Multirotor DIY

Like many other segments of the RC Hobby, after building a few kits, some people like to try designing and building their own models from scratch. Modelers have been doing this for decades with fixed wing aircraft by scratch building from plans, building their own designs or kit-bashing and modifying existing designs. When it comes to helicopters, because of the complex mechanics that are used in them, unless you own, or have access to, a machine shop, it is pretty tough to design and build your own models. Multirotors on the other hand, are more like aircraft in this respect, and do offer modelers the option of quickly and easily designing and building their own multirotor aircraft. For modelers on a tight budget, scratch building can often be a good way to get into multirotors for a minimum investment. In this installment of Multirotor Flight, we will take a look at some of the different options that are available, as well as some general rules of thumb for designing and building your very own multirotor aircraft.

Virtually all multirotors share the same common group of electrical components. These components include a flight controller board, a battery, motors, speed controllers and props. The multirotor frame basically serves as a way to mount these components together in a specific alignment, with the motors spaced far enough apart so the props can rotate freely. This basic arrangement can take the form of a Tri-copter, Quad-copter or Hex-copter platform, and each type has its specific benefits. The most common and simplest of these configurations is the Quad-copter platform, so we will focus our attention on that design for now.

The cool thing about multirotors is that you can use virtually anything to build the frame, as long as it is light enough to be carried by the thrust of the propellers. People have actually taken 4 motors and mounted them to the corners of a Fed-Ex shipping box and built a multirotor from it! Obviously this is not a very efficient design, because over 1/4 of the prop arc on each motor is blocked by the corners of the box, but it does show how simple a quad can be built.

Figure 1
Figure 1
Figure 2
Figure 2
Figure 3
Figure 3

When considering quad frames, there are basically three different types, the X-style, where all the motors are spaced equally from a central body as shown in Figure 1, a Spider style, which is shown in Figure 2, and an H-style, which is shown in Figure 3. The Spider style frame is most commonly used for FPV or aerial photography, since this design provides a way to mount a camera out ahead of the front props to eliminate them from the view of the camera. The commonality of all three of these designs is that the four motors basically form a square, with the electronics mounted somewhere in the middle.

Figure 4
Figure 4

The frame itself can be constructed from a wide variety of materials including wood, plastic, metal, fiberglass, carbon fiber or any combination of these materials. An extremely simple and inexpensive frame can be constructed by taking a wooden yard stick from a home improvement store, cutting it in half, and epoxying the two pieces together into an X shape. The motors can be attached to the ends of the arms with wood screws, and the remaining electronics can be mounted with hook and loop fastener or double sided servo tape. Other wood designs can be constructed from 3/8” to 1/2” square pine or spruce stock combined with 1/8” plywood for motor mounts and center plates. Figure 4 shows an example of this type of frame with a small plastic food storage box added to the center to house all of the electronics.

Figure 5
Figure 5

Lightweight metal is also a good material to make rugged, easy to assemble frames. Most home improvement stores carry 3 foot, 4 foot and 8 foot lengths of square aluminum tubing in several sizes, and this can be used to assemble multirotor frames. Center plates and motor mounts can be fabricated from either sheet aluminum or fiberglass G-10 or FR4 material, and everything can be held together with sheet metal screws or pop rivets. These types of frames are extremely crash resistant, and are easy to fabricate with common hand tools. Figure 5 shows an example of a typical metal quad-copter frame.

Once you have your frame materials picked out, the actual design is only limited by your imagination, but remember, for best operation you do want the motors to end up in as close to a square pattern as possible. Once you pick a frame shape, you need to decide on a frame size. For multirotors, the frame size is usually listed as the center to center distance in millimeters between two motors on opposite corners of the frame. Most commercially available flight controller boards are designed to work best with mid-size multirotors with a frame size of between 400mm and 800mm. For a first try at scratch building, something in the 500mm to 600mm size is a great place to start.

Once you decide on a frame size, the next thing you want to do is select the proper motors for your quad. In any quad design, you want to make sure that you have enough power to fly properly and efficiently. In order for any multirotor to fly well, you need to have a thrust to weight ratio of at least 2 to 1. If your machine weighs 40 ounces ready to fly, including batteries and all payloads, then you want to make sure that the combined thrust of all the motors at full throttle is at least 80 ounces. At this point you may ask, “How do I know how much thrust my motor will produce?” If you purchase good name-brand motors that have published propeller data charts, this is easy. You simply look up a motor and prop combo, and see how much thrust it produces. If you choose motors with no test data from the manufacturer, then you have to take an educated guess as to the thrust they will produce.

When picking out motors, low Kv models that spin larger props are always more efficient and quieter than higher Kv motors spinning smaller props. For the best thrust efficiency, you should use the largest prop your motors can safely run. Most mid-sized multirotors run well on 3-cell Li-Po batteries, and motors with Kv values in the 800 to 1000 RPM/volt range do very well on 3 cells. If you want to use a 4-cell Li-Po battery, then motors with a Kv value in the 650 to 800 range would work best.

Here are a couple examples of how much thrust you can get with different size motors. Running on 3 Li-Po cells, a motor with a stator size of 2212 or 2213 and a Kv value of around 950 will produce 24 ounces of thrust from a 9×4.7 prop, and would work on a quad that weighs up to 48 ounces. A 2215 to 2217 size motor with a Kv of around 950 will produce around 35 ounces of thrust from a 10×4.7 prop, and would work on a quad that weighs up to 70 ounces. It is OK to have greater than a 2 to 1 thrust to weight ratio on your model, it will just result in longer flight times due to more efficient operation, but do not take this to extremes. If you get much more than a 3 to 1 thrust to weight ratio, the extra available power can result in difficulty in tuning the PID settings or gain settings in your flight controller board due to the high amount of thrust that is available.

