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What is back-to-back and face-to-face testing and which configuration shall I choose to test?

 In a dual-motor testing environment, you may have two motors and propellers to be mounted on a propulsion system in order to increase the output thrust and neutralize the torque without adding much extra structure on the drone. 

 The following versions may allow you to run coaxial tests: 

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The nature of the dual-motor testing introduce some new parameters:

  • Axial distance: shown as Z in the figure above, physical offset between two propellers in the axial direction
  • Radial offset: shown as e in the figure above, physical offset between two propellers in the radial direction
  • Diameter difference: two propellers may have different diameters
  • Pitch difference: two propellers may have different pitches

There are also a few extra parameters when it comes to controlling two motors and ESCs: the rotation speed difference between two propellers and the air speed and pressure generated by the propeller in the front. It is complicated to model all these parameters in a single test and we suggest to first find out the target setup you may need to start a test.

Generally speaking, most of the coaxial propulsion systems on a UAV use the back-to-back design. The advantage of a back-to-back design is mainly that both motors are mounted onto the same drone frame structure and may potentially reduce the material and weight on the drone structure.

If you wish to use the Flight Stands to run back-to-back tests, please keep in mind that there is a minimum coaxial distance imposed on the thrust stand. You may not be able to reach the desired distance between your two propellers due to the distance restriction. 

When a distance cannot be reached in a back-to-back setup, we recommend using the face-to-face setup as an alternative. Face-to-face setup allows you to reach a minimum distance of 0 in theory. The disadvantage of this setup is that the aerodynamic effect may not be exactly the same as a back-to-back setup.

From what Tyto Robotics has tested internally, we found out that the difference between a back-to-back setup and a face-to-face setup is not significant when all propellers are large enough. The smaller the propellers are, the smaller the thrust they may generate and thus make the drag effect more obvious and the test result less accurate.