High-altitude Long-endurance Aircraft Research Articles

Consistent Structural Linearization in Flexible Aircraft Dynamics with Large Rigid-Body Motion

This paper investigates the linearization, using perturbation methods, of the structural deformations in the nonlinear flight dynamic response of aircraft with slender, flexible wings. The starting point is the coupling of a displacement-based geometrically nonlinear flexible-body dynamics formulation with a three-dimensional unsteady vortex lattice method. This is followed by a linearization of the structural degrees of freedom, which are assumed to be small in a body-fixed reference frame. The translations and rotations of that reference frame and their time derivatives, which describe the vehicle flight dynamics, can still be arbitrarily large. The resulting system preserves all couplings between rigid and elastic motions and can be projected onto a few vibration modes of the unconstrained aircraft with geometrically nonlinear static deflections at a trim condition. Equally, the unsteady aerodynamics can be approximated on a fixed lattice defined by the deformed static geometry. Numerical studies on a representative high-altitude long-endurance aircraft are presented to illustrate the approach. The results show an improvement compared to those obtained using the mean-axes approximation.

Passive morphing of flying wing aircraft: Z-shaped configuration

High Altitude, Long Endurance (HALE) aircraft can achieve sustained, uninterrupted flight time if they use solar power. Wing morphing of solar powered HALE aircraft can significantly increase solar energy absorbency. An example of the kind of morphing considered in this paper requires the wings to fold so as to orient a solar panel to be hit more directly by the sun's rays at specific times of the day. An example of the kind of morphing considered in this paper requires the wings to fold so as to orient a solar panel that increases the absorption of solar energy by decreasing the angle of incidence of the solar radiation at specific times of the day. In this paper solar powered HALE flying wing aircraft are modeled with three beams with lockable hinge connections. Such aircraft are shown to be capable of morphing passively, following the sun by means of aerodynamic forces and engine thrusts. The analysis underlying NATASHA (Nonlinear Aeroelastic Trim And Stability of HALE Aircraft), a computer program that is based on geometrically exact, fully intrinsic beam equations and a finite-state induced flow model, was extended to include the ability to simulate morphing of the aircraft into a “Z” configuration. Because of the “long endurance” feature of HALE aircraft, such morphing needs to be done without relying on actuators and at as near zero energy cost as possible. The emphasis of this study is to substantially demonstrate the processes required to passively morph a flying wing into a Z-shaped configuration and back again.

The equivalence of gravitational potential and rechargeable battery for high-altitude long-endurance solar-powered aircraft on energy storage

Applying solar energy is one of the most promising methods to achieve the aim of High-altitude Long-endurance (HALE) flight, and solar-powered aircraft is usually taken by the research groups to develop HALE aircraft. However, the crucial factor which constrains the solar-powered aircraft to achieve the aim of HALE is the problem how to fulfill the power requirement under weight constraint of rechargeable batteries. Motivated by the birds store energy from thermal by gaining height, the method of energy stored by gravitational potential for solar-powered aircraft have attracted great attentions in recent years. In order to make the method of energy stored in gravitational potential more practical in solar-powered aircraft, the equivalence of gravitational potential and rechargeable battery for aircraft on energy storage has been analyzed, and four kinds of factors are discussed in this paper: the duration of solar irradiation, the charging rate, the energy density of rechargeable battery and the initial altitude of aircraft. This work can provide some governing principles for the solar-powered aircraft to achieve the unlimited endurance flight, and the endurance performance of solar-powered aircraft may be greatly improved by the application of energy storage using gravitational potential.

Energy management strategy for solar-powered high-altitude long-endurance aircraft

Development of solar-powered High-Altitude Long-Endurance (HALE) aircraft has a great impact on both military and civil aviation industries since its features in high-altitude and energy source can be considered inexhaustible. Owing to the development constraints of rechargeable batteries, the solar-powered HALE aircraft must take amount of rechargeable batteries to fulfill the energy requirement in night, which greatly limits the operation altitude of aircraft. In order to solve this problem, a new Energy Management Strategy (EMS) is proposed based on the idea that the solar energy can be partly stored in gravitational potential in daytime. The flight path of HALE aircraft is divided into three stages. During the stage 1, the solar energy is stored in both lithium–sulfur battery and gravitational potential. The gravitational potential is released in stage 2 by gravitational gliding and the required power in stage 3 is supplied by lithium–sulfur battery. Correspondingly, the EMS is designed for each stage. The simulation results show that the aircraft can always keep the altitude above 16km with the proposed EMS, and the power consumed during night can be also alleviated. Comparing with the current EMS, about 23.5% energy is remained in batteries with the proposed EMS during one day–night cycle. The sensitivities of the improvement of crucial technologies to the performance of aircraft are also analyzed. The results show that the enhancement of control and structural system, lithium–sulfur battery, and solar cell are ranked in descending order for the performance improvement of solar-powered HALE aircraft.

