AeroClub Indian Institute of Space Science and Technology, Thiruvananthapuram

HOVERCRAFT DESIGN WORKSHOP Organized by AeroClub

Procedural Manual for the Hovercraft Design and its components

27th February – 1st March 2015

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INTRODUCTION TO HOVERCRAFT Hovercraft is a vehicle that is used for moving on water, land as well as on mud and even ice. A hovercraft behaves more like an aircraft than a ship, involving aerodynamic concepts of lift and drag. It uses powerful flow of air through a skirt so as to generate a small pressure difference below the body of the hovercraft that eventually lifts the vehicle. The same airflow may be used to guide and control the hovercraft, while there are many designs that use other mechanisms to do so. The first practical design of a hovercraft dates back to 1950s-1960s. Now thoroughly revised and perfected, hovercrafts now serve a wide range of applications, ranging from ferrying people and goods, to active military warfare as well as in sports. Let’s now explore them.

HISTORY & DEVELOPMENT OF HOVERCRAFTS

There were numerous attempts in the period of 1910-1940s to understand the physics behind the fluid behaviour under the hulls or wings of crafts. Many of them tried making what we currently known as ‘hydrofoils’. However they were still far from hovercrafts. It was Sir Christopher Cockerell in 1955, a British that got the first success. The principle behind today’s hovercraft was first demonstrated by Cockerell in 1955, using a contraption constructed with a cat food can, a coffee can and a set of kitchen scales. Sir Cockerell himself coined the term ‘hovercraft’. Christopher Cockerell's idea was to build a vehicle that would move over the water's surface, floating on a layer of air. This would reduce friction between the water and vehicle. To test his hypothesis, he put one a smaller can inside a larger can and used a hairdryer to blow air into them. The downward thrust produced was greater when one can was inside the other rather than air just being blown into one can. Cockerell was knighted for his achievement in 1969. In June 1959, the first successful over was made by SR .N1 in British waters with due efforts from Cockerell himself. Speedy improvements soon followed, increasing the performance of

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the hovercraft. Now the vehicle could climb obstacles as high as its skirt, 1960s. Several companies jumped into the efforts as they saw a promising business in hovercrafts.

Figure : Cockerell devised SR.N1 in British Channel.

UNDERSTANDING THE PHYSICS INVOLVED Making a hovercraft becomes quite exciting and innovative when we are clear with the underlying physical principles behind it. A hovercraft is a vehicle lifted by air cushion. To make this happen, obviously there has to be a pressure difference at its base to lift the entire vehicle. When you build your own hovercraft, it’s important to understand the control of flow of air underneath it. These are the basic principles: Air Pressure - The mechanics of a hovercraft involves mainly of air, of course. You need to pressurize the air and send it through appropriate channels to create the cushion below.To create the air flow, you need device such as a fan, or a vacuum cleaner or hair blower. Hull - Next thing to have is a board or plywood that acts as a framework for the hovercraft... Another purpose for the hull is to help distribute air beneath its surface. With the right velocity, a thin air film below the hull is created and the pressure builds up. At certain pressure difference, the hovercrafts lifts up from the ground/surface. Velocity - In fluid mechanics, velocity of fluid increases when an orifice opening is decreased. You have to simulate the pressure increase by limiting the passage of air flow and channel it through a hole in the plywood. Skirt - A heavy plastic sheet is wrapped around the perimeter of the board. This is called the skirt. This helps in building the air cushion below. It traps the air and only allows a little amount released below it. If you build your own hovercraft, it is important that you make an appropriate skirt with required dimensions and openings.

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Figure : A simple demonstration of air cushion.

Generating the lift for the vehicle- A forced stream of air is passed through the perforated skirt of the hovercraft, which vents out of the skirt at a pressure slightly higher than that of the atmospheric pressure, which creates a net upward force on the hovercraft, due to Newton’s 3rd law and lifts the vehicle. So, the lift for the hovercraft is achieved by a propeller motor system in the horizontal plane. The air cushion that develops considerably decreases the friction between the vehicle and the surface. Forward motion of the vehicle- Now as a very thin air cushion layer is developed, it is evident that forward motion becomes quite easy as it feels like moving butter on a china dish. Commercial hovercrafts have two rear motor-propeller assemblies that propel the hovercraft ahead. So, the thrust for the hovercraft is generated by the propeller motor system in a vertical plane. However the thrust and lift mechanisms may be clubbed together to reduce the complexities in the vehicle.

OUR DESIGN Materials used:

1. Coroplast 2. Wood 3. Spokes and rods 4. Flexikwik 5. Plastic bag for skirt 6. Thermocol

The CAD model for the design is as shown in figure. The body is entirely made up of

coroplast, a light weight material commonly used for fabricating RC planes, gliders etc. Its

light weight makes it ideal for its selection. The body consists of one sheet mounted over

another using some support blocks made of thermocol. The body on the top consists of the

following electronic equipment:

a) Motor: To propel the hovercraft, a high rpm motor is used. Generally to attain high

rpm, brushless motors are used. The usual rating is 1100kv.

b) The electronic speed controller (ESC): To get variable input voltage to motor which

enables it to attain different rpm during its working is achieved using this component.

The details regarding ESC can be found in section…..

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Figure : Rudder control mechanism (Kinematic diagram))

c) The receiver: To receive the transmitted signal from the transmitter, receiver is used.

We have used a seven module receiver i.e. seven controls can be received at a time.

d) Battery: Used as a power source.

e) Transmitter: To send the signal to the electronic equipment when connection is

made wireless, a transmitter is used to this task.

