Pete O Shea
Mar 01, 1997, 01:00 AM
<p><font size="4">March 1997: Simulation of Electric Power Systems</font> </p>
<p>This last week simulation software has been a hot topic on the eflight message list.
After some thought, I decided to make available a spreadsheet I've used to determine
some performance parameters. Some of the equations and thought behind them I've found
in the AstroFlight Electric Motor Handbook, by Robert Boucher. This book is a
valuable resource for electric fliers, and I recommend it. Other valuable rules of thumb
are ones pioneered by Keith Shaw, and written in several of his articles. </p>
<hr>
<p>The first page of the spreadsheet starts with a calculation of wing loading, starting
from a known wing area and a total model weight. Wing loadings for sailplanes are
about 10 to 15 oz/square foot, for sport planes 15 to 20 oz/square foot, and for heavily
loaded scale planes 20-30 oz/square foot. Higher wing loadings increase the stall
speed of a model, making landings more difficult (and exciting). </p>
<p>Next the watts/pound is presented based on various parameters entered throughout the
sheet. This is a figure of merit made well-known by Keith Shaw. In summary,
acceptable performance for an electric airplane requires a minimum of 50 watts of input
power to the motor for each pound of total model weight. Greater values of watts per
pound will result in higher climb rates and more aerobatic performance. </p>
<p>Next is a calculation of the power system weight relative to the total airplane weight.
This is another useful approximation from Keith Shaw: the weight of the cells,
motor, prop, and controller should be around half the weight of the total airplane. Then a
calculation of the minimum power needed for level flight and an approximation of stall
speed, again from Master Shaw. </p>
<p>The second page of the spreadsheet calculates the power system parameters.
Starting with the number of cells, the motor parameters, and the prop used, the
static power is calculated. A macro is used to calculate the motor power and prop power,
and stop when they are equal. This is under the static condition when the airspeed is
zero, such as when running the system on the bench. Current required during level
flight drops somewhat from the static condition as the prop unloads in the air.
Boucher gives as an approximation that a correctly chosen prop will unload about 20%
in the air. </p>
<hr>
<p>Below is a link to the Excel spreadsheet I put together to try out these equations.
It was created using Excel version 5.0a for the PC. Any equal or newer version
of Excel, for the PC or the Mac, should be able to import this file. If anyone needs
it, I can send them a copy saved in an earlier Excel format, or as delimited text. </p>
<p>The values in the spreadsheet are for my 28% scale Yellow Jacket. I relied
heavily on spreadsheet analysis to determine the best scale size for this plane, given the
Aveox brushless motor I was starting with. I've since bench tested the power system,
and the static motor current was within 5% of the predicted value. </p>
<p><a href="/articles/ezonemag/sporttop/ftp://ftp.ezonemag.com/misc/calcul8.xls">Download Pete's Excel Spreadsheet:
calcul8.xls</a></p>
<hr>
<p><font size="4">A Bit More on Arming Switches</font> </p>
<p>Last month I covered safety issues, including fuses and arming switches. There's
a few pieces of data I'd like to add: I measured a few of the arming switches I use.
These are black plastic rocker switches marketed by Jomar, and are sold through New
Creations R/C, although similar switches can be found through Digi-Key and other sources.
They are DPDT (double-pole, double-throw) switches, and I use two of the poles in
parallel for minimum resistance. I weighed a new one with a triple-beam balance and
its weight was 5.8 grams (0.20 ounces). I weighed an amount of solder sufficient to
solder to four of its terminals at 0.6 grams (0.02 ounces). At a total weight of 6.4
grams (0.22 ounces) this will not be a problem with most planes. </p>
<p>I also measured the resistance of a few of these switches, one relatively new, and one
that's been used in hundreds of flights. At 23 to 30A amp loads, their resistances
were approximately 0.3 milliohms. At 30 amps, that's a power loss of slightly more
than 1/4 watt. </p>
<hr>
<p><font size="4">Update on my winter/spring project, the Yellow Jacket</font> </p>
<p>Unfortunately, I haven't had any building time this month, so the wing is still in raw
balsa form. I have decided which covering material to use, though. I got a
sample of polyester dress lining cloth from Herm Perez and the stuff is great.
Light, strong, flexible - it should go over curves no problem. Herm has
written about his application technique in several eflight messages, and if we're lucky,
we'll get a full article from him for the EZone. </p>
<hr>
<p>See you next month, </p>
<p>Pete </p>
<p><a href="http://rcgroups.com/shared/nospam.php?u=poshea&d=ezonemag.com">E-mail</a> the author </p>
<p><a href="/articles/ezonemag/sporttop/petebio.htm">About </a>the author </p>
<p>This column is copyrighted (c) 1997 by Peter O'Shea and may not be reprinted or
retransmitted without proper attribution to the author and the E-Zone.