Evaluation of Quasi-Static Gust Loads Certification Methods for High-Altitude Long-Endurance Aircraft

Aeroelastic gust loads analysis can be approached using quasi-static, transient, or continuous methodologies. Pratt method is a quasi-static approach that forms the basis for Federal Aviation Regulation Part 23 gust loads certification requirements. This work evaluates the usefulness of Pratt method for unconventional high-altitude long-endurance aircraft. The derivation of Pratt method is reviewed, and all assumptions are identified. Error of a key curve fit equation is quantified directly. A state-of-the-art nonlinear aeroelastic code was upgraded and used to facilitate quantification of application-dependent errors by comparing quasi-static results to results from nonlinear transient analysis. Application-dependent errors are presented in the context of a SensorCraft-inspired joined-wing model and a Helios-aircraft-inspired flying-wing model. Recommendations are made on the usability of Pratt method for aircraft similar to the two high-altitude long-endurance models. It is concluded that Pratt method is useful for preliminary design of the joined-wing model but inadequate for the analysis of the flying-wing model. Further recommendations are made regarding the subtleties in the implementation of Pratt method for unconventional configurations.

Theoretic analysis and experiment on aeroelasticity of very flexible wing

A high-altitude long-endurance aircraft with high-aspect-ratio wing usually generates large deformation, which brings the geometric nonlinear aeroelastic problems. In recent decades, it has become a key focus of the international researchers of aeroelasticity. But some critical technologies are not developed systematically, such as aerodynamic calculation methods of the curved wing with deformation, moreover, there are few experimental validations of these technologies. In this paper, we established the steady aerodynamic calculating method of the curved wing with quite large deformation based on the extended lifting line method, and calculated the unsteady aerodynamics using the strip theory considering curved surface effects. Combining the structure geometrical nonlinear finite element method, we constructed a systematic analytic approach for the static aeroelasticity and flutter of very flexible wing, and further designed the ground vibration and wind tunnel test to verify this approach. Through the test and the theoretic results comparison, we concluded that the extended lifting line method has adaptable precision for the static aeroealsticity and the strip theory considering curved surface effects for flutter analysis can give exact critical speed and flutter mode when the dynamic stall does not happen. The work in this paper shows that the geometric nonlinear aeroelastic analytic approach for very flexible wing has very high efficiency and adaptable precision. It can be used in the engineering applications, especially the iterated design in preliminary stage.

Validation Studies for Aeroelastic Trim and Stability of Highly Flexible Aircraft

High-altitude, long-endurance (HALE) aircraft are highly flexible, requiring nonlinear aeroelastic analysis. NATASHA (Nonlinear Aeroelastic Trim and Stability of HALE Aircraft) is a computer program developed to analyze the aeroelastic behavior of HALE aircraft. The underlying formulation is based on the published geometrically exact, fully intrinsic beam equations. This paper presents a wide range of results from NATASHA, comparing them with well-known solutions of beam stability and vibration problems, experimental data, or results from well-established computer codes. Despite the simplicity of the underlying formulation, results obtained confirm the accuracy of the analysis and its suitability for conceptual and preliminary design of HALE aircraft.

Nonlinear-Aerodynamics/Nonlinear-Structure Interaction Methodology for a High-Altitude Long-Endurance Wing

Nonlinear aeroelastic analysis is essential for high-altitude long-endurance (HALE) aircraft. In the current paper, we have presented a computational aeroelastic tool for nonlinear-aerodynamics/nonlinear-structure interaction. Specifically, a consistent nonlinear time-domain aeroelastic methodology has been integrated via tightly coupling a geometrically exact nonlinear intrinsic beam model and the generalized unsteady vortex-lattice aerodynamic model with vortex roll-up and free wake. The effects of discrete gust as well as flow separation at various angles of attack from attached flow to the stall and poststall ranges are also included in the nonlinear aerodynamic model. A HALE-wing model is analyzed as a numerical example. The trim angle of attack is first found for the wing, and the results show that aeroelastic instability could occur at higher angles of attack. The HALE-wing model under the trim condition is then analyzed for various gust profiles to which it is subject. It is found that for certain gust levels, the elastic deformations of the HALE wing tend to become unstable: notably, the in-plane deflections become very significant. It is noted for the unstable solution of the HALE wing that the flow may be well beyond the stall range. An engineering approach with the use of the nonlinear sectional lift is attempted to consider such stall effects.

