Electric Motors

Electric Motors Electric powered model aircraft has gained popularity, mainly because the electric motors are more quiet, clean and often easier to start and operate than the combustion motors. They need batteries to operate and despite some developments in this area; the batteries still are somewhat heavier as energy source compared with the gas fuel. Thus, the electric flier has to strive to build the model as light as possible in order to obtain a reasonable wing loading and/or a reasonable flight time. The electric motor's operation is based on the electromagnetic principle. When electric current flows through a coil it creates a magnetic field with a strength proportional to the current's value, the number of windings of the coil and is inversely proportional to the coil's length. The strength of the magnetic field will further increase by introducing a so-called ferromagnetic material inside the coil. An electromagnetic device only gets magnetic when electric current is applied, whereas a permanent magnet doesn't need electric power to be magnetic. Both electromagnets and permanent magnets have the so-called poles at either end. One is called N (north) and the other S (south). When two magnets get close together the N and the S poles attract, whereas the same poles (N N or S S) will repel each other. The electric motor functions according to the same principle. There are two main different motor types used in model aircraft: The brushed and the brushless. A brushed motor consists mainly of a cylindrical metal case containing a stator and a rotor. The rotor is part of the motor shaft, which rotates inside the stator. The rotor has several coils (poles) that may either have an iron core or are coreless. The stator consists usually of two permanent magnets mounted close to the metal case. The rotor coils receive electric current via a so-called commutator, which is connected to a DC voltage through two brushes (hence the name). The commutator changes the voltage polarity to the coils at a certain instant once every turn of the motor shaft, thereby keeping the motor running. The motor shaft is supported by two bearings, which may be of plastic, porous brass bushes or ball bearings (more expensive). The coreless motor has the rotor coils not wrapped around an iron core but just fastened into shape with glue, which makes the rotor much lighter and faster to accelerate and thus suitable for servos. Since the coreless don't have iron core they have much less iron losses, which make them more efficient than cored motors. However, the coreless motors will not stand continuous high RPM and/or loads without falling apart. That's why they are generally rather small, with low speed and low power. As flight power motors the corless are only used with small indoor planes. A DC motor converts the electric current into Torque and the voltage into rotations per minute (RPM). Torque is a twisting force measured at a certain radial distance from the shaft's centreline. For example: Newtons*meters (Nm) Motor's Output Power (W) = Torque(Nm) * 2p*RPM / 60 The power consumption of a DC motor (Input Power) is equal to its terminal voltage times the current. However, every motor has losses, which means that the motor consumes more power than it delivers at its shaft. The motor's Output Power is equal to the Input Power minus the Power Loss. Most of Power Loss is equal to the sum of the Copper Loss plus Iron Loss. Copper Loss = Coil's Resistance Rm * Current Iin2 Iron Loss = Vin * Idle Current Io The following equation can also be used to calculate the motor's Output Power: Pout = (Vin - Iin * Rm) * (Iin - Io) The motor's Efficiency (h) is the ratio of the Output Power to the Input Power: % h = 100 * Pout / Pin Efficiency is a measure of how much of the Input Power (the power that the battery delivers to the motor) is actually used to turn the propeller (Output Power) and how much is wasted as heat. A motor with higher efficiency delivers more power to the prop, and wastes less. Assuming the same current, increasing the voltage increases the motor efficiency until the RPM limit is reached above which the efficiency falls. A further parameter is the motor's Kv, which refers to the ratio of the RPM to the Voltage at the motor's terminals minus the Voltage loss inside the motor due to the coil's resistance Rm. Kv = RPM / (Vin - Vloss) Thus: RPM = Kv * (Vin - Vloss) And since: Vloss = Iin * Rm The RPM will decrease as the current Iin (load) increases. For instance, a motor with a Kv of 1000, a coil resistance Rm of .04 ohms and with a terminal voltage of 8 volts at 12 amps will have the following RPM: 1000 * (8 - 12 * .04) = 7520 RPM instead of 8000 RPM (if the motor coils had no resistance Rm) that means a loss of 480 RPM from the ideal in this case. RPM Loss = Kv * (Iin * Rm) If a larger propeller is used the current will increase, thereby further decreasing the motor's RPM. So, in high current applications a low resistance Rm is needed in order to prevent too much loss of RPM. In reality the coil's resistance Rm increases as the temperature increases, which means that the RPM will decrease over time even if the input voltage is constant. If the motor shaft is held so that it cannot move at all, it is in stalled condition. In such a condition the motor will draw the maximum current possible from the battery and will most likely be destroyed. The current drawn in stalled condition is calculated according to , Istall = Vin / Rm Another parameter is the motor Torque as a function of Current. It is called Kt and is expressed in inch-ounces per ampere (imperial units): Kt = 1352 / Kv The amount of Torque per ampere depends on the motor's Kv. The higher the Kv, the lower the Torque per ampere. High Kv = Low Torque per ampere Low Kv = High Torque per ampere Like the actual RPM is less than the ideal due to the resistance Rm, the actual torque is also less than the ideal due to the idle (no load) current Io. The actual Torque is calculated as follows: Torque = Kt * (Iin - Io) For the same Torque: High Kv - needs more Current Low Kv - needs more Voltage The motor's Kv is much dependent on the coils’ number of turns. A high number of turns gives a low Kv and vice-versa. So one may ask, which Kv is the best? The answer is; it depends on the sort of plane and on the type of flying. For instance: For the same power, a lower Kv allows the use of a larger diameter prop, giving higher thrust at the expense of top speed, whereas a higher Kv requires a smaller prop, spinning it at higher RPM resulting in a higher top speed but in lower thrust. So, if you intend to hover, have fast climb, good acceleration, are able to use a larger diameter prop and the top speed is not of concern, the low Kv is preferable. Increasing the current increases the RPM Loss, but decreases the Torque Loss. Motor's maximum efficiency occurs when the RPM loss equals the Torque loss. Every motor type has an ideal voltage, current and RPM at which the motor's max efficiency is obtained. These values are often shown in the manufacturer's data sheets. Brushed motors' efficiencies are normally between 30 and 80% depending on the type and