Multirotors and the Law

There has been a tremendous amount of new developments since the last installment of Multirotor Flight, so it seemed appropriate to go into some of the new rules, regulations, exemptions and landmark decisions that have been made over the past few weeks.

Thursday September 25^th, 2014 marked the beginning of a new era in aerial cinematography. During a 24 minute long press conference, the FAA granted permission for the motion picture industry to use aerial platforms, such as R/C Helicopters and Multirotors, on closed sets for the purpose of aerial photography. Speaking on behalf of the FAA, Michael Huerta began his statement with the following words. “We recognize the potential unmanned aircraft bring to business, such as surveying, movie making, farming, monitoring pipelines and electric lines, as well as countless other industries. Our challenge at the FAA is to integrate unmanned aircraft into the busiest, most complex airspace system in the world—and to do so while we maintain our mission—protecting the safety of the American people in the air and on the ground. We are taking a reasonable and responsible approach. We are introducing unmanned aircraft into America’s airspace incrementally and with the interest of safety first.

This process opens up a whole new avenue for companies and organizations wishing to safely integrate unmanned aircraft into their business. In addition, it’s a major step forward in our plan for safe and staged integration.”

During the press conference, the FAA announced that it has granted special exemptions under section 333 of the FAA Modernization and Reform Act to 6 companies who have already provided all of the necessary paperwork and have been working on this with the FAA for some time now. In order to be able to operate, these companies will be required to operate under the following set of guidelines.

1. All pilots of the UAS craft must hold a minimum of an FAA Private Pilots License
2. All Aircraft must pass a safety inspection before flight
3. All aircraft must be operated within the guidelines of a Pilots Operational Handbook (POH) that is generated for each specific aircraft
4. All flights must take place in a “Sterile environment” on a closed set away from the general public
5. The aircraft must be operated within the line of sight of the pilot at all times
6. The aircraft must stay below an altitude of 400 feet AGL at all times during the flight
7. All aircraft must maintain maintenance logbooks describing any and all work done to the aircraft
8. All operations must be conducted during daylight hours

The 6 companies that have been granted exemptions by the FAA thus far are: Astraeus Aerial, Aerial MOB. LLC, Pictorvision Inc., HeliVideo Productions, LLC, Snaproll Media, LLC and RC Pro Productions Consulting LLC, dba Vortex Aerial. A seventh company has also submitted paperwork to the FAA and is still waiting for final approval of their revised documents.

In addition to the 6 exemptions that have already been granted, the FAA is currently considering requests from 40 other commercial entities that have expressed an interest in conducting operations under the new guidelines. This is a huge step forward, and just the beginning of an ongoing integration of UAS aircraft into the Federal Airspace System.

Obviously there are mixed reactions within the modeling community regarding the FAA’s ruling in this matter. Some feel that it is a huge victory to have any exemptions granted that will begin the trial period in which the FAA will be able to see how this technology progresses in a controlled manner. Others are upset that the guidelines seem to be extremely restrictive, and this is putting aerial cinematography out of the grasp of the majority of pilots.

The bottom line is that the FAA has an obligation to the general public to insure that the Federal Airspace System is kept free of any and all possible hazards to both public and private air transportation. In order to be able to do that, the FAA needs to make sure that the pilots who are operating the UAS aircraft are both competent pilots, and have a good working knowledge of the Federal Airspace System and all the rules and regulations that govern the operation of aircraft within the system.

According to the latest statistics gathered by the FAA, on average, approximately two million people fly within the Unites States every single day! Each and every one of these people expects to take off, travel for several hours, and then land safely at their destination. It is the job of the FAA and the United States Air Traffic Control System to make sure that this happens each and every day on every single flight.

It is very unsettling when an individual does something with a small UAS aircraft, whether it is a Multirotor, Helicopter or Aircraft, that jeopardizes the safety of this system. We have all heard of people flying multirotors near airports, sporting events, over crowds of people and other places that they simply do not belong. Many people will actually brag on the RC forums how they got their multirotor up to 10,000 feet of altitude during a flight, without any regard to the potential for disaster that they are creating. Everyone needs to be responsible for the safety of other people around them while they fly. All too often, pilots simply do not consider the damage or injury that can be caused by our RC model aircraft should something go wrong.

Figure 1

Figure 2

Figure 3

In general aviation, there are several instances every year of in-flight damages caused by aircraft striking objects in flight. Normally this is caused by something as simple as hitting a bird in flight. The amount of damage that a simple bird strike can do to a general aviation aircraft is mind boggling. Figure 1 shows the result of a relatively small bird impacting the windshield of a private jet. Figure 2 shows the type of damage that a turbine engine can sustain when ingesting a larger bird, such as a Canada Goose during flight. Figure 3 shows the damage that a bird strike caused on the nose of a jet aircraft. The one thing to remember is that all this damage was caused by the relative soft tissue and light-weight hollow bone structure of birds. Imagine the damage that could be caused by a 10 or 15 pound Octo multirotor with a large digital camera hanging underneath it!

This type of interaction between UAS aircraft and passenger aircraft is exactly what the FAA is so concerned about. In addition to the structural damage that could be caused to an aircraft, the secondary damage can be even worse. In the event of a collision like this, you can be guaranteed that the Li-Po battery powering the multirotor will be severely damaged, and will most likely result in a secondary Li-Po fire. In a collision like that shown in Figure 3, this could result in an immediate electrical fire right behind the instrument panel of the aircraft. In a windshield collision, like that shown in Figure 1, you could end up with a Li-Po battery on fire inside the cockpit of the aircraft. In most cases, the damage caused by a bird strike to an aircraft does not cause the aircraft to crash. The pilot is usually able to make a controlled emergency landing at the nearest airport and get the plane and everyone on board down safely.

