Estimation of Performance Airspeeds for High-Bypass Turbofans Equipped Transport-Category Airplanes

Journal of Aviation Technology and Engineering, Jun 2016

Conventional Mach-independent subsonic drag polar does not replicate the real airplane drag characteristics exactly and especially not in the drag-divergence region due to shock-induced transonic wave drag. High-bypass turbofan thrust is a complicated function of many parameters that eludes accurate predictions for the entire operating envelope and must be experimentally verified. Fuel laws are also complicated functions of many parameters which make optimization and economic analysis difficult and uncertain in the conceptual design phase. Nevertheless, mathematical models and predictions have its important place in aircraft development, design, and optimization. In this work, airspeed-dependent turbofan thrust and the new fuel-law model were used in combination with an airplane polynomial drag model to estimate important performance speeds. Except for the airframe-only dependent control airspeeds, all performance speeds are airframe powerplant dependent. In all analytical considerations one ends up with polynomials of the 4th order that have no closed-form solutions. A real positive-root seeking numerical procedure based on the family of Newton-Raphson methods was used to extract performance airspeeds for variable in-flight weights and altitudes in the ISA troposphere. Extensive testing of the accuracy and convergence of the Newton-Raphson nonlinear equation solvers was conducted before performance speed calculations. A fictitious long-range wide-body transport-category airplane was modeled in combination with a pair of high-bypass and ultra-high bypass ratio flat-rated turbofans. Procedure employed here can be easily extended to cases when fitted, measured drag and thrust data is given in arbitrary polynomial forms. Sensitivity analysis is performed on minimum-drag airspeed and maximum aerodynamic efficiency. Transonic wave drag considerations are introduced.

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Estimation of Performance Airspeeds for High-Bypass Turbofans Equipped Transport-Category Airplanes

Journal of Aviation Technology and Engineering Estimation of Performance Airspeeds for High-Bypass Turbofans Equipped Transport-Category Airplanes Nihad E. Daidzic AAR Aerospace Consulting Conventional Mach-independent subsonic drag polar does not replicate the real airplane drag characteristics exactly and especially not in the drag-divergence region due to shock-induced transonic wave drag. High-bypass turbofan thrust is a complicated function of many parameters that eludes accurate predictions for the entire operating envelope and must be experimentally verified. Fuel laws are also complicated functions of many parameters which make optimization and economic analysis difficult and uncertain in the conceptual design phase. Nevertheless, mathematical models and predictions have its important place in aircraft development, design, and optimization. In this work, airspeed-dependent turbofan thrust and the new fuel-law model were used in combination with an airplane polynomial drag model to estimate important performance speeds. Except for the airframe-only dependent control airspeeds, all performance speeds are airframepowerplant dependent. In all analytical considerations one ends up with polynomials of the 4th order that have no closed-form solutions. A real positive-root seeking numerical procedure based on the family of Newton-Raphson methods was used to extract performance airspeeds for variable in-flight weights and altitudes in the ISA troposphere. Extensive testing of the accuracy and convergence of the Newton-Raphson nonlinear equation solvers was conducted before performance speed calculations. A fictitious long-range wide-body transport-category airplane was modeled in combination with a pair of high-bypass and ultra-high bypass ratio flat-rated turbofans. Procedure employed here can be easily extended to cases when fitted, measured drag and thrust data is given in arbitrary polynomial forms. Sensitivity analysis is performed on minimum-drag airspeed and maximum aerodynamic efficiency. Transonic wave drag considerations are introduced. Transport-category airplane; High-bypass turbofan; Thrust; Fuel law and TSFC; Drag polar; Performance airspeeds; Newton-Raphson nonlinear equation solvers; Transonic wave drag - About the Authors Dr. Nihad E. Daidzic is president of AAR Aerospace Consulting, LLC. He is also a full professor of aviation, adjunct professor of mechanical engineering, and research graduate faculty at Minnesota State University. He has a PhD in fluid mechanics and ScD in mechanical engineering. He was formerly a staff scientist at the National Center for Microgravity Research and the National Center for Space Exploration and Research at NASA Glenn Research Center in Cleveland, OH. He has also held various faculty appointments at Vanderbilt University, University of Kansas, and Kent State University. His current research interest is in theoretical, experimental, and computational fluid dynamics; micro- and nano-fluidics; aircraft stability, control, and performance; and mechanics of flight, piloting techniques, and aerospace propulsion. Dr. Daidzic is ATP and ‘‘Gold Seal’’ CFII/MEI/CFIG certified with flight experience in airplanes, helicopters, and gliders. Introduction In order to optimize airplane operation and predict its performance in the conceptual design phase, early estimates of control and performance airspeeds are important. Much of the aircraft field and cruise performance capabilities depend on the set of control and performance airspeeds, such as, rotation, takeoff safety, climb, maximum- and long-range cruising, and reference landing speeds. Pilots essentially fly airplanes by reference to set of optimum airspeeds. Best flight practices depend much on the ability of pilots to maintain set airspeeds optimized for each phase of flight. Completed aircraft prototypes must undergo experimental verification before being certified. Aircraft manufacturers obtain such specific information by performing numerous repetitive, tedious, and expensive flight tests (Daidzic, 2013; FAA 2011) . Flight testing campaigns do not normally contribute much to understanding of flight physics, but are a required step toward particular airplane certification (EASA, 2007; FAA, 2013; JAA 2007) . Indeed, all limitations, control, and gross performance figures entering approved airplane operational/flight manuals (Airplane Flight Manual and Flight Crew Operations Manual) must be based on measured data (Daidzic, 2013; Eshelby 2000) . Airframe and engine characteristics cannot be presently modeled and simulated with fidelity, reliability, and accuracy required to substitute measured test data for certification purposes (Eshelby, 2000) . Although validation of analytical and computational calculations and wind-tunnel scale experiments must be verified during flight tests, nevertheless, the analytical methods provide deeper understanding of the fundamental flight physics and enable local an (...truncated)


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Nihad E Daidzic. Estimation of Performance Airspeeds for High-Bypass Turbofans Equipped Transport-Category Airplanes, Journal of Aviation Technology and Engineering, 2016, pp. 4, Volume 5, Issue 2,