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NASA NTRS2018

Mars Helicopter Technology Demonstrator

Aerial exploration of Mars with helicopters could provide mission capabilities that go beyond that of orbiting satellites, landed spacecraft and rovers. Helicopters allow examination of Mars at visual resolutions comparable to landers and rovers but over much longer ranges. They could access and land at designated targets in a controlled manner and could be used to carry or retrieve small payloads. Helicopters could enhance rover missions by quickly scouting out safe traverse routes or providing reconnaissance on possible science target destinations and, as standalone systems, could be used to explore areas that may not be reachable by rovers. Mars helicopters may also be considered as elements of a sample return architecture where they could be used for timely retrieval of small science samples back to a Mars ascent vehicle for return to Earth. The challenge to helicopter use on Mars is the thin carbon dioxide atmosphere with approximately 1% of the density of Earth’s atmosphere. Much like the Sojourner rover on the Pathfinder mission paved the way for the Spirit, Opportunity, Curiosity and the Mars 2020 rovers, an initial demonstration on Mars is desirable so as to inform the development of future helicopter missions. The Jet Propulsion Laboratory is leading a collaborative effort with AeroVironment Inc., and NASA centers Ames, Langley and Glenn to develop a small helicopter as a technology demonstrator. In this paper we briefly describe the results of this effort including results from controlled free-flight of a full-scale (approx 850 g) prototype flown in a test chamber under Mars conditions, the design and development of the 1800 g (not-to-exceed mass) technology demonstrator helicopter, and the operation of the helicopter.

Balaram, J. (Bob)↗

NASA NTRS1978

Helicopter external noise requirements: FAA perspective

Enactment of helicopter noise certification standards for the control of noise impact contributing to community annoyance is considered in terms of the development of helicopters as an environmentally compatible air transportation mode. Increased use of helicopters for commercial applications and public awareness of aircraft noise are cited as factors making development of helicopter noise standards necessary both for the protection of the environmental interest of the community and to ensure the orderly growth of the helicopter industry itself. Noise sources, technology trends in helicopter design, and design concepts to control helicopter noise are discussed along with the regulatory background and specific helicopter regulatory concepts.

Foster, C. R.↗

NASA NTRS2011

The Effects of Ambient Conditions on Helicopter Rotor Source Noise Modeling

A new physics-based method called Fundamental Rotorcraft Acoustic Modeling from Experiments (FRAME) is used to demonstrate the change in rotor harmonic noise of a helicopter operating at different ambient conditions. FRAME is based upon a non-dimensional representation of the governing acoustic and performance equations of a single rotor helicopter. Measured external noise is used together with parameter identification techniques to develop a model of helicopter external noise that is a hybrid between theory and experiment. The FRAME method is used to evaluate the main rotor harmonic noise of a Bell 206B3 helicopter operating at different altitudes. The variation with altitude of Blade-Vortex Interaction (BVI) noise, known to be a strong function of the helicopter s advance ratio, is dependent upon which definition of airspeed is flown by the pilot. If normal flight procedures are followed and indicated airspeed (IAS) is held constant, the true airspeed (TAS) of the helicopter increases with altitude. This causes an increase in advance ratio and a decrease in the speed of sound which results in large changes to BVI noise levels. Results also show that thickness noise on this helicopter becomes more intense at high altitudes where advancing tip Mach number increases because the speed of sound is decreasing and advance ratio increasing for the same indicated airspeed. These results suggest that existing measurement-based empirically derived helicopter rotor noise source models may give incorrect noise estimates when they are used at conditions where data were not measured and may need to be corrected for mission land-use planning purposes.

