Army Aeroflightdynamics Directorate Research Articles

Aircraft and Rotorcraft System Identification: Engineering Methods with Flight Test Examples, Second Edition [Bookshelf

This book is largely devoted to the practical engineering aspects of aircraft system identification using real-world flight tests, and it indeed includes an impressive array of examples. The text presents examples involving 17 different airframes, including conventional fixed-wing aircraft, helicopters, tilt-rotors, vertical and/or short take-off and landing (V/STOL) aircraft, and small unmanned aerial vehicles (UAVs). The author is most well known in the rotorcraft community due to his affiliation with the U.S. Army AeroFlightDynamics Directorate (AFDD) and their involvement in numerous military and civil rotorcraft programs over the last few decades. The frequency-domain approach is well suited for rotorcraft system identification and is prevalent throughout the rotorcraft industry. Although the examples in the text are weighted more toward rotorcraft, it should be noted that the second edition includes a variety of new fixed-wing examples, making the latest edition more suitable for the general aircraft community. The material in the book is very much linked to the software package Comprehensive Identification from Frequency Responses (CIFER), which was developed by the author's group at AFDD. Several chapters in the book have a direct correlation with specific components of the software, and virtually all of the example results are produced by CIFER. The second edition has been updated to be compatible with the latest version of CIFER, including its updated user interface. The text was never intended to be a software user’s manual, but since the examples in the book are inherently tied to CIFER, reference to the detailed operation of the software is inevitable. In general, the improvements in the software also translate into improved readability and accessibility of the text.

Autonomous Black Hawk in Flight: Obstacle Field Navigation and Landing‐site Selection on the RASCAL JUH‐60A

This paper describes the development and flight test of autonomous obstacle field navigation and safe landing area selection on the U.S. Army Aeroflightdynamics Directorate RASCAL JUH‐60A research helicopter. Using laser detection and ranging (LADAR) as the primary terrain sensor, the autonomous flight system is able to avoid obstacles, including wires, and select safe landing sites. An autonomous integrated landing zone approach profile was developed and validated that integrates cruise flight, low‐level terrain flight, and approach to a safe landing spot determined on the fly. Results are presented for a range of sites and conditions. Approximately 750 km of autonomous flight was performed, 230 km of which was at low altitude in mountainous terrain using the obstacle field navigation system. This is the first time a full‐scale helicopter has been flown fully autonomously a significant distance in low‐level flight over complex terrain, basing its planning solely on sensor data gathered from an onboard sensor. These flights demonstrate tight integration between terrain avoidance, control, and autonomous landing.

Handling qualities studies into the interaction between active sidestick parameters and helicopter response types

Active inceptors offer great potential for improving the handling qualities of fly-by-wire rotorcraft. In a cooperative research effort, the DLR Institute of Flight Systems in Germany and the US Army Aeroflightdynamics Directorate (AFDD) conducted several in-flight experiments to study the influence of the dynamic characteristics (natural frequency and damping) of the cyclic stick on the overall handling qualities of a rotorcraft. Experiments were performed looking at sidestick (DLR) and centerstick (AFDD) inceptors for Rate and Attitude Command response types. The results of two different experiments are presented in this paper. The first experiment evaluated the roll handling in forward flight and was only performed with a sidestick inceptor. The task used in this experiment was designed during exercises of the Empire Test Pilots’ School (ETPS) on DLR’s Flying Helicopter Simulator EC135 ACT/FHS. ETPS also contributed to the optimization of the static sidestick characteristics that were used for the testing. The second, more comprehensive experiment, evaluating ADS-33 Hover and Slalom Mission Task Elements, was performed both with a sidestick and a centerstick allowing a direct comparison of both types of inceptors. Regression analyses were performed on the collected pilot ratings to gain a systematic insight into the preferred stick characteristics for the different inceptor and response types. The test results consistently suggest that the preferred characteristics are best described by a first-order response model.

Flight Test and Simulation of a Cargo Container Slung Load in Forward Flight

This article presents some recent results from a cooperative effort by the U.S. Army Aeroflightdynamics Directorate, the Technion Israel Institute of Technology, and the Northern Arizona University. The objective of the article was to study and simulate the behavior of the 6 x 6x 8 ft CONEX cargo container in forward flight suspended beneath a UH-60 Black Hawk helicopter. This load, like other cargo containers, is subject to massively separated unsteady flow and is limited by stability to operational airspeeds well below the power-limited speed of the configuration. The article makes use of aerodynamic data from wind tunnels, flight tests, and computational fluid dynamics. The objective is a simulation of the helicopter-slung load system validated over the complete flight envelope. The principal remaining technical challenge is a mode of the unsteady load aerodynamics that is capable of predicting the critical unstable speed and some progress has been made in meeting this challenge.

Hover Performance Correlation for Full‐Scale and Model‐Scale Coaxial Rotors

The hover performance data of full-scale and model-scale coaxial rotors have been compared with CAMRAD II predictions having a free vortex wake analysis. Performance correlations of a coaxial rotor were made with a variation of key parameters including the rotor spacing and height. To understand aerodynamic behavior of the U.S. Army Aeroflightdynamics Directorate(AFDD)coaxialrotoroperatingoverarangeofReynoldsnumbersfrom36,000to180,000,theReynoldsnumber scaling effect was explored using an airfoil design code, MSES. It was found that the coaxial rotor spacing effect on hover performance was minimal for the rotor spacing larger than 20% of the rotor diameter. The measured performance data showed that more thrust was lost from the lower rotor of a coaxial than the upper rotor due to a larger rotor-to-rotor wake interference effect, and the lower rotor kept only an 81% of the single rotor OGE (out-of-ground effect) thrust whereas the upper rotor maintained a 90%. The lower rotor IGE (in-ground effect) thrust increased quickly by 26% as the rotor approached to the ground from the position of an 80% of the rotor diameter to 10%, and the corresponding IGE power increased by 17%. These thrust and power characteristics were well predicted. Overall, the performance prediction for the coaxial rotor was satisfactory when compared with the measured data.

