Pete O Shea
Jan 01, 1997, 01:00 AM
<p>Welcome back, electric fliers. I hope everyone found cool and useful airplane
stuff under their Christmas tree! </p>
<hr>
<h3>Some ways to optimize the efficiency of your power system, Part II </h3>
<p><b>Motor</b> </p>
<p>The motor is usually the first component chosen for a plane. There are three
classes of motors available today for planes: brushed motors with ferrite magnets, brushed
motors with cobalt magnets, and brushless motors. Ferrite motors are the least
expensive, and have the lowest efficiency. That is, the output mechanical power
delivered to the prop is low relative to the electrical power put into the motor.
For ferrite motors, efficiency can be as high as 70%. Cobalt motors have more
powerful (and more expensive) magnets, and have efficiencies up to about 80%.
Brushless motors, the most expensive choice, eliminate contact losses from the brush
and commutator, and have efficiencies up to about 90%. </p>
<p>The choice of motor for a given plane first depends on how much power is needed to fly
the plane. Keith Shaw's watts-per-pound approximations are the usual starting point.
For good sport performance, 50 to 60 watts of input power per pound of all-up
airplane weight is the usual starting point. Higher performance sufficient for
aerobatics requires perhaps 80 watts per pound. These approximations are based
on input power to the motor, P = V * I. </p>
<p>A comprehensive table of motor data can be found on Ken Myer's web site <a href="http://members.aol.com/kmyersefo/mtrdata.pdf">here</a>, in Adobe pdf format.
Information on Astro motors can be found on the Astro web page at <a href="http://www.astroflight.com">www.astroflight.com</a>. Information on Aveox
brushless motors can be found on their web page at <a href="http://www.aveox.com">www.aveox.com</a>.
Choosing just the right motor for a given plane involves balancing many factors,
some of them conflicting. If you're unsure, ask the experts on the <a href="/articles/ezonemag/sporttop/../../pages/maillist.htm">eflight mailing list. </a> </p>
<p><b>Cells</b> </p>
<p>We all want our airplanes to stay up indefinitely (or at least until we get tired).
The first way to increase the flight time is to use a cell with a high capacity,
measured in amp-hours (Ah) or milliamp-hours (maH). This is the amount of current the cell
can deliver for a one-hour period. A cell with a 2000 maH rating can deliver 2000
milliamps (2 amps) for one hour. The same cell can nominally also deliver twice the
current (4 amps) for half an hour, or 8 amps for fifteen minutes, etc.
Unfortunately, it's not quite that simple, because of internal resistance. </p>
<p>The primary way efficiency is lost in a cell is by its internal resistance. The
power loss due to this is expressed as P = I<sup>2</sup> * R, where P is power lost in
watts, I is current in amps, and R is cell resistance in ohms. This power
loss will be converted to heating the cell, which will reduce its capacity from the
nominal rated value. So the 2000 maH cell, instead of delivering 8 amps for fifteen
minutes, may only deliver the 8 amps for fourteen minutes. Heating the cell also shortens
its lifetime somewhat. Obviously, keeping the current low helps greatly, because of
the squared term. But the current needed to fly a plane can only be varied over a
certain range. Using a cell with a low internal resistance is very important. </p>
<p>So when choosing a cell, we only need to look for high capacity and low internal
resistance, correct? Wrong, because of our number-one nemesis in electric flight, <b>weight</b>.
High-capacity cells with low internal resistance also weigh more. The weight of a
nicad cell increases with increasing capacity, so we need to choose a cell that has
sufficient capacity for our flight target times, but has low weight and low internal
resistance. </p>
<p>How to do that? Well, the E-zone's got you covered. A good table of data on
all Sanyo cells can be found <a href="/articles/ezonemag/sporttop/../1996/sanyodat.htm">here</a>. Most electric
fliers use Sanyo cells because they have high-quality cells with specifications that are
suitable for electric flight. If you've seen the table, you'll have noticed that
Sanyo makes over fifty different kinds of cells. Don't be confused, there are only a
few different types that are commonly used for electric flight. The most common
right now is the N1700SCRC, which offers relatively high capacity, low weight, and low
internal resistance. It was only a few years ago that Sanyo's best comparable cell
was the 1200SCR, so you can see that the electrochemists and engineers have found nifty
ways to give us cells with higher capacity. Sanyo is just now introducing a 2000SCR
cell with reportedly the same size as the 1700SCRC, a weight only a gram or two more, and
comparable internal resistance. If you can find them and afford them, they will be
great for general-purpose sport flying! </p>
<p>For planes that can't carry the weight of the 1700SCRC and don't need the capacity,
such as electric-powered gliders, the 1000SCR cell is often used. Small planes, such
as Speed 400 types, need even lighter cells. For these planes the KR600AE and the
N500AR are typically used. </p>
<h4>Prop </h4>
<p>Ah, the propeller. This is the one element of the power system that has a big
influence on how the plane flies, can be changed easily, and is relatively cheap.
