January 2

Understanding RC Electronics

Electronics_schematic_UBEC
Motor/ESC/Lipo circuit with UBEC receiver supply.

The growth in the number of electrically powered radio controlled model aircraft being purchased is quite amazing. Most newcomers to electric rc model flying have virtually no knowledge of electrics to back up their choice. This can lead to frustrations when things start to go wrong and components cause problems.

I believe that understanding rc electronics in its simplest form is important for your success in flying electric rc model planes. To this end I am going to  give you some basics to help you  along the road to your success.

Measuring Electricity

 We can measure electricity in a number of different ways, but there are certain units of measurement that are particularly important.

 VoltageVolt drop

The voltage is a kind of electrical force that makes electricity move through a wire and we measure it in volts. The bigger the voltage, the more current will tend to flow. So a 12-volt car battery will generally produce more current than a 1.5-volt flashlight battery.

Current

Voltage does not, itself, go anywhere: it’s quite wrong to talk about voltage “flowing through” things. What moves through the wire in a circuit is electrical current: a steady flow of electrons, measured in amperes (or amps).Current mearurement

Power

Together, voltage and current give you electrical power. The bigger the voltage and the bigger the current, the more electrical power you have. We measure electric power in units called watts.

The electric power in a circuit is equal to the voltage × the current (in other words: watts = volts × amps). So if you have a 100 watt (100 W) light and you know your electricity supply is rated as 120 volts (typical household voltage in the United States), the current flowing must be 100/120 = 0.8 amps.

If you’re in Europe, your household voltage is more likely 230 volts. So if you use the same 100-watt light, the current flowing is 100/230 = 0.4 amps.

The light burns just as brightly in both countries and uses the same amount of power in each case; in Europe it uses a higher voltage and lower current; in the States, a lower voltage and higher current.

Energy

Power is a measurement of how much energy you’re using each second. To find out the total amount of energy an electric appliance uses, you have to multiply the power it uses per second by the total number of seconds you use it for.

The result you get is measured in units of power × time, often converted into a standard unit called the kilowatt hour (kWh).

If you used an electric toaster rated at 1000 watts (1 kilowatt) for a whole hour, you’d use 1 kilowatt hour of energy; you’d use the same amount of energy burning a 2000 watt toaster for 0.5 hours or a 100-watt lamp for 10 hours. See how it works?

The Difference Between Electrics and Electronics

Now we need to differentiate between Electrics and Electronics.

Electronics is the technology of electrical circuits that are made up from active electrical components such as transistors, diodes and integrated circuits.

It is distinct from electrical technology which deals with the generation, distribution, switching and conversion of electrical energy into other forms (e.g. light and motion) using wires, motors, generators, batteries, switches, relays, transformers, resistors and other passive components.

Electronics is the knowledge of how individual components work and how to assemble them together to make a working device, while electrical technology is the knowledge of how to connect together different devices to turn electrical energy into useful stuff like heat, light and motion.

This means that anyone with a basic knowledge of ‘model electrics’ can install the necessary circuitry to make a model plane do what he (or she) wants it to do without actually understanding the clever workings of the electronic bits in the circuit.

Our radio control electrical circuit involves a power supply, conductive wiring, a load and a switch. The circuit is dead until either the switch is made or the power supply is connected directly to the circuit, at which stage a current of electricity flows through the wiring of the circuit and energises the load.

The load does whatever it does for as long as the power is supplied and can be anything that consumes electrical energy such as a receiver and servos or a receiver, servos, ESC and motor. When the switch is returned to its original position or the power supply is disconnected, the circuit becomes dead again and the load stops whatever it was doing.

What is a Circuit Schematic Diagram?

A circuit diagram, or a schematic diagram, is a technical drawing of how motor esc lipo receiver layout the various components of the system are connected together to achieve the end result.

Each electric component is drawn connected to the next by a line or lines representing the conductive wires that link the installation together as in the diagram on the right here.

Let’s now deal with the individual components of our circuit installation one at a time:

Battery

This is our power source and is essentially a store of electrical energy. It has two poles, or terminals, which are termed negative and positive.

The battery has chemicals inside it which react to produce little particles of electrical charge called ‘free electrons’ and drive them all toward the negative pole. This is insulated internally from the positive pole, which has a deficiency of electrons and whose sole function in life is to grab them from the other end of the battery.

If a metallic conductor, in the form of a thin metallic rod called a wire or a bundle of very thin rods called a cable, is connected between the two poles then the electrons will flow along it from negative to positive. This flow of electrons is called a current.

All batteries are identified by two very important properties. The voltage it holds puts a value on the electrical ‘pressure’ exerted between the negative and positive poles, i.e. the higher the voltage then the more electrical pressure it can exert on a load.

Increasing the voltage will, for example, makes a bulb glow brighter or a motor turn faster. The battery also has a ‘capacity’ which is a measure of how much electrical energy it holds and can supply before it becomes discharged, or ‘flat’.

The rate of flow of electrical current along the conductor is measured in Amps or, to give it its full name, Amperes.

The more amps a load draws from the battery then the quicker it will discharge.

The value of a battery’s capacity is the arithmetical product of the current and the amount of time for which it can be supplied and is quoted in Amp-Hours (AH), or Milliamp-Hours (mAH) which are 1000 times smaller.

