TITLE: Electric Flight FAQ VERSION: V1.04 DATE: November 25, 1996 MAINTAINER: Tim McDonough, tpm@inw.net FAQ OWNER: Jim Bourke, jbourke@world.std.com COMMENTS ON THIS REVISION (Tim McDonough) Doug Ingraham has provided a new article describing a technique for matching nicad cells. If there is more material you'd like to see please let me know. I'm open to suggestions on how we can make the FAQ the best electric flight resource available. INTRODUCTION 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. Submission of questions (and answers!) to the FAQ are encouraged, as well as any thoughts on how it can be improved. Please send FAQ information to: Tim McDonough tpm@inw.net This collection of documents is Copyright (c) 1996, Jim Bourke and Tim McDonough. 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. Tim McDonough ====================================================================== ELECTRIC FLIGHT FREQUENTLY ASKED QUESTIONS An asterisk (*) indicates a new or updated article in this version of the FAQ. CONTENTS [Beginner Issues] Why fly electric? What kind of equipment do I need? Can you suggest a few beginner setups? How do I get started? What kind of planes can fly with electric power? [Batteries] How is the voltage of a pack determined? What are milli-amp hours? How fast can I charge my batteries? What is Nicad memory? What is Cell reversal? Should I cycle my packs? Can I deep discharge an individual cell safely? What is the discharge of a Nicad like? Black Wire Syndrome * Low Tech Cell Matching [Electric motors] What does "breaking-in" a motor actually do? How do I break-in a motor? What are turns and windings? What is "timing"? What does gearing do? How do I compare an electric motor to an IC engine? What are motor constants? What's the difference between Delta and Wye Brushless Motors? [Speed Controls] How does a speed control work? What is the advantage of a High Rate Control? What is the best switching rate for a hi rate ESC? How does a motor brake work? What is a BEC and how does it relate to the speed control? What are the disadvantages of a BEC? What is a Cutoff? How long can I fly once the cutoff takes place? What is opto-isolation and what does it do? How do I disable the BEC on my speed control? [Chargers] How does charging current relate to capacity? How can I make certain my packs are fully charged? [Formulas] How do I calculate duration? How do I calculate Watts? * How do I convert units of measure? [Design Issues] What is the watts/pound rule? How do I match an electric power system to a given airframe? How do I convert a gas powered plane to electric? * Should I wire two motors in series or parallel? [Motor Data] [What Works] [Glossary] [Internet Resources] Mailing lists FTP sites Web pages Newsgroups [Credits] [Regards] [Beginner Issues] ====================================================================== Q. Why fly electric? A. (Jim Bourke, jbourke@world.std.com) The primary draw of electric aircraft is the clean, quiet operation of the equipment. Unlike a noisy internal combustion engine, electric aircraft 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@world.std.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 800 to 1700 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@world.std.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@world.std.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 $42 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@world.std.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. [Batteries] ====================================================================== Q. How is the voltage of a pack determined? A. (Jim Bourke, jbourke@world.std.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). Q. What are milli-amp hours? A. (Jim Bourke, jbourke@world.std.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@world.std.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. Q. What is Nicad memory? A. (Jim Bourke, jbourke@world.std.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@world.std.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@world.std.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@world.std.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@world.std.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? (Red Scholefield, redscho@afn.org) A. I thought no one would ever ask! The Black Wire Syndrome The black wire syndrome is an occupance 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. 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! [Electric motors] ====================================================================== Q. What does "breaking-in" a motor actually do? (tg) 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? (tg) 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@world.std.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. Q. How do I compare an electric motor to an IC engine? A. (Jim Bourke, jbourke@world.std.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@world.std.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 many 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. [Speed Controls] ====================================================================== 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. [Chargers] ====================================================================== Q. How does charging current relate to capacity? A. (Jim Bourke, jbourke@world.std.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 Q. How can I make certain my packs are fully charged? A. (Jim Bourke, jbourke@world.std.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. [Formulas] ====================================================================== Q. How do I calculate duration? A. (Jim Bourke, jbourke@world.std.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 Q. How do I calculate Watts? A. (Jim Bourke, jbourke@world.std.