Consequently, for each of my planes I have a large collection of props of different
sizes, and from different manufacturers. Why do I have lots of props? Because
it's difficult, if not impossible, to quantify in advance just which propeller will give
the best balance of speed, climb performance, and run time for a given airplane.
Trial-and-error experimentation is the only way to find the perfect prop. </p>
<p>However, there are some approximations that give a place to start. The ratio of
the pitch to the diameter of the prop is usually between 0.5 and 1.0. As a very
rough approximation, a lower ratio will give better climb, but limited full speed in level
flight. A higher ratio will give a greater maximum speed, but with less climbing
ability. Again, getting advice from someone with a similar plane is a good place to
start. And remember to balance your props to get smoother running and better
efficiency. </p>
<p><b>Simulation and prediction software</b> </p>
<p>Software is available that can predict, with acceptable accuracy, some of the flight
parameters of a given airplane, motor, cell, and propeller combination. Using
prediction software, many different combinations of equipment can be simulated on the
ground, until an acceptable solution is found. This can save time and money over
buying the equipment and testing it out in an actual airplane. Many electric fliers
use Electricalc, written by Sid Kaufman, and available through Aveox, New Creations RC,
and other places. Some (like me) like to tinker with the equations themselves in a
spreadsheet. </p>
<hr>
<p align="center"><font size="4">My winter project, the Yellow Jacket</font> </p>
<p>My current building project is a 28% scale model of a Formula I racer, the Yellow
Jacket. I had just won an Aveox motor and controller system in the KRC raffle, and
was looking for a new plane to build for it. I came across a 3-view in a book about
racing planes I borrowed from Jim Bourke, and couldn't resist the yellow paint job with
the black curvy bee-like stripes. Here's the 3-view: </p>
<p align="center"><img src="http://static.rcgroups.com/articles/ezonemag/sporttop/../../images/yellow.gif" width="677" height="900"> </p>
<p>After a bunch of number-crunching, I decided to build it with the following specs:
<ul>
<li>Wingspan 63.8" (162 cm) </li>
<li>Root chord 14.3" (36.3 cm) </li>
<li>Length 59.4" (151 cm) </li>
<li>Wing area 745 sq. in (4800 sq. cm) </li>
<li>Target weight 9.0 lb (4.0 kg), wing loading 27.5 oz/sq. ft (not sure of metric units) </li>
</ul>
<p>With the generous help of Tom Hunt, I've got a 3-view scaled up to the correct size,
and am working on designing the structure. I'm planning on a one-piece wing with the
cockpit area pulling away from the fuse. The fuse will be formers and stringers, but
I'm not sure if I'll build it left-and-right side, or top-and-bottom halves. I'm
going to try to make carbon fiber plug-in landing gear, and will have lots of fun making
molds and fiberglass parts for the cowl and cheeks. I haven't decided on covering
yet - perhaps preprimed Micafilm painted with the yellow and black, if the weight will be
low enough. As on all of my other planes, I'll have a battery hatch, probably on
the fuse bottom, for changing packs between flights. </p>
<p>The power system will be an Aveox 1412/4y with a Model-Air Tech belt drive with a 4.0/1
ratio. I'll start with 28 cells and a 17x12 prop. I've bench-tested this
power system with an 18x12 prop and found a static current of just under 30 amps (within a
few percent of the predicted value!). </p>
<p>The amount of power produced by this system is impressive and a bit scary. Next
month I'll be writing about electric system safety. I feel better already. </p>
<p>See you next month. </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>
<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.
stuff under their Christmas tree! </p>
<hr>
<h3>Some ways to optimize the efficiency of your power system, Part II </h3>
<p><b>Motor</b> </p>
<p>The motor is usually the first component chosen for a plane. There are three
classes of motors available today for planes: brushed motors with ferrite magnets, brushed
motors with cobalt magnets, and brushless motors. Ferrite motors are the least
expensive, and have the lowest efficiency. That is, the output mechanical power
delivered to the prop is low relative to the electrical power put into the motor.
