Comparisons of computational fluid dynamics solutions of static and manoeuvring fighter aircraft with flight test data

Abstract

As the capabilities of computational fluid dynamics (CFD) to model full aircraft configurations improve, and the speeds of massively parallel machines increase, it is expected that CFD simulations will be used more and more to steer or in some cases even replace traditional flight test analyses. The mission of the US Air Force SEEK EAGLE office is to clear any new weapon configurations and loadings for operational use. As more complex weapons are developed and highly asymmetric loadings are requested, the SEEK EAGLE office is tasked with providing operational clearances for literally thousands of different flight configurations. High-fidelity CFD simulations employing the turbulent Navier-Stokes equations are in a prime position to help reduce some of the required wind-tunnel and/or flight test workload. However, these types of CFD simulations are still too time consuming to populate a full stability and control parameter database in a bruteforce manner. This article reviews results previously published by the authors, which validate the ability of high-fidelity CFD techniques to compute static force and moment characteristics of aircraft configurations. A methodology to generate efficient but non-linear reduced-order aerodynamic loads models from dynamic CFD solutions, which in-turn may be used to quickly analyse various stability and control characteristics at a particular flight condition, is introduced, and the results based on the US Air Force F-16C fighter aircraft that exemplify the process are discussed.

Transonic Aerodynamic Load Modeling of X-31 Aircraft Pitching Motions

“…The pitch moment is only considered in this paper because it is the more-difficult parameter to predict. For forced oscillations about the pitch axis, the input to the model includes angle of attack and pitch rate, i.e., xt αt; qt (10) Note that the response functions are time-dependent and have the effects of _ α and _ q; therefore, these terms are not included in the input vector. If the time responses in aerodynamic coefficients due to the step changes in angle of attack α and angular velocity q are known, then the total produced pitch moment at time t can be obtained using Eq.…”

“…An alternative approach to creating the full-order model is to develop a reducedorder model (ROM) that seeks to approximate CFD results by extracting information from a limited number of full-order simulations [9,10]. Ideally, the specified ROM can predict aircraft responses over a wide range of amplitude and frequency within a few seconds without the need of running CFD simulations again.…”

Improved Methodologies for Maneuver Design of Aircraft Stability and Control Simulations

“…The final goal is to develop a methodology for efficient and accurate screening of nonlinear aerodynamic phenomena such as spin, tumble, lateral instabilities, limit-cycle oscillations, and tail buffet of full scale aircraft early in the design process. To date, we have adopted the innovate approach introduced by McDaniel et al 10 which is graphically represented in figure 1, with a summary as follows: (a) CFD simulations are performed using computational training maneuvers designed to excite the relevant flow physics encountered during actual missions, (b) a mathematical Reduced Order Model (ROM) is built of the aircraft response using system identification methods, (c) the model is validated by comparing CFD simulations against model predictions, and (d) predictions of all flight test points are made using the model to determine the expected behavior of the aircraft.…”

“…10,11,[13][14][15][16][17][18][19][20][21] This was partially to validate that computational techniques have progressed to the point where turbulent Navier-Stokes solvers are now capable of capturing the unsteady nonlinear aerodynamic behavior that leads to the various static and dynamic instabilities mentioned above. However, the cost of this approach was computationally expensive CFD simulations with a slow turn-around time, which severely hindered development.…”

Unsteady Aerodynamics Modeling for Aircraft Maneuvers: a New Approach Using Time-Dependent Surrogate Modeling

“…To make timely progress in the use of CFD for aircraft design, efforts over the last few years have been spent mainly on the development of a Reduced Order Model (ROM) using CFD from an appropriate training maneuver(s) and an accurate System IDentification (SID) approach [28,30,5]. The objective of the ROMs is to develop a model that significantly reduces the CFD simulation time required to create a full aerodynamics database, making it possible to accurately model aircraft static and dynamic characteristics (within the range of data used for model generation) from a number of time-accurate CFD simulations.…”