A battery which can supply one amp for one hour has a capacity of one amp-hour. To give a figure more typical of an rc model plane application, a 3000mAh (3AH) battery will supply a current of 2 Amps for 1.5 hours (1.5 x 2 = 3).

I hope you will have noted that this does not depend upon the voltage of the battery. This is a different thing altogether.

There is an arithmetical relationship between voltage and current in a circuit Ohms Law triangleand it’s called Ohm’s Law and states that the voltage across a load divided by the current flowing through it is called its resistance and its formula is:

R=V ÷ I.

Also by re-arranging the formula in line with the triangle:

V = I x R  and  I = V ÷ R

R = Resistance, V = Voltage and I is the usual symbol for current. For most practical purposes you won’t need Ohm’s Law on a day to day basis but it’s useful to be aware that it exists, if only to know what ‘resistance’ means.

Batteries For RC Model Aircraft

Basically there are two types of battery in common use for rc airborne applications. These are Nickel Metal Hydride (NiMH) types and Lythium-Polymer (LiPo) types.

Nickel Metal-Hydride (NiMH) batteries are generally made up in packs of end-to-end joined-up cells. 4.8V NiMH batteryThis arrangement is called ‘series’ connected.

Each cell has a nominal voltage of 1.2 volts and the cells are available in different case sizes. The smaller case sizes have the smallest capacity; typically around 850mAH for an AAA pack, while the largest cells, ‘F’ size, go up to 10000 mAH (or 10AH). 

NiMH cells can be fast-charged at the field from a portable charger. When fully charged, a NiMH cell can have as much as 1.55 volts across the poles. This will quickly settle down to its ‘nominal voltage’ of around 1.2v and stay at that level until it is almost fully discharged where the voltage starts to drop quickly and should not be allowed to go below 1v per cell or damage will occur.

Lithium Polymer (LiPo) batteries are the latest technolgy and are lighter and more powerful even than NiMH cells. Their nominal voltage is 3.7v per cell, BUT they do have to be carefully handled and monitored. Careless handling or overcharging/discharging can in the extreme case, cause them to catch fire and even destroy your model. Lipo battery

They have become the main choice of power source for electric flight. Treated with care and respect, they deliver excellent power to weight ratio so long as their sensitivity to over-discharge problems is taken account of.

For this reason it’s always advisable to use a speed controller which will monitor the battery voltage and cut off the power to the motor before it reaches a critically low value.

That said, LiPo batteries are in use all over the world and instances of such accidents are becoming rare.

It is essential that you purchase the correct type of balancing charger and follow the instructions, then you will have no problems.

LiPo packs come in multiples of 3.7v and are quoted in the form ‘XS’ where X is the number of cells. Thus a ‘3S’ pack is 3 x 3.7 = 11.1 volts.

The capacity is quoted in mAHs, and the maximum current which can safely be delivered is given in the form ‘YC’, where Y is the value of the capacity in mAH/ 1000 = AH

As an example, a 1700 mAH LiPo pack has a corresponding ‘C’ value of 1.7 and so a pack rated at 20C can supply a maximum of 20 x 1.7 Amps = 34 Amps.

Polarity

Previously I described how there is a flow of electric current when a load is connected between the two poles of a battery. Unfortunately some loads are sensitive to which way around they are connected (polarised) while others aren’t.

Examples of non-polarised loads are conventional bulbs, relays, switch terminals and fuses.

Practically any unit which includes semiconductors (transistors, Integrated circuits, etc.) will be polarised, so make sure that you connect items such as speed controllers and receivers the right way around. They are usually marked with + (positive) and – (negative) signs/labels or at least the instructions will tell you how to connect them.

Be warned, if you connect up a polarity-sensitive device the wrong way round, even for a split second, then you will probably damage it and often fatally.

Conductors

All wiring used to connect the various components of our rc and power system are forms of conductor and usually of the insulated flexible variety. This enables complicated circuits to be accommodated inside our fuselages and wings with lots of formers, ribs and electronic gizmos to be negotiated.

A wire, or cable, is made up of a central conductive core, usually multiple strands of thin copper wire, and an insulating outer sleeve.

The sleeve is typically either PVC in the case of the rc linkages or a silicone-based compound, which is more flexible and resistant to heat for the motor power circuitry.

The most crucial factor as far as wiring is concerned is to use the right thickness/diameter. Too high a current passing through a thin cable will increase its temperature to the point where the insulation breaks down and melts and the conductor inside can short out against other ‘live’ components.

Circuits in rc planes are typically of two types; those which provide power to motors and those which don’t. The former will be subject to high currents while the latter will only carry a few amps at most.

The accepted standard is to use thick multi-strand silicone-coated cable for power wiring, and thinner multi-strand PVC ‘hook-up’ wire for such items as receivers and servos. Most products bought ‘over the counter’ will come fitted with the appropriate gauge and type of wire and often with the appropriate connector attached.

The gauge of the silicone covered wire is usually quoted in AWG (American Wire Gauge) and the following table will be useful when working out what gauge of wire you need:

 

AWG               Conductor dia.(mm)          Max current (Amps)

10                                2.59                                        55

12                                2.05                                        41

14                                1.63                                        32

16                                1.29                                        22

18                                1.02                                        16

20                                0.81                                        11

22                                0.66                                        7

 

Switches

I know it may seem obvious but as we are discussing all of the major components in the system, we should also include these items.

