Suhail Akhtar

CFD-based adaptive flow control using ARMARKOV disturbance rejection

In this paper we demonstrate adaptive flow control for an incompressible viscous fluid through a two-dimensional channel without the use of an analytical model. An adaptive disturbance rejection algorithm is implemented within a CFD simulation to reduce the effects of an unknown velocity disturbance on the performance variable z, which is the transverse velocity component of the flow at a downstream location. The algorithm requires minimal knowledge of the system, specifically, the numerator coefficients of the transfer function from the control input to the performance variable. System identification, based on CFD simulations prior to the disturbance rejection simulations, is used to identify the required parameters

Optimization of dynamic soaring maneuvers to enhance endurance of a versatile UAV

Dynamic soaring is a process of acquiring energy available in atmospheric wind shears and is commonly exhibited by soaring birds to perform long distance flights. This paper aims to demonstrate a viable algorithm which can be implemented in near real time environment to formulate optimal trajectories for dynamic soaring maneuvers for a small scale Unmanned Aerial Vehicle (UAV). The objective is to harness maximum energy from atmosphere wind shear to improve loiter time for Intelligence, Surveillance and Reconnaissance (ISR) missions. Three-dimensional point-mass UAV equations of motion and linear wind gradient profile are used to model flight dynamics. Utilizing UAV states, controls, operational constraints, initial and terminal conditions that enforce a periodic flight, dynamic soaring problem is formulated as an optimal control problem. Optimized trajectories of the maneuver are subsequently generated employing pseudo spectral techniques against distant UAV performance parameters. The discussion also encompasses the requirement for generation of optimal trajectories for dynamic soaring in real time environment and the ability of the proposed algorithm for speedy solution generation. Coupled with the fact that dynamic soaring is all about immediately utilizing the available energy from the wind shear encountered, the proposed algorithm promises its viability for practical on board implementations requiring computation of trajectories in near real time.

Track fusion of legacy radars using weighted averages

In air defense systems, multiple radars with overlapping area of coverage are deployed for surveillance and tracking of aircraft. In such a situation, multiple tracks on the same target may appear. The problem of determining whether two tracks have originated from the same target is termed as Track-to-Track Association (T2TA). This process is followed by a Track-to-Track Fusion (T2TF) process which is done to fuse the tracks of multiple radars. Many multiple radar systems employ centralized T2TA and T2TF which takes place in a fusion center while the radars and their trackers are remotely located from the fusion center. Ideally, the radar trackers must supply target state estimates as well as the covariance and cross-covariance to the fusion center for optimal T2TA and T2TF. However, in case of legacy radars, only the target state estimates are provided to the fusion center. We propose a suboptimal but simple-to-implement technique based on weighted averages for T2TA and T2TF of legacy tracks. In the proposed technique, weighted averages are based on the reliability of the radar, track quality and deviation of target trajectory estimate from its smoothed value.

Dynamic analysis and nonlinear simulation of aircraft flat spin

Abstract In this paper we investigate the dynamic characteristics of flat spin of a fighter aircraft in open-loop configuration. The aircraft model under study is of the multi-role fighter configuration co-sponsored by China and Pakistan. The aerodynamic model used in the study is in the form of look-up tables that have been developed from rotary balance steady coning and oscillatory coning motion wind tunnel data. The set of all possible equilibrium spin states is numerically computed for various values of control settings using eighth-order aircraft equations of motion. Results from dynamical systems theory are applied to investigate local stability characteristics of aircraft around steady spin state. The complete set of dynamic modes of aircraft in spin is evaluated and mode content in each of the motion variable is determined using modal decomposition. This analytical technique is complemented by performing numerical simulations to investigate flat spin dynamic features. The methodology is applied to investigate dynamic behavior of two flat spins: right flat spin at 72.1° and much flatter left spin at 84.4°. The presented work provides insight into the global overview of aircraft flat spins as a function of various control settings, and their dynamic characteristics and it can facilitate the designing of flight control laws for spin recovery/prevention.