In the design, there is a motor mount which forces the air to go through a rectangular pocket

into the hollow base of hovercraft. This opening allows the air to do a cushioning effect and

thus air fills inside the skirt. Above this opening, we have rudder mounting controlled using a

servo motor. The following four bar mechanism is used (also called as “double rocker”) as

shown in figure below.

Inflating the skirts is the first and foremost task to be done in the design of any hovercraft.

This design uses two sheets mounted one over another thus the space between them is

empty. Thus for proper cushioning to occur, flow must be directed so that uniform inflation is

achieved in all places else, the hovercraft will lose stability while in motion.

Figure : Schematic to represent the generation of air cushion as well as throttle

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Proper skirting involves its folding, making it leak proof to avoid any unnecessary air leakage

and its fixture on the craft’s body. To achieve this follow the procedure given in the later

section.

The air cushioning provides advantage in terms of reducing the friction, it also allows a kind

of flexibility for the craft to move on any terrain.

The actual physics working is making holes (small) to allow some air to come out of cushion

and lift the body up and thereby reducing the friction to move on the terrain. If holes are not

provided, inflation alone can do nothing alone. Moreover the hovercraft actually runs in air

slightly above ground due to the holes provided. The same property is used while moving

over water. But the air cushion there also provides buoyancy which helps it to float over

surface.

PROCEDURE FOR CONSTRUCTION

The hovercraft we made was made in the way we thought and according to the challenges

we posed and solved. One is always encouraged to present or try his/her own idea after one

understands the basic principle and idea about design.

The hovercraft was designed through the following stages:

I. Body design:

a. Two sheets of dimensions 45 cm X 30 cm are cut from the Coroplast sheet

given in the kit to prepare top and bottom sheets.

b. On one of the sheets cut out a rectangle of size 26cm X 15 cm (as shown

below).

Figure : Making the Base of the hovercraft

c. Cut another long strip of length 72 cm X 6 cm to make rudder envelope. At

positions 21, 36 & 57, make folding to obtain four walls of a cuboid (as shown

below).

d. On one strip of length 21 cm, cut out a rectangle of size 2 cm X 1 cm to provide

place to mount servo motor for rudder control.

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e. Cut out two small pieces of equal sizes of dimension 12 cm X 15 cm for making

rudder. Fold the piece at 6 cm from one end for ease of fixity of rudder above it.

(as shown below)

f. Fix the rudder flaps in between the rectangle made earlier using flexkwik.

g. Cut out two quadrants with radius as 15 cm from the available coroplast sheet

(as shown in figure). Allow a small part to be straight (around 6 cm) so as to fix

rudder above it.

h. Again from the remaining sheet cut out a rectangular piece of dimension 25 cm X

15 cm to make the circular covering for air passage.

i. Fix it on two quadrants using flex quick to make a quadrant of a hollow cylinder

as shown below.

j. Fix the rudder over the air passage covering on the rectangular base made at the

top.

k. In the rudder flaps itself, make provision to insert the “push rod” in between the

two flaps.

l. Cut out 4 cubical blocks of side 3 cm from the thermocol sheet available for

supporting the top plate over the base. Also cut out a cuboidal block of size 3 cm

X 4 cm X 4 cm to provide strength at the base for motor mount.

m. Fix the base sheet and on the corners fix the cubical blocks using double sided

tape. Apply the tape on both the ends.

n. Also cut wood available (plywood sheet) into 3 cm X 4 cm sized 4 pieces and

glue them together to form walls of a cuboid.

o. At the distance of 19 cm from the front edge, place the wooden cuboid so as to

give structural support to the body. Glue it there.

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Figure : Circular Covering for air passage

The body is ready!!!

II. Mount design:

1. Cut the plywood sheet into two pieces of dimensions 20 cm X 5 cm and 15

cm X 5 cm.

2. On the 20 cm long strip cut, mark a point at 14.5 cm from the base along the

axis of symmetry of strip (lengthwise).

3. Drill a hole of 4 mm diameter and fix motor on to the hole to mark positions

for the other holes.

4. Once marking is made, drill 4 holes each 3 mm diameter at the correct

position.

5. Now make an inverted T-shape using both strips by making 20 cm strip erect

over the 15 cm strip exactly at its center. (as shown below)

6. Apply M-seal (sealant) to fix the mount in the position.

7. At the position 19 cm from the front end of the top sheet, mark position for

two holes on the coroplast sheet, each 5 cm on side of marking.

8. Now pierce four holes at a distance of 10 cm as shown (two hole of 5 cm on

either side of mount) on the top sheet of hovercraft and pass two “zip ties”

clamping the top layer of coroplast sheet and mount’s base plate.

Motor mount is ready for use!!!

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III. Skirt design and folding:

1. Cut a rectangle of size 60 cm X 50 cm from the given polybag.( plastic sheet)

2. First, fix the polybag to the sides of the hovercraft’s top sheet using cello tape.

3. On the back side of hovercraft’s body, make a fold to fix it to the last edge of

the coroplast sheet using tape.

4. For the front face, fold the skirt to get a triangular piece out and fix it. Then

fold the remaining sheet to the front edge of the coroplast sheet.

5. On the base, punch holes on the coroplast sheet and cut out small rectangles

for allowing air to take up the gap to lift up the hovercraft. There is no such

ideal size but a hole of 2 cm diameter on polybag sheet would be sufficient

enough for the job.

6. Also a hole of 1 cm diameter on the coroplast sheet would be fine enough to

allow air to come out.