Switches are devices usually operated manually and used to turn electrical circuits on and off. Slide Switch

In its simplest form, a switch has a lever which moves a conducting piece of metal into a position where it makes contact with another fixed conductor, thus bridging the two and allowing an electrical current to flow between them.

Each of these conductors is called a terminal (or pole) and is connected to the wires within the circuit, thus controlling the current in that circuit,

slide switches are usually fitted by radio manufacturers in the wiring harnesses that they supply to connect receivers to battery packs.

 

Series versus Parallel Connection and Loads

At this point I will have to explain those confusing terms parallel and series. There are two very simple rules here.

Loads connected ‘in parallel’ go alongside each other in side-by-sideParallel Resistors (parallel) cables which are then connected at each end (see diagram right). The voltage across each load will equal the total voltage across the circuit, Figure 6.

Loads connected ‘in series’ are connected one after the other along the same cable. The voltages across each load will add up to the total voltage across the circuit.Series Resistors

The terms ‘series’ and ‘parallel’ are also applied to connecting batteries, but beware! While you can connect two batteries with different voltages and capacities in series, you must NEVER connect two batteries of a different voltage in parallel or one will discharge into the other, with potentially serious damage to both batteries and possibly the model.

The total voltage of a series pair of batteries will be the total of the two battery voltages and the capacity will be that of the individual batteries.

The voltage of a parallel pair will be the same as the two individual batteries and the capacity of a parallel pair will be twice that of the individual batteries.

Be sure to always make sure that the two batteries in the pair are the same type (e.g. NiMH or LiPo) and the same capacity (e.g. 3000mAh), irrespective of whether you are wiring them in series or parallel.

If you are using two battery packs of the same voltage and capacity to power a plane, you should disconnect pairs of batteries from each other when charging. Charge them separately as there’s always the risk that one will charge at a different rate and that one will become the weaker of the two if you charge them together as a pair.

Brushless motors

Over recent years there has been a major revolution in model motor technology with the advent of brushless motors. As the name suggests, these don’t rely on fixed brushes to transmit the power to the central armature. Rather they are made ‘inside out’ with the wire coils fixed around the inside of the motor casing and a permanent magnet rotating the shaft inside them.

The power to energise the coils is transmitted by some very clever, fast-switching electronics outside the motor case. Because there are no brushes, these motors are much more efficient than their brushed cousins, so a small and light brushless motor will do the same job as a bigger, heavier brushed one.

To learn more about brushless motors visit my main website page on Electric Motors  where everything to do with these motors is explained in detail.

Electronic Speed controllers

There are two distinct parts to the inside of an ESC; the logic circuitry and the power circuitry.ESC

The logic components are the ones which are connected to the receiver via the 3-wire lead with a plug on the end. Its operating voltage is dictated by the receiver and is generally of the order of 4.8v.

The purpose of the logic circuitry is to detect and decode the signal coming from the receiver and to switch the high-speed, high-current semiconductors in the power circuit which control the speed and direction of the motor.

The ESC manufacturer will state in the technical information the range of main motor battery voltages that the ESC is designed to handle. They should also state a value for the maximum motor current (in Amps) which the ESC will handle under continuous operation.

You should be guided by those two values when choosing your ESC, after first ascertaining the working voltage of the motor and its current consumption under load as described earlier.

I’ve emphasised the words ‘under continuous operation’, because the current rating often causes confusion . When you switch on an ESC and bang open the throttle there will be a sudden inrush of current from the battery to the motor to get it spinning. This will always be a larger value than that consumed when the motor is running at full speed, i.e. continuously.

Modern switching semiconductors therefore have two current rating values; one for continuous and one for ‘inrush’ or pulsed currents. The value we should be concerned with is the continuous current rating.

My rule of thumb is to allow a 30% margin over and above the motor manufacturers recommended maximum continuous current rating. E.g. If the maximum recommended current for a particular motor is stated as 30Amps I would select an ESC that has a continuous current rating of at least 40Amps.

Always buy a known unit from a reliable source with a good reputation and a clearly stated service/warranty policy.  Incidentally it’s quite okay to run a low-current motor on an ESC which has a much higher rating, but not the other way around.

The power circuit connections of an ESC will always comprise a pair of thick battery cables. These are usually made in red and black, for positive and negative connections respectively. Always ensure they are correctly connected (+ to + and – to -)

Brushless motor ESC’s have three motor wires. It doesn’t matter which way round you connect the motor wires to the motor unlike the battery connections which should NEVER be reversed.

If you wish to reverse the direction in which the motor is rotating then just swap over any two of the three wires from the ESC to the motor.

Most standard ESCs for brushless motors will have been set by the manufacturer to a pre-determined operating mode and, when you switch on, will ‘Auto-set’ themselves.

If changes to operating modes need to be made, these can be done from your transmitter following the instructions supplied with the ESC or you may require a separate programming card to make changes to their working parameters.

Many ESCs contains a battery eliminator circuit (BEC) If it does, then you must NOT connect an additional power supply to the receiver.

To learn more about ESCs and BECs go to my main website page about ESCs & BEC circuits.