For ferrite motors, efficiency can be as high as 70%. Cobalt motors have more
powerful (and more expensive) magnets, and have efficiencies up to about 80%.
Brushless motors, the most expensive choice, eliminate contact losses from the brush
and commutator, and have efficiencies up to about 90%. </p>
<p>The choice of motor for a given plane first depends on how much power is needed to fly
the plane. Keith Shaw's watts-per-pound approximations are the usual starting point.
For good sport performance, 50 to 60 watts of input power per pound of all-up
airplane weight is the usual starting point. Higher performance sufficient for
aerobatics requires perhaps 80 watts per pound. These approximations are based
on input power to the motor, P = V * I. </p>
<p>A comprehensive table of motor data can be found on Ken Myer's web site <a href="http://members.aol.com/kmyersefo/mtrdata.pdf">here</a>, in Adobe pdf format.
Information on Astro motors can be found on the Astro web page at <a href="http://www.astroflight.com">www.astroflight.com</a>. Information on Aveox
brushless motors can be found on their web page at <a href="http://www.aveox.com">www.aveox.com</a>.
Choosing just the right motor for a given plane involves balancing many factors,
some of them conflicting. If you're unsure, ask the experts on the <a href="/articles/ezonemag/sporttop/../../pages/maillist.htm">eflight mailing list. </a> </p>
<p><b>Cells</b> </p>
<p>We all want our airplanes to stay up indefinitely (or at least until we get tired).
The first way to increase the flight time is to use a cell with a high capacity,
measured in amp-hours (Ah) or milliamp-hours (maH). This is the amount of current the cell
can deliver for a one-hour period. A cell with a 2000 maH rating can deliver 2000
milliamps (2 amps) for one hour. The same cell can nominally also deliver twice the
current (4 amps) for half an hour, or 8 amps for fifteen minutes, etc.
Unfortunately, it's not quite that simple, because of internal resistance. </p>
<p>The primary way efficiency is lost in a cell is by its internal resistance. The
power loss due to this is expressed as P = I<sup>2</sup> * R, where P is power lost in
watts, I is current in amps, and R is cell resistance in ohms. This power
loss will be converted to heating the cell, which will reduce its capacity from the
nominal rated value. So the 2000 maH cell, instead of delivering 8 amps for fifteen
minutes, may only deliver the 8 amps for fourteen minutes. Heating the cell also shortens
its lifetime somewhat. Obviously, keeping the current low helps greatly, because of
the squared term. But the current needed to fly a plane can only be varied over a
certain range. Using a cell with a low internal resistance is very important. </p>
<p>So when choosing a cell, we only need to look for high capacity and low internal
resistance, correct? Wrong, because of our number-one nemesis in electric flight, <b>weight</b>.
High-capacity cells with low internal resistance also weigh more. The weight of a
nicad cell increases with increasing capacity, so we need to choose a cell that has
sufficient capacity for our flight target times, but has low weight and low internal
resistance. </p>
<p>How to do that? Well, the E-zone's got you covered. A good table of data on
all Sanyo cells can be found <a href="/articles/ezonemag/sporttop/../1996/sanyodat.htm">here</a>. Most electric
fliers use Sanyo cells because they have high-quality cells with specifications that are
suitable for electric flight. If you've seen the table, you'll have noticed that
Sanyo makes over fifty different kinds of cells. Don't be confused, there are only a
few different types that are commonly used for electric flight. The most common
right now is the N1700SCRC, which offers relatively high capacity, low weight, and low
internal resistance. It was only a few years ago that Sanyo's best comparable cell
was the 1200SCR, so you can see that the electrochemists and engineers have found nifty
ways to give us cells with higher capacity. Sanyo is just now introducing a 2000SCR
cell with reportedly the same size as the 1700SCRC, a weight only a gram or two more, and
comparable internal resistance. If you can find them and afford them, they will be
great for general-purpose sport flying! </p>
<p>For planes that can't carry the weight of the 1700SCRC and don't need the capacity,
such as electric-powered gliders, the 1000SCR cell is often used. Small planes, such
as Speed 400 types, need even lighter cells. For these planes the KR600AE and the
N500AR are typically used. </p>
<h4>Prop </h4>
<p>Ah, the propeller. This is the one element of the power system that has a big
influence on how the plane flies, can be changed easily, and is relatively cheap.