price. Most motors supplied in kits for beginners have the stator made of low cost ferrite magnetic material. They are called ferrite or "can" motors. "Can" motors are rather inefficient and cannot be opened and serviced like other higher quality motors. However they are cheap and most kits will fly just fine with these motors, so it's ok to use a "can" motor for your first plane. "Rare hearth" motors such as Cobalt and Neodymium are considered to be far superior to ferrite motors, but they are also much more expensive. Unlike ferrite magnets, the "rare earth" magnets withstand high temperatures without losing their magnetic properties. Electric motors have several designations such as 280, 300, 400, 480 and 600, which refer to the case length and also give an idea of their power and weight. For example a 480 motor has about 48mm case length, is heavier and is able to deliver more power than a 280 motor. Generally a 280 motor is suitable to power models up to 400gr and a 480 motor may be suitable to models up to 800gr, while a 600 motor may power models up to 1200gr, assuming direct drive (without gearbox reduction). As a rule of thumb, the input power for a sports plane (no EDF) should be about 110 W/kg (50 W/lb) in order to get reasonable flying characteristics. Gliders and parkflyers may need much less power, 65 W/kg (30 W/lb), while the scale and aerobatics may need much more power, e.g. > 200 W/kg (90 W/lb). This assuming that the motor has about 75% efficiency. Power/Weight : Estimated Performance (Sports Plane) : Watts / lb Watts / kg   For wingloading about 60g/sq.dm (20oz/sq.ft) However, the power to weight ratio recommended above is by itself not enough to guarantee whether the plane will fly at all or its flight performance, as other factors have to be taken into account, such as the pitch speed of the propeller, which refers to propeller's rpm times the pitch. Note that the static rpm is lower than when the model is flying. The minimum pitch speed recommended is 2 to 3 times the plane's stall speed. The stall speed of an aircraft in mph (both model and full-scale) is approximately equal to four times the square root of the wing loading in ounces per square foot. To calculate the aircraft's approximate stall speed , To calculate the aircraft's approximate level flight speed. Another factor is the Static Thrust, which refers to how much the aircraft is pulled or pushed forward by the power system when the aircraft is stationary. The Static Thrust should be at least about 1/3 of the aircraft's weight. However, in order to be able to hover (3-D models), the Static Thrust should be greater than the plane's weight. To estimate the prop's approximate Static Thrust, Note that the Static Thrust alone is not enough to predict how the aircraft will fly, as other factors like the prop pitch speed should also be considered. Measuring and comparing the propellers' Static Thrust may be misleading, as the blades of a given prop may stall, resulting in a low static thrust on the test bench, while it may give excellent performance in flight and even outperform others that have a better Static Thrust. Output Power = Thrust * Pitch Speed So, with a given power, the more thrust you have, the less top speed you get. In other words, assuming the same power: Large diameter & small pitch = more thrust, less top speed (like low gear in a car). Small diameter & large pitch = less thrust, more speed (like high gear in a car). The prop diameter-to-pitch ratio for sport models should be between 2:1 and 1:1 In case the pitch is too high related to diameter, the prop becomes inefficient at low forward speed, as when during the take-off and/or climbing. At the other end of the scale, a propeller designed for greatest efficiency at take- off and climbing (low pitch & large diameter), will accelerate the model very quickly from standstill but will give lower top speed. The performance of an electric powered model is also greatly affected by the batteries' internal resistance. The lower the battery's internal resistance, the less restriction it has in delivering the needed power. For the same capacity, the battery with higher recommended max discharge rate has lower internal resistance. Gearboxes are often used to reduce the motor's rpm at the propeller shaft, increasing their torque and allowing the use of larger propellers. Since the propeller blades also are more efficient at moderate rpm, this combination is often worth- while despite the increased weight. Indoors and slow flier models have often a gearbox which allows the use of relatively smaller and lighter motors improving the slow flight performance and prolonging the flight time. The drawback is that the top speed is reduced. High-speed models such as those powered by Electric Ducted Fans, (EDF) require high Kv motors that have max efficiency at high RPM (typical above 25.000 RPM).        Some factors have to be taken into account when designing an EDF propulsion system, such as the intake (inlet) should have about the same area as the Fan Swept Area FSA, in order to prevent efficiency loss. Also care should be taken during the design of both the intake (inlet) and the exhaust (outlet) ducting. In order to reduce efficiency losses due to turbulence and drag, the duct internal surfaces should be as smooth and straight as possible. Circles are the best duct cross sections to minimise surface drag. The exhaust area is usually about 85 to 95% of FSA for best performance. Some examples of ducting are shown in the pics on right.         For a given power, the EDF propulsion system has often lower thrust/weight ratio compared with a conventional propeller system, and some EDFs need to be hand- launched or bungee-launched since they can't take-off the ground. Once in the air the EDF may reach rather high speed though. The flight time of an electric powered model depends on some variables like: Aircraft's flight characteristics (based on wing loading and lift), the combination motor/propeller, the motor's efficiency (Pout/Pin) and last but not the least, the batteries energy/weight ratio. Flight time in minutes = (battery capacity / average current drawn) x 60. Electric flight models may be built small and lightweight enough to fly inside a sports hall. They are the so-called Indoor Models, having approx. 