How to start coaxial tests?

To start a design:

1. Determine the motor layout: back-to-back or face-to-face on your drone.
2. Make a target thrust output that you wish to reach for the total system.
3. Assuming that you will use the same motor and propeller on the propulsion system, then there can be a 25% to 35% thrust loss in the downstream propeller. Calculate the individual thrust output required.
4. Select a few motors and propellers that may achieve the required thrust while respecting the dimension restrictions on your drone: use the thrust stand to run single-motor tests in order to first characterize the general performance of your chosen motors and propellers.
5. Try to pick multiple propellers that can adapt on the same motor and test those: for example, you may adapt a 26”x8.5” and 28”x9” and 30”x10” propellers on the same motor, make extra tests with all the possible combinations.
6. In a back-to-back design, measure the height of your motors and the minimum thickness of the mounting frames in order to calculate the minimum Z (axial distance) that you may achieve.
7. Place the smallest propeller possible (that can generate adequate thrust and with good efficiency tested in a single-motor) as the upstream propeller and use the same one for downstream.
8. Derive the Z/D factor and see what is the minimum to be achieved, mark down the number and design a few groups of tests with an increment. For example, when you find out the minimum Z/D to be 0.17, you may try to test the stand in 0.17, 0.20, 0.23, 0.26, etc. We regard over 0.5 as over-distanced and it makes no sense to approach or exceed that.
9. Generally speaking, the bigger the Z is, the better it becomes for the downstream propeller. But due to the material thickness and drone frame limitations, the bigger the Z is the heavier the drone will become. Try to find the distance that compromises for a better efficiency and overall thickness.
10. Test coaxial face-to-face or back-to-back for the target distance and run full power: see if the dual-motor system can generate enough thrust for take-off. Test further to obtain the individual performance of each motor and propeller in the dual-motor setup: efficiency, power consumption and calculate flight time.
11. Try to run the previous step with a higher pitch propeller in the downstream position: see if the individual efficiency gets improved;
12. If you wish to achieve a better efficiency, you may first try to increase diameter and pitch of the downstream propeller; and then to increase Z distance; for most of the applications, the least ideal approach is to add radial offset as it is complicated and extremely hard to predict the overall optimization in performance.