Aeromechanics Analysis of a Heavy Lift Slowed-Rotor Compound Helicopter

A heavy lift slowed-rotor tandem compound helicopter was designed as a part of the NASA heavy lift rotorcraft systems investigation. The vehicle is required to carry 120 passengers over a range of 1200 nautical miles and cruise at 350 knots at an altitude of 30,000 feet. The basic size of the helicopter was determined by the United States Army Aeroflightdynamics Directorate's design code RotorCraft. Then performance, loads, and stability analyses were conducted with the Comprehensive Analytical Model of Rotorcraft Aerodynamics and Design II. Blade structural design (blade inertial and structural properties) was carried out using the loading condition from the Comprehensive Analytical Model of Rotorcraft Aerodynamics and Design II. A rotor parametric study was conducted to investigate the effects of the twist, collective, tip speed, and taper on aircraft performance. Designs were also developed for alternate missions to explore the influence of the design condition on performance.

Analysis of SPR measurements from HART II

The wind tunnel test of HART II (Higher Harmonic Control Aeroacoustic Rotor Test), performed in October 2001 in the Large Low-Speed Facility (LLF) of the German-Dutch Wind-tunnel (DNW), is part of an international cooperative program by the German DLR, French ONERA, DNW, NASA Langley and the US Army Aeroflightdynamics Directorate (AFDD). The main objective of the program is the investigation of rotor wake and its influence on rotor blade–vortex interaction (BVI) noise with and without higher harmonic pitch control (HHC). For blade position and deflection measurements the Stereo Pattern Recognition (SPR) technique was used for the first time. This technique is based on a 3-dimensional reconstruction of visible marker locations by using stereo camera images. An evaluation of these images leads to the spatial position of markers which are attached to each of the four blades and to the bottom of the fuselage and thus to the blade motion parameters in flap, lead-lag and torsion. In this paper the different analysis methods and post-processing of SPR data and the final results for all four rotor blades are presented and the advantages, drawbacks and the potential of this technology are shown.

Prediction and Measurement of Blade - Vortex Interaction

An extensive quantity of airload measurements was obtained for a pressure-instrumented model of the BO-105 main rotor for a large number of higher harmonic control settings at Duits-Nederlandse Wind Tunnel. The wake geometry, vortex strength, and vortex core size were also measured through a laser light sheet technique and laser Doppler velocimetry. These results are used to verify the blade-vortex interaction (BVI) airload prediction methodologies developed by Army Aeroflightdynamics Directorate, Deutsche Forschungsanstalt fur Luft-und Raumfahrt, NASA Langley Research Center, and the Office National d'Etudes et de Recherches Aerospatiales. The comparisons show that an accurate prediction of the blade motion and the wake geometry is the most important aspect of the BVI airload predictions

HHC aeroacoustic rotor tests in the German-Dutch wind tunnel: Improving physical understanding and prediction codes

Within an international research program jointly conducted by the US Army's Aeroflightdynamics Directorate (AFDD), NASA Langley, DNW, ONERA and DLR, the effects of a higher harmonic blade pitch angle variation on the rotor noise and vibrations were intensively studied. The program, denoted with Higher Harmonic Control Aeroacoustic Rotor Tests (HART) comprised both theoretical and experimental activities, the latter ones being represented by a comprehensive wind tunnel test campaign. Besides aeroacoustic measurements it comprised flow visualization tests and flow field measurements which were applied along with optical methods to determine the blade deflections. The measurements were preceded by extensive predictions performed with the analytical models of each organization. Within these simulations different models were used, leading to a very comprehensive theoretical data base. It gave an impression of the rotor codes' uncertainties which were tried to be eliminated by means of the wind tunnel findings. Although these activities yielded general improvements of the simulation codes they demonstrated also that further work is required in order to accurately predict the effects of a higher harmonic blade pitch angle on the rotor noise and vibrations.

Flight‐Testing and Frequency‐Domain Analysis for Rotorcraft Handling Qualities

A demonstration of frequency-domain flight-testing techniques and analysis was performed on a U.S. Army OH-58D helicopter in support of the OH-58D Airworthiness and Flight Characteristics Evaluation and of the Army's development and ongoing review of Aeronautical Design Standard 33C, Handling Qualities Requirements for Military Rotorcraft. Hover and forward flight (60 kn) tests were conducted in 1 flight hour by Army experimental test pilots. Further processing of the hover data generated a complete database of velocity, angular-rate, and acceleration-frequency responses to control inputs. A joint effort was then undertaken by the Airworthiness Qualification Test Dirtectorate and the U.S. Army Aeroflightdynamics Directorate to derive handling-quality information from the frequency-domain database using a variety of approaches. This report documents numerous results that have been obtained from the simple frequency-domain tests; in many areas, these results provide more insight into the aircraft dynmamics that affect handling qualities than do traditional flight tests. The handling-quality results include ADS-33C bandwidth and phase-delay calculations, vibration spectral determinations, transfer-function models to examine single-axis results, and a six-degree-of-freedom fully coupled state-space model. The ability of this model to accurately predict responses was verified using data from pulse inputs. This report also documents the frequency-sweep flight-test technique and data analysis used to support the tests.