Full scale nonlinear dynamic inversion based control for an experimental glide platform

Under-actuated aerial vehicles in addition to providing significant advantages in terms of design efficiency pose a considerable challenge in identifying and formulating an effective control scheme. Strong coupling among different axes in addition to under-actuation results in configurations more suited to unconventional design methodologies. This work presents a dynamic inversion based nonlinear control for an under-actuated glide vehicle designed with an inverted-V tail. The complexity of the problem is compounded by the fact that the platform comprises of only two control surfaces with the absence of any other input. Dynamic inversion has been applied to develop an effective stabilization scheme ensuring the stability of internal dynamics as well as to achieve limited decoupling among the axes in a high-fidelity environment. A single loop configuration has been utilized for this research which aims to stabilize the platform using controlled variables by direct utilization of states or state errors. A full scale six-DOF nonlinear mathematical model for the configuration, developed through CFD simulations, has been used for analysis and simulations to ensure accurate analysis of the effectiveness of the designed control law.

Analysis of Spin Characteristics of a High Performance Aircraft with High Alpha Yawing Moment Asymmetry

This paper presents methodology of spin analysis of a multirole fighter aircraft, in open loop configuration, having aerodynamic asymmetry in yawing moment at high angles of attack. High fidelity aerodynamic model is developed in the form of lookup tables from static, coning and oscillatory coning test data obtained from Rotary Balance wind tunnel tests. Steady spin modes are predicted by solving 3-DOF aircraft model in conjunction with developed aerodynamic model using Nelder-Mead Simplex Optimization Routine. The natural tendency of an aircraft to yaw to the right at high angles of attack has resulted in prediction of significantly higher number of right spins as compared to left spins. 6-DOF time history simulations performed from selected initial conditions of steady spins showed that flat spins, both right and left are oscillatory and unstable. Proposed spin recovery strategies using aerodynamic control surfaces are found to accelerate spin recovery of left flat spins. However right flat spins which span across the whole range of aileron and elevator deflections do not respond to attempted recovery actions.

Part-II : Effect of Design Modifications on Computed Trajectory of a Large-Caliber Spinning Projectile

The desired characteristics of Artillery Projectiles is to have maximum range at a given muzzle velocity. It can only be achieved provided the projectile has stable dynamic flight configuration, i.e., it suffered minimum launch and terminal phase oscillations, minimum drift and inherent angle of attack, to reduced aerodynamic loading. The mass distributions, CG location, static margin and mass moment of inertias, are the critical factors in deciding the most economic flight characteristics. In this paper, which is the second part of a two-part study 1 , four different configurations of 203mm HE M106projectile has been simulated for various flight parameters in order to ascertain the effect of CG and mass moment of Inertias, on stability and flight performance. The computed strategy has already been verified against experimental and theoretical results 1 . The results of study depict that, the stable flight is a tradeoff between CG (i.e. static margin) and the mass moment of inertia, in a particular axis. Interesting fact revealed in parametric sensitivity analysis which highlight the flight envelop range for a particular projectile configuration, with best performance.

Part-I : Aerodynamic Data Generation and 6 DOF Trajectory Calculation of a Baseline Large-Caliber Spinning Projectile

External Ballistics is a blend of complex physics especially in case of unguided spinning projectiles. Since unguided projectiles are fire and forget, their inflight characteristics are of much significance, to be predicted beforehand to ascertain stability and flight dynamics before prototyping or manufacturing. In this paper, the aerodynamic characteristics and trajectory of a large caliber spinning projectile has been computed and compared with experimental results. Since the spinning projectile has continuously varying orientation in all translational and rotational axes, while passing through Troposphere/ Stratosphere, with nonlinear density variation, a 6 Degree of Freedom analysis is required to predict the trajectory and stability characteristics. Real time aerodynamic forces and moments are required to replicate the actual behavior. Hence, extensive CFD analysis has been carried out for entire mach regime, to generate the aerodynamic static and dynamiccoefficients library, for further integration with 6 DOF solver. The analysis strategy has been validated with available experimental results.The study has been covered in two Parts. Current paper covers Part I only, which analyzes theexisting 203mm HE M106 shell inherent launch instabilities, transonic region flight disturbances, total aerodynamic angle of attack variation and drift effects due to projectile inherent equilibrium yaw angle. In Part II, CG optimization has been carried out while analyzing multiple configurations, to develop a design strategy for smart ammunition.