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. It pays to buy a good motor! Q. How do I convert between different units of measure? A. (Tim McDonough, tpm@inw.net) 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 millimeters (mm) in 1 centimeter (cm) There are 1000 grams (g) in a kilogram (kg) There are approximately 28g in 1oz. (Actually the value is closer to 28.34952g but it's not going to matter much given the precision to which we can most likely measure other parameters in our systems.) Wing loading is usually expressed in ounces per square foot or in grams per square centimeter. To convert ounces to/from grams: weight_in_ounces * 28 = weight_in_grams weight_in_grams / 28 = weight_in_ounces To convert inches to/from centemeters: length_in_inches * 2.54 = length_in_centemeters length_in_centemeters / 2.54 = length_in_inches To calculate wing loading (oz/ft^2) when you have the wing area in inches and the weight in ounces: wing_loading = weight_in_ounces / (area_in_square_inches / 144) [Design Issues] ====================================================================== Q. What is the watts/pound rule? A. (Jim Bourke, jbourke@world.std.com) Basically, the rule states that 50 or 60 watts per pound is needed to produce good sport flying characteristics. Some airplanes, such as gliders, can use a smaller ratio (30 watts/lb), while others, like pylon racers, may need a much higher ratio (80 watts/lb.). The watts/pound 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@world.std.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) sport plane with a 600 sq. in. (4.2 sq. ft.) wing. I desire mild aerobatics (50 watts/pound) and a low wing loading (< 20 oz/ sq. ft.). I want at least 5 minutes of full power. Lets see how close I can get. We'll try three different power systems. 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 (sq. ft.) #1 Astro 15 with a 10 cell, 1700 mAh pack: The weight of this power plant is roughly 2 lbs. We now have enough information to figure out the aircraft weight and wing loading. Aircraft weight = 6 lbs (96 oz) Wing Loading = 96 oz / 4.2 sq. ft. = 22.9 oz/sq. ft. 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: amps = watts / # of cells watts = watts/pound ratio * aircraft weight (in pounds) Therefore: amps = (watts/pound * aircraft weight) / 10 volts = (50 watts/pound * 6 pounds) / 10 volts = 300 watts / 10 volts = 30 amps 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/ sq. ft. 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). Aircraft weight = 6.75 lbs (108 oz) Wing Loading = 108 oz / 4.2 sq. ft. = 25.7 oz/ sq. ft. 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). Aircraft weight = 6 lbs (96 oz) Wing Loading = 96 oz / 4.2 sq. ft. = 22.9 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? (Doug Ingraham, dpi@rapidnet.com) A. 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. Cuting 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. [Weights and Measures] See [Formulas] [Motor Data] ====================================================================== 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 40Cobalt 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=Astro Flight, GR=Graupner Kv: Voltage constant, in RPM/volt Rm: Terminal Resistance, in Ohms Io: No-load current, in Amps Remarks: Astro Flight motor data comes from the manufacturer. [What Works] ====================================================================== This section of the FAQ lists *some* combinations of motor, prop, battery, etc. that will provide a flyable e-powered plane. There's much work to be done here but I thought it was important to get the section kicked off with an example or two. TPM >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 [Glossary] ====================================================================== 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 2000 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. [Internet Resources] ====================================================================== Mailing lists ------------- EFLIGHT This is an electronic mailing list devoted to the discussion of electric model aircraft. Subscriptions are free. To subscribe, send an email to "majordomo@world.std.com" with the body "subscribe eflight". Further instructions are available on The E Zone. FTP sites --------- ftp://ftp.luth.se/pub/misc/rc This site is the unofficial model aircraft plans site for the internet. Web pages --------- http://world.std.com/~jbourke/ezone.html The E Zone is the official home of the faq, and of the EFLIGHT mailing list. http://members.gnn.com/KenMeyers/homepage.htm 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.inw.net/~tpm/freeflight.html There's been a lot of interest in free flight planes lately both electric and non-electric powered. If you're interested this site has information on subscribing to the Free Flight electronic mailing list, some links to other free flight related sites, and a photo gallery of pictures of various types of free flight planes. 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. Non-Electronic Resources ------------------------ 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 to me from Wil Beyers. 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. Wil [Credits] ====================================================================== Many thanks to the following faq contributors: Jim Bourke, Doug Ingraham, Terry Gamble, Matthew Orme, Red Scofield. [Regards] ====================================================================== Special thanks to: Jim Bourke for taking on the task of running the E Zone and starting off the FAQ. Don Granlund for being a patient mentor to Jim Bourke. Keith Shaw for writing excellent articles, and for proving over and over again that electric sport scale aircraft are practical endeavors. Ken Myers, for maintaining a great web page.