• biplane
• box wing
• HALE
• remote sensing
• solar
• surveillance
• telecommunication
• UAV

In the last few years, the interest regarding solar powered aircraft has increased significantly and, in particular, the attention has been focused on Unmanned Aerial Vehicles (UAV), able to perform High Altitude and Long Endurance (HALE) missions for telecommunication, remote sensing, surveillance and control, both for civil and military applications.

The present dissertation shows the results of a research about Solar Powered Biplanes (SPBs), dealing with the definition of a design methodology and the introduction of new design tools for this kind of aircraft.
Chapter 1 illustrates the purpose of thesis and the potential benefits of choosing a biplane architecture, among which the most important is the increase of structural stiffness without reducing the aerodynamic efficiency.

In order to understand the challenge represented by the solar powered flight and to give a picture of the context in which the present research is placed, the history of the solar powered flight and the state of the art about solar powered aircraft are presented in Chapters 2 and 3.
Chapter 4, then, gives an overview of the research activities about the box-wing systems and presents the state of the art on the solar powered biplanes, which have been studied in the past years at the Department of Aerospace Engineering (DIA) of Pisa.

Chapter 5 introduces the mission requirements and the definition of two types of mission, conceived in order to perform the two main tasks of such kind of UAVs: Telecommunication (TLC) and Intelligence, Reconnaissance and Surveillance (IRS) operations.

The selection of system components is presented in Chapter 6, where the characteristics of different types of solar cells, solar panels technologies and rechargeable batteries are illustrated.

Chapter 7 provides the geometric definition of a generic SPB configuration and some simplifications are introduced in order to define the main design parameters.

The design procedure is illustrated in Chapter 8, which opens with a description of the effects of each design parameter on the SPB design.
A method for the preliminary aircraft sizing, based on design curves, is proposed and the other steps of the procedure are introduced: a section for input data, one for the definition of additional requirements, one for the procedure settings, and the last one for the optimization method adopted.
This latter is the most important part of the procedure and it is presented with particular attention to the related objective function, defined as the aircraft mass multiplied by a penalty function, which is activated when the aforementioned additional requirements are not fulfilled.

The objective function evaluation is performed by means of the so-called "sizing process", which is described in Chapter 9. It is composed of several models for Aerodynamics, Flight Mechanics, energy balance evaluation, structural analysis and propulsion system sizing.
When describing such models, the thesis focuses on some important aspects, such as the definition of wings' airfoils, the method for the minimum required power and cruise speed calculation and the fulfillment of Flight Mechanics requirements by means of a mass balance strategy.
The models for the evaluation of accumulators, structures and motors masses are presented as well.

Finally, Chapter 10 presents the designed SPBs and, by means of a tool introduced in this work and called "Operating Domain", the flexibility in terms of altitude-latitude-day conditions is analyzed for such configurations.
Among the achieved results, it is worth of notice a SPB configuration able to fly in each year's day, at latitudes up to 45° and altitudes up to 18000 m. Such performances meet some of the requirements defined by the U.S. Defense Advanced Research Projects Agency (DARPA) for the ongoing Vulture Program and indicated as goals of the HALE flight.
In conclusion, an example of a multi-purpose configuration is presented to show the flexibility of the SPB concept. The presented aircraft can perform both TLC and IRS missions, flying at altitudes up to 17000 m, and, since its modular geometry, it can be modified to fly above 18000 m during Summer.