In Summary

In  this post we have covered just about everything you need to know about the control and power system of your electric rc airplane. If you familiarise yourself with the content you will have the basis of understanding rc electronics. There should be very little else you will require other than the information provided by the product manufacturers.

If you have found this post helpful why not visit my main website www.rookiercflyer.com for all the help you need to become a competent flyer. Any comments will be appreciated or I will be happy to answer any questions you want to ask via the comment facility below.

Happy New year everyone.

Colin

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

November 20

Choosing an RC Brushless Motor

Weight and Dimensions

Apart from Power there are two important things to keep in mind whenrc brushless motor installation choosing an rc brushless motor. These are a) its Weight and b) its Dimensions. Having to add extra weight at the front of a model to achieve the correct centre of gravity (CofG) is far from desirable. If we can incorporate at least some of this extra weight by using a heavier and more powerful motor, this is preferable to having a smaller less powerful motor and a lump of lead at the front of a model.

The trainer on the right would fly with a smaller motor but this selection fits well, suits the installation, and provides a surplus of power for emergency situations whilst helping bring the CofG to its desired position.

Occasionally there is no choice but to use additional weight to achieve the correct CofG, but don’t forget about its relationship to the weight of your motor. The dimensions of a motor are obviously important, as it needs to fit within the space provided.

Power Output

So now we come on to the amount of Power (Watts) required for the model to perform as it should. 3D models need thrust to weight ratios greater than 1:1, whilst scale WW1 biplanes need considerably less. The table below provides recommendations of performance in Watts per pound for the styles of model commonly flown. It is important to understand that if you run your motor above its maximum rated efficiency the Watts per pound rule won’t be accurate. The motor will tend to overheat as a result of a higher percentage of the Watts going into the motor producing heat instead of power. This is something that should be avoided at all cost.

  • 70-90 watts/lb.            Trainers and slow flying Aerobatic models.
  • 90-110 watts/lb.           Sport Aerobatic and fast flying Scale models.
  • 110-130 watts/lb.          Advanced Aerobatic and high speed models
  • 130-150 watts/lb.          Lightly loaded 3D models and Ducted Fans.
  • 150-200+ watts/lb.       Unlimited performance 3D models.

These figures are generally optimum requirements but many model planes will actually fly quite successfully with a lower power to weight ratio. It is not uncommon for lightly loaded vintage style model to fly on 50 watts/lb.

RC Brushless Motor Formats

Brushless Motors are available in two formats. Inrunners and Outrunners. Let us take a look at each type in turn so that we can decide which is best for our application.

The electromagnetic windings of a brushless motor are made stationary (the stator) and the permanent magnets made to revolve (the rotor). The wiring connections are permanent and fixed.

Inrunner brushless motor
Inrunner Brushless Motor

Unlike brushed motors there is no need to make any electrical connection to  the revolving parts and two configurations are possible.

Inrunner Brushless Motor

In this type the stator is in the form of a cylinder around the outside of the motor and the rotor spins inside it. The resulting motor is known as an ‘Inrunner’. They look more like conventional motors, with a fixed cylindrical casing. With a small rotor, they turn at a high rpm but do not produce as much torque as outrunner types.

Outrunner Brushless Motor
Outrunner motor
Outrunner brushless Motor

If the stator with its windings is in the centre, the result is an ‘Outrunner’. Now the outer casing becomes the rotor, with the permanent magnets arranged around the inside of its rim. The casing is fixed to the motor shaft at one end whilst the other end runs in a bearing mounted in the non-moving part from which the coil lead out wires protrude. They turn more slowly and because the magnetic interaction occurs at a greater distance from the motor axis, they develop more torque.

 

 

Brushless Motor Benefits

There are major benefits to be had using brushless motors as an alternative to the brushed types.

  • They are more efficient – Brushless motors are much more efficient. This efficiency has been measured to be between 85% to 95% better than brushed motors.
  • Less electrical energy is wasted as heat – This means that more is used to do useful work.
  • Reduced Noise – Brushless motors have fewer mechanical parts than brushed motors, so they emit less sound.
  • Longer Lifetime – The only parts that are in mechanical contact in brushless motors is the shaft bearings, compared to both bearings and brushes in brushed motors. This results in a considerable reduction in wear and tear.
  • Reduced Electro Magnetic Interference – Brushless motors emit less energy as electromagnetic (EM) waves than brushed motors do. As a result they are more efficient, and create less radio interference.
  • Torque, RPM and Voltage are Linearly Related – This means that the amount of Torque or RPM produced by the motor divided by the Voltage supplied is a constant value. This makes it easy to predict how much power your motor is going to produce.

 Selecting The Best RC Brushless Motor For your Plane

The choice of motor type, either Inrunner or Outrunner, will depend on the requirement of your particular model. Inrunners are more efficient and powerful. Generally they produce higher revs per volt (Kv) compared to outrunners. For models requiring a small prop running at high speed like pylon racers and ducted fans, inrunners are a popular choice.

It is possible to use inrunners with gearboxes to turn larger propellers but this is a more expensive option that adds extra weight and is less efficient due to mechanical losses within the gearing.

For most normal applications the outrunner is a better option. Its ability to turn larger propellers in direct drive mode reduces both mechanical losses and cost and provides greater efficiency.

