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Flight Control System Design for a High Altitude, Long Endurance Airplane: Sensor Distribution and Flexible Modes Control

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Flight Control System Design for a High Altitude, Long Endurance Airplane: Sensor Distribution and Flexible Modes Control Flight Control System Design for a High Altitude, Long Endurance Airplane: Sensor Distribution and Flexible Modes Control Hamidreza Bolandhemmat* Abstract— This manuscript outlines the control system design Figure 1: The undefomed Aquila airplane shape (gray) is compared to the process for a solar-powered unmanned high-altitude long deformed shape under the first symmteric bending and torsion elastic modes endurance flying wing aircraft, called Aquila, which was (coloured as function of the deformation magnitude); Left picture portrayes the front view while the right shows the side view. developed by the Facebook Connectivity Lab to serve as communication backhaul for remote and rural connectivity. With 400kg mass and 42m wing-span, it was designed to autonomously fly at stratospheric altitudes of the atmosphere between 18 and 26km for months at a time. I. INTRODUCTION Performance of the solar-powered High-Altitude Long Endurance (HALE) aircrafts in closing the energy cycle on winter solstice days could be notably enhanced by increasing the aspect-ratio1 (L/D>35) and decreasing the weight. Both, however, contribute to higher structural flexibility and thereby existence of lightly damped “coupled” rigid body and elastic This article aims at demonstrating such a methodology in modes which could result in adverse Aeroservoelastic (ASE) the design of flight control and active flutter suppression phenomena such as Limit Cycle Oscillations (LCO), and systems for the Aquila aircraft. Challenges in specifying (body-freedom) flutter. LCO is used to describe sustained, effective sensor and servo placements on the aircraft to provide periodic, but not catastrophically divergent aeroelastic sufficient observability and controllability of the prevalent motions. It results from a nonlinear coupling of aircraft under-damped elastic modes are also addressed. In order to structural response and the unsteady aerodynamics. Flutter is reduce complexity, and increase the design transparency, the 3 characterized by an unstable interaction between the elastic classical frequency loop-shaping techniques are adopted . modes and the unsteady aerodynamic loading. The ASE Other design techniques including OBLTR, and MIDAAS are phenomena are usually excited by either control surface explored in references [1]. Both techniques utilize feedbacks actuations or by the atmospheric disturbances at critical only from the output measurements. OBLTR is an LQG/LTR dynamic pressures. If not controlled effectively, both can based method with adaptive terms to account for the modeling cause structural failures leading to loss of the aircraft. uncertainties, whereas MIDAAS is LQR based with an optimal output blend to isolate and suppress the problematic In the conventional flight control design philosophy, aircraft elastic mode(s). is considered to be a lumped mass in the air with only 6 Degree This manuscript is organized as follows. Section II presents of Freedom (DOF) motion variables being the rigid-body characteristics of the Aquila’s open loop flight dynamics modes. These modes are measured by installing an IMU inside across its flight envelope. Section III deals with the sensing the fuselage. However, due to their light weight and high and actuation system design for the Aquila control. The control aspect-ratio wing and fuselage, the HALE airplanes2 behave requirements are given in Section IV. Section V covers design as multi-body dynamic systems comprised of distributed of the primary flight control laws for the rigid body modes lumped masses which are connected with aeroelastic spring control. Section VI reviews design of an active flutter and dampers. Therefore, a distributed sensing and control suppression control laws for the elastic modes control. The design methodology must be pursued to measure and control Gust Load Alleviation (GLA) control loop is presented in the rigid body and elastic modes, simultaneously. In other Section VII. words, with increasing the aircraft structural flexibility, the elastic modes vibrate at frequencies much closer to the rigid II. OPEN LOOP FLIGHT DYNAMICS ANALYSIS body modes and therefore must be considered in the control system design process. Figure 1 compares the Aquila aircraft Aquila is a 400kg, unmanned backward sweep flying-wing jig-shape with the deformed shape under the first symmetric aircraft with 42m wingspan. More than half of the aircraft bending elastic mode oscillating at below 1Hz. weight is devoted to batteries to store the solar energy *Research Scientist at Facebook Connectivity Lab 2 The next generation of the passenger aircrafts and cargo transports are ([email protected]). also designed with slender high aspect ratio wings, and therefore a similar flight control design philosophy must be considered. 