Handling Qualities Testing of a Highly Unstable Fighter

摘要

The Eurofighter "Typhoon" is a modern swing-role fighter aircraft jointly designed and built by the UK, Germany, Italy and Spain. The aircraft features a delta-canard configuration; to optimise performance and agility, it was designed highly unstable in pitch.The flight control system (FCS) has undergone a continuous evolution. Step by step, new features were introduced, teething problems were solved and the full performance spectrum, including the additional air-to-surface role with a new array of external stores, was cleared.While one objective during this development process was to provide the safest possible system which delivers maximum aircraft performance, a second, equally important objective was to optimise the handling qualities (HQ) of the Eurofighter.When digital flight control technology became available in the 1970's, some renowned engineers stated that it would soon be a "piece of cake" to perfectly tailor the FCS to the pilots' needs on the drawing board, thus rendering HQ flight test virtually redundant. This optimistic viewpoint was defeated in the Eurofighter program, where the FCS of all systems consumed the greatest amount of flight test hours.One important issue was that wind tunnel testing and numerical methods were not capable of providing certain flight mechanical derivatives with the necessary fidelity. These were, however, essential for the optimum scheduling of the controller gains. As a consequence, substantial in-flight gathering of aerodynamic data had to be conducted so as to provide the precise derivatives. Further, it turned out that a significant disparity was present between open-loop "flying qualities" (FQ) and closed-loop "handling qualities" (HQ). Even where the design engineers were able to adequately model and predict the FQ, the pilot in the (control) loop proved to be the big unknown; his behaviour during certain tasks was found to be far from linear, and gains as well as control strategies varied significantly between individuals.Such complex HQ issues put classic test methods to their limits. The test team decided to test modern techniques like HQDT (Handling Qualities During Tracking), and, after some initial trial and error, obtained extremely valuable results. Furthermore, innovative devices like GRATE II (Ground Attack Test Equipment II) were introduced in an attempt to gather meaningful and systematic data from HQ testing.In summary, the FCS of the highly unstable Eurofighter proved to be a very complex system, both from an engineering and testing point of view. Perhaps the most difficult issue was to understand the pilot in the loop, his characteristics and how he interacted with the FCS; however, an innovativeand open-minded approach to closed-loop testing provided a better understanding of the underlying problems, and thus paved the way for a mature system. As a result, the operational users from the five customer nations now enjoy the luxury of a safe and agile fighter with pleasant handling qualities and a low piloting workload.