On the other hand, if any of these accidents were caused by a large multirotor instead of a bird, and the Li-Po batteries on board created an in-flight fire as a result of the collision, the odds of survival go down dramatically. These are the types of things that need to be considered when Multirotors are flown any where near the flight path of general aviation or commercial aircraft. None of us want to see anything like this happen, but all it takes is the careless actions of one individual to create a problem that would devastate both the aviation and modeling communities.

There are of course other considerations to the use of Multirotor aircraft, and these need to be addressed as well. Right now, there is nothing stopping the casual sport flyer from flying a multirotor aircraft for fun and recreation. You can even use the craft to take still and motion pictures from the air, as long as it is not done for profit or commercial purposes. This does not mean that you can, or should, take your newly purchased multirotor down to beach on Labor Day weekend and fly it over crowds of thousands of people. At all times during your flight you should be able to ask yourself, “If I lost power right now, and the multirotor fell straight down out of the sky, is there a potential for someone to get injured?” If the answer to that question is yes, then you should not be flying there under any circumstances.

Many people will say, “The chances of that happening are one in a million, so why worry about it.” That very well may be true, but it is that mindset that separates the true professional aerial cinematographers from the weekend wannabees that do not have safety as a paramount concern.

In addition the concerns of injury and property damage are the concerns for privacy. Nobody wants to be spied on, and some people think that this is exactly what is happening. Recently there was a story in the news about a man who used his DJI Phantom to take some aerial photos of a friend’s house that was under construction. A few minutes into the flight they heard a shotgun go off. Then they heard someone from the neighboring property yell, “Get that Drone off my property!” Shortly after that 3 more shots were fired, one of which struck the craft and caused it to fall out of the sky. The police were called, and the crazy neighbor was hauled off to jail and charged with assault with a deadly weapon. Unfortunately there are people out there that are scared to death of the new multirotor technology, and as operators of these craft, it is extremely important to make sure that no one sees us or our aircraft as a threat.

The truth of the matter is that as a new technology, the use of multirotor aircraft for aerial photography and other commercial applications will be a slow, steady uphill pursuit. It will take some time for the FAA to keep an eye on the use of these aircraft in a very controlled environment, and to log enough flights to establish safety records and common practices that all operators can follow. In the mean time, it is up to the people that do get their aircraft certified for use during the early stages of this new legislation that must maintain constant vigilance to ensure that everything is done “by the book” in a safe and controlled manner.

Other organizations are already taking form to help the motion picture industry be able to operate multirotors in a safe manner while filming. The Society of Aerial Cinematographers (SOAC), was founded by Robert Rodriguez in July of 2014 as a community organization to train and educate the camera operators, directors, producers, as well as other people involved in the motion picture industry. Located in Burbank, California, the SOAC will be right in the heart of Hollywood, and will work with the cinematographers in that area to insure the safe integration of this new technology into the film industry. The SOAC is working in conjunction with the AMA and will be setting up educational programs to meet the needs of Hollywood. A copy of the SOAC logo can be seen in Figure 4.

Figure 4

During this time of controlled integration of UAS aircraft into US airspace, the FAA and other regulatory agencies will be watching all of us with a microscope. We need to be on our best behavior, and also work with one another to make sure that all flying is done in a safe manner, and that all of the equipment used is in top operating condition. Hopefully, within a few years, the FAA will see how safely these aircraft can be operated, and as a result, the current restrictions will be relaxed to make it easier for more people to enjoy this section of the hobby, and possibly turn it into a money making career!

Till next time, fly safe and fly often!

Using Performance Graphs

Picking out a power system for a multirotor aircraft can be a bit overwhelming for pilots that are new to this segment of the hobby. Trying to figure out what motor, prop, ESC’s and battery to use can be a difficult task without the proper documentation. Fortunately, many of the better motor companies are now providing prop charts and performance data graphs for their motors that make this process much easier. Unfortunately, if you do not know how to read the data from these charts, they are of little help. In this installment of Multirotor Flight we will go through a step by step approach to explain what is represented in these data charts, and how to use that data to select a power system.

For the sake of this example, let us assume that we have a 500mm quad frame that should weigh about 56 ounces, or 3-1/2 pounds, ready to fly including all the electronics and a 3-cell 5000mah Li-Po battery. In order to be able to hover, the combined thrust of all 4 motors needs to be equal to the weight of the craft. In this case, the multirotor weighs 56 ounces and we have 4 motors, so each motor needs to make 14 ounces of thrust, since 4 x 14 equals 56.

When selecting motors for a multirotor, the total thrust that the motors are capable of producing needs to be at least double the weight of the craft to allow excess power for climb and maneuvering. For decent flight times, your motors should put out 3 times more thrust than the craft weighs, and for racing, you may want to go to 4 or 5 times the weight of the craft when calculating total thrust. For the sake of this example, we will use a motor that puts out 3 times the thrust at full throttle that we need to hover.

Many motor manufacturers have prop charts available for their motors. These charts show the full throttle performance of the motors with a variety of props. While this data is not sufficient to calculate flight times or motor performance, it does provide enough information to select a motor for a multirotor. As we said earlier, each motor in our example quad needs to produce 14 ounces of thrust in a hover. If we want a 3 to 1 power to weight ratio for our machine, then each motor should be capable of producing 3 x 14 or 42 ounces of thrust at full throttle.

Figure 1 shows an excerpt of the prop chart for a Cobra CM-2217-20 motor. If you look at the line for the APC 12×4.5-MR prop, you will see that this combination produces 41.2 ounces of thrust at full throttle when run at 11.1 volts, which is what you can expect from a 3-cell Li-Po battery. Since this value is extremely close to our desired thrust of 42 ounces, this motor and prop looks like a good combination to use.

