American Helicopter Research Articles

American Helicopter Emergency Medical Service Pilots Report to Work Despite High Rates of Sleepiness

ObjectivePrevious studies on helicopter emergency medical service (HEMS) pilots found a positive correlation among fatigue, nodding off in flight, and accidents. We sought to quantify the amount of sleepiness in HEMS pilots using the Epworth Sleepiness Scale (ESS). MethodsAn anonymous survey was sent via the National EMS Pilots Association emergency medical services listserv including demographics, the ESS, and subjective effects of fatigue on flying. Statistical analyses were performed using the t-test and analysis of variance. ResultsThirty-one surveys were returned. Twenty-one (65%) reported an ESS > 10, indicating excessive daytime sleepiness. Twelve (39%) reported nodding off in flight; 20 (65%) indicated that they should have refused to fly, but only 14 (45%) actually did. En route was the most likely phase of flight to be affected by fatigue (23 [74%]), whereas takeoff (2 [7%]) and landing (2 [7%]) were the least likely to be affected. ConclusionMany HEMS pilots in this small study reported excessive daytime sleepiness. Most respondents indicated that they should have turned down a flight because of fatigue. More research is necessary to quantify the burden of fatigue among HEMS pilots.

Load Reduction During Resonance Crossing of a Variable-Speed Rotor

The efficiency of the rotorcraft could be improved significantly by varying the speed of the rotor based on the flight condition. Due to this reason, variable-speed rotor (VSR) technology is becoming popular, especially among urban air mobility vehicles. However, one of the biggest challenges of a VSR is the vibration of the rotor blade when it passes through a resonance point. A detailed study on the characterization of these resonances was performed by the authors (“Performance Advantages and Resonance Analysis of a Variable Speed Rotor Using Geometrically Exact Beam Formulations,” Journal of American Helicopter Society, Vol. 67, May 2022, Paper 042006). In this study, methods of reducing loads during resonance crossings are explored. The rotor model is built in Dymore, which is a multibody dynamics simulation tool. The geometrically exact beam theory is used to model the rotor blades as one-dimensional beams. Peters et al.’s (“Finite State Induced Flow Models Part I: Two-Dimensional Thin Airfoil,” Journal of Aircraft, Vol. 32, No. 2, March–April 1995, pp. 313–322) unsteady aerodynamics model with finite-state inflow and dynamic stall is used to obtain the aerodynamic loads acting on the rotor. Load reduction studies were carried out by varying the transition time, structural damping, and lag stiffness of the blade. The longer the rotor took to traverse a resonance region, the greater were the resonance loads. However, there were torque limitations on how quickly a rotor could pass through resonance. Increasing the structural damping was a very effective way of reducing the resonance loads. Increasing the lag stiffness of the rotor blade deteriorated the torque response due to 4/rev crossings. Combination studies were performed by combining the ideal transition times with 7% lag damping.

Erratum to: Analytical Estimate of Rotor Controls Required for a Straight Vortex Disturbance Rejection

Correction to:Journal of the American Helicopter Society, Vol. 62, (1), January 2017, pp. 1-4,http://dx.doi.org/10.4050/JAHS.62.015001

Erratum to: How Big Is a Lock Number?

Correction to:Journal of the American Helicopter Society, Vol. 66, (1), January 2021, pp. 1–3, DOI 10.4050/JAHS.66.012001

Study on Blade Property Measurement and Its Influence on Air/Structural Loads

In this study, the structural properties of Second Higher-Harmonic Aeroacoustic Rotor Test blades are determined using state-of-the-art test techniques. The measurement includes bending and torsion stiffnesses, section geometric offsets, and mass and inertia properties. Several Second Higher-Harmonic Aeroacoustic Rotor Test blades, including the original instrumented blade used for the wind-tunnel test campaign in 2001, as stated by Yu et al. (The HART-II Test: Rotor Wakes and Aeroacoustics with Higher-Harmonic Pitch Control (HHC) Inputs—The Joint German/French/Dutch/US Project, Proceedings of the 58th American Helicopter Society (AHS) Annual Forum, American Helicopter Soc., Montreal, June 2002) are used for the activity. A finite element-based cross-section analysis combined with an x-ray computer tomography technique is employed for the cases where no mechanical measurement is available or attempted. The resulting structural properties are correlated against the earlier estimated values, which have extensively been used in the literature for the validation of the Second Higher-Harmonic Aeroacoustic Rotor Test rotor. A substantial deviation is observed between the present measurement and the earlier property result. The comprehensive rotor dynamics analysis is performed to quantify the impact of the measured blade properties on the aeromechanics behavior of the rotor. The location of the center of gravity is demonstrated to be the most influential factor affecting the deviation and the sensitivity of the aeroelastic response of the rotor.

