Forward Flight Performance Research Articles

Assessment of Predictive Capability of Aeromechanics Methods

A technique is described, based on simple statistics, that can be used to assess the accuracy of analytical predictions compared to measurements. It is demonstrated that the approach can be applied to a broad range of problems in the area of helicopter aeromechanics, including hover and forward flight performance, blade aerodynamic and structural loads, vibratory forces, stability, and rotorcraft icing. The method described has both strengths and weaknesses and these are shown with examples and discussed. It is also shown that there is currently a hierarchy of accuracy in aeromechanics problems, ranging from the necessarily accurate methods for performance prediction to the inaccurate and untrustworthy calculations of fixed-system vibration.

Analysis and Design of a Flap-Equipped Low-Twist Rotor for Hover

This study evaluates the potential benefits that fixed but deployable trailing-edge flaps may have on a low-twist hovering rotor using a blade-element method combined with two-dimensional and three-dimensional computational fluid dynamics. Low blade twist is beneficial in terms of forward-flight performance, which, combined with a deployable flap, could also offer good hover performance, resulting in an overall better design in comparison with a plain twisted blade. To evaluate this concept, a parametric study of flap configurations was conducted using a simple blade-element method based on computational-fluid-dynamics-generated two-dimensional aerodynamics. This simple model indicated that up to 6 deg of blade twist could be recovered at high rotor thrust by using an optimized flap configuration. Performance improvements were also obtained for outboard slotted-flap configurations. The optimum slotted-flap designs were also evaluated using three-dimensional computational fluid dynamics, which confirmed that an inboard flap combined with a low-twist ( 7 deg) rotor blade matched the performance of a plain blade with -13 deg of twist. A blended-flap configuration was also applied and evaluated. This simpler design demonstrated the equivalent performance of a clean rotor with 10 deg of blade twist, providing further evidence of the potential of the fixed inboard flap concept.

앞전 Droop과 Gurney 플랩을 이용한 동적 실속 제어

본 연구에서는 헬리콥터의 전진비행성능 향상에 필수적인 로터블레이드의 동적실속성능을 향상시키기 위한 수동제어기법에 대한 연구를 수행하였다. 로터블레이드의 동적실속성능을 향상시키기 위해서는 블레이드 익형에 발생하는 유동박리에 대한 제어를 통해 양력 특성과 피칭모멘트 특성을 동시에 향상시켜야만 한다. 본 연구에서는 실제구현이 용이한 고정 앞전Droop과 Gurney 플랩을 심한 동적실속영역에 대해 동시에 적용하여 기존의 동적실속 제어기법에 비해서 탁월한 양력성능 향상 및 피칭 모멘트 성능 향상을 얻을 수 있음을 확인하였다. To achieve the advanced forward flight performance of helicopter, the passive control methods for enhancement of the dynamic stall characteristics of rotor blades are studied. To enhance the dynamic stall characteristics of the rotor blades, it is essential to improve the lift performance and the pitching moment performance simultaneously with the control of the separation on the rotor blades. For this point of view, both the fixed droop leading edge and the Gurney flap which are simply realized are used for control of the dynamic stall in severe dynamic stall conditions. From this study, the combination of both passive control methods showed dramatic enhancement of lift and pitching moment performance in dynamic stall than previous research results.

Tilt Duct Vertical Takeoff and Landing Uninhabited Aerial Vehicle Concept Design Study

¨A new autonomously controlled tilt-duct vertical takeoff and landing uninhabited aerial vehicle concept is proposed. This design combines the vertical flight capability of a helicopter and forward flight performance of a fixed-wing conventional aircraft. The two main engines and propellers are located inside the tilting ducts attached to the wing tips. There is a third engine‐propeller combination located inside the aft fuselage for pitch and yaw control during hover and transition. The advantages and disadvantages of the ducted propellers are discussed. A conceptual design study is performed including airfoil and geometry selection, initial sizing calculations, estimation of stability and control parameters, etc. Drawings of the aircraft in hover, transition and forward flight modes are presented.