Figure 1

Once we have selected a motor, the ESC’s must be chosen. Even though most of a multirotors time is spent running in the 40 to 60% throttle range, you still need to size the speed controllers for the maximum rated current of the motor to be able to handle the full throttle bursts you will experience from time to time during flight. The Cobra CM-2217-20 motor has a max current rating of 20 amps, so that is the absolute minimum size ESC that should be used. It is always a good idea to use ESC’s that are a size bigger than actually needed, as this give a bit of a buffer zone to ensure they are never overworked. Because of this, ESC’s in the 25 to 30 amp range would be a perfect fit for this power system.

Now that the Motor, Prop and ESC have been chosen, it is time to use the motor performance graphs to see exactly the power needed to fly our Quadcopter. Different companies provide slightly different performance data in their charts, but since we are using the Cobra motor in this example, we will use the charts published on their website. Each motor and prop combination has a set of 4 data charts available for download. These charts provide Prop Thrust vs Throttle, Motor current vs Throttle, Propeller RPM vs Throttle and Prop Efficiency vs Throttle.

The data for these performance graphs comes from the prop chart shown in Figure 2. This chart shows the performance of the motor and prop, running at 11.1 volts, over the entire range of operation from 10% to 100% throttle in 10% increments. Since multirotors spend most of their time in a hover, where the motors are only putting out 40 to 60% throttle, the middle area of these curves is most important to us.

Figure 2

Figure 3 shows the first curve that we will look at, the Propeller Thrust Vs Throttle Position graph. This graph will tell us how much throttle will be needed to produce the required thrust for hovering flight in our Quadcopter. Once we know the throttle level required, that value will be used to read the other graphs. Earlier we determined that each motor needed to make 14 ounces of thrust. So Step 1 of the process will start at the 14 ounce point on the left side of this graph. A line is drawn straight to the right from the 14 ounce point until it intersects the blue curve on the graph. From this point, for Step 2 another line is drawn straight down from the intersection point to the Throttle axis. This second line is a little to the left of the 50% throttle point, so we will call it 49% throttle. Now we know that in a hover, our motors will be running at 49% throttle to produce 14 ounces of thrust to maintain a stable hover.

Figure 3

To find out how much current our motors will pull in a hover we need to look at the next graph, Motor Current vs Throttle Position, which is shown in Figure 4. To use this graph, we start at the bottom at the 49% throttle position and draw a line straight up until we intersect the blue curve. This is shown as Step 3. Then a second line is drawn straight left from the intersection point until it reaches the Motor Current axis. In this case, the line ends up half way between the 3 amp and 4 amp lines, which is 3.5 amps of current. This is the current that each motor will pull from the battery during hovering flight. Since we have 4 motors on our Quadcopter, the total current used will be 3.5 x 4 or 14 amps.

Figure 4

Next, if you desire, you can find out how fast the props will be spinning with the Propeller RPM vs Throttle Position graph shown in Figure 5. Once again we start at the 49% throttle point at the bottom of the graph and draw a line straight up until it intersects the blue curve. From that point, a second line is drawn to the left until it intersects the Propeller Speed axis. In this example, the line is just a little bit below the 4000 RPM line, or approximately 3950 RPM.

Figure 5

Finally, as a double check that you drew the lines correctly on the first 2 graphs, the Propeller Efficiency vs Throttle Position graph shown in Figure 6 can be used. If you start at the bottom of this graph at the 49% throttle point, and draw a line up until it intersects the blue curve, and then draw a second line to the left, the line hits the Prop Efficiency axis half way between the 10.0 and 10.5 lines, or approximately 10.25 grams of thrust per watt of input power. To use this data, we must first convert the weight of our Quadcopter from pounds to grams. Since there are 454 grams to the pound, if you take 3.5 pounds x 454 you get 1589 grams. To calculate the power required we then divide the weight of our multirotor by the prop efficiency to see how many watts of power are needed in a hover. 1589 divided by 10.25 is equal to 155 watts. Since we have a 3-cell Li-Po battery, which puts out 11.1 volts, to calculate the current used you take 155 divided by 11.1, and that equals 14 amps for all 4 motors combined. This is the same value that we got from the Motor Current vs Throttle Position graph earlier, so we know that the math is right!

Figure 6

Now that we have all this data, what can be done with it? We know that our 56 ounce machine will require 14 amps of current from the battery to stay in a stable hover, and we will be at 49% throttle to achieve this. From this information we can calculate flight time, based on our battery size. Most Li-Po batteries have their capacity stated in milli-amp hours or mah. In this example, we are using a 3-cell 5000 mah battery. For doing flight time calculations, it is best to have the battery capacity stated in Amp-hours. This is pretty easy to do, since there are 1000 ma to the Amp, our 5000 mah battery can also be called a 5 Ah battery.

Before we calculate flight times a brief discussion of the terms “C” and “C-rate” are in order. Battery discharge is commonly measured in multiples of C, which stands for the Capacity of the battery. By definition a 1-C discharge rate will drain a battery completely in 1 hour. A 2-C discharge rate will drain the battery in ½ of an hour or 30 minutes. A 3-C discharge rate will drain the battery in 1/3 of an hour or 20 minutes, and so on. To calculate how long our battery will last in our Quadcopter, we need to determine the C-rate of discharge.

Earlier we calculated that our total current draw from all 4 motors in a hover was 14 amps. Since our battery capacity is 5 Ah, the C-rate of discharge is 14 divided by 5 or 2.8-C. Since the discharge of a battery is always expressed with C being the 1-hour rate of discharge, which is also 60 minutes, to calculate the discharge time you simply take 60 minutes and divide it by the C-rate of discharge. In this case, 60 divided by 2.8 is equal to 21.4 minutes. Now remember, this is the time to completely drain the battery, which is something you NEVER want to do with Li-Po batteries. You should always leave 20% of the energy in the pack at the end of the flight to prevent damaging the cells. This means that we should never use more than 80% of the total battery energy. To correct the flight time to use only 80% of the battery capacity, take 21.4 x 0.8 and you will get 17.1 minutes.

