Pete O Shea
Dec 01, 1996, 01:00 AM
<p>Greetings, electric fliers, and welcome to my first column. I plan to write
about miscellaneous issues related to sport electric flying, such as system efficiency,
converting fuel-powered kits, safety subjects, etc. I want this column to reflect
the general interests of the electric flyer, so feel free to <a href="http://rcgroups.com/shared/nospam.php?u=pete_oshea&d=msn.com">email</a> me with column suggestions, or if you want to
add to what I've written. I also participate in the eflight mailing list, so we can
carry on discussions there also. Not on the eflight list? Go <a href="/articles/ezonemag/sporttop/../../pages/maillist.htm">here</a> now! If you want to read about me, go <a href="/articles/ezonemag/sporttop/petebio.htm">here</a>. </p>
<hr>
<p><b>December 1996: Some ways to optimize the efficiency of your power system, Part
I</b> </p>
<p>In this column, I want to describe some design considerations in how to choose and
arrange the components in the power system of an electric plane to achieve the best
possible system efficiency and lowest weight. </p>
<p>Why, if we don't plan to compete, should we try to optimize efficiency? Why don't
we just plug it in and fly? Electric planes have a limited energy source. Some
people want to get every second of duration out of every flight. Some people enjoy
the challenge of getting the maximum performance out of an airplane. Some people
want to show the fuel-powered guys that electric planes fly quieter, cleaner and <u>better</u>
than glow planes. </p>
<p>Typical sport electric planes are designed to have a balance between performance (climb
rate, speed, etc.) and duration. Since some of my calculations will be based on
assumptions or targets that may not be universally shared, please feel free to give me <a href="http://rcgroups.com/shared/nospam.php?u=pete_oshea&d=msn.com">feedback</a> on any topic covered. </p>
<h4>Component arrangement </h4>
<p>In general, arrange the motor, arming switch, speed control, fuse, and battery to
result in the shortest length of wiring possible. This will, of course, reduce weight and
electrical resistance. The motor position is pretty much fixed and the battery position
will need to be in a certain place for balancing the plane. The shortest wiring harness
will result when the arming switch, fuse, and speed control are somewhere between the
battery and the motor. It is very commonly advised that the speed control should not be
placed in front of the battery, since in a crash the battery will shift forward and crush
the speed control. I ignore this advice. I think that the probability that a shifting
battery will directly impact the speed control is fairly low, and requires a high-speed
crash at a particular angle. I haven't yet lost a speed control to a flying battery, and
I've had a few high-speed head-first crashes. </p>
<p>The arming switch and fuse, if used, should be located so that the wiring can be the
shortest possible. I plan to cover details on arming switches and fuses in an
upcoming column on safety. I solder my fuses directly into the electrical center of
the battery, which eliminates the need for fuse connectors (extra weight and contact
resistance). Since I use removable battery packs in all of my planes, the fuse gives
protection when the battery is in my flight box or being handled. </p>
<h4>Wiring harness </h4>
<p>The chief design consideration for the wiring harness is "what gauge wire do I
use?" To minimize resistive losses in the wire, the wire should be a large diameter
(low gauge). However, the weight of the wire gets larger as the size increases. What size
is big enough? Calculate the power lost in the wiring harness: </p>
<p>P = I<sup>2</sup> * R </p>
<p>For a sport plane drawing 25 amps, with a wiring harness of 20 inches of 13-gauge wire,
</p>
<p>R = 20 * 0.167 = 3.34 milliohms </p>
<p>P = 25<sup>2</sup> * 3.34x10<sup>-3</sup> = 2.09 watts </p>
<p>Is this a lot? Should we go to 12-gauge wire? Well, if the plane is running on 20 cells
with a motor input power of 560 watts or so, this loss represents about 0.4% of the total
input power. Going to 12-gauge wire would reduce the power loss to 1.65 watts, or about
0.3% of the input power. Not a significant increase. If the plane is a 7-cell
trainer or glider running at about 210 watts, this loss represents 0.9% of the input
power. So the <u>smaller</u> airplane actually has <u>more</u> need for a larger
wire diameter! But the smaller airplane can less afford the increased weight.
