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The coaxial distance, defined as Z, is the physical distance between two propellers on the axial direction. This parameter is significant to the dual-motor testing because it determines the airflow inlet from the upstream propeller towards the downstream propeller.
When doing coaxial testing, it is extremely important to have two motors counter-rotating. This means one propeller in clockwise direction (CW) and another in counterclockwise direction (CCW). They must generate either push or pull (thrust) in the same direction, while attempting to neutralize the torque.
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Figure copyright: NASA@SC15
When it comes to airflow, the upstream propeller will generate air flowing through both axial and radial directions as you may see in the figure above. This will require the downstream propeller being able to attack the incoming airflow in both directions.
The smaller the Z is, the closer those two propellers become. After the acceleration and twisting, air flows out from the upstream propeller with axial and radial speed and pressure and the closer the downstream propeller is placed, the more turbulence and more severe radial resistance it must suffer.
In coaxial testing, we use the term Z/D factor to represent the relationship between the axial distance Z between two propellers and the propeller's diameter D. If you have two propellers in different sizes, use the smaller one. The Z/D factor is a significant factor to be controlled and observed in coaxial testing and we suggest this factor shall not be smaller than 0.15.
The smaller the Z/D factor is, the more easily the downstream propeller being affected by the turbulence. You may observe negative thrust or torque and other phenomena related to the low Z/D factor.
To start the test, we suggest to first try testing in a face-to-face setup and reach the minimum Z/D factor of 0.15. Gradually increase this factor and make measurements until you can reach the best efficiency, mark down the Z distance and see if you may reach it with the back-to-back coaxial.
It’s hard to predict the overall performance of a coaxial system without testing realistically different Z/D factors. Please also keep in mind that when two propellers have different diameters and pitches, the optimum Z/D value may also differ.
In a common application, some theories indicate that we shall have the downstream propeller larger in diameter and higher in pitch: it can theoretically avoid the downstream propeller under full turbulence and an increased attack angle can also help to tackle the counter-rotating air flow. You may see some of our tests on the Tyto Robotics database: https://database.tytorobotics.com/tests.