75cm wingspan (30") with a weight less than 200gr (7oz) and flying no faster than 8-16Km/h (5-10mph). The so-called Park Fliers are somewhat faster. They are often made of foam material and may fly at speeds anywhere from 25Km/h up to about 40Km/h (16 to 25mph). They are rather sensitive to strong winds, so it's recommended to fly them during calm weather. For further pictures and info about Indoors/Park Fliers check: Aeronutz Of course, it's also quite possible to build much bigger electric powered aircraft models. As the motor rpm increases it requires the rotor coils to be energised sooner so that they get the full magnetic field strength in time to react with the stator's magnetic field. Also when the load increases, the magnetic field in the rotor coils increases, which interacts with the stator's magnetic field, producing a rotated resultant magnetic field. Some motors allow the brushes' angle to be changed by the same amount as the field rotation, thereby increasing the motor's efficiency under a given load. That's called for motor "timing". An electric motor may be timed under load by slowly changing the brush holder's angle while measuring the current. The ideal brush angle is when the motor draws less current. There is no fixed ideal timing angle, since the best timing angle changes as the motor load and speed changes. If the motor has been timed at clockwise rotation it has to be re-timed in case the rotation needs to be reversed. The motor's direction of rotation may be reversed by inverting the voltage polarity at the supply terminals. A timed motor gets higher idle current (with no load). Brushed motors need some maintenance, since both the brushes and the comm. will wear after a while due to the friction. Most quality motors allow brush replacement. The commutator itself also needs cleaning as it gathers deposits of carbon and gunk due to the graphite powder from the brushes. It may be cleaned by a very light polishing action with scotchbrite or with a so- called commutator stick. The gunk can also be cleaned off while the motor is running manually, using a few drops of alcohol. If commutator is pitted or shows brush skipping and chattering means that it has been overheated and got deformed (out of round). It needs to be repaired, as polishing will not cure the deformation. Brushes are usually made of three different compounds: Graphite, Copper and Silver. Brushes made of silver are normally used in competitive racing as they have low resistance, but they produce the highest commutator wear and also have medium brush wear and lubrication. Silver brushes produce sludge that only can be removed by lathing the commutator. Copper brushes don't produce sludge and work best at high rpm. These brushes produce medium commutator wear and have high brush wear and low lubrication. Graphite brushes produce low commutator wear, have low brush wear and high lubrication but have high resistance, which means that they are not suitable for racing. Usually it's necessary to "break-in" a new brushed motor so that the flat brushes get a curved surface and thus increasing the contact area with the commutator. Running a motor with new flat brushes at full load will cause a lot of arcing, which pits the contact surfaces and degrades performance. The "break-in" may be done by running the motor without load (without prop), at about 1/2 its rated voltage for about a hour or two. The brushes should get a curved surface without sparks/arcing. Some high-quality motors do not need to be "broken-in". This will be mentioned in the respective motor's manual. In case of doubt, just break it in. Sparks that occur between the brushes and the commutator can cause radio interference. In order to prevent radio interference it is recommended the use of ceramic capacitors soldered between each motor terminal and the motor case. For extra security against interference, a third capacitor should also be fitted between the motor terminals. Note: many Graupner Speed xxx motors have the first 2 of these capacitors already fitted internally. A common way to control the electric motor's speed is by using an Electronic Speed Controller (ESC). The Electronic Speed Controller is based on Pulse Width Modulation (PWM), which means that the motor's rpm is regulated by varying the pulses' duty-cycle according to the transmitter's throttle position. For example, with the throttle at the minimum position, there will be no pulses, while moving the throttle to the middle will produce 50% duty-cycle. With the throttle at the max position the motor will get a continuous DC voltage. Most ESCs have a facility known as Battery Eliminator Circuit (BEC). These controllers include a 5V regulator to supply the receiver and servos from the same battery that is used to power the motor, thereby eliminating the weight of a second battery only to power the radio and servos. The motor power is cut-off when the battery voltage falls, for example below 5V. This prevents the battery from getting totally flat allowing the pilot to control the model when the motor stops. Some controllers also include a brake function that prevents the propeller from keeping spinning when the motor power is cut-off. Electronic Speed Controllers are available in different sizes and weights, which depends on their max output current capabilities. Another important characteristic of an ESC is the on-resistance of the output power switching transistor(s). The on-resistance should be as low as possible, since its value is proportional to the power loss dissipated by the output transistor(s): P = R x I2 The on-resistance is normally between approx. 0.012 and 0.0010ohm. The value depends on how many output parallel-connected transistors the actual ESC has. The higher the current capability the lower the on-resistance should be. These figures are normally shown on the ESC data sheet along with the BEC voltage cut-off value and the max. output current to the receiver and servos. As a safety measure many ESCs have a function that won't allow the motor to start running unless the throttle is initially set in the minimum position. Another safety device is the so-called arming switch connected between the motor and the controller. The arming switch should be off until the plane is ready to taxi out on the runway or be hand-launched. After the flight, the arming switch should be turned off as soon as possible. This will prevent the motor from start running in case the throttle stick is moved forward unintentionally. In order to keep the arming switch contacts in good shape (lowest resistance) it's advisable to never switch it on/off under power. This means that the arming switch should be only turned on/off when the throttle is in the minimum position. The more powerful the motor, the more need for the safety of an arming switch. A reasonable approach is using an arming switch on flight models larger than speed 400 size (approximately 100 watts and above). Large batteries are capable of delivering very high currents when shorted or when the propeller gets blocked. Such high currents are enough to overheat and melt components/wiring, which may lead to a fire. Some organisations that provide insurance for modellers require a fuse in electrically powered models. To choose the correct rating for the fuse just put the largest and highest-pitch prop that you expect to fly with. Measure the current draw of your power system on the bench and multiply the value by about 1.25. This 25% margin should prevent nuisance blows. Find the fuse with a rating at or just above this current level. Another type of electric motors for model aircraft are the so-called brushless. These motors are little more expensive but they have higher efficiency. Typically between 80 to 90%. Since they have no brushes, there is less friction and virtually no parts to wear, apart from the bearings. Unlike the DC brushed motor, the stator of the brushless motor has coils while the rotor consists normally of permanent magnets. The stator of a conventional (inrunner) brushless