Propellers

Without them you couldn’t get off the ground and without knowing which one to use for your particular application you wouldn’t be any better off.  Props come in many shapes and sizes.  They are also made from many different materials such as abs plastic, wood, and carbon fiber.

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Finding out which is right for your application is very important and there are a few simple ways you can make that determination.  The first thing to check are the specifications of your motors.  The documentation for most motors will generally come with recommended propeller sizes as well as thrust and amperage draw measurements for each different size propeller they recommend.  Propellers come marked from the manufacturer using a standardized sizing system.

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You will notice on the propeller above it names the manufacturer and also the number 10×4.7.  The first number will always be the length of your propeller in inches.  The second number signifies that this propeller in a perfect world would give you 4.7 inches of forward travel for every rotation.  Typically a number of 4.7 or below is known as a “slow fly” propeller and is generaly used with motors which have a lower kv rating and more torque.  Propellers with a pitch larger than 4.7 are typically for faster higher kv motors which have less torque.

Multi-rotor aircraft add another layer of complexity to propellers in that they use both clock wise rotating and counter clockwise rotating propellers.  In the case of a quad copter they have two of both.  This is a necessity to maintaining stable flight and attitude.  On a quad copter the torque of the clock wise rotating and the counter clockwise rotating motors acts to cancel out any tendency the aircraft might have to spin either left or right though no control input had been given.  This allows the quad copter to achieve yaw movement by alternating the opposing motors RPM.  Propellers are usually designated with a CW or CCW marking.

Much more information on propellers and motors can be found online.  In the resource section of the blog you can find some great places to look.

Resources

For any person who might be interested in creating a multi-rotor for themselves I have compiled a collection of vendors where you can find everything that you will need.  Prices will generally be reasonable but shipping might take longer than most are accustomed to.  This is due to the fact that most of these vendors are out of China.  China, for some time now, has been a leader in the multi-rotor industry providing low cost and surprisingly high quality components.  Below are a list of vendors all of which have been reliable and I use on a regular basis.

Other very helpful sites for information include:

There are many more resources out there but these have been some of the most helpful to me.  Shop around and do some reading.  You will be surprised how easy you can pick this up with a little research.

 

Learning to Fly

One of the largest hurdles to overcome when getting into multi-rotors is learning how to fly.  Initially it can be frustrating and it can be expensive if you crash frequently – and you will.  Like any other skill, it requires a lot of time in the saddle to get good at it.  In order to save time and frustration, I recommend several different methodologies of initial flight training.

Simulators

Computer based simulators are probably one of the most cost effective ways of learning how to fly.  For a minimal initial investment you can crash as much as you want and not have to worry about repairing anything except your ego.  Most simulators do a very good job of recreating the feel of actual flight and many of them allow you to use your actual transmitter as the controller which really helps with getting comfortable.  Once you get comfortable hovering around in the simulator you can make things more challenging by adding environmental factors to the experience such as different flying locations, varying weather conditions etc.

A couple of the most popular flight simulators are:

Wikipedia also has a comprehensive list of RC flight simulators.  Most of these options will be worth the money because it will save you tremendous amounts of time, frustration, and trips to the piggy bank.

Toy Multi-Rotors

Another great and inexpensive option for learning to fly is small toy multi-rotors.  In the last couple of years there have been several small multi-rotors released which have the stabilization and flight time of their big brothers but at a fraction of the cost.  Because the controls and physics are basically the same as their larger counter parts, they make for great learning platforms.  Also because of their small size and light weight they can take crashes much better than their larger counterparts.

Both models are made by the same company and one is slightly more advanced than the other.  Either one would make a great learning platform.

These are not the only options available and in recent months the market has been flooded by clones and variations on the theme.  So do some reading and find out which one best suits you.  Once you make your choice, get it and fly as often and as much as possible.  Also, don’t be afraid: it is much easier to fly better and learn faster when you are relaxed and your movements are fluid.

YouTube

One of your greatest allies when embarking on this journey will be YouTube.  Not only can you find information on just about any individual component you can think of and get lots of free assistance from members of the community, but you can also learn to fly.

There are several common ways to start learning multi-rotor flight.  The first step is usually maintaining a simple hover, then you move to hovering in different shapes and so on.  There are many very qualified pilots on You Tube who can take you through the process step by step.  So get online and look around.  There is a large support group out there and it is getting larger.

Above all, practice.

Components Needed for Your Build

If you are going to get into the world of multi-rotors and you have no idea where to start here I will explain all of the basic components needed for flight.  I will not go into tremendous detail because there are many resources for such knowledge available online. I will, however, give you a good idea of where to start and what you will need.