7. Position of holes is very crucial and its position will accordingly vary to

compensate for non-uniform distribution of mass.

IV. Assembly:

1. On the mount, fix the motor using nuts and bolts available.

2. Connect the motor to the electronic speed controller (ESC).

3. Connect the ESC to the receiver in a convenient way.

4. Now turn ON the transmitter.

5. Connect the ESC with the battery, a beep sound comes. Wait till three beeps.

(If beep continues contact the instructors present.)

6. Fix the battery in such a way that it remains perpendicular to ground and

stays with the mount. Similarly do for ESC as well.

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TESTING

After assembling all the components in correct manner and order, the Hovercraft is ready to

take off from the ground.

1. Start giving the throttle to the motor which starts to rotate and provide air for

propelling the system.

2. To control the directions, rudders are employed and are controlled using servo motor.

3. Before starting make sure that there is no air leakage occurring from the side skirts.

4. See if the system is imbalanced due to weight or other symmetry reasons. Try to

remove such imbalances by countering them.

5. Give throttle to propel.

References: 1. http://www.discoverhover.org/abouthovercraft/history.htm 2. http://inventors.about.com/library/inventors/blhovercraft.htm

Figure : Hovercraft design after completion

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ELECTRICAL COMPONENTS Some Useful Terminologies regarding batteries: Cell arrangement: Described using the format xSyP (where x and y are integers), this tells you how the cells in the battery are wired up. Batteries are made up of cells, whose voltage is determined by cell chemistry and whose capacity is determined by energy density and physical size of the cell. S stands for series and P stands for parallel. As you may know, series connection adds the voltage of the cells and connecting in parallel adds the capacity of the cells, so a combination of cells in series and parallel results in a battery. The battery shown in the second image reads that it has an arrangement of 3S1P, meaning, it has 3 cells that are all in series with no parallel wiring. This may seem confusing because it says "1P," but think of the arrangement as a grid. By multiplying 3 and 1, you get the total number of cells in the battery, which in this case is 3. If it were a 3S2P battery, there would be 2 sets of 3 series-wired cells in parallel, resulting in 6 cells in total. Often, the parallel arrangement is omitted when discussing batteries, because most packs are 1P (so instead of saying you're using a 3S1P pack, you may as well just say 3S). Capacity: Usually measured in mAh (milliamp hours), this is determined by the cell arrangement (parallel) and tells you how long you can expect the battery to last on a charge (although it's not that simple). 2600mAh as shown on the battery in the picture is equal to 2.6Ah (amp hours), a format you may be more familiar with on larger batteries, like the SLA (sealed lead acid) one in your car, which is probably around 50Ah. A capacity of 2600mAh means that the battery can discharge at 2.6 amps for one hour (hence "amp hours"), 1.3 amps for 2 hours, etc., before it runs out of "juice." Because the battery shown has a 1P arrangement, each cell has a capacity of 2600mAh. Voltage: The voltage of a battery is also determined by the cell arrangement (series), and there are a few common voltage measurements worth noting: Charged - The voltage of a fully-charged LiPo cell is 4.20V, and charging above this will damage the cell. Nominal - This can be considered as a sort of "half-charged" voltage, as it is 3.70V, in between charged and discharged. Nominal voltage is what manufacturers use when describing the voltage of their batteries. Discharged - The voltage of a discharged LiPo cell is 3.00V, and discharging below this will definitely damage the cell. Because the battery shown has a 3S arrangement, it is marked with its nominal voltage of 11.1V (3.70V*3 cells). A fully charged 3S pack is 12.60V and a fully discharged 3S pack is 9.00V. Constant C Rating (Discharge): The constant C rating (in relation to discharge) tells you how many amps can be safely drawn from the battery constantly. The "C" in a rating of xC (where x is an integer) actually stands for the capacity of the battery in Ah. By multiplying the C rating's coefficient by capacity of the battery in Ah, you can determine the sort of amperage you can draw. In the case of this battery, with a capacity of 2600mAh (2.6Ah) and a C rating of 55C (that's pretty high, FYI), you can multiply 55*2.6 and get the max constant output of the battery, which is 143A.

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Burst C Rating (Discharge): In addition to the constant C rating, there is also a burst C rating, which is higher. Most of the times, the "burst" is rated for 10 seconds. Although it is not marked on the battery in the picture, it says in the documentation that this battery's 10 second burst rating is 80C. So, 80*2.6 is 208A burst. That's a lot! It's worth noting that your LiPo won't last long when that many amps are being drawn from it. At 208A, a 2600mAh LiPo will last approximately for 45 seconds. C Rating (Charge): Determined in the same fashion as the C ratings for discharge, the C rating for charge tells you at what amperage you can safely charge your battery. This information is generally listed on the back of the battery with all the safety information. For the battery shown, it happens to be 5C, which means that it can be charged at 13A (2.6*5). Li-Po battery: Lithium-Polymer Battery

Lithium-polymer batteries can be dated back to the 1970’s. Their first design included a dry solid polymer electrolyte that resembled a plastic film. Therefore, this type of battery can result in designs thin as a credit card while still holding relatively good battery life. In addition, lithium-polymer batteries are very lightweight and have improved safety. However, these batteries will cost more to manufacture and have an energy density inferior to lithium-ion batteries. The battery All LiPo batteries (should) have 2 sets of wires coming out of them: discharge leads and balance leads (sometimes called balance taps). The discharge leads are the thicker wires of which one is positive (red, +, anode) and the other negative (black, -, cathode), and are used to discharge the LiPo as their name suggests. The balance leads are used when charging the battery to ensure that all the cells in the battery are charged equally. There is generally a common ground connection on one side of the balance connector, as well as a positive connection to each cell in the battery. Therefore, depending on the number of cells the battery has, it will have a balance connector with a different number of pins.

