Problem Description

A FBW-System forms an electronic link between the pilot and an aircraft’s components. This serves to increase controllability and safety of any given system.


The intention of this project was to create a functioning fly by wire system (FBW system) with a dedicated aircraft. To achieve this all relevant physical aspects regarding rigid body movement and fluid dynamics where considered. Said information was then used to construct a model of a fixed-wing aircraft. The resulting airframe was then fitted with the appropriate electrical systems to serve as a testing platform for the proposed flight controller and its associated algorithms. These form the main part of the project, ranging from telemetry handling, data acquisition and extended linear quadric state estimation to solution approaches for optimal control problems via closed loop policies. This allowed the implementation of a two axis attitude tracking servo as well as a yaw damping mechanism that fully manages the rudder during flight maneuvers. After the model aircraft was equipped with the FBW system, numerous flight tests were made to determine the tuning parameters. The most successful flight extended over about 200 meters.

Fixed wing airframe

The mechanical design of the unmanned aerial vehicle (UAV) was centered around maximizing the vehicle’s efficiency to increase possible real-world applications. The airframe had to be modeled around a large payload carrying capacity and should offer good fuel efficiency. The set design criteria are best met by a fixed-wing design, which creates lift through immovable airfoils. This also comes with the added benefit of greater stability compared to rotary wings, further increasing system reliability.

Mechanical Aspects

The design process of an airframe involves many important decisions that dictate the flight characteristics.

Wing Configuration

The wing configuration is a major aspect of fixed-wing airframe design, dictating most of the plane's roll dynamics. If the wings of an aircraft are located above the center of mass (on top of the fuselage) a roll stability is induced. Other benefits of this design decision are better ground clearance and easier access for equipment under the wings such as engines. On the contrary wings placed below the center of mass have an opposite effect, making the aircraft's roll axis more unstable but also more maneuverable. The dynamics of this can be compared to an inverted pendulum.

The angle between the wings and the horizontal reference line is referred to as dihedral (angled upwards) or anhedral (angled downwards). A positive dihedral angle has a stabilizing effect, as it creates a rolling torque that forces the airplane to level its wings horizontally.

The proposed design featured high-mounted wings and wing dihedral to form a stable testing platform. It should be kept in mind however, that this configuration decreases roll authority, making fast maneuvers difficult.

Wing Configuration

Adverse Yaw

Adverse Yaw

An aircraft controls its roll angle by deflecting the ailerons. This requires one to be deflected upwards while the other one is inverted, resulting in asymmetrical drag on the wingtips, as a rotational moment acts on the airframe, causing it to rotate around the yaw axis.

This behavior is undesired because it moves the plane's forward axis out of the airstream which has negative effects on aerodynamics and controllability. The angle between an aircraft's heading and the relative wind is referred to as sideslip.

A method to overcome this would be to use the rudder for compensation. This approach was combined with the implementation of differential ailerons. These deflect further upward than downward, balancing the drag at each wingtip while still creating a rolling motion.

Navigation

The navigation of a fixed-wing aircraft can be divided into longitudinal and lateral components. The first movement plane contains the aircraft's altitude. It is mainly a result of the lift generated by the wing and and gravity. The lift is dependent on the specific wing design, (relative) airspeed and the angle of the airstream to the wing, referred to as angle of attack (AoA). An aircraft's altitude is mostly controlled over changing the angle of attack by pitching the plane using the elevator or changing the airspeed by altering the engine power.
The lateral component involves the airplane's roll axis, which is essential for performing turns on the horizontal plane. The direction an aircraft is heading towards on said plane is referred to as its heading and is normally referenced to north. Rolling a plane causes the lift force, that always acts perpendicular to the wings, to point sideways, allowing to perform the turning maneuver. This however results in a smaller part of the lift acting against gravity which causes the aircraft to loose altitude. The most common way to counteract this effect is by increasing the lift again through use of the elevator. The radius of a turn maneuver is dependent on the current velocity and the roll angle. It should be noted that excessive roll angles create large G-Forces that pose a lot of stress on potential passengers and the airframe itself.

Longitudinal Plane

Lateral Plane