Guiding Principles for Flight Test

21 May 2022

The previous post asked the question, “what is flight test?” In addition to offering a definition, it promised a methodology for professional flight test that can be learned and practiced. Let’s dive into that methodology, with an overview of the two guiding principles for flight test.

The two foundational concepts for flight test are model validation and the build-up approach. Model validation is related to the concept of digital engineering or a digital twin. The planned aerospace system is defined in terms of mechanical structure, aerodynamics, electrical, thermal, data systems, and so on. This model of the system may be as simple as first-principles, derived on the back of an envelope and roughly calculated to few significant figures – or may include computational fluid dynamics. The model is exercised to predict how the system is expected to respond in the planned flight conditions. These calculations are often generated before the physical prototype is fully manufactured, in the course of engineering iterations and design decisions. Model predictions should include uncertainty factors for the calculated responses, to account for desired performance margins but also to identify potential exceedances or divergent response. Flight test uses the modeled performance of the system to target test resources to the critical conditions as well as to identify safety considerations. These test results are used to confirm the accuracy of the model or, alternatively, to re-tune the model to match test results. By spot-checking at inflection points and the edges of the envelope, the development process can be confidently and safely accelerated towards operational fielding. Interpolation is sometimes acceptable, but extrapolation is generally not. Model validation is often described by flight testers using the mantra, “predict – test – validate”. The model predicts system behavior, test evaluates the predictions, and the resulting data is used to validate and refine the model.

When choosing conditions for flight test to address model validation, it is important to prioritize and sequence test points using the build-up approach. The term “build-up approach” is a simple way to express that initial flight test conditions should be identified with lower risk and higher confidence in the model, in order to progressively work out to the edge of the envelope. This is not a one-dimensional tactic, but requires a collaborative and multi-disciplinary team to consider how to intelligently move into areas of lower model confidence, higher safety risk, higher energy states, lower terrain clearance, or reduced engineering margin. The build-up approach sequences pre-requisite tests that contribute certainty through model validation. By starting with test conditions that have lower model uncertainty and lower risk, the refined model predictions can be fed-back into subsequent tests to increase safety and efficiency.

Using the model validation method and build-up approach, a flight test team may safely, efficiently, and effectively prove the capability of an aerospace system. Now the question is, since you can’t spend the time and money to test everything – what should you flight test? How do we determine which requirements should drive flight test efforts?