Thesis. 1977. Ph.D.--Massachusetts Institute of Technology. Dept. of Aeronautics and Astronautics. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND AERONAUTICS. / Vita. / Includes bibliographical references. / Ph.D.

Many situations arise in engineering where it is desired to model a system of complicated input and output variables. However, analytical difficulties arise when these systems exhibit nonlinear behavior. Neural networks have proven useful for such applications because they are able to model complicated nonlinear systems through exposure to a database including input parameters and the desired outputs. One such complicated system consists of the unknown relationships between flight variables and structural loads on helicopters. The development of an accurate neural network based model would allow indirect monitoring of these loads so that fatigue-damaged components could be replaced according to load history. In this thesis, an extensive database of real-time flight records has been effectively used to teach a multilayer feedforward artificial neural network nonlinear relationships between common flight variables and the resulting component loads. The trained network predicts time-varying mean and oscillatory load records corresponding to flight variable histories. Component loads in both the fixed and rotating systems of a military helicopter have been resolved over a variety of standard maneuvers. Predictions under the present conditions are on the order of 90 to 100% accurate. Although the range of maneuvers presently considered is rather limited in comparison to the total helicopter flight spectrum, the present results justify further pursuit of this neural network application. / Master of Science

A theoretical investigation of the asymmetry of induced flow in the vicinity of a lifting rotor in forward flight has been conducted. The analysis is based upon an asymmetric wake which is a logical extension of that used for previous investigations. Equations for the induced velocities at an arbitrary point in space are presented in a form suitable for numerical integration. Numerical results for the normal induced velocity in the lateral plane of the rotor are presented in the form of tables and charts. Comparison with previously available measurements indicates an improvement in accuracy over older theories. The results should be useful in estimating the interference between wing and rotor of compound helicopters and convertiplanes. In addition the results should be applicable to the problem of mutual interference between rotors of multi-rotor helicopters. / Master of Science

