Constructal design: Aeroelastic instability and flow of stresses

Aeroelastic phenomena involve aerodynamics, dynamics, and elasticity. Our recent research shows “why” a certain design parameter (engine placement) influences the aeroelastic flight envelope of the aircraft. Our approach is based on the Constructal Law and the principle that the better design provides better access for the flows that inhabit the system. This is in sharp contrast with trial and error techniques, such as optimization — which entails choosing from different choices, cases, and designs. Heeding the flutter property of different configurations, we have chosen some important cases that address maximum and minimum flutter areas to investigate the flow of stresses through aircraft wings.

The results reveal that when the stresses flow smoothly, the stability of the structure improves. On the other hand, in cases where the location of the engine causes stress strangulation, flutter speed decreases considerably. The worst stress strangulation occurs when the engine is placed at the area of minimum flutter speed. Conversely, the smoothest flow of stresses occurs for the engine placement at the area of maximum flutter speed.

Body-Freedom Flutter Suppression by Engine Placement

Nonlinear Aeroelastic Trim And Stability of Hale Aircraft, NATASHA, is the computer program used to simulate the aeroelastic behavior of these aircraft. NATASHA uses the Nonlinear Composite Beam Theory of Hodges and the finite state induced airflow model of Peters et al. The engines were modeled as follower forces with mass, inertia, and angular momentum, and the flying wing, whose geometry was similar to Horten IV, was modeled as a beam-like structure with initial twist and curvature. Extensive parametric studies were carried out on two-engine and four-engine configurations, and NATASHA’s results showed that the aeroelastic flight envelope could be extended up to four times that of the base model by the right choice of engine placement. A thorough flutter calculation for all possible engine placements is computationally expensive. My research provides a methodology for finding the highest flutter zone with the potential to increase the flutter speed while using the area of minimum kinetic energy of the unstable mode.

Time Dependent Engine Excitations, High-Aspect-Ratio Wing

Another aspect of our research concerns the transient excitation of an aircraft’s engines that can only be studied using time domain analysis. These excitations are large and cannot be simulated with small perturbations about an equilibrium state (i.e., the Hopf bifurcation approach in the assessment of stability). In my research, the dynamic behavior of a lightweight, small-class thrust, turboshaft engine (JetCat SP5) is simulated by the JetCat SP5 engine simulator for both rectangular pulse and ramp fuel inputs. The nonlinear aeroelastic response of the wing to these kinds of excitations is examined for different engine placements along the span with offsets from the elastic axis. It was shown that these excitations are prone to limit cycle oscillations (LCO) even below flutter speed.

Passive morphing of flying wing aircraft

A morphing flying wing can maximize the energy absorption of solar panels on the wing surfaces by changing its configuration such that the panels have the highest exposure to the sun. This change in the geometry of the flying wing is highly effective in energy absorption during times just before sunset and just after sunrise, and consequently, the aircraft can endure longer flight. Our aeroelastic simulations have shown that near zero-energy morphing is feasible. With NATASHA, we have demonstrated that it is possible to morph a folding wing configuration using only the flight control flaps and aerodynamic forces.