Dynamic Stability Analysis of a Tethered Aerostat

Abstract

incorporates the concepts of apparent mass, dynamic tether and allows 6 degrees of freedom for the motion of the aerostat. Estimation of aerodynamic coefficients is based on empirical relations and curves available in literature. Weight and buoyancy are calculated based on geometry of the aerostat. Appropriate values for operational altitude and desired angle of attack of the aerostat are assumed. Moment balance about confluence point gives the optimal location of the confluence point. Equations of motion for the aerostat and dynamic tether are simulated and appropriate boundary conditions are applied. Force balance gives the tether tension force and its orientation at the confluencepoint.Basedonthetethertensionanditsorientationattheconfluencepoint,thetetherprofileisestimated bybreakingupthetetherintoseveralelasticsegments,eachinequilibrium.Onceequilibriumisestablished,thewind isperturbed andthe response of the aerostat issimulated. Thepaper reportsthe results of simulation carriedout for the TCOM 365Y aerostat and the aerostat response to various ambient velocity profiles. I. Introduction T ETHERED aerostats fall under the category of lighter-than-air systems.Agashavinglowerdensitycomparedwithambientair (usually hydrogen or helium) is enclosed in an envelope and the difference in their densities gives rise to buoyancy. In an aerostat, buoyancy is the major source of lift, whereas in heavier-than-air systems (e.g., fixed-wing aircraft or rotorcraft), aerodynamic lift produced due to relative motion between the ambient air and the vehicle is the major source of lift. The various components of a typical aerostat system are outlined in Fig. 1. The hull or envelope is a bag containing the lifting gas. Fins are attachedattherearendofthehullandprovidestabilitytotheaerostat; they are usually in the form inflated structures, filled with lifting gas orair.Thepayload,whichisusuallyasurveillancecameraoraradar, is mounted one the envelope. A series of ropes called confluence lines connect the hull to a single point called confluence point, to which the main tether is attached. Aerostats can remain stationary for long duration in reasonable weather, which makes them a very good choice for surveillance, advertising, and raising antennae for wireless communication, to name a few. In real life, aerostats have to operate in highly varying weather conditions and winds. Aerostat failures have occurred because of abrupt changes in the wind, which result in shock loads. Estimation of these shock loads is an important requirement in aerostat design, and it can be accomplished by modelling the dynamics of an aerostat and predicting its response to sharply fluctuating winds. This paper starts with a brief history on the development of modeling and simulation of aerostats. The next section deals with equilibrium analysis of tethered aerostats, in which the angle of