Flight Mechanics, Flight Control and Aeroelasticity


Environmental and commercial requirements are shaping a new generation of more efficient and eco-friendly airplanes. These new requirements have impulsed the evolution of Flexible Aircraft (FA) and Very Flexible Aircraft (VFA). The main characteristic of FA/VFA are high aspect ratio wings. Increasing aspect ratio leads to the increase of the aerodynamic efficiency. This means that a greater lift to drag ratio will be generated, which causes a reduction in fuel consumption and carbon-emission reducing the environmental impact of the aircraft. To attain structural requirements and reduce as much as possible the structural weight, designers use composite materials; nevertheless,
these materials include nonlinearities to the structural behavior. The result are very long and flexible wings. Increasing the aspect ratio and structural efficiency of the wing tends to decrease the frequencies of aeroelastic modes. This leads to a coupling between the aeroelastic and the flight dynamics of the aircraft. As a result, the performance
characteristics, flight dynamics and handling qualities of the aircraft change completely with respect to the rigid-body approximation.

Modelling the dynamics of flexible aircraft depend on its level of flexibility. For slightly flexible aircraft (wing tip deformation of up to 10% of the wing semi-span), formulations based on a linear structural behavior are used, e.g. based on mean-axis approximation, and they can be coupled with different aerodynamic models. If the deformations
go above 10%, the aircraft enters a new level of flexibility and it is classified as VFA. These aircraft demand the use of nonlinear structural dynamics and nonlinear aerodynamics. This is normally done by using strain based and geometrically-nonlinear beam formulations. All these aeroelastic formulations have something in common. The lack of available data for validation. To complete an appropriate model validation, it is necessary to have reliable aeroelastic data. To this date, there is a lack of flying platforms that resembles airliners or jet transport aircraft with flexible or very flexible wings that display coupling of flight dynamics and structural modes in flight.

The idea within this project is to develop an aircraft prototype capable of displaying coupling between rigid-body and structural dynamics during flight. This testbed will allow to better understand the implications of increasing flexibility in the ultimate goal of designing greener aircraft. The conceptual vehicle has been called TU-FLEX.

The conceptual design of the TU-FLEX already includes a maximum wing tip displacement as a boundary condition in the optimization process. Initially, there will be two versions of the TU-FLEX, each with a different set of wings. The FA version of the vehicle, where the deformation of the structure should remain inside the linear flexible-boundary of 10% during the maneuvers. The VFA version of the vehicle will be designed to show geometrical nonlinear behavior. The aircraft should operate regularly above the 10% limit and a maximum deformation of 20% with respect to the span will be allowed. The TU-FLEX multiple-wings conceptual design is presented in Fig. 1. In this illustration is possible to compare the rigid body wing with the flexible and the very flexible ones.

TU-FLEX will be defined as a flexible testbed with a configuration that permits to trace conclusions applicable to both, transport and commercial airliners. The design and construction of this platform would allow:

  • To complete flight experiments to collect coupled motion between flight and aeroelastic dynamics in response to control inputs;
  • To serve as a ground experiment testbed to define new procedures to characterize the elastic and aeroelastic properties of FA/VFA by static, ground-vibration and wind-tunnel testing;
  • To implement structural regulators or structural controllers that allow FA/VFA to flight safely;
  • To carry different control architectures to stabilize and navigate FA/VFA;
  • To evaluate a variety of sensors and different sensors-configurations to rebuilt the shape of FA/VFA;
  • To evaluate different control surfaces architectures to better regulate and control FA/VFA;
  • To implement novel system identification techniques to capture FA/VFA aeroelastic-dynamics.