Kenichi Saitoh

Transonic Flutter Characteristics of an Arrow Wing Mounted with Engine

Transonic flutter wind tunnel tests with a flexible scaled model of the Jet-powered Supersonic Experimental Airplane were conducted at Japan Aerospace Exploration Agency (JAXA). The test results would be used for establishing and verifying analysis tools to evaluate transonic aeroelasticity. The test article was designed as 11% scaled model of the experimental airplane which length and width are 11.5m and 4.93m respectively. The wind tunnel was operated by sweeping either total pressure or by sweeping Mach number, or both proportionally. Typical dip shape boundary of transonic flutter was obtained and limit cycle oscillation was also revealed below the flutter boundary. In this paper, the summary of the flutter tests is reported. I. Introduction eroelastic flutter evaluation plays a significant role in the structural design of aircraft since flutter phenomenon can cause serious damage or loss of aircraft. Structural requirements for less weight and less cost make the role more important on high-speed aircraft, especially a supersonic transport (SST) due to the complexities in aerodynamic and structural aspects in transonic region. Transonic Flows are characterized by the presence of adjacent regions of subsonic and supersonic flow, usually accompanied by shock waves. Nonlinear behavior of the shock waves causes unexpected unstable vibrations such as transonic flutter (transonic-dip) and limit cycle oscillation (LCO) which are inconvenient for performance of high speed aircraft. In the past, there have been many activities, not only numerically but also experimentally, in investigation of this kind of nonlinear aeroelasticity. Since 1997, the National Aerospace Laboratory (NAL), presently the Japan Aerospace Exploration Agency (JAXA), has been conducting the National Experimental Supersonic Transport (NEXST) program to prepare the nations technical base for coming international development of the next generation SST (Ref. 1). Major objectives of the NEXST program are as follows; a) to develop system integration technology for aircraft b) to establish CFD-based aerodynamic design technology with inverse and optimization method c) to progress component technologies in aerodynamic and composite structures design In order to demonstrate these technologies, the developments of two types of scaled supersonic experimental aircraft are promoted; one is non-powered experimental airplane and the other is jet-powered one. The non-powered airplane (NEXST-1) was developed to demonstrate the inverse aerodynamic design technology to realize a low drag configuration. The cranked arrow wing, area rule and warp concept are integrated with the natural laminar wing technology. During the development of the NEXST-1, the aeroelastic characteristics of a clean arrow wing are investigated elaborately (Ref.2). Following the NEXST-1, aeroelastic investigation of the jet-powered experimental airplane (NEXST-2) was conducted in the basic design phase. The NEXST-2 has a role to demonstrate the optimized-inverse design system and some component technologies including engine-airframe integration, high performance stable air-intake and composite structure for the wing. The NEXST-2 is characterized as unique

Numerical simulation for random aeroelastic responses using inverse fourier transform

This paper reports a new simulation technique for an aeroelastic system which responds to random external forces. Since the aeroelastic system including the effects of unsteady aerodynamics is ordinarily described in the frequency domain, the Inverse Discrete Fourier Transform (IDFT) can be utilized to simulate its random response. The response caused by the external random noise is calculated through a transfer function first in the frequency domain and then converted to the time domain. The objective of the present study is to provide mathematical time history data for evaluating the various estimation methods of the flutter boundary from subcritical responses in flight and/or wind tunnel testing. An example application to the method of flutter prediction is shown. The technique can also be used to evaluate the effects of the active control device coping with atmospheric turbulence.

Unsteady transonic aerodynamics during wing flutter

Unsteady pressure distributions of a two-dimensional super-critical wing while it was fluttering were measured in the transonic flow regime. The results were compared with those by the Navier-Stokes code which includes wind-tunnel wall effects. Although there were discrepancies between the experimental results and the analytical model for the pressure phase delay distribution, no disagreements were observed for the pitching first harmonics provided that there was no large flow separation. In the tests, the flutter was forced to be suppressed soon after its onset before it reached a limit cycle oscillation (LCO) where the amplitude of the pitching angle was supposed to be over 2 degrees.

Development of flutter margin prediction program

A research to predict flutter margin from experimental signals has been conducted in JAXA. Some signal processing methods has been tried: e.g., the Peak-Hold method, the Random Decrement, RD, method, and the Auto-Regressive Moving Average, ARMA, method. Recently, we tried to apply the Eigen-system Realization Algorithm, ERA, method instead of the ARMA. The discrete flutter margin, Fz, was calculated with the ERA identified system model, and the flutter condition was predicted with the set of Fz. Adding to this process, we used the RD technique to pre-process the system output signal. As a result, the Fz calculated by the present method agrees well with that calculated by the ARMA method. In addition, the present method can also process faster than the ARMA method.

Structural Design and Flight Verification of Unmanned Supersonic Experimental Airplane

The flight trial of the unmanned experimental airplane was conducted by Japan Aerospace Exploration Agency (JAXA) in order to substantiate supersonic drag reduction technology with a CFD (Computational Fluid Dynamics)-based optimum aerodynamic design procedure and to establish the experimental system with the non-powered and unmanned supersonic experimental airplane (NEXST-1). The NEXST-1 is the scaled airplane which has a length of 11.5m, a wing span of 4.7m, and about 2,000kg in weight. The airplane configuration was designed using the inverse method giving the preferable pressure distribution to minimize the aerodynamic drag force at Mach Number 2. One solid rocket booster was used to launch the NEXST-1 giving the speed of MACH 2 and the altitude of 18km. Inner wing of the NEXST-1 has multi-spar box structure. Outer wing, tail wings, ailerons and rudder were machined from aluminum plates. Concept of multi-frame and skin structure with longerons has been adopted for the fuselage. On 10 October 2005, the NEXST-1 with the booster was launched. After burn out of the booster, the NEXST-1 was separated from the booster. The total flight time was about 15 minutes. Every system had accomplished each work successfully and the NEXST-1 including a data recorder was recovered safely. From a structural point of view, this flight could show us the compliance with the structural design criteria, such as loadings, vibration, aeroelasticity and structural temperature. The successful flight trial shows not only substantiation of supersonic drag reduction technology but also the verification of the experimental system design and structural design of NEXST-1.