motor is part of its outer case, while the rotor rotates inside it. The metal case acts as a heat-sink, radiating the heat generated by the stator coils, thereby keeping the permanent magnets at lower temperature. They are 3-phase AC synchronous motors. Three alternated voltages are applied to the stator's coils sequentially (by phase shift) creating a rotating magnetic field which is followed by the rotor. It's required an electronic speed controller specially designed for the brushless motors, which converts the battery's DC voltage into three pulsed voltage lines that are 120o out of phase. The brushless motor's max rpm is dependent on the 3-phase's frequency and on the number of poles: rpm = 2 x frequency x 60/number of poles. Increasing the number of poles will decrease the max rpm but increase the torque. A brushless motor's direction of rotation can be reversed by just swapping two of the three phases. Earlier speed controllers needed an additional set of smaller wires connected to the motors' internal sensors in order to determine the rotor position to generate the right phase sequence. New controllers read the so-called "back EMF" from each phase, which allows the motor to be controlled without the need of the extra wires and sensors. These new controllers are called "sensorless" and can be used to control motors with or without internal sensors. At less than full throttle the 3-phase pulses are chopped at a fixed frequency with a duty-cycle depending on the throttle position. At full throttle the phase pulses are no longer chopped giving the max rpm and torque. The ESC's 3-phase actual output frequency and thus the motor's rpm depend on motor's Kv (rpm / volt), the actual load and the voltage applied, as the ESC needs the EMF positioning pulses back from the motor before it sends the output pulses. Many brushless ESC allow the user to set the Electronic Advance Timing. High advance timing (hard timing) is suitable for high pole count motors (above 6 poles, such as Jeti, Mega, Plettenberg). High advance timing gives more output power at expense of efficiency. Low advance timing (soft timing) is suitable for low pole count motors. It gives higher efficiency with some loss of output power and is recommended when long run-time is the primary goal. A recent type of brushless motor is the so-called "outrunner". These motors have the rotor "outside" as part of a rotating outer case while the stator is located inside the rotor. This arrangement gives much higher torque than the conventional brushless motors, which means that the "outrunners" are able to drive larger and more efficient propellers without the need of gearboxes.

Gas Turbines

Gas Turbines Another well-known propulsion system is the gas turbine or jet engine. There are several types of gas turbine engines, but the simplest ones are the so-called turbojets. These engines are shaped like a cylinder containing several parts inside, which rotates on a central shaft. An auxiliary electric motor is needed to start the turbine engines. The outside air enters the engine through the inlet into the compressor, which consists of one set of fixed blades (stator) and another of rotating blades (rotor). The air is then compressed at the compressor section and enters thereafter the burner where the fuel/air mixture is ignited. This creates a hot gas passing through the turbine and out the nozzle, which is shaped to accelerate the hot exhaust. The turbine uses the energy from the hot exhaust to rotate and since the turbine is linked to the compressor by the central shaft, it will also keep the compressor rotating, thus no longer needing the electric motor. Normally the model aircraft turbines use propane/butane gas along with a glow plug to start the ignition and rise the burner's temperature above 100oC before liquid fuel is injected through small holes into the burner. Once the combustion gets started, the glow plug is no longer needed.  The combustion process may be controlled or stopped by regulating the amount of the fuel available, the amount of oxygen available or the source of heat. Unlike the conventional combustion/piston engines, the jet engines don't have a natural limitation of the rpm. This means that the rpm will keep rising as more fuel is fed to the engine until the materials no longer withstand the high temperature and/or the high rpm and will breakdown. Therefore, an Electronic Control Unit (ECU) is required to limit the max fuel flow. The max value is set by using an external device called Ground Support Unit (GSU). Since model aircraft powered by gas turbines usually fly very fast, with speeds above 500Km/h (300mph), these type of engines are definitely not recommended to beginners

Pulsejet Engines

Pulsejet Engines Aircraft model builders have always strived to emulate the full-sized aircraft, as well as their propulsion systems. The word "pulse" engine may be tracked back to around 1880 - 1890 and it is claimed that a Frenchman has build a pulsejet engine in the beginning of 1900 however, it's unknown whether he was successful. The Germans used this type of engine during the W.W.II to power the well-known V-1 flying bomb. This power concept was eventually proven to be relatively inefficient, terribly noisy and also having a very short lifetime. The valves on the V-1 engine lasted no longer than 30 minutes continuous use. The pulsejet was therefore abandoned as a full-size aircraft propulsion system. Nevertheless, it has been used on model aircraft by some enthusiasts until now. A model pulsjet engine is basically made of a tube consisting of a head with a venturi shaped air-intake, a diffuser, a combustion chamber, reed valve plates, a spark plug and an exhaust. In order to start the engine, a compressed air from an external pump or air bottle is fed to the angled pipe located near the diffuser while a pulsed high voltage supply is applied to the sparkplug. The air/fuel mixture is pushed through the valve into the combustion chamber and ignited, which causes a noisy explosion that closes the valve plates while the expanding gasses escape trough the exhaust. This produces a low pressure inside the combustion chamber that opens the valves and new air/fuel mixture enters the chamber again, which is ignited by the residual heat and gasses from the previous explosion. The high temperature developed keeps the motor running without the spark plug and the compressed air, which are only needed at the start moment. Some types have no sparkplug attached. The initial ignition is then obtained by introducing external sparking wires through the exhaust. The pipe has an acoustic resonant frequency depending on its length, which must be close to the valves' working frequency in order to get a reliable operation. The extreme heat developed means that this engine needs a lot of air cooling and cannot stand static running on a test-bench for longer period than about 10 seconds. It must also be mounted outside the model to prevent burning damage to the structure. Due to the extreme noise and the high temperature involved, this engine is absolutely not recommended for beginners and should not be used near residential areas.

