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At least 1,135 records · Page 63

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NASA NTRS1992

A finite-volume Euler solver for computing rotary-wing aerodynamics on unstructured meshes

An unstructured-grid solver for the unsteady Euler equations has been developed for predicting the aerodynamics of helicopter rotor blades. This flow solver is a finite-volume scheme that computes flow quantities at the vertices of the mesh. Special treatments are used for the flux differencing and boundary conditions in order to compute rotary-wing flowfields, and these are detailed in the paper. The unstructured-grid solver permits adaptive grid refinement in order to improve the resolution of flow features such as shocks, rotor wakes and acoustic waves. These capabilities are demonstrated in the paper. Example calculations are presented for two hovering rotors. In both cases, adaptive-grid refinement is used to resolve high gradients near the rotor surface and also to capture the vortical regions in the rotor wake. The computed results show good agreement with experimental results for surface airloads and wake geometry.

Strawn, Roger C.↗

NASA NTRS1992

Shadowgraph flow visualization of isolated tiltrotor and rotor/wing wakes

Shadowgraph flow visualization images were acquired from a 0.184-scale tiltrotor and wing in hover. Measurements and details of the vortex core structure were examined as a function of thrust condition and wake age. Experimental data for the isolated rotor wake geometry and rotor wake interactions with a semi-span wing and image plane were acquired. Quantitative measurements and comparisons of wake geometry and distortion were made for three configurations: the isolated rotor, rotor/wing, and rotor/wing/image plane. Comparisons between tiltrotor and helicopter rotor wake geometry measurements were made. Experimental wake geometry data were also compared with two wake models. Suggestions for improvements to existing prescribed-wake and free-wake models are proposed.

Swanson, Alexandra A.↗

NASA NTRS1993

Effect of ground and ceiling planes on shape of energized wakes

Energized wakes expelled by devices like thrust augmentors, propellers and rotors are studied theoretically in order to explore how their shape changes as they interact with ground and ceiling planes. It is found that when the airstream is stationary and the vehicle in hover, the presence of a ceiling plane causes an energized wake to constrict even more than when in an unbounded medium. The presence of a ground plane is found to cause the wake to constrict less than in free space. The computations also show that the vortex sheets first move inward before moving downward. Along their trajectory, the vortex sheets that separate the energized fluid from the ambient fluid have a nearly constant strength and velocity which indicates that the velocity just inside the wake is approximately constant. When a velocity is given to the wind tunnel airstream, the forward edge of the wake first rises and then descends so that it spends more time in the vicinity of the actuator disk. Implications of these results on measurements obtained in wind tunnels are discussed.

Rossow, Vernon J.↗

NASA NTRS1992

Multispectral, multialtitude observations of the KUREX-91 experiment site from a helicopter platform

An eight-channel modular multiband radiometer, a 252-channel SE-590 spectroradiometer, and a single-channel infrared thermometer were mounted in a nadir-looking attitude on a Soviet MI-8 helicopter to collect visible, infrared, and thermal measurements of the KUREX-91 study area. These measurements were coincident with surface, other airborne, and spaceborne measurements collected during the July 1991 field campaign. The helicopter missions included both hovers and slow-flight transects over specific sites. Multiple altitude flights over several of the study sites were also made. Study sites included grasslands, forest stands, and agricultural fields. Data collection was executed so that observations from the multispectral devices could be compared with multispectral and biophysical observations made on the surface as well as with multispectral observations from other airborne and spaceborne systems. A preliminary analysis of multispectral and multialtitude data of several study sites is presented.

Walthall, Charles L.↗

NASA NTRS1993

Free-wake computation of helicopter rotor flowfields in forward flight

A new method has been developed for computing advancing rotor flows. This method uses the Vorticity Embedding technique, which has been developed and validated over the last several years for hovering rotor problems. In this work, the unsteady full potential equation is solved on an Eulerian grid with an embedded vortical velocity field. This vortical velocity accounts for the influence of the wake. Dynamic grid changes that are required to accommodate prescribed blade motion and deformation are included using a novel grid blending method. Free wake computations have been performed on a two-bladed AH-1G rotor at low advance ratios including blade motion. Computed results are compared with experimental data. The sudden variations in airloads due to blade-vortex interactions on the advancing and retreating sides are well captured. The sensitivity of the computed solution to various factors like core size, time step and grids has been investigated. Computed wake geometries and their influence on the aerodynamic loads at these advance ratios are also discussed.

