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!
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.
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.
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.
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.