An aerostat is helium buoyant, aerodynamically shaped and tethered lighter-than-air vehicle. The aerostat has been used for many observation and measurement missions, such as border surveillance, scientific measurement and radio transmission. The aerostat has no propulsion system for flight. Because of this, the aerostat is emission and noise free during the flight. Recently, the aerostat becomes a more attractive platform for military and civilian missions and many people consider aerostat as an alternative cheap and environmentally friendly platform for their applications. The aerostat, however, is not a popular topic for engineering study. Only a few researchers have given their attention to the aerostat related subjects. To make the aerostat as an alternative and attractive vehicle compared to other competing aerial platform, recently developed engineering tools and techniques should be applied in the aerostat development.
In the structural point of view, the aerostat has many unique features compared to the fixed wing aircrafts and even other lighter-than-air vehicle such as an airship. The aerostat is tethered to the ground by a flexible cable, and has no propulsion device and control unit. It makes it very hard to apply conventional aircraft design tools and requirements directly to the aerostat development. In the development of aircraft, a structural designer uses deterministic approaches in many ways. Factor of safety, limit load factor determining design loads are usually fixed as given conditions by an airworthiness requirements. The airworthiness requirements itself are product of long time trials and errors by aircraft designers. These given requirements, however, can’t be applied to aerostat directly because of the unique features of the aerostat. Instead of deterministic way, a new approach for determining design loads should be developed. The new approach should consider the aerostat vehicle and the tether cable dynamics all together. Additionally, aerostat operating environment such as atmospheric turbulence should be included in the new design tool also.
In this dissertation, the aerostat dynamic equation of motion has been built including the tether cable dynamic effects. A numerical program to solve the derived equation of motion has been developed. The dynamic motion of the 32m aerostat, which is under development at KARI, has been analyzed under discrete gust and continuous turbulence. The aerostat behaviors under discrete gust which represents a deterministic approach for determining design loads for manned aircraft are solved to verify the effect of aerostat mechanical properties on the aerostat dynamic behavior. Continuous turbulences are simulated for each given altitude, translational mean wind velocity and gust intensity. Dynamic behaviors of the 32m aerostat are simulated for each continuous turbulence conditions. Translational and vertical velocity and pitching behavior and tether reaction force are monitored for each simulation. To estimate design loads for the tether cable, each simulated tether reaction forces are counted by peak-over-threshold method. Each counted tether reaction forces are approximated by 3-parameter weibull distribution curves which represent the probability of occurrence of maximum tether reaction force under given continuous turbulence. Finally, all 3-parameter weibull curves are assembled considering the occurring probability of each continuous turbulence conditions. The assembled 3-paramenter weibull curve is used to calculate maximum peak load of the tether reaction force during expected life time of the 32m aerostat.
에어로스탯은 헬륨기체의 부력을 이용하여 공중에 체공하는 무동력 비행체이다. 지상으로 연결된 테더케이블을 통하여 에어로스탯은 일정한 범위 내에서 제공하며, 전력과 통신을 공급받는다. 에어로스탯의 운용범위를 확장하고 보다 경쟁력 있게 제작하기 위해서는, 보다 최적화된 설계가 요구된다. 본 논문에서는 기존에 항공기 개발에 적용되었던 결정론적 방법이 아닌, 통계적인 기법을 이용하여 테더 반력의 최대치를 산출할 수 있는 방법을 구성하고 이를 32M 에어로스탯에 적용하였다. 테더 케이블의 비선형 운동을 포함하는 중형급 에어로스탯인 32M 에어로스탯의 운동방정식이 도출하였으며, 이를 바탕으로 무작위적인 돌풍(random gust)에 대한 32M 에어로스탯의 응답을 해석하였다. 무작위적인 돌풍은 von-karman의 돌풍 PSD를 바탕으로 1km 상공의 바람환경의 통계적 데이터를 만족하는 시간이력(time history)이 구성되었다. 시공간에서 구성된 돌풍이력에 대한 32M 에어로스탯의 거동특성이 산출되었다. 에어로스탯의 거동특성 중 테더반력, 자세각 및 각속도, 수평/수직 속도 등에 대한 특정 동 운동 계수들에 대한 시간이력(time history)이 산출되었다. 이중 테더 반력의 시간이력을 분석하여 테더 반력의 하중 크기별 분포데이터를 구성하였다. 구성된 테더반력의 국부최고점 분포도에 대한 세가지 모멘트인 평균값, 표준편차 그리고 스큐니스(skewness)를 이용하여 이 분포를 3-매개변수(parameter) Weilbull 함수로 근사하였다. 특정한 수평속도와 돌풍세기에 대해 구성된 테더 반력의 분포를 예상되는 모든 돌풍의 수평속도와 돌풍세기에 대하여 적분함으로써 테더 반력의 장기간 분포를 예측하였다. 이를 바탕으로 32M 에어로스탯의 수명기간동안 예상되는 최대 테더 반력을 성공적으로 예측하였다.