In the earlier stages of your model flying experience it is unlikely that you will be getting involved with high performance aeroplanes. For this reason I propose to concentrate on outrunner motors suitable for sport and basic aerobatic flying performance.

If we refer to the chart above it is very convenient that a figure of around 100 watts/lb. of model weight is appropriate. I like round figures, they make calculations very simple!

The three most significant figures published by brushless motor manufacturers are:-

  • Kv – This refers to the RPM (Revolutions Per Minute) constant of a motor. It approximates to the number of revolutions per minute that the motor will rotate at when 1V (one volt) is applied to the motor. This figure is called ‘revs per volt’ designated the abbreviation Kv. (Be very careful not to confuse this with kV which is a totally different abbreviation for ‘kilo-volts’ or multiples of 1000 volts). Please understand that you will never obtain the revs calculated from this constant. As soon as you attach a propeller to the motor the load will reduce the RPMs your motor can attain.
  • Continuous Current Rating – This is the maximum value of current that the motor is capable of handling continuously during operation without incurring damage through overheating and potential bur- out of the wire coils. They will also quote a figure of Maximum Current the motor is capable of withstanding for a very limited duration. this figure  should never be used for the purpose of calculating the power figures you will need for your plane.
  • Cell Count – This will tell you the range of voltages appropriate to the motor’s operation. these figure will often include both NiMH cell counts and Lipo cell counts. Not many people use NiMH batteries nowadays so the significant information for us is the Lipo cell count. Most Lipo cells give a typical average voltage under load of 3.3 volts.

Power (Watts) = Volts (Cell Count x 3.3V) x Continuous Current Rating (Amps)

 Using this equation it is possible to calculate the working power figure for any motor. With this figure as a maximum beyond which it is undesirable to operate, the only other variable capable of increasing or decreasing the power consumption of the motor is the size of propeller fitted.

Propeller Considerations

The amount of power required of the motor to turn a given propeller is affected by both pitch and diameter. Below is the formula for calculating power requirements for a given size of propeller:-

power = k x rpm x diameter4 x pitch

 The factor k is a constant depending on the units used to define power, pitch and diameter as well as the airfoil, thickness and shape of the propeller. Because RPM is controlled by the Kv of the motor, the only variables we can control are diameter and pitch.

You can see that changing the pitch of the propeller has only a small effect on power but a change in diameter has a major effect. Because pitch has such a small effect on power, it can be used very effectively to improve model performance at the expense of very little change in current. If we are looking for an increase in top end speed increasing the propeller pitch by a factor of one (e.g. from a 10 x 6 to a 10 x 7) will only increase the power required by 14% hence current will increase by 14%. So if the motor was drawing 25A with propeller one then propeller two will require 29A, a small penalty for the improved performance.

If, however, we increase the diameter of the propeller from a 10 inch to an 11 inch of the same pitch, it would require 1.46 times the power to maintain the same rpm (11/10 to the fourth power i.e. 1.1 x 1.1 x 1.1 x 1.1 = 1.46). If we continue to use the same battery the voltage remains the same so the current would have to increase by a factor of 1.46, i.e. 25A x 1.46 = 37A. An increase of this magnitude could push the current beyond the safe operating parameters of the motor.

This leads us to consider that maybe we can get the same amount of performance from our motor by reducing the propeller diameter but increase the pitch. Lets use a 9″ diameter propeller instead of the 10″. This will reduce the current consumption by a factor of 0.9 x 0.9 x 0.9 x 0.9 = 0.66. So if we take the 25A and multiply it be 0.66 = 16.5A. Now lets increase the pitch from 6″ to 8″ we increase the current by 8/6 = 1.33, i.e. 16.5 x 1.33 = 22A.

Clearly just by reducing the diameter by 1″ and increasing the pitch by 2″ we have been able to reduce the current consumption by 3A. This means that our flight time can be longer for a minimal reduction in overall power consumption. Performance will suffer very little as it is not normal to operate a plane at maximum power continuously. Besides which it is unlikely that the actual power loss will be noticeable  for sport flying applications.

It is always a good thing to experiment with different propellers

Digital Wattmeter
Digital Wattmeter

to find the one that gives the best performance for a particular model. It is normally best to start with the size recommended by the motor manufacturer then make small variations either side of this size.

It goes without saying that you are going to need our faithful servant the Wattmeter to check how the changes are affecting current and power figures. You can buy yours through these links:

RC Wattmeter (USA) or RC Wattmeter (UK)

Summarising  Our Choice Of RC Brushless Motor

Although there are a considerable number of factors to be considered when deciding on which motor to install in your plane, the principle one is the Power to Weight Ratio. By referring to the chart at the beginning of this post you can decide the sort of performance you need and select your motor based on this consideration.

It is always good to err on the high side of the power capability of the motor. You can always throttle back for normal flight leaving a reserve of power for  those occasions when it is needed. A motor that is having to perform at its maximum capacity continuously will not last very long and will not provide the reserve you may need.

The performance you get from your rc brushless motor results from a combination of motor, propeller and lipo capacity and rating, given that your ESC has at least a 30% margin above the anticipated maximum current handling capacity.

Don’t forget to visit our main website at www.rookiercflyer.com for all the help you will need if you are just getting started. If I can help you pleas don’t hesitate to ask your questions via the comments facility below.