To calculate how much power you need for flying a multirotor, a good rule of thumb is 60 watts per pound to maintain a stable hover. This lets you know approximately how much total power you will need, and gives a starting point for picking out motors and calculating flight times. If you have a machine that weighs 3 pounds, ready to fly with batteries, then you will need approximately 180 watts of power in a hover. If you have 4 motors on your multirotor, then each one will be producing 180/4 or 45 watts of power. If you are running a 3-cell battery pack, which produces around 11.1 volts under load, then each motor will pull 45/11.1 or 4.05 amps of current.

The 60 watts per pound rule can also be used to calculate flight times. In this example, 180 watts divided by 11.1 volts gives a total current draw of 16.2 amps and this value can be used to get a rough flight time calculation based on the size of your battery pack. For the sake of this example, let’s assume that you have a 3-cell 3300mah battery pack for your model. Since a 3300mah battery can also be called a 3.3 Ah battery, a 16.2 amp current draw is equal to a discharge rate of 16.2/3.3 or 4.9C. From earlier columns you may remember that flight time can be calculated by taking 60 minutes, and dividing that by the C-rate of discharge. Using this formula, 60/4.9 is equal to 12.25 minutes. This formula assumes full discharge of the pack, which you really never want to do. You should always leave 20% of the energy in a battery pack at the end of each flight, so if you take 80% of the calculated flight time of 12.25 minutes you get 9.8 minutes of flight time from the 3300mah 3-cell Li-Po battery.

Once the motors and props have been selected, the next item to pick is the speed controller. All of the speed controllers in a multirotor should be identical to one another. They should be the same brand, the same size and have the same firmware. Using different brand speed controllers mixed together can make it very hard for the flight controller to do its job properly. When sizing speed controllers for multirotors, it is a good idea to have the speed controllers be able to handle double the full throttle current of your motors and props that you are using. If you have motors that draw 9 amps at full throttle, use 20 amp speed controllers. If you have motors that pull 14 amps at full throttle, then use 30 amp speed controllers.

There are several reasons for over-sizing the speed controllers. First and foremost is reliability. If a speed controller never sees more than half of its rated current, the chance of having an in-flight failure is practically zero. In multirotors, especially quads, the loss of power to one motor will always result in a crash, so you want to make sure that the motors stay running! Another big reason to over-size your speed controllers is cooling. In many cases, the speed controllers are mounted in the center of the frame, often under a cover or inside a radio equipment box where they get little or no air flow for cooling. If you run the speed controllers at less than 50% power at all times, they will run cool enough to be able to operate with little or no airflow across them.

Finally, the last piece of equipment you will need for your project is the flight controller board. This is where the do-it-yourself idea can be taken to extremes if desired. If a modeler is adept in soldering and building electronic circuits, then a flight controller board can also be fabricated from scratch by starting with an Arduino board, or other microprocessor development board, and adding all the sensors themselves. Going this route would only be for the truly hard-core electronics buff, sine it does require a completely different set of skills and electronic knowledge to pull off successfully. For most people, buying an off the shelf, fully assembled Flight Controller is really the only way to go.

Figure 6
Figure 6

When picking a Flight Controller, there are a lot of options available over a wide range of prices. As it is with most things, you get what you pay for, and this is especially true when it comes to flight controllers. A quick look at eBay or overseas online stores will uncover dozens of different Multi-Wii based controller board clones, some for under $20.00. Figure 6 shows a typical Multi-Wii type flight controller board that is available for about $35.00 on line. This is an area where spending a little extra money will pay for itself in the long run. Many of the cheaper boards do not come with software pre-loaded, and require some knowledge of computer coding in order to get them to operate correctly. Many have very basic computer interfaces, or rely on interfaces from open source projects to operate and program the controller boards. Some of the higher priced flight controllers, such as the DJI Naza series, come with all the software pre-loaded and feature a polished, easy to use computer interface that makes setting up the board a very simple procedure. For first time scratch builders, you should probably stick with a board that you are already experienced with that already works reliably, since that is one less thing that you will have to worry about when setting up and flying your model.

Whether you are on a tight budget, and want to build a multirotor as cheaply as possible, or have some radical new design that you want to build and see if it will actually fly, scratch building can be a very fun process, and offer a new challenge for multirotor pilots. By taking the time to figure out a few basic design rules, a successful project can be easily realized. One of the coolest things about scratch building is the sense of pride that you have at the flying field when someone asks you, “What multirotor kit is that?”, to which you can proudly reply, “It is not a kit, it is my own design!”

Multirotors and Cameras

Once pilots get familiar with flying their multirotor aircraft, they look for interesting new things to do with them. Many times, they will carry some type of camera up to get an eye in the sky perspective. Whether this is done for aerial photography, FPV flights, movie making or any other reason, multirotors can be a very effective tool for getting a shot that would normally require a full size aircraft or helicopter. In this installment of Multirotor Flight, we will take a look at some of the various types of cameras and camera gimbals that are currently being used on Multirotors, as well as some of the differences between the various items.

With a camera on a multirotor you can capture a lot of interesting shots. These include aerial views of a home or property, overhead images of sporting events or to get an in-cockpit FPV view while you fly. There are an infinite number of ways to put a camera on a multirotor, but for the sake of this article, we will look at some of the most common things that are being done right now. To begin, let’s take a look at some of the popular cameras that available and suitable for use on multirotors.

Figure 1
Figure 1

The first type is the smaller sport action cameras that are very popular today. One of the first models to come to mind in this type is the ubiquitous “GoPro” series of cameras. These little movie-making gems have been around for several years now and have proven themselves through millions of hours of videos. The HD version of this camera was first released in January of 2010 with a 5MP image sensor. This model was followed up in October of 2011 with the improved HD Hero2, which had a higher resolution 11MP sensor in it. The latest version of this camera, the HD Hero3, has a 12 MP sensor, and is available in 3 different models, the White, Silver and Black Editions. The top of the line Black Edition can take video at 240 frames per second, for super slow motion shots, or take regular speed video at up to 4096 x 2160 pixel resolution. These newest versions even include a long range Wi-Fi wireless remote that allow you to start and stop video, or take still shots, from up to 600 feet away! Figure 1 shows this newest addition to the GoPro lineup.

