cheers and happy flying.
Last edited by mike_kelly; Aug 12, 2017 at 08:15 AM.
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Nov 03, 2014, 01:33 AM
Waste Nothing
RickC_RCAV8R's Avatar
Kudos Mike ! As usual , you tend to nail all of the heads that I am trying to police up . This better explains all of the stuff that I was either assuming , or flat out doing wrong when using Ecalc . Thanx for this tutorial post .
Latest blog entry: Filament Fridge ?
Nov 03, 2014, 10:54 AM
Thread OP
Originally Posted by RickC_RCAV8R
Kudos Mike ! As usual , you tend to nail all of the heads that I am trying to police up . This better explains all of the stuff that I was either assuming , or flat out doing wrong when using Ecalc . Thanx for this tutorial post .
Thanks again Rick. Nice to know it is worth the trouble to write it down! Thanks to my wife too who edits the stuff and makes it readable!
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