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The Charger In order to charge LiPo batteries, you must use a LiPo-compatible charger. If you try to charge a LiPo with a non-LiPo charger, it will catch fire. Balancing Balancing cells is quite possibly the most important part of charging a LiPo battery. As LiPo batteries are used, their cells may discharge unevenly and become "unbalanced." To combat this, balancing chargers are plugged into the balancing leads of the LiPo battery as well as the discharge leads, allowing them to individually charge and "balance" the cells within the LiPo battery so that all the cells are at the same voltage by the end of charging. Some LiPo chargers don't have balancing capabilities, and when this is the case, it is necessary to buy and use a separate balancer. Balance Charging Setup When setting up a charger to balance charge LiPo batteries, you are presented with 2 main parameters: current and voltage. Charge Current: The current at which you should charge your LiPo battery depends on the battery's capacity and charge C rating. Regardless of charge C rating, though, most people charge their LiPos at 1C, as that is the safest rate, both from a fire danger and battery longevity standpoint. Charging your LiPo at a higher rate will make it charge faster, but charging at high rates will also decrease the life of the battery in the long run. Charge Voltage: This is the nominal voltage of the battery you want to charge. Often, the charger states the cell arrangement (such as "3S") next to its nominal voltage for easier recognition. The chargers check the battery by counting its cells via the balance plug and do not charge if your selected voltage and the battery's voltage don't match, which is a very good safety feature. Here are a few real-life LiPo balance charging scenarios: 2600mAh 3S LiPo charged at 1C 1C*2.6Ah = 2.6A charge current 3S*3.7V = 11.1V charge voltage 2.6A*12.6V (fully charged voltage) = 32.76W power drawn 1800mAh 2S LiPo charged at 1C 1C*1.8Ah = 1.8A charge current 2S*3.7V = 7.4V charge voltage 1.8A*8.4V (fully charged voltage) = 15.12W power drawn 5000mAh 2S LiPo charged at 1C 1C*5.0Ah = 5.0A charge current 2S*3.7V = 7.4V charge voltage 5.0A*8.4V (fully charged voltage) = 42.00W power drawn

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All these charges of the respective batteries are very safe and within the realm of the charger's capability. Additionally, each of these charges, because they are being used at a 1C charge rate, theoretically take 1 hour to charge each battery from 3.00V per cell "dead" to 4.20V per cell "full." In real life, charge time varies depending on the degree of discharge of the battery (most of the time you will stop using the battery before it hits 3.00V/cell) and the degree of imbalance between the cells (the more imbalanced they are, the longer it takes the charger to balance them). Storage Proper storage voltage for a LiPo is 3.85V per cell. Most LiPo chargers have a storage function that will either charge or discharge the battery until it hits 3.85V per cell. After the LiPo is at a proper 3.85V per cell for storage, you can find a good place for it to stay. LiPos are best stored in relatively low temperatures (40-45 degrees F), so a refrigerator is an excellent place for them.

In some instances, you will need to completely discharge the LiPo. The most likely reason for this is to measure capacity, because charging from 3.0V per cell to 4.2V per cell (or discharging from 4.2V per cell to 3.0V per cell) is the only way to accurately judge capacity. Think back to the constant C rating for discharge of the battery, and get as close to its max constant discharge current as you can. Safety If you take care of your LiPo battery, it will take care of you. Or not burn your house to the ground at the very least. Here are some guidelines to follow for safe usage of LiPos: -don't poke it or puncture it. -don't drop it. -don't short it out. -don't overcharge it. -don't let it overheat. -don't throw it in a fire.

Discharging

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Summary

Type Secondary Chemical Reaction Operating

Varies, depending on electrolyte.

Temperature Improved performance at low and high temperatures.

Recommended for

Cellular telephones, mobile computing devices.

Initial Voltage 3.6 & 7.2 Capacity Varies depending on the battery; superior

to standard lithium-ion. Discharge Rate Flat

Recharge Life Charging

300 – 400 cycles

Temperature 32º F to 140º F (0º C to 60º C)

Storage Life Loses less than 0.1% per month. Storage Temperature -4º F to 140º F (-20º C to 60º C)

Can be recycled by dropping them off at any of our over 7,200 stores nationwide.

Disposal Should be recycled through your local RadioShack store.

Other Notes

Typically designed to be recharged in the device rather than in an external charger. Lighter than nickel-based secondary batteries with (Ni-Cd and NiMH). Can be made in a variety of shapes.

Electronic Speed Control (ESC) Speed Control Fundamentals Early electric R/C car speed controls consisted of nothing more than a hefty variable resistor, the wiper of which was moved by a servo. This had the advantage of being simple, but was very inefficient at partial throttle settings. Such a control works by reducing the voltage to the motor, but this means that any voltage that does not appear across the motor terminals must appear across the speed control. For example, at half throttle, a resistor speed control that is controlling a motor drawing 10A from a 6-cell pack will have 3.6V across it, and 10A flowing through it. From second law, that is 36W, all of which becomes useless heat. This would be like running a 40W light bulb in the radio compartment of your plane. Furthermore, half the power produced by the battery is being wasted. A resistor speed control is only efficient at zero throttle (when no current is flowing), and at full throttle (when there is no voltage drop across the speed control).

