Rafmagnsflug
Eins og sumir vita þá eru rafmagnsmódel ekki síðri en bensínmódelin
nema náttúrulega þau eru hljóðlátari, eða hvað?
F5B er keppnisflokkur rafmagnsvéla, þær geta náð talsverðum hraða, eða
uppfyrir 150 mílur sem er í kringum 240 km/klst.
Eins og gefur að skilja hvín talsvert í þeim á þessari ferð.
Mjög skemmtileg módel ef þú ert í pyloni.
Hljóðdæmi
Hér fyrir neðan getur fundið svör við nokkrum spurningum(FAQ) sem koma upp
þegar að fólk er að spá í rafmagnsflug.
Listanum er haldið við af E Zone.
FAQ
ISSUE DATE: Feb 1999
MAINTAINER: Steve Lewin, faq-maintainer@ezonemag.com
COMMENTS ON THIS REVISION (Steve Lewin, faq-maintainer@ezonemag.com)
I have taken over the maintenance of the FAQ from Craig Limber to whom
many thanks for his efforts to date. You might find the spelling of some
of the newer bits is a bit variable. I'll try to remember to write in
American but I'll probably slip into British English from time to time.
This FAQ has been reformatted to make more use of HTML features like
tables and hyperlinks. For example you can now jump direct from the
contents list to the various sections.
"What does "C" mean?" added to the Battery section.
"Where do I plug my BEC controller in" added to the Speed Controls
section.
Information about what sensor/sensorless means has been added to the
Electric Motors section.
Additional example models have been added to the "What works" section.
I have corrected a few out of date references in the Internet resources
section and added one or two of my own favorites.
An entry in bold text in the contents list means that
entry has been changed or added in the latest version of the FAQ.
Back to
Contents
The following articles contain the answers to some Frequently Asked
Questions (FAQ) about electric flight. They attempt to collect much of the
common wisdom useful for the construction and successful operation of
electrically powered flying models into one place as well as providing
some "food for thought" regarding why you might choose electric power over
glow fuel, gas, etc.
Each question or article is attributed to the individual or group who
contributed the article/answer. In many cases Q&A was a discussion
thread on the Eflight electronic mailing list. Unless otherwise specified,
all monetary amounts are in US dollars.
You are encouraged to submit any questions on electric flight subjects.
If you have the answers too that is even better but questions that you
have not been able to find answers for are still welcome. Also any
suggestions as how the FAQ can be improved. Please send FAQ information
to:
faq-maintainer@ezonemag.com
This collection of documents is Copyright (c) 1996-1999 by Jim Bourke,
Tim McDonough, Craig Limber and Steve Lewin. All rights reserved.
Permission to distribute the collection is hereby granted providing that
distribution is electronic, no money is involved, reasonable attempts are
made to use the latest version and all credits and this copyright notice
are maintained. Other requests for distribution will be considered. All
reasonable requests will be granted.
All information here has been contributed with good intentions, but
none of it is guaranteed either by the contributors, Jim Bourke, or myself
to be accurate. The users of this information take all responsibility for
any damage that may occur.
Steve Lewin (faq-maintainer@ezonemag.com)
Credits:
Thanks to Tim McDonough, Doug Ingraham, Aveox, Bob Boucher and many
others for their help with answers.
Back to
Contents
Q. Why fly electric?
A. (Jim Bourke,
jbourke@ezonemag.com)
The primary draw of electric aircraft is the clean, quiet operation of
the equipment. Unlike a noisy internal combustion engine, electricaircraft
can often be flown at small parks or football fields without disturbing a
community (please check local laws before doing this).
An electric power system is also a very versatile piece of equipment. A
single high quality electric motor can be used to power a wide variety of
aircraft simply by choosing different propeller/battery pack/ gearing
combinations.
Other reasons that people enjoy electric aircraft include:
- a feeling that one is a "pioneer"
- a desire to do something different
- an appreciation of the unique challenges presented
Q. What kind of equipment do I need?
A. (Jim
Bourke, jBourke@ezonemag.com)
Generally speaking, you need equipment that is very similar to what
other RCers require. There are only a few primary components: the radio,
the battery, the charger, the aircraft, the speed control, and the motor.
The amount of accessories you purchase are up to you, but most people
typically buy things like a soldering iron, flight box, volt/amp meter,
etc.
You probably already know that the radio is used to transmit the
control inputs to the aircraft. I'll describe the electric RC specific
components below:
BATTERY:
The battery pack is what provides power to the motor. Typical packs are
composed of 500 to 2000 mAh cells. We can discuss the meaning of mAh (read
"milli-amp hour") later, but for now just understand that the higher the
number, the more charge the battery can hold. The weight of the pack is
proportional to its capacity.
CHARGER:
The charger is used to charge the battery packs. There are three
primary charging methods: trickle, fast, and peak. Trickle charging is a
low-current charge that takes several hours to perform but is guaranteed
to not hurt the battery. Fast charging involves stuffing energy into the
pack at a high rate so it is charged in as little as 15 minutes, with some
danger that the pack will be damaged if it is not monitored. Most low-end
chargers provide both fast and trickle charging. The high-end chargers use
a type of fast charging called peak charging. Peak chargers simply monitor
the charge automatically so the pack cannot be hurt by fast charging. If
you are going to be at all serious about electric flight, buy a peak
charger right away.
AIRCRAFT:
For the most part, the aircraft is the same as ones powered by internal
combustion. However, the electric systems are heavier than their
equivalent IC counterparts, so electric aircraft are usually built much
lighter than IC aircraft. Due to the high vibration caused by internal
combustion, most IC planes are overbuilt anyway and can be easily
lightened.
SPEED CONTROL:
The speed control provides proportional throttle control by varying the
amount of power that is transferred from the battery to the motor. Not all
planes have a speed control. Some use a simple on/off switch or just leave
the throttle on full blast until the battery is exhausted.
MOTOR:
Today's electric R/C modeler has a vast supply of different motors to
experiment with. Cobalt motors are considered to be far superior to
ferrite motors, but are much more expensive. A new type of motor is the
"brushless" motor. These are very expensive, but provide an even wider
range of output possibilities at high efficiency. Most motors that are
supplied in beginners kits are a type of ferrite motor called "can"
motors. These motors are very inefficient and cannot be serviced like a
higher quality motor. However, most kits will fly just fine with the motor
provided so its ok to use a "can" motor for your first plane.
Q. Can you suggest a few beginner setups?
A. (Jim
Bourke, jBourke@ezonemag.com)
This should give you an idea of what it takes to get started.
Configuration #1 2 meter wingspan electric glider kit $65
try the Electra, Skimmer, etc.
building materials $40
6-cell battery pack $20
AC/DC fast charger $30
2 channel radio $65
----
total $220
If you intend to continue with the hobby, then buy an inexpensive 4
channel radio instead of the two channel shown above. That would add about
$65 to the cost. Notice that the above configuration also does not include
a speed control. One can be added for around $60, but you may want to
consider an on/off switch instead for $30.
Configuration #2 High wing electric trainer $65
try the Electri-Cub, Mirage, PT-E, etc.
building materials $40
battery packs (2 7-cell 1400 mah SCR packs) $50
Speed control $70
(Ace Smart Throttle, Astro Flight model 210, etc)
Peak charger $110
4 channel radio $130
----
total $465
This configuration will give you a nice flying airplane with plenty of
room to grow. You could do with somewhat less expensive components, but
this way you will be able to use the same radio, charger, speed control,
and batteries on your next plane. You do not need two battery packs, but
having two packs is nice because one can charge while you are flying with
the other.
Configuration #3: High wing .15 to .25 sized electric trainer $65
(Sig Seniorita, Astro Flight Porterfield)
Cobalt geared motor $125
building materials $40
battery packs (2 packs) $120
Speed control $70
(Ace Smart Throttle, Astro Flight model 210, etc)
Peak charger $110
6 channel radio $220
----
total $750
This is the way to go if you can afford the equipment.