Schmitz, Frederic H.↗

NASA NTRS2022

Henrich Focke —Inventor of the First Successful Helicopter

In 1924, Henrich Focke, Georg Wulf and Dr. Werner Naumann started Focke-Wulf Flugzeugbau GmbH (Focke-Wulf Aircraft Manufacturing Ltd.). Focke-Wulf constructed Juan de la Cierva’s C.19 and C.30 Autogiros under license from 1930 until 1937. This led Focke to design the first successful and fully controllable helicopter, the Fw-61. Model tests in his wind tunnel supported the configuration and in 1932 Focke proposed a helicopter project to the ministry with drawings and calculations, which was granted in that same year. Impressed with the success of the Fw-61, the German Air Ministry suggested that Focke establish a new company dedicated to helicopter development and issued him a requirement for an improved design, capable of carrying a 700 kg (1,500 lbs) payload. Focke and Gerd Achgelis (a test pilot) partnered to establish the Focke–Achgelis company in 1938. The Fw-61 became the Fa-61. Historically, Henrich Focke and his Focke-Achgelis company must surely be considered as the developer of the world’s first successful helicopter. They began development of the Focke Wulf company’s Model Fw-61 helicopter, which had a side-by-side rotor system, in 1932. First flight was achieved on June 26, 1936 with test pilot Ewald Rohlfs at the controls. With the Fw-61’s flight demonstrated success, Professor Focke went on to present three early papers: 1. Das Trag- und Hubschrauberproblem (Autogyro and Helicopter Problems), November 1937 2. Forschung und Entwicklung der Drehflügelflugzeuge (Research and Development of Rotating Wing Aircraft), April 1942 3. Fortschritte des Hubschraubers (Progress of the Helicopter), October 1943 The first paper was also published by the German Museum (Munich) in 1938 and it was translated into English on the order of the Ministry of Aircraft Production by J. Helledoren as R.T.P No. 2128 at the end of 1937. The other two reports, however, appear not to have been translated during or after World War II and thus are made public here – probably for the first time. Translations of the three Focke papers are provided in this report. A careful translation from German to English of the papers has been made. Many of original photos have been recovered from the German Museum (Munich). In several cases, the original graphs have been redrawn to enhance each paper’s technical value. The editors have added footnotes where appropriate. The editors consider Focke’s papers to be of such immense technical and historical value that they deserve wide distribution to the rotorcraft industry. Finally, the editors believe that readers of the many technical points Henrich Focke makes about his development program will see that Focke’s program anticipated even today’s best rotorcraft industry efforts.

Inventor↗

NASA NTRS1987

Helicopter main-rotor speed effects on far-field acoustic levels

The design of a helicopter is based on an understanding of many parameters and their interactions. For example, in the design stage of a helicopter, the weight, engine, and rotor speed must be considered along with the rotor geometry when considering helicopter operations. However, the relationship between the noise radiated from the helicopter and these parameters is not well understood, with only limited model and full-scale test data to study. In general, these data have shown that reduced rotor speeds result in reduced far-field noise levels. This paper reviews the status of a recent helicopter noise research project designed to provide experimental flight data to be used to better understand helicopter rotor-speed effects on far-field acoustic levels. Preliminary results are presented relative to tests conducted with a McDonnell Douglas model 500E helicopter operating with the rotor speed as the control variable over the range of 103% of the main-rotor speed (NR) to 75% NR, and with the forward speed maintained at a constant value of 80 knots.

Mueller, Arnold W.↗

NASA NTRS1986

General equilibrium characteristics of a dual-lift helicopter system

The equilibrium characteristics of a dual-lift helicopter system are examined. The system consists of the cargo attached by cables to the endpoints of a spreader bar which is suspended by cables below two helicopters. Results are given for the orientation angles of the suspension system and its internal forces, and for the helicopter thrust vector requirements under general circumstances, including nonidentical helicopters, any accelerating or static equilibrium reference flight condition, any system heading relative to the flight direction, and any distribution of the load to the two helicopters. Optimum tether angles which minimize the sum of the required thrust magnitudes are also determined. The analysis does not consider the attitude degrees of freedom of the load and helicopters in detail, but assumes that these bodies are stable, and that their aerodynamic forces in equilibrium flight can be determined independently as functions of the reference trajectory. The ranges of these forces for sample helicopters and loads are examined and their effects on the equilibrium characteristics are given parametrically in the results.

Cicolani, L. S.↗

NASA NTRS2012

Conceptual Design and Performance Analysis for a Large Civil Compound Helicopter

A conceptual design study of a large civil compound helicopter is presented. The objective is to determine how a compound helicopter performs when compared to both a conventional helicopter and a tiltrotor using a design mission that is shorter than optimal for a tiltrotor and longer than optimal for a helicopter. The designs are generated and analyzed using conceptual design software and are further evaluated with a comprehensive rotorcraft analysis code. Multiple metrics are used to determine the suitability of each design for the given mission. Plots of various trade studies and parameter sweeps as well as comprehensive analysis results are presented. The results suggest that the compound helicopter examined for this study would not be competitive with a tiltrotor or conventional helicopter, but multiple possibilities are identified for improving the performance of the compound helicopter in future research.