Ramachandran, K.↗

NASA NTRS1993

Benefits of variable rotor speed in integrated helicopter/engine control

Current helicopter flight and propulsion controls are typically designed with the assumption that rotor speed will be held to a constant setpoint. A new flight and propulsion control system using a continuously variable rotor speed command is proposed to improve the maneuverability and agility of helicopter systems. In this new approach, the flight control system generates an optimal variable rotor speed command in addition to conventional control commands in a framework of integrated flight/propulsion control. The benefits (i.e. improved maneuverability and agility) of varying rotor speed during transient maneuvers are demonstrated using a bob-up maneuver as an example. In particular, two types of benefits are identified in different maneuver conditions. One comes from a thrust augmentation, while the other comes from an exchange of rotational and translational energy. In the example, a simple linear dynamic hover model is used with an optimal control design method to generate the optimal rotor speed command.

Iwata, Takanori↗

NASA NTRS1993

Data acquisition for aeroelastic testing at the NASA Langley Transonic Dynamics Facility

Aeroelasticity is defined and different types of aeroelastic tests are described. The instrumentation and data acquisition requirements dictated by the time varying characteristics of aeroelastic phenomena are pointed out. Instrumentation needs for various types of tests are discussed. The hardware components and operational features of the real-time, digital-computer-controlled data acquisition system used to support testing at the Langley Transonic Dynamics Facility, which includes a major wind-tunnel, a helicopter-model hover facility, and two model setup and calibration laboratories are described.

Doggett, Robert V., Jr.↗

NASA NTRS1930

Simplified aerodynamic analysis of the cyclogiro rotating wing system

A simplified aerodynamic theory of the cyclogiro rotating wing is presented herein. In addition, examples have been calculated showing the effect on the rotor characteristics of varying the design parameters of the rotor. A performance prediction, on the basis of the theory here developed, is appended, showing the performance to be expected of a machine employing this system of sustentation. The aerodynamic principles of the cyclogiro are sound; hovering flight, vertical climb, and a reasonable forward speed may be obtained with a normal expenditure of power. Auto rotation in a gliding descent is available in the event of a power-plant failure.

Wheatley, John B↗

NASA NTRS1937

Static Thrust Analysis of the Lifting Airscrew

This report presents the results of a combined theoretical and experimental investigation conducted at the Georgia School of Technology on the static thrust of the lifting air screw of the type used in modern autogiros and helicopters. The theoretical part of this study is based on Glauert's analysis but certain modifications are made that further clarify and simplify the problem. Of these changes the elimination of the solidity as an independent parameter is the most important. The experimental data were obtained from tests on four rotor models of two, four, and five blades and, in general, agree quite well with the theoretical calculations. The theory indicates a method of evaluating scale effects on lifting air screws, and these corrections have been applied to the model results to derive general full-scale static thrust, torque, and figure-of-merit curves for constant-chord, constant-incidence rotors. Convenient charts are included that enable hovering flight performance to be calculated rapidly.

Knight, Montgomery↗

NASA NTRS1951

Empirical Relation Between Induced Velocity, Thrust, and Rate of Descent of a Helicopter Rotor as Determined by Wind-tunnel Tests on Four Model Rotors

The empirical relation between the induced velocity, thrust, and rate of vertical descent of a helicopter rotor was calculated from wind tunnel force tests on four model rotors by the application of blade-element theory to the measured values of the thrust, torque, blade angle, and equivalent free-stream rate of descent. The model tests covered the useful range of C(sub t)/sigma(sub e) (where C(sub t) is the thrust coefficient and sigma(sub e) is the effective solidity) and the range of vertical descent from hovering to descent velocities slightly greater than those for autorotation. The three bladed models, each of which had an effective solidity of 0.05 and NACA 0015 blade airfoil sections, were as follows: (1) constant-chord, untwisted blades of 3-ft radius; (2) untwisted blades of 3-ft radius having a 3/1 taper; (3) constant-chord blades of 3-ft radius having a linear twist of 12 degrees (washout) from axis of rotation to tip; and (4) constant-chord, untwisted blades of 2-ft radius. Because of the incorporation of a correction for blade dynamic twist and the use of a method of measuring the approximate equivalent free-stream velocity, it is believed that the data obtained from this program are more applicable to free-flight calculations than the data from previous model tests.

Castles, Walter, Jr↗

NASA NTRS1956

Flight Investigation of the Stability and Control Characteristics of Vertically Rising Airplane Research Model with Swept or Unswept Wings and X- or +-tails

The take-off, landing, hovering, and transition flight characteristics of a propeller-driven, fighter-type, vertically rising airplane model were obtained from time histories of the flights and from the pilots' opinions of the flight characteristics. In general, there were no major differences in the behavior of any of the configurations, all of which performed satisfactorily.