Comparison of Theoretical Structural Models with Experiment for a High-Aspect-Ratio Aeroelastic Wing

M ATERIALS of various densities and stiffnesses typically constitute the experimental slender wing structures designed for flutter and limit cycle oscillation experiments of high-altitude long-endurance (HALE) aircraft. Recent research performed atDuke University includes a study inter alia of the effects of structural nonuniformity on the natural frequencies and mode shapes of very flexible beams, which are frequently used in geometrically nonlinear structural analyses of HALE wings [1–5]. Jaworski and Dowell [6] demonstrated that such beamlike structures with spanwise discontinuities could be represented modally as a uniform beam for sufficiently small spanwise discontinuities. However, determining the set of natural frequencies requires knowledge of any such discontinuities or variations. This work addresses the effects of these nonuniformities on the four lowest natural frequencies of a cantilevered HALE-type wing. Here, the experimental wing model used by Tang and Dowell [2–4] is analyzed using the commercial finite element program ANSYS. As shown in Fig. 1, the HALE wing is composed of a steel spar with multiple steps, periodically-spaced aluminum ribs, balsa wood fairings to fill the space between ribs, and a tip store. Specifically, the effects examined herein are the step depth for the particular step distribution used by Tang and Dowell [2–4]; the addition of periodically-spaced ribs; the presence of balsa wood fairings with varying levels of rigid connection to the central spar; and the addition of a tip store.

Geometrical Nonlinear Aeroelastic Stability Analysis of a Composite High-Aspect-Ratio Wing

A composite high-aspect-ratio wing of a high-altitude long-endurance (HALE) aircraft was modeled with FEM by MSC/NASTRAN, and the nonlinear static equilibrium state is calculated under design load with follower force effect, but without load redistribution. Assuming the little vibration amplitude of the wing around the static equilibrium state, the system is linearized and the natural frequencies and mode shapes of the deformed structure are obtained. Planar doublet lattice method is used to calculate unsteady aerodynamics in frequency domain ignoring the bending effect of the deflected wing. And then, the aeroelastic stability analysis of the system under a given load condition is successively carried out. Comparing with the linear results, the nonlinear displacement of the wing tip is higher. The results indicate that the critical nonlinear flutter is of the flap/chordwise bending type because of the chordwise bending having quite a large torsion component, with low critical speed and slowly growing damping, which dose not appear in the linear analysis. Furthermore, it is shown that the variation of the nonlinear flutter speed depends on the scale of the load and on the chordwise bending frequency. The research work indicates that, for the very flexible HALE aircraft, the nonlinear aeroelastic stability is very important, and should be considered in the design progress. Using present FEM software as the structure solver (e.g. MSC/NASTRAN), and the unsteady aerodynamic code, the nonlinear aeroelastic stability margin of a complex system other than a simple beam model can be determined.

Impact of solar storms on high altitude long endurance unmanned aircraft and airship design and operations

AbstractThis paper applies existing information on solar storms to unmanned aviation; no new research data is presented. The purpose of this paper is to alert the unmanned aviation community to the potential hazards posed by solar storms, to familiarise it with the effects of solar storms and how to mitigate them, and to encourage research on solar storm effects on high altitude long endurance (HALE) aircraft and airship design and operations. As unmanned aircraft and airships move increasingly into high altitude (50,000+ft), endurance (24+ hr) roles, they will become vulnerable to the effects of space weather, specifically that of solar storms. Although solar storms are commonly associated with their impact on satellites, they affect the routing and timing of airline flights flying for six to eight hours at 30,000 to 40,000ft. Operating twice as high and with flight times twice as long (or longer) than those of airliners, HALE aircraft and airships occupy a middle zone of vulnerability, being more so than airliners but less so than satellites. A key difference however is that satellites are designed for space weather, whereas some current HALE vehicles are not. The paper concludes that unmanned HALE aircraft and airships can be one to three orders of magnitude more vulnerable to solar storms than a trans-Pacific airliner.

