How to measure top speeds of multirotors

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How fast can your multirotor fly? The simplest solution for this question would probably be to attach a GPS device (tracker / OSD) to your multirotor and to read out the maximum speed.

But hey, this is pretty imprecise...!

GPS devices suffer from measurement noise (e.g. through signal degradation), which becomes problematic on rapidly accelerating objects like our multirotors. More advanced GPS chipsets (e.g. u-blox LEAx) have the option to choose between different filtering modes (e.g. pedestrian, car, airborne), that make some assumptions on the maximum accelerations and movements of the sensor and filter the results accordingly. That will most likely improve the accuracy of the measurements, but still noise might remain. Here is an interesting article on GPS speed measurements, which states that the classical methods to calculate velocities in GPS receivers have an accuracy in the order of a few meters per second, due to significant noise.

I was recently attaching a pretty good GPS logger to a quadrotor (HUMs with Cobra 2204, 75C 4S 1300 mAh, C-prop 5x3). There seemed to be hardly any wind. The "Top Speed" value reported by the tracker was 113 km/h. But this value includes the influence of measurement noise and wind, and it is therefore not precise.

The only way that I know of how to deal with these problems and how to get the real (air)speed out of a GPS logger is to do the following:


  • Fly full speed at a constant height, on a straight line and against the wind for at least 300 meters. 
  • Then, turn around and fly the same straight line but in the other direction. 
  • Download the data from the logger and calculate the average of each of these two straight line runs (this will remove or at least attenuate noise). 
  • Now, calculate the average of the two runs (this will remove the influence of wind and yield the true airspeed).


The true airspeed of my copter was calculated to be 95 km/h, which is 16 % lower than the maximum speed reported by the logger. And I must add that this was a day where I felt hardly any wind. Still the wind velocity was about 10 km/h according to the GPS measurements. More wind will of course make the difference even bigger.

So if you hear people saying "Hey, my quadrotor is flying 150 km/h!", then be careful until you saw the measurements...


Today, I'll hopefully measure the top speed of my Shrediquette DERBE, let's see what comes out...!

Comments

  1. Jerome Demers3 October 2015 at 15:00

    I saw some pictures of guys using radar gun to know the speed of their miniquad.
    and yes we all saw youtube video of people claming 150km/h with there quad.

    The radar gun measure a brief moment and you could go slow in that measurement period. With your air sensor, you actually measure the entire flight and can do much more with the data.


    I see the gun radar been cheap, easy to use and a reliable solution.

    What do you think?


    ReplyDelete

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