Results from a linear analysis of the heave dynamics of an air-cushion vehicle equipped with a bag-and-finger skirt are described. A two-dimensional section of the cushion is subject to pure heave or long-wave surface motion inputs. The skirt mass is lumped in the fingers, with the bag being modelled as a combination of massless inelastic membranes and links. The airflows from bag to cushion and from cushion to atmosphere are assumed quasisteady, and the bag and cushion volumes are modelled as lumped pneumatic capacitances. For a configuration representative of a 37t vehicle, frequency response characteristics show the effect of skirt geometry and mass changes, and cushion capacitance. The results suggest that changes in skirt geometry cannot be used to radically modify an undesirable heave response, but reducing the skirt mass may be effective. The air compressibility also affects heave response at high frequencies, with the effect becoming more prominent at the low cushion-flow rates now used in practice.
Traditionally, Euler’s equations are commonly employed for the analyses of interesting gyrodynamics problems. Nevertheless, they generally give no idea how these complicated gyroscopic forces are mutually interacting. Thus, we may risk missing some insights regarding working forces. Therefore, in this study, we present a so-called “finite particle method, ” which simulates a gyroscope by a small number of dynamically equivalent particles (usually 2 to 8 only) rigidly connected. This method largely degenerates complicated 3-D gyrodynamics to a particle dynamics problem in a rotating frame. The finite particle method has elegantly demonstrated its validity by successfully deriving the same steady gyrodynamics equations as that derived from Euler’s approach, yet only in amazingly minimal steps for some cases. Surprisingly enough, by this method, one can swiftly understand some delicate gyrodynamics phenomena more deeply by merely inspecting centripetal and Coriolis forces particle by particle, without even knowing what angular momentum is. In short, this finite particle method is characterized by its simpler concept, succinct derivation, and possibly insightful understanding of intrinsic force interactions for some gyrodynamics problems as demonstrated in the retrograding phenomenon of disk-shaped satellites and other examples.
A new optimal guidance law (OPG) for air-to-air missiles, based on the design concept of decreasing the acceleration requirement commanded in the final phase of engagement, is presented. The closed-form solution of the OPG is derived analytically from the time-varying linear state equations composed of the line-of-sight angle and line-of-sight rate. The optimal proportional navigation law and augmented proportional navigation law, where both the navigation constants are 3, have been proven to be simplified versions of the OPG. The performance of the OPG is evaluated and compared with that of the true proportional navigation law (TPN) and augmented proportional navigation law (APN) in terms of the miss distance, interception time and energy expenditure.
In this paper, a study on designing a thick supercritical airfoil by utilizing Takanashi’s inverse design method is discussed. One of the problems to design a thick supercritical airfoil by Takanashi’s method is that an oscillation of the geometry may occur during the iteration process. To reduce the oscillation, an airfoil parameterization method is utilized as the smoothing procedure. A guideline to determine the target pressure distribution to realize the thick airfoil is also discussed.
Choked flux rates and shock jump relations were calculated both analytically and computationally for a converging-diverging nozzle. High inlet stagnation pressure with a maximum of 6.45MPa was applied to the cryogenic vapor-liquid nitrogen flow. The total inlet-to-static exit pressure ratio range was between 3.5 and 15.3. The analytical shock jump expressions, which were derived, are exact extensions of the Rankine-Hugoniot single-phase gas dynamic relations. Accordingly, a consistent agreement was obtained between analytical and computational pressure, Mach number and void fraction shock jumps. The numerical mass flux values were also successfully compared with experimental data and two separate analytical results, both obtained from a modified Henry-Fauske model and a homogeneous mass flux equation.
The spatial temperature and atomic number density of the plume generated by a 3-kW-class arcjet thruster are determined with a small number of absorption measurements across the plume based on diode-laser absorption computerized tomography. The maximum temperature and atom number density increase with the massflow rate and discharge current. The increase trend is not always found with the specific input power, although there exists an increasing trend with it at a fixed massflow rate. The development of the plume is studied with the measurements of the plume at different locations. The technique should also be applicable to arcjet or other hypersonic flow systems without much modification.
A modified Newton-Raphson minimization technique for determining aerodynamic coefficients and stability derivatives of spin-stabilized projectiles with a six-degree-of-freedom nonlinear dynamical model was developed. The dynamical model for the projectiles is constructed having process noise in the system, and the instrumentation noise of the system outputs is simulated by a data model statistically similar to the measured data. The state equations of the dynamical system are continuous types while the measurement data are discrete. A continuous-discrete estimation model for the motion of the projectiles is constructed in this paper. The state variables of the system were estimated by the extended Kalman filter, and the system parameters were identified by the modified Newton-Raphson technique based on the maximum likelihood criterion. Research results show that parts of the parameters can be identified under proper noise intensity. However, the accuracy of identification is strongly influenced by both process and measurement noise, Moreover, parameter sensitivity to the system behavior is crucial for the success of identification. Two typical aerodynamic characteristics of projectiles, 105 and 20mm, are imposed to investigate the applicability of state estimation and parameter identification. It is found that the drag coefficient of zero angle-of-attack and the rolling moment derivative and identified with effective accuracy in a wide range of noise levels. On the other hand, other parameters are more difficult to identify, but the causes of deficiency for particular parameters in identification are discussed.
Concerning a tension-controlled flexible solar paddle, thermomechanical analyses are made to predict maximum/minimum possible temperatures, resulting thermal expansion/contraction lengths, and thermal bending stress generated. Paddle tension-control-mechanism pull-out strokes are then calculated to discuss possibilities of an overstroke that may stress the paddle. Calculated results substaniate the rationality of presumed causes of paddle damage. Strain measurements were also conducted at component/piece/segment levels to determine the paddle equivalent expansion coefficient. Emphasized is that a reasonable estimate of thermal expansion/contraction should be made under temperature-dependent coefficient values.