Russell, Carl↗

NASA NTRS1950

An Analysis of the Autorotative Performance of a Helicopter Powered by Rotor-Tip Jet Units

The autorotative performance of an assumed helicopter was studied to determine the effect of inoperative jet units located at the rotor-blade tip on the helicopter rate of descent. For a representative ramjet design, the effect of the jet drag is to increase the minimum rate of descent of the helicopter from about 1,OO feet per minute to 3,700 feet per minute when the rotor is operating at a tip speed of approximately 600 feet per second. The effect is less if the rotor operates at lower tip speeds, but the rotor kinetic energy and the stall margin available for the landing maneuver are then reduced. Power-off rates of descent of pulse-jet helicopters would be expected to be less than those of ramjet. helicopters because pulse jets of current design appear to have greater ratios of net power-on thrust to power-off, drag than currently designed rain jets. Iii order to obtain greater accuracy in studies of autorotative performance, calculations in'volving high power-off rates of descent should include the weight-supporting effect of the fuselage parasite-drag force and the fact that the rotor thrust does not equal the weight of the helicopter.

AUTOROTATION↗

NASA NTRS1993

L-Band Mobile Terminal Antennas for Helicopters

The feasibility of using a low gain antenna (LGA) as a mobile terminal antenna for helicopters is described in this paper. The objectives are 1) to select the lowest cost antenna system which can be easily mounted on a helicopter and capable of communicating with a satellite, and 2) to determine the best antenna position on the helicopter to mitigate the signal blockage due to rotor blades and the multipath effect from the helicopter's body. The omni-directional low gain antenna (LGA) is selected because it is simple, reliable, and low cost. The helix antenna is selected among the many LGAs because it is the most economical one and has the widest elevation beamwidth. Both 2-arm and 4-arm helices are studied experimentally to determine the antenna's performance and the scattering effects from the helicopter's body. It is found that the LGA should be located near the tail section and at least 8 inches above the helicopter.

helicopter↗

NASA NTRS1973

Results of the noise measurement program on a standard and modified OH-6A helicopter

A field noise measurement program has been conducted on a standard OH-6A helicopter and one that had been modified by reducing the rotor speed, altering rotor tip shape, and treating the engine exhaust and inlet to reduce the external noise levels. The modifications consisted of extensive aircraft design changes resulting in substantial noise reductions following state-of-art noise reduction techniques. The purpose of this study was to document the ground noise characteristics of each helicopter during flyover, hover, landing, and take-off operations. Based on an analysis of the measured results, the average of the overall on-track noise levels of the final modified helicopter was approximately 14 db lower than that for the standard helicopter. Narrow-band-spectra data of the hovering helicopter show a reduction in the overall noise due to the reductions achieved for the lifting main and antitorque tail rotor, engine exhaust, and gear box noise for the modified helicopter. The noise results of the test program are found to correlate generally with noise measurements made previously on this type of aircraft.

Henderson, H. R.↗

NASA NTRS1973

Results of the flight noise measurement program using a standard and modified SH-3A helicopter

A field noise measurement program has been conducted using both a standard SH-3A helicopter and an SH-3A helicopter modified to reduce external noise levels. Modifications included reducing rotor speed, increasing the number of rotor blades, modifying the blade-tip shapes, and acoustically treating the engine air intakes and exhaust. The purpose of this study was to document the noise characteristics recorded on the ground of each helicopter during flyby, hover, landing, and take-off operations. Based on an analysis of the measured results, the average of the overhead, overall, ontrack noise levels was approximately 4 db lower for the modified helicopter than for the standard helicopter. The improved in-flight noise characteristics, and associated small footprint areas and time durations, were judged to be mainly due to tail-rotor noise reductions. The noise reductions were obtained at the expense of required power increases at airspeeds greater than 70 knots for the modified helicopter.

Pegg, R. J.↗

NASA NTRS1981

Subjective field study of response to impulsive helicopter noise

Subjects, located outdoors and indoors, judged the noisiness and other subjective noise characteristics of flyovers of two helicopters and a propeller driven airplane as part of a study of the effects of impulsiveness on the subjective response to helicopter noise. In the first experiment, the impulsive characteristics of one helicopter was controlled by varying the main rotor speed while maintaining a constant airspeed in level flight. The second experiment which utilized only the helicopters, included descent and level flight operations. The more impulsive helicopter was consistently judged less noisy than the less impulsive helicopter at equal effective perceived noise levels (EPNL). The ability of EPNL to predict noisiness was not improved by the addition of either of two proposed impulse corrections. A subjective measure of impulsiveness, however, which was not significantly related to the proposed impulse corrections, was found to improve the predictive ability of EPNL.

Powell, C. A.↗

NASA NTRS1981

Comparison of predicted engine core noise with proposed FAA helicopter noise certification requirements

Calculated engine core noise levels, based on NASA-Lewis prediction procedures, for five representative helicopter engines are compared with measured total helicopter noise levels and proposed FAA helicopter noise certification requirements. Comparisons are made for level flyover and approach procedures. The measured noise levels are generally significantly greater than those predicted for the core noise levels, except for Sikorsky S-61 and S-64 helicopters. However, the predicted engine core noise levels are generally at or within 3 db of the proposed FAA noise rules. Consequently, helicopter engine core noise can be a significant contributor to the overall helicopter noise signature and, at this time, will provide a limiting floor to a further decrease in future noise regulations.