Neural Network Research on Validating Experimental Tilt-Rotor Performance

Results from a neural network validation study with full-scale XV-15 tilt-rotor experimental hover and forward flight data are presented. Two test data bases, acquired during separate tests conducted at NASA Ames, were used. These two isolated XV-15 rotor test data bases were: the 80by 120-Foot Wind Tunnel hover and forward flight test data base, and secondly, an outdoor hover test data base. For this neural-network-based data validation study, the objective associated with the wind tunnel test data was: obtain neural network representations, conduct data quality checks, and demonstrate sensitivity to test conditions. Neural networks were successfully used to represent and assess the quality of full-scale tilt-rotor hover and forward flight performance test data. The neural networks accurately captured tilt-rotor performance at steady operating conditions and it was shown that the wind tunnel forward flight performance test data were generally of very high quality. The second objective, using outdoor hover test data, was to formulate and implement a neural-network-based wind correction procedure. Compared to existing, momentum-theorymethod based wiad corrections to outdoor hover performance, the present neural-network-procedure-based corrections were better. The present wind corrections procedure, based on a well-trained neural network, captured physical trends present in the outdoor hover test data that had been missed by the existing, momentum-theory-based method. Copyright © 1998 by the American Institute of Aeronautics and Astronautics, Inc. No copyright is asserted in the United States under Title 17, U.S. Code. The U.S. Government has a royalty-free license to exercise all rights under the copyright claimed herein for Governmental Purposes. All other rights are reserved by the copyright owner. A a

Optimization Methods Applied to the Aerodynamic Design of Helicopter Rotor Blades

This paper describes a formal optimization procedure for helicopter rotor blade designs which minimizes hover horsepower while assuring satisfactory forward flight performance. The approach is to couple hover and forward flight analysis programs with a general purpose optimization procedure. The resulting optimization system provides a systematic evaluation of the rotor blade design variables and their interaction, thus reducing the time and cost of designing advanced rotor blades. The paper discusses the basis for and details of the overall procedure, describes the generation of advanced blade designs for representative Army helicopters, and compares designs and design effort with those from the conventional approach which is based on parametric studies and extensive cross-plots.

Performance degradation of helicopter rotor in forward flight due toice

This study addresses the analytical assessment of the degradation in the forward flight performance of the front rotor Boeing Vertol CH47D helicopter in a rime ice natural icing encounter. The front rotor disk was divided into 24 15-deg sections and the local Mach number and angle of attack were evaluated as a function of azimuthal and radial location for a specified flight condition. Profile drag increments were then calculated as a function of azimuthal and radial position for different times of exposure to icing, and the rotor performance was re-evaluated including these drag increments. The results of the analytical prediction method, such as horsepower required to maintain a specific flight condition, as a function of icing time have been generated. The method to illustrate the value of such an approach in assessing performance changes experienced by a helicopter rotor as a result of rime ice accretion is described.

Some Results of the Testing of a Full-Scale Ogee-Tip Rotor

Full-scale tests were utilized to investigate the effect of the Ogee tip on helicopter rotor acoustics, performance, and loads. Two facilities were used for this study: the Langley whirl tower and a UH-1H helicopter. The test matrix for hover on the whirl tower involved thrust values from 0 to 44,480 N (10,000 Ib) at several tip Mach numbers for both standard and Ogee rotors. The full-scale testing on the UH-1H encompassed the major portion of the flight envelope for that aircraft. Both near-field acoustic measurements as well as far-field flyover data were obtained for both the Ogee and standard rotors. Data analysis of the whirl-tower test shows that the Ogee tip does significantly diffuse the tip vortex while providing some improvement in hover performance at moderate thrust coefficients. Flight testing of both rotors indicates that the strong impulsive noise signature of the standard rotor can be reduced with the Ogee rotor. Forward flight performance was significantly improved with the Ogee configuration for moderate lift coefficients. Further, rotor control loads and vibrations were reduced through use of this advanced tip rotor.

Some Effects of the Shroud on the Forward Flight Performance of the Lifting Ducted Fan

Some Effects of the Shroud on the Forward Flight Performance of the Lifting Ducted Fan

Some Effects of the Shroud on the Forward Flight Performance of the Lifting Ducted Fan

Some Effects of the Shroud on the Forward Flight Performance of the Lifting Ducted Fan