Figure 2
Figure 2

Other miniature sports cameras that are commonly used include the Contour HD series, the Sony Action Cam, the Boscam HD19 and Drift HD series of cameras. All of these cameras are small and light weight, typically 4 ounces or less, and can be directly hard-mounted to the frame of a multirotor. These cameras are priced in the $150 to $400 range, and come with a variety of mounting brackets making it easy to attach the camera to a multirotor frame. Figure 2 shows, clockwise from the top right, several different cameras from Contour, Sony, Boscam and Drift.

Figure-3

The one drawback to any of these types of cameras is that they have an extremely wide field of view, up to 170 degrees on some models, so you essentially see everything from straight up to straight down, and from straight left to straight right, all at the same time. This wide angle of view does introduce some curvature distortion into the picture, especially around the top and bottom of the image. The photo in Figure 3 shows these curvature distortions. The horizon in the center of the photo is straight, while the power lines in the top of the photo bend down on the ends and the railing in the bottom of the image curve up on the ends.

The wide angle of view on these cameras can introduce other problems as well. A common issue is being able to see the spinning propellers in the corners of the video when filming from a multirotor. Unless the camera is mounted out in front of the props, you will always see a little bit of prop in your videos. For some modelers, this is OK, but if you intend on using the video for commercial purposes, the props can ruin the shot. Most of these style cameras have the option to crop the image to a lower resolution, and limit the field of view of the camera down to 120 degrees or even 90 degrees. They do this by just recording the center portion of the image and throwing away the part around the edges. While this can get rid of the props in the edges of the shot, they resulting video is usually a lower resolution image, typically 1280 x 720 pixels instead of 1920 x 1080 pixels. For some this is acceptable, but for others it is not an option.

Moving up in size, the next type of camera commonly used are the mid-size DSLR models. These cameras have much larger image sensors than the ones found in the small action cameras mentioned earlier, so you can get a much clearer, well defined image from them. Most of the smaller action cameras have an image sensor that is only about 6 x 4.5mm in size while most of the mid-size DSLR cameras have an APS-C size sensor, which typically measures about 23 x 15mm. This gives about 14 times more surface area on the image sensor, and this can capture more light for better photos.

Figure 4
Figure 4

Several cameras that have become very popular in this size are the Sony NEX-5 and NEX-7 series, the Panasonic Gh3, the Nikon D7000 and Canon Rebel series including the T2i, T3i and T4i models. Figure 4 shows the Sony NEX-5 camera, which is very popular due to its small body size. These types of cameras feature removable lenses that allow the user can select the best one for the job, depending on the type of shot they need. Since these cameras can all shoot in both still photo and video modes, you can use them to take excellent quality HD video or still shots with 16 to 24 MP resolution, depending on the camera image sensor. These cameras are priced in the $600 to $1500 range, and typically weigh between 12 and 24 ounces, depending on the camera body and lens used. Because of their larger size and weight, these cameras are best suited for larger multirotors in the 650 to 800mm size range.

Figure 5
Figure 5

Going up another size, we come to the full frame DSLR cameras. These cameras have large image sensors that usually equal in size to a 35mm film camera negative, roughly 36 by 24mm, so they are capable of capturing very clear images for both video and still photos. Figure 5 compares the size of full frame sensors to those found in the mid-size DSLR and sports cameras. Here you can see that the surface area of the full frame sensor more than double the area of an APS-C sensor, and 34 times more area than the sensor used in a GoPro camera.

Figure-6

Two of the more popular cameras in this size range are the Canon 5D or the Nikon D4. Figure 6 shows the Canon 5D camera, which is typical of this type. These cameras feature metal bodies, and take larger full size lenses, so they are quite a bit heavier. Depending on the lens used, these cameras weigh between 2.5 and 4 pounds, so you do need a larger multirotor to carry them. Typically, 800mm to 1000mm size hex or octo frames are used for carrying this size camera. These full frame DSLR’s can also get fairly expensive, costing upwards of $5000 for just the camera body alone. Because if this, you will want to make sure that you have an extremely reliable multirotor setup before you risk putting one of these large cameras up in the air!

Moving up from the full frame DSLR cameras are the full cinema grade cameras such as the Canon C300 or Red’s Epic and Scarlett models. The price tag on these cameras, with a matching lens, can top $40,000, so they are typically used only by professional motion picture companies with larger custom built multirotors up to 1500mm in size.

Now that we have looked at several types of cameras, let’s see how to attach them to your multirotor. There are many ways to mount a camera to a multirotor, but it breaks down to two basic types. You can either hard mount the camera in a fixed position to the frame, or mount the camera in some type of gimbal mount, which allows the camera to be moved in flight relative to the airframe. The smaller action cameras, such as the GoPro series, are often hard-mounted to the frame of a multirotor with the mounting kit that comes with the camera. Most of these mounts have a square of double stick adhesive on the base that allows the user to simply stick the camera wherever they need to on the multirotor frame. Most of the camera mounts have the ability to tip the camera up and down a little bit once mounted, but once the angle is set it is fixed for the flight. This does limit the types of shots you get, and as you maneuver the multirotor, the image will tilt and roll with the airframe as the multirotor flies.

To eliminate these issues, and to stabilize the camera, some type of gimbal assembly can be used. A gimbal allows the camera to move with respect to the multirotors airframe, so you can vary the shot without having to actually move the multirotor. This comes in very handy if you want to park your multirotor in one spot using GPS hold, and then be able to pan across a scene in a video. In many cases the gimbal can also be gyro stabilized, so as the multirotor pitches and banks while it flies, the camera stays level with respect to the ground at all times for a level image. Most gimbals have two directions of movement, typically pitch and roll. For yaw control you can simply rotate the entire multirotor. For more control of a shot, some gimbals can be controlled and stabilized in all three flight axes, pitch, roll and yaw.

