Single Main Rotor Research Articles

Modeling and Analysis of Helicopter Flight Mechanics in Autorotation

Autorotation is an abnormal mode of operation for a helicopter because no powerplant torque is applied to the main rotor. Thus, descending e ight requires an upward e ow of air through the rotor to sustain rotation and, therefore, lift. There is a negligible literature on the e ight mechanics of helicopter autorotation, although the aerodynamic phenomenon is well understood. The objectiveof this paper is, therefore, to examinehelicopter e ight mechanics across the autorotation e ight envelope, while addressing the modeling requirements for simulation in autorotation. A nonlinear individual blade/blade element model of a conventional single main and tail rotor helicopteris used to generatea variety ofdata, including trim states, time,and frequency responses. It isconcluded that contemporary mathematical modeling can mimic the general performance characteristics of helicopters in autorotation with no special development. Although rotorspeed can vary signie cantly, even during maneuvers that embody only small perturbations in the body states, linearized models can remain an appropriate basis for analysis. Finally, distinctive aspects of helicopter e ight mechanics in autorotation, dissimilar to level e ight, are readily explained, and it is suggested that they are benign in nature.

Helicopter Flight-Control Reconfiguration for Main Rotor Actuator Failures

Although reconfigurable flight control has been well demonstrated on fixed-wing aircraft using existing control surface redundancies, such failure accommodations were widely believed to be impossible for single main rotor helicopters. In fact, on any such existing helicopter, the failure of a main rotor actuator is catastrophic, resulting in a complete loss of control. An actuator geometry that enables reconfigurable helicopter flight control is presented, and, from this, reconfigurable flight-control methods are developed to accommodate main rotor actuator failures. The geometry provides control axes coupling that is key to solving the reconfigurable flight-control problem. Such coupling enables the development of a swashplate reconfiguration strategy, where any two of the three control axes can be controlled at a time, despite a failure in any one of the main rotor actuators. Of most interest is the particular swashplate reconfiguration solution where attitude control (pitch and roll) is retained, whereas vertical control is sacrificed. Vertical control is then achieved by one of two methods: flight to a longitudinal velocity that supports the desired vertical velocity or the use of rotor speed to change the main rotor's thrust to perform limited closed-loop vertical control. These methods are combined to form a robust reconfigurable flight-control method. All of these methods are independent of the underlying flight-control system. Such independence is demonstrated by evaluation of the reconfigurable flight-control methods for two different flight-control systems: a classic proportional-integral-derivative controller and neural dynamic programming, a neural network based controller, All designs are tested by the use of a sophisticated nonlinear validated model of the Apache helicopter.

A nonlinear inverse simulation technique applied to coaxial rotor helicopter maneuvers

As compared with the maneuvering flight studies for single main rotor helicopters, the corresponding studies for coaxial rotor helicopters are relatively poor. In this paper, the maneuvering flight governing equations for coaxial rotor helicopters is established. By introducing induced velocity interference factor analysis, the coaxial rotor aerodynamic interference can be taken into account. With the combination of coaxial rotor helicopter control features and nonlinear inverse simulation technique, the governing equations for maneuvering flight can be solved so as to determine helicopter control input, control force and moment, and helicopter body attitudes which are needed for performing a specified maneuver. Good results of the sample calculations of level turn and lateral jink maneuvers are obtained and simply analyzed.

Selected aerodynamic design issues for the single‐rotor remotely piloted helicopter

Some of the fundamental problems associated with the aerodynamic configuration of a single‐rotor remotely piloted helicopter are discussed in this paper. A method for selecting the aerodynamic shape of a fuselage and determining the locations and parameters of a horizontal stabilizer and a tail rotor in the preliminary design analysis is given. The application of this method to a remotely piloted single main rotor and tail rotor helicopter developed at the Nanjing University of Aeronautics and Astronautics is described. A raindrop shaped fuselage is chosen for the remotely piloted helicopter and the low drag characteristics of the fuselage are demonstrated from the wind‐tunnel experimental data. The experimental results on the pitching moment characteristics of the horizontal stabilizer are also presented and analyzed. The design considerations of the horizontal stabilizer and tail rotor of the helicopter are discussed in detail.

Helicopter station-keeping: comparing LQR, fuzzy-logic and neural-net controllers

Station-keeping is a difficult flight control mode for helicopters since they have little forward speed and are dynamically unstable in that mode. In this paper the designs for a fuzzy-logic controller and neural-network (NN) controllers which are used to provide acceptable station-keeping performance for a single main rotor helicopter are presented. The effectiveness of the proposed designs is established from an examination of a number of results obtained from a simulation of the controlled helicopter. The required training and recall data needed for the NN were obtained from the responses of the same helicopter type being controlled in a station-keeping mode by a continuous, linear, optimal feedback control law, obtained by solving a linear quadratic regulator problem.

The principles and practical application of helicopter inverse simulation

Inverse simulation is a technique whereby the control actions required for a modelled vehicle to fly a specified manoeuvre can be established. In this paper the general concepts of inverse simulation are introduced, and an algorithm designed specifically to achieve inverse simulation of a single main and tail rotor helicopter is presented. An important element of an inverse simulation is the design of the input functions i.e. manoeuvre definitions, and the methods used are also detailed. A helicopter mathematical model is also discussed along with the validation and verification of the inverse simulation. Finally, the applicability of the method is demonstrated by illustration of its use in two flight dynamics studies.

Helicopter flight control using individual channel design

Using the multivariable analysis framework known as individual channel design (ICD), it is shown that for a typical strongly cross-coupled single main rotor helicopter at 80 knots forward flight, the standard 4-input 4-output multivariable control problem, for design purposes, decomposes without significant loss of structural (interaction) information into two simpler 2-input 2-output multivariable problems: one for the longitudinal dynamics and one for the lateral dynamics. Following on this analysis of multivariable structure, the control systems design is built up in a systematic fashion and a novel type of feedforward control is used to overcome a severe lack of robustness as well as to decouple the lateral dynamics into two SISO subsystems round crossover frequency. A diagonal (4*4) feedback controller matrix is then used, the elements of which are designed on the basis of the decoupled lateral and longitudinal dynamics. Resulting closed-loop bandwidths of all four channels are within Level 1 handling quality specifications and step responses are satisfactory with acceptably low cross-coupling.

Flight Requirements for the Operation of Rotorcraft.

Throughout the years that rotorcraft have been on the British Register of Civil Aircraft, the single reciprocating engined, single main rotor helicopter has been the predominant representative of this particular family of aircraft. These helicopters were certificated to the British Civil Airworthiness Requirements (BCAR), Section G, “Rotorcraft,” issued in January 1954, which was written in terms of such helicopters.During the past few years there have been significant changes in rotorcraft design—the turbine engine has been introduced and the multi-engined type has become a reality.Basic changes in the design trends of any type of aircraft must inevitably involve a re-appraisal of the appropriate requirements by the responsible airworthiness authority. The BCAR are subjected to a constant process of refinement and revision and, as a result of the development of new types, the Air Registration Board has been considering the airworthiness implications and preparing amendments to most parts of Section G, but I wish to discuss only Flight Requirements which are in the main contained in one Sub-Section of Section G—Sub-Section G2. From the recent revisions I have extracted only the significant changes, although there are still problem areas I would like to put on record.