The first things we will cover are the components which make up the multi-rotor itself.  They come in many shapes and sizes and have varying levels of complexity but the basic parts will always be the same just in varying quantities.  Below are a few examples of multi-rotor flight configurations.

Arducopter Layouts

These examples are from the mission planner software which configures the flight controller I used for my project, the APM 2.5.  As you can see there are many different configurations for a multi-rotor platform, each with its own advantages and drawbacks.  I will be focusing on the X configuration for this project also known as a quad copter.  It is a good middle of the road configuration mixing lifting ability with decent stability and agility.  Below is a functional diagram of how the different components of a quad copter work together to achieve flight.

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At this point I can begin to describe each one of these components in detail.

The Battery

I start with the battery because it is probably the most important part of the multi-rotor in more than one way.  This is because multi-rotor aircraft require a tremendous amount of electricity to operate.  Every time you leave the ground all of your motors are working very hard to defy gravity and because they are not airplanes they cannot glide nor can they rely on aerodynamic forces to keep them aloft.  Multi-rotors apply pure brute force to keep themselves flying and as such, require large amperages to keep flying.  Until very recently there was no battery technology on the market which could provide enough amps for a significant enough amount of time for this type of machine to be viable.  Another problem was that even if you had a battery that could provide you with the amount of amps you needed, the battery would inevitably be too heavy and thus weight prohibitive.

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Then came the commercialization of Lithium Polymer batteries (LiPO).  These batteries have very good power density and are light weight while also providing tremendous amperage for twenty minutes or more depending on the application.  Because of these advances small multi-rotor aircraft which could carry payloads for a significant amount of time became possible.  The industry has progressed very quickly since then due in large part to China’s rise as an industrial power house.

Most LiPO batteries used in multi-rotor applications come as cells configured in parallel giving you different voltages.  One LiPO cell has a voltage range between 2.7 to 4.23 volts.  Below you can see the cells from a 3 cell LiPO.

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Care must be exercised in charging and maintaining these batteries as they should not be over charged or discharged.  Over discharging will lead to a change in the cells’ chemistry which creates gas that will “puff” your battery (as seen in the photo above) as well as ending the cells’ active life prematurely.  Overcharging can lead to excessive heat , fire, and gas discharge.  Because these batteries can be so volatile, one must always use special balance chargers that can charge and discharge each cell individually to ensure safe operation.  Such as the one pictured below:

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For the purposes of this project we will be using mainly three and four cell batteries.

The ESC

The ESC is an electronic speed control unit.  It sits between the battery which uses DC current and the brushless motors which run off of multiphase AC current.  You can see an example below:

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The red and black wires on the left side of the previous image are for the DC current in.  The three black wires on the right side of the ESC are for AC current out.  The multicolored wire with plug on the left side is used by your Flight Control Board to vary the speed of the motor and also provides a five volt rail which can in turn power the Flight Control Board.  That feature is included in ESC units which also have a Battery Eliminator Circuit or BEC built in.

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Each ESC unit comes rated to handle a range of amps passing through them.  The above example for instance is rated to thirty amps.  This means that if the motor/propeller/load combination you are using draws more than thirty amps of current for any amount of time you run the risk of burning out your ESC which in turn would result in a catastrophic failure. This would would be an especially horrible scenario if you were flying your multi-rotor at the time.

The Motors

There are two types of brushless motors used for remote control applications, brushless outrunners and inrunners.  The term outrunner comes from the fact that the motor’s bell housing and the shaft rotate together.  The interior of the bell housing has evenly spaced rare earth magnets attached.  The center part of the motor, which includes the motor mount, has arms that are wrapped in copper wire wound in a specific manner.  When the copper wires are energized by the ESC they act as electromagnets which switch on and off rapidly, interacting with the magnets creating motion.  They come in many shapes and sizes.

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The wider the bell, the more torque the motor will have.  In the case of multi-rotors which will be carrying payloads, a wider bell with more torque is preferred.  This gives the multi-rotor increased lift capacity.  The motors are rated by their size in millimeters and by a kv rating system.  Kv stands for revolutions per volt and in a perfect world if you had a 1000 kv motor you would get 1000 revolutions per volt.  The kv number to torque ratio has an inverse relationship, in that the larger a motor is and the more torque it has, the lower its kv rating will be.  Conversely, smaller motors with less torque but more revolutions per minute will have higher kv ratings.  A good starting point for a quad copter with the capacity to carry a medium sized camera would be a motor with a kv rating between 700 and 1000.