Q. How do I get started?
A. (Jim Bourke,
jBourke@ezonemag.com)
Go to a local hobby store and find out the clubs in your area. R/C
clubs are the best resource you can have. Clubs provide training, flying
fields, auctions, competitions, fun-fly events, newsletters, and, best of
all, lots of advice for beginners. Most clubs require you to become an AMA
member. AMA membership costs $48(1999) and provides you with a free
magazine subscription and insurance coverage.
Q. What kind of planes can fly with electric
power?
A. (Jim Bourke, jBourke@ezonemag.com)
Any plane that can fly with an IC engine can fly with electric power.
However, there are tradeoffs that must be considered with electric power
that aren't a factor with IC systems.
For example, In order to increase duration, you must be willing to
accept increased weight (wing loading) and/or decreased power. The three
characteristics are interdependent. Since the weight per cell is
proportional to capacity, the relationships can be expressed as shown:
Weight = # of cells * weight per cell
Power = # of cells * current
draw
Duration = weight per cell / current draw
Therefore:
INCREASING the number of cells, INCREASES power and INCREASES weight
INCREASING the weight per cell INCREASES duration and INCREASES weight
INCREASING the current draw, INCREASES power and DECREASES
duration
Stated differently:
To decrease weight, I must DECREASE power and/or duration
To
increase power, I must INCREASE weight and/or DECREASE duration
To
increase duration, I must INCREASE weight and/or DECREASE power
The Moral: Electric aircraft are quite capable of outperforming even
the hottest glow aircraft, provided the pilot is willing to live with a 1
minute flight time.
The challenge electric flyers face is that, even though electric
systems are much more efficient than IC, we are not currently able to
store nearly as much energy in our power supply at an equivalent weight.
This is why electric flyers try to build their aircraft as light as
possible. If wing loading and duration aren't a concern, however, electric
flight can be used to power any kind of aircraft.
Until we have a breakthrough in battery technology that allows us to
store 4 or 5 times more energy in a cell, these tradeoffs will be the
central issue of electric aircraft design.
Q. How can I sign up to the AMA or MAAC?
A. Craig
Limber (craig.limber@west.gecems.com)
The AMA (Academy Of Model Aeronautics) and MAAC (Model Aeronautics
Association of Canada) are two of the organizations which provide
insurance and services to all forms of modeling. Most clubs in the U.S.
and Canada require membership to this type of organization (usually
because of the insurance).
The AMA can be reached at:
Tel: 1-765-287-1256. FAX: 1-765-289-4248
Web: http://www.modelaircraft.org/
MAAC address:
Unit 9, 5100 South Service Rd, Burlington,
Ontario,Canada L7L 6A5
Tel: (905) 632-980
Web:
http://www.maac.ca/
Back to
Contents
Q. What safety procedures should I follow when
flying?
A. (Peter O'Shea, poshea@ezonemag.com)
Some common-sense safety precautions are:
1. Don't connect the motor battery until you have your frequency pin
(or other frequency clearance means) and are ready to put the plane on the
runway or hand-launch.
2. Don't turn on the radio system until you are ready for your flight.
Turn off the radio system as soon as possible after the flight.
3. Make sure that the throttle is set to off before turning on your
transmitter. Many digital speed controls have a function that won't allow
the motor to turn until the throttle stick has been in, or moved to, the
low position. Don't make it a habit of testing this function.
4. Motor-on radio checks must be done with the aid of a helper.
5. Check the leading edge and tip of your prop for molding flash, if
it's not a wood prop. Carefully sand away molding flash with fine
sandpaper.
Q. What is an arming switch?
A. (Peter O'Shea,
poshea@ezonemag.com)
An arming switch is, simply, a cutoff switch that goes between the
battery and anything else in the plane. The arming switch serves the
purpose of making it very unlikely for the motor to start up when the
airplane is not in use. Radios can be inadvertently left on, throttle
sticks can be accidentally bumped, but if the arming switch is used, the
motor will not start up. The arming switch is typically left off until the
plane is ready to taxi out to the runway (or be hand-launched). When
retrieving the plane after the flight, the arming switch is the first
thing turned off.
Q. When should I use an arming switch?
A. (Peter
O'Shea, poshea@ezonemag.com)
The more powerful the motor, the more need there is for the safety
redundancy of an arming switch. Just where to draw the line is debatable,
but a conservative approach would be to use one on any plane larger than
speed 400 size (approximately 100 watts and above).
Q. How much power will I lose? How much weight will I
gain?
A. (Peter O'Shea, poshea@ezonemag.com)
A commonly-available double-pole, double-throw plastic rocker switch
has been measured at 0.3 milliohms after hundreds of flights. This same
arming switch has a weight of 5.8 grams (0.20 ounces). When used properly,
an arming switch will not cause any noticeable drop in system power. The
simple secret to a long-lasting, low-loss arming switch is this: NEVER
SWITCH CURRENT with the arming switch. The switch should only _carry_
current. This means that you never turn the switch to "on" until the
throttle is in the low position, and similarly after the flight the switch
is only turned "off" after the throttle is again in the low position.
Q. What is a fuse? Why use one?
A. (Peter
O'Shea, poshea@ezonemag.com)
A fuse is an electrical component that cuts off the flow of current
when the current in a circuit exceeds the fuse's rated value. A fuse is
used in electric airplanes because nicad batteries are capable of
delivering very high currents when shorted. These currents are sufficient
to overheat components and wiring and potentially start a fire. The cause
of the high current could be a failed speed controller, a short in the
system wiring, or even simply a stalled motor. Note that some
organizations who provide insurance for modelers REQUIRE a fuse in
electrically powered models (MAAC in Canada is good example).
Q. What type of fuse should I use?
A. (Peter
O'Shea, poshea@ezonemag.com)
For speed 400 systems: the Littelfuse Picofuse, available through
Digi-Key (1-800-DIGIKEY) for about a dollar apiece. These are the size of
a 1/4 watt resistor and have ratings up to 15 amps.
For higher-powered planes: the automotive blade fuse, available at your
local auto parts dealer, K-Mart, Wal-Mart, or even Radio Shack. These come
in ratings from fractional amps up to 60 amps.
Q. How do I choose the fuse rating?
A. (Peter
O'Shea, poshea@ezonemag.com)
Choosing the correct rating for the fuse is not difficult. Put the
largest and highest-pitch prop on that you expect to fly with. Measure the
current draw of your power system on the bench, or calculate it using
software such as ElectriCalc. Take the static current you've just
measured/derived and multiply by about 1.25. This 25% margin will allow
for taking off in tall grass and should prevent nuisance blows. Find the
fuse with a rating at or just above this current level.
Q. How and where do I install the fuse?
A.
(Peter O'Shea, poshea@ezonemag.com)
Where is the fuse installed? Most people put them in series in the
wiring harness between the battery and the speed control, as long as the
speed control is not being used for BEC. If BEC is being used, the fuse
should go between the speed control and the motor. For installation, blade
connectors can be used for the auto fuses. Another way is to solder the
fuse into the motor battery itself, which provides protection for the pack
when it is removed from the plane. The only drawback to this method is
that it isn't field-replaceable. If you use the Sermos-style fuse holder
you can have the fuse double as an arming switch if the holder is mounted
so that it is easily accessible from the outside.
Q. Are there any safety precautions for the motor
battery?
A. (Peter O'Shea, poshea@ezonemag.com)
Absolutely.
Many early electric planes were designed to hold one fixed nicad pack,
which was charged while in the plane. Many, if not most, electric fliers
now have multiple packs for each plane to get more flying time at the
field. These packs are removed from the plane for cooling and recharging.
The high-current capability of nicad cells demands careful attention to
safety when constructing, handling, and storing these packs. The pack
should be constructed so that a minimum of one durable layer of insulation
is between each cell and its neighbor, and the joint area between the
positive top of the cell and the negative case should be protected
carefully. The overall pack should be insulated so that no metal is
exposed anywhere.
