## Battery Charger

LiPo charger needs to be capable of charging a 3-cell battery.  A battery charger capable of balance charging the battery is also recommended.  Balance charging reads the voltage of the individual cells of the battery and charges each cell equally to the other cells.   This feature will increase charging time but will prolong the life of the battery and provide slighter longer flight times.

## Calculate Flight Time of LiPo Battery

Before we can calculate flight time, we need to know the average amperage the quadcopter will draw.  Once we have the average amperage draw we can then calculate flight.

To calculate flight time, take the battery’s capacity in amp hours, then divide that into the average amp draw of the quadcopter and then multiply it by 60.  The total is the flight time in minutes.

For Scout, I chose a 11.1 volt 30C 3000 mAh LiPo battery.  I calculated the average amp draw of Scout to be around 20 amps.  This will give me a flight time of 9 minutes.

### Flight Time Calculator

Battery Capacity: mAh
Average Amp Draw: A
Flight Time: Minutes

## Batteries

Hobby King Zippy 11.1v 30C 3000mAh LiPo Battery

LiPo batteries (short for “Lithium Polymer”) are the latest and greatest when it comes to battery technology.  When considering power to weight ratio, LiPos are far superior compared to NiCad (Nickel-Cadmium) or NiMH (Nickel-Metal Hydride) batteries.

There are several options when choosing a LiPo battery.  The first is the voltage level that the propulsion system runs at.  A single cell LiPo battery outputs 3.7 volts.  To increase the overall voltage, LiPo cells are connected in serial.  Connecting the LiPo cells in serial, cumulatively adds to the overall voltage.  For example, two 3.7 volt LiPo cells will output 7.4 volts and a three LiPo cells will output 11.1 volts and so on.  For Scout, the motors and Electronic Speed Controllers that I have chosen run at 11.1 volts and so I chose 11.1 volt battery.

The second option is capacity.  Capacity is how much power the battery can hold measured in milliamp hours (mAh).  For example, a LiPo battery rated at 1000 mAh will discharge in one hour with a 1000 milliamp load placed on it.  If the same battery had a 4000 milliamp load placed on it, the battery will discharge in 15 minutes.  The higher the capacity of a battery, the longer the flight time, but also the bigger and heavier the battery will be.

The third option is discharge rate.  Discharge rate is how many amps can be discharged at a time.  The discharge rate is multiple of the total capacity of the battery and represented as the “C” rating.  For example, a battery with a capacity of 2000 mAh and a discharge rate of 10C is capable of withstanding 20,000 milliamp or 20 amp loads.

## Propellers

Propellers commonly come in 2, 3 and 4 blades.  The more blades on the propeller, the less efficient they become.  However, more blades produce less noise and are able to handle higher power requirements.

Propellers are specified by their diameter and pitch.  The diameter is measured length of the propeller.  The pitch is how far the propeller will advanced in one revolution.  For example, a 10×4 propeller has 10 inch diameter and will travel 4 inches in one revolution.

## Propeller Size

The diameter of a propeller dictates how much thrust can be generated.  The larger the propeller the more thrust can be generated and also the more energy is needed to spin the propeller.

Propellers come in two spinning directions: clockwise and counterclockwise.  The spinning direction is also referred to as “tractor” (counterclockwise) and “pusher” (clockwise) propellers.  Tractor propellers are more common than pusher propellers.  A quadcopter needs a matched set of tractor and pusher propellers.   Because pusher propellers are less common than tractor propellers, propeller choice will be dictated by which propellers are available in pusher configuration.

## Importance of Propeller Pitch

I discovered that my initial propeller choice of a 3-blade 8×6 propeller was the root of all my frustration in trying to stabilize Scout’s flight.  After weeks of tuning Scout’s stability, I began to hit a wall.   Even with the best tuning, Scout would still drift and sway during flight.  I could not get Scout to hover in one place.   I began to track down why Scout was so unstable.  I initially thought it was too much vibration that was overloading the sensors.  I added more foam padding to the sensor board and balanced the propellers.  The stability marginally improved, but not as much as I would like.

I then thought it was the ArduPirates code that was the problem so I switched to the ArduCopter code.   Scout was still unstable.  I then remembered I had bought a set of 2-blade 8×4 propellers.  I decided to give them a try.  Eureka!  Scout’s performance was remarkable.  Scout transformed into a different animal.  Without changing the tuning settings from the previous propellers, Scout’s stability is as smooth as glass.  I surmised that the issue was not the 3-blade to 2-blade choice but the pitch of 6 inches was creating choppy turbulent air and the quadcopter could not stabilize.

I recommend using APC propellers.  They are both rugged and perfectly balanced from the factory.