Unfortunately we are not quite finished yet. These calculations that were done so far assume that our Quadcopter is in a stationary, stable hover. Any time you maneuver a multirotor of any kind, one or more of the motors need to speed up to provide the force to move the craft to a new location. Any time a motor spins faster it uses more current, and that will cut into your flight time. Just how much this cuts into the flight time depends on the flying style and the weather conditions. If you are doing basic aerial photography work, with a minimal amount of maneuvering, taking 75% of the calculated flight value is a good starting point. With our 80% battery use time of 17.1 minutes, if this value is multiplied by 0.75 you get 12.8 minutes.

Other considerations include wind conditions during the flight. If you are doing aerial photography, and there are winds blowing steady at 15 MPH, the extra work that the motors must do to keep the multirotor in one place can dramatically cut your flight times, sometimes by as much as half! If you are doing pylon racing or FPV racing, and your motors are running up at 75% throttle or higher, the current draw can be 3 to 4 times higher than it is in a hover, and this would reduce the flight time down to as little as 4 or 5 minutes! To monitor actual battery voltage, and to make sure that you never over-discharge your batteries, a small battery alarm device, such as the one shown in Figure 7 can be used. These plug into the balance lead of your Li-Po battery and monitor each individual cell. As soon as the voltage in any cell drops below a pre-set level, an alarm will sound to let you know it is time to come back and land.

Figure 7

Hopefully the preceding discussion has taken some of the mystery out of selecting a power system for your multirotor aircraft, and provided the information to properly use the performance graphs that are being provided with multirotor motors.

Multirotor Maintenance

Multirotor aircraft are relatively low-maintenance models by design. Unlike helicopters, which have a large number of moving parts such as swash-plates, gears, pulleys, pushrods, blade holders and thrust bearings to name a few, conventional multirotors are rather simple machines with very few moving parts. The standard quad or hex configuration multirotor has just 4 or 6 rotating propellers which provide all the flight control. Because of the simplicity of multirotors, pilots often see them as “maintenance-free” models, but this can lead to problems down the road, and cause costly in-flight failures which could have been easily avoided. In this installment of Multirotor Flight, we will take a look at some of the important aspects of routine multirotor maintenance, and how this can prevent costly damage to your model.

Any multirotor that has an even number of motors requires nothing more than the spinning props to provide all of the models flight control. With half of the props spinning clockwise and the other half spinning counter-clockwise, the motor torques cancel each other out and the craft can be stable in yaw in a hover. With tri-copter designs, two of the motors can cancel out each others torque, but the third motor will require a tilt servo to provide torque cancellation and yaw control. Since the majority of multirotors in use today are either quad or hex configurations, we will concentrate on those in this discussion. Everything covered here does also apply to tri-copter designs, you just have the added complexity of the tilt mechanism and its control servo and associated pushrods.

One of the most important things to watch for in a multirotor, and often the most neglected, is the condition of your propellers. In multirotors, everything happens in the props. By constantly changing the speed of the motors, and thus the RPM of the props, multirotors maintain controlled flight and amazing maneuverability. Props should always be checked for balance before they are installed onto any multirotor. If the prop is out of balance you can bring it into balance by carefully scraping away material on the heavy blade, or by adding tape to the light blade to bring the prop into balance. Running out of balance props can cause issues with control stability and can lead to failure of the flight controller board.

Even with experienced pilots, a multirotor will occasionally make a rough landing and flip over causing the props to strike the ground. If this is done in tall grass, chances are you will not suffer any prop damage. However, you should always inspect your multirotor carefully any time a prop strikes the ground. One of the worst possible things to happen to a multirotor is to have an in-flight prop failure. This is especially true if you throw one blade off of a prop and have the other one left intact. In a quad, the loss of a prop results in an uncontrollable flight situation that will cause the machine to fall out of the sky and hit the ground. In most hex models, proper tuning of the flight controller will allow for flight with an inoperative motor. The flight controller senses the loss of thrust from the inoperative motor and compensates for it by slowing down the motor directly opposite it. The craft can then continue to fly like a quad on the four remaining motors and make it back to the takeoff point safely. On the other hand, if you lose a prop blade on a motor, and the motor continues to run, the vibration caused running with just one prop blade can be extremely high. In some cases, the vibration can be so bad that the solid state MEMS gyros in the control board can no longer function properly and the craft can completely lose control.

If your model ever does flip over at the end of a flight, the props can sustain hidden internal damage that may not cause a failure right away. This is especially true of plastic props and fiber reinforced plastic props. Figure 1 shows some of the types of internal stress damage to look for on plastic props. If you look closely at this prop you can see a white line that has formed at the right edge of the hub where it meets the blade. This is the type of damage you can see after a multirotor flips over on a landing. A prop like this will continue to operate fine for several flights, but eventually it will fail and the prop blade will tear off at the hub leaving a horribly out of balance condition on that motor. You can also see some lighter discoloration of both blades on this prop as indicated in the photo. Whenever a prop shows a lightening of the color, it means that the plastic itself has stretched, and the glass fibers in the prop have torn loose from the surrounding plastic. This substantially weakens the prop and will cause a failure later on during a flight. If you ever see a prop that shows this type of discoloration, do not fly the multirotor before replacing the prop!

Figure 1

Another common type of prop damage comes from incidental contact with objects during a flight. With the extreme FPV type of flying that is common now, people are flying very close to buildings, fences, trees and other obstacles. While this makes for extremely exciting flights, and great flight videos, it can be tough on the props if you bump into things during the flight. When doing this type of flying, you should always inspect the props after every flight. Figure 2 shows a prop that has come into contact with a few obstacles during a flight, and shows a few battle scars to prove it. The scratches on this prop create areas of stress concentration that can later lead to in-flight failures. The ground off tips of the prop can cause the prop to go out of balance and put a lot of vibration into the airframe. This vibration can cause screws to loosen up in the frame over time and also damage the sensors in the gyros and accelerometers on your flight controller board. If any of your props look like the one shown here, replace them before flying again.