This is an example of how higher-voltage airplanes can attain greater efficiency,
because resistive losses are less significant when the voltage is higher but the current
is roughly the same. For the best efficiency, keep the current down! </p>
<table border="1" cellpadding="2">
<caption valign="top">Copper wire resistance and weight </caption>
<tr>
<td><p align="left">Gauge </td>
<td>Resistance,<br>
microohm/in </td>
<td>Resistance,<br>
microohm/cm </td>
<td>Weight, oz/in </td>
<td>Weight, g/cm </td>
</tr>
<tr>
<td>12 </td>
<td>132 </td>
<td>39.5 </td>
<td>0.0297 </td>
<td>0.331 </td>
</tr>
<tr>
<td>13 </td>
<td>167 </td>
<td>65.7 </td>
<td>0.0305 </td>
<td>0.339 </td>
</tr>
<tr>
<td>14 </td>
<td>210 </td>
<td>82.7 </td>
<td>0.0165 </td>
<td>0.183 </td>
</tr>
<tr>
<td>16 </td>
<td>332 </td>
<td>131 </td>
<td> </td>
<td> </td>
</tr>
<tr>
<td>18 </td>
<td>540 </td>
<td>213 </td>
<td> </td>
<td> </td>
</tr>
<tr>
<td>20 </td>
<td>843 </td>
<td>332 </td>
<td> </td>
<td> </td>
</tr>
</table>
<p>The resistance data in the above table is from various wiring manufacturers, and is not
specific to wiring available for electric planes. The resistance of wiring available
for electric planes may vary somewhat, since the resistivity of the copper depends on the
way the wire was processed (annealed, cold-drawn, etc.). </p>
<p> The weight data for 13-gauge wire is from measurements of Astro
silicone-insulated wire. The weight data for 12 and 14-gauge wire is from
measurements of Jomar wire, which has thinner insulation than the Astro wire. Note
that the Jomar 12-gauge wire is actually lighter than the Astro 13-gauge. The Astro
wire, however, has the advantage that the insulation is silicone, and can withstand
soldering temperatures without melting. I personally think the insulation on the Astro
wire is thicker than it needs to be, since all electric-powered planes are relatively
low-voltage devices and don't need insulation designed to withstand kilovolt-level
potentials. </p>
<h4>Fuse </h4>
<p>Most sport planes should have a fast-blow fuse in the wiring harness. Watch for an
upcoming column explaining my reasoning behind this. In that column, I'll go over
fuse selection. Fuse mounting, however can be done several ways. If the
typical auto blade fuse is used, spade-type connectors can be soldered into the wiring
harness. This gives the advantage of easily being able to change the fuse, but the
disadvantage of some contact resistance in the fuse-connector interface. There is
also a special Sermos connector available for mounting the fuse and using it as a type of
arming switch, which kills two birds with one stone. Soldering the fuse into the battery
pack, as I do, saves the weight and electrical resistive loss of connectors, and also
protects the battery pack against shorting at its connector terminals when it's out of the
plane. </p>
<h4>Speed control </h4>
<p>It is relatively easy to find a suitable and efficient speed control. There are
three characteristics desired: low electrical resistance in the full-on state, efficiency
in partial-throttle operation, and low weight. The on-state resistance is determined
by how many power mosfet transistors are used, and by the quality of the transistors used.
More transistors will result in lower resistance, but add weight. Most speed
controls have two to four transistors in the current path. Most speed controls have
an on-resistance of about 5 milliohms (0.005 ohms), and typical weights are about ten to
forty grams without wires. The efficiency of the speed control at partial throttle
settings is a complex interaction between the speed control and the motor, and is not
specified by speed control manufacturers. A high-rate speed control, which switches
the motor power at frequencies of a few kilohertz, has much better partial-throttle
efficiency than a frame-rate speed control. Most sport planes will be flown at
varying throttle settings, so a high-rate speed control is a must. </p>
<p>Some speed controls have a BEC design (battery eliminator circuitry). This is a
circuit that regulates the power from the motor battery to a constant voltage for powering
the radio receiver and servos, allowing the radio battery to be left out and saving
weight. BEC designs have some restrictions: typically the cell count for the motor
can't be below 6 cells or above 10 cells. In addition, high numbers of servos in the
plane draw extra power, increasing the load on the BEC regulator. The use of BEC
circuitry is debatable: some weight savings is achieved, but at the expense of the
possible loss of power to the radio system if the BEC regulator is overtaxed. </p>
<p>Part II of this column will cover cells, motor choices, and prop selection. See
you in January, and may you all find neat airplane goodies under the Christmas tree. </p>
<p><a href="http://rcgroups.com/shared/nospam.php?u=pete_oshea&d=msn.com">E-mail</a> the author </p>
<p><a href="/articles/ezonemag/sporttop/petebio.htm">About</a> the author </p>
<hr>
<p>This column is copyrighted (c) 1996 by Peter O'Shea and may not be reprinted or
retransmitted without proper attribution to the author and the E-Zone.