Thesis (MScEng)--University of Stellenbosch, 2004. / ENGLISH ABSTRACT: A need exists for the development of an unmanned rotorcraft capable of autonomous flight, as would be required for the survey of high voltage electricity supply lines. A program was initiated at the University of Stellenbosch in December 2002 in order to develop such an aircraft. The first goal of this thesis was the development of software that could calculate the stability and control derivatives of a model helicopter. These derivatives could then be used in the formulation of an appropriate helicopter control strategy. The second goal of the thesis was an investigation of the stability and control characteristics of model helicopters. The trim settings of the helicopter were required in the calculation of the stability and control derivatives. A computer program was developed to determine the trim settings of a helicopter in forward flight. Another program was developed to calculate the stability and control derivatives, using the results of the trim analysis. The trim analysis was based on the assumption of negligible coupling between the longitudinal and lateral modes of motion. The method proposed by Bramwell (1976) was used to perform the trim analysis. The stability and control derivatives were calculated by obtaining the trim settings from the trim analysis. These derivatives were then used to solve the roots of the characteristic equations of the longitudinal and lateral modes of motion. The stability of the helicopters were investigated firstly by examining the stability derivatives and secondly through root-loci analyses. The most important results were the following: • The root-loci analyses indicated that a helicopter without a horizontal stabiliser suffered from instability of the phugoid mode. It was also found that the short-period motion of these helicopters was heavily damped. Fitting a horizontal stabiliser to these helicopters caused the phugoid motion to become stable even at low speeds. This was achieved at the cost of a reduction in short-period motion damping. • The periods of the lateral and longitudinal motions were smaller than those found on full-scale helicopters. This was attributed to the small mass and inertia properties of the model helicopters. An increase in speed is generally accompanied by an increase in the stability of the helicopters. This could be attributed to the effective operation of the tail surfaces at higher speeds. • The axial climbing speed of a helicopter is influenced by the rotor speed. A low rotor speed allows higher climbing velocities at a given power setting. This was due to lower induced power losses at low rotor speed, assuming that no blade stall occurs. • The rotor speed does not influence the incremental amount of power (M:,) required to achieve a certain climbing velocity, due to the fact that the profile power losses are constant for a certain rotor speed. • The simplified horseshoe-vortex theory can be used to analyse the downwash angle at the horizontal stabiliser if the helicopter is in high-speed forward flight. / AFRIKAANSE OPSOMMING: Daar is tans 'n vraag na die ontwikkeling van onbemande rotor-vlerk vliegtuie wat die vermoë beskik om hulself te beheer. Hierdie tipe vliegtuie sal gebruik word om byvoorbeeld hoë-spannings elektrisiteitverskaffingsdrade na te gaan. 'n Program is in Desember 2002 by die Universiteit van Stellenbosch begin om sulke vliegtuie te ontwikkel. Die eerste doel van hierdie tesis was om sagteware te ontwikkel wat die stabiliteit- en beheerafgeleides van 'n model helikopter kon bereken. Hierdie afgeleides kan dan gebruik word om 'n gepaste helikopter beheerstrategie saam te stel. Die tweede doel van die tesis was om die stabiliteit- en beheerseienskappe van model helikopters te ondersoek. Die berekening van die stabiliteit- en beheerafgeleides van die helikopter berus op die beheerinsette benodig om die helikopter in ewewig te hou (trim). 'n Rekenaarprogram is ontwikkelom hierdie beheerinsette vir 'n helikopter in voorwaartse vlug te bereken. 'n Ander program is ontwikkelom die stabiliteit- en beheerafgeleides te bereken met behulp van die ewewig beheerinsette. Die analise van die helikopter in ewewig berus op die aanname dat die grootte van die koppeling tussen die longitudinale en laterale beweginsmodusse weglaatbaar klein is. Die beheerinsette van die helikopter in ewewig tydens voorwaartse vlug is bereken deur van Bramwell (1976) se metode te gebruik. Die stabiliteit- en beheerafgeleides is bereken deur van hierdie beheerinsette gebruik te maak. Die afgeleides is gebruik om die wortels van die karakteristieke vergelykings van die longitudinale en laterale bewegingsmodusse te bereken. Die stabiliteit van die helikopters is eerstens beoordeel deur die stabiliteitsafgeleides te ondersoek en tweedens deur middel van 'n wortel-lokus analise. Die belangrikste resultate is as volg: • Die wortel-lokus analise toon dat 'n helikopter sonder 'n horisontale stabiliseerder phugoid-onstabiliteit (Iangperiode onstabiliteit) het. Die kort-periode beweging van hierdie helikopters het verder groot hoveelhede demping aangetoon. Die phugoid-beweging kon selfs teen lae snelhede gestabiliseer word deur 'n horisontale stabiliseerder aan te heg. Hierdie stabiliteit is egter bereik ten koste van die demping van die kort-periode beweging wat verminder is. • Die periodes van die longitudinale en laterale bewegings is kleiner gewees as vir volskaal helikopters. Dit kan toegeskryf word aan die klein massa en inersie van die model helikopters. • Die stabiliteit van die helikopter is in die algemeen verbeter soos die snelheid verhoog. Dit kan toegeskryf word aan die beter werking van die stert teen die verhoogde snelhede. • Die klimtempo van die helikopter word beïnvloed deur die hoofrotor snelheid. 'n Lae hoofrotor snelheid laat 'n hoër klimptempo toe teen 'n spesifieke drywinginset. Dit is as gevolg van die laer geïndusseerde drywingsverliese teen die laer hoofrotor snelheid. Daar word aanvaar dat die lugvloei oor die lem nie staak nie. • Die hoofrotor snelheid beïnvloed nie die inkrimentele drywing (M,,) wat benodig word om 'n sekere klimtempo te bereik nie. Dit is as gevolg van die konstante drywings verliese teen 'n sekere hoofrotor snelheid. • Die vereenvoudigde perdeskoenwerwel teorie kan gebruik word om die afspoel hoek by die horisontale stabiliseerder te bereken indien die helikopter in hoë-spoed voorwaartse vlug is.

No description available.

The concept of applying several disciplines to the design and optimization processes may not be new, but it does not currently seem to be widely accepted in industry. The reason for this might be the lack of well-known tools for realizing a complete multidisciplinary design and analysis of a product. This study aims to propose a method that enables engineers in some design disciplines to perform a fairly detailed analysis and optimization of a design using commercially available software as well as codes developed at Georgia Tech. The ultimate goal is when the system is set up properly, the CAD model of the design, including all subsystems, will be automatically updated as soon as a new part or assembly is added to the design; or it will be updated when an analysis and/or an optimization is performed and the geometry needs to be modified. Such a design process takes dramatically less time to complete; therefore, it should reduce development time and costs. The optimization method is demonstrated on an existing helicopter rotor originally designed in the 1960's. The rotor is already an effective design with novel features. However, application of the optimization principles together with high-speed computing resulted in an even better design. The objective function to be minimized is related to the vibrations of the rotor system under gusty wind conditions. The design parameters are all continuous variables. Optimization is performed in a number of steps. First, the most crucial design variables of the objective function are identified. With these variables, Latin Hypercube Sampling method is used to probe the design space of several local minima and maxima. After analysis of numerous samples, an optimum configuration of the design that is more stable than that of the initial design is reached. The process requires several software tools: CATIA as the CAD tool, ANSYS as the FEA tool, VABS for obtaining the cross-sectional structural properties, and DYMORE for the frequency and dynamic analysis of the rotor. MATLAB codes are also employed to generate input files and read output files of DYMORE. All these tools are connected using ModelCenter.

