Know How... 117

Using Project Loon as a mesh network, electric motors and which you should use for your quad, Google Plus questions, propeller basics, and more.

Remember Project Loon?

It's a Google R&D Project to provide Internet connectivity to rural areas. The idea was to use high-altitude balloons, flown 20 miles up, to create an airborne wireless network. The balloons used a tank of helium to inflate & rise to the desired alt, then dumped helium to maintain. The gas canister could allow the balloon to stay airborne for up to 100 days. It used the shifting wind patterns at various altitudes to keep station.

It was tested in New Zealand. The balloons created a mesh using 24.2 Ghz radios that could provide 1.4Gbps of bandwidth

Now it's coming to the United States! Last week, Google filed documents with the FCC to test a super-fast wireless technology in San Mateo and Mountain View, California. Google is planning to combine 5.8, 24.2 and 71-76 Ghz radios into arrays that would beam high-speed wireless networking around a city.

• 5.8Ghz 802.11A/G/N/AC
• 24.2Ghz
  • Mesh Wireless
  • Ubiquity makes 25Ghz Point-to-Point wireless that can deliver 1.4Gbps up to 13km away
• 71-76Ghz "Millimeter Wave" Radio
  • MM Wireless is "Backhaul" wireless
  • "BridgeWave" a MM Wireless vendor, was able to get full-duplex 3Gbps over a single, zero-footprint unit.
  • Some MM Wireless systems can deliver 10Gbps more than 25KM.
  • They could combine this new wireless with a backhaul buildout of fiber.

The idea comes directly from Google's experience with Project Loon. The 24.2Ghz Ubiquity radios in the arrays were first tested on the balloons. If they combine the frequency trifecta with beam-forming antennas, they could blanket a city

This is in addition to the 34 cities that Google is considering for Google Fiber rollouts.

Electric Motors

Brushless Motors for your Quad:
Let's talk about how Brushless motors work:

A Brushless motor is very much like a brushed motor:

• There is a power source
• There is an Armature
• There is shaft
• There is a coil that generates an electrical field
• There is a permanent magnet
• There is a commutator that flips the current flowing through the coil and therefore flips the electromagnetic field.

Where it differs is in what part of the motor moves.

• In a brushed motor, the permanent magnet is stationary and the armature rotates, driven by the current flowing through the coil attached to the armature.
• In a brushless motor, the armature is STATIC. The permanent magnet rotates around or within the armature, driven by the current flowing through the coil attached to the armature.
• In other words... In a brushed motor, the armature rotates. In a brushless motor, the permanent magnet rotates.

The big difference is in the commutator:

• A brushed motor uses a mechanical commutator. As the shaft turns, it MECHANICALLY turns the commutator, which changes it position relative to the brushes, which allows it to flip current polarity.
• A brushless motor uses a SENSOR to determine the position of the shaft, and the current is flipped ELECTRONICALLY by a controller.

Brushless Advantages

1. Longer life - No brushes means no brushes to wear down.
2. Less maintenance - It doesn't use a mechanical process to switch current through the coil, but an ELECTRIC controller that uses a sensor within the motor to know the position of the armature.
3. More efficient b/c there is no friction from the brushes
4. Decreases the amount of EMI generated by the motor

Brushless Motor Disadvantage

1. More expensive than a Brushed Motor
2. More complicated
3. Slighly more prone to damage (because of the sensor)

There are two types of brushless electric motors you can choose for your QuadCopter: Inrunners and Outrunners

Outrunners:

• Low RPMs but high Torque
• Direct Drive: Shaft is connected directly to the Prop.
• Quiet

Inrunners:

• High RPM, but low torque
• Geared Drive: Shaft is connected to motor through a gearbox
• More efficient than Outrunners
• Wider selection of props
• Noisy
• More parts to break

As a general rule, the faster that a motor spins, the more efficient it will be. However, the faster an electric motor turns, the less torque it generates.

• Inrunning motors turn VERY fast, and produce little torque. Instead, they go through a reduction gearbox that spins the prop at 1/x times as fast as the motor shaft
• This means you can choose from a variety of propellers by changing the gear ratio of the gearbox. (i.e. you can make the propeller spin at its most efficient speed while not changing the RPMs of the motor.