about miscellaneous issues related to sport electric flying, such as system efficiency,
converting fuel-powered kits, safety subjects, etc. I want this column to reflect
the general interests of the electric flyer, so feel free to <a href="http://rcgroups.com/shared/nospam.php?u=pete_oshea&d=msn.com">email</a> me with column suggestions, or if you want to
add to what I've written. I also participate in the eflight mailing list, so we can
carry on discussions there also. Not on the eflight list? Go <a href="/articles/ezonemag/sporttop/../../pages/maillist.htm">here</a> now! If you want to read about me, go <a href="/articles/ezonemag/sporttop/petebio.htm">here</a>. </p>
<hr>
<p><b>December 1996: Some ways to optimize the efficiency of your power system, Part
I</b> </p>
<p>In this column, I want to describe some design considerations in how to choose and
arrange the components in the power system of an electric plane to achieve the best
possible system efficiency and lowest weight. </p>
<p>Why, if we don't plan to compete, should we try to optimize efficiency? Why don't
we just plug it in and fly? Electric planes have a limited energy source. Some
people want to get every second of duration out of every flight. Some people enjoy
the challenge of getting the maximum performance out of an airplane. Some people
want to show the fuel-powered guys that electric planes fly quieter, cleaner and <u>better</u>
than glow planes. </p>
<p>Typical sport electric planes are designed to have a balance between performance (climb
rate, speed, etc.) and duration. Since some of my calculations will be based on
assumptions or targets that may not be universally shared, please feel free to give me <a href="http://rcgroups.com/shared/nospam.php?u=pete_oshea&d=msn.com">feedback</a> on any topic covered. </p>
<h4>Component arrangement </h4>
<p>In general, arrange the motor, arming switch, speed control, fuse, and battery to
result in the shortest length of wiring possible. This will, of course, reduce weight and
electrical resistance. The motor position is pretty much fixed and the battery position
will need to be in a certain place for balancing the plane. The shortest wiring harness
will result when the arming switch, fuse, and speed control are somewhere between the
battery and the motor. It is very commonly advised that the speed control should not be
placed in front of the battery, since in a crash the battery will shift forward and crush
the speed control. I ignore this advice. I think that the probability that a shifting
battery will directly impact the speed control is fairly low, and requires a high-speed
crash at a particular angle. I haven't yet lost a speed control to a flying battery, and
I've had a few high-speed head-first crashes. </p>
<p>The arming switch and fuse, if used, should be located so that the wiring can be the
shortest possible. I plan to cover details on arming switches and fuses in an
upcoming column on safety. I solder my fuses directly into the electrical center of
the battery, which eliminates the need for fuse connectors (extra weight and contact
resistance). Since I use removable battery packs in all of my planes, the fuse gives
protection when the battery is in my flight box or being handled. </p>
<h4>Wiring harness </h4>
<p>The chief design consideration for the wiring harness is "what gauge wire do I
use?" To minimize resistive losses in the wire, the wire should be a large diameter
(low gauge). However, the weight of the wire gets larger as the size increases. What size
is big enough? Calculate the power lost in the wiring harness: </p>
<p>P = I<sup>2</sup> * R </p>
<p>For a sport plane drawing 25 amps, with a wiring harness of 20 inches of 13-gauge wire,
</p>
<p>R = 20 * 0.167 = 3.34 milliohms </p>
<p>P = 25<sup>2</sup> * 3.34x10<sup>-3</sup> = 2.09 watts </p>
<p>Is this a lot? Should we go to 12-gauge wire? Well, if the plane is running on 20 cells
with a motor input power of 560 watts or so, this loss represents about 0.4% of the total
input power. Going to 12-gauge wire would reduce the power loss to 1.65 watts, or about
0.3% of the input power. Not a significant increase. If the plane is a 7-cell
trainer or glider running at about 210 watts, this loss represents 0.9% of the input
power. So the <u>smaller</u> airplane actually has <u>more</u> need for a larger
wire diameter! But the smaller airplane can less afford the increased weight.