To summarize: use an arming switch and a fuse. Insulate your packs
completely. Give our power sources the respect they deserve, and use
common sense too.
Back to
Contents
Q. How is the voltage of a pack determined?
A.
(Jim Bourke, jBourke@ezonemag.com)
A battery pack consists of a number of cells, wired in series.
Therefore, the voltage for the pack is equal to the number of cells times
1.2 volts (Nicad cells provide 1.2 volts of electricity). However, because
of a cell's internal resistance, the actual voltage you are getting is
slightly lower; more like 1.1 volts per cell or even down to 1 volt per
cell in the higher current installations.
Q. What are milli-amp hours?
A. (Jim Bourke,
jBourke@ezonemag.com)
The milli-amp hour is the standard unit of storage capacity for a
cell.
It is analogous to "gallons of fuel" for an internal combustion engine.
The milli-amp hour rating of a cell tells how many constant milli-amps of
current can be supplied by the pack for one hour. This rating can be used
to find the duration that a battery pack can provide given a certain
draw.
Because cells are wired in series, the milli-amp hour rating of a pack
is the same as the milli-amp hour rating of a single cell.
Q. How fast can I charge my batteries?
A. (Jim
Bourke, jBourke@ezonemag.com)
Different cells can withstand different charging rates. Check with the
manufacturer to make certain you don't damage your pack.
For fast charging, most packs can be safely charged in 15 minutes,
which requires a charging current of 4 times the capacity of the pack.
Trickle charging is usually done at a rate of 1/10th the capacity, or
C/10. Cell manufacturers list this as the charge rate in which the cells
will not vent (release gases that build up from overcharging). However,
even this charge rate can reduce the life expectancy of a cell if left on
after the cell is fully charged.
Q. What is Nicad memory?
A. (Jim Bourke,
jBourke@ezonemag.com)
Besides a hot topic of debate, you mean?
Nicad memory is commonly explained as a loss of cell or pack capacity
after repeated charges and discharges to the same level. It is actually a
voltage depression in the cell that causes it to appear as if the cell
isn't charged. The cause of memory is beyond my capacity to understand or
explain, but suffice to say that repeated charges and discharges to the
same level have been reproducibly shown by battery makers to cause voltage
depression. Therefore, Nicad memory is a very real phenomenon. How often
it occurs is a subject of much debate. Don't bring it up unless you are
prepared.
To avoid Nicad memory, simply fast charge your batteries regularly and
provide an occasional deep discharge to 1.0 volts/cell under a light load.
Also, avoid trickle charging batteries for long periods. Fast charging
negates the effect of Nicad memory.
GE did a study on Nicad memory and concluded that memory cannot occur
if any one of the following conditions are met:
1. Batteries achieve full overcharge (peak charge)
2. Discharge is
not exactly the same each cycle- plus or minus 2-3%
3. Discharge is to
less than 1.0 volt per cell.
Much of this information comes from the Nicad faq referenced in the
Internet Resources section of this faq.
Q. What is Cell reversal?
A. (Jim Bourke,
jBourke@ezonemag.com)
From the nicad faq:
"In a battery, not all cells are created equal. One will be weaker than
the others. So, as the battery is discharged, the weakest cell will use up
all its active material. Now, as discharge continues, the current through
the dead cell becomes a charging current, except that it is reversed. So,
now reduction is occurring at the positive terminal. As there is no more
nickelic hydroxide, it reduces the water, and produces hydrogen. Cell
pressure builds, and it vents. The cell has lost water and the life of the
cell has been shortened
"This is the big danger of battery cycling to prevent memory.
Invariably, unless one is very careful, one ends up reversing a cell. It
does much more harm than the cycling does good. Also, keep in mind that
cells do have a finite life. Each cycle is a bit of life."
Q. Should I cycle my packs?
A. (Jim Bourke,
jBourke@ezonemag.com)
Weigh the dangers of cell reversal versus the dangers of Nicad memory
and decide for yourself.
Some people discharge their packs to 0 volts per cell and say they have
never had a problem. Others say that cycling below 1.0 volt is damaging. I
have never witnessed Nicad memory, but I have never witnessed cell
reversal either. Use your best judgement.
Q. Can I deep discharge an individual cell
safely?
A. (Jim Bourke, jBourke@ezonemag.com)
Individual cells (i.e. NOT IN PACKS) can be discharged to 0 volts per
cell safely. Cell reversal can't occur with individual cells. In fact,
cycling an individual cell is a good way to determine its exact capacity.
This is how packs are "matched".
Q. What is the discharge of a Nicad like?
A.
(Jim Bourke, jBourke@ezonemag.com)
Well, look at the following graph and you'll get an idea. |
|--
| \
V | ---------------------------
o | -----
l | \
t | \
a | |
g | |
e | |
| |
| |
| |
+------------------------------------------------
Time
The graph tries to show that a Nicad provides an initial surge of power
(at around 1.2 volts or higher), then provides a pretty much constant
number of volts until its capacity is almost entirely depleted. This means
that the voltage level of a cell is NOT proportional to remaining charge.
By the time a cell reaches 1.0 volt, it is almost entirely discharged.
Q. What is "blackwire" on the negative leads of nicad
battery packs?
A. (Red Scholefield, redscho@afn.org)
I thought no one would ever ask!
The Black Wire Syndrome
The black wire syndrome is an occurance in battery packs (Ni-Cds) where
the negative wire becomes corroded (turns from shinny copper to
blue-black). This is the result of either a shorted cell in the pack, the
normal wearout failure mode of Ni-Cds, or cell reversal when a pack is
left under load for an extended period. The sealing mechanism of a Ni-Cd
cell depends to some degree on maintaining a potential across the seal
interface. Once this potential goes to zero the cell undergoes what is
called creep leakage. With other cells in a pack at some potential above
zero, the leakage (electrolyte) is "driven" along the negative lead. It
can travel for some distance making the wire impossible to solder and at
the same time greatly reducing its ability to carry current and even
worse, makes the wire somewhat brittle.
A switch left on in a plane or transmitter for several months can cause
this creepage to go all the way to the switch itself, destroying the
battery lead as well as the switch harness. There is no cure. The effected
lead, connector, switch harness must be replaced. This leakage creep takes
time so periodic inspection of the packs, making sure that there are no
shorted cells insures against the problem.
The cells should also be inspected for any evidence of white powder
(electrolyte mixed with carbondioxide in the air to form potassium
carbonate). In humid conditions this can revert back to mobile electrolyte
free to creep along the negative lead. Some "salting" as this white powder
is referred to, does not necessarily mean that the cell has leaked. There
may have been some slight amount of residual electrolyte left on the cell
during the manufacturing process. This can be removed with simple
household vinegar and then washed with water after which it is dried by
applying a little warmth from your heat gun..
Q. How do I match cells without spending a fortune on
expensive equipment?
A. Since few of us can afford a turbo matcher
and it really isn't that important for sport flying here is what I do to
make semi- matched packs from inexpensive cells.
The lower tech way requires only a DVM and your regular charging gear
plus a notepad.
1) Check the individual cell voltage with a DVM. If the cell is at Zero
volts then it will probably never be any good for any use.
2) Discharge the individual cells to <0.5 volts. I use a 0.5 ohm
power resistor for this but you can use around 30 ohms overnight.
3) build a pack so that cells can be easily changed out. I like 7 cells
as a max for this as too many cells makes it hard to do some of the
testing.
4) Charge the pack at c/10 for 24 hours. The cells should be warm at
the end of this period.
5) Discharge the pack into a load. A motor with a light load works
pretty well. 5-10 amps is a good enough load. Watch the voltage of each
cell. You are looking for cells that have lower voltage than the others.
Keep an eye on it and when it starts to slow down figure out which cells
died first. Mark those some way. Stop discharging when one cell drops
below 0.5 volts. I usually put a little number on the cell to tell me
which ones died in which order.
