Some approximations to the flapping stability of helicopter rotors

The flapping equation for a helicopter in forward flight are reported which have coefficients that are periodic in time, and this effect complicates the calculation of stability. A constant coefficient approximation which will allow the use of all the well known methods for analyzing constant coefficient equations are presented. The flapping equation is first transformed into the nonrotating coordinate frame, where some of the periodic coefficients are transformed into constant terms. The constant coefficient approximation is then made by using time averaged coefficients in the nonrotating frame. Stability calculations based on the approximation are compared to results from a theory which correctly includes all of the periodicity. The comparison indicates that the approximation is reasonably accurate at advance ratios up to 0.5

Application of a parameter identification technique to a hingeless helicopter rotor

A mathematical model of a gyro-controlled, three-bladed hingeless helicopter rotor was developed and parameters of the model were estimated using a parameter identification technique. The flapping and feathering degrees of freedom of the blades were modeled. The equations of the model contain time-varying, periodic coefficients due to the forward speed of the rotor. A digital simulation of the analytical model was compared with wind-tunnel measurements to establish the validity of the model. Comparisons of steady-state and transient solutions of the analytical model with the tunnel measurements gave reasonably good matching of gyro angle but less satisfactory matching of hub moment measurements. Further improvements were obtained by use of a parameter identification technique to adjust as many as 10 parameters of the analytical model. The sensitivity of the blade response to small changes in the parameters was also calculated

Sizing-stiffened composite panels loaded in the postbuckling range

Stiffened panels are widely used in aircraft structures such as wing covers, fuselages, control surfaces, spar webs, bulkheads, and floors. The detailed sizing of minimum-weight stiffened panels involves many considerations. Use of composite materials introduces additional complexities. Many potential modes of failure exist. Analyses for these modes are often not trivial, especially for those involving large out-of-plane displacements. Accurate analyses of all potential failure modes are essential. Numerous practical constraints arise from manufacturing/cost considerations and from damage tolerance, durability, and stiffness requirements. The number of design variables can be large when lamina thicknesses and stacking sequence are being optimized. A significant burden is placed on the sizing code due to the complex analyses, practical constraints, and number of design variables. On the other hand, sizing weight-efficient panels without the aid of an automated procedure is almost out of the question. The sizing code postbuckled Open-Stiffener Optimum Panels (POSTOP) has been developed to aid in the design of minimum-weight panels subject to the considerations mentioned above. Developed for postbuckled composite panels, POSTOP may be used for buckling resistant panels and metallic panels as well. The COPES/CONMIN optimizer is used in POSTOP although other options such as those in the ADS system could be substituted with relative ease. The basic elements of POSTOP are shown. Some of these elements and usage of the program are described

Design and analysis of a stiffened composite fuselage panel

The design and analysis of stiffened composite panel that is representative of the fuselage structure of existing wide bodied aircraft is discussed. The panel is a minimum weight design, based on the current level of technology and realistic loads and criteria. Several different stiffener configurations were investigated in the optimization process. The final configuration is an all graphite/epoxy J-stiffened design in which the skin between adjacent stiffeners is permitted to buckle under design loads. Fail safe concepts typically employed in metallic fuselage structure have been incorporated in the design. A conservative approach has been used with regard to structural details such as skin/frame and stringer/frame attachments and other areas where sufficient design data was not available

Orson Fowler\u27s Influence in the Shenandoah Valley: Gravel Wall Buildings in Augusta County, Virginia.