Figure 7
Figure 7

There are 2 basic types of controls for gimbals, servo driven systems and direct drive brushless motor systems. The servo driven gimbals are typically designed for smaller cameras such as the GoPro and small point and shoot digital cameras. With high quality servos, these gimbals can be set up to operate smoothly, but it is tough to get rid of all the play in the linkages so there can be a little camera vibration. The servos also take a finite amount of time to respond to changes in attitude, so there can be a small amount of lag in the control loop. These gimbals are good for simple camera systems, and can be found in the $100 to $300 range, including servos. When set up properly, these simple gimbals can provide good results for basic shots. Figure 7 shows a simple servo driven 2-axis gimbal.

Brushless direct-drive gimbals are a more recent development, and there have been a large number of them introduced recently. Because the brushless motors that drive the gimbal are directly connected to the camera mounts, the response of these gimbals is virtually instantaneous and very fluid. When properly set up, the multirotor can make violent changes in pitch and roll, but the view from the camera remains unchanged. DJI has several brushless gimbals available that are popular. Their Zenmuse gimbals are available for several different midsize DSLR cameras including the Sony NEX-5 and NEX-7, as well as the Panasonic Gh2. These gimbals work exceptionally well, but they are a bit on the expensive side with a $3,500 retail price tag. DJI recently released a new brushless gimbal for the GoPro Hero cameras that allows them to be attached to their smaller RTF Phantom quadcopter, or any other smaller size multirotor frame. The retail price of this gimbal is $699, and can be seen in Figure 8. There are many other brushless gimbals that are currently in development, and like everything else, as more manufacturers make these items, the prices of them will come down.

Figure 8
Figure 8

Another type of camera used on multirotors are the FPV systems. These have been around for quite a while and are extremely popular for getting a “from the cockpit” view as you fly your multirotor. Many of the smaller sport action cameras have the ability to record HD video and simultaneously send out a lower resolution live video stream that can be broadcasted back to the pilot via a telemetry transmitter, so they can double as an FPV camera. Back on the ground, the pilot can see the video from the camera in several different ways including small video monitors attached directly to the transmitter, larger video monitors set up for multiple people to view, or in video goggles that are directly worn over the pilots eyes while flying. This type of flying give a very interesting perspective, as if you are sitting in the aircraft while you fly it, so if you ever get the chance to go on an “FPV ride” with someone’s model, be sure to give it a try. One word of caution though, if you have never done this before, it is best to sit down in a chair for your first FPV ride. When you lose your normal view of the outside world while wearing FPV goggles, the banking of the view can trick your mind into thinking that you are falling over with some rather embarrassing results!

Cameras can add an entire new perspective to your multirotor experience, and provide some interesting new ways to use your model. If you have not tried FPV flying or filming from your multirotor, be sure to give it a try. It adds an entire new dimension to your flying experience.

Multi-Rotor Efficiency

One topic that seems to be on the mind of every Multirotor pilot is efficiency. One of the most commonly asked questions is “How long can I fly per charge” or “How big of a battery do I need to fly for 15 minutes.” In this installment of Multirotor Flight, we will look at how to optimize the efficiency of your multirotor, and how to calculate flight times from a given battery size.

There are several key components that contribute to the efficiency of multirotors, but the most important ones are motor efficiency, prop efficiency and overall weight. The weight part of the equation is easy, the lighter the craft is, the longer it will fly. No real mystery there, a lighter machine takes less energy to keep aloft, and will draw less power from the batteries, so naturally flight times will be longer. The other two components, Motor efficiency and Prop efficiency are a bit more detailed, so we will spend more time looking at them.

Motor efficiency can be looked at as the ratio of the amount of power you get out of a motor versus the amount of power that you put into the motor. Every motor has internal losses, and these are due to several causes. First are the frictional losses, which result from drag in the motor bearings and the drag caused by the air as it is pulled through the motor by the cooling fan or cooling holes. In a modern motor with ball bearings these losses are small, but do account for some of the motor’s inefficiency.

Next are the heat losses, which show up in the form of resistive, or I-Squared-R losses, and hysteresis losses in the core of the motor. Whenever you run current through a wire there will be heat losses. Copper is an excellent conductor of electricity, but it does have some resistance. The number watts of power lost to heat in a motor can be calculated by taking the number of amps of current running through the motor, multiplying that value by itself, and then multiplying the result by the resistance of the motor windings. Mathematically this can be expressed as I x I x R and is commonly called I-Squared-R losses. If you have a motor that is running 20 amps of current through it, and the resistance, or Rm value, of the motor is 0.1 ohms, the power lost in the motor is equal to 20 x 20 x .1 or 40 watts. One thing to notice is that this value is independent of voltage. If you are running a motor on 3 cells, 4 cells or 5 cells, and you prop the motor so that the current is the same in all 3 cases, the heat loss in the motor will be 40 watts, regardless of the voltage. In the case of 3 Li-Po cells, if you have 12 volts at 20 amps you are pushing 240 watts of power into the motor. With 40 watts of energy lost to heat, the heat losses are 40/240 or 16.7% of the total power. If you use 4 cells and have 16 volts, then you are putting in 16 x 20 or 320 watts of power. In this case, the losses to heat are 40/320 or only 12.5%. If you go up to 5 cells, then the total power into the motor is 20 x 20 or 400 watts, and the 40 watts of heat loss only represents 10% of the total input. That is why higher voltage set-ups are more efficient than lower voltage set-ups.

Figure 1
Figure 1

Hysteresis loss is the energy dissipated in the stator core of the motor. The coils of wire in a motor turn the sections of the stator into alternating electro-magnets that go back and forth from a positive charge to a negative charge, over and over again. Ideally, when the current stops flowing, the stator pole should go to a completely neutral charge. In actual practice however, there is a small amount of residual magnetic force that stays in the stator and this must be overcome in the next transition. This causes heating in the core of the motor, and contributes to the overall losses. As the motor warms up during use, this heat also causes the resistance value of the copper wire to go up, so the I-Squared-R losses go up as well. Larger motors that are being run at a fraction of their full capability will run cooler and be more efficient. At this point you are probably wondering “What can be done about these losses?”