1 Induced drag is inversely proportional to span. 3 The corresponding design toolbox can be found @ FBHALEControl. throughout the day for the night flight. The aircraft is powered Roll (��) mode frequency is low and close to the Phugoid by four electric motors and propellers4 mounted under the (�ℎ) frequency at roughly 0.45rad/sec. This is mostly due to wings. The motors differential thrust is also utilized to control the large roll stability coefficient (�ℓ(), and similar inertia the aircraft in the roll and yaw channels. Figure 2 illustrates the properties around the � and � body axes (�,, ≅ �.. ≅ 47000 5 Aquila configuration . kg.m2). As a result, these modes can interact at non-zero bank flight conditions. Figure 2: Aquila unmanned aircraft configuration. Figure 3: Unstable spiral (�), Dutch-Roll (��) and Phugoid (�ℎ) modes at 10m/s and sea level. Spoilers are mounted to effectively control the rate of �C, �, �, �, ���I, �J12, �̇J12 ≝ �� descent of the aircraft during the approach and landing. They are also deployed when the motors differential thrust authority �?, �, ℎ, �A, �12, �̇12 ≝ �ℎ drops under a threshold. To enhance the aircraft directional stability, the aircraft is equipped with winglets. The longitudinal pitch channel is controlled by approx. 2m-long �C, �, �, ���I ≝ � elevon control surfaces installed at the end of each wing. The elevons can also move differentially to assist in controlling the roll and yaw motions. Their travel range is limited to a specified range. Bandwidth of the elevon servo actuators is 3Hz (-3dB). Note that the roll control authority is limited due to the high aerodynamic roll damping Figure 4 shows an elastic Short Period (���) mode at The aircraft flight envelope covers altitudes of up to 28km, 5rad/s. This mode consists of a conventional Short Period with a low Equivalent Airspeed (EAS) range of 9 to 15m/s mode coupled with the first symmetric bending and torsion (dynamic pressure range of between 50 to 110 Pa). However, degrees of freedom6 represented by the generalized modal speeds above 12m/s are within the flutter boundary at higher states �12, �̇12 and �̇15, respectively. There is another coupled altitudes (>7km). The nominal (weight dependent) stall speed elastic and rigid body mode at 6.5rad/s which is mostly is at 8.5m/s EAS. dominated by the first asymmetric bending and torsion degrees The aircraft structure is modeled by using the shell and of freedom. beam finite element models. The aerodynamic behavior is captured by using the Doublet-Lattice aerodynamic models. Figure 4: The elastic Short Period (���), first asymmetric bending (���) The Blade Element Theory is utilized to model the propulsion mode, Plunge and pitch (�� + �), and Roll (�) modes at 10m/s and sea level. system thrust and torque at various flight conditions. The resultant model used for the control laws design consists of �J12, �̇J12, �̇12, �12, � ≝ ��� 112 states which includes the aeroelasticity effects of the flexible structure and aero-lags of the unsteady flow. The nonlinear model is trimmed and linearized at various flight conditions across the flight envelope. The linearized models �12, �̇12, �, �, �̇15 ≝ ��� are then used for the open loop flight dynamics analyses and the control system design. A summary of the modal analysis depicting the interconnections between the states and various �OPQ, �12, �̇12, ℎ, �A, � ≝ (�� + �) modes, the eigen vectors, are given in the next subsection. �OPQ, �J12, �̇J12, �, � ≝ � A. Modal analysis Eigenstructure analyses are carried out to determine The first few elastic degrees of freedom of the aircraft interconnection between the states, each degree of freedom including the symmetric bending and torsion are given by and various dynamic modes. This is necessary to determine the Figure 5. relative significance of the rigid body and elastic degrees of freedom which is helpful for the (flutter) control system design and sensor selection. The preliminary analysis is performed by using the aircraft full-order model at 10m/s and sea level. As illustrated in Figure 3, the aircraft has a slightly unstable Spiral (�) mode with a time constant larger than a minute. The Dutch 4 5 The propellers can be unidirectional without any significant adverse Note that the Center of Gravity (CG) is not on the airplane (�;< ≅ 4� effect from the P-factor (asymmetric disc effect) and the gyroscopic moment. aft measured from the nose). The blade pitch angles can be also controlled to control the glide slope angle. 6 An indication that the rigid body and elastic modes are coupled. Figure 5: 1st symmetric bending (right), first asymmetric bending (left), 2nd case, the Dutch Roll mode becomes slightly unstable as symmetric bending (bottom left) and first symmetric
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