Chat soon.

Colin

 

October 23

RC Brushless Outrunner Motors Testing – Wattmeter

The Essential Digital Power Meter

Digital Wattmeter
Digital Wattmeter

A “Wattmeter” is an invaluable, and inexpensive piece of measuring equipment. It is the best way of ensuring that your rc brushless outrunner motors and associated electric power train is correctly matched.  All of the components within an electric power train (Motor, ESC & Lipo) are designed to operate within specific parameters, and we need to ensure that our system is going to produce the power we are looking for without exceeding the upper limits of these parameters.

In order to test your setup before you commit the model to flight you need to have some method of checking the various power consumption components. This is exactly what a Digital Power Meter or Wattmeter does.

Using Your Wattmeter

Here’s how it works. You will have seen in the main website under “Installing Electric Motors” how the motor, battery and ESC all connect up.

The Wattmeter is temporarily inserted between the battery and the speed controllers’ main cables and will provide you with a readout of exactly how much Current your power train is consuming, how many Watts of power the system is providing, and also how many Volts the battery is maintaining when under full load.

watts up meterIf you look at the image on the right you will see that the meter is reading a Voltage of 11.93V. This probably tells us that a 3cell lipo battery is supplying volts to the circuit.

Next we see that the current being consumed is at 2.87 Amps. So if we multiply 2.87 x 11.93 we get a value of Watts = 34.2391 Watts. Our meter only has three digits to display the watts so it gives us 34.2W. this is more than adequate for our purpose.

You can ignore the Ah figure at this stage but for the inquisitive amongst you, this is the value of current being consumed each hour and is determined by the length of time this current has been drawn from the power system.

Let us  quickly look at the circuit that is used to take these measurements.Wattmeter Diagram

This simplified diagram shows the supply from our Battery and the load which comprises the ESC and Motor. The current (A) in Amps is measured in the positive line. The Voltage (V) is measured across the positive and negative lines. The product of these two values appears as W (Watts). All of this takes place inside the Wattmeter and is shown digitally in the display.

A Practical Exercise

Normally the rig is secured safely on the bench or in the plane, and the throttle is advanced (ensure you stand behind the prop / fan and that if the worst happens and the prop flies off, it will not damage anything).

The following diagram shows the correct position of the Wattmeter in relation to the other components.

Wattmeter circuit

Let the motor run for around 30 seconds or so, and make a note of the figures mentioned above as shown on the wattmeter display.

Current (Amps) =

Volts (V) =

Watts (Volts x Amps) =

The figures are likely to fluctuate a little during the test, but you will get the trend. One of the main things this will show for you is the power (Watts) that the set up is producing. You will see that I have given you the components that need to be multiplied together to give the Watts

Watts are derived from multiplying the Volts of the battery (under load) by the Amps consumed.

W =V x I (I is the symbol for current, or amps).

The more Volts you have the higher the Watts or the more Amps you have the higher the Watts.

Selecting A Propeller Using Your Wattmeter

Assuming that the battery you select is not likely to be changed for a higher voltage version, then changing the propeller is the biggest single factor in affecting the current that the motor will consume.

Let’s say your motor is designed for a 6 X 4 prop, on the chosen battery, but you want to try and get a little more thrust, or climbing performance from your setup. With the meter in circuit, you could now fit a slightly larger diameter propeller, and monitor the current being drawn.

Now you can watch the display and see the Amps that the motor is pulling and the Watts being produced, and therefore ensure you do not “over prop” the motor to the point where the maximum figures allowable are exceeded.

These maximum figures are also applicable to your battery, and your speed controller, and indeed exceeding any or all the limits of these items could prove very expensive, or even dangerous.

Saving Money

The modest cost of a suitable digital power meter or wattmeter will be recouped the very first time you use it and discover that your current consumption and resultant power figures are beyond the maximums allowed for your power train. Without it you could be in for a great deal of heat and smoke and a major replacement expense for your rc outrunner motors circuit components.

You can obtain a Wattmeter on-line by clicking this link:-

RC Wattmeter (USA) or RC Wattmeter (UK)

This is probably one of the most important posts I will offer, especially if you intend to fly electric planes. I hope you have found it useful and will share it with others. Please don’t forget to visit my website: www.rookiercflyer.com for everything you need to know about starting up with RC model planes.

Be safe and Enjoy your flying.

Colin

 

October 9

Understanding Wire Gauge Current Rating

The wire most frequently used, and recommended, for electric motor power systems is often just called Silicone Wire.

8 awg silicone wire

The wire is a flexible, multi-strand wire with a silicone insulation sleeve that gives it its name.

This post is aimed at you understanding wire gauge current rating when connecting Lipo Batteries to Electronic Speed Controllers (ESCs)

Determining Wire Gauge Rating

The size or “gauge” of the power wires between the Lipo Battery and Electronic Speed Controller (ESC) is based on:

1)  The application

2)  The anticipated Maximum current

3)  The length of the wire from the BATTERY TO THE ESC AND BACK TO THE BATTERY ( In other words, it is the total length of the positive and negative leads combined).

This is an important consideration because the resistance of wire is directly proportional to this length and is responsible for reducing the voltage (volt drop) over longer lengths.