The best answer is to buy better quality motors. Brushless motors vary widely in the materials used, and in the quality of construction. Cheap motors will use thicker stator plates and cheaper grades of magnets, and this will reduce the overall efficiency. By using thinner stator plates, the hysteresis losses can be greatly reduced. Figure 1 shows close-up views two different stators. The one on the left has stator plates that are 0.5mm thick, and is fairly inefficient. The stator on the right has stator plates that are only 0.2mm thick and have a lot higher efficiency. Some of the worst quality motors out there will have an overall efficiency of as little as 65%, and some of the top quality premium motors can have efficiencies as high as 90% . In the cheap motor, 35% of the power you put into the motor gets wasted as heat, and in the high quality motor only 10% of the energy goes to waste heat. In real world numbers, if the good motors got you 9 minutes of flight from a charge, the cheap motors would only get you 6.5 minutes of flight. That is a huge difference!

Figure 2
Figure 2

The next big thing for efficiency is the propeller. The propellers job is to convert the rotational energy of the motor into thrust, and then use this thrust to lift the multirotor off the ground. With all propellers, the maximum efficiency point occurs at relatively low thrust, around 10% of the thrust that is available at full throttle. The efficiency of the prop drops off in a fairly linear manner as the thrust increases to full power. Figure 2 shows a typical efficiency curve for several props on a brushless motor. From this graph you can see how much the efficiency of a propeller varies over the throttle range of a motor. In this example, the best prop reaches an efficiency level of a little over 12 grams of thrust per watt of input power at 20% throttle and falls of to only 5 grams of thrust per watt at full throttle. At 50% power, which is where a properly set up multirotor spends most of its time, the thrust efficiency is around 9 grams per watt.

What this graph shows you is that as a prop spins faster and faster, its thrust efficiency goes down. This is why running a larger prop at a lower RPM on a multirotor is better than running a smaller prop at a higher RPM from an efficiency standpoint. For example, on a smaller machine, you might have a choice of running 10 inch props or 8 inch props. The 10 inch props will probably generate the required thrust at about 40% throttle, and have an efficiency close to 10 grams per watt, while the 8 inch props might have to spin up to 70% throttle to generate the same amount of thrust, and at that speed the prop efficiency will be down to about 7 grams per watt of power.

If you have a multirotor that weighs 1000 grams, and you are running 10 inch props that produce 10 grams of thrust per watt of input power, it will take 100 watts of power to make the craft hover. On the other hand, with the 8 inch props that only produce 7 grams of thrust per watt of power, to get 1000 grams of thrust it would take 143 watts of input power! Again, a huge difference in efficiency just by changing the props you use. There are also props being developed specifically for multirotors now that have blade profiles optimized for hovering flight, and this can increase the prop efficiency even further. Figure 3 shows examples of propellers that are specifically designed for multirotors.

Figure 3
Figure 3

Looking at the two extremes presented here so far, you can see how the flight time you get from a battery can vary dramatically depending on the motors and props that are used. If you took low quality motors and matched them up with the smaller props, you will get a certain amount of flight time. If you took the same multirotor and put on the high quality motors, and matched it with the larger, more efficient props, you can increase your flight time by as much as 50% or more! Now let’s take this newly acquired knowledge about motor and propeller efficiency and put it to good use to calculate actual flight times for a multirotor.

Whenever I make rough calculations about flight times, I like to use a prop efficiency number of 8 grams per watt of input power. This assumes a decent quality motor with an efficiency of around 80% and a properly sized power system that requires 50% throttle to maintain a hover. To properly size your motors, you want to see how much thrust each motor makes at full throttle with the prop you intend to use, and multiply that value by the total number of motors on your multirotor. Then, take half of that number and make sure the weight of your multirotor does not exceed that value. For example, let’s assume that we have a motor that spins a 10×4.7 prop and produces 35 ounces of thrust at full throttle. If we are building a quad-rotor machine we need 4 motors, so if we take 35 ounces times 4, we get a total of 140 ounces of available thrust at full throttle. Based on this thrust, we want our multirotor to weigh no more than 70 ounces, ready to fly, with batteries and any cameras or FPV gear. With these assumptions, we will now go through the step by step procedure for calculating flight time or required battery size for a given multirotor airframe.

In this example our craft will weigh 70 ounces including a 3-cell 5000mah Li-Po battery and a GoPro camera. Converting this weight to grams we take 70 ounces x 28.35 grams per ounce, and get 1985 grams. If we use the prop efficiency number of 8 grams of thrust per watt then we divide 1985 by 8 to get the required number of watts, and this equals 248 watts. Since we are using a 3-cell Li-Po battery we will have around 11.1 volts on average for the flight. To calculate the number of amps we need to pull from the battery you take 248 watts and divide it by 11.1 volts to get 22.3 amps.

Now that we know the current draw we can convert that to the C-rate of discharge. A 5000mah battery can also be called a 5 Amp-Hour battery because there are 1000 milliamps to the amp. If we are pulling 22.3 amps from the battery, and have a 5 Amp-Hour battery, then the C-rate of discharge is 22.3 divided by 5, which equals 4.46. At a discharge rate of 1-C a battery will take 1 hour or 60 minutes to fully discharge. To calculate the number of minutes of flight time, you simply take 60 minutes and divide that by the actual C-rate of discharge. In this example that would be 60 divided by 4.46 which equals roughly 13.5 minutes.

With the calculations we just made, we now know that our 70 ounce quad-copter that is powered with a 3-cell 5000mah battery pack will fly for 13.5 minutes to completely discharge the battery. In actual use however, you never want to fully drain the battery pack. You always want to leave about 20% of the energy in the battery at the end of the flight. Doing this will greatly extend the life of your batteries and prevent a situation where you actually run out of power and fall from the sky. If we abide by this “80% Rule” and leave 20% of the energy in the battery pack at the end of the flight, our actual flight time would be 13.5 minutes times 0.8 which equals 10.8 minutes.