Figure 2

Another area that should be looked at from time to time is the tightness of your prop retaining nuts. Figure 3 shows a typical motor and prop combination with a prop washer and prop nut holding the propeller in place. Because the motors in multirotor are constantly speeding up and slowing down, the changing torque forces can cause a prop to slip on the prop adapter and cause the prop nut to loosen over time. The prop nut only has to loosen up about ¼ of a turn to allow the prop to slip on the motor, and then you can lose control of the model. This is especially true with wood props. Because of the open cell nature of wood, these types of props will crush slightly over time and cause the prop nut to loosen up. Many times at the end of a days flying, we will take a prop off a motor and put the nut on just finger tight, and then get distracted and forget to tighten it up all the way. If this happens, it is pretty likely that the prop will spin off on the next flight and cause a crash. If you carry a small 4-way wrench in your tool box, it is very quick and easy to check the tightness of your prop nuts before each days flying, and make sure that nothing has worked lose over time.

Figure 3

While we are talking about props, one thing that is very important to check for is interference between the prop washer and the prop itself. On many Slow-Fly style props, the blades have a lot of pitch near the hub and sweep up above the front edge of the prop hub. When the prop washer is put onto the motor shaft the prop washer can hit the blades of the prop before it touches the prop hub. The left side of Figure 4 shows this very clearly. When a prop washer hits the prop in this way, the edges of the prop washer can cut into the prop and cause a weak spot in the prop blade. This will cause a crack to start to form in the prop and can lead to an unexpected blade failure several flight down the road. The good news is that most prop washers have a taper cut into one side of them, and by flipping the prop washer over, as seen in the right side of Figure 4, the prop washer will hit the hub of the prop first and clear the blades quite nicely.

Figure 4

By following these suggestions, and keeping a close eye on your propellers, you can avoid a lot of problems with your multirotors. Now that we have covered all of the prop maintenance basics, let’s move on to the multirotor frame.

Most multirotor frames are held together with a series of nuts, bolts and screws. The motors are also attached to the frames with some type of mounting screw. With 4 or more motors running at various speeds all the time, you can induce some rather strong harmonic vibrations into your multirotor frame if all the props are not balanced properly. These vibrations can cause screws to vibrate loose on frame arms or motors, and eventually lead to catastrophic failures.

Figure 5 shows a typical small multirotor frame that is held together with socket head screws. When building frames of this type, always use a drop of medium (blue) thread locking compound on each and every screw as the frame is put together. This will lock the screws into place and prevent them from backing out later on during a flight. The same holds true for your motors when they are mounted to the frame. By using thread locking compound on your motor mount screws, you will ensure that a motor does not loosen up in flight and cause problems for you down the road. Always remember that thread locking compound is much like CA glue, it cures in the absence of oxygen and is a one-time use product. If you ever have to take your frame apart later for repairs, you will need to re-apply the thread locking compound to each screw as you put the frame back together.

The last maintenance item to watch for on your multirotor is all the wiring associated with the motor and control systems. With 4 or more speed controllers, flight controllers, camera gimbals and optional lighting, there can be quite a bit of wiring in a multirotor. Carbon fiber or fiberglass frame plates can have very sharp edges, and over time, these can cut right through the insulation on electronic wiring. The fibers in carbon fiber plates are electrically conductive, so if the plates cut deep enough into the insulation to expose bare copper wire, you can get a short in your power system that can potentially lead to an electrical fire. Be sure to pay close attention to your wire routing and inspect all of your battery and motor leads from time to time for any signs of chafing on the insulation. Control leads that run from your flight controller to your receiver and speed controllers are typically quite small in diameter, and easily pinched or cut. Be sure to route these wires carefully through your frame, and use plenty of zip-ties to secure and support the wiring to the frame wherever possible.

Quite often, motor lead extensions are used on multirotors when the speed controllers are mounted in the center frame. This adds extra connectors that need to be plugged in and can be failure points if not monitored. If you are adding motor lead extensions to a motor, it is a good idea to add an extra piece of heat shrink tubing over the bullet connectors where they plug together. This provides an extra layer of insulation to avoid electrical shorts, and prevents the wires from pulling apart inside the arms of your multirotor if you ever have to pull a motor off to change a motor mount.

With these few simple maintenance tips, you can ensure that you get many hours of safe, trouble-free operation from your multirotor, and protect the investment that you have made in your equipment. Always remember to fly safe, and drop by next time for another installment of Lucien’s Corner.

Maximizing Flight Time

Once pilots get their multirotors operating well, and start actually doing things with it other than tearing holes in the sky, one of the first hurdles faced is increasing the available flight time. Most pre-packaged, ready-to-fly multirotor models come with a battery pack that will only last from 8 to 10 minutes of normal flying. For many, this is long enough, but for others that wish to do aerial photography, survey work or other professional type applications, there is always a desire to extend the flight times. Getting flight times in the 20 to 30 minute range is achievable with many multirotors that are available today, but there is a bit more to it than simply strapping more batteries on the model. In this months installment of Multirotor Flight we will take a look at how to maximize the efficiency of the multirotor power system in order to get the longest possible flight times.

First and foremost, there are a few basics about flight that must be considered. Number one is the fact that “Lighter Always Flies Better”. This is especially true in multirotors since the weight of the machine takes a certain amount of power to hold in the air. Heavier machines require more power and therefore they will have shorter flight times than lighter machines.

The second major thing to consider is the fact that larger props are more efficient than smaller props at converting rotational energy into thrust. This is because larger props can move larger volumes of air more efficiently.

