Competing for Compromise:Air Frame Design Considerations
In this lesson students will learn about the different parts of the quadcopter air frame: their functions, and things to consider when designing these parts to achieve sufficient lift to get airborne. Students will make use of an online calculation tool that helps guide their choices based on parameters selected by the instructor. Here they learn first hand the art and science of compromise based on the goals of an engineering project. First, an introduction to the air frame components.
Most multirotor airframes have several common components, some of which may be separate or combined as modules. They include:
The center stack links and supports the arms. It also provides a mounting point for the flight controller, battery, receiver and other electronics. The weight of these components adds up and these parts must be quite resistant to bending and breakage. The stack may be built with as little as two levels. Its design needs to take into account access holes for wire routing, holes for mounting electronics and the attachment of the four arms of the quadcopter.
The arms of the copter are often its most most distinguishing feature. They can be made from simple materials such as square tube towel rods from the hardware store, carbon tubes, or custom cnc machined aluminum like the one we build in this tutorial. The length of the arms must allow for propeller clearance, room for landing gear, reasonable weight, and sufficient strength. Arm design should consider the location of motors, controllers and wire routing. Each arm should mount to the center stack with a minimum of three bolts, four if possible.
The motors will be mounted to the ends of the arms in some fashion. With simple square tube, it is possible to drill holds and mount the motors directly to the arms. With round tube and/or fragile materials, a separate motor mount must be fabbed or purchased. The machined aluminum arms in the TSN quad incorporate motor mounts into the design. Proper hole alignment and sizing is crucial which ever technique is used. The spacing of the holes on the motor should be determined in advance of the final design of the mount. This information is often available on the manufacturer’s web site, or if you have the motors in hand that makes a precise measurement a simple matter.
Landing gear will set the airframe itself above the ground for several reasons. The battery is typically mounted on the bottom of the craft and needs to avoid ground contact due to its fragile structure. Landing gear also isolates the spinning props from the ground making it less likely to break one in a minor tipping incident. Finally, landing gear permits the addition of undercarriage accessories such as a camera that can be placed under the field of view of the propellers. The design and overall height of the landing gear must will depend on the design goals of the craft. These must be established before manufacturing and construction take place. Will the robot be used for acrobatics, aerial photography, or as a learn to fly platform?
The Great Compromise: Strength vs Weight
This is an ideal subject for students to be exposed to the process of engineering compromise. Here students are engaged in engineering practices with an activity that has no predetermined outcome. Often in class when students work problems with the forces of motion, certain facts are ignored for the sake of simplicity and convenience. Wind resistance is often assumed to be absent when learning how to calculate the speed of a falling object. Friction is ignored when calculating acceleration of a moving vehicle. With aerial robots, these forces and many others can not be ignored or there will be expensive consequences.
It will become obvious to any student that the design of the motor mounts, arms and chassis do need to be as light as possible. But along with weight considerations the job of these components must be considered. Their task is to not only carry the weight of their respective component parts, but also to withstand the forces involved when countering gravity, wind, and various twisting forces that accompany the complex movements of a multirotor. Additional strength is required to handle the occasional rough landing, or yes, even crashes. Strength therefore is important, and mass can add strength-but the machine must be able to rise of the ground and maneuver in three dimensions.
So where do we start off? This is the most difficult question, because committing to one aspect of the multirotor design narrows down the options for the rest of the design and components.
A good rule of thumb is to choose a combination of parts that allow thrust be at least double the weight of the ‘copter-just to get off the ground. In other words, after all the parts of the airframe, battery, motors, escs, props, etc are tallied (referred to as AUW or all up weight), the upward thrust of the motors together should at minimum double the downward force of gravity acting upon them. Achieving this goal used to be more difficult than it is now thanks to some remarkable online calculation tools. Therefore my students always begin this process with some simulation time in front of the computer. Here there is a tool that can add virtual components from their online catalog, and run calculations with various component combinations. The calculator will will automatically tally up machine weight, as well as theoretical thrust, estimate flight time among many other things. The tool we like to use is called Ecalc. A limited(but very useful) version is free, but its only a few dollars for the full version.
Visiting the ecalc site and Hobby King or another vendor with commonly available components will give students a pretty good representation of what their components can do before their machine is even designed. Let’s run through a scenario. For teachers, setting some parameters for students in addition to budget is easier from a management perspective, and you can still allow for advanced designers to customize airframe components during the process. Let’s say your budget is $150.00 for components. In a classroom, it is easier to have as many common parts as possible. In our example we will choose:
2100 mah battery
1100 kv 28/36 motors
20 amp speed controllers
I also tell students they should strive for at least 6 minutes of hover time.
Student Exercise: All Up Weight
One of the parameters to enter first is the weight of the model with or without components. Look at the motors we have chosen. You can see they can, according to the manufacturer achieve a maximum thrust of 1000 grams. Multiply that by four and we have a maximum of 4000 grams of thrust with the right props. Using our rule of doubles, that means theoretically we can have a 2000 gram aircraft.
Using ecalc to add up the masses of the components shows that they add up to 613 grams. These do not include flight controller, power distribution board, wires etc. Add ten percent for those. This gives 674.3 grams, leaving us with 253 grams for our chassis, including materials and fasteners.
In the TSN Quadcopter design, the arms with motor mounts weigh 28 grams each if made from aluminum. The 3 centers weigh 12 grams each if made from plastic and 8 grams from garolite. That puts us at 112 + 24= 136 grams. Adding fasteners will still put us well under acceptable weight.
Once the calculations are run, the results give a great deal of insight into what to expect. Here we see that our flight time estimate shows our goal of 6 minutes can be achieved with this configuration.
There are loads of variables to explore in the data stack generated by ecalc. One estimates how much extra payload can be carried by the aircraft. If you want to add a camera, extra battery or retractable landing gear, this figure comes in really handy. In our example, you can see that we have some room for some goodies with an additional payload of 420g.
You can also see that there is room for improvement as well. Students should be encouraged to try different combinations of propellers, including those recommended by the motor manufacturer and those that might not be. Remember you can’t break or burn components that are not real! If students choose a particular prop size, remind them that there must be space between the arms sufficient to clear the props from hitting each other should they embark on their own designs.
Whether they work alone or you pair up students on the computers, a great deal of classroom energy starts to generate. This amazing transformation takes place as students realize they are in an impromptu engineering competition as they crunch number, look up specs and compare notes. You’ll quickly hear kids turn in their chair saying “I got 6 minutes!” and another says “I got 6.5 minutes!” as others frantically check their figures and try to find the perfect combination of parts that will yield the longest flight time. This really gets them excited to start the design a build process! Have students document several different combinations in their engineering journal so they don’t forget what works in the simulation. If you have a projector, share out a couple of combinations that work and some that didn’t for discussion.
Depending on your students’ design skills, they can use software to do some analysis of different arm designs. They can apply different forces to the arms and see how much they bend and twist. They will be able to adjust and redistribute mass, material types, and length of the arms themselves before ever machining a single part. The ‘ready to manufacture’ design of the TSN Quad is a starting point for teachers and students to get off the ground. After that, the sky’s the limit.
Next: CNC Machining the Air Frame