This number gives you a starting point to work from, but your power usage will vary depending on how aggressively you fly. If you are doing aerial photography and fly smoothly in a hover the entire flight, then you will get a nice long flight. If you fly aerobatics the entire flight, looping, rolling and performing high speed passes, this can cut your flight time in half. The calculations above are for estimating purposes only and get you in the ball park for your size multirotor. If you want to find out how much flight time you will actually get for your specific flying style you can use the following method.

First, estimate your flight time based on the calculations shown above and then take about half of that value. If you figure you can hover for 12 minutes, you could use 5 minutes for your test flight. Start with a fully charged battery and set the timer on your transmitter for 5 minutes, or have a friend time you with a stopwatch. Take off, fly around for 5 minutes in the style you typically fly and try to land as close to exactly 5 minutes as you can. If your friend is timing you have him call out 4:30, 4:45, 4:50 and count you down the last 10 seconds to exactly 5 minutes. After you land, shut down your multirotor and unplug the battery. When you re-charge this pack take note of the number of mah of charge you put back into the pack. For example, if you recharge the pack and put back in 1900mah of charge, then you can divide that value by 5 and calculate that you are using an average of 380mah per minute for your typical flying style. If you are running a 5000mah battery, and want to use 80% of that per flight, then you have 4000mah to work with. If you take 4000mah and divide that by your calculated energy consumption of 380mah per minute you get 10.5 minutes as your recommended flight time on that battery. At that point you can set the timer in your transmitter for 10 minutes per flight and virtually never have to worry about over-discharging a battery pack.

Hopefully that helps answer some of the questions about Multirotor efficiency, and allows you to calculate the flight time for any multirotor model. If you have any comments about this column, or have a topic that you would like to see covered in the Multirotor Flight column, you can send an email to the author at lmiller@innov8tivedesigns.com. See you all next time!

Multirotors… Now What?

For the past few months we have been talking about the History of Multirotors, the Physics of Multirotor flight, different multirotor configurations and flight stabilization boards. This basic overview of Multirotors should give the readers enough information to understand how these incredibly interesting aircraft work, and how we are now able to fly them with our standard aircraft and helicopter radios. The big question that a lot of people may ask is, “Now that they are available, what can someone do with one of these Multirotor aircraft?” In this month’s installment of Multirotor Flight, we will look at some of the applications for Multirotors, and how we can benefit from their use.

The first and most obvious use for these craft is the simple joy of flying them. In the same way that we currently fly airplanes and helicopters for the pure joy of flying, Multirotors offer a way to enjoy the RC Hobby in a new and exciting way. Multirotors can provide an excellent transition for fixed-wing pilots that want to try rotary-wing flight, but are intimidated by the complexity and steep learning curve of modern helicopters. For heli pilots, multirotors offer a more relaxing approach to rotary-wing flight that offers a completely new set of flying experiences. With Multirotors now available in a wide variety of sizes, from tiny indoor models that rest in the palm of your hand, to huge outdoor systems that can carry 10 pound cinematography cameras, there is something available to fit any modeler’s needs or budget.

Currently, one of the most popular uses for Multirotor aircraft is aerial photography. Multirotors provide a unique perspective for both still and motion picture photography, and allow the users to get an “Eye in the Sky” view that can be quite amazing in most cases. A lot of videos are popping up on YouTube and other video file sharing sites that show coverage of RC Model events from and aerial perspective, that were shot with a Multirotor aircraft. A wide range of small, light weight sports cameras are now available from companies such as GoPro, Sony, Contour, Drift and others, that can be quickly and easily strapped onto a Multirotor and carried up to get an aerial shot of an event. Figure 1 shows an 825mm Quad model with a Contour HD sports camera attached.

Figure 1
Figure 1

Amateur photographers are using these machines to take aerial photos of parties, sporting events, trips to the sand dunes and scenic vacation sites to name a few. Professional level photographers are now using Multirotors to take photos and videos of weddings, construction site progress, agricultural fields, real estate and land development, accident scenes and nature documentaries. The list goes on and on and is only limited to the imagination of the user. In short, virtually any application that needs to have a view from above that was traditionally done with a helicopter or crane, can now be accomplished with a Multirotor. Figure 2 shows a Y-6 Multirotor with a DSLR camera on a gimbal mount that can be used for capturing high resolution video and still images.

Figure 2
Figure 2

Another use for Multirotor aircraft, that is beginning to see more popularity, is providing aerial surveillance and tactical awareness for police and sheriff departments. There have been several national conferences held this year for organizations such as the National Sheriffs Association, The National Organization of Police Chiefs, AUVSI and others where this very topic is discussed in great detail. In the United States right now, there are approximately 18,000 police agencies, of which only around 300 have aviation units. Helicopters are the vehicle of choice for most of these departments, but the cost of full size helicopters has become difficult to justify for many municipalities. A turbine helicopter can cost over a million dollars to purchase, and ongoing operational costs can be several thousands of dollars per hour. Typically, for the cost of a few hours of time in a full size helicopter, a complete Multirotor system can be purchased, and the on-going operational cost of a Multirotor is typically measured in dollars per hour instead of thousands of dollars per hour for a full size helicopter, so from a cost standpoint, Multirotors can bring the ability to have an aerial view of a situation to virtually any police or sheriff department in the country.

There are so many practical uses for this type of aircraft that it is almost a forgone conclusion that we will all be seeing more Multirotor craft being used by law enforcement personnel in the very near future. Here are a few examples of how Multirotors can be used in every day applications.