The third thing to consider is slower turning props are more efficient at creating thrust that faster turning props. Props are essentially little wings going round and round generating lift. Basic aerodynamic principles show that the drag on a wing increases exponentially as speed increases, and this is the same with props. Props that are spinning faster have greater drag losses than slower spinning props, and this robs a model of flight time.

Finally, the efficiency of the motors and speed controllers themselves will play a role in the overall efficiency of the power system and can have a big impact on the flight time of a multirotor. Now that each of the components of the efficiency equation have been introduced, let’s take a closer look at each one and how it affects the overall flight time of multirotor aircraft.

Weight is always a factor in considering flight times for electric powered models of any kind. Design engineers will always say, “Design an aircraft to fly, not to survive a crash”. There is always a trade-off between structural strength and weight when designing aircraft. All aircraft must be built strong enough to survive any of the forces they will see in the air, but also be able to handle a bit of “hanger rash” and the occasional not-so-perfect landing without falling apart. Modern carbon fiber composites, light-weight aluminum alloys and molded plastic parts work together in many multirotor frames available today providing an airframe that is not only strong, but also light weight. The best starting point to maximize flight times is to use as light a frame as you can to get the job done.

Another consideration in the weight part of the equations is the total weight of the power system. Careful attention needs to be paid when selecting motors to make sure they are matched to the power that is actually needed to do the job. If a machine needs motors capable of producing 200 watts each, and larger motors capable of producing 400 watts are used, the total weight of the multirotor will be higher than necessary and this will require more power to keep the craft flying. Always remember, lighter always flies better, and also longer!

Next let’s take a look at how propellers factor into the flight time and efficiency of multirotor power systems. Larger props will always be more efficient than smaller props, because they move a larger column of air downwards as the props rotate. From 7^th grade geometry class, we all know that the area of a circle is equal to Pi times the radius squared. For a 10 inch diameter prop, the cross sectional area of the column of air moved down by the propeller is equal to 3.14159 x 5 x 5 or 78.5 square inches. If you go to a 14 inch prop, then the area is equal to 3.14159 x 7 x 7 or 153.9 square inches or roughly double that of the 10 inch prop. With the larger 14 inch prop, you only have to move the air downward at half the speed to move the same volume of air compared to the 10 inch prop. Since less energy is required to accelerate the air, less current is pulled from the battery and flight time increases.

Figure 1

Prop efficiency is commonly expressed in the number of grams of thrust created per watt of power consumed by the motor, such as 8.5 g/W. The higher this number is, the more efficient the prop is at converting the rotational energy of the motor into thrust. Figure 1 shows a chart of the efficiencies of several different size props running on the same motor. Each of these props is spinning at an RPM that produces 500 grams of thrust. As can be seen from the data, the larger props turn at a lower speed to generate the thrust AND require less power to do so. When you take the grams of thrust created, and divide that by the number of watts used by the motor, you get the efficiency factor of that prop at one specific thrust value.

From this data, you can quickly see how prop diameter affects flight time. If you had a quadcopter that weighed 2000 grams (4.4 pounds) you would need to make 500 grams of thrust from each motor to hold the craft in a stable hover. From the chart you can see that the 10 inch props have an efficiency of 9.08 grams per watt while the 14 inch props have an efficiency of 11.55 grams per watt. This is an increase in efficiency of 27%, so all other things being equal, switching from 10 inch props to 14 inch props would give you an increase in flight time of 27%. Of course, this is assuming that the motors are actually capable of running the larger props without exceeding the maximum current rating of the motor. In some cases, it is necessary to use a lower Kv motor to have the torque needed to spin the larger prop without exceeding the motors maximum current recommendations.

Figure 2

If this data is taken one dimension further, we can see how rotational speed affects the efficiency of propellers. Props act much like wings from an aerodynamic standpoint. The parasitic or skin friction drag of any wing increases as a function of the square of the speed. Because of this, if a prop is rotating at twice the speed, the drag is 4 times higher. Figure 2 shows a set of data collected with a brushless motor running on 4 Li-Po cells with an APC 12×4.5 Multirotor prop. This table shows the performance of the motor over its full range of operation at throttle settings from 20% up to 100% throttle. From this chart, if a graph is created that plots prop efficiency versus throttle position the data set will look like the one shown in Figure 3. From this graph, if the machine is hovering at 50% throttle the efficiency of the props is approximately 8.2 grams per watt. At 40% throttle this value increases to about 9.3 grams per watt, and at 60% throttle it decreases to 7.2 grams per watt. If your multirotor is heavier, not only do you have to generate more lift to keep it flying, the props are also less efficient at producing thrust, so you get hit even harder on the current draw from the battery. On the other hand, if your machine is lighter, you need less power and the props are working more efficiently which contributes to longer flight times.

Figure 3

The final part of the equation is the actual power system itself. Higher quality motors that are made with better grades of materials will be inherently more efficient than cheaper motors using inferior components. If you have one motor that has an efficiency of 80% compared to another motor that has an efficiency of 90% there is a huge difference between the two. At first glance some will say that one motor is 10% more efficient that the other. The truth is that one motor is twice as efficient as the other! If you look at the inefficiency of the motors, the 80% efficient motor is 20% inefficient and the 90% efficient motor has an inefficiency of only 10%. The inefficiency is what causes the losses in the motor. The motor that is 80% efficient has double the losses of the motor that is 90% efficient, and will cost you flight time across the board.

The other thing to consider in power systems is the voltage that the motors are running at. The electrical losses in a power system are directly related to the current flowing through the system. From Ohm’s law we know that power is equal to voltage times current. Voltage can also be expressed as current times resistance, so if we substitute this for voltage, power can also be expressed as current times current times resistance, which is commonly called “I-Squared-R”. This is why electrical losses in a system go up as a function of the square of the increased current.

