Aircraft Longitudinal Stability Research Articles

Aero-Propulsive Interactions between UAV Wing and Distributed Propellers Due to Their Relative Position

The purpose of this paper is the evaluation of the aero-propulsive effects on a UAV wing model with distributed propulsion. An array of three propellers is placed ahead of the leading edge of a rectangular wing with flap. The investigation was performed with high-fidelity numerical analyses to provide insights into the phenomenology and to screen the interesting positions to be validated in the wind tunnel. The propellers’ array is moved into twelve different positions, allowing longitudinal and vertical translations. The wing has an untwisted and constant section profile, with a single slot trailing-edge flap that is deflected into three positions. The flap span is entirely covered by the propellers’ blowing. Results show an increment of lift, drag, and pitching moment coefficients with distributed propellers enabled. For a given thrust level, the magnitude of such increments depends on the propellers’ positions, the flap configuration, and the angle of attack. The lift enhancement sought in distributed propulsion applications comes at the expense of a significant increase in drag and pitching moment magnitude. In some combinations, the wing’s contribution to the aircraft longitudinal stability is severely affected. Conversely, the propellers’ inflow is altered such that thrust is increased in all the investigated configurations, with a small reduction of propulsive efficiency.

Heavy rain effects on aircraft longitudinal stability and control determined from numerical simulation data

In this study, the effects of heavy rain on the longitudinal stability and control performance of the DHC-6 Twin Otter aircraft are determined based on data from numerical simulation. A two-way momentum coupled Eulerian–Lagrangian approach for two-phase flows developed in the authors’ previous work is adopted to obtain the basic aerodynamic coefficients in rain. Then the aerodynamic derivatives of the aircraft in rain are evaluated on the basis of the narrow-strip theory. Finally, the longitudinal stability and control performance of the aircraft in rain are analyzed. Our research results suggest that heavy rain can cause degradations in both of aerodynamic and flight mechanics performances to an aircraft. In practice, these short-duration rainfall encounters should be avoided as far as possible since rain mainly has significant adverse effects on the short-period mode characteristics.

Altitude tracking controller design for a small unmanned aerial vehicle based on linear-quadratic-Gaussian-integral control

This paper presents the design procedure of a digital altitude tracking controller based on optimal control theory. The controller governs the longitudinal dynamics of a small-scale fixed-wing unmanned aerial vehicle (UAV) via an inner-outer loop control structure. In particular, the inner feedback loop maintains the aircraft's longitudinal stability and controls the climb rate (vertical velocity) while the outer-loop tracks the desired altitude through barometric altitude feedback. The inner-loop controller is designed based on the linear-quadratic-Gaussian-integral (LQGI) approach which necessitates the aircraft's longitudinal model to be expressed by a state-space representation. In this work, the model is obtained via a system identification technique called the observer/Kalman filter identification (OKID) method. Actual flight test data is collected and used in the system identification algorithm. On the other hand, the outer-loop controller simply features a proportional-gain control which eliminates the error between a reference altitude and the aircraft's altitude measured in real-time. The proposed altitude tracking controller is implemented on the National Cheng Kung University's Swallow UAV system and has been validated through a series of successful flight tests.

Identification of Aircraft Longitudinal Stability and Control Derivatives Combining Global Search Natural Algorithms and a Gradient Based Algorithm

In this work, two optimization methods are investigated to accomplish the parameter identification of the longitudinal motion of a military aircraft within the framework of the Output Error methodology. One of these methods is based on natural algorithms, in particular, the genetic algorithms and particle swarm optimization, which combined constitute the so called Life Cycle Method. The other is the Levenberg-Marquardt optimization algorithm, which is a gradient based method. Since the methodologies differ in the way they perform the optimization, being the first based on search and so appropriate for global minima search and the second gradient based, thus, good for local minima, both are compared and used in such a way that they complement each other.

Aircraft longitudinal stability and control derivatives identification by using life cycle and Levenberg–Marquardt optimization algorithms

In this work, two optimization algorithms are investigated to accomplish the parameter identification of the longitudinal motion of a real aircraft by using the output error method. The first algorithm is the nature-inspired algorithm named the life cycle model, which is a composed strategy based on other heuristics such as genetic algorithms and particle swarm optimization. The second one is the gradient-based technique named Levenberg–Marquardt algorithm, which is a variant of the Gauss–Newton method. Flight test data, performed with a training jet aircraft (Xavante AT-26), were used to feed the output error method. In this context, both optimization algorithms were tested, in solo performance and in a cascade-type approach. Results are reported, aiming to illustrate the success of using the proposed methodology.

Effects of horizontal tail ice on longitudinal aerodynamic derivatives

Flight tests were performed with the NASA Lewis Research Center's DH-^6 icing research aircraft to determine the accuracy with which the effects of ice on aircraft longitudinal stability and control could be measured. Flights were made with both a clean (baseline) configuration and with simulated ice on the horizontal tail. At a given trim speed 45 repeat doublet maneuvers were performed in each of four test configurations to determine the ensemble variation of the estimated stability and control derivatives. Additional maneuvers were also performed in each configuration to determine the variation in the longitudinal derivative estimates over a wide range of trim speeds. Stability and control derivatives were estimated by a modified stepwise regression (MSR) technique. A measure of the confidence in the derivative estimates was obtained by comparing the standard error for the ensemble of repeat maneuvers to the average of the estimated standard errors predicted by the MSR program. The magnitude of icing effects on the derivative estimates was strongly dependent on flight speed and aircraft wing flap configuration. With wing flaps up, the estimated derivatives were degraded most at lower speeds corresponding to that configuration. With wing flaps extended to 10 deg, the estimated derivatives were degraded most at the higher corresponding speeds.

Development Program for the F-15 Inlet

This paper describes the F-15 variable capture area inlet and the associated control system. Factors that influenced integration of the inlet/inlet control system with the total airframe are discussed. The methodology of using instantaneous total pressure patterns at the engine face to evaluate the effect of inlet distortion on engine operation is briefly described. Several inlet design variables that were tested are discussed. In addition to its effects on inlet behavior, the variable capture area reduces cruise drag, favorably influences aircraft longitudinal stability at low speeds, and reduces loads on the inlet backup structure at certain supersonic conditions. The boundary-layer bleed system optimization demonstrated that a slot in the throat was the single most effective location for bleed removal. Comparative results are presented for the final configuration between model and full-scale wind-tunnel testing, and subsequent flight testing. Results show that full-scale instantaneous distortion typically is equal to or less than that measured on a l/6th scale model.