Motors are also rated by amperage draw.  This is more of a linear relationship in that the greater the torque and thrust and the larger the propeller the more amps the motor will draw under load.  Always make sure that the ESC units you use are rated to sustain the amount of amps your motor and propeller combination will draw.  Failure to remember this will result in a crashed multi-rotor.

Flight Control Units

Flight Control Units go by many names and vary widely in pricing and features.  The FCU is what controls all of the attached ESC units, which in turn control the motor’s revolutions per minute, thus making controlled level flight possible.

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But that is only scratching the surface of what they can do.  Their prices are based on their features and you can get good FCU for prices starting around $15 and ranging up into the thousands.  Which one is right for you depends entirely on what type of mission you are trying to accomplish with your multi-rotor.  Even the least expensive FCU will feature integrated 3 axis gyros.  The FCU software interpretation of the input data from the gyros is what allows the board to have a type of spatial awareness.  It will in turn use this information to control power output to each motor.  It does this continuously in a loop allowing for stable flight of a multi-rotor aircraft.

Mid range Flight Control Units will usually incorporate additional sensors.  These usually include a GPS unit with antenna and a barometer.  The GPS unit adds geospatial awareness to the FCU.  In this way the unit can incorporate its GPS position, speed, heading, and altitude data.  This increases the accuracy and stability of the multi-rotor.  One of the down sides with GPS data is the fact that it is very inaccurate, especially in regards to altitude.  GPS has a tendency to wander both horizontally and vertically.  Hence the introduction of the barometer.  When the FCU is armed before take off the barometer on the board will take a reading.  That reading becomes 0 ft. altitude.  As the multi-rotor lifts off, the values sampled by the barometer change and the FCU can use this data in addition to the GPS data to make very accurate altitude predictions.  On average the altitude prediction will be within the range of plus or minus 1 ft.

Higher end Flight Control Units add a variety of additional features.  These include digital compasses and software suites for modifying settings and planning way point based missions.  Missions which the multi-rotor can fly from take off to landing completely autonomously.

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The ability to stream commands and real time telemetry data between your laptop to your multi-rotor is also possible.  There are many different possibilities and many different controllers for many tasks.  Figure out what you want to do and shop around.

Some popular and cost effective boards are:

It is important to read as much about these boards as you can find.  It will make finding the right board for your application much easier.  Probably the best repository for all things remote controlled is RcGroups.  The RC Groups forum contains a wealth of knowledge and helpful people.  You will be able to find highly detailed threads on every one of the Flight Control Boards and much more.

For the purposes of this project I chose to use the ArduPilot Mega 2.5 from DIYdrones.  It has a robust feature set, a large active community and the software is created and maintained by that community.  Because of this world wide cooperation the APM 2.5 performs on a level that matches that of controllers which cost ten times its price.

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The Transmitter and Receiver

Arguably the most important part of the system is the transmitter and receiver.  This system is how your physical inputs become motion with the multi-rotor.  The better the system the further your multi-rotor can go.  For this project and all other RC related projects I use a modified version of the Turnigy 9x which is an inexpensive, fully featured RC controller which runs on the 2.4 GHz radio band.

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The modifications I have done include adding the custom thumb stick ends, a back light for the LCD screen, and aftermarket transmitter module, a high gain panel antenna, and a LiFe battery.  Below you can see the upgraded module.

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This allows me to use custom antennas and much more reliable receivers which feature telemetry data.  I also use a custom firmware (operating system) on the transmitter called Er9x.  It expands the transmitter model memory and makes complex configurations much easier.  Many people who have been in the RC world for years swear by this setup.  It is very cost effective as well; my radio with all the modifications cost around $100 dollars.  Other radios with the same feature set can cost $400 dollars and up into the thousands.  Here is an example of a receiver for this system.

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The majority of transmitters and receivers run on two different frequencies – 2.4 GHz and 433 MHz.  Both frequencies have their advantages and disadvantages and for the most part 2.4 GHz is used for short range and 433 MHz for long range.  2.4 GHz users can expect ranges with stock transmitters of about 3 to 5 kilometers.  People who use the 433 MHz band with stock transmitters can achieve distances of up to 20 kilometers.  The vast disparity in range comes down to wave lengths.  The higher the frequency the shorter the wave length the more likely it is the signal will bounce off of an object rather than permeate it.  The lower the frequency the longer the wavelength, the farther it will travel at the same transmission power and the more likely it will be to penetrate solid objects.  The systems are also dramatically different in price, with the long range systems costing 3 to 10 times more.  For the purposes of this project I will be using the 2.4 GHz system because I wont usually be flying beyond the line of sight and it is more cost effective.