CONTROL, LATERAL↗

NASA NTRS1954

Flight Tests of a Man Standing on a Platform Supported by a Teetering Rotor

Because previous flight tests of a man standing on a jet-supported platform were very successful, a series of flight tests were made to determine the flying qualities of a man standing on a platform supported by a teetering rotor. The rotor was 7 feet in diameter and was driven by air jets at the rotor tips fed by hollow blades and air hoses connected to an external air supply. The machine was tested with a free-teetering rotor and with a spring-restrained rotor, and, as an alternate arrangement, with the rotor and foot platform isolated from the framework by rubber connections to reduce the inertia of the moving parts. The machine was tested indoors in hovering and in limited translational flight and outdoors in light and in strong gusty winds. The stability and controllability with the spring-restrained rotor were uniformly good. The stability with the free-teetering rotor was marginal, but, when an experienced flyer placed his hands on a hand rail, the stability and controllability were good. The stability and controllability with the rubber-isolated frame and spring-restrained teetering rotor were very good and quite like the jet-supported platform.

Paul R Hill↗

NASA NTRS1950

An introduction to the physical aspects of helicopter stability

In order to provide engineers interested in rotating-wing aircraft, but with no specialized training in stability theory, some understanding of the factors that influence the flying qualities of the helicopter, an explanation is made of both the static stability and the stick-fixed oscillation in hovering and forward flight in terms of fundamental physical quantities. Three significant stability factors -- static stability with angle of attack, static stability with speed, and damping due to a pitching or rolling velocity -- are explained in detail.

Gessow, Alfred↗

NASA NTRS1946

Static-thrust Investigation of Full-scale PV-2 Helicopter Rotors Having NACA 0012.6 and 23012.6 Airfoil Sections

An investigation was conducted to compare the performance of two 25-ft-diam rotors which had identical dimensions and were similar in construction but different in blade airfoil-sections. Tests were conducted at indicated blade pitch angles from 3 degrees to 11.5 degrees and rotor speeds of 200, 290, and 371 rpm. The 23012.6 rotor required 2 percent less power to hover than the 0012.6. At thrust coefficients above design, the performance of the 23012.6 became better than the 0012.6 rotor.

PROPELLERS, HELICOPTER - P-V ENG FORUM 2↗

NASA NTRS1944

Effect of Helicopter Performance of Modifications in Profile-Drag Characteristics of Rotor-Blade Airfoil Sections

Rough conventional, smooth conventional, and laminar-flow or low-drag sections were tested. The items covered are rotor thrust for fixed power in hovering, range and endurance at cruising speed, and power required at high-forward speed. Calculations indicated that a smooth conventional section gives marked performance gains. Smaller gains are obtainable by using a low-drag section. At high speeds or loads the low-drag section is inferior to the smooth conventional section.

Gustafson, F.B.↗

NASA NTRS1941

Contribution to the aerodynamics of rotating-wing aircraft. Part II

The interrelations established in an earlier report (NACA-TM-921) are used to study the best assumptions for hovering and horizontal flight. The effect of the twisted and tapered blade on the rotor efficiency is analyzed and the gliding coefficient at different stages (from autogyro to helicopter) of horizontal flight compared. To the extent that model or full-scale test data are available, they are included in the comparison.

Sissingh, G↗

NASA NTRS1939

Dynamic stability of a helicopter with hinged rotor blades

The present report is a study of the dynamic stability of a helicopter with hinged rotor blades under hovering conditions. While in this case perfect stability can in general not be obtained it is possible by means of design features to prolong the period of the spontaneous oscillations of the helicopter and reduce their amplification, and so approximately assure neutral equilibrium. The possibility of controlled stability of a helicopter fitted with hinged blades is proved by the successful flights of various helicopters, particularly of the Focker FW61 helicopter.

Hohenemser, K↗

NASA NTRS1938

Distribution of Temperatures over an Airplane Wing with Reference to the Phenomena of Ice Formation

The results obtained from the present study of temperature distribution over an airplane wing afford means for making the following statements as regards the conditions of ice accretion and the use of a thermic anti-icer or de-icer: 1) Ice can form on a wing only when the temperature is below or hovering around zero. 2) The thermic effects produced on contact of the air with the moving wing rather oppose ice accretion. 3) The thermic procedure in the fight against ice accretion on the wing consists in electrical heating of the leading edge. 4) It seems that the formation of ice on the wing ought to be accompanied by a temperature rise which brings the accretion to 0 degrees. 5) If the thermic effects of friction favor the operation of the thermic anti-icer, the functioning of the de-icer is facilitated by the release of heat which accompanies the deposit of ice.

Brun, Edmond↗