Glow Engines

Glow Engines There are two main propulsion systems used by R/C models today: The internal combustion systems (glow engines) and the electric motors. Combustion engines' energy source has so far a higher energy/weight ratio than the batteries used to power the electrics. However, the combustion engines are usually more noisy and more prone to oil spillage than the electric motors. There are two types of glow engines: The four-stroke and the two-stroke. Two-stroke engines are the most used, mainly because they are simple made, light, easy to operate, easy to maintain, and are usually inexpensive. Two-stroke engines operate at a high RPM and therefore can be quite noisy without a good silencer.  Nevertheless, the four-stroke engines also enjoy some popularity, mainly because they produce a lower, more scale-like sound and consume less fuel. They have lower power/weight ratio and lower RPM, but provide more torque (use larger propellers) than theirs two- stroke counter-parts. However, since the four-stroke engines require high precision engineering and more parts to manufacture, they are usually more expensive. They also need more maintenance and adjustment than the two-stroke, yet they are not too difficult to operate and maintain. A glow engine consists basically of: - Crankcase: which is the main body of the engine and houses the internal parts. - Head: mounted on the top of crankcase. It has fins to provide engine cooling. - Muffler: damps the exhaust noise as it exits the combustion chamber. - Carburettor: to control the amount of fuel and air that enters the engine. - Prop Shaft: is a part of the Crankshaft that protrudes from the crankcase. - The Crankshaft transforms the movements of the Piston into rotational motion. - The Piston has a cylindrical form and operates by an up/down movement (assuming the engine is viewed upright) inside a sleeve, which is called Cylinder. The glow motor's Carburettor consists basically of: - Rotating barrel, which controls the amount of fuel/air mixture going to the combustion chamber. - Throttle arm connected to the barrel, which enables the engine's speed to be controlled by a servo. - Idle Stop Screw to adjust how far the throttle barrel closes. - Idle Mixture Screw to adjust the amount of fuel entering the carburettor while the engine is idling. - Needle Valve to adjust the amount of fuel entering the carburettor during medium and high-speed operation. All glow engines require a special fuel, called "glow fuel." It consists of methanol as base, with some amount of nitromethane to increase the energy and pre-mixed oil into the fuel, which lubricates and protects the engine parts. Two-stroke engines operate by igniting the fuel in its combustion chamber once every turn of its crankshaft. The fuel is mixed with air at the carburettor and forced into the cylinder during the down movement of the piston (1st stroke). While the piston moves up, the mixture is compressed and when the piston reaches the top, the glow plug ignites the compressed gases, forcing the piston down (2nd stroke). On the way down exhaust gases escape through the exhaust port while the fuel mixture enters the cylinder again. In a four-stroke engine the fuel/air mixture enters the combustion chamber during the down movement of the piston through a valve operated by the camshaft (1st stroke). When the piston moves up, the valve closes and the mixture is compressed (2nd stroke). When the piston reaches the top, the glow plug ignites forcing the piston down (3rd stroke). On the next up movement of the piston, a second valve opens and allows the exhaust gases to escape (4th stroke). The piston moves down and the fuel mixture enters the combustion chamber again, repeating the 1st stroke. The glow engines usually have a simple ignition system based on a glow plug made up of a little coil of platinum wire rather than a spark plug. A 1.5V battery is used to heat the glow plug only during the starting procedure and is removed when the motor reaches a certain rpm. This is possible because the glow plug keeps glowing by the heat produced during the compression and combustion without needing the battery. There are two lengths of glow plugs available. The short ones are normally used on engines smaller than 2.5cc (.15cu in). Some have a metal bar across the bottom of the plug called for Idle Bar, which prevents raw fuel from dousing the heat from the element during idle. There are also the so-called "hot" and "cold" glow plugs, which refer to their effective coil operating temperature. The glow plug's temperature depends on several factors, such as the coil's alloy, thickness and length, the size of the hole in which the coil is located as well as which material the glow plug's body is made of. Usually smaller engines and those that run on less nitro prefer hotter plugs. In case of doubt just follow the engine manufacturer's recommendation. Turbo glow plugs have a chamfered end that matches the threaded hole on the engine's head. It is claimed to give less compression leakage around the glow plug and less disruption of the combustion chamber. Also the hole in the cylinder head, which exposes the glow plug to the air/fuel mixture in the cylinder is much smaller, resulting in fewer rough edges that could create unwanted hot spots. The turbo plug is shown on the left of the picture below. Glow engines may have plain bushed supported crankshaft or ball bearings. Ball bearing engines usually have a better performance, run smoother, and last longer but are more expensive than those with bushings. The model engines' piston and cylinders construction are usually based in two methods: Ringed engines or ABC. Ringed engines have been the main method of construction until recently. It consists of an aluminium or iron piston with a ring moving in an iron sleeve. The ring provides the compression when operating. Ringed engines are inexpensive to restore its compression after long usage by simply replacing a ring, and are generally slightly cheaper. They require an extended break-in period where the motor is run very rich to provide lots of lubrication while the ring fits itself to the cylinder. They are also more easily damaged if the engine is run too lean. A more recent method is the ABC, which stands for Aluminium, Brass, Chrome where an aluminium piston runs in a chrome plated brass sleeve. The piston and cylinder are matched at the factory to give a perfect fit and good compression. ABC engines start easily by hand, give more power than the ringed engines, have a good life-span and are less prone to damage with a lean run. Schnuerle ported engines have several fuel inlet ports on three sides of the cylinder allowing more fuel to flow to the combustion chamber. This gives somewhat more power than with standard porting, which has only one fuel inlet port on the side of the cylinder opposite the exhaust outlet. A Schnuerle ported engine is usually slightly more expensive due to higher manufacturing costs involved. The fuel tank size and location affects the engine operation during the flight. A typical tank placement is shown on the picture below: When the engine is in the upright position, the fuel tank's centreline should be at the same level as the needle valve or no lower than 1cm, (3/8in) to insure proper fuel flow. A too large fuel tank may cause the motor to run "lean" during a steep climb and "rich" during a steep dive. Normal tank size for engines between 3.5cc (.21) and 6.5cc (.40) is 150 - 250cc. Ducted Fans In order to emulate the full-size aircraft jet-power systems, it is often used the so-called Ducted Fan, there a glow-engine drives a fan fitted inside the model. There are also glow engines specially designed for Ducted Fans, which have a special shaped head, also having the exhaust port facing towards the rear of the model. These engines are often equipped with a tuned pipe exhaust in order to improve their efficiency at high rpm.