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A typical (in 1997) high-rate analog speed control connected to a Graupner Speed 600 motor. Notice the fuse in the positive lead from the battery connectors. An electronic speed control (the photo shows a typical high-rate speed control) works by applying full voltage to the motor, but turning it on and off rapidly. By varying the ratio of on time to off time, the speed control varies the average voltage that the motor sees. Since at any given instant, the control is either fully off (no current flowing, so P = 0 × V = 0W) or fully on (no voltage drop across the speed control, so P = I × 0 = 0W), this kind of control is theoretically 100% efficient. In reality, electronic speed controls are not 100% efficient. Ignoring the factors introduced by switching rate (discussed later), the loss in efficiency is due to the fact that the components doing the actual switching are not perfect. They are not mechanical switches, and therefore have significant resistance. Whenever there is current flowing through a resistance, there is power loss. Some early electronic speed controls used ordinary (bipolar) transistors to switch the motor current. These generally have a 0.7V drop, regardless of the current flowing through them. This means a power loss. For example, at 20A (full throttle on a small 05 sized sport plane), this would result in a 14W loss (P = I × V = 20A × 0.7V = 14W). Modern speed controls use MOSFETs (Metal Oxide Semiconductor Field Effect Transistors). Rather than having a fixed voltage drop like a bipolar transistor, a MOSFET has a fixed resistance when turned on. Therefore, the voltage drop depends on the current flow. A typical MOSFET used in inexpensive speed controls has 0.028 Ohms resistance. Using Ohm’s law, we can determine the voltage loss. At 20A, this produces a 0.56V drop (V = I × R = 20A × 0.028 Ohms = 0.56V). We can use the second law to compute that the power loss would be 11.2W (P = I × V = 20A × 0.56V = 11.2W). The power loss can be reduced by using more MOSFETs in parallel, or using modern lower resistance MOSFETs. For instance, an Astro 211 speed control has a resistance of only 0.002 Ohms. At 20A, this would result in a 0.8W power loss. If it were being used with 10 cells at 20A that would be less than a 0.4% loss (10 cells at 20A produces about 220W). Theoretically, the speed control will be equally efficient at all throttle settings. (One could argue that it is more efficient at lower settings, because it spends more of its time in the 100% efficient off state.) The rate at which a speed control turns the motor on and off is also important. Early speed controls, including some made even today, were low-rate controls. These turn the motor on and off at the same rate that your radio sends pulses to the servos (usually 50 to 60 times per second). The simple theory presented above breaks down at these low rates, and such speed controls are very inefficient at partial throttle settings. There are many technical reasons for this, involving factors like motor coil inductance, impedance, and so on. There is another simple reason, and that is bad timing. Consider a typical low-cost motor with a three slot armature. As this motor rotates, each of the three commutator segments passes each brush three times per revolution. Each

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armature winding is energized in a given direction once per revolution. Now suppose that the speed control is being operated at 1/3 throttle (so it is on for 1/3 of the time and off for 2/3 of the time), and that this results in the motor turning at 60 revolutions per second (3,600 RPM). If the speed control is pulsing the motor 60 times per second, then each pulse corresponds exactly to the beginning of one revolution. Since the power is on only for 1/3 of the time, only one armature winding is energized in each revolution, and it will always be the same one. Therefore, this one winding is doing all the work, and will get much hotter than if the work were shared by all three windings. The rotation will also not be smooth, as the motor accelerates and decelerates with each revolution. If you used such a speed control with a geared motor, the gears would take quite a beating and quickly wear out. Modern speed controls turn the motor on and off at a much higher rate (typically 1,000 to 4,000 times per second, with 2,500 being typical). Even at 1,000 cycles per second, the problem described above would not happen until the motor reached 60,000 RPM, which is beyond the reach of most motors. This results in much smoother operation and due to a better match of the switching frequency to armature winding characteristics, results in less heat loss within both the motor and the speed control. Speed Control Features The ads and literature describing the numerous speed controls in the market today list many features. We will briefly examine some of them here. Brake A brake forces the motor to stop turning once the speed control stops delivering power. Electric motors become generators when being driven by their output shaft (for example, by a wind-milling propeller). The more load you put on a generator, the harder it is to turn. A speed control brake simply places a load (a low resistance) across the motor terminals, making it difficult for the motor to turn. This is generally sufficient to stop it completely. If a folding propeller is being used, this will allow it to fold. If a fixed propeller is being used, it will produce less drag than if it were spinning. Soft Start This term describes both speed controls and a special kind of on/off-only motor switch. In both cases, it indicates that the control will go from off to full throttle slowly (for example, over the course of one second) instead of going instantly. This is very important if using a gearbox or folding propeller, since an instant start can strip gear teeth, or shear propeller hinge pins. Some speed controls let you adjust the soft start time interval. Digital or Microprocessor Until fairly recently, the majority of speed controls were analog, meaning they worked with voltages and pulse widths, and had dedicated circuitry to perform each of their functions. Most modern speed controls are digital. These controls use a microprocessor to measure the incoming pulse width from the radio, and to generate the pulses to the MOSFETs. Digital designs have the advantage of being adjustment free, and of being able to provide sophisticated safety features. For example, most digital controls refuse to turn on until the throttle stick has been moved completely to off first. Battery Eliminator Circuit (BEC) In small planes, it is advantageous to eliminate the weight of a receiver battery. Many speed controls provide a BEC feature that provides power to the receiver and servos from the motor battery. There is still a great deal of debate as to whether this is safe, primarily due to the danger of electrical noise getting into the receiver and causing reduced radio range. The other danger of course is that the motor battery could run down to the point that the BEC cannot provide power to the receiver. BEC is very popular with the electric pylon racing crowd, where the planes never get very far away, and land immediately after the race.

