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.

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.