6) Charge the pack at c/10 for 16 hours. repeat 5 and 6 a few times.
note any change in cell order. Sometimes after exercising the cells a few
times the worst ones become good. Change out any weak cells and start
over. After a few iterations you will have a pack that with cells that
will dump close to each other.
7) Fast charge the pack at a 3c or even 4c rate. Touch the cells often
during the charge to see if any cells get hot during charge. If all cells
get hot then reduce the charge rate. If only one or two you might want to
swap out those cells and start over. The cells you pull out are probably
fine for the most part. You can generally make a pack out of the culls
that works fine but has slightly reduced capacity. I like to take these
cells and charge them up and let them sit a couple of weeks. If they still
have nearly a full charge I make 4 and 5 cell receiver packs out of them
and sell the packs to the 1/4 scale gas guys. They love them cause I only
charge $15 for such a pack and they last for years. If you have a charger
like the 110d or the 112d that will tell you the energy put into a cell
you can do this a different way. This is a slightly higher tech way but
requires a smarter charger. You can substitute the charge time for the amp
hours figure if your charger displays that.
Another method:
1) Discharge the cell to zero volts. I use a 0.5 ohm power resistor. a
25 to 30 ohm resistor overnight will do this as well.
2) Charge the cell in the peak detecting charger at a 3c rate. Use the
same rate for all cells of a given type. Watch the temp of the cell and if
it gets hot during the charge cull that one out. When the cell peaks you
need to record the peak voltage and the ah that went into the cell.
3) Repeat 1 and 2 a couple of times. The values you get will become
fairly consistant unless the temperature changes a lot.
4) Select cells with similar capacities and peak voltages in that
order. The cells with the lowest peak voltage are the better cells. They
will have the higher voltage under load. You learn a lot about batteries
when you play with them like this. I hope this helps!
Q. What does "C" mean?
a. Steve Lewin
"C" is the charge rate which should fully charge a battery in one hour.
It is the same number as the cell capacity but in milli-amps (mA) i.e. for
a 500 maH cell, the C rate is 500 mA, for a 1700 mAH cell C is 1700 mA (or
1.7A). It is convenient to give charging rates in terms of C. For example,
the standard slow charge rate for all Ni-Cds is C/10 (one tenth of C). For
an 800mAH battery that's 80mA. At that rate the battey will be charged in
around 10 hours (actually a little longer for a full charge). Fast charge
rates are around 3C or 4C. At 4C a cell will be charged in approximately
1/4 hour.
Back to
Contents
Q What does "breaking-in" a motor actually do?
A.
Terry Gamble
Basically, a new motor comes with flat brushes and a round commutator.
the idea is to wear the brushes down in such a manner that you have a
curved surface (and thus more contact area) at the commutator/brush
interface. if you try to break it in by simply flying it at full load, you
create a lot of arcing which pits the surfaces and degrades performance.
note that you must break in the motor prior to using it. if you've already
pitted the brushes and commutator, it is too late. you then have to settle
for what you have, or buy a new motor. you can expect this procedure to
improve power output 10-30 %. inexpensive ferrite "can" motors need all
the help they can get so don't forget this step! some motors do not need
to be broken in. manufacturers of these high-quality motors will mention
this in the motor's manual. if in doubt, break it in.
Q. How do I break-in a motor?
A. Terry Gamble
Ideally you'd like to run the motor at about 1/3-1/2 it's rated voltage
with no load (without prop) for an hour or two. long enough to wear the
brushes down without arcing. the r/c car guys have special transformers
for this, but all you really need for a typical 05 can motor is 2 alkaline
d cell batteries and some spare 12 gauge wire. simply hook the batteries
up in series so you have a 3 volt power source and hook the wires to the
appropriate terminals on the motor. let the motor run until the batteries
are dead, and presto...you have a broken in motor. if you have an old
electric train transformer and a voltmeter, you can also dial in 3-4 volts
with the transformer and save the cost of 2 batteries.
Q. What does gearing do?
A. (Jim Bourke,
jBourke@ezonemag.com)
Gearing allows a motor to turn a larger prop at a lower rpm. This
allows the system to produce more thrust while drawing the same number of
amps. The trade-off is that top speed is reduced, which makes gearing
suitable mostly for slow-flying aircraft. Sport electric planes are
usually run with a direct drive system. See Bob Boucher's book "The
Electric Motor Handbook" for a more detailed explanation.
Q. How do I compare an electric motor to an ic
engine?
A. (Jim Bourke, jBourke@ezonemag.com)
If all you are looking for is a watts to horsepower conversion, then
the formula is: 1 brake horsepower = 750 watts. The problem is that
electric motors have many more variables than ic engines. In order to
determine the performance of an electric motor, you must first answer
questions such as how much duration you want, how much power you need,
etc. Gearing also heavily influences the comparison.
Q. What are motor constants?
A. (Jim Bourke,
jBourke@ezonemag.com)
Motor constants are used to define the characteristics of a motor in
quantifiable terms. Every motor can be accurately defined using exactly 3
motor constants: kv (rpm/volt), rm (terminal resistance), and io (no-load
current). The kv constant is the rpms produced by a motor per volt
applied. A 100% efficient motor could be described using this constant
alone, but there are losses in the motor that make this impossible. If the
kv is known, then we can determine another constant called kt. Kt is the
torque produced per amp. kv and kt are proportional as shown: kt = 1355 /
kv This relationship between kt and kv is a law for every motor. The rm
constant is called the "terminal resistance" of the motor. This is the
loss inside the motor due to the wiring in the armature. The rm constant
represents a loss of power due to imperfect materials inside the motor.
The final constant, io, is the no-load current. The motor table at the end
of this faq shows several popular motors and their constants.
Q. In a brushless motor system, what's the difference
between a delta wind and a wye wind? what are the advantages and
disadvantages?
A. (Matthew Orme, Aveox Inc. email:
102252.401@compuserve.com)
Get a pencil and paper for this.
"Wye" wind Motor
Draw 3 resistors (or coils) radiating from a central point (The Wye
tie). label the three ends A, B, and C. These represent the three phase
connections in the Wye motor.
In the controller, each of these has 2 pair of MOSFETs connected to it,
a pair to source the current, and a pair to sink the current. The motor
fires something like this (simplified for clarity) A-B, A-C, B-C, B-A,
C-B, C-A ad nauseam. The Magnets 'chase' the rotating magnetic field.
Notice that there are always 2 phases 'commutated' at the same time, but
the mix differs, and the current direction will reverse every other time.
The motors resistance is the sum of any two phases ie. measure from any 2
phases. the third phase is open electrically when any other 2 are
commutated.
"Delta" wind Motor
Draw 3 resistors connected in a triangle (delta). Each of the vertices
is a phase. When you commutate CA-AB, you get most of the energy on one
coil, (A), but some on (A-C-B) side. (mostly losses imo). The net result
of most of the current going through one set of coils at a time, instead
of 2 is that the Kt is cut in half and Kv doubles.
At Aveox, we have essentially deemed the Deltas as secondary to Wye
winds in any application, except where very high degree of uniformity in
both directions is very important. Things like robots that move in both
directions equally put up with the efficiency losses. Since the motors are
very insensitive to timing changes (unlike the Wye winds), you don't have
great performance in one direction, and poor in another (without adjusting
the timing), you have good performance in both (but it ain't worth the
losses in a model).
They have been discontinued at Aveox for a couple of years. We do
whatever we can to get them out of circulation, by changing them over at a
loss. (But they are really easy to make if you insisted, and I would feel
guilty afterwards). When you finish winding a stator, you have 6 wires
coming out, the start and finish of each phase. connect every other one
together to make the wye tie, or adjacent pairs to make the delta.
Q. How does Timing effect the power of an electric
motor?
