Studies on Optimum Shapes of Envelopes for Aerostats and Airships

Principal Investigator: Prof. Rajkumar S. Pant

Co-Investigator: Prof. A. G. Marathe

Aerospace Engineering Department,

IIT-Bombay

 

OBJECTIVE:

           The objective of this project is to obtain optimum shape of the envelope of Aerostats and Airships from aerodynamic, structural and manufacturing considerations.

 

INTRODUCTION:

           The envelope is the most important component of an aerostat or airship. The selection of the shape of the envelope is principally governed by aerodynamic considerations, apart from structural and manufacturability considerations. Though the most common shape for aerostats and airships is ellipsoidal, various other unconventional shapes and configurations have also been employed such as Spherical, Dart, Deltoid, Flat Body, Lenticular, Toroidal, Multi-balloon, Multi-hull etc, each having some advantages and disadvantages. Modification to the conventional ellipsoidal shape have also been tried to improve the aerodynamic performance, such as inclusion of parabolic and circular arc components.

           A preliminary investigation of the suitability of GNVR shape for the airship envelope was carried out at IIT Bombay through the development and flight testing of the Micro-Airship. The airship was able to attain a maximum speed of around 25 kmph, which indicated that it had good drag characteristics. The airship demonstrated excellent controllability during the flight tests and response of the control system was found to be very positive, since several descending approaches and touch-downs were successfully accomplished. However, the ratio of the stabilizer area to the envelope surface area was found to be slightly higher than that employed on similar airships. This indicated that there might be a possibility of reduction in the stabilizer size (and hence weight) if the L/D of the vehicle is increased, with a consequent increase in payload.

           In a recent study by Prof. GNV Rao, the benefit of insertion of a constant diameter insert in the GNVR shape (which results in an increase of L/D from 3.0 to 4.5) was investigated. The study revealed that an airship hull with a larger L/D than the generally accepted value of about 3.0 for minimum drag coefficient can result in lesser drag force for the same volume and speed at L/D more than 3.0, if the diameter is reduced slightly and an additional cylindrical insert is introduced.

           These studies bring out the need for a systematic evaluation of various modifications and improvements that can be brought about to the conventional ellipsoid shape, to investigate the aerodynamic advantages of the same. Further, a study of the merits and demerits of the various unconventional shapes for aerostats and airships will also be helpful in proposing a shape suitable for a specific application.

 

SPECIFIC TASKS TO BE CARRIED OUT:

1. Literature survey and collection of data related to various envelope shapes that have been employed for airships and aerostats.

2. Prediction of aerodynamic coefficient for GNVR and SAC shapes using FLUENT/STAR-CD software package, and validation with known data.

3. Development of a code for optimisation of envelope shape from aerodynamic, structural and fabrication considerations, and coupling it with FLUENT/STAR-CD.

4. Determining the optimum shapes for one aerostat and one airship envelope to meet the user specified operational requirements.

 

PROJECT COMMENCEMENT DATE: 1st April 2004

DURATION: Two years

PROGRAM ON AIRSHIP DESIGN AND DEVELOPMENT

PADD

Research And Development Projects