A new yaw probe

This post covers the design and testing of a new yaw probe design carried out by a first-year aero team member. The intention was to investigate a small vertical symmetric aerofoil design that utilises differential static pressures in an attempt to improve the somewhat poor precision of the current probe that relies solely on dynamic pressure components (briefly discussed in It's off to track we go).

The first tests were run in Java Foil as a batch run of 2D NACA profiles of varying thickness to find the best trade-off between maximising static pressure difference for a given change in angle of attack, while avoiding separation near or upstream of the ideal pressure tap location. Separation will be reduced on a 3D profile though, so with this in mind, the target for the 2D profiles was to find a case where the separation point coincided with the ideal pressure tap location at 10 degrees incidence (towards the upper bounds of the yaw angles we expect to see under aggressive but controlled driving) and at Re=50000. The best candidate ended up being 0014 (Figure 1). 

The 0014 profile was then run through CFD as a finite 3D profile with the pressure taps at 50% vertical span. I decided Ansys Fluent should be used instead of our usual Star-CCM+ as it's a bit more user-friendly for beginners and it's quicker for setting up one-off simple simulations like this one. Velocity was set to 10m/s (the slowest speed that we would be interested in looking at yaw angles) and chord length was 100mm, giving the Re of 50000 as mentioned above. The aim of these simulations was to get a more accurate estimate for pressure differentials at different angles to see if the concept was worth continuing with. The results here were pretty linear, and suggested 12-13 Pascals increase in differential pressure for each degree increment, tested up to 13 degrees (Figure 2). With the 10 Pa uncertainty (+/- 5 Pa) previously calculated for the pressure sensors, this gives a simulated 0.8 degree precision at 10m/s. This is nearly at the original goal of 0.5 degree precision, and is significantly better than the 2 degree precision measured on the existing yaw probe, so the go-ahead was given for manufacture (Figures 3, 4, 5). Given how far forward the pressure ports are, we probably could have cut the aerofoil chord length in half and reshaped the profile to reduce its size without affecting the readings. I decided not to though, partly so it was easier to mount the pitot-static probe on top, but mostly just for simplicity of the task for the first-year student. 

The finished yaw probe (Figure 6) was calibrated in the wind tunnel at yaw angles from 0-9 degrees in 1-degree increments, and velocities of 5, 10, 13, 16, and 19 m/s. The results showed similar trends across all airspeeds and a generally smooth surface which was nice. All tests were run on a single differential pressure sensor, with serial outputs read through a laptop and converted to Pa using the previously calibrated conversion for that particular sensor. The results are expressed as a surface in Figure 7. 

We found later that true 0-degree yaw was achieved closer to the wind tunnel's 1.5-degree mark, suggesting the mount plate was rotated slightly. Consequentially, we effectively only tested up to 7.5 degrees, so future calibration testing may be required if we see the car regularly exceeding this at track (we haven't had enough testing with the current yaw probe to know this with much certainty, especially given its low precision). 

Figure 7: Surface plot of the corrected raw wind tunnel test results from multiple view angles

Out of interest, I compared the experimental results (corrected for the 1.5 degree offset) to the 3D CFD results, as shown in Figure 8. The agreement was very good considering the slightly imperfect resin-coated surface finish on the manufactured part, and the lack of consideration of y+ values and any sort of mesh independence tests in the CFD model. 

The last step was to figure out where the best spot to mount it on the nose of the car is. Given the response curve of the probe was close to linear (with only a slight sensitivity decrease around the 4 degree mark), it didn't really matter where it was mounted, so we just went with the most logical place, in-line with the front tyres/axle. Our IMU gives a very good estimate of instantaneous corner radius (and a useless slip angle estimate), so converting the yaw probe reading to an effective centrepoint of the corner should be easily achievable. Ideally the yaw probe would just focus on wind effects, with the IMU doing the heavy lifting for slip angle relative to the ground, but our IMU slip data is so heavily discritised that the resulting low precision renders it unusable, so having a good yaw probe is extra critical for us.