Design and Development of the Atlas Human-Powered Helicopter

AeroVelo initiated the Atlas Human-Powered Helicopter Project in August 2011 to win the American Helicopter Society Sikorsky Prize, which despite prior attempts had remained unclaimed for over 30 years. The Sikorsky Prize required a human-powered helicopter to sustain flight for 60 s, momentarily reach a height of 3 m, and maintain position within a 10-by-10-m area. A configuration study was undertaken using low-fidelity aerodynamic analysis and estimated mass figures. An aerostructural optimization framework was developed for rotor design, including a novel vortex-ring aerodynamic model with included ground effect prediction, finite element analysis including integrated composite failure analysis, and a detailed weight estimation scheme. The airframe comprised a wire-braced truss structure, and innovative designs were developed for many of the aircraft’s lightweight-focused subsystems. After initial flight testing in August 2012, experimental optimization and performance improvement led to a second testing program beginning in January 2013. Testing in 2013 led to a reduction in required power, and improved understanding of the structural dynamics and control strategy. The project culminated with the successful American Helicopter Society Sikorsky Prize flight on 13 June 2013.

Biodynamic Model of a Seated Occupant Exposed to Intense Impacts

Quantitative comparison of biodynamic responses, simulated using the Patil or Liu lumped parameter models, were carried out with respect to experimental anthropomorphic test dummy response data obtained from the Sikorsky advanced composite airframe program helicopter crash test. The biodynamic responses obtained from these models were characterized by large offsets from the experimental peak magnitudes, large perturbations in biodynamic displacements, and delayed response. The inability to accurately predict biodynamic response using either model led to development of a new four-degree-of-freedom nonlinear biodynamic model corresponding to a Hybrid II 50th-percentile male occupant, the parameters of which were identified by minimizing the least-square error between the simulated and advanced composite airframe program experimental responses. The new model accurately predicts peak magnitude, overall shape, and duration of the biodynamic transient response, with minimal phase shift. The biodynamic model was further validated using data from the crashworthy composite fuselage drop experiment conducted by Fasanella and Jackson (“Impact Testing and Simulation of a Crashworthy Composite Fuselage Section with Energy-Absorbing Seats and Dummies,” Journal of the American Helicopter Society, Vol. 49, No. 2, 2004, pp. 140–148). The biodynamic model overpredicted the peak lumbar loads by 1.06% when compared to the pilot’s anthropomorphic test dummy response from the Sikorsky advanced composite airframe program experiment, and it predicted lumbar loads within when compared to crashworthy composite fuselage experiments.

Haunting Legacy: Vietnam and the American Presidency from Ford to Obama. By MarvinKalb and DeborahKalb. Washington, DC: Brookings Institution Press, 2011. 355 pp.

Haunting Legacy: and the American Presidency from Ford to Obama. By Marvin Kalb and Deborah Kalb. Washington, DC: Brookings Institution Press, 2011. 355 pp. Thirty-seven years after the last American helicopter flew away from the U.S. Embassy in Saigon, the War still lingers in America's subconscious. Haunting Legacy begins where David L. Anderson's edited volume, Shadow on the White House: Presidents and the War, 1945-1975 (University Press of Kansas, 1993) ended. Anderson's book argued that no president could ignore Vietnam, as Southeast Asia pulled each commander in chief, starting with Harry Truman, into its vortex, and in Haunted Legacy, the father and daughter journalist team of Marvin and Deborah Kalb conclude, Vietnam has infiltrated the presidential DNA (p. 3). The Kalbs establish their central thesis in the introduction and then examine each president from Gerald Ford to Barack Obama. They contend that beginning with Ford, all post-Vietnam presidents live with the legacy and burden of the War--each president fears to sink into another quagmire, by applying military force against another country, and especially hesitates to deploy ground troops into hostile territory. The authors establish two presidential foreign policy models: the Gerald Ford and the Jimmy Carter. The former consists of President Ford and both Bushes, men more aggressive and willing to use military force, while the latter model consists of Presidents Carter, Ronald Reagan, and Bill Clinton--men more reluctant to use military offense. The Kalbs show an interesting unwillingness to categorize the incumbent; they believe that President Obama fits both models, so far. The authors state that President Ford did his best to help South Vietnam, but Congress (and society) refused. Not wanting anyone to perceive him or the country as a paper tiger, Ford utilized military force against the Khmer Rouge after they seized the SS Mayaguez. The authors venerate President Carter for his success with the Panama Canal Treaty, the Camp David Accords, SALT II (never ratified), and the strengthening of ties with China, but then blame bad luck rather than indecisiveness with Carter's failure to control the Iranian hostage crisis. In addition, Carter and National Security Advisor Zbigniew Brzezinski receive gentle treatment for their backing of the Mujahideen in Afghanistan, which would have disastrous consequences in the future, a point that the Kalbs establish well but do not unpack further. A shift in tone occurs when the Kalbs analyze President Reagan. The authors throw subtle jabs at Reagan, and present him as the Republican mythologized Cold War winner, an aloof former B-movie actor, who failed on all counts in Lebanon. They determine, He still talked a good game, but when it was time for a decision, he punted (p. …

