Sunday, February 11, 2018

Yaw bias due to asymmetric mounting

One of the things we noticed when we flew yesterday is that the ball was consistently deflected left (i.e. showing the airplane yawed to the right) throughout the flight.

The probe was mounted on the right hand side of the plane. We can imagine the airflow probably looks something like this:



It's not surprising, therefore, that the probe "sees" an airflow that tends to come from the left.

The problem, though, is whether this "yaw bias" is consistent across flight conditions. If it is consistent, then for a given probe mounting position, on a given airplane, all we need do is add an offset in the display software, and we're done. If it's not consistent, then we need to do something more fancy.

We do have fancy tricks we can pull out, where we "fake out" a good-enough yaw reading using an accelerometer and scaling to the current IAS, but we'd rather not play these games. The closer we are to a simple glider yaw string, the better.

Now a word about estimating angle of yaw. It turns out (we describe this in our original 2016 paper) that the ratio of pressure difference between left and right holes and the pressure difference between center hole and static is a nondimensional quantity that, according to potential flow theory, is a predictor of yaw. (The same is true for upper and lower holes, and AoA.) It also turns out that the relationship to angle is very nearly linear, with a scaling factor of about 11.5 degrees for each unit of ratio difference. With that, we can do a very rough computation of angle of yaw based on our pressures, and this is what we'll be using here.

Our intention is to get a general clue as to whether we have a project, given our current setup, or whether we need to seriously go back to the drawing board.

My conclusion at the moment is that we still have a project. Let me show you the data. First, here is the center hole dynamic pressure plotted at the top, and rough-calculated angle of yaw below, for the entire test:


What a mess, right? When we're sitting on the ground, the data is all over the place. Notice though that, halfway into the first flight, we do some maneuvers, including slowing down (this is when we tried stalls). Zooming into this maneuvering period, we have:


This looks more like data, and you can see the near-constant offset of the yaw signal. If I had to guess, I'd say it's about negative 2.5 degrees or so. The question is how this yaw signal varied with conditions. We can zoom into the area near the left of the plot where we are slowing down graduallyin preparation for the stall to see more:


There is a trend towards the right-yaw signal correcting to the left towards the low end of the speed region -- but is that real yaw that happened as we slowed down?

Finally, for this section of the data, we can plot center hole dynamic pressure versus yaw to see if there's a correlation:
Clearly there is some sort of correlation. It seems to go from a rough average of about -3.2 degrees at high speed to -2 degrees at low speed, a difference of 1.2 degrees. Again, I have no idea if this is a variation in actual yaw or the "fake" yaw due to our mounting.


In the big picture, we are building an operational instrument for pilots, not a flight test instrument. The gold standard for a flight-test-worthy airdata probe is a large boom that sticks out in front of the airplane. We can't tell every pilot to install one of those. So what's good enough for piloting?

While this question remains open, we certainly can do more data acquisition to study this issue. There are various avenues, at least a few of which are:

  • Trying out various installation locations on the same airplane;
  • Trying maneuvers with an experienced pilot remaining coordinated, and looking for correlations in that data; and
  • Acquiring data from an accelerometer and correlating with our yaw signal.
One more thing that's worth mentioning is that the standard inclinometer ball is already "wrong". For  a given angle of yaw, the ball is less sensitive at lower airspeeds (in fact its sensitivity should follow the square of the airspeed, so the difference is not trivial). Our yaw signal should be scaled properly at all airspeeds -- and it does not lose sensitivity at the low speeds when you need it most.

We hope to figure out what is the "best" yaw estimation method given convenient mounting on everyday airplanes. It might be our current aero-derived yaw, or it may be something else. Stay tuned!

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