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At least 559 records · Page 31
All sourcesNASA NTRSDOE OSTI
NASA NTRS1973

Evaluation of an aeroelastic model technique for predicting airplane buffet loads

A wind-tunnel technique which makes use of a dynamically scaled aeroelastic model to predict full-scale airplane buffet loads during buffet boundary penetration is evaluated. A 1/8-scale flutter model of a fighter airplane with remotely controllable variable-sweep wings and trimming surfaces was used for the evaluation. The model was flown on a cable-mount system which permitted high lift forces comparable to those in maneuvering flight. Bending moments and accelerations due to buffet were measured on the flutter model and compared with those measured on the full-scale airplane in an independent flight buffet research study. It is concluded that the technique can provide valuable information on airplane buffet load characteristics not available from any other source except flight test.

Hanson, P. W.↗
NASA NTRS1973

A parametric study of planform and aeroelastic effects on aerodynamic center, alpha- and q-stability derivatives

Summarized are the aerodynamic center, alpha and q- aeroelastic effects on fighter-type aircraft in the 18,700 N gross range. The results indicate that with proper tailoring of planform (fixed or variable sweep), stiffner and elastic axis location it is possible to minimize trim requirements between selected extreme conditions. The inertial effects were found to be small for this class of aircraft.

Roskam, J.↗
NASA NTRS1973

Studies in tilt-rotor VTOL aircraft aeroelasticity, volume 1

Aeroelastic and dynamic studies which complement and extend various aspects of technology applicable to tilt-rotor VTOL aircraft are discussed. Particular attention is given to proprotor/pylon whirl instability, a precession-type instability akin to propeller/nacelle whirl flutter. The blade flapping and pitch-change freedoms of a proprotor are shown to lead to a fundamentally different situation as regards the manner in which the precession-generated aerodynamic forces and moments act on the pylon and induce whirl flutter relative to that of a propeller. The implication of these forces and moments with regard to their capacity for instigating a whirl instability is examined, demonstrating why a proprotor can exhibit whirl flutter in either the backward or forward directions in contrast to a propeller which is found to always whirl in the backward direction. Analytical trend studies delineating the effect of several system design parameters on proprotor/pylon stability and response are shown.

Kvaternik, R. G.↗
NASA NTRS1974

Rotation in vibration, optimization, and aeroelastic stability problems

The effects of rotation in the areas of vibrations, dynamic stability, optimization, and aeroelasticity were studied. The governing equations of motion for the study of vibration and dynamic stability of a rapidly rotating deformable body were developed starting from the nonlinear theory of elasticity. Some common features such as the limitations of the classical theory of elasticity, the choice of axis system, the property of self-adjointness, the phenomenon of frequency splitting, shortcomings of stability methods as applied to gyroscopic systems, and the effect of internal and external damping on stability in gyroscopic systems are identified and discussed, and are then applied to three specific problems.

Kaza, K. R. V.↗
NASA NTRS1974

Rotor aeroelastic stability coupled with helicopter body motion

A 5.5-foot-diameter, soft-in-plane, hingeless-rotor system was tested on a gimbal which allowed the helicopter rigid-body pitch and roll motions. Coupled rotor/airframe aeroelastic stability boundaries were explored and the modal damping ratios were measured. The time histories were correlated with analysis with excellent agreement. The effects of forward speed and some rotor design parameters on the coupled rotor/airframe stability were explored both by model and analysis. Some physical insights into the coupled stability phenomenon are suggested.

Miao, W. L.↗
NASA NTRS1974

Experimental and analytical studies in tilt-rotor aeroelasticity

An overview of an experimental and analytical research program underway for studying the aeroelastic and dynamic characteristics of tilt-rotor VTOL aircraft is presented. Selected results from several investigations of scaled models in the transonic dynamics tunnel, as well as some results from a test of a flight-worthy proprotor in the full-scale wind tunnel are shown and discussed with a view toward delineating various aspects of dynamic behavior peculiar to proprotor aircraft. Included are such items as proprotor/pylon stability, whirl flutter, gust response, and blade flapping. Theoretical predictions are shown to be in agreement with the measured stability and response behavior.

Kvaternik, R. G.↗
NASA NTRS1973

Some conclusions regarding the aeroelastic stability of hingeless helicopter blades in hover and in forward flight

In this paper results and conclusions obtained from the study of the aeroelastic instability of hingeless helicopter blades are presented. First, the large amplitude coupled flap-lag equations of motion of a hingeless elastic helicopter blade are solved using an asymptotic expansion procedure in multiple time scales. Both hover and forward flight cases are considered. Stability boundaries and amplitudes of nonlinear response are obtained. From these, the importance of the nonlinear coupling and the effect of the periodic coefficients is determined. Next, using a system of linearized coupled flap-lag-pitch equations in hover, various divergence mechanisms for hingeless blades are shown. Finally, the flutter boundaries for coupled flap-lag-pitch are obtained. The effect of the torsional degree of freedom on the flap-lag type of instability is investigated. Similarly the effect of lag on the flap-pitch type of instability is considered. In addition, the effect of various blade parameters on the stability boundaries is shown.

Friedmann, P.↗
NASA NTRS1974

Unsteady aerodynamics could dominate the space shuttle booster aeroelastic stability

Quasi-steady techniques are used in a preliminary, order of magnitude, analysis of the aeroelastic stability of a number of space shuttle boost configurations. The configurations analyzed all have at least one low frequency free-free bending mode that is aerodynamically undamped due to aerodynamic interference effects. Typically, the aerodynamic interference effects involve flow conditions at one vehicle station influencing the loads at another. Due to the finite flow convection velocity and the aerodynamic coupling of flow conditions at different locations on the vehicle, these interference loads result in aerodynamic undamping of one or more of the low frequency free-free elastic modes. The implication of these results, i.e., the possibility of undamped, divergent, oscillations leading to structural failure, (if the aerodynamic undamping should dominate the structural damping) is serious enough to warrant further analytic and experimental investigations.

Reding, J. P.↗
NASA NTRS1974

Aeroelastic stability of periodic systems with application to rotor blade flutter

The dynamics of a helicopter blade in forward flight are described by a system of linear differential equations with periodic coefficients. The stability of this periodic aeroelastic system is determined, using multivariable Floquet-Liapunov theory. The transition matrix at the end of the period is evaluated by: (1) direct numerical integration, and (2) a new, approximate method, which consists in approximating a periodic function by a series of step functions. The numerical accuracy and efficiency of the methods is compared, and the second method is shown to be superior by far. Results illustrating the effect of the periodic coefficients and various blade parameters are presented.

Friedmann, P.↗
NASA NTRS1974

Optimal aeroelastic design of an oblique wing structure

A procedure is presented for determining the optimal cover panel thickness of a wing structure to meet specified strength and static aeroelastic divergence requirements for minimum weight. Efficient reanalysis techniques using discrete structural and aerodynamic methods are used in conjunction with redesign algorithms driven by optimality criteria. The optimality conditions for the divergence constraint are established, and expressions are obtained for derivatives of the dynamic pressure at divergence with respect to design variables. The procedure is applied to an oblique wing aircraft where strength and stiffness are critical design considerations for sizing the cover thickness of the wing structure.

Gwin, L. B.↗