The role of shock induced trailing-edge separation in limit cycle oscillations

The potential role of shock induced trailing edge separation (SITES) in limit cycle oscillations (LCO) was established. It was shown that the flip-flop characteristics of transition to and from SITES as well as its hysteresis could couple with wing modes with torsional motion and low damping. This connection led to the formulation of a very simple nonlinear math model using the linear equations of motion with a nonlinear step forcing function with hysteresis. A finite difference solution with time was developed and calculations were made for the F-111 TACT were used to determine the step forcing function due to SITES transition. Since no data were available for the hysteresis, a parameter study was conducted allowing the hysteresis effect to vary. Very small hysteresis effects, which were within expected bounds, were required to obtain reasonable response levels that essentially agreed with flight test results. Also in agreement with wind tunnel tests, LCO calculations for the 1/6 scale F-111 model showed that the model should have not experienced LCO

Vortex flow hysteresis

An experimental study was conducted to quantify the hysteresis associated with various vortex flow transition points and to determine the effect of planform geometry. The transition points observed consisted of the appearance (or disappearance) of trailing edge vortex burst and the transition to (or from) flat plate or totally separated flows. Flow visualization with smoke injected into the vortices was used to identify the transitions on a series of semi-span models tested in a low speed tunnel. The planforms tested included simple deltas (55 deg to 80 deg sweep), cranked wings with varying tip panel sweep and dihedral, and a straked wing. High speed movies at 1000 frames per second were made of the vortex flow visualization in order to better understand the dynamics of vortex flow, burst and transition

Space shuttle wing leading edge heating environment prediction derived from development flight data

An analytical program is in progress at Rockwell International to revise wing leading edge heating predictions in order to improve correlation with STS-1 to -5 flight radiometer data. This paper discusses the methods that have been used to improve agreement between prediction and flight and summarizes the aerodynamic correlations which, when updated, will be used to analyze future orbiter missions

An investigation of wing buffeting response at subsonic and transonic speeds. Phase 2: F-111A flight data analysis. Volume 1: Summary of technical approach, results and conclusions

A detailed investigation of the flight buffeting response of the F-111A was performed in two phases. In Phase 1 stochastic analysis techniques were applied to wing and fuselage responses for maneuvers flown at subsonic speeds and wing leading edge sweep of 26 degrees. Power spectra and rms values were obtained. This report gives results of Phase 2 where the analyses were extended to include maneuvers flown at wing leading edge sweep values of 50 and 75.5 degrees at subsonic and supersonic speeds and the responses examined were expanded to include vertical shear, bending moment, and hingeline torque of the left and right horizontal tails. Power spectra, response time histories, variations of rms response with angle of attack and effects of wing sweep and Mach number are presented and discussed. Some Phase 1 results are given for comparison purposes

Fuzzy Modeling and Parallel Distributed Compensation for Aircraft Flight Control from Simulated Flight Data

A method is described that combines fuzzy system identification techniques with Parallel Distributed Compensation (PDC) to develop nonlinear control methods for aircraft using minimal a priori knowledge, as part of NASAs Learn-to-Fly initiative. A fuzzy model was generated with simulated flight data, and consisted of a weighted average of multiple linear time invariant state-space cells having parameters estimated using the equation-error approach and a least-squares estimator. A compensator was designed for each subsystem using Linear Matrix Inequalities (LMI) to guarantee closed-loop stability and performance requirements. This approach is demonstrated using simulated flight data to automatically develop a fuzzy model and design control laws for a simplified longitudinal approximation of the F-16 nonlinear flight dynamics simulation. Results include a comparison of flight data with the estimated fuzzy models and simulations that illustrate the feasibility and utility of the combined fuzzy modeling and control approach

Book Review: The Moral Decision

The Moral Decision has been reviewed on many occasions since its first publication in 1955. In this current review, a former student of Professor Cahn has reflected and drawn upon his personal knowledge of Professor Cahn to offer the reader a new perspective of both the book and its author

An Investigation of Littoral Transport Between Virginia Beach and Sandbridge, Virginia

A method for calculating littoral transport, direct determination of the sediment flux, has been proposed and evaluated. Three sampling stations, located between Virginia Beach and Sandbridge, Virginia, were occupied for a period of thirteen months (September, 1972 to September, 1973). Measurements include velocity of the littoral current, cross-sectional area of the surf zone, and in situ samples of suspended and bottom sediment load. Observations of sea and atmospheric conditions were obtained. Two ways of calculating littoral transport were derived from the same basic method, thus two sets of transport values were generated. Two annual littoral transport values were calculated by use of these figures and regional wave data. The numbers were 1.2 x 1012 and 9.6 x 1011 grams per year or 355,000 and 294,000 cubic yards per year. Tidal effects on the rate of littoral transport were examined. Transport was greatly affected by changes in cross-sectional area and subsequently in the shape of the zone of littoral transport due to the tidal fluctuations. In addition, beach cusps in conjunction with tidal rise and fall were found to affect the littoral current; cusps were found to retard littoral current at high tide, but were ineffectual at low tide. It is felt that more work needs to be done in both evaluation of the method and investigation of problems that were encountered