Batteries

Batteries Batteries are available in different sizes, weights, voltages and capacities C, which refer to their stored energy expressed either in amps-hour Ah or milliamps-hour mAh. For example, a battery with a capacity of 500mAh should deliver 500mA during one hour before it gets totaly discharged (flat). Radio control sytems are usually powered by rechargeable batteries. Rechargeable battery types available on the market today are: Nickel-Cadmium (NiCads) Nickel-Metal hydride (NiMH) and Lithium-Polymer (Lipo) batteries. Even Lead-Acid batteries are also used as ground power source. Normally the NiCads stand more "abuse" which means that they may be charged at higher rate (normally 2 - 4C) and have the ability to deliver higher current, i.e. discharge rates up to 2C continuous or 8 to 10C during 4 - 5 minutes and even up to 100C during very short time. They have some designations such as the Sanyo AE for high capacity and AR or SCR for quick charge/discharge. A NiCad cell consists basically in a positive plate foil of nickel metal with nickel oxide/hydroxide, a negative plate foil of cadmium metal with cadmium hydroxide and an isolating porous separator film moistened with an electrolyte of potassium hydroxide (caustic potash). The two plates are sandwiched between the isolating porous separator films, rolled up and enclosed in a nickel-plated steel can. A spring-loaded vent is fitted at the positive terminal end in order to release the electrolyte and/or gasses, in case overpressure occurs due to overcharge. See picture below. The NiMH have higher capacity/weight compared with the NiCads but are more sensitive to high charge rates (max recommended 1C) and normally it is not recommended to discharge the NiMH batteries at higher rates than 3 - 5C. The NiMH self-discharge rate is also about 50% higher than the NiCads. However, the NiMH are more environment-friendly. A new type of NiMH battery known as HeCell has recently been developed, which is claimed to allow higher discharge rates than the conventional ones (about 12 - 16C). Both battery types lose their stored charge due to internal chemical action, even when not in use. Normally the NiCads lose around 10% of its charge in the first 24 hours after been charged and keep losing it by 10% per month. The rate of self-discharge doubles for a rise in temperature of 10 degrees C. Some NiCads can discharge themselves completely in a period of six months. The best way to keep batteries which are not in use for a long time, is by having them stored in the refrigerator (not in the freezer). Just allow the battery to reach the ambient temperature before using/recharging. Some manufacturers claim that these battery types are able to stand at least 1000 charges/discharges during their lifetime, assuming they have been subject to the ideal charging and handling methods. In practice however, we may expect about 600 - 800 charges/discharges. A safe method to charge both the NiCads and the NiMHs is by using a constant charge current (CC) at 1/10 of their capacity (0.1C) during 14 hours. For other charge current values one may use the following formula: Charge Time (Hours) = 1.4 x Battery Capacity / Charge Current (assuming that a constant charge current is used). However, low cost CC chargers provide no way of detecting when the battery is fully charged. The user is then expected to estimate the charging time based on the constant charging current value and the battery capacity, according to the formula above. And providing the NiCads' are discharged to about 1.1V p/cell each time before recharging, this charging method can be used to achieve a reasonably long battery life. Since repeatedly recharging an already fully charged NiCad or one with a large part of its charge remaining will degrade its performance. Some chargers provide the option to discharge the batteries down to about 1.1V per cell before starting the charging process. There are also fast battery chargers on the market charging from 1C up to 4C. But due to the high charging current level, it is required a reliable method of stopping the charge once the battery is fully charged, otherwise overheating and battery damage may occur. Since the NiMHs' and NiCads' voltage actually starts dropping after they have reached the fully charged state, the fast chargers use the so-called Delta Peak detecting method. There are "negative delta V (-DV)" and "zero delta V (0D)" detectors. Also "change of temperature (dT/dt)" detectors are commonly used. Some manufacturers use negative or zero delta V together with change of temp. detection, in case of one method fails to detect. Since NiMHs' voltage drop (delta V) after the fully charged state is lower than the NiCads, a more sensitive delta V charger is required for the NiMH batteries. Some chargers allow the user to set the value of the delta peak detection, which may be between 10 - 20mV per cell for NiCads and 5 - 10mV for NiMHs. A too low value may cause false peak detection due to electric noise, preventing the batteries from getting fully charged, whereas a too large value may result in overcharge, which reduces the batteries' life. Some fast chargers offer the possibility to automatically change over to slow charge (trickle-charge, for ex. at 0.05C) when the fully charge status is detected. The graph on the right shows the voltage and temperature variation of a four cell NiCad during charging at 1C constant charge current. Notice how the voltage drops after it has reached a top value, whereas the temperature keeps rising. The battery is considered fully charged when the temp. rises about 10°C above the ambient temp. (e.g. 24 + 10 = 34°C ) The NiMH batteries tend to dissipate heat during all the charging process, while the NiCads get warm only when they reach the full charge point. The nominal voltage is 1.2V per cell for both battery types and a charged cell may have about 1.45 - 1.50V. It's not possible to know exactly the NiCad's or NiMH's cell charge status by only measuring it's terminal voltage, as the cell's charge status is not a linear function of the cell's voltage. A reliable method to know how much charge is left or whether a cell still has its nominal capacity, is by discharging it with a known constant current and measure the time until the cell voltage reaches about 1.1V. For example, it should take about two hours to discharge a fully charged 500mAh cell by using a constant discharging current of 250mAh. Battery researchers have in the recent years come to conclusion that NiCads respond better to a pulsed charging waveform than to a steady DC current. By applying the charge current in one-second pulses with brief "rest" periods between them, ions are able to diffuse over the plate area and the cells are better able to absorb the charge. This is particularly true at the higher charge rates used by fast chargers. These chargers have a microprocessor that samples the "rest" periods between the