A. Tom Cimato, MaxCim Motors
In general terms this describes motor action:-) But (bet you knew that
was coming) more specifically you need to first visualize that the
permanent magnets create a fixed field in magnitude and position (for this
discussion). This field has a shape with a maximum intensity exactly
between the magnets (in a 2 pole motor - most DC motors - directly
opposite each other). To generate the maximum torque from the motor, the
electric field generated by applying current to the windings must be
located 90 ELECTRICAL degrees from the center of the magnet field (90
physical deg. in a 2 pole motor). This is defined as the max. Kt (torque
constant - oz.in./A)
What happens in a real motor under load is that the electric field lags
the magnet field due to inductance effects in the windings and possibly
some saturation effects in the steel parts (magnetic iron). The amount of
lag varies with load (and rpm), so is a complex variable.
We try to preset the motor timing (brush position) so that at a
selected load current, the fields are at the 90 E deg point for max. Kt.,
and max. magnetic power conversion efficiency. At no-load (without a prop)
this will be an advanced position and since it is ahead of the 90 E Deg.
point, will have a reduced Kt (take the SIN of the angle x KT90, eg SIN108
x 1 = .951 x Kt), which requires more current to turn the shaft (at
no-load. The current will pull into phase at the selected load current.
This will actually look like the current decreasing as the load increases,
up to a point.
In a brushless motor the same thing is happening, but we advance the
hall sensors for this effect. The big difference is that we have to
visualize the analogy while sitting on the rotor (stationary frame of
reference) and look at the stator field.
This is explained in most texts of ac and dc electric machines. Check
your university book store:-)
It's a lot more complex than this, so I hope I've given you some
insight as to the goings on in that little bundle of wire and magnets.
Q. How much should I advance the timing on my speed 400
motor?
A. 5mm.
Q. How do I use the "speed 400 timing
tool?"
A. Craig Limber with thanks to several eflight members.
The tool comes with two brass rings, two bolts and a metal rod. One of
the rings is supposed to be bolted onto the front of the motor (the bolts
I got with mine were too long; I ended up using some shorter ones from my
supply of metric fasteners) whereupon the ring can be placed in a clamp.
The other ring has two pegs on it which are plugged into the two holes in
the back of the motors. The metal rod is then slide through the two holes
in the sides of the ring and used as a lever to rotate the plastic back of
the motor to advance it. You do not need to bend the metal tabs on the
motor's case. Note that you should rotate the end plate in the OPPOSITE
direction the armature will turn. DON'T FORGET to place witness marks on
the case and plastic end BEFORE making any changes!
Q. How much heat can our motors dissipate?
A.
Matthew Orme
The industry standard is 1 watt per square inch for continuous
operation (24 hours), at room temperature.
We say about 3 watts for ours for as long as a nicad pack can run
it
Q. How do I keep my motors clean?
A. Bob Boucher
If commutator has deposits of carbon and gunk on it you can clean it
with scotchbrite or a com stick. Very light polishing action. You can also
clean off gunk when motor is running by a few drops of alcohol.
If com is pitted or shows brush skipping and chattering your com has
been overheated and needs to be returned. It is out of round. polishing
will not cure out of round.
You need a lathe with a ball bearing in the tail stock and a diamond
tool..or at least a sharp cutting tool.
On 05 motors you can use a hobby com lathe. On Astro and Plattenburg
motors you need a real lathe. Astro and New creations can do this for a
small fee.
Q. What is a a sensorless brushless
motor/controller?
A. Steve Lewin
"Conventional" brushless motors have small internal sensors which
provide signals to sense the position of the armature. These are used by
the controller to allocate the drive current to the correct windings.
Sensorless motors do not have these internal sensors. The controllers used
with them use a different method to work out where to assign the winding
currents (see below). Sensorless controllers are obviously required for
motors without sensors but can also be used with other brushless motors.
This allow the user to choose motor and controller separately as with
brushed motors to get the best of each rather than being obliged to buy
the controller from the motor manufacturer as is the case with
conventional brushless motors and their dedicated controls.
Q. How does a sensorless brushless motor/controller
work?
A. Doug Ingraham
The simplest description I can come up with follows:
A reference
voltage can be derived by summing the two driven windings and the undriven
winding is compared to the reference. The undriven winding will have a
voltage induced upon it because it is being moved through a magnetic
field. When a zero crossing is detected it is the time to rotatethe
magnetic field to the next position. The above works once the motor is
spinning. Before it is spinning there is no zero crossing to detect so you
have to resort to interesting techniques like ringing the windings to try
to figure out the position of the motor so you can perform a clean start.
It is much easier to do all this if the motor characteristics are known by
the controller. If they are not known, the controller must learn them." Back to
Contents
Q. Why doesn't my folding prop extend all the way out? Do
I need to shave plastic away from the slots in the spinner?
A. Jim
Bourke
There is no fix. The blades are opening as they should. Hold the motor
horizontal and you will see that the blades do not hang straight down.
This is roughly the attitude they should have when unfolded.
Back to
Contents
Q. How does a speed control work?
A. (Doug Ingraham, Lofty Pursuits, dpi@rapidnet.com)
An ESC (Electronic Speed Control) is a device that controls the speed
of the motor by turning the motor on and off. Consider this simple
circuit, in which a motor and battery are controlled by a switch. ------------------------------------
| |
--- ----+----
- | |
--- Battery | Motor |
- | |
--- | |
- / Switch ----+----
| / |
---------------/ +----------------
To turn on the motor you close the switch which allows current to flow
to the motor. If you open the switch you stop the flow of current and the
motor will slow down and eventually stop turning. Proportional throttle
control is achieved by varying the amount of time the switch is on
relative to the amount of time it is off. For example, for 1/2 throttle,
the switch is on half the time.
In order to achieve smooth throttle response, this switching must occur
several times per second. Inexpensive speed controls typically perform
this switching 50 times per second. The reason 50 times a second was
chosen is because this is the rate that control pulses are sent to each
servo and the electronics are greatly simplified if this rate is used.
This is called frame rate because the ESC operates at the same rate as the
radio control frames are updated. All electronic speed controls operate
pretty much like this.
Q. What is the advantage of a High Rate Control?
A.
(Doug Ingraham, Lofty Pursuits, dpi@rapidnet.com)
The simple answer is the efficiency is greatly increased over that of a
frame rate control.
An electric motor wants to turn at a certain speed that depends on the
voltage that is being supplied to it. The best speed control would provide
a nice clean DC voltage that varies from a maximum that is the same as the
battery down to zero volts. The frame rate control fails to do this
because the motor sees full voltage during the on times and zero voltage
during the off times.
If you were able to measure the amps that flow into the motor during
the on time you would see that they are much higher than expected. These
high currents end up heating the motor and battery almost as much at every
throttle setting above 20% as at full throttle.
One way to make low-rate controls more efficient is to place a good
size capacitor across the motor terminals. This tends to average the
voltage somewhat.
The high rate controls work by reducing the voltage that goes to the
motor. The reason they can do this is because the motor is not a pure
resistive load. It has some of the same characteristics of an inductor and
it is these characteristics that make a high rate control more efficient.
An inductor that has current flowing through it tends to want that current
to continue to flow. If the voltage is turned on and off fast enough the
current will continue flowing smoothly through the motor.
Q. What is the best switching rate for a hi rate
ESC?
A. (Doug Ingraham, Lofty Pursuits, dpi@rapidnet.com)
There is little difference between a control that operates from about
1000hz up to around 5000hz. The exact best rate would depend heavily on
the motor. For our hobby motors about 3000hz is near optimum. As the
switching rate increases, the losses due to turning the switch on and off
start to go up as the losses in the motor go down. The crossover point for
these losses is about 3000hz for most motors.
Q. How does a motor brake work?
A. (Doug Ingraham,
Lofty Pursuits, dpi@rapidnet.com)
When a DC motor is spinning with the speed control turned off it is
acting as a generator. With the prop windmilling the motor (generator now)
is producing a voltage at the motor terminals but is doing no work. If you
place a short across the motor terminals the motor must now work hard to
try to generate the same voltage across a dead short. This will cause the
generator (motor) to slow down. The motor short is provided by an
electronic switch in the speed control.
Q. What is a BEC and how does it relate to the speed
control?