 Electrical Wire Gauge Chart

In North America and the UK, American Wire Gauge (AWG) is used to identify the wire ‘size’. The table below gives the conversion from AWG to Metric cross sectional area.

American Wire Gauge
(AWG)
Diameter
(inches)
Diameter
(mm)
Cross Sectional Area
(mm2)
0000 0.46 11.68 107.16
000 0.4096 10.40 84.97
00 0.3648 9.27 67.40
0 0.3249 8.25 53.46
1 0.2893 7.35 42.39
2 0.2576 6.54 33.61
3 0.2294 5.83 26.65
4 0.2043 5.19 21.14
5 0.1819 4.62 16.76
6 0.162 4.11 13.29
7 0.1443 3.67 10.55
8 0.1285 3.26 8.36
9 0.1144 2.91 6.63
10 0.1019 2.59 5.26
11 0.0907 2.30 4.17
12 0.0808 2.05 3.31
13 0.072 1.83 2.63
14 0.0641 1.63 2.08
15 0.0571 1.45 1.65
16 0.0508 1.29 1.31
17 0.0453 1.15 1.04
18 0.0403 1.02 0.82
19 0.0359 0.91 0.65
20 0.032 0.81 0.52
21 0.0285 0.72 0.41
22 0.0254 0.65 0.33
23 0.0226 0.57 0.26
24 0.0201 0.51 0.20
25 0.0179 0.45 0.16
26 0.0159 0.40 0.13

You will notice that the smaller the gauge number, the larger the wire diameter.

Large gauge wire (small gauge number) can safely handle more current, over longer distances, with less voltage drop than smaller gauge (large gauge number) wire, but it is heavier. Wire that is capable of ‘handling’ the current (amps) without too much voltage drop also has to be sized for the aircraft.

Selecting Wire Gauge Amp Rating

AWG 12 wire would be useless in an indoor flier requiring only a couple of amps of current as it would be far too heavy. On the other hand, a giant scale model requiring 100 amps at full power would not work with AWG 12 wire. The resistance of the wire would create an unacceptable voltage drop and, depending on the wire’s insulation, it could melt.

Because we are usually running only a couple feet of wire in our applications, we can get away with using much smaller wire than we would if we were installing long cables.  The cross sectional area of cables is measured in “Circular mils”.

The ‘Circular Mil’ unit is calculated by taking the diameter of the wire, in thousandths of an inch, and multiplying it by itself. This gives a value that accounts for the cross-sectional area of the wire without involving π (Pi – 3.142). For example, a 20 gauge wire measures 0.032″ in diameter which is 32 thousandths of an inch, also known as 32 mils. If we take 32 x 32 we get 1,024 circular mils.)

Often, in RC applications, we can use 100 “circular mils” for every Amp of current or even 75 “circular mils” per Amp is acceptable in some circumstances.

Based on 100 circular mils per amp, an application requiring a maximum 50 amps needs 5000 circular mils of wire ( 50 x 100), which is equal to a 13 gauge wire. (From the above chart we can see that 13 gauge wire has a diameter of 0.072″  or 72 mils so 72 x 72 = 5184 which is the nearest size to the 5000 we require).

To be on the safe side, I would step that up to a 12 gauge wire which has 6,530 circular mils, and would provide 130.6 circular mils per amp with minimal weight penalty.

Wire Gauge/Current Rating

The above table gives wire gauges for specific current carrying capacity based on a very conservative 120 circular mils which give a very safe margin for error should larger currents occur in extreme circumstances.

The Power Wire and Power Connector Relationship

The list below shows the maximum wire gauge a given connector will physically accept. The manufacturer or supplier does not specify the current rating of their connectors based on the wire gauge current rating they accept. These ratings are specific to the connector only. It is always acceptable to use a smaller gauge wire with most connectors.

AWG       Name Of Connector
4              Progressive RC (PRC) 10mm bullet
6              6.5mm Castle polarized bullet, PRC8 polarized bullet, PRC6 bullet
8              PRC 8mm bullet, PRC6 polarized bullet, 6.5mm Castle bullet, 8mm Castle bullet
10            HXT 6mm, 6mm bullet, EC5, Anderson Power Pole (PP45), 5.5mm Castle bullet
12            Deans Ultra, XT-60, HXT 4mm, 4mm bullet, EC3, PP30
13            4mm Castle polarized bullet
16            PP15
20            JST-RCY
(PRC = Progressive RC)       (PP = Anderson Power Poles)

Additional Information

Anderson Power Poles call their connectors PP15, PP30 and PP45 and they are all rated to 55 amps

PP15s are suitable for AWG 20 -16,

PP30s are suitable for AWG 16 – 12,

PP45s are suitable for AWG 14 – 10,

Most ESC suppliers DO NOT state the wire gauge of the power leads. Some Lipo Battery suppliers DO advise the power lead gauge.

I hope this information will prove useful for some of you. If you like this post you will probably like my website: www.rookiercflyer.com especially if you are new to our hobby. Please feel free to share this with anyone you think may find it helpful.

October 2

RC Planes Beginners Choice – Seagull E-Pioneer

Seagull E-Pioneer

Electric flight grows stronger and stronger as each month passes. The new E-Pioneer from Seagull EP E-PioneerSeagull addresses the needs of the rc planes beginners fraternity who want a trainer that has the appearance of a typical Nitro/Glow powered model.