This is an example of how higher-voltage airplanes can attain greater efficiency,
because resistive losses are less significant when the voltage is higher but the current
is roughly the same. For the best efficiency, keep the current down! </p>
<table border="1" cellpadding="2">
<caption valign="top">Copper wire resistance and weight </caption>
<tr>
<td><p align="left">Gauge </td>
<td>Resistance,<br>
microohm/in </td>
<td>Resistance,<br>
microohm/cm </td>
<td>Weight, oz/in </td>
<td>Weight, g/cm </td>
</tr>
<tr>
<td>12 </td>
<td>132 </td>
<td>39.5 </td>
<td>0.0297 </td>
<td>0.331 </td>
</tr>
<tr>
<td>13 </td>
<td>167 </td>
<td>65.7 </td>
<td>0.0305 </td>
<td>0.339 </td>
</tr>
<tr>
<td>14 </td>
<td>210 </td>
<td>82.7 </td>
<td>0.0165 </td>
<td>0.183 </td>
</tr>
<tr>
<td>16 </td>
<td>332 </td>
<td>131 </td>
<td> </td>
<td> </td>
</tr>
<tr>
<td>18 </td>
<td>540 </td>
<td>213 </td>
<td> </td>
<td> </td>
</tr>
<tr>
<td>20 </td>
<td>843 </td>
<td>332 </td>
<td> </td>
<td> </td>
</tr>
</table>
<p>The resistance data in the above table is from various wiring manufacturers, and is not
specific to wiring available for electric planes. The resistance of wiring available
for electric planes may vary somewhat, since the resistivity of the copper depends on the
way the wire was processed (annealed, cold-drawn, etc.). </p>
<p> The weight data for 13-gauge wire is from measurements of Astro
silicone-insulated wire. The weight data for 12 and 14-gauge wire is from
measurements of Jomar wire, which has thinner insulation than the Astro wire. Note
that the Jomar 12-gauge wire is actually lighter than the Astro 13-gauge. The Astro
wire, however, has the advantage that the insulation is silicone, and can withstand
soldering temperatures without melting. I personally think the insulation on the Astro
wire is thicker than it needs to be, since all electric-powered planes are relatively
low-voltage devices and don't need insulation designed to withstand kilovolt-level
potentials. </p>
<h4>Fuse </h4>
<p>Most sport planes should have a fast-blow fuse in the wiring harness. Watch for an
upcoming column explaining my reasoning behind this. In that column, I'll go over
fuse selection. Fuse mounting, however can be done several ways. If the
typical auto blade fuse is used, spade-type connectors can be soldered into the wiring
harness. This gives the advantage of easily being able to change the fuse, but the
disadvantage of some contact resistance in the fuse-connector interface. There is
also a special Sermos connector available for mounting the fuse and using it as a type of
arming switch, which kills two birds with one stone. Soldering the fuse into the battery
pack, as I do, saves the weight and electrical resistive loss of connectors, and also
protects the battery pack against shorting at its connector terminals when it's out of the
plane. </p>
<h4>Speed control </h4>
<p>It is relatively easy to find a suitable and efficient speed control. There are
three characteristics desired: low electrical resistance in the full-on state, efficiency
in partial-throttle operation, and low weight. The on-state resistance is determined
by how many power mosfet transistors are used, and by the quality of the transistors used.
More transistors will result in lower resistance, but add weight. Most speed
controls have two to four transistors in the current path. Most speed controls have
an on-resistance of about 5 milliohms (0.005 ohms), and typical weights are about ten to
forty grams without wires. The efficiency of the speed control at partial throttle
settings is a complex interaction between the speed control and the motor, and is not
specified by speed control manufacturers. A high-rate speed control, which switches
the motor power at frequencies of a few kilohertz, has much better partial-throttle
efficiency than a frame-rate speed control. Most sport planes will be flown at
varying throttle settings, so a high-rate speed control is a must. </p>
<p>Some speed controls have a BEC design (battery eliminator circuitry). This is a
circuit that regulates the power from the motor battery to a constant voltage for powering
the radio receiver and servos, allowing the radio battery to be left out and saving
weight. BEC designs have some restrictions: typically the cell count for the motor
can't be below 6 cells or above 10 cells. In addition, high numbers of servos in the
plane draw extra power, increasing the load on the BEC regulator. The use of BEC
circuitry is debatable: some weight savings is achieved, but at the expense of the
possible loss of power to the radio system if the BEC regulator is overtaxed. </p>
<p>Part II of this column will cover cells, motor choices, and prop selection. See
you in January, and may you all find neat airplane goodies under the Christmas tree. </p>
<p><a href="http://rcgroups.com/shared/nospam.php?u=pete_oshea&d=msn.com">E-mail</a> the author </p>
<p><a href="/articles/ezonemag/sporttop/petebio.htm">About</a> the author </p>
<hr>
<p>This column is copyrighted (c) 1996 by Peter O'Shea and may not be reprinted or
retransmitted without proper attribution to the author and the E-Zone.