A. (Doug Ingraham, Lofty Pursuits, dpi@rapidnet.com)
BEC stands for Battery Eliminator Circuit. The battery it is
eliminating is your receiver pack. The BEC is a completely separate
circuit from the rest of the speed control. It is generally a one to two
amp linear regulator that converts the motor battery voltage to a
regulated 5 or 6 volts to power the receiver and servos.
Q. What are the disadvantages of a BEC?
A. (Doug
Ingraham, Lofty Pursuits, dpi@rapidnet.com)
A BEC using a linear regulator has some very strict limits over which
it can operate. Modern regulators only need about 0.25 volts more input
voltage than output voltage. Therefore, a BEC that supplies 5 volts to the
radio gear needs 5.25 volts from the battery. So a 6 cell pack operated at
reasonable current levels (<30 amps) is the minimum needed to be safe
in powering a radio from a bec. Strange things happen when the input
voltage drops below 5.25 volts. On the other end of the scale one
shouldn't try to operate a linear regulator type bec on more than 10
cells. At ten cells, the regulator may overheat and shut off. Of course
when they turn off the radio is turned off and the plane generally
crashes. I have seen a number of crashes that I am certain were caused by
the bec regulator chip overheating. The characteristic of this event will
be that operation is fine for a couple of minutes into the flight and then
radio loss is complete. It is most likely to occur on high cell counts
(8+) with many servos (4 or more) and a fast airplane (the speed of the
plane affects the amount of current that the regulator must provide to the
servos). The pilot generally thinks he got interference and doesn't blame
the real culprit. There are other types of regulators that don't have this
problem but so far they are not in use because they are larger, weigh more
than a small four cell receiver pack, and cost more as well.
Q. What is a cutoff?
A. (Doug Ingraham, Lofty
Pursuits, dpi@rapidnet.com)
A cutoff is a circuit that is added to an esc equipped with a bec to
try to prevent the motor battery from being run dead and causing a crash
when the input voltage goes below 5.25 volts. Car type speed controls
often have a bec but never have a cutoff circuit. That is why you
shouldn't use a car type control in a plane with the bec active.
Q. How long can I fly once the cutoff takes
place?
A. (Doug Ingraham, Lofty Pursuits, dpi@rapidnet.com)
This depends on so many factors that there is no good way to answer it.
On my (Doug Ingraham's) bec equipped speed 400 motor glider I have run
tests to attempt to answer this. The plane has 2 micro servos, receiver
and speed control. The idle current is about 60ma for this setup. When the
motor is running full throttle and the cutoff takes place there is about
25mah remaining in the 6 cell 600mah pack. The cutoff takes place at 5.6
volts with the speed control in that model. That means that the pack would
be dead in less than 25 minutes in flight. In fact the flight loads are
about 100ma with this plane. So about 15 minutes of flight time. That
sounds pretty good except that I have flown pure gliders for 2.5 hours and
not noticed the time. in that context it doesn't seem very safe to be
sport flying with a bec in a glider. What about a 7 cell pack? I used the
same setup to try a 7 cell pack. The 7 cell 600mah pack has only 4mah
remaining at a cutoff voltage of 5.6 volts. This is 2.4 minutes of
relative safety.
Q. What is opto-isolation and what does it do?
A.
(Doug Ingraham, Lofty Pursuits, dpi@rapidnet.com)
Opto isolation is a technique used to prevent noise on one side of an
electronic circuit from affecting another. The only connection is by
pulses of infrared light like that used in tv remote controls. the way it
works in a speed control is the servo signal connects to an infrared light
emmiting diode which turns on and off with the servo pulses. In the same
sealed package there is an infrared sensitive photo transistor that senses
the pulses of light from the diode and turns on the transistor when the
light is shining. this gives thousands of volts of isolation between the
receiver and the speed controls electronics thus helping to prevent
interference from affecting the radio link. This works very well since the
bulk of the interference comes through the power connections to the radio.
A speed control equipped with a bec cannot have effective opto isolation
unless both the positive and negative connections to the regulator are
broken. A speed control cannot have a functional bec and opto isolation at
the same time. they are mutually exclusive.
Q. How do I disable the BEC on my speed
control?
A. (Doug Ingraham, Lofty Pursuits, dpi@rapidnet.com)
The trick is to get the servo signal to the speed control without
getting power from it. this is usually accomplished by disconnecting the
red wire from the servo cord that goes to the speed control. most servo
connectors allow you to pull the pins out of the connectors. it is
recommended that you do this rather than clip the wires in case you later
want to re-connect the bec. wrap the metal pin with black electrical tape
after you pull it out of the plug. one good reason to do this is if you
have a speed control that has bec, but no cut-off. many car escs can be
converted to aircraft use with this procedure. it is dangerous to use an
esc without a cut-off circuit in an r/c airplane.
Q. Where do I plug in my BEC speed control ?
A.
(Steve Lewin, slewin@clara.co.uk)
Plug it into the receiver's throttle channel. The 3 wires on the plug
are +ve power(often a red wire), ground (often black wire) and signal
(often white or orange). All of the sockets on the receiver are able to
take power so the speed control plugs into the same channel whether it has
BEC or not. Since the signal you need to drive the speed control is the
throttle signal that's where it goes.
Back to
Contents
Q. how does charging current relate to
capacity?
A. (Jim Bourke, jBourke@ezonemag.com)
To determine the rate for a given length of Charging, use the following
formula: amps = Capacity / time to charge e.g. to charge a 1200 mah
battery in 20 minutes requires a current setting of 3.6 amps: amps = 1200
mah / .33 h = 1.2 ah * 3 h =3.6 amps
The same formula can also be reworked to determine how long it will
take to charge a battery at a given current: time to charge = Capacity /
amps e.g. The time it takes to charge a 1500 mah battery at 5 amps is 18
minutes: time to charge = 1500 mah / 5 amps = 1.5 ah / 5 a = .3 h = 18
minutes. Note that is is the minimum time to perform the charge. Because
the charging process is not totally efficient some of the energy is lost
as heat and the charging takes place a little longer than this. However,
if you are using a timed charger, stick with this formula.
Q. How can I make certain my packs are fully
charged?
A. (Jim Bourke, jBourke@ezonemag.com)
A peak charger automatically does this. If you don't have a peak
charger, then it is possible to monitor the charge yourself. Simply stop
charging when one of the following two things occur: 1. The pack starts to
get warm. 2. The charging voltage starts to drop. SAFETY WARNING: if you
are doing a manual fast charge (i.e. by watching the temperature and/or
voltage yourself: PAY ATTENTION! If the batteries get too much charge they
will overheat and that could damage or even destroy your batteries!
Back to
Contents
Q. How do I calculate duration?
A. (Jim Bourke,
jBourke@ezonemag.com)
Use the battery pack's mah rating to determine how long the needed
current can be delivered in minutes:
duration = 60 * (capacity/1000) /
current
eg: to calculate the duration of a 1700 mah pack for a 30 amp
draw:
duration = 60 * 1.7ah / 30 amp
duration = 3.4 minutes.
While it is very difficult to calculate the exact current draw in
flight we can get a rough but useful estimate by finding it on the ground
and then multiplying by 0.75. If your prop is highly pitched enough so
that it is stalled when running static this number will be far less
accurate.
Q. How do I calculate watts?
A. (Jim Bourke,
jBourke@ezonemag.com)
Watts is a measure of power. There are two kinds of watts that we are
concerned with: watts "in" and watts "out". The watts coming "in" is the
power going into the motor, and watts "out" is the power coming out of the
propeller. Watts "in" is what most modelers measure, as it is much easier.
Watts "in" is simply the product of volts and current drawn. Each cell
will produce 1.2 volts without load, but less at maximum efficiency. You
can count on 1.0 volt per cell, which means that: watts in = # of cells x
amps drawn
watts "out" will be some fraction of watts in, depending on the power
system's efficiency. Most high quality ferrite, brushless, and cobalt
motors are at least 70% efficient. Cheap ferrite motors range from 20% to
60% efficient, depending on load. Brushless motors can push 90% on a good
day. It pays to buy a good motor!