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Automatic Cut-Off This feature is generally used with a BEC, so that the motor will shut down before the battery is depleted, thus reserving some power for the radio. Optical Isolation To reduce the possibility of the speed control interfering with the radio receiver, some controls use an optoisolator chip. This is basically an LED (light emitting diode) and phototransistor encased in plastic. The signal from the receiver drives the LED, which optically transfers the signal to the rest of the speed control. There is no electrical connection between the receiver and the main part of the speed control. Obviously, this eliminates the possibility of providing a BEC. Selecting a Speed Control Selecting a speed control is a matter of determining the conditions under which it must operate, and then choosing one with specifications that fit those conditions and your budget. The parameters to consider are: number of cells expected current draw space available weight limits need for a BEC need for a brake other desired features Most speed controls operate over a range of cell counts, such as 6 to 12 cells. You must choose a control that covers the range with which you want to use it. Do not go below or above the manufacturer’s specified range, or you will damage the speed control. Determine the current draw that you will get at full throttle. If you have no idea, you can measure it on the bench (without a speed control, although this is hard on the gearbox if you using one). Alternatively, consult with the manufacturer of your motor, or with other modellers. You can also use one of the motor performance prediction programs, like MotoCalc or ElectriCalc, to get fairly accurate predictions. Many speed controls have both a continuous current rating (the current level that the control can handle indefinitely), and a peak current rating (the level it can handle for a short time, usually less than 30 seconds). For sport flying, select your speed control based on the continuous rating. This rating should be higher than or same as your expected maximum current draw. Be careful if you are considering any of the R/C car speed controls. Most of these have grossly overstated continuous current ratings. For example, one popular control is advertised to have a 250A continuous rating, when in fact it would fry in seconds at 80A. Installation Installation of the author’s high rate speed control with brake in the nose of a modified Great Planes Spectra. Again, notice the fuse in the positive battery lead. The motor is a Great Planes Goldfire, reworked with a car motor end-bell and replaceable brushes, turning a Master Airscrew 12×8 folding propeller through a 2.5:1 gearbox. Current draw is about 30A, and the speed control barely gets warm. Installing a speed control is simple, provided you follow the manufacturer’s instructions. Pay special attention to the details of motor interference suppression. Doing this properly can make the difference between perfect operation and an unusable speed control. Interference suppression, using a 0.1µF and two 0.047µF capacitors. Also notice the diode just below the rear motor bearing.

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If there are no instructions about interference suppression, solder a 0.1µF 50V capacitor between the two motor terminals, and a 0.047µF 50V capacitor between each terminal and the motor case, for a total of three capacitors (see the photo at left). Many speed controls also require you to install a Schottky diode across the motor terminals. Install this diode with the banded end towards the positive terminal. The diode is an important part of the efficient operation of the speed control, and those that do not require one will already have one installed on the control itself. Finally, if you are using a speed control with a BEC, do not install the fuse between the battery and the speed control; do install it between the speed control and the motor. Otherwise, if your fuse blows, you will lose control of your aircraft since the radio will no longer have power. Interfacing the ESC with a microcontroller (with an example interfacing with AVR microcontroller) DC ESCs in the broader sense are PWM controllers for electric motors. An electronic speed control or ESC is an electronic circuit with the purpose to vary an electric motor's speed, its direction and possibly also to act as a dynamic brake. The ESC generally accepts a nominal 50 Hz PWM servo input signal whose pulse width varies from 1ms to 2ms. When supplied with a 1 ms width pulse at 50Hz, the ESC responds by turning off the DC motor attached to its output. A 1.5ms pulse-width input signal results in a 50% duty cycle output signal that drives the motor at approximately half-speed. When presented with 2.0ms input signal, the motor runs at full speed due to the 100% duty cycle (on constantly) output. The correct phase varies with the motor rotation, which is to be taken into account by the ESC: Usually, back EMF from the motor is used to detect this rotation, but variations exist that use magnetic or optical detectors. Computer-programmable speed controls generally have a user-specified option which allows setting low voltage cut-off limits, timing, acceleration, braking and direction of rotation. Reversing the motor's direction may also be accomplished by switching any two of the three leads from the ESC to the motor. Interfacing with an AVR microcontroller: In the AVR, the timer/counter 1 is used to generate PWM signals. This signal is emitted from OC1A pin of ATmega8535 microcontroller and fed to an ESC to drive two brushless DC motors. The width of the PWM pulse is defined by the value of OCR1A. The maximum value of this register will set the motor at low voltage cut-off limits. This value is defined by OCR1_HIGH. And the minimum value of this register will set the motor run at highest speed. This value is defined by OCR1_LOW. To speed up motor, you can use a command or make port C pin 2 high; and to slow down motor, you can use a command or make port C pin 3 high. Motors:

A study of brushed and brushless DC motors: Brush DC Motors Around since the late 1800s, dc brush motors are one of the simplest types of motors. Sans the dc supply or battery required for operation, a typical brush dc motor consists of an armature (a.k.a., rotor), a commutator, brushes, an axle, and a field magnet. The motor’s properties are determined by the material it is made of, the number of coils wound around it, and the density of the coils. The armature or rotor is an electromagnet, and the field magnet is a permanent magnet. The commutator is a split-ring device wrapped around the axle that physically contacts the brushes, which are connected to opposite poles of the power source.

