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 aerofoil design in an attempt to improve the somewhat poor precision of the current probe that relies solely on dynamic pressure components (shown in It's off to track we go).

The first tests were run in Java Foil as a batch run of 2D symmetric NACA profiles of varying thickness to find the best trade-off between maximising static pressure difference for a given angle of attack, while avoiding separation near or upstream of the ideal static port location on the aerofoil. Given separation will naturally be reduced on a 3D profile, the target for the 2D profiles was to find a case where the separation point coincided with the ideal pressure tap location at a mid-high yaw angle of 12 degrees, with Re=50000. The best candidate ended up being 0014. 12 degrees is towards the maximum yaw angle expected based on corner geometry assuming no tyre slip for typical slalom corners, although this doesn't consider the influence of wind. 

The 0014 profile was then run through CFD in 3D, with the pressure measurement points positioned at 50% vertical span. Velocity was set to 10m/s (the slowest speed that we would be interested in looking at yaw angles). 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 a 12 Pa increase in differential pressure for each degree increment, simulated up to 13 degrees. With an estimated uncertainty of 15 Pa for the sensors and yaw probe combined, this gives a 1.25-degree precision at 10m/s. This still didn't achieve our previous 0.5-degree goal, but is nearly three times better than what the previous yaw probe could manage.

The finished yaw probe 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. 0.2-degree precision was achieved at 19 m/s. 

Higher angles should have been tested but time was cut short, so another calibration session will be needed.

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.

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 good considering the perfectly smooth surface assumed in CFD, the lack of any sort of mesh independence test, and the relatively large experimental uncertainty at this velocity.

For us, the best spot to mount the probe is as far rearward on the nose as possible. This will minimise incident yaw due to steering, keeping the maximum expected yaw angles as low as possible to leave a buffer for wind influence, and also just to keep the aerofoil from completely stalling.