Testing of untethered subscale models, often referred to as subscale flight testing, has traditionally had a relatively minor, yet relevant use in aeronautical research and development. As recent advances in electronics, rapid prototyping and unmanned-vehicle technologies expand its capabilities and lower its cost, this experimental method is seeing growing interest across academia and the industry. However, subscale models cannot meet all similarity conditions required for simulating full-scale flight. This leads to a variety of approaches to scaling and to other alternative applications. Through a literature review and analysis of different scaling strategies, this study presents an overall picture of how subscale flight testing has been used in recent years and synthesises its main issues and practical limitations. Results show that, while the estimation of full-scale characteristics is still an interesting application within certain flight conditions, subscale models are progressively taking a broader role as low-cost technology-testing platforms with relaxed similarity constraints. Different approaches to tackle the identified practical challenges, implemented both by the authors and by other organisations, are discussed and evaluated through flight experiments.

Unless a segregated airspace and the corresponding clearances can be afforded, flight testing of remotely piloted aircraft is often done near the ground and within visual line-of-sight. In addition to the increased exposure to turbulence, this setup also limits the available time for test manoeuvres on each pass, especially for subscale demonstrators with a relatively high wing loading and flight speed. A suitable testing procedure, efficient excitation signals and a robust system identification method are therefore fundamental. Here, the authors use ground-based flight control augmentation to inject multisine signals with low correlation between the different inputs. Focusing on initial flight-envelope expansion, where linear regression is common, this paper also describes the improvement of an existing frequency-domain method by using an instrumental variable (IV) approach to better handle turbulence and measurement noise and to enable real-time identification analysis. Both simulations and real flight tests on a subscale demonstrator are presented. The results show that the combination of multisine input signals and the enhanced frequency-domain method is an effective way of improving flight testing of remotely piloted aircraft in confined airspace.

The following publications are not directly included in this thesis, although they constitute an important part of the background.

The decades-old idea of electric-powered commercial flight has re-emerged alongside high expectations for greener CO2 emission-free air transportation. But to what extent can electric aircraft reduce the energy and environmental footprint of aviation? What should such aircraft look like, and how does their operation compare to conventional jet aircraft? What technologies are needed and which of them are already in place? This paper goes back to the basics of flight physics and critically analyzes some of the unresolved challenges that lay ahead. Current commercial operations are examined and the short-term effects of any electrification of short-range flights are quantified. Fundamental system components and basic design and operating concepts are analysed to highlight unavoidable constraints that often seem to be misunderstood or overlooked. These limitations are illustrated with a conceptual study of a full-electric FAR/CS-23 commuter aircraft and realistic estimations of its performance. It becomes clear that electric propulsion alone will not fully meet society’s expectations, even if key enabling technologies continue to develop as forecast. Nevertheless, this paper suggests that electrification may instead become one piece of a propulsion-technology mix that would more effectively address our short- and long-term emission goals.

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