Even electric flight cynics and die-hard petrol heads are starting to admit that the gulf is narrowing and many are embracing the attraction of clean flight.

Seagull have made a few adjustments to reduce airframe weight a little. The E-Pioneer could just as easily have been designed for nitro/glow power, but isn’t.

This model has been designed as a dedicated electric powered trainer from the outset and despite the built-up construction, it lacks the fragility often associate with electric powered models of this class.

This Is Different

There is no foam in this plane, it is totally built up from balsa and ply. E-pioneer Nose attachmentThere’s a substantial dedicated space for the battery and motor drive components. This is definitely an out and out electric design.

As you open the box you’d be forgiven for thinking that you had received a damaged kit. Not so! It is intended that this model be broken down into its component parts for transport or storage.

The design of the model demonstrates some innovative thinking. Take, for example, the way the whole front-end of the fuselage (forward of the wing seat) is removable.

Similarly, the rudder and elevator servos are contained within the bolt-on fin and tailplane assembly.

The two-piece bolt-on wing panels and the large central fuselage access hatch all contribute to one of the most easily built aircraft I have seen.

Assembling the E-Pioneer

Our motor of choice, the N3548-800KV is ideal for this plane and E-pioneer Motor installationwas fitted to the set of pre-fabricated motor mounts  supplied to suit either an inrunner or, as in this case, outrunner motor. An APC-E 12″ x 6″ propeller was fitted.

Attaching the motor mount to the front bulkhead is the one and only necessary gluing experience in the entire build.

The lightweight wide chord wing measures over 61” in span making the E-Pioneer a good sized model.

The heavy duty undercarriage wires, decent size wheels and steerable nose leg mechanism all feel right for the model. There’s a reassuringly feeling of strength in these key areas. All of these features result in a model fit for purpose, be it bumpy grass strip or tarmac runway.

The removable front section, two-piece wing and tail mounted servos mean that you will need extension leads for your servos and possibly to take the signal feed from the receiver to the ESC. Be sure to use extension lead retainers on these joints to avoid loss of control mid-flight should connectors separate accidentally.

To avoid having to use a servo reverser you will need to select two servos that operate in opposite directions for the rudder and steerable nose wheel if you wish to operate them via a “Y” lead from the rudder output of the receiver.  Alternatively you can use one servo but ensure that the rudder horn is mounted on the appropriate side of the control surface to give the correct movement.

Nose wheel/rudder servo

Holes for the plastic control horns are pre-drilled and hidden under the covering so you’ll need to look closely to find them.

With the captive nuts for the tailplane pre-fitted and the top fuselage centre-section fitting nicely onto its magnetic catch, the model can be rigged in a little less than five minutes.

Select your servos carefully to fit the pre-cut holes in the airframe. The servo mounting plates are made from substantial plywood so enlarging these holes would not be easy.

Power Considerations

A three-cell Li-Po and a 12 x 6” prop needs a speed controller that will handle the job. A 60 Amp ESC is ideal for this power train and has proved to be so, providing a good margin for overload should it be necessary.

Seagull’s stated flying weight of 4 – 4.5 lbs is a little on the conservative side but so long as you achieve a weight of around 4.5 – 5 lbs you will be absolutely fine. In actual fact, the 600+ square inches of wing still make the E-Pioneer a bit of a floater.

 In The Air

The model is ideally sedate on three-cells and this would be my battery recommendation. The light weight and the low inertia means the E-Pioneer can be really flung around and a rookie pilot will have no difficulty in progressing from the first flight at the patch right up to ‘B’ certificate level. E-Pioneer in flight

The low weight means that the effect of low level turbulence in higher winds has to be watched. Aileron authority afforded by the narrow control surfaces mean the requirement to suddenly pick up a wing can cause moments of consternation when landing. Having said this, the yaw and pitch controls are very crisp, so the rate switches were employed for the first flights. The E-Pioneer’s direct rudder and elevator to servo control linkage system to allow a smooth, accurate feel when flying. 

The model tracks well and is easy to land. It is well suited for use on club grass strips and firm smooth runways alike and with its direct linkage steerable nose wheel, the ground handling is excellent.

It’s a little more control sensitive than an equivalent i.c. powered model which is down to the lower weight, but the smooth reliability of the electric powertrain generates real confidence.

The landing flare-out can cause the model to float on a little in a low headwind. Be aware that the externally mounted elevator servo is open to water spray. I gave it a quick spray of a silicone based spray to prevent ingress of any such water. Other than that the Seagull E-Pioneer is a fantastic model for the newcomer who wants to follow the electric route. Sticking with three cells and sensible servos will provide an ideal rc planes beginners choice.

 Datafile

  • Name: Seagull E-Pioneer
  • Aircraft type: ARTF electric trainer
  • Manufactured by: Seagull
  • Wingspan: 61/2″ (1560mm)
  • Wing area: 606 sq. in.
  • All-up weight: 5 lbs
  • Wing loading: 19oz / sq. ft.
  • Functions (servos): Aileron (2); elevator (1); rudder (1); nose wheel steering (1); throttle (0)

Should you wish to purchase this model, you can do so by clicking this link:-

Seagull E-Pioneer (UK)

or

Seagull E-Pioneer (USA)