Helicopters are versatile flying machines that have capabilities that are unparalleled by fixed wing aircraft, such as operating in hover, performing vertical take-off and landing on unprepared sites. However, modern helicopters still suffer from high levels of noise and vibration caused by the physical phenomena occurring in the vicinity of the rotor blades. Therefore, improvement in rotorcraft design to reduce the noise and vibration levels requires understanding of the underlying physical phenomena, and accurate prediction capabilities of the resulting rotorcraft aeromechanics. The goal of this research is to study the aeromechanics of rotors in steady and maneuvering flight using hybrid Computational Fluid Dynamics (CFD) methodology. The hybrid CFD methodology uses the Navier-Stokes equations to solve the flow near the blade surface but the effect of the far wake is computed through the wake model. The hybrid CFD methodology is computationally efficient and its wake modeling approach is non-dissipative making it an attractive tool to study rotorcraft aeromechanics. Several enhancements were made to the CFD methodology and it was coupled to a Computational Structural Dynamics (CSD) methodology to perform a trimmed aeroelastic analysis of a rotor in forward flight. The coupling analyses, both loose and tight were used to identify the key physical phenomena that affect rotors in different steady flight regimes. The modeling enhancements improved the airloads predictions for a variety of flight conditions. It was found that the tightly coupled method did not impact the loads significantly for steady flight conditions compared to the loosely coupled method. The coupling methodology was extended to maneuvering flight analysis and the flight test control angles were employed to enable the maneuvering flight analysis. The fully coupled model provided the presence of three dynamic stall cycles on the rotor in maneuver. Analysis of maneuvering flight requires knowledge of the pilot input control pitch settings, and the vehicle states. As the result, these computational tools cannot be used for analysis of loads in a maneuver that has not been duplicated in a real flight. This is a significant limitation if these tools are to be selected during the design phase of a helicopter where its handling qualities are evaluated in different trajectories. Therefore, a methodology was developed to couple the CFD/CSD simulation with an inverse flight mechanics simulation to perform the maneuver analysis without using the flight test control input. The methodology showed reasonable convergence in steady and maneuvering flight regimes and control angle predictions compared fairly well with test data. In the maneuvering flight regions, the convergence was slower due to relaxation techniques used for the numerical stability. Further, the enhancement of the rotor inflow computations in the inverse simulation through implementation of a Lagrangean wake model improved the convergence of the coupling methodology.

Dynamic stall deeply affects the response of helicopter rotor blades, making its modeling accuracy very important. Two commonly used dynamic stall models were implemented in a comprehensive code, validated, and contrasted to provide improved analysis accuracy and versatility. Next, computational fluid dynamics and computational structural dynamics loose coupling methodologies are reviewed, and a general tight coupling approach was implemented and tested. The tightly coupled computational fluid dynamics and computational structural dynamics methodology is then used to assess the stability characteristics of complex rotorcraft problems. An aeroelastic analysis of rotors must include an assessment of potential instabilities and the determination of damping ratios for all modes of interest. If the governing equations of motion of a system can be formulated as linear, ordinary differential equations with constant coefficients, classical stability evaluation methodologies based on the characteristic exponents of the system can rapidly and accurately provide the system's stability characteristics. For systems described by linear, ordinary differential equations with periodic coefficients, Floquet's theory is the preferred approach. While these methods provide excellent results for simplified linear models with a moderate number of degrees of freedom, they become quickly unwieldy as the number of degrees of freedom increases. Therefore, to accurately analyze rotorcraft aeroelastic periodic systems, a fully nonlinear, coupled simulation tool is used to determine the response of the system to perturbations about an equilibrium configuration and determine the presence of instabilities and damping ratios. The stability analysis is undertaken using an algorithm based on a Partial Floquet approach that has been successfully applied with computational structural dynamics tools on rotors and wind turbines. The stability analysis approach is computationally inexpensive and consists of post processing aeroelastic data, which can be used with any aeroelastic rotorcraft code or with experimental data.

No description available.

Thesis (M.S.M.E.)--Baylor University, 2008. / Includes bibliographical references (p. 101-102).