Propeller and Motor Balancing

One of the biggest hindrances to the success of any multi-rotor system is unwanted vibration.  Vibration interferes with the delicate sensors on your controller board and can lead to unpredictable and possibly dangerous behavior from your multi-rotor.  In addition it makes for bad video and photos; this is especially the case when using cameras such as the gopro or any other camera which uses a CMOS sensor.  CMOS sensors write image data one pixel at a time starting from the top left of the sensor and continuing across in rows until the entire image is captured.  This is done on average about 30 times per second and hence is not visible to the naked eye.  Under standard conditions CMOS sensors can achieve beautiful and fluid video and photos.  But when vibration is introduced to the cameras it leads to a “Jello” effect on the video that makes the video unusable.

Below is an example of “Jello” video.

Vibrations also cause unnecessary wear and tear on your equipment and can loosen components.  So it is very important to minimize the causes and effects of vibration on your multi-rotor.

There are a couple factors that lead to excessive vibration and they are mostly motor and propeller related.  The first of these factors we will be looking into is motor vibration.  Most beginners in multi-rotor assume that the motors that they buy from their local retailer come pre-balanced.  I have in my experience found that more often than not that is not the case.  This has even been true with motors that are advertised as balanced and were more expensive than similar models.  So our first step in removing vibration from our multi-rotor will be balancing our motors.

The most efficient way I have found to do this is with the motors already mounted on the multi-rotor frame with the ESCs connected either to a controller board or to a servo tester that you can use to control the motors speed.  You will also need some way to detect and visualize the vibrations.  This is where your smart phone comes in handy.  Because most modern cell phones have three access accelerometers built in to them there are free software solutions that will help you quantify the vibrations much like a seismograph.  I have an android base phone and I use Seismos which is available in the Google Play store.  If you have an Iphone there are many solutions available to you as well.

Step 1 The Rig

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As you can see, we have assembled all of the aforementioned components into a rig that makes it easy to run the motor at varying speeds and determine how much vibration is being produced.

The first step is to use the servo controller to slowly run the motor up to full speed while running Seismos on your smartphone.  You will notice that there is not a direct linear relationship between the speed of the motor and the vibration produced.  In fact it varies by RPM and every motor will be different.  What is important is running the motor up and down several times until you have a good idea, on average, of how much vibration are being produced by watching Seismos and keeping an idea in your head.

Step 2

Look at the motor from the top down and create a cross section in your mind.  I usually split the motor into quadrants.

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The next steps are trial and error.  You need to cut a small square of electrical tape about the height of the bell of the motor.  Then choose one of your quadrants and apply the tape to that side.  In the photo above that would be where one of the lines intersects the motor bell.  Also as shown below.

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Once you have your tape applied, run the motor up to speed again and see if the vibration has decreased.  If it has not, move the tape to another quadrant and test it again.  Continue in this manner until you have found a location for the tape that decreases the vibrations the most.  When you find the best spot, you have basically found a place on the motor bell that is lighter than the rest.  You might also want to try adding another piece of tape on top of the one you already have applied just to see if the added weight decreases vibration even further.

Once you have done this for all of your motors you are ready to move on to the next task which is balancing your propellers.  There are different sorts of do-it-yourself prop balancing methods out there but to get the best results you need a proper propeller balancing tool such as the Top Flight Propeller Balancer.

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This type of propeller balancer gives you the best results.  This is because the shaft that you mount the propeller to is suspended between two magnets.  This allows the propeller to move back and forth with almost no friction.

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Once you have placed the propeller in the balancer, wait until the propeller has settled and stopped moving.  If the propeller is vertical that means that the blade that is at the bottom is heavier than the blade at the top.  In order to fix this we need to either remove material from the bottom blade or add material to the top.  I use the subtractive method and will remove the propeller from the balancer and lightly sand the back side of the bottom blade.

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I will then place the propeller back in the balancer and I will continue this process until the propeller is balanced.  You will know your propeller is balanced when no matter what position you place the propeller in the balancer the propeller will stay in that position and not move.

Very often it is possible to get the blades balanced but the propeller will still not be.  This is because the hub of the prop may also be unbalanced.  If your propeller will rest in the horizontal position parallel to the table surface then your blades are most likely balanced.  To determine which side of the hub is heavier you can place the propeller vertically in the balancer and then let it go.  Generally which ever side of the hub comes to rest at the bottom is the hub that needs some material removed.

This is a process which can be quite tedious and requires patience and practice to get good at it.  It is, however, of paramount importance as you build and fly your multi-rotor – this cannot be overstated.  Spend a lot of time balancing your motors and props.