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The brushes charge the commutator in polarity reverse of the permanent magnet, in turn causing the armature to rotate. The rotation’s direction, clockwise and/or counter clockwise, can be reversed easily by reversing the polarity of the brushes, i.e., reversing the leads on the battery.

BRUSHLESS DC MOTORS In terms of differences; the name is a dead giveaway. BLDC motors lack brushes. But their design differences are bit more sophisticated. A BLDC motor mounts its permanent

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magnets, usually four or more, around the perimeter of the rotor in a cross pattern. Efficiency is a primary selling feature for BLDC motors. Because the rotor is the sole bearer of the magnets, it requires no power, i.e., no connections, no commutator, and no brushes. In place of these, the motor employs control circuitry. To detect where the rotor is at certain times, BLDC motors employ, along with controllers, rotary encoders or a Hall sensor. BLDC motors are synchronous motors, which means their rotors and stators turn at the same frequency. They come in single-, dual-, and tri-phase configurations. To Brush When it comes to a loosely defined range of basic applications, one could use either a brush or brushless motor. And like any comparable and competing technologies, brush and brushless motors have their pros and cons. On the pro side, brush motors are generally inexpensive and reliable. They also offer simple two-wire control and require fairly simple control or no control at all in fixed-speed designs. If the brushes are replaceable, these motors also boast a somewhat extended operational life. And because they need few external components or no external components at all, brush motors tend to handle rough environments reliably. For the downside, brush motors require periodic maintenance as brushes must be cleaned and replaced for continued operation, ruling them out for critical medical designs. Also, if high torque is required, brush motors fall a bit flat. As speed increases, brush friction increases and viable torque decreases. However, torque may not be an issue in some applications and could actually be desirable. For example, electric toothbrushes require higher speeds with decreasing torque, which is good for the brush and your teeth and gums. Other disadvantages of brush dc motors include inadequate heat dissipation caused by the rotor limitations, high rotor inertia, low speed range due to limitations imposed by the brushes, and electromagnetic interference (EMI) generated by brush arcing. Or Not to Brush BLDC motors have a number of advantages over their brush brothers. For one, they are more accurate in positioning apps, relying on Hall Effect position sensors for commutation. They also require less and sometimes no maintenance due to the lack of brushes. They beat brush motors in the speed/torque trade-off with their ability to maintain or increase torque at various speeds. Importantly, there’s no power loss across brushes, making the components significantly more efficient. Other BLDC pros include high output power, small size, better heat dissipation, higher speed ranges, and low-noise (mechanical and electrical) operation. Nothing is perfect, though. BLDC motors have a higher cost of construction. They also require control strategies that can be both complex and expensive. And, they require a controller that can cost almost as much as if not more than the BLDC motor it governs. Summary of motors: Brushed Motor Pros Two wire control Replaceable brushes for extended life Low cost of construction Simple and inexpensive control No controller is required for fixed speeds Operates in extreme environments due to lack of electronics Brushed Motor Cons Periodic maintenance is required Speed/torque is moderately flat. At higher speeds, brush friction increases, thus reducing useful torque Poor heat dissipation due to internal rotor construction Higher rotor inertia which limits the dynamic characteristics

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Lower speed range due to mechanical limitations on the brushes Brush Arcing will generate noise causing EMI BLDC Motor Pros Electronic commutation based on Hall position sensors Less required maintenance due to absence of brushes Speed/Torque- flat, enables operation at all speeds with rated load High efficiency, no voltage drop across brushes High output power/frame size. Reduced size due to superior thermal characteristics. Because BLDC has the windings on the stator, which is connected to the case, the heat dissipation is better Higher speed range - no mechanical limitation imposed by brushes/commutator Low electric noise generation BLDC Motor Cons Higher cost of construction Control is complex and expensive Electric Controller is required to keep the motor running. It offers double the price of the motor.

TRANSMITTER: A transmitter or radio transmitter is an electronic device which, with the aid of an antenna, produces radio waves. The transmitter itself generates a radio frequency alternating current, which is applied to the antenna. When excited by this alternating current, the antenna radiates radio waves. In addition to their use in broadcasting, transmitters are necessary components of many electronic devices that communicate by radio, such as cell phones, wireless computer networks, Bluetooth enabled devices, etc. Basics: A transmitter can be a separate piece of electronic equipment, or an electrical circuit within another electronic device. A transmitter and receiver combined in one unit is called a transceiver. The term transmitter is often abbreviated "XMTR" or "TX" in technical documents. The purpose of most transmitters is radio communication of information over a distance. The information is provided to the transmitter in the form of an electronic signal, such as an audio (sound) signal from a microphone, a video (TV) signal from a video camera, or in wireless networking devices, a digital signal from a computer. The transmitter combines the information signal to be carried with the radio frequency signal which generates the radio waves, which is often called the carrier. This process is called modulation. The information can be added to the carrier in several different ways, in different types of transmitter. In an amplitude modulation (AM) transmitter, the information is added to the radio signal by varying its amplitude. In a frequency modulation (FM) transmitter, it is added by varying the radio signal's frequency slightly. Many other types of modulation are used. Working: A radio transmitter is an electronic circuit which transforms electric power from a battery or electrical mains into a radio frequency alternating current, which reverses direction millions to billions of times per second. The energy in such a rapidly reversing current can radiate off a conductor (the antenna) as electromagnetic waves (radio waves). The transmitter also impresses information, such as an audio or video signal, onto the radio frequency current to be carried by the radio waves. When they strike the antenna of a radio receiver, the waves excite similar (but less powerful) radio frequency currents in it. The radio receiver extracts the information from the received waves. A practical radio transmitter usually consists of these parts:

