You can extract a ton of value out of the force and moment data that nearly every test collects. But like I said before, you can measure all sorts of things during a test, including more detailed pressure data than just the general tunnel conditions alone:
- Measuring air pressures across a surface quantifies how the distribution changes with different flow conditions and angles—and can help identify why a certain design feature isn’t producing the aerodynamic impacts you expected.
- Including air data systems where they’d be on the real aircraft makes it easy to generate corrections for any unique setups—and prevent aircraft loss due to the autopilot using an unreliable airspeed.
Measuring air pressures across a surface:
To get raw air pressures in a small localized area, you can build a model with little pinhole openings on some surfaces, like the wings or tails. These openings go through to the interior of the model and have thin flexible tubes connected. The tubes then route to a pressure measurement device held either inside or outside of the model.
These openings, called pressure ports, allow you to measure the air pressure on that surface at each port. You can then capture how the pressure distribution changes with different flow conditions and angles.
This is great for troubleshooting why a certain design choice isn’t creating the behavior you wanted. Usually though, it’s used to more precisely tune CFD modeling, because it gives you the truth data to fix specific regions of the CFD model instead of doing an overall tweak and hoping it does what you want.
Including air data systems:
An aircraft’s onboard air data system is what lets it measure external air pressures to figure out how fast it’s flying and at what altitude.
Having an accurate conversion of air pressure to airspeed is absolutely critical: if your aircraft thinks it’s flying a few knots faster than it actually is, you’re just baiting disaster.
What makes this worse is some air data systems will have one of the two pressure measurements somewhere on the fuselage, where the measurements are impacted by the shape of the fuselage.
Thankfully, it’s often straightforward to incorporate a few pressure ports in the exact places they would be on the real aircraft. Especially if your test article is the same size as your real UAV, you’ll collect data on what all your air data ports are reading, and you’ll know what the true airspeed is because the tunnel is collecting that data anyways.
From this information you can then generate an accurate correction to the air pressures the aircraft reads in flight. With just a bit of extra effort and cost, you can make sure that one of the most reliably punishing risks to your aircraft is substantially mitigated.