WORK IN PROGRESS
This document goes through the IQ design process for designing drones for intelligent missions. All of the autonomous drones I have designed have been for research purposes. From doing this, I have noticed there is not a lot of material about designing and building your custom drone for this purpose. I have tried to aggregate all of the information I have learned over the years into one document. I have also developed a fairly robust design process to help guide new and old drone engineers through their design choices.
- Figure out what you want your drone to do
- Figure out your sensor package
- Design a frame to carry your payload
- Estimate the weight of the frame and payload
- Do power thrust analysis and select motors, escs and batteries
This is where you examen the mission and decide what you will design to accomplish the goal. Start thinking of how your drone will fly, navigate, and take data.
You should figure out all of the mission specific hardware computers and sensors before you design the vehicle to move your payload.
- FCC
- Companion Computer
- Camera(s)
- RC receiver
- Telemetry Radio
- Navigation equipment (GPS, OF, altimeter)
- Avoidance sensors (lidar, sonar, more cameras)
How do we best arrange our sensors, compute and comms to accomplish the mission. Make a CAD to best lock down the spacial arrangement of the components. Use the CAD to get a better weight estimate of your frame.
This part of the process is the most tricky. All of these bits and pieces are coupled, so changing one affects the other. This is not a well-defined process, and in its current state, most hobbyists rely on trial and error, as well as word of mouth. It is more of an art than a science.lege. I will attempt to make this more based on my experience to be more scientific and use some aerospace principles I learned in college.
It is helpful to think of what our ultimate goal is when designing a drone. We want to carry a payload, and we want to be able to carry it for as long as possible usually. While we may think we have constraints on the flight time and payload, ultimately, people want to add more sensors and make the vehicle do more things. As the designer to the aircraft, we should try to give the mission designers as much margin as possible. To get the most out of our system, we should analyze the efficiency of our system and try to optimize each component. The two main design principles I will try to optimize are
- Aerodynamic/Mechanical Efficiency
- Electrical Efficiency
The Aerodynamic efficiency of your multi-rotor is affected by a couple different parameters.
- The propeller disk
- The pitch of the propeller
- Blade count
- The mass of the vehicle
A multi-rotor works by taking mass(air) and throwing it down so the vehicle can hover or ascend. This is Newton’s third law in a nutshell: for every action, there is an equal and opposite reaction. To generate more thrust, we want to throw as much air as possible down, one way to do this is to increase the propeller disk diameter. When we look at the equation for a circle, we notice that the area increases by the square of the radius, so a small increase in the radius will net us a whole heap more of air!
Principal:
- A bigger disk area equals a lot more thrust
By the logic of wanting to throw as much air down as possible, we can increase thrust by throwing the air down faster. This can be accomplished by increasing the pitch of your blade. So great, let’s have a large rotor and high pitch! Not so fast. Increasing the pitch of the propeller increases the drag of the rotor, and the added pitch doesn’t transfer the energy from the rotor to the air very well.
Principal:
- Higher pitch equals more thrust
- Higher pitch equals low efficiency
Similar to blade pitch, more blades will give higher thrust, but will also increases the drag of the blade as well as the mass.
Principal:
- More blades equal more thrust
- More blades equal lower efficiency
Basically there is a decent amount of math that goes into this, but the big takeaway is this equation.
We notice that the power to hover increases to the power of 1.5, which means adding double the mass requires almost 3 times the power! This is a huge mistake people make: they say “I’m only going to add a little bit of weight”, but then their flight time decrease quite a bit.
Principal:
- Required power increase to the 1.5 power of mass
- A bigger disk area equals more thrust by the square of the radius
- Higher pitch equals more thrust
- Higher pitch equals low efficiency
- More blades equal more thrust
- More blades equal low efficiency
- Required power increase to the 1.5 power of mass
The basic idea behind analyzing the electrical efficiency is to minimize the resistance loss in the circuit. Since P=I^2R, this means that the power consumed by the resistance in the wires increases with the square of the current. This means having a higher voltage system, which in turn reduces the current, is a more efficient electrical system.
Now that we understand how to best use the energy stored within our drone, we can look at selecting motors and escs.
The first thing to do is take the weight estimate you calculated from our CAD and parts spreadsheet and use this as the minimum thrust our motors will need at 50% throttle. The best thing to do is take the estimated weight of the done and add a healthy margin; the more you add, the better off you will be. This is especially important if this a developmental drone. As you continue to develop your application, you will want to add more sensors and actuators to the aircraft, which will kill your flight time quickly. As we noted above, the power required to hover is to the 1.5 power of your mass!