With the rise in the popularity of personal drones, or UAVs; incidents involving UAVs that have put people and property at risk are on the rise as well. Be sure to follow all regulations for R/C and unmanned vehicles and fly responsibly. Some regulations to be aware of before continuing to read this article are to keep a direct line of sight on your vehicle at all times; avoid flying near airports and emergency personnel, avoid manned aircraft, and do not fly anything weighing more than 55lbs.
For more information please visit https://www.faa.gov/uas
Designing a Quadcopter UAV
Designing something that flies can be a daunting task. The purpose of this article is to help with the physical design of the UAV with the intention of using a 3D printer in the fabrication process. This process can vary depending on the type of UAV you decide to make. To begin, you must answer a few questions.
- What type of UAV do you plan to build?
- How much of the UAV do you want to be able to print and how much of the UAV should you print?
- Will the 3D printed part be strong enough yet light enough?
Some other things to keep in mind when you design your parts are: how big of a part can the printer you plan on using make, what material you plan to make your parts out of, and how will all of the non-printed components fit?
What Can’t You Print?
This article assumes that you only have a plastic-depositing 3D printer at your disposal. Because of this, certain components of any UAV will simply be impossible to print. These include the control board and radio, flight motors, control motors, cameras, batteries, and props.
DO NOT TRY TO PRINT PROPS! Props are put under extreme stresses in flight conditions, spinning at thousands of RPM. A typical 3D printer would also struggle with the basic shape of a prop, as it is mostly overhang. Even if you could print the prop with the correct shape, the layers inherent to 3D printing not only reduce the prop’s efficiency, but are structural weak points that could break as soon as the prop was spun up to flight speed. Depending on the material used this could result in a range of different sized pieces of shrapnel flying off in any direction. DO NOT TRY TO PRINT PROPS, it is much safer and easier to buy them instead.
What Can You for a Quadcopter?
The best thing to use a 3D printer for is the fuselage or frame of whatever style UAV you are building. Flaps, landing gear, baffles, tails, struts, and more can all be easily printed and calculated. Depending on your design these elements can be printed separately or all at once.
What Type of UAV Should You Make?
There are two main types of UAVs that you can make: rotor craft and winged craft. Rotor craft are vehicles like helicopters, tricopters, quadcopters, etc. Winged craft are your typical R/C aircraft, gliders, and kites.
3D Printing a Quadcopter
Quadcopters are the most common UAV on the market right now. They are a type of helicopter that uses four static motors to provide both lift and control. They are related to the tricopters which use three motors, two static and one pivoted. A quadcopter is controlled by varying the amount of power to each motor, which are set up in counter-rotating pairs to prevent the ‘copter from spinning out of control.
The quadcopter uses differential thrust to pitch and roll and the momentum of the blades to yaw. A tricopters can’t use momentum of its blades to yaw (one motor does not have a counter-rotating companion) so they use a pivoting motor to provide a counter force in the yaw axis. Some other variants are the hexa- and octo-copters, which use six and eight motors respectively. These control similarly to the quadcopter due to the balanced number of motors, but are much more complex and larger than their four motored cousins.
A quadcopter is usually laid out in a symmetrical design that keeps the torque of each motor around the center of mass equal. Some look like an ‘H’ while others look like an ‘X’. Tricopters will usually end up looking like a ‘T’ or a ‘Y’. When designing a ‘copter, you always want to make sure that the center of mass is in the center of all the motors, otherwise one side will have to work harder than the other and the vehicle’s performance will suffer. Also keep the center of mass lower than the center of lift, this will make your vehicle more stable and easier to control. The center of lift on any ‘copter is on the plane at which all the props spin.
Make it Light Yet Strong
The goal when designing the structure of anything meant to fly is to make it as light as possible yet remain as strong as possible. When printing a frame for a quadcopter your frame will generally not be the heaviest part, but the heavy stuff will be directly attached to it.
The batteries will likely be the heaviest part of your UAV. This means you want them at the center and low, but not hanging off the bottom so that they get damaged if you land hard or crash. Adding landing skids or a shield are good ideas to protect the batteries.