<p>This last week simulation software has been a hot topic on the eflight message list.
After some thought, I decided to make available a spreadsheet I've used to determine
some performance parameters. Some of the equations and thought behind them I've found
in the AstroFlight Electric Motor Handbook, by Robert Boucher. This book is a
valuable resource for electric fliers, and I recommend it. Other valuable rules of thumb
are ones pioneered by Keith Shaw, and written in several of his articles. </p>
<hr>
<p>The first page of the spreadsheet starts with a calculation of wing loading, starting
from a known wing area and a total model weight. Wing loadings for sailplanes are
about 10 to 15 oz/square foot, for sport planes 15 to 20 oz/square foot, and for heavily
loaded scale planes 20-30 oz/square foot. Higher wing loadings increase the stall
speed of a model, making landings more difficult (and exciting). </p>
<p>Next the watts/pound is presented based on various parameters entered throughout the
sheet. This is a figure of merit made well-known by Keith Shaw. In summary,
acceptable performance for an electric airplane requires a minimum of 50 watts of input
power to the motor for each pound of total model weight. Greater values of watts per
pound will result in higher climb rates and more aerobatic performance. </p>
<p>Next is a calculation of the power system weight relative to the total airplane weight.
This is another useful approximation from Keith Shaw: the weight of the cells,
motor, prop, and controller should be around half the weight of the total airplane. Then a
calculation of the minimum power needed for level flight and an approximation of stall
speed, again from Master Shaw. </p>
<p>The second page of the spreadsheet calculates the power system parameters.
Starting with the number of cells, the motor parameters, and the prop used, the
static power is calculated. A macro is used to calculate the motor power and prop power,
and stop when they are equal. This is under the static condition when the airspeed is
zero, such as when running the system on the bench. Current required during level
flight drops somewhat from the static condition as the prop unloads in the air.
Boucher gives as an approximation that a correctly chosen prop will unload about 20%
in the air. </p>
<hr>
<p>Below is a link to the Excel spreadsheet I put together to try out these equations.
It was created using Excel version 5.0a for the PC. Any equal or newer version
of Excel, for the PC or the Mac, should be able to import this file. If anyone needs
it, I can send them a copy saved in an earlier Excel format, or as delimited text. </p>
<p>The values in the spreadsheet are for my 28% scale Yellow Jacket. I relied
heavily on spreadsheet analysis to determine the best scale size for this plane, given the
Aveox brushless motor I was starting with. I've since bench tested the power system,
and the static motor current was within 5% of the predicted value. </p>
<p><a href="/articles/ezonemag/sporttop/ftp://ftp.ezonemag.com/misc/calcul8.xls">Download Pete's Excel Spreadsheet:
calcul8.xls</a></p>
<hr>
<p><font size="4">A Bit More on Arming Switches</font> </p>
<p>Last month I covered safety issues, including fuses and arming switches. There's
a few pieces of data I'd like to add: I measured a few of the arming switches I use.
These are black plastic rocker switches marketed by Jomar, and are sold through New
Creations R/C, although similar switches can be found through Digi-Key and other sources.
They are DPDT (double-pole, double-throw) switches, and I use two of the poles in
parallel for minimum resistance. I weighed a new one with a triple-beam balance and
its weight was 5.8 grams (0.20 ounces). I weighed an amount of solder sufficient to
solder to four of its terminals at 0.6 grams (0.02 ounces). At a total weight of 6.4
grams (0.22 ounces) this will not be a problem with most planes. </p>
<p>I also measured the resistance of a few of these switches, one relatively new, and one
that's been used in hundreds of flights. At 23 to 30A amp loads, their resistances
were approximately 0.3 milliohms. At 30 amps, that's a power loss of slightly more
than 1/4 watt. </p>
<hr>
<p><font size="4">Update on my winter/spring project, the Yellow Jacket</font> </p>
<p>Unfortunately, I haven't had any building time this month, so the wing is still in raw
balsa form. I have decided which covering material to use, though. I got a
sample of polyester dress lining cloth from Herm Perez and the stuff is great.
Light, strong, flexible - it should go over curves no problem. Herm has
written about his application technique in several eflight messages, and if we're lucky,
we'll get a full article from him for the EZone. </p>
<hr>
<p>See you next month, </p>
<p>Pete </p>
<p><a href="http://rcgroups.com/shared/nospam.php?u=poshea&d=ezonemag.com">E-mail</a> the author </p>
<p><a href="/articles/ezonemag/sporttop/petebio.htm">About </a>the author </p>
<p>This column is copyrighted (c) 1997 by Peter O'Shea and may not be reprinted or
retransmitted without proper attribution to the author and the E-Zone.