Q. How do I convert between different units of
measure?
A. (Tim Mcdonough, tpm@inw.net, Craig Limber,
craig.limber@aurean.ca)
Depending on where you live or where you went to school weight may be
expressed in grams, ounces, kilograms, or pounds. Lengths may be expressed
in meters, centimeters, millimeters, feet, or inches. The following
conversions are not exact in some cases but 'close enough' for modeling
purposes. Some of these things may seem real obvious. Real obvious things
used incorrectly sometimes cause errors that are very difficult to track
down!
Basic information:
- there are 16 ounces (oz) in 1 pound (lb)
- there are 12 inches (in) in 1 foot (ft)
- there are 144 square inches (in^2) in a square foot (ft^2)
- there are 100 centimeters (cm) in 1 meter (m)
- there are 10 centimeters (cm) to 1 decimter (dm)
- there are 10 millimeters (mm) in 1 centimeter (cm)
- there are 1000 grams (g) in a kilogram (kg)
| Measurement |
from |
to |
multiply by |
| weight |
ounces |
grams |
28.34952 |
| weight |
pounds |
kilograms |
.454 |
| length |
inches |
cm |
2.54 |
| density |
lb/ft^3 |
g/liter |
16.018 |
| density |
oz/ft^3 |
g/liter |
1.001 |
| wing loading |
oz/ft^2 |
g/dm^2 |
3.051 |
| wing area |
in^2 |
dm^2 |
.0645 |
| wing area |
ft^2 |
dm^2 |
9.29 |
A more detailed and complete conversion table can be had on the ezone
at: http://www.ezonemag.com/articles/1996/convtab.htm
Another table which lists all of the common measurements, drill bit
sizes and wire gauges up to one inch, in both metric and imperial, can be
found at: http://www.ezonemag.com/articles/1996/permtab.htm
Back to
Contents
Q. What is the watts/pound rule?
A. (Jim Bourke,
jBourke@ezonemag.com)
Basically, the rule states that 50 or 60 watts per pound (110 watts/kg)
is needed to produce good sport flying characteristics. Some airplanes,
such as gliders, can use a smaller ratio (30 watts/lb or 66 watts/kg),
while others, like pylon racers, may need a much higher ratio (80 watts/lb
or 176 watts/kg.). This ratio is normally computed using watts "in". There
is an implicit assumption that the motor being used is at least 70%
efficient. cheap "can" type motors, or motors that are being operated
beyond their specified current rating, will produce a misleading
ratio.
Q. How do I match an electric power system to a given airframe? Or, how
do I convert a gas powered plane to electric?
A. (Jim Bourke,
jBourke@ezonemag.com)
Matching an electric power system to a plane is not as straightforward
as many would expect. The problem is that electric power is very versatile
so it is necessary to do a bit of juggling to find out which system will
work best. For example, pretend that I have a 4 lb (64 oz or 1814g) sport
plane with a 600 sq. in. (4.2 ft^2 or 39 dm^2) wing. I desire mild
aerobatics (50 watts/pound) and a low wing loading (< 20 oz/ft^2 or 60
g/dm^2). I want at least 5 minutes of full power. Lets see how close I can
get.
We'll try three different power systems. For simplicity we'll assume
that a cell delivers 1V so the total voltage is the same as the # of
cells. The formulas we need are:
- watts = # of cells x amps (amps = watts / # of cells)
- duration = 60 * (capacity/1000) / amps
- wing loading = aircraft weight (ounces) / wing area (ft^2)
#1 astro 15 with a 10 cell, 1700 mah pack: the weight of this power
plant is roughly 2 lbs (900g). We now have enough information to figure
out the aircraft weight and wing loading.
Aircraft weight = 6lbs (96 oz)
wing loading= 96 oz / 4.2 ft^2 =
22.9 oz/ft^2
In order to figure out the duration, we need to know how many amps of
current the motor will draw. We can find out how many amps we need the
motor to draw using the watts/pound ratio we chose:
Total watts=watts/pound ratio * aircraft weight (in pounds) therefore:
watts=(50 watts/pound * 6 pounds) = 300
amps= total watts / # of cells therefore:
amps= 300 / 10 = 30A
At this point you may have to stop and try a different motor if the
amps you need aren't realistic. In this case we can continue because an
astro 15 is still at least 70% efficient at 30 amps. Now that we have the
amps we can figure out the duration.
Duration=60 * (capacity/1000) / amps
= 60 * (1700/1000) / 30
=
102/30 = 3.4 minutes
We have just found that an astro 15 motor with a 10 cell 1700 mah pack
will fly this 4 lb plane for 3.4 minutes at 50 watts/pound with a wing
loading of 22.9 oz/ ft^2 If we use this combination, we will have a short
flying time and no reserve power. Lets try a larger motor.
#2 astro 25 motor with a 16 cell, 1700 mah pack this power system
weighs roughly 2 3/4 pounds (44 oz or 1247g).
Aircraft weight=6.75 lbs (108 oz or 3061g)
wing loading=108 oz /
4.2 ft^2=25.7 oz/ sq. ft (77.1 g/dm^2).
amps needed=(watts/pound * aircraft weight) / 16 volts
= (50
watts/pound * 6.75 pounds) / 16 volts
= 337.5 watts / 16 volts = 21
amps
duration=60 * (capacity / 1000) / amps
= 60 * (1700 / 1000) / 21
= 102 / 21 = 4.9 minutes
This motor raises the wing loading a little bit, but it gives us the
duration we want and quite a bit of reserve power. We can choose a prop
that draws 25 amps at full throttle, but throttle back most of the time
for 4.5 minutes of powered flight. Lets see what happens when we decrease
the cell capacity. It will lower the wing loading, but it will also
decrease duration:
#3 astro 25 motor with a 16 cell, 1400 mah pack. This power system
weighs roughly 2 lbs (32 oz or 900g).
Aircraft weight=6 lbs (96 oz or 2720g).
wing loading=96 oz / 4.2
ft^2=22.9 or 68.7 g/dm^2
amps needed=(50 * 6) / 16 = 19
duration=(60 * 1.4) / 19 = 4.4 minutes
Now that you have the general idea, you know enough to run your own
numbers. One thing I didn't examine was the effect of gearing in the above
example. When I learn of a way to incorporate the effects of gearing in
the above calculations, I'll present an example. Unfortunately, the
watts/pound rule does not take gearing into account.
Q. I'm building an electric twin. Should I wire my motors in series or
parallel?
A. (Doug Ingraham, dpi@rapidnet.com)
There are several reasons why series connections are better than
parallel but the biggest reason is efficiency. If you hook the motors in
series the noload current of both motors together is equal to the noload
current of the individual motors so you don't suffer in that area. The rm
is twice as much but this doesn't matter since the voltage is doubled, and
the current is the same as a single motor so the losses in series are the
same as for single motor systems. If you parallel the motors the noload
current is doubled and in fact all currents are doubled. This is very bad
because losses are based on the square of the current.
If your system resistance (batteries+esc+wiring) is 0.045 ohms and you
draw 30 amps from a single motor with the losses in the system are going
to be (i^2)*r=40.5 watts with a single motor or with motors in series. In
parallel the current will be 60 amps so the losses will be 162 watts for
all losses up to the motors. As for failure modes, in parallel if a motor
shorts both motors stop. If a motor opens one motor stops and the other
continues normally. In series if a motor opens up both motors stop. If one
motor shorts the other motor will try to go twice as fast (throttle back
quick).
In parallel you need a controller that can handle twice the current.
The batteries will go dead twice as fast so your duration will be half. In
series you need twice as many cells so the weight is going to go up. You
need a speed control that will handle the extra voltage. You also will
need a charger that can charge twice as many cells. Usually the extra
cells won't matter in a twin setup because you are doubling the power.