charging pulses to read the battery terminal voltage. Another interesting discovery is that the charging process actually improves even further if during the "rest period" between charging pulses, the cells are subject to very brief discharging pulses with an amplitude of about 2.5 times the charging current, but lasting only about 5mS. It is claimed that these short discharge pulses actually dislodge oxygen bubbles from the plates and help them diffuse during the "rest period". The use of these brief discharge pulses is known as "burp charging". Tests done by both US military and NASA have shown that NiCads charged by using fast chargers employing the burped pulse system tend to last up to Twice as long as those charged by traditional CC chargers. Many of the high-end fast pulse chargers for NiCads use a charging method according to those findings. A battery pack consists of several cells connected in series, which inevitably age at different rates and gradually develop individual different charge status, and since the battery pack as a whole is charged and discharged repeatedly, these differences may become accentuated. The result is that some weaker cells can eventually be discharged well below 1.0 V and even driven into reverse polarity before the others reach the fully discharged state. During the recharging process, the weaker cells will be improperly recharged and tend to suffer increased crystal growth, while the others will absorb most of the charge and overheat, which dramatically degrades the whole battery pack performance. It's therefore advisable checking if the battery cells get different temperatures during the charging process, specially when high charge current rates are used. It's claimed that individual cell differences may level out by slow charging the battery pack from time to time at 0.1C during 14h or so.   TRITON Charger, Discharger   Handles 1-24 NiCd or NiMH cells, 1-4 Li-Ion cells   or 6,12, and 24V Lead Acid batteries.    For those who like to tinker with electronics and can't afford an expensive and sophisticated charger, there's a cheap alternative based on the National Semiconductorâ LM317 low cost regulator. The circuit diagram below shows a constant current charger using the LM317. The constant current may be set anywhere between 10mA and 1.5A by choosing the appropriate resistor R. R = 1.25 / I Where R is the resistor value in ohms, 1.25 is a reference drop voltage in Volts and I is the constant current in Amps. For example, to charge a 500mAH battery at 0.1C, (50mA) the R value will be: 1.25 / 0.05 = 25ohm. The dissipated power on the resistor R in this example is: P = V x I = 1.25 x 0.05 = 0.0625W or 62.5mW. The dissipated power on the LM317 IC is: (Vin - Vout) x Charging Current. It's advisable to use a heatsink to prevent the IC from getting too hot. Notice that the IC's metal package or tab also carries the Vout, so it's necessary to use isolating washers in case you attach the heatsink to a metal case. NiCads and NiMHs may be on charge during relatively long time without the risk of overcharging damage when using a constant current equal or less than 0.1C. However, it is not advisable to have the batteries continuously on charge longer than 24h, so one may connect the charger to a timer in order to cut the charging after about 14 -18h. For those who prefer a more sophisticated D.I.Y. NiCad charger based on delta peak method, as well as other interesting circuits, New rechargeable battery types, such as the Li-Ion (liquid electrolyte), the Lithium-Ion-Polymer (gel flat electrolyte) and especially the Lithium-Polymer (solid polymer electrolyte) are now often used with slow-flyers, indoors and even in much bigger models. A Lithium-Polymer cell (Li-poly or Lipo) has 3.7V nominal voltage, 4.2V max and 3.0V minimum. Other types may have different nominal voltages. These battery types have much higher energy density than NiCads and NiMHs. The max charge rate recommended is 1C, while the discharge rate should not be higher than 3 - 4C continuous or 5 - 6C during short time for the earlier types. Nowadays however some manufacturers offer discharge rates up to above 20C. For the same capacity, the battery with higher recommended max discharge rate has lower internal resistance, which provides better ability to deliver power. The self-discharge rate is claimed to be very low, typically 5% per year. These batteries cannot be charged with the same chargers that are designed for only NiCads or NiMH. In order to correctly charge the Li-ion/Lithium-polymer batteries, it must be taken into account the number of cells in the actual battery pack, since both the max charging current and voltage have to be set according to the cells' specifications. Charging these batteries with a wrong charger may cause them to explode! Also a short circuited pack may easily catch fire. According to Kokam, the Lithium-polymer batteries should not be discharged below 2.5V per cell, otherwise a rapid deterioration will occur. The basic charging procedure is by limiting the current (from 0.2 C to max 1C depending on manufacturer) until the battery reaches 4.2 V/cell and keeping this voltage until the charge current has dropped to 10% of the capacity C. Since the batteries only have 40 to 70% of full capacity when 4.2V/cell is reached, it's necessary to continue charging them until the current drops as described above. A charge timer should be used to terminate the charge in case the top voltage and/or termination current never reach their values within a certain time, which depends on the initial charging current, (e.g. 2 hours at 1C or 10 hours at 0.2C). Trickle charging is not good for Lithium batteries, as the chemistry cannot accept an overcharge without causing damage to the cells. Panasonic's charge curve for their 830mAh cells is shown below: The circuit diagram below shows a simple Li-ion/Lithium-polymer charger based on National Semiconductor LM317 low cost regulator. Before connecting the cells to the charger the max charging voltage has to be set by adjusting P1 (2k potentiometer). The max charging voltage must not exceed 4.2V per cell (Kokam), e.g. 8.4V for two serial connected cells. It is recommended using a digital voltmeter. The max charging current is set by choosing the value of Rx. Rx = 0.6 / max charging current For example, for a max charging current of 600mA, Rx should be 0.6 / 0.6 = 1ohm, while for a max charging current of 1.2A it should be 0.6 / 1.2 = 0.5ohm. The dissipated power on Rx at a charging current of 1.2A is: P = V x I = 0.6 x 1.2 = 0.72W The dissipated power on the LM317 IC is: (Vin - Vout) x Charging Current. It's advisable to use a heatsink to prevent the IC from getting too hot. Notice that the IC's metal package or tab also carries the Vout, so it's necessary to use isolating washers in case you attach the heatsink to a metal case. The LM317's max output current is 1.5A. For higher charging currents one may use the LM350 