The battery compartment can be printed as part of the frame or separately and attached later. That depends on how the other side of the frame is laid out. If you have a battery compartment hanging under your frame the top of the frame is perfectly flat, you can print it upside down as one and save on support material. However if the other side of the frame is not flat, you can separate the battery compartment to make one side flat and attach it later. Of course it is possible not to have a compartment as well, but with the LiPO batteries typically used, it is best to protect them from harm.
The frame itself is actually very straight forward. As long as you have all the mounting for your electronics, the only design elements you need to pay attention to are how thick the frame is and that it is symmetrical. Symmetry is important for the control of the quadcopter; while thickness is important in making sure it stays together.
To illustrate, image each arm that the motors are attached to is a lever. When the motor is switched on, it applies a force to that lever which pivots at the center of mass. You need to make sure that each arm can support the force applied by your motor, and then some. This is called the factor of safety and it is very important. A factor of 2 is generally used; going too high will make your vehicle too heavy while going to low will mean that it breaks.
Resources for 3D Printing a Quadcopter
For example, if you need an arm that can support a maximum force of 100 newtons at one end with a factor of safety of 2, you should design the arm to support 200 newtons. How thick you end up making your frame will depend on how powerful your motors are as well as how long the arm is and how strong the material you use is. Some good resources are the engineering toolbox, which has the equations needed to make the calculations, and makeitfrom.com, which lists the properties of hundreds of common materials, including PLA and ABS. Links to these sites are below.
Most CAD suites also include ways of testing the strength of designs, which can be a very useful tool when used properly. When designing the arms of your ‘copter, be sure to keep in mind that the inside of the arm is follow with any infill less than 100%, making them lighter but making the calculations more difficult due to the internal structure of the infill.
A good, low tech test of your design would be to print a single arm and mount it to the side of a counter or table. Be sure to wear safety glasses as you add weights to the motor mount until the arm fails (when cracks form). If the weight added at failure is twice the force that a motor will produce, you are all set.
Winged UAVs are a much different beast from rotorcraft. Most only have one or two engines, which provide forward thrust only. The wings produce all the lift. They also require control surfaces to pitch, roll, and yaw.
Design of a 3D printed winged UAV is very similar to the design of non-3D printed winged aircraft. The same laws of Aerodynamics apply. The challenge with 3D printed UAVs is their actual structure, specifically in the wings.
A first step would be to find an airfoil (the cross section of the wing) that best fits your specific model. http://airfoiltools.com/ has a catalogue of NACA and other airfoils to choose from as well as tools to move them into CAD.
3D Printer is good for Winged Craft!
A 3D printer is actually one of the easiest ways to produce an airfoil. To do this though, the wing needs to be printed from its root (where it meets the body of the plane) to its tip. The wing will be exactly the shape you need, however printing in this orientation produces the weakest wing due to the orientation of the layers to the forces that would be applied to the win in flight. Ideally you want the layers to be co-planar with the wing itself (print the wing bottom to top or top to bottom) however this is impossible to do with most airfoils without getting a rough surface and reduced efficiency.
One way to combat this is to print the wing root to tip and then coat the wing in something to s=hold it together, such as a resin. Another tactic would be to not print the entire wing, but instead print the frame of the wing and cover it with a surface material like a plastic film or fiberglass (depending on your UAV’s scale and price range) This second option will also make the overall wing lighter.
As for the fuselage, any orientation you can print it in should work. It may be worth buffing the final surface into something smoother in order to reduce drag and make your UAV more efficient. Be sure to include mounting points for the wings (or even print with the wings attached) Electronics, and to keep the center of mass in the proper spot with regard to your wing design.
One final item to mention are ducts. Ducts go around props and fans to increase their thrust without adding power. They also serve to protect a prop from braking in the event of a mild crash. Ducts can be designed to be fitted onto just about any prop or fan, including those on commercial UAVs. Since they are also basically hollow cylinders they are easy and fast to print.
3D printing a vehicle that can fly is a very tricky task. Each design will have its own quirks, faults, issues, and challenges. Hopefully this article has provided some insight on the process involved and will make designing your UAV easier and safer.