Group-delay measurement of frequency-converting devices using a comb generator

We propose a new method for the measurement of (group) delay from the radio-frequency (RF) input to the intermediate-frequency (IF) output of a mixer or a receiver. The method is particularly convenient for measuring the change in group delay with the local-oscillator (LO) tuning frequency of the receiver since the method does not require access to, or even knowledge of, the LO signal. The method employs a calibrated comb (impulse) generator. Other required equipment is limited to a reference signal generator and a digitizer of modest bandwidth, allowing the measurement to rely on a low-frequency generator and an oscilloscope. Simulated and measured data are presented to verify the approach

Titanium and advanced composite structures for a supersonic cruise arrow wing configuration

Structural design studies were made, based on current technology and on an estimate of technology to be available in the mid 1980's, to assess the relative merits of structural concepts and materials for an advanced arrow wing configuration cruising at Mach 2.7. Preliminary studies were made to insure compliance of the configuration with general design criteria, integrate the propulsion system with the airframe, and define an efficient structural arrangement. Material and concept selection, detailed structural analysis, structural design and airplane mass analysis were completed based on current technology. Based on estimated future technology, structural sizing for strength and a preliminary assessment of the flutter of a strength designed composite structure were completed. An advanced computerized structural design system was used, in conjunction with a relatively complex finite element model, for detailed analysis and sizing of structural members

Active controls technology to maximize structural efficiency

The implication of the dependence on active controls technology during the design phase of transport structures is considered. Critical loading conditions are discussed along with probable ways of alleviating these loads. Why fatigue requirements may be critical and can only be partially alleviated is explained. The significance of certain flutter suppression system criteria is examined

Letter to the Honorable W. B. Hoggatt, Governor of Alaska

Correspondence regarding a legal case in the Alaska territory. PLEASE NOTE: This letter includes discussion of the rape of an indigenous person below the age of 18

Farm Tractor Rollover Protection: Why Simply Getting Rollover Protective Structures Installed on All Tractors is Not Sufficient

In North America, agricultural is one of the most hazardous occupations, trailing only mining and construction in likelihood of experiencing a serious injury or fatality. Without quoting specific statistics, roughly one-half of the serious injuries experienced each year involve an agricultural tractor, and roughly one-half of the serious injuries involving tractors are rollovers. It is well established that when a tractor is equipped with a rollover protective structure (ROPS) and seatbelt, and both are utilized, serious injuries rarely, if ever occur, in the event of an overturn. In fact, tractor overturns are the leading cause of agricultural fatalities in the United States. Loringer and Myers (2008) have studied the population of ROPS-equipped tractors and have estimated that as of 2004, only 51% of tractors in service were equipped with ROPS. Predictions based on past experience suggest that it will be 2024 before 75% of the tractors in service will be equipped with ROPS if no other actions are taken. While this problem is indeed a large rock protruding through the water, it is worthwhile to begin to explore what other rocks lay below the surface so that effective strategies can be developed to address them as the water level drops. Myers has collected and presented data on the ages of tractors and whether or not they were ROPS equipped. In 1975, OSHA promulgated rules requiring ROPS on agricultural tractors. In 1985, ASAE S318.10 was introduced to require ROPS on all tractors for the first time. It would seem, therefore, reasonable to assume that all tractors placed in service since 1985 are ROPS equipped. However, a review of the Myers data shows that this is not the case. For tractors in service for 0 to 4 years, only 93.8% are reported to be ROPS equipped. For tractors in service for 5 to 9 years, only 92.8% are reported to have a ROPS. It would appear that if all tractors in service were to be sold with ROPS, then long term 6% to 7% would be operated without ROPS. Using the Myers data and assumptions, this would lead to 18 to 20 deaths per year due to rollovers

Topological analysis of polymeric melts: Chain length effects and fast-converging estimators for entanglement length

Primitive path analyses of entanglements are performed over a wide range of chain lengths for both bead spring and atomistic polyethylene polymer melts. Estimators for the entanglement length N_e which operate on results for a single chain length N are shown to produce systematic O(1/N) errors. The mathematical roots of these errors are identified as (a) treating chain ends as entanglements and (b) neglecting non-Gaussian corrections to chain and primitive path dimensions. The prefactors for the O(1/N) errors may be large; in general their magnitude depends both on the polymer model and the method used to obtain primitive paths. We propose, derive and test new estimators which eliminate these systematic errors using information obtainable from the variation of entanglement characteristics with chain length. The new estimators produce accurate results for N_e from marginally entangled systems. Formulas based on direct enumeration of entanglements appear to converge faster and are simpler to apply.Comment: Major revisions. Developed near-ideal estimators which operate on multiple chain lengths. Now test these on two very different model polymers

Viscoplasticity and large-scale chain relaxation in glassy-polymeric strain hardening

A simple theory for glassy polymeric mechanical response which accounts for large scale chain relaxation is presented. It captures the crossover from perfect-plastic response to strong strain hardening as the degree of polymerization $N$ increases, without invoking entanglements. By relating hardening to interactions on the scale of monomers and chain segments, we correctly predict its magnitude. Strain activated relaxation arising from the need to maintain constant chain contour length reduces the $N$ dependence of the characteristic relaxation time by a factor $\sim \dot\epsilon N$ during active deformation at strain rate $\dot\epsilon$. This prediction is consistent with results from recent experiments and simulations, and we suggest how it may be further tested experimentally.Comment: The theoretical treatment of the mechanical response has been significantly revised, and the arguments for coherent relaxation during active deformation made more transparen