  1. Taking aerial photographs of an accident scene to be able to see the entire area as a whole: This is much quicker and easier than tracing tire locations in the street with paint and getting out tape measures to document the scene. Other hazards surrounding the accident scene, which may have been involved in the accident, can also be easily seen in an aerial view of the area.
  1. Finding lost children or older individuals that have wandered off and gotten lost: We have all heard news stories of small children or elderly people with Alzheimer’s that have wandered off and gotten lost, with night time fast approaching, and the need to find the person as quickly as possible before they end up spending a cold night in the woods alone. Having several Multirotor aircraft available with infrared night vision cameras can quickly comb an area and help to find people quickly and direct rescuers to their position.\
  1. Providing a tactical advantage in finding and apprehending fleeing fugitives: Quite often the news will cover a car-jacking or other event that leads to a car chase. In many cases, the driver will flee the scene on foot after crashing the car and hide in a residential neighborhood. Police, with an available Multirotor in the trunk of their car, can quickly get into the air, find the suspect and direct arresting officers to their hiding location, while minimizing the risk to everyone involved.
  1. Assist Fire Departments on scene as they fight fires: There are Fire Departments that are starting to employ Multirotor aircraft to provide a tactical view of structure fires. Fire Captains can get real-time video of the entire scene from above, and be able to direct their firefighter’s resources to the best locations. The Multirotors can also be used to fly up to the windows in high-rise buildings and see if anyone is inside that needs assistance or evacuation.

In addition to Police and Fire Departments, many other organizations can benefit from the use of Multirotor aircraft. Border Patrol agencies can quickly and easily look for people trying to sneak in or out of countries under the cover of darkness. Security companies can use Multirotors to provide perimeter security of sensitive sites or buildings that are of a national interest. They can also be used to patrol major sporting events such as the Superbowl or the Olympics to provide security for the buildings and parking lots. Lifeguards can use Multirotors to patrol beaches to look for swimmers that get swept out in the currents or to alert swimmers of oncoming sharks or other dangers. Power companies can also utilize Multirotors to do inspections of power lines, wind turbines and other parts of the power system infrastructure without the need to have people working closely to high voltages. Figure 3 shows a close-up inspection photo of a high voltage insulator on an electrical tower that was taken with a Multirotor aircraft. These are just a few of the different applications that Multirotors can be used for, and the list goes on and on.

Figure 3
Figure 3

Looking back to our own personal use as modelers, there are even more interesting applications available. At the recent AMA Expo, held in Ontario, California January 11th, 12th and 13th, keynote speaker Chris Anderson, former editor of Wired Magazine and current CEO of DIY Drones and 3D Robotics, gave a talk about the state of the art of modern small Multirotor aircraft and Unmanned Aerial Systems or UAS’s. During his talk, Chris spoke of a new product that they are developing called the Follow-Me Box. This device is designed to be carried by a user, and send their GPS coordinates up to the flight control system in a Multirotor or UAS system. The Multirotor or UAS would then take this information and fly a prescribed distance from the user and follow them wherever they went, filming all of the action from a small sports camera that is attached underneath. The use of this technology opens an entire new area of aerial photography. For example, a surfer could strap the Follow-Me Box to his surf board and go out to hit the waves, while a friend stands by with the Multirotor aircraft on the beach. When the surfer gets out to his location to catch a wave, he can press a button on the box and have the Multirotor take off and fly to a position that was 100 feet above and 100 feet East of wherever he is, or whatever distance and direction he chooses to program into the device. Then, as he surfs back to shore, the Multirotor will stay 100 feet up and 100 feet East of his location, always pointing the camera back towards the surfer. This process could be repeated until the batteries hit a preset critical level, at which time the Multirotor would fly back to shore and auto land in the same spot it took off from. This almost sounds like science fiction, but the ability to do this type of controlled flight is now a reality! Figure 4 shows the type of image that you could capture with this new technology.

Figure 4
Figure 4

This same technology could also be used by people doing nature documentaries or other types of photography. Hikers, bikers, hunters or anyone else that could benefit from having an “Eye in the Sky” perspective of what they were doing can also benefit from this type of technology. Someone that was cruising around the sand dunes on a motorcycle or in a dune buggy could have a Multirotor follow them around everywhere they go and film the action from above. This new technology will open up all kinds of different opportunities for people to explore. In a few years time, you will see people with Multirotors at their son’s or daughter’s soccer game, following the ball around videotaping all of the action that takes place, and then uploading the video to the team webpage so all the parents can see the action. The possibilities are truly endless!

Flight Controllers

In this edition of Multirotor Flight we will take a look at some of the more popular Flight Controller Boards (FCB’s) that are currently available on the market, and compare the features that each one has to offer. As we learned in the last Multirotor Flight column, the FCB is basically the “Brains” of a Multirotor model. It sits in between the radio receivers and speed controllers, and uses an array of sensors to detect the position and attitude of the craft, interpreting inputs from the radio receiver, and converting those inputs into the control signals needed to tell the speed controllers how fast to spin each of the propellers. The variation in thrust that is caused by changing the speeds of the motors is what allows us to control a multirotor model that has fixed pitch propellers.

The main characteristic that sets flight controllers apart from one another is the number of sensors they contain and the number of control moments or Degrees of Freedom (DOF) that the controller is capable of. The simplest flight controllers only have 3-axis gyro sensing, which is accomplished by using either three single axis sensors, a two axis sensor paired with a single axis sensor or a single sensor that is capable of measuring Pitch, Roll and Yaw at the same time. With only gyros for attitude sensing, the craft will hold position in the short term, but over time, it can drift and constantly needs to be flown to maintain a level attitude. The early KK-Multicopter and GAUI GU-344 are examples of these 3-DOF type controller boards. Figure 1 shows one of the early KK-Multikopter boards that used 3 separate single axis gyros for attitude sensing.

Figure 1
Figure 1

The next class of Flight Controller Boards adds accelerometers that can sense changes in movement front to back, side to side and up or down. Accelerometers can also sense the force of gravity, so when the craft is in a stable hover, it knows which way “Down” is. This information can be used to automatically level the craft when the control sticks are released when the FCB is operated in an “Auto-Leveling” or “Stability” mode. These types of FCB’s can sense movement in 6 different directions and are referred to as 6 Degree of Freedom or 6-DOF boards. The OpenPilot CC3D, Hoverfly Sport, and most basic MultiWii boards are examples of this type of controller.

