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.

Experiments with downscaled or subscale physical models have traditionally been an essential source of information in aerospace research and development. Physical models are very effective at revealing unforeseen issues and providing confidence in design predictions or hypotheses. While computational methods are predominant nowadays, experimental methods such as wind-tunnel testing still play a critical role as verification and calibration tools. However, wind-tunnel testing is often too expensive, too slow or unavailable during aircraft conceptual design or the early development of immature technologies. It is here that testing free-flight subscale models - referred to as subscale flight testing (SFT) - could be an affordable and low-risk complementary method for obtaining both qualitative and quantitative information.

Disruptive technological innovations have significantly altered both the cost and the capabilities of SFT during recent decades. Such innovations include the price performance of miniaturised electronics and communication systems, advances in rapid prototyping techniques and materials, the availability of specialised components from the booming drone market and the rapid development of open-source software and hardware, allowing for sophisticated and capable test platforms at a fraction of the cost compared to a few decades ago. It is therefore necessary to re-evaluate the benefits and limitations of SFT, as well as its role in contemporary aircraft design and technology development processes.

This dissertation aims to contribute to knowledge on the use of the SFT method for research and development, focusing on low-cost, time-efficient solutions that are particularly suitable for small organisations and limited resources. The method’s challenges, needs and limitations are identified through a critical study of the physical similarity principles, an in-depth review of the experiences of other organisations, and practical field experiments with different subscale models in real conditions. Some of the proposed solutions include a low-cost data acquisition system with custom-made instruments, a novel method for automatic execution of excitation manoeuvres, specific techniques and parameter-identification methods for flight testing in confined airspaces, and a set of tools for the analysis and visualisation of flight data. The obtained results may serve as proof of the current possibilities to evaluate and demonstrate new technology through SFT using very limited economic and human resources.

Following the electrification of the automotive sector, the idea of electric-powered flight for commercial air transportation is becoming, in the eye of the public, the main hope for greener air transportation. To what extent can electric aircraft reduce the energy and environmental footprint of aviation? How should they 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 analyses critically some of the unresolved challenges that lay ahead. Current commercial operations are examined and short-term effects of electrification are identified. Fundamental components, basic design and operating concepts are analysed to highlight unavoidable constraints that seem often overlooked. It becomes clear that electric propulsion alone will not fully meet society’s expectations even if key enabling technologies develop as forecasted. Nevertheless, this paper suggests that electrification may instead become one piece of a propulsion-technology mix that would address more effectively our short- and long-term goals.

The decades-old idea of electric-powered commercial flight has re-emerged along with high expectations for greener air transportation. To what extent can electric aircraft reduce the energy and environmental footprint of aviation? How should they 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 basics and analyses critically some of the unresolved challenges that lay ahead. Current commercial operations are examined and the short-term effects of electrification are identified. Fundamental components, basic design and operating concepts are analysed to highlight unavoidable constraints that seem often overlooked. These limitations are illustrated with a conceptual study of a full-electric FAR/CS-23 commuter and realistic estimations of its performance. It becomes clear that electric propulsion alone will not fulfil the expected goals, but it might be one more step on the way.

Remotely piloted scaled models not only serve as convenient low-risk flying test-beds but also can provide useful data and increase confidence in an eventual full-scale design. Nevertheless, performing advanced flight tests in a safe and cost-effective manner is often a challenge for organizations with limited resources. A typical scenario is testing within visual line-of-sight at very low altitude, a type of operation that offers major cost advantages at the expense of a reduced available airspace. This paper describes some of the authors' work towards efficient performance evaluation and system identification of fixed-wing, remotely piloted aircraft under these challenging conditions. Results show that certain techniques, manoeuvre automation, and platform-optimised multisine input signals can improve the flight test efficiency and the modelling process. It is also probable that some of the benefits observed here could be extrapolated to flight testing beyond visual line-of-sight or even to full-scale flight testing.

Downscaled physical models, also referred to as subscale models, have played an essential role in the investigation of the complex physics of flight until the recent disruption of numerical simulation. Despite the fact that improvements in computational methods are slowly pushing experimental techniques towards a secondary role as verification or calibration tools, real-world testing of physical prototypes still provides an unmatched confidence. Physical models are very effective at revealing issues that are sometimes not correctly identified in the virtual domain, and hence can be a valuable complement to other design tools. But traditional wind-tunnel testing cannot always meet all of the requirements of modern aeronautical research and development. It is nowadays too expensive to use these scarce facilities to explore different design iterations during the initial stages of aircraft development, or to experiment with new and immature technologies.

Testing of free-flight subscale models, referred to as Subscale Flight Testing (SFT), could offer an affordable and low-risk alternative for complementing conventional techniques with both qualitative and quantitative information. The miniaturisation of mechatronic systems, the advances in rapid-prototyping techniques and power storage, as well as new manufacturing methods, currently enable the development of sophisticated test objects at scales that were impractical some decades ago. Moreover, the recent boom in the commercial drone industry has driven a quick development of specialised electronics and sensors, which offer nowadays surprising capabilities at competitive prices. These recent technological disruptions have significantly altered the cost-benefit function of SFT and it is necessary to re-evaluate its potential in the contemporary aircraft development context.

This thesis aims to increase the comprehension and knowledge of the SFT method in order to define a practical framework for its use in aircraft design; focusing on low-cost, short-time solutions that don’t require more than a small organization and few resources. This objective is approached from a theoretical point of view by means of an analysis of the physical and practical limitations of the scaling laws; and from an empirical point of view by means of field experiments aimed at identifying practical needs for equipment, methods, and tools. A low-cost data acquisition system is developed and tested; a novel method for semi-automated flight testing in small airspaces is proposed; a set of tools for analysis and visualisation of flight data is presented; and it is also demonstrated that it is possible to explore and demonstrate new technology using SFT with a very limited amount of economic and human resources. All these, together with a theoretical review and contextualisation, contribute to increasing the comprehension and knowledge of the SFT method in general, and its potential applications in aircraft conceptual design in particular.

This paper reports the current status of a joint Swedish-Brazilian research project aiming at exploring sub scale flight testing. A 13% scale fighter aircraft is used as a test bench for developing methods and procedures for data acquisition. This paper will present an aerodynamic database as a partial result of the collaborative project.

Thispaper presents various techniques and procedures that aim to simplify flighttesting of fixed-wing, remotely-piloted aircraft with the purposes ofperformance evaluation and system identification. These methods have beenspecifically developed for flight within visual line-of-sight, a type ofoperation that limits the available airspace severely but offers major costadvantages considering the current regulations for unmanned flight in mostWestern countries.

This work has the objective to show development of a non-conventional subscale aircraft tests and compare a preliminary numerical analysis with wind tunnel tests. The subscale flight tests can be used as a low cost tool and that can improve the learning curve, considering the future aircrafts are characterized by the use of new technologies and radical configurations. For this validation was built the ITA-BWB aircraft, a single engine and tailless concept. This project demonstrates all steps of preliminary analysis with VLM software, preliminary flight tests and wind tunnel tests, where these data were used for simulations of specific maneuvers used in parameter identification. Finally, the simulations demonstrated consistency between the static derivatives acquired by VLMsoftware and wind tunnel tests, which were used to estimate dynamic derivatives that allowed the simulation of specific maneuvers for subscale flight tests.