Wind tunnels can give us an immense variety of data types, qualitative insights, and also just plain pretty visuals. Almost anything you can think of to measure in a wind tunnel can be measured in a wind tunnel.
There are a few data types that are nearly universal to all tunnels:
- Measurements of air pressure and temperature are taken as the airflow enters the test section, to record exactly the conditions the test article (or model) is “seeing.”
- A device called a balance reads the forces and moments the model experiences due to the airflow over it. These are specially designed to isolate the model from everything else so they measure exclusively what the airflow does to the model. (Balances are super impressive in their own right, so I’ll talk more about them in a later email.)
If you were to go to a test and only collect these two kinds of data, it would still be incredibly successful. These quantities alone allow you to take the output forces and moments (lift, pitching moment, etc) and convert them into coefficients.
But what do we do with those coefficients?
One of the most useful things about wind tunnel testing is that it provides us with what is more or less the most accurate data we will ever get about our vehicle’s aerodynamics. We can then use this data as a baseline for every other analysis we do in our design process.
What is most common is taking results from a computational fluid dynamics (CFD) simulation and comparing them to the wind tunnel data of the exact same configuration.
Plotting these two data sources on top of each other will reveal ways in which the CFD model doesn’t behave the same way as the wind tunnel model. The CFD may predict the aircraft stalls at a higher angle of attack, or generates more drag, than is real.
Having the “truth” data allows an engineer to adjust the settings of the CFD run to have it match the wind tunnel data. This then lets the CFD simulation be actually trustworthy going forward when analyzing other incremental changes to the vehicle.
Maybe a new payload is added that sticks out awkwardly, or the vertical tail increases in height. Wind tunnel tests are expensive and time-consuming, so it’s not feasible to do one for every single change to an aircraft. But if you have a CFD model that’s been precisely tuned using wind tunnel test data, you can use that instead to figure out the impact of a modification to your aircraft.
This is one of the most valuable payoffs of a wind tunnel test, and why I think they’re such an excellent investment for most design programs.
You don’t get data for only the configurations you tested. You get a truth source to use to improve your toolsets and analysis, one that will serve you and your vehicle well into the future.