When we consider total power in a system, we normally figure Volts x Amps. In a 3-cell power system (11.1 volts) if we are pulling 20 amps all together, the total power is 222 watts. In a 6-cell power system (22.2 volts) we can get the same power from only 10 amps. If the wiring in a power system had a resistance of 0.1 ohms, the total power losses in the 3-cell system would be 20 x 20 x 0.1 or 40 watts. In the 6-cell system, the power losses would be 10 x 10 x 0.1 or only 10 watts. This is why higher voltage power systems are inherently more efficient. In this example switching the power system from 3-cells to 6-cells cuts the electrical loses by 75%!

Batteries are another important consideration when calculating flight times on multirotors. In many cases, the batteries make up the highest percentage of the payload weight of the craft. Quite often, pilots make the mistake of thinking that adding a second battery will double the flight time. This is incorrect for 2 reasons. First, when a second battery is added, the overall weight of the aircraft increases, and this requires more power to hold the craft in a hover all the time. The other thing that people forget to consider is the fact that the props also operate less efficiently at the higher loading, so the efficiency takes a double hit! In most cases, adding a second battery will not double the flight time, but will instead give a 70 to 75% increase in flight time. Each additional battery has a smaller impact in the increase of flight time, and eventually a point is reached where adding another battery will actually decrease the flight time due to the high weight and lack of efficiency in the power system.

When all of these different aspects are combined, it is possible to double the flight time of a multirotor by simply optimizing every part of the power and propulsion system. Putting all of these different considerations together, you will get the longest flight times from a multirotor if you start with the lightest airframe possible, use the largest props that will fit on the frame and power them with high quality, low Kv motors, using higher voltage battery packs. Hopefully this information will help to get the power system in your multirotor optimized for the longest possible flight times and allow you to get the maximum benefit from your multirotor power system.

The ABC’s of PID’s

If you have ever worked with any open-source multirotor controller boards in the past, then you have probably needed to tune the PID settings in the feedback loops a few times. For some, these settings make perfect sense, but for most, they are a complete mystery that seemingly requires the combined skills of a computer programmer and an electrical engineer to figure out. In this installment of Multirotor Flight, we are going to take a look at control feedback loops, the PID settings that adjust them, and how they all work together to make a multirotor aircraft controllable.

All closed loop control systems are designed to keep a specific device, or system of devices, operating at a pre-determined setpoint. Whether it is the thermostat on the wall that controls the temperature of your home, the cruise control in your car that keeps it traveling down the highway at a specific speed, or the gyro sensors in your multirotor that keep it level when you let go of the controls, all closed loop systems rely on PID settings to keep the system in equilibrium. Understanding PID settings, and how they affect the response of closed loop systems, will help out greatly in understanding how to properly tune a multirotor aircraft flight controller.

So what are PID’s? Some people refer to them as “pid” settings, said like the word “kid”, while others will refer to them as 3 individual letters, calling them “P-I-D” settings. While either one works, the letters do mean something. The letter “P” stands for “Proportional Control”, the letter “I” refers to “Integral Control” and the letter “D” stands for “Derivative Control”. Any of the readers that remember taking a calculus class in high school or college are familiar with the terms Proportional, Integral and Derivative, and when talking about PID settings, these terms do have similar meanings. To understand PID settings, we need to discuss each of these terms in brief detail in order to see how each one affects the control of our multirotor aircraft.

In every closed loop control system there are three terms that must be considered: A particular variable, a desired setpoint and an error value. When talking about multirotors, the variable could be the roll angle, with a desired setpoint of 0 degrees, or altitude with a desired setpoint of 50 feet above the ground. The error value only exists if the variable is NOT at the desired setpoint. To clarify this a little more, here is an example. If you are looking at the roll axis in a multirotor, and at one instant in time, the frame is rolled 4 degrees to the right, then the variable is Roll, the desired setpoint is 0 degrees, and the error value is 4 degrees. The purpose of the control loop is to sense the 4 degrees of error in roll, and then tell the system what to do in order to put the multirotor frame back to the desired set-point, which would be 0 degrees, or level, for a hover.

Figure 1

Figure 1 shows a block diagram for the operation of a typical PID closed loop control system. Starting at the left side of the diagram, we have two values that are compared against one another, namely, the desired Setpoint value and the current Output value of the system. Any difference or “error” between these two values is then fed into the 3 sections of the PID control loop for processing. In any PID control loop, the “P” term is related to the present error, the “I” term is related to the accumulation of past errors, and the “D” term is related to prediction of future errors. The corrections that are generated by each of these three sections are then added together to form a total correction value, which is finally fed into the Process section of the control loop.

For a multirotor controller, the process section creates each of the control pulse signals that get sent out to the speed controllers, which then drive the motors in the aircraft. Finally, the props that are attached to the motors provide the corrective forces needed to move the multirotor back to the desired setpoint. The output from the gyro sensors in the multirotor controller are looped back and compared once again to the setpoint value, and the entire process starts all over. This control loop runs continuously, hundreds of times per second, trying to keep the multirotor level when no control input is made, or moving the multirotor to a new attitude whenever a control input is given.

When all three sections of the PID control loop work together, and are properly tuned with respect to one another, you get a system that responds incredibly well to the constantly changing environment that is inherent in multirotor flight. The truly amazing thing is that there are multiple PID loops in a multirotor controller, all running together at the same time! Each axis of flight requires a separate control loop that is driven by the various sensors. Pitch, Roll, Yaw, Altitude, Heading, and Position are all continuously monitored and maintained at some specific set point as directed by the internal sensors, or to new set points as commanded by the pilot. When you stop and think about it, multirotor controllers do a lot of stuff that we simply take for granted!