Column-4-Fig-2

The next step up in features for FCB’s typically adds a Magnetometer and a Pressure Sensor to bring the total Degrees of Freedom up to 10. Figure 2 shows a Quadrino Zoom board, which has 10-DOF, with all of the various sensors labeled. The pressure sensor can accurately measure the minute changes in air pressure that occur as you go up and down in altitude, and use this information to accurately hold the craft at a desired altitude. The one downside to using air pressure to calculate altitude is that during a long flight, or if a pressure front comes through the area where you are flying, the barometric pressure can change and this can introduce considerable error into the altitude calculations.

The Magnetometer can sense the magnetic field of the earth and give you compass heading capability. Some of the newer boards and newer firmware options take advantage of this ability to sense compass directions and offer what is called a “Head Free” or “Intelligent Orientation Control” mode. When a multirotor gets too far away from the pilot, it is easy to lose orientation and not know which way the craft is pointing. With a “Head Free” mode, the FCB remembers which way North is when the craft is powered up, and constantly compares this to the orientation of the gyros. The FCB then compensates for this an automatically offsets the controls based on the direction the multirotor is pointing. Normally, if the craft is pointing straight away from you, and you push the control stick forward, the craft goes forward. Push the stick left, and the craft goes left. However, if the craft is in front of you pointing to your left and you give it forward stick, it will go “Forward” from its perspective, but that is actually to your left. Likewise, if you move the stick to the right, the craft will go to its “Right” which is actually away from you. Experienced pilots compensate for this in their head without having to think about it, but for new pilots that are just learning to fly, it can cause serious orientation problems and lead to crashes.

With a Head Free mode, in the above example, if the craft is pointing to your left, and you move the control stick forward, the Magnetometer senses that the craft is rotated 90 degrees from the orientation it originally took off from, and it compensates for this and actually sends a “Right” command to the motors, and from the pilots perspective, the craft goes away from them in the desired direction. This feature can take a lot of workload off a pilot that is trying to hold a position while rotating the craft in different directions. This comes in very handy when you are doing aerial photography and need to point the camera in specific direction while you fly in a different direction. When using this mode there is one important thing to remember, you must always keep the craft in front of you. If you let the craft get behind you and you have to turn around, all of the controls will be backwards! The newest firmware for the Quadrino Zoom, DJI Naza and DIY-Drones APM 2.x series boards all allow this feature.

The top end controller boards will also include a GPS antenna, and this gives these FCB’s the ability to sense and know exactly where they are in space at any point in time. The GPS feature, with the right firmware, can enable the multirotor to accurately hold position with an accuracy of just a few feet. It can also allow the craft to fly a pre-programmed course to specific waypoints, at specific altitudes, pointing in specific directions. Since the GPS system can also accurately record the exact position of the craft just before takeoff, most of these types of FCB’s can also perform an automatic “Return To Home” command and safely land the craft right where it took off from with no input from the pilot. Many of the 10-DOF FCB’s have a GPS module available that can be added to gain GPS functionality. The DJI Naza, DIY-Drones APM 2.x, and the Quadrino Zoom board all have GPS add-ons or options available. Other higher end systems, such as the DJI WooKong controller, come with the GPS antenna included.

Another feature that is very important when using Multirotor Flight Controller Boards is the type of user interface that is available. There is a huge variation in the types of Graphical User Interfaces, or GUI’s, that are available for programming the options on the FCB’s. The most basic interfaces are simple DOS Command Line prompts that only allow the user to import a pre-written version of firmware to the board. All of the changes have to be made to the code itself before it is loaded into the microprocessor on the FCB. These types of interfaces are the most complicated to use, but offer total control of all programming aspects of the craft. These types of interfaces are most commonly used by developers of the FCB or hard core programmers that are familiar with the actual firmware code.

Figure 3
Figure 3

Most multirotor pilots prefer a GUI that allows a simple USB interface to the control board, and the ability to simply click and drag the cursor to make changes to the programming. The MultiWii project has a pretty basic GUI, shown in Figure 3, which allows the user to change PID gain settings, program control switch positions and flight mode assignments. This interface also will stream output from all of the sensors, so you can verify that they are working, as well as give a basic graphical representation of the pitch, roll and yaw of multirotor model.

The DJI products have a nice GUI that also includes built-in instructions for each section of the interface. All you have to do is hover your mouse cursor over a parameter that can be changed and the appropriate instructions pop up on the right side of the display. The transmitter stick calibration screen for the DJI Naza FCB is shown in Figure 4.

Figure 4
Figure 4

One of the better GUI’s that I have seen is the Mission Planner interface for the APM 2.0 and 2.5 boards from DIY Drones. With the GPS add-on installed, when you boot up the board with the interface running, it will give you Google Maps view of exactly where you are. Figure 5 shows the GUI and gives the exact location of the tech lab in my office building on Google maps! It is truly amazing how accurate these new GPS Systems are. Figure 6 shows the configuration screen with all of the various PID setting and options that you can set. Once all the adjustments are made, you simply click the “Write Params” button and all the new variables are loaded into memory.

Figure 5
Figure 5
Column-4-Fig-6
Figure 6

In the end, the FCB that you choose for a specific project depends on the mission of the model, and any future upgrades that you may want to add at a later date. If you are simply flying a multirotor for fun, and just want to tear holes in the sky, any good 6-DOF board will work great. If you want to do FPV work or aerial photography, then one of the 10-DOF boards will provide you with more control over your flight, and if you choose one that has an optional GPS add-on available, you can upgrade it later if you find that you need the ability to do position holds, or want to be able to return-to-home at the flick of a switch. Figure 7 is a chart that gives a brief comparison of several popular Flight Controller Boards that are currently available, and are being used by a large number of people. Depending on your project, one of these will certainly work well for you, and there will be plenty of information on the internet in various forums where you can get questions answered.

Figure-7