Orson Squire Fowler remains a well-known name within the field of architectural history, thanks to his octagonal house designs which enjoyed a degree of popularity during the second half of the nineteenth century. An avid phrenologist, Fowler’s concern for healthy living environments influenced his house designs. In the first edition of his book A Home For All (1848), Fowler advocated for board wall construction for houses, a method he abandoned in the third edition (1853) in favor of concrete. The material he described as concrete would be laid in a “gravel wall plan” and consisted of a lime-based mortar with larger aggregate than modern-day mixes. Scholarly literature has focused primarily on the proliferation of the octagon house design and less on the materials. This thesis remedies this gap by analyzing gravel wall buildings constructed in Augusta County, Virginia during the second half of the nineteenth century. Orson Fowler’s influence in Augusta County, Virginia, is evident in forty-eight “gravel wall” houses and buildings constructed between 1859 and ca. 1900. Augusta County’s architectural history has received a great deal of study over the past decades, but gravel wall construction has been misidentified or ignored. While the county’s gravel wall buildings seem to closely follow Fowler’s prescribed material, they entirely reject the octagon form and instead follow local plan types. These plan types vary widely, from symmetrical to asymmetrical, and single-pile to double-pile, all of which are consonant with long-standing vernacular plan types identified by earlier scholarship. These buildings link Augusta County with the progressive construction methods advocated by Fowler and others. While progressive construction methods were adopted, traditional plan types were retained. Mortar analysis undertaken on mortar samples from various sites indicates a great deal of variation between sites and with Fowler’s prescribed ratios and suggests that variation not only existed in plan, but also material. The gravel wall method of construction faded out around the turn of the twentieth century, replaced by cheaper and quicker alternatives like concrete block

Skin-stiffener interface stresses in composite stiffened panels

A model and solution method for determining the normal and shear stresses in the interface between the skin and the stiffener attached flange were developed. An efficient, analytical solution procedure was developed and incorporated in a sizing code for stiffened panels. The analysis procedure described provides a means to study the effects of material and geometric design parameters on the interface stresses. These stresses include the normal stress, and the shear stresses in both the longitudinal and the transverse directions. The tendency toward skin/stiffener separation may therefore be minimized by choosing appropriate values for the design variables. The most important design variables include the relative bending stiffnesses of the skin and stiffener attached flange, the bending stiffness of the stiffener web, and the flange width. The longitudinal compressive loads in the flange and skin have significant effects on the interface stresses

A Protection And Detection Surface (PADS) for damage tolerance

A protection and detection surface (PADS) concept was studied for application to composite primary aircraft structures. A Kevlar-epoxy woven face sheet with a Rohacell foam core was found to be the most effective PADS configuration among the configurations evaluated. The weight of the PADS configuration was estimated to be approximately 17 pct of the structural weight. The PADS configuration was bonded to graphite-epoxy base laminates, and up to a 70 pct improvement in compression-after-impact failure strains was observed

Direct Democracy and the Culture Wars

Proponents of direct democracy maintain that this institution can facilitate political participation in the United States. By providing citizens with a greater role in the legislation-making process, these supporters assert, the initiative and/or referendum may heighten the importance assigned to political action and engage those who do not consistently vote. Substantial empirical evidence supports this relationship, and the positive relationship between direct democracy and turnout is the most consistent finding in all of the literature on this institution. I contend, however, that the existing literature is both theoretically and methodologically incomplete. Theoretically, scholars have yet to identify the exact causal mechanism that explains why any ballot measure might bring citizens to the polls, and this failure has led to the employment of multiple incomplete measurements of the direct democracy process. I attempt to rectify this concern by positing two key requirements necessary for any proposition to influence the decision to vote. These criteria lead to the conclusion that we must look at the issue content of each individual ballot measure to identify its effect on participation. Using moral issue propositions as an example of those that consistently possess the potential to raise voting rates, I illustrate the factors necessary for direct democracy to fulfill the expectations of its proponents. Across a number of contexts, I find substantial evidence for the ability of moral issue propositions to habitually engage citizens, mobilize them above normal turnout rates, and even increasing levels of political knowledge. In contrast, the average ballot measure rarely maintains this capacity, and even others that address salient and/or controversial issues (such as tax matters) exhibit difficulties in doing so more than episodically. These findings provide significant insights into the consequences of permitting the public to legislate via the ballot, how institutions shape the size and composition of the electorate, and what might be done to increase turnout in this country

Linear analysis of a force reflective teleoperator

Complex force reflective teleoperation systems are often very difficult to analyze due to the large number of components and control loops involved. One mode of a force reflective teleoperator is described. An analysis of the performance of the system based on a linear analysis of the general full order model is presented. Reduced order models are derived and correlated with the full order models. Basic effects of force feedback and position feedback are examined and the effects of time delays between the master and slave are studied. The results show that with symmetrical position-position control of teleoperators, a basic trade off must be made between the intersystem stiffness of the teleoperator, and the impedance felt by the operator in free space