By the Numbers:
Kv = "Voltage Constant

• This is the THEORETICAL number of times the motor turns for every volt it is supplied
• In reality, the number is slightly less because no motor is 100% efficient.

Example:

• Turnigy 1704-1900Kv Outrunner
  When we give it 11.1 volts, it will turn 21,090 times per minute (1900 X 11.1)
• Neewer A22112/13 KV1000 Brushless Motor
  When we give it 11.1 volts, it will turn 11,100 times per minute
• Emax MT2213 935kv
  When we give it 11.1 volts, it will turn 10,378.5 times per minute

Choosing your Motors:
This is a problem --- because other than the Kv, motors from different manufacturers don't really report their specs in the same way:

• Some give you thrust (like the Emax with 850g of thrust)
• Others just give you the max power (like the 1740, rated for 49 watts)
• Still others will give you NOTHING

But there are a few things to remember:

1. You need to choose your shaft
2. Size the motor to your frame
   • If you have a x250, then you're probably going to want a smaller, faster-spinning motor
   • Larger craft, like a x350, x450 or x525 will need larger motors, with more turns, that spin more slowly, but provide more torque, which you need for the bigger props.
3. Feedback: Look at the forums... see what people like... because the manufacturer numbers are pretty much useless.
4. Choose bullet connectors and wire type

Props

Propeller Talk:
There are two variables to every propeller: Diameter and Pitch

• Diameter refers to the length of the propeller from tip to tip
• Pitch refers to the distance the propeller would travel forward, assuming that it was a perfect medium, in one rotation. The more angled the blade, the higher the pitch.

Choosing your prop is all about balancing top speed, lift capacity, and power drain.

• Increasing the DIAMETER of the prop increases your lift capacity (because there's more surface area to the propeller)
• Increasing the PITCH of the prop increases your top speed (because it increases the distance the prop would move forward in one revolution)
• Increasing either the Diameter or Pitch of a propeller increases the current draw on your electrical system.

Example:
Disclaimer: None of the following examples are intended to be numerically accurate. They are representations of the relationship between diameter, pitch and current draw.
Let's say that you've got a Quadcopter using 5x5 propellers (5" Diameter, 5" pitch) moving at 15 kph at 100% power output (meaning that you have the ability to spin the prop faster)

• Going with a larger diameter prop (6x5) but keeping the pitch the same will still top out at 15 kph, but you'll be able to lift more mass and accelerate more quickly..
• Going with a prop of the same diameter, but higer pitch (5x8) will increase your top speed, but will not increase your acceleration or lift capacity.
• BOTH of these modifications would adversely affect the longevity of your battery.

The larger the propellor's diameter and pitch, the more current the motor will draw for a given RPM. In our example, the NEEWER 2212-100KV will turn at (near) 12,000 rpm mo matter how large the prop, but as the prop gets large and more pitched, the more CURRENT it will draw through the system, from the battery.

Important note: If all the propellers on a QuadCopter were turning in the same direction, the craft would spin uncontrollably in the rotation AWAY from the rotation of the propellers. (Much like in a helicopter that has lost its tail rotor.) To keep that from happening, we spin the propeller in pairs: either clockwise or counter-clockwise. Think of it like a box: Top Left and Bottom right will spin clockwise. Top right and Bottom Left will spin CCW.

The Takeway:
There is a temptation to get the largest motor you can, and pair it with the biggest, more aggressive prop you can fit. You might think that it will give you the highest carrying capacity and the best top speed.

But that usually means your motor is going to always be running BELOW its "gold band" of efficiency (remember, the faster a motor is running, the more efficient it is)

• That usually means you're killing your flight time
• Also, flying a Quad is all about throttle management. It's how you control altitude, speed and direction.
• The larger your prop, the slower it is to respond to throttle changes because it has more mass.
• The slower the prop is to change its velocity, the less control you have over the altitude, speed and direction of the Quad.

There is another temptation: to get the smallest motor you can, and pair it with the most efficient prop.

• But that usually means your motor is going to be running hot (and heat is wasted energy)
• And it also means you're not going to get much performance out of your craft.

The best way to decide your best setup, it to practice your setup. You'll find the best combination of motor/prop/battery for your flying skills

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• Transcripts: Know How... 117 (Transcript)