Consequently, for each of my planes I have a large collection of props of different
sizes, and from different manufacturers. Why do I have lots of props? Because
it's difficult, if not impossible, to quantify in advance just which propeller will give
the best balance of speed, climb performance, and run time for a given airplane.
Trial-and-error experimentation is the only way to find the perfect prop. </p>
<p>However, there are some approximations that give a place to start. The ratio of
the pitch to the diameter of the prop is usually between 0.5 and 1.0. As a very
rough approximation, a lower ratio will give better climb, but limited full speed in level
flight. A higher ratio will give a greater maximum speed, but with less climbing
ability. Again, getting advice from someone with a similar plane is a good place to
start. And remember to balance your props to get smoother running and better
efficiency. </p>
<p><b>Simulation and prediction software</b> </p>
<p>Software is available that can predict, with acceptable accuracy, some of the flight
parameters of a given airplane, motor, cell, and propeller combination. Using
prediction software, many different combinations of equipment can be simulated on the
ground, until an acceptable solution is found. This can save time and money over
buying the equipment and testing it out in an actual airplane. Many electric fliers
use Electricalc, written by Sid Kaufman, and available through Aveox, New Creations RC,
and other places. Some (like me) like to tinker with the equations themselves in a
spreadsheet. </p>
<hr>
<p align="center"><font size="4">My winter project, the Yellow Jacket</font> </p>
<p>My current building project is a 28% scale model of a Formula I racer, the Yellow
Jacket. I had just won an Aveox motor and controller system in the KRC raffle, and
was looking for a new plane to build for it. I came across a 3-view in a book about
racing planes I borrowed from Jim Bourke, and couldn't resist the yellow paint job with
the black curvy bee-like stripes. Here's the 3-view: </p>
<p align="center"><img src="http://static.rcgroups.com/articles/ezonemag/sporttop/../../images/yellow.gif" width="677" height="900"> </p>
<p>After a bunch of number-crunching, I decided to build it with the following specs:
<ul>
<li>Wingspan 63.8" (162 cm) </li>
<li>Root chord 14.3" (36.3 cm) </li>
<li>Length 59.4" (151 cm) </li>
<li>Wing area 745 sq. in (4800 sq. cm) </li>
<li>Target weight 9.0 lb (4.0 kg), wing loading 27.5 oz/sq. ft (not sure of metric units) </li>
</ul>
<p>With the generous help of Tom Hunt, I've got a 3-view scaled up to the correct size,
and am working on designing the structure. I'm planning on a one-piece wing with the
cockpit area pulling away from the fuse. The fuse will be formers and stringers, but
I'm not sure if I'll build it left-and-right side, or top-and-bottom halves. I'm
going to try to make carbon fiber plug-in landing gear, and will have lots of fun making
molds and fiberglass parts for the cowl and cheeks. I haven't decided on covering
yet - perhaps preprimed Micafilm painted with the yellow and black, if the weight will be
low enough. As on all of my other planes, I'll have a battery hatch, probably on
the fuse bottom, for changing packs between flights. </p>
<p>The power system will be an Aveox 1412/4y with a Model-Air Tech belt drive with a 4.0/1
ratio. I'll start with 28 cells and a 17x12 prop. I've bench-tested this
power system with an 18x12 prop and found a static current of just under 30 amps (within a
few percent of the predicted value!). </p>
<p>The amount of power produced by this system is impressive and a bit scary. Next
month I'll be writing about electric system safety. I feel better already. </p>
<p>See you next month. </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>
<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.