Fatigue–Strength Surface: basis for structural analysis under dynamic loads

The relation between fatigue strength and number of load cycles to failure is usually represented by Wohler curves. The four-parametric Weibull function shows a good relation between the static and dynamic strength in the low- and high-cycle fatigue range for a constant stress ratio R (minimum stress/maximum stress) (see Jarosch and Stepan in Fatigue properties and test procedures of glass reinforced plastic rotorblades. American Helicopter Society, 25th Annual National Forum, Paper No. 370, 1969; Och in Fatigue strength, AGARDograph No. 292, helicopter fatigue design guide, 1983; Bansemir and Emmerling in Fatigue substantiation and damage tolerance evaluation of fiber composite helicopter components, applied vehicle technology panel: applications of damage tolerance principles for improved airworthiness of rotorcraft, Corfu, Greece, 1999). However, in addition to the number of load cycles to the failure, fatigue strength also depends on the stress ratio R as well. In predicting the lifetime of a component, a more proper way for the presentation of fatigue life test data is the Goodman approach. This often used method affords the user to predict lifetime at any stress ratio, but does not represent the real material behaviour fully. The Goodman diagram does not take into account the combined effect of low-cycle fatigue and high-cycle fatigue. One better way to build an accurate relation between the fatigue strength, number of load cycles to failure and the stress ratio is to add the third dimension. The result is a three-dimensional view of the fatigue strength as a function of number of load cycles to failure and stress ratio R. Hence, the mathematical description of the fatigue strength as a kind of a surface function depending on the number of load cycles to the failure and the stress ratio is of high interest and indeed the focus of this study. The surface function represents the real material behaviour in low-cycle fatigue range as well as in high-cycle fatigue range more accurate compared to basic Goodman approach. For the determination of the Fatigue–Strength Surface, a so-called F–S Surface, test points with different stress ratios are necessary. The surface function can be adapted to the test points using a nonlinear regression analysis based on least square method. Therefore, it is advisable to use either the three-dimensional Weibull function or a surface function which consists of Tschebyscheff Polynomials. To enhance the regular two-dimensional Weibull function to a three-dimensional model, the four Weibull parameters are described as functions of the stress ratio. This method can be used to analyse the material behaviour based on the results of coupon or component tests. In case of coupon tests, this method is applied and validated as described in the paper. Furthermore, the use of material F–S Surface is compared with Goodman approach. For this purpose, mean surfaces of coupon test specimens with a survival probability of 50% are used. Especially in high-cycle fatigue range and for the stress ratios greater than zero, Goodman approach shows a slight deviation from the actual fatigue strength. In order to consider such effects, lifetime calculations are performed in Eurocopter Deutschland GmbH by using the reduced, so-called working S–N curves which are based on component test results. It has been found that the F–S Surface tends to describe the real fatigue strength in a more accurate way. This may lead to a longer lifetime of components and the possibility to extend the margins of components. This paper is developed from the Diploma thesis “Prediction of the dynamic strength behaviour of structural elements based on S–N–R surfaces created by nonlinear regression analysis of experimental data” (see Weinert in Prediction of the dynamic strength behaviour of structural elements based on S–N–R-surfaces created by nonlinear regression analysis of experimental data Technische Universitat Dresden 2011).