rated at 3A or the LM1084 rated at 5A. Note: if a Li-ion battery gets discharged below 2.9V/cell, it needs to be slow charged at 0.1C until 3.0V/cell is reached before a higher charging current rate may be used. Also discharging below 2.3V/cell will damage the battery. According to the manufacturers the Li-ion batteries should be stored charged to about 30 - 50% of capacity at room temperature. For prolonged storage periods, store discharged (i.e. 2.5 to 3.0V/cell) at -20° to 25° C. Important! Make sure to set your charger to the correct voltage according to the number of cells. Failure to do this may result in battery fire! Before you charge a new Lithium pack, check the voltage of each cell individually. This is absolutely critical as an unbalanced pack may explode while charging even if the correct cell count was chosen. If the voltage difference between cells is greater than 0.1V, charge each cell individually to 4.2V so that they are all equal. If after discharge, the pack still is unbalanced you have a faulty cell that must be replaced. Do not charge at more than 1C. NEVER charge the batteries unattended. Caution: If you crash with Lithium cells there is a risk that they get a latent internal short-circuit. The cells may still look just fine but, if you crash in any way remove the battery pack carefully from the model and place it on a non-flammable place, as these cells may catch fire later on. (A box with sand is a cheap fire extinguisher). Don't use Lithium batteries when flying in areas with large amounts of dry vegetation, as a crash may result in a serious forest fire. A new sort of Lithium (Saphion) cells has now been introduced into the market. These cells are claimed safe since they don't burst into flames when abused like the traditional Li-Ion-Polymer do. Their safety aspects result from the incorporation of phosphates as the cathode material, which are stable in overcharge or short circuit conditions and also have the ability to withstand high temperatures without decomposing. When abuse occurs, phosphates are not prone to thermal runaway and don't burn. These cells have a nominal voltage of 3.2V, can be discharged down to 2V and charged to 4.2V. The recommended discharge rate is 5 to 6C continuous for a long life or higher discharge rates for a shorter life. For further details check out the manufacturer Valence Technology Inc The lead acid batteries have much lower energy/weight ratio than all those previously mentioned. Which means that the lead acid batteries are heavier for the same capacity. They are not suitable to be used airborne, but since they are rather cheap, they are often used on the flying fields as ground power supply for engine starters and/or to charge the smaller ones. There are various versions of lead acid batteries: The Gel-Cell, the Absorbed Glass Mat (AGM) and the Wet Cell. The Gel-Cell and the AGM batteries cost about twice as much as the Wet Cell. However, they store very well and do not tend to sulfate or degrade as easily as the Wet Cell. Lead acid batteries get "sulfated" when the soft lead sulfate normally formed on the positive and negative plates' surfaces re-crystallises into hard lead sulfate when the batteries are left uncharged during long time. This reduces the battery's capacity and ability to be recharged. Adding Silica Gel to the sulphuric acid turns the electrolyte into a solid mass that looks like jelly, hence the name Gel-Cell. This prevents acid spillage even when the battery is broken. The sulphuric acid in AGM batteries is absorbed into fine fibreglass mats, they have the same advantage of the gelled batteries but can take more abuse. Both the Gel-Cell and AGM are the safest lead acid batteries one can use. However, Gel-Cell and some AGM batteries require a slower charging rate. These batteries may be damaged if fast charged on a conventional car charger. There are sealed (maintenance free) and serviceable non-sealed Wet Cell batteries. Non-sealed batteries are recommended in hot climates since distilled water can be added through the filler caps when the electrolyte evaporates due to the high environment temperature. The lead acid batteries have a self - discharge rate of about 1% to 25% a month. They will discharge faster at higher temperature. For example, a battery stored at 35°C (95°F) will self-discharge twice as fast than one stored at 24°C (75°F). Lead acid batteries left uncharged during long time will become fully discharged and sulfated. The best way to prevent sulfation is by periodically recharging the battery when it drops below 80% of its charge. It is possible to determine a non-sealed battery's charge status by measuring the concentration of the sulfuric acid of the battery electrolyte ("battery acid") with a hydrometer. These batteries are built with different characteristics depending on application. For example, starting batteries (also called SLI - Starting, Lightning, Ignition) have the ability to deliver large starting current during very short time (cranking amps). They have many thin plates of Lead "sponge", which gives a large surface area. Starting batteries are mainly intended to start engines when the batteries seldom get deep discharged, because if they often get deep discharged the Lead sponge falls faster to the bottom of the cells, significantly reducing their lifespan. Another type are the deep cycle batteries, which may be discharged down to 20% of the full charge, time after time, without reducing their lifespan as their plates are much thicker, however, these batteries lack the ability to deliver large current during short time compared with the starting batteries. Deep cycle batteries are therefore used where current is needed during long time, such as in forklifts, golf carts or solar electric backup power. If a deep cycle battery is also going to be used as a starting battery, it should be oversized about 20% in relation to the recommended starting battery's size in order to provide the same cranking amps. The lead acid batteries have normally 3 or 6 cells connected in series. Each cell has a nominal voltage of 2V resulting in a nominal pack voltage of 6V and 12V respectively. They are usually charged with a constant voltage of 2.4 - 2.5V per cell having the charging current limited to 1/10C. It is not recommended charging these batteries with a charging current exceeding 1/3C. A lead acid battery pack is considered fully charged when the charging current falls below 10mA and/or the cell voltage reaches 2.4 - 2.5V. Should a lead acid battery be continuously left on charge (when used as power backup); the charging voltage should not exceed 2.25 - 2.30V per cell. It is also advisable to charge these batteries in a well-ventilated area/room, since it produces hydrogen-oxygen gases that can be explosive and also the electrolyte contains sulfuric acid that can cause severe burns. Lead acid batteries' lifespan is about 4 to 8 years depending on the treatment.