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  • 1990-1994  (5)
  • 1985-1989  (1)
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  • 1
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    The quest for stall-free dynamic lift (1992)
    In:  CASI
    Details
    Publication Date: 2013-08-31
    Description: During the past decade, numerous major effects have addressed the question of how to control or alleviate dynamic stall effects on helicopter rotors, but little concrete evidence of any significant reduction of the adverse characteristics of the dynamic stall phenomenon has been demonstrated. Nevertheless, it is important to remember that the control of dynamic stall is an achievable goal. Experiments performed at the US Army Aeroflight-dynamics Directorate more than a decade ago demonstrated that dynamic stall is not an unavoidable penalty of high amplitude motion, and that airfoils can indeed operate dynamically at angles far above the static-stall angle without necessarily forming a stall vortex. These experiments, one of them featuring a slat that was designed from static airfoil considerations, showed that unsteadiness can be a very beneficial factor in the development of high-lift devices for helicopter rotors. The experience drawn from these early experiments is now being focused on a program for the alleviation of dynamic-stall effects on helicopter rotors. The purpose of this effort is to demonstrate that rotor stall can be controlled through an improved understanding of the unsteady effects on airfoil stall and to document the role of specific means that lead to stall alleviation in the three dimensional unsteady environment of helicopter rotors in forward flight. The first concept to be addressed in this program will be a slatted airfoil. A two dimensional unsteady Navier-Stokes code has been modified to compute the flow around a two-element airfoil.
    Keywords: AERODYNAMICS
    Type: NASA. Ames Research Center, Physics of Forced Unsteady Separation; p 277-295
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  • 2
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    Effect of an extendable slat on the stall behavior of a VR-12 airfoil (1993)
    In:  CASI
    Details
    Publication Date: 2019-06-28
    Description: Experimental and computational tests were performed on a VR-12 airfoil to determine if the dynamic-stall behavior that normally accompanies high-angle pitch oscillations could be modified by segmenting the forward portion of the airfoil and extending it ahead of the main element. In the extended position the configuration would appear as an airfoil with a leading-edge slat, and in the retracted position it would appear as a conventional VR-12 airfoil. The calculations were obtained from a numerical code that models the vorticity transport equation for an incompressible fluid. These results were compared with test data from the water tunnel facility of the Aeroflightdynamics Directorate at Ames Research Center. Steady and unsteady flows around both airfoils were examined at angles of attack between 0 and 30 deg. The Reynolds number was fixed at 200,000 and the unsteady pitch oscillations followed a sinusoidal motion described by alpha = alpha(sub m) + 10 deg sin(omega t). The mean angle (alpha(sub m)) was varied from 10 to 20 deg and the reduced frequency from 0.05 to 0.20. The results from the experiment and the calculations show that the extended-slat VR-12 airfoil experiences a delay in both static and dynamic stall not experienced by the basic VR-12 airfoil.
    Keywords: AERODYNAMICS
    Type: AD-A274150 , NASA-TP-3407 , A-93056 , NAS 1.60:3407 , ATCOM-TR-93-A-002
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  • 3
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    Preliminary study of a wing-tip vortex using laser velocimetry (1987)
    In:  CASI
    Details
    Publication Date: 2019-06-28
    Description: Measurements have been made in the wake of a semi-span NACA 0015 airfoil with emphasis on the region of the wing tip vortex. The spanwise and streamwise velocity components were measured using a two-component laser Doppler velocimeter. The purpose of the study was to initiate the operation of a laser velocimeter system and to perform preliminary wake measurements in preparation for a more extensive study of the structure and near field development of a tip vortex.
    Keywords: AERODYNAMICS
    Type: NASA-TM-88343 , A-86207 , NAS 1.15:88343 , AVSCOM-TM-86-A-2
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  • 4
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    Characteristics of deformable leading edge for high performance helicopter rotor (1993)
    In:  Other Sources
    Details
    Publication Date: 2019-06-28
    Description: The deformable leading edge (DLE) concept to improve the blade capability in lift, drag and pitching moments has been investigated for the purpose of meeting new rotor maneuverability and susceptibility requirements. The advantages and disadvantages of this concept have been carefully examined with limited computational and experimental results. This work showed that this concept achieves a substantial improvement in lift capability and also reduces the drag and pitching moment at the same time. Effects of various parameters, such as Reynolds number, reduced frequency, mean angle of oscillation, and airfoil shape, on the performance of these airfoils were also investigated.
    Keywords: AERODYNAMICS
    Type: AIAA PAPER 93-3526 , AD-A268648 , In: AIAA Applied Aerodynamics Conference, 11th, Monterey, CA, Aug. 9-11, 1993, Technical Papers. Pt. 2 (A93-47201 19-02); p. 968-988.
    Format: text
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  • 5
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    Suppression of dynamic stall with a leading-edge slat on a VR-7 airfoil (1993)
    In:  CASI
    Details
    Publication Date: 2019-06-28
    Description: The VR-7 airfoil was experimentally studied with and without a leading-edge slat at fixed angles of attack from 0 deg to 30 deg at Re = 200,000 and for unsteady pitching motions described by alpha equals alpha(sub m) + 10 deg(sin(wt)). The models were two dimensional, and the test was performed in a water tunnel at Ames Research Center. The unsteady conditions ranged over Re equals 100,000 to 250,000, k equals 0.001 to 0.2, and alpha(sub m) = 10 deg to 20 deg. Unsteady lift, drag, and pitching-moment measurements were obtained along with fluorescent-dye flow visualizations. The addition of the slat was found to delay the static-drag and static-moment stall by about 5 degrees and to eliminate completely the development of a dynamic-stall vortex during unsteady motions that reached angles as high as 25 degrees. In all of the unsteady cases studied, the slat caused a significant reduction in the force and moment hysteresis amplitudes. The reduced frequency was found to have the greatest effect on the results, whereas the Reynolds number had little effect on the behavior of either the basic or the slatted airfoil. The slat caused a slight drag penalty at low angles of attack, but generally increased the lift/drag ratio when averaged over the full cycle of oscillation.
    Keywords: AERODYNAMICS
    Type: NASA-TP-3357 , ATCOM-TR-92-A-013 , A-93006 , NAS 1.60:3357 , AD-A271988
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  • 6
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    NACA 0015 wing pressure and trailing vortex measurements (1991)
    In:  CASI
    Details
    Publication Date: 2019-06-28
    Description: A NACA 0015 semispan wing was placed in a low-speed wind tunnel, and measurements were made of the pressure on the upper and lower surface of the wing and of velocity across the vortex trailing downstream from the tip of the wing. Pressure data were obtained for both 2-D and 3-D configurations. These data feature a detailed comparison between wing tips with square and round lateral edges. A two-component laser velocimeter was used to measure velocity profiles across the vortex at numerous stations behind the wing and for various combinations of conditions. These conditions include three aspect ratios, three chord lengths, a square- and a round lateral-tip, presence or absence of a boundary-layer trip, and three image plane positions located opposite the wing tip. Both pressure and velocity measurements were made for the angles of attack 4 deg less than or equal to alpha less than or equal to 12 deg and for Reynolds numbers 1 x 10(exp 6) less than or equal to Re less than or equal to 3 x 10(exp 6).
    Keywords: AERODYNAMICS
    Type: NASA-TP-3151 , A-91056 , NAS 1.60:3151 , AVSCOM-TR-91-A-003 , AD-A257045
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