Cutting the duration in half usually makes people unhappy. So why does
anyone use parallel? because with speed 400 size you can get away with it
and it doesn't kill you. 20 amps is no big deal from 1700scrc cells and
speed controllers are common that can handle this. Expect to have
excessive losses with any motor setups larger than these.
Back to
Contents
Here is a little motor data for just a few of the hundreds of motors
out there. man cat# name kv rm io
---------------------------------------------------------------
af 603 035 cobalt 2765 0.04 2.5
af 605 05 cobalt 2125 0.045 2.5
af 615 15 cobalt 1488 0.069 2
af 625 25 cobalt 971 0.093 2
af 640 40 cobalt 682 0.121 2
af 661 60 cobalt 11t 347 0.103 2.5
af 662 60 cobalt 13t 293 0.15 2
af 690 90 cobalt 11t 230 0.55 2.5
af 691 90 cobalt 10t 256 0.111 2.75
af 604 035 cobalt fai 4285 0.017 5
af 608 05 cobalt fai 3214 0.021 5.25
af 627 25 cobalt fai 1592 0.039 4.5
af 642 40 cobalt fai 5t 1161 0.05 4.5
af 643 40 cobalt fai 4t 1452 0.034 5.5
af 660 60 cobalt fai 651 0.045 4.5
gr 1740 speed rx 540 bb vz 2740 0.007 1.7
gr 1788 speed 500 e 12v 1040 1.2 0.4
gr 1789 speed 500 race 2850 0.075 2
gr 1799 speed 500 7.2v 2360 0.122 1.5
gr 3305 speed 500 bb race vs 3100 0.0064 1.4
gr 3322 speed 500 8.4v 2000 0.16 1.2
gr 1780 speed 600 bb 9.6v 1584 0.194 1.8
gr 1786 speed 600 9.6v 1979 0.265 1.37
gr 1787 speed 600 bb 7.2v 2638 0.096 2.8
gr 1793 speed 600 7.2v 2526 0.085 2.8
gr 3301 speed 600 8.4v 1890 0.125 2.3
gr 3302 speed 600 bb turbo 12 1491 0.285 1.1
gr 3316 speed 600 bb 8.4v 1932 0.125 1.95
gr 3323 speed 600 eco 7.2v 1583 0.156 1.5
gr 6314 speed 600 bb turbo 14 993 0.44 0.7
key: manufacturer: af=AstroFlight, gr=Graupner
kv: voltage
constant, in rpm/volt
rm: terminal resistance, in ohms
io: no-load
current, in amps
Back to
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This section of the faq lists *some* combinations of motor, prop,
battery, etc. that will provide a flyable e-powered plane. Most of this
information has been derived from discussions on the EFlight mailing list.
There are many other combinations possible but this may give you some
ideas. For starter setups see also the Beginner Issues
section.
From Jack Sowle
Great Planes Electricub
Trinity Saphire 17 turn modified 05 car motor
Gearbox of your
choice, 3.5:1 ratio
Master Airscrew Wood Electric Series 12 x 8 prop
6 or 7 cell 1400 mah battery pack will get you 8-10 minutes of putzing
around or 5-6 minutes of minor aerobatics. (It is a Cub after all)
From Bart de Ruijter
Model: Graupner Rowdy ARF
Weight: 1800 grams
Wing span: 1,4 meter
Motor: Graupner Ultra
930-6 8 Volt
Prop: Graupner Slimprop 8x4
Battery: 8 cells 1700 Mah
Sanyo (giving 1900+ Mah)
Amps: 18 A
Climb: 25-30 0
Motor run
time: 5 minutes
Flying time: 7-10 minutes
Flying style: moderate
aerobatics, nice loops, Take-off (grass): 10 meters
From Bruce Cronkite
Model: SIG LT25
Wing span:
62 in
Motor: AstroFlight 15G
Prop: Master Airscrew 12x8 wood
Battery: 14 cells
Flying time: approx 10 minutes
Flying style:
Stable trainer with reasonable aerobatics
From Dereck Woodward
Model: SIG 4 Star 40
Wing
span: 60 in
Motor: MaxCim MaxNeo 3:1 geared
Prop: Zinger 13 x 6/10
Battery: 20 cells
Flying time: approx 10 minutes
Flying style:
Take off in a couple feet, roll inverted then go vertical. Carry on from
there
Back to
Contents
- BEC:
- Battery Eliminator Circuitry. This feature enables a speed
controller to operate a receiver from the same battery that the motor
uses. BEC saves weight, as it eliminates the receiver battery.
- hi-rate:
- A hi-rate speed control switches at a high frequency, usually at
around 2 to 3000 times per second.
- IC:
- IC stands for "internal combustion." I use this term to refer to all
the various kinds of fuel-driven engines: gas, diesel, glow, etc...
(i.e. the "normal" R/C power plant).
- lo-rate:
- A lo-rate speed control switches at the same frequency as the servo
signal. Roughly 50 times per second.
- Opto-coupling:
- Indicates that the electrical current for the power system is
isolated, which makes motor-induced radio interference less likely. The
design of opto-coupling makes it impossible to incorporate with BEC.
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Mailing lists
EFLIGHT
This is an electronic mailing list
devoted to the discussion of electric model aircraft. Subscriptions are
free. To subscribe, surf over to the EZone web page http://www.ezonemag.com/ and follow
the links to subscribe.
FTP sites
ftp://ftp.luth.se/pub/misc/rc
This site is the unofficial model aircraft plans site for the
internet.
Web pages
http://www.ezonemag.com/
The E
Zone is the official home of the faq, and of the EFLIGHT mailing
list.
http://members.aol.com/KMyersEFO/
Ken
Myers electric flight page. Ken is the editor of The Ampeer, a very
popular electric flight newsletter. Back issues of The Ampeer are
available at Ken's page, as well as some great advice and information
for beginners. Recently added are several of Keith Shaw's article on
electrics. These article have been made available to the Internet
community with the permission of Tom Atwood at Airage Publishing (Model
Airplane News.)
http://www.math.niu.edu/~behr/misc/RC/speed-ctl.html
Eric
Behr has placed several plans for electronic speed controls on this
page.
http://www.paranoia.com/~filipg/HTML/FAQ/BODY/F_Battery.html
Various
battery-related information. Check out the Nicad faq!
http://loke.as.arizona.edu/~ckulesa/flight.html
Craig
Kulesa's excellent silent flight web page. This page contains a table
that matches the popular Astro electric motors to IC equivalents.
http://www.net101.com/dedola/conversion.html
This
site has a lot of information related to english-metric conversion. Very
useful if you want to build from plans that were drawn in units you're
not too familiar with. (Thanks to Dennis Weatherly for this tip.)
http://asp1.sbs.ohio-state.edu/gifs/forecasts/ngm/vectors.gif
This
site has 12 to 48 hour wind vector predictions. (Thanks to Corky Boyd,
corky@mail.wwnet.com for this tip.)
Newsgroups
rec.models.rc.air
This is an internet newsgroup
devoted to radio control aircraft.
rec.models.rc.soaring
This internet newsgroup is
mainly concerned with R/C gliders but also contains some discussion
about electric flight.
rec.models.rc.helicopter
The newsgroup for radio
control helicopters including more and more about
electrics.
Back to
Contents
Sailplane Modeler is expanding its coverage to include electric powered
planes of all sorts. The new name will be Sailplane and Electric Modeler.
The following information came from Wil Byers.
It will cover all aspects of electric including electric sailplanes.
Additionally, we will retain our sailplane writers, but the magazine is
definately growing in the direction of electrics. So, please spread the
word.
Also, you can subscribe to our magazine at:
Sailplane & Electric Modeler
P.O. Box 4267
W. Richland, WA
99353-0024
$22 3rd class $27 1st class.
Electric Flight International is another excellent magazine for
eflyers. Here is the North American contact address:
Traplett Distribution, USA
144 West Sierra Madre Blvd.
Sierra
Madre, CA 91024-2435
PH: (626) 836-6931
FX: (626) 836-6941
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