One issue reported by some early-adopters of the Vortex is ‘Yaw Slippage’. This doesn’t affect many quads, but for those that are affected, the information below should help to diagnose, and resolve it.
This is a fairly long document, for an executive summary, scroll to the bottom. 

There are several reasons for yaw slippage, including PID/FC parameters incorrectly set, center-points of the radio incorrectly set, and one which will be discussed here, the alignment of the motors.

The Vortex uses a tubular motor arm as opposed to the flat Carbon Fiber arms used on many other racing quads. These tubes offers several advantages, one of which is of course the ability to hide the ESC in the tube, where it remains protected from the elements (and passing branches), another being better airflow as the prop wash hits the arm.

The disadvantage of a tubular motor arm is that it makes perfect alignment of the motors a challenge.

Each of the arm tubes is drilled with three holes, one which pierces through the entire tube, for the arm folding pivot mount, and two which are used to key the motor mount. The tolerances of the plastic parts, and the size of the holes themselves result in almost zero ‘play’, once the quad is assembled (unlike the early prototypes which needed aligning after every crash).

These holes are drilled on a custom-made ‘jig’ which ensures that they are as precise as possible, but unfortunately not perfect (as is the case with any mechanical production process).

What analysis of many early production Vortexes taught us is that the acceptable error in the angular ‘skew’ of the drilled holes is less than we had assumed.

When, for example, the rear arm tubes have a slight ‘twist’ in the same direction (i.e. both rotated forwards, or both backwards’), the flight characteristics are still good, without demonstrating ‘yaw slippage’. When, however, both arms are twisted in opposite directions (i.e. left rear arm rotated forwards, and right rear arm rotated backwards), some degree of yaw slippage can be observed.

After the first production build, ImmersionRC tightened up the accepted tolerance of this skew in the arm holes, to +/- 0.5 degrees. With this tolerance, the yaw slippage is not perceivable.

For quads which are already shipped, this blog post will document a simple method for measuring, and correcting, any arm twist.

First step in measuring arm twist is to either get a ruler out, or for those of you with a 3d printer (or a mate with a 3d printer), print the part shown to the right.

The STL file to print this may be downloaded from here: Download (58.8 KB)

To measure the arm twist using a ruler, install a 5″ prop, place the quad on a flat surface, and measure the distance between the tip of the prop, and the flat surface, with the prop perpendicular to the tubular arm.

Use the same tip of the prop to measure both sides (eliminating the error caused by bent props).

To use the 3d-printed alignment tool, every step is 1mm, slide the tool under the tip of the prop, and note which step it sits on. Then rotate the prop 180 degrees and measure the opposite side.

For every 2mm of difference between the height of the two prop tips there is approx. a 1 degree twist in the arm, which should be corrected.

Once the error is measured, it can easily be fixed using readily available materials.

Using a bit of trigonometry, a 2mm difference in height between the two blade tips of a 5″ prop, requires a 0.14mm thick piece of tape, or thin washer, under the low side of the motor mount (between the motor mount plate, and the motor itself). A typical piece of ‘Scotch Crystal’ tape is 0.02-0.04mm thick, just for reference.

Several layers of very thin tape can easily be stacked to achieve the correct thickness.

Note: The image shows a copper foil being used, which is a little thicker than most plastic-tapes. Any tape which doesn’t easily compress under pressure will work. 

Another Note: Take care when repeatedly removing the motor. If the copper wires that exit the motor are flexed too much, too many times they can fracture (as is the case with any brushless motor). 

Executive Summary

  • Vortex ‘Yaw slippage’ can be caused by slight errors in the position of the holes milled in the tubular arms.
  • A small number of quads seem to be affected from the first batch, the tolerance of the holes in the arms of the second batch of Vortexes, shipping now, is much improved.  
  • Measuring the twist, and fixing it using readily available materials is easy following the recipe provided.

ImmersionRC is committed to delivering a quality product. If you have received a quad which presents a large arm skew (> 2mm height error between tips), please contact repairs@immersionrc.com and a replacement arm will be arranged.