Now that we have a basic understanding of what each of the P-I-D terms means, and how they relate to one another, let’s look at each one in a little more detail to get a better understanding of how they actually work. As we said earlier, the P part of the PID loop stands for Proportional Control. Like the name suggests, proportional control responds proportionally to the amount of error in the system. If there is a small error in the system, then the feedback loop will generate a small force to correct it back to the desired setpoint. If there is a large error detected, then a large force will be applied to correct the system back to the desired setpoint.

This process can be pictured easily from the standpoint of driving a car down a highway. If you are driving down a straight section of highway and your car is drifting slightly to the right, you will turn the steering wheel just a few degrees to the left to get the car headed back to its desired path. If the car is drifting very quickly to the right, then you would turn the steering wheel a lot more to the left to make the correction. When setting the Proportional part of a PID loop, the idea is to apply the proper amount of correction force to get the system back to the desired setpoint. With the P value set too high, the system will overcorrect and can oscillate back and forth around the desired setpoint, and with too low of a P value, the system will slowly wander towards the setpoint, without ever getting there.

The second part of the PID control loop, the “I” component, is the Integral control. Back from the calculus days you may remember that if you integrate a mathematical formula, you get the area under the curve, which is the accumulation of a value over time. The purpose of the Integral component of the PID control loop is to respond to how the error occurs over time. If something happens to push the system out of equilibrium, and the Proportional part of the control loop is taking too long to respond, the Integral part of the loop will add in more corrective force to bring the system back into equilibrium quicker. If the gain is set too high, too much correction will be applied and the system will oscillate back and forth around the desired set point. If the gain in the Integral portion of the control loop is set too low, it will take a long time for the system to get exactly back to the desired set point, and will always drift slightly one direction or the other.

The third and final part of the PID loop is the Derivative control. If you remember from calculus, if you take the derivative a formula, you get the slope of the curve, which is an indication of how rapidly the value is changing. Likewise the “D” portion of the PID settings, or Derivative Control, responds to how quickly the error value is changing. For our multirotors, if a machine is in a hover, and gets hit by a huge gust of wind that rapidly flips it up on its side, you want the corrective forces to be applied quickly, and in a large enough amount to bring the frame back to a level state. So while the Proportional control gives a lot of corrective force if the frame is displaced a large amount, the Derivative Control will add in additional corrective force if the movement happens quickly. When adjusting the Derivative portion of the PID loop, if you have too much gain, the response will happen too quickly, and correct too much, causing the machine to overshoot past the desired “level” point which can lead to oscillation, and if you do not have enough gain, the response will be sluggish and not respond quickly enough.

With any PID control system, you can tune the performance to meet the type of flying that you are doing and the mass of your specific airframe. When the parameters are set up perfectly, you will always have a small amount over correction in the system that dampens our after 1 or 2 small oscillations. This is great for an aerobatic machine where you want quick, crisp controls. However, for a big camera rig multirotor, you would want a much smoother response and set the gains a little lower.

Figure 2

Figure 2 shows the response characteristics of 4 different amounts of gain in a PID control loop with respect to time. The origin of the graph indicates a current steady state of control, the thin gray line represents the new desired setpoint value, and each of the colored lines represents how the system gets from the current value to a new desired setpoint value with respect to time. Curve A, the red line, shows the response of a system with not enough gain. On this curve it takes a long time for the system to respond to the change, and it never quite gets all the way there. In a multirotor this would create a very sluggish feel to the controls, a machine that wanders around a lot and has very slow response to external forces that disrupt the position of the craft.

Curve B, the green line, represents what is known as Critical Gain. This is the condition that gets you to the new set point as quickly as possible, but without overshooting past the desired setpoint. This would be an ideal condition for a large camera platform multirotor where you want smooth steady response, without any jerkiness in the controls.

Curve C, the blue line, represents a system that has a little too much gain, but is still stable. This system gets you to the new set point as quickly as possible while remaining stable, but with this much gain in your control system, you will actually go a little past the desired set point, which is called overshoot, and then wobble a tiny bit back and forth before settling on the new desired setpoint. This amount of gain would be good for an aerobatic type multirotor, where you want the response to be as quick as possible, and you do not mind if it overshoots and wobbles a tiny bit from time to time.

Finally, Curve D, the orange line, shows a system that has way too much gain and cannot stabilize itself. In this condition, the corrections happen too fast and are too great to ever allow the multirotor to level out. A multirotor that is set up like this will never stabilize, and will do the “Hula Dance” or “Toilet Bowl” maneuvers all the time. In extreme cases the oscillations will actually get greater and greater with each cycle until the machine becomes completely uncontrollable and smashes into the ground. If you ever increase the gain in one of the PID loops and the machines starts oscillating right after takeoff, set it back down immediately! If you continue flying, you will probably regret it.

Most flight controller boards come with some PID values preset in them, which are usually pretty safe and slightly conservative. If you want to fine tune the settings a bit to suit your flying style, then by all means do it. There are a few things you should always remember when setting PID settings though. First, only vary one setting at a time. If you do change more than one setting at a time, you never know which one is actually causing the changes you see. Second, it is best to move in small steps, no more than about 10% of whatever the current value is. For example, if you have an I setting of 0.050 in a control loop, and want to raise it, try 0.055 first, then 0.060, then 0.065 and so on. If you try changing a setting too much at once, you can end up with a completely unstable controller that destroys your multirotor. Third, when you find a set of variables that works well for you, WRITE THEM DOWN! Files get easily overwritten, so if you have a combination that works well, keep a little logbook with your PID settings written down in it so you can refer back to them later on if you need to.

Each and every flight controller board has its own unique set of PID settings, so you will have to do a little research, check the on-line forums, and generally ask around to find good starting points for your PID settings. From there, you can fine tune the values to get the board to meet the requirements of your particular frame and flying style. Hopefully this has helped you understand the mystery of PID settings, and enable you to try fooling around with them yourself.