Trajectory Optimization Procedures for Rotorcraft Vehicles, Their Software Implementation, and Applicability to Models of Increasing Complexity

This paper describes numerical procedures for the solution of trajectory optimization problems in rotorcraft flight mechanics. We specifically consider procedures which can be easily interfaced with black-box flight simulators, with minimal assumptions on the functionalities of such third-party software components, and which can cater to a wide range of vehicle models of varying complexity. At first we identify in the direct approach to the solution of maneuver optimal control problems the Presented at the American Helicopter Society 64th Annual Forum, Montreal, Canada, April 29 – May 1, 2008.

Influence of Dynamic Stall and Dynamic Wake Effects on Helicopter Trim and Rotor Loads

Flight test data of helicopters indicate that vibratory levels in the fuselage exhibit a wide spectrum of frequencies including the dominant blade passage frequency and its integer multiples. The present work attempts to understand the reason for the existence of several frequencies in the response of the fuselage and possible cause for this observed phenomenon by formulating a computational aeroelastic model. In this theoretical study, a systematic approach has been undertaken to identify the effects of inflow modeling and sectional aerodynamic load evaluation, on helicopter trim, rotor blade response, and hub loads. Five different combinations of aerodynamic models of increasing complexity, representing rotor inflow and sectional aerodynamic loads, have been proposed. The differential equations of motion are solved in time domain in a sequential manner to obtain the response of all the blades in the rotor system, the inflow variables, and the sectional loads at every time step. The results of the present study show that the aerodynamic model incorporating dynamic wake and dynamic stall effects introduces a wide spectrum of harmonics in the hub loads including blade passage frequency and its integer multiples. The influence of aerodynamic modeling on the variation of trim parameters with forward speed has also been brought out. It is observed that the aerodynamic model incorporating dynamic wake and dynamic stall effects predicts the trim parameters whose variation with forward speed resemble qualitatively similar to those obtained in flight test. A comparison of the variation of blade sectional lift for various aerodynamic models indicates that in the advancing side of the rotor, a dynamic stall model introduces a shift in the azimuth angle at which the minimum lift occurs. The effect of structural flap—lag coupling due to blade pretwist on trim and rotor loads has been studied, and these results are compared with those pertaining to a straight blade configuration. ©2009 American Helicopter Society

An Effective Crashworthiness Design Optimization Methodology to Improve Helicopter Landing Gear Energy Absorption

Crashworthiness design optimization problems are continuously challenged by inherent complexity of long simulation time, numerical instability, and unavailability of sensitivity analysis. In recent years, surrogate-model-based design optimization using design of experiments and response surface methodology has proven to be a promising way to achieve the improved design for crashworthiness problems. However, the method is still hampered by a large number of function evaluations for a large number of design variables or low accuracy for small sample size. This paper presents an effective simulation-based optimization algorithm for optimal design of large-scale, computationally expensive crashworthiness problems. The proposed optimization algorithm is called the sequential regularized multiquadric regression with output space mapping (SRMQ/OSM). The proposed method is demonstrated on two simple numerical problems and is subsequently extended to a generic helicopter skid landing gear optimization problem to improve the energy absorption efficiency with prescribed design constraints. The results show that the proposed approach converged quickly to a feasible design with no constraint violation. The improved skid gear design obtained by the proposed SRMQ/OSM method exhibits better impact performance in terms of energy absorption, peak acceleration, onset rate, and pulse duration. ©2009 American Helicopter Society.

Errata: Compressible Dynamic Stall Performance of a Variable Droop Leading Edge Airfoil with a Gurney Flap [Journal of the American Helicopter Society Vol. 53, No. 1, January 2008, pp. 18–25[

Errata: Compressible Dynamic Stall Performance of a Variable Droop Leading Edge Airfoil with a Gurney Flap [Journal of the American Helicopter Society Vol. 53, No. 1, January 2008, pp. 18–25[

Rotor Performance Enhancement and Vibration Reduction in Presence of Dynamic Stall Using Actively Controlled Flaps