Vonglahn, U.↗

NASA NTRS1981

ATC simulation of helicopter IFR approaches into major terminal areas using RNAV, MLS, and CDTI

The introduction of independent helicopter IFR routes at hub airports was investigated in a real time air traffic control system simulation involving a piloted helicopter simulator, computer generated air traffic, and air traffic controllers. The helicopter simulator was equipped to fly area navigation (RNAV) routes and microwave landing system approaches. Problems studied included: (1) pilot acceptance of the approach procedure and tracking accuracy; (2) ATC procedures for handling a mix of helicopter and fixed wing traffic; and (3) utility of the cockpit display of traffic information (CDTI) for the helicopter in the hub airport environment. Results indicate that the helicopter routes were acceptable to the subject pilots and were noninterfering with fixed wing traffic. Merging and spacing maneuvers using CDTI were successfully carried out by the pilots, but controllers had some reservations concerning the acceptability of the CDTI procedures.

Tobias, L.↗

NASA NTRS1981

Comparison of predicted engine core noise with proposed FAA helicopter noise certification requirements

Calculated engine core noise levels, based on NASA-Lewis prediction procedures, for five representative helicopter engines are compared with measured total helicopter noise levels and proposed FAA helicopter noise certification requirements. Comparisons are made for level flyover and approach procedures. The measured noise levels are generally significantly greater than those predicted for the core noise levels, except for the Sikorsky S-61 and S-64 helicopters. However, the predicted engine core noise levels are generally at or within 3 dB of the proposed FAA noise rules. Consequently, helicopter engine core noise can be a significant contributor to the overall helicopter noise signature and, at this time, will provide a limiting floor to a further decrease in future noise regulations.

Von Glahn, U.↗

NASA NTRS1982

Helicopter engine core noise

Calculated engine core noise levels, based on NASA Lewis prediction procedures, for five representative helicopter engines are compared with measured total helicopter noise levels and ICAO helicopter noise certification requirements. Comparisons are made for level flyover and approach procedures. The measured noise levels are generally significantly greater than those predicted for the core noise levels, except for the Sikorsky S-61 and S-64 helicopters. However, the predicted engine core noise levels are generally at or within 3 dB of the ICAO noise rules. Consequently, helicopter engine core noise can be a significant contributor to the overall helicopter noise signature.

Vonglahn, U. H.↗

NASA NTRS1985

Optimal landing of a helicopter in autorotation

Gliding descent in autorotation is a maneuver used by helicopter pilots in case of engine failure. The landing of a helicopter in autorotation is formulated as a nonlinear optimal control problem. The OH-58A helicopter was used. Helicopter vertical and horizontal velocities, vertical and horizontal displacement, and the rotor angle speed were modeled. An empirical approximation for the induced veloctiy in the vortex-ring state were provided. The cost function of the optimal control problem is a weighted sum of the squared horizontal and vertical components of the helicopter velocity at touchdown. Optimal trajectories are calculated for entry conditions well within the horizontal-vertical restriction curve, with the helicopter initially in hover or forwared flight. The resultant two-point boundary value problem with path equality constraints was successfully solved using the Sequential Gradient Restoration Technique.

Lee, A. Y. N.↗

NASA NTRS1986

New capabilities and recent research programs of the NASA/Army CH-47B variable-stability helicopter

The CH-47B is the third in a series of variable-stability helicopters developed and operated by NASA since 1952 to investigate helicopter and VTOL handling qualities. Recently, several new capabilities were added to this helicopter to enable it to better support new and evolving research requirements. The installation of a programmable force-feel system for the evaluation pilot's conventional cyclic stick, and a four-axis side-stick controller permit a range of in-flight investigations concerning manipulator characteristics and augmentation system features that had not been possible with earlier NASA research helicopters. A recently installed color electronic display system with a programmable symbol generator will permit the investigation of display formats for a variety of VTOL and helicopter missions. Finally, a powerful new general-purpose flight computer is now in operation. It is programmable in high-level languages and will provide more efficient support of research programs. In addition to these new hardware capabilities, flight-control software has been developed to improve the in-flight simulation capability of the aircraft. A brief description of the CH-47B's variable-stability research equipment is provided, recent research programs are summarized, and some remarks concerning the potential of the helicopter are presented.

Hindson, W. S.↗