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A power supply circuit to transform the input electrical power to the higher voltages needed to produce the required power output. An electronic oscillator circuit to generate the radio frequency signal. This usually generates a sine wave of constant amplitude often called the carrier wave, because it serves to "carry" the information through space. In most modern transmitters this is a crystal oscillator in which the frequency is precisely controlled by the vibrations of a quartz crystal. A modulator circuit to add the information to be transmitted to the carrier wave produced by the oscillator. This is done by varying some aspect of the carrier wave. The information is provided to the transmitter either in the form of an audio signal, which represents sound, a video signal, or for data in the form of a binary digital signal. In an AM (amplitude modulation) transmitter the amplitude (strength) of the carrier wave is varied in proportion to the modulation signal. In an FM (frequency modulation) transmitter the frequency of the carrier is varied by the modulation signal. An RF electronic amplifier to increase the power of the signal, to increase the range of the radio waves. An impedance matching (antenna tuner) circuit to match the impedance of the transmitter to the impedance of the antenna (or the transmission line to the antenna), to transfer power efficiently to the antenna. If these impedances are not equal, it causes a condition called standing waves, in which the power is reflected back from the antenna towards the transmitter, wasting power and sometimes overheating the transmitter. In higher frequency transmitters, in the UHF and microwave range, oscillators that operate stably at the output frequency cannot be built. In these transmitters the oscillator usually operates at a lower frequency, and is multiplied by frequency multipliers to get a signal at the desired frequency. RECEIVER: In radio communications, a radio receiver is an electronic device that receives radio waves and converts the information carried by them to a usable form. It is used with an antenna. The antenna intercepts radio waves (electromagnetic waves) and converts them to tiny alternating currents which are applied to the receiver, and the receiver extracts the desired information. The receiver uses electronic filters to separate the desired radio frequency signal from all the other signals picked up by the antenna, an electronic amplifier to increase the power of the signal for further processing, and finally recovers the desired information through demodulation. The information produced by the receiver may be in the form of sound (an audio signal), images (a video signal) or data (a digital signal). A radio receiver may be a separate piece of electronic equipment, or an electronic circuit within another device. Devices that contain radio receivers include television sets, radar equipment, two-ways, cell phones, wireless computer networks, GPS navigation devices, satellite dishes, etc.

SERVO: A Servo is a small device that has an output shaft. This shaft can be oriented to specific angular positions by sending the servo a coded signal. As long as the coded signal exists on the input line, the servo will maintain the angular position of the shaft. As the coded signal changes, the angular position of the shaft changes. In practice, servos are used in radio controlled airplanes to position control surfaces like the elevators and rudders. They are also used in radio controlled cars, puppets, and of course, robots.

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A Futaba S-148 Servo Servos are extremely useful in robotics. The motors are small, as you can see by the picture above, have built in control circuitry, and are extremely powerful for their size. A standard servo such as the Futaba S-148 has 42 oz/inches of torque, which is pretty strong for its size. It also draws power proportional to the mechanical load. A lightly loaded servo, therefore, doesn't consume much energy. The guts of a servo motor are shown in the picture below. You can see the control circuitry, the motor, a set of gears, and the case. You can also see the 3 wires that connect to the outside world. One is for power (+5volts), another for ground, and the white wire is the control wire.

A disassembled servo So, how does a servo work? The servo motor has some control circuits and a potentiometer (a variable resistor, aka pot) that is connected to the output shaft. In the picture above, the pot can be seen on the right side of the circuit board. This pot allows the control circuitry to monitor the current angle of the servo motor. If the shaft is at the correct angle, then the

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motor shuts off. If the circuit finds that the angle is not correct, it will turn the motor the correct direction until the angle is correct. The output shaft of the servo is capable of travelling somewhere around 180 degrees. Usually, it is somewhere in the 210 degree range, but it can be varied by manufacturer. A normal servo is used to control an angular motion of between 0 and 180 degrees. A normal servo is mechanically not capable of turning any farther due to a mechanical stop built on to the main output gear. The amount of power applied to the motor is proportional to the distance it needs to travel. So, if the shaft needs to turn a large distance, the motor will run at full speed. If it needs to turn only a small amount, the motor will run at a slower speed. This is called proportional control. How do you communicate the angle at which the servo should turn? The control wire is used to communicate the angle. The angle is determined by the duration of a pulse that is applied to the control wire. This is called Pulse Coded Modulation. The servo expects to see a pulse every 20 milliseconds (0.02 seconds). The length of the pulse will determine how far the motor turns. A 1.5 millisecond pulse, for example, will make the motor turn to the 90 degree position (often called the neutral position). If the pulse is shorter than 1.5 ms, then the motor will turn the shaft to closer to 0 degrees. If the pulse is longer than 1.5ms, the shaft turns closer to 180 degrees.

As you can see in the picture, the duration of the pulse dictates the angle of the output shaft (shown as the green circle with the arrow). Note that the times here are illustrative and the actual timings depend on the motor manufacturer. The principle, however, is the same.

A 7 * 6 E propeller means that the diameter of the propeller is 7 inches and the pitch is 6, meaning the plane progresses 6 inches per rotation. More the diameter, more is the load it can carry and the slower it is. Smaller the diameter, faster is the propeller. More pitch means more load on the motor.