A computational study of helicopter vibration and rotor shaft power reduction is conducted using activelycontrolled trailing-edge flaps (ACFs), implemented in both single and dual flap configurations. Simultaneous vibration reduction and performance enhancement is demonstrated under level flight condition at high advance ratios, where dynamic stall effects are significant. Power reduction is achieved using the adaptive Higher Harmonic Control (HHC) algorithm in closed loop, with 2-5/rev flap control harmonics. This approach is compared with an off-line, nonlinear optimizer available in MATLAB, and favorable comparisons are obtained. A parametric study of flap spanwise location is also conducted to determine its optimal location for power reduction. The effectiveness of the ACF approach for power as well as simultaneous vibration and power reduction is also compared with conventional individual blade control (IBC) approach. Rotor power reduction and simultaneous reduction of vibration and power are shown to be larger at higher rotor thrust and advance ratio. Finally, the effect of active flap on dynamic stall is examined to determine the mechanisms of rotor power reduction. The simulation results clearly demonstrate the potential of the ACF system for power reduction as well as simultaneous vibration and power reduction. Nomenclature c Blade chord cc Flap chord CT Rotor thrust coefficient D Matrix defined as TTQT+R fb(.) Blade equations of motion ft(.) Trim equations FHX4,FHY4, FHZ4 4/rev hub shears, nondimensionalized by MbΩR MHX4,MHY4, MHZ4 4/rev hub moments, nondimensionalized by MbΩR J Quadratic-form objective function to be minimized Presented at the American Helicopter Society 62nd Annual Forum, Phoenix, AZ, June 9-11, 2006. 1 Mb Blade mass mc Flap mass per unit length, nondimensionalized by Mb/R MHz1 Yawing moment about rotor hub Nb Number of rotor blades PR Rotor shaft power, nondimensionalized by MbΩR qt Vector of trim variables qti Vector of trim variables at i-th control step Q Weighting matrix for objectives to be minimized R Weighting matrix on control input R Rotor blade radius Rt Trim residuals vector T Sensitivity, transfer matrix between control inputs and objective function uk Control input vector, kth control step uk,opt Optimum value of control input vector xc Spanwise location of center of control surface zk Objective vector, kth control step αR Rotor shaft angle of attack δ f Flap deflection angle ∆Cd,flap Additional drag due to flap deflection δNc,δNs N/rev cosine and sine amplitude of δ βp Blade precone angle γ Lock number μ Helicopter advance ratio ωF , ωL, ωT Blade flap, lead-lag and torsional natural frequencies Ω Rotor angular speed ψ Rotor azimuth angle φR Lateral roll angle θ0,θ1c,θ1s Collective and cyclic pitch components θ0t Tail rotor collective pitch θtw Built-in twist angle σ Rotor solidity Introduction and Background Specifications for noise and vibration levels in rotorcraft are continuously increasing in stringency, thus motivating research related to active noise and vibration reduction. Desirable vibration levels have been identified to be below 0.05g to provide passengers with “jet smooth” ride (Ref. 1). A number of active control techniques have emerged for effective vibration reduction (Refs. 1,2), as illustrated schematically in Fig. 1. These approaches generally fall into one of two categories: (a) active control approaches aimed at reducing vibrations in the rotor before they propagate into the fuselage, and (b) active control approaches

Cross-Sectional Design of Composite Rotor Blades

An important step of rotor blade design is to design cross-sections that possess specified stiffness and inertia properties. This paper describes an innovative methodology for Based on a paper presented at the 63 Annual Forum of the American Helicopter Society, Virginia Beach, Virginia, May 1 – 3, 2007.

Editorial: Impact Factor and Its Significance for the Journal of the American Helicopter Society

Editorial: Impact Factor and Its Significance for the Journal of the American Helicopter Society

Editorial: The Early Editors of the Journal of the American Helicopter Society

Editorial: The Early Editors of the Journal of the American Helicopter Society

50th Anniversary Perspectives on the American Helicopter Industry

50th Anniversary Perspectives on the American Helicopter Industry

Errata: Generalized Timoshenko Theory of the Variational Asymptotic Beam Sectional Analysis [Journal of the American Helicopter Society Vol. 50, No. 1, January 2005, pp. 46–55

Errata: Generalized Timoshenko Theory of the Variational Asymptotic Beam Sectional Analysis [Journal of the American Helicopter Society Vol. 50, No. 1, January 2005, pp. 46–55

Design of a Martian Autonomous Rotary-Wing Vehicle

The design of a Martian autonomous rotary wing vehicle (MARV) is described. MARV is a 50-kg gross takeoff mass, coaxial helicopter designed for Mars exploration. Powered by a fuel cell system, it carries a payload of 10.8 kg over a range of 25 km with an endurance of 39 min including hover capability for 1 min. MARV is designed in response to the Request For Proposal from NASA/Sikorsky for the Year 2000 American Helicopter Society student design competition. The design covers aerodynamic and structural design of rotor blades, vehicle power plant, fuselage and landing gear, control system, transmission, and vehicle lander communications. A detailed mechanism for autonomous deployment of the vehicle from the lander is also described. This preliminary design study indicates that controlled vertical flight on Mars is feasible with existing technology