Supercritical Airfoil Design for Future High-Altitude Long-Endurance Concepts

The design and analysis is presented of a new laminar flow airfoil for a future high-altitude long-endurance aircraft that has an operational condition at supercritical speeds. The XFOIL and MSES computational codes were used to design, modify, and analyze the airfoil. The airfoil has enough thickness and performance to meet the requirements set for one of the U.S. Air Force Research Laboratory's SensorCraft concepts: a joined-wing configuration with a diamond shape in planform and front views. This SensorCraft concept's geometry and operational altitudes and speeds were used to determine the airfoil design conditions. The airfoil has a drag bucket over a large range of lift coefficients

Novel Strategy for Suppressing the Flutter Oscillations of Aircraft Wings

A new strategy, based on the nonlinear phenomenon of saturation, is proposed for controlling the flutter of a wing. The concept is illustrated by means of an example with a rather flexible, high-aspect wing of the type found on such vehicles as high-altitude long-endurance aircraft and sailplanes. The wing is modeled structurally as an Euler-Bernoulli beam with coupled bending and twisting motions. A general unsteady nonlinear vortex-lattice technique is used to model the flow around the wing and provide the aerodynamic loads. The structure, the flowing air, and the controller are considered the elements of a single dynamic system, and all of the coupled equations of motion are simultaneously and interactively integrated numerically in the time domain. The results indicate that the aerodynamic nonlinearities alone can be responsible for limit-cycle oscillations and that the saturation controller can effectively suppress the flutter oscillations of the wing when the controller frequency is actively tuned.

Nonlinear Aeroelasticity and Flight Dynamics of High-Altitude Long-Endurance Aircraft

High-Altitude Long-Endurance (HALE) aircraft have wings with high aspect ratios. During operations of these aircraft, the wings can undergo large de∞ections. These large de∞ections can change the natural frequencies of the wing which, in turn, can produce noticeable changes in its aeroelastic behavior. This behavior can be accounted for only by using a rigorous nonlinear aeroelastic analysis. Results are obtained from such an analysis for aeroelastic behavior as well as overall ∞ight dynamic characteristics of a complete aircraft model representative of HALE aircraft. When the nonlinear ∞exibility efiects are taken into account in the calculation of trim and ∞ight dynamics characteristics, the predicted aeroelastic behavior of the complete aircraft turns out to be very difierent from what it would be without such efiects. The overall ∞ight dynamic characteristics of the aircraft also change due to wing ∞exibility. For example, the results show that the trim solution as well as the short-period and phugoid modes are afiected by wing ∞exibility.

Subsonic and transonic low-Reynolds-number airfoils with reduced pitching moments

A subsonic and a transonic airfoil are presented for application in a high-altitude long-endurance aircraft and a very-high-altitude aircraft, respectively. The subsonic airfoil is designed for a lift coefficient c(l) = 1.4 at a chord Reynolds number Re = 700,000 and a very low Mach number. The transonic airfoil is designed for c(l) = 1.0 at Re = 500,000 and a transonic Mach number M = 0.7. Both airfoils are developed to perform as well or better than previously designed airfoils. However, the present airfoils are developed for a constrained pitching moment to reduce aircraft trim drag and to relieve, to some extent, the torsional loads in the typically high-aspect-ratio wings. The beneficial effects of a cruise flap and of boundary-layer transition control on the off-design performance characteristics are illustrated.

Aeroelastic characteristics of a highly flexible aircraft

This paper describes an aeroelastic analysis performed on a human-powered aircraft. The structural characteristics of this aircraft (namely, very flexible wings, high aspect ratio, and low wing loadings) are typical of proposed high-altitude long-endurance aircraft. Because of these unique features, the aircraft exhibits uncommon aeroelastic characteristics. The structural dynamics of the airplane are obtained from a finite element model. In an assumed mode approach, a subset of the natural mode shapes is used to calculate the quasisteady and unsteady generalized modal forces using a two-dimensional strip model, which includes unsteady drag and leading-edge suction forces. The aeroelastic model predicts the airplane to be stable (at both sea level and high altitude), except for a mildly unstable phugoid mode. The results indicate that unsteady aerodynamics are significant for this type of aircraft and that the flexibility of the aircraft must be included to correctly model the dynamics for control purposes. A final conclusion is that the aircraft dynamics are highly dependent on the flight conditions (flight speed and altitude). d,D e J k,K Nomenclature = area of span station i • parasite drag coefficient = slope of lift curve = aerodynamic moment about the aerodynamic center at zero angle of attack = half chord (ct/2) at span station i - damping, damping matrix = Oswald's efficiency factor