Preliminary Analysis of an Aircraft Capable of Deploying and Retracting a Helium Balloon for Long Term Hover Introduction J2 Aircraft Dynamics were approached by a company who were interested in the development of a UAV with exceptionally long endurance as an observation platform. The way of achieving this endurance, was to deploy a helium balloon to provide buoyancy when loitering, and then to retract this balloon to provide a high speed dash to a new location. The company had investigated a variety of options when considering the feasibility of this project, none of which were able to provide a suitable modelling environment when considering evaluating the behavioural characteristics of the aircraft. They then approached J2 Aircraft Dynamics and we confirmed that through the use of the J2 Universal Tool-Kit we would easily be able to evaluate their design. Constructing an Aircraft Model An existing UAV was to be used as the base platform for this project, and so the first stage was to build a model using g the J2 Universal Tool-Kit and assess its characteristics to ensure it behaved as the existing aircraft. The aircraft was to be based on the one shown above with several modifications 1) The twin fin on top was to be replaced by a single fin underneath 2) A balloon mount was to be added down the upper spine of the aircraft that would be used to house the balloon and associated equipment, and to ensure the balloon would remain clear of the propeller.
The power of the J2 Universal Tool-Kit meant that this model could be constructed from a series of hierarchical components including internal equipment, and engine characteristics, very rapidly and could be flown in minutes to assess its basic characteristics. Adding the Balloon Once satisfied that it was behaving correctly, the next stage was to add in the balloon. This was done by adding another propulsion item to the model where the thrust was directly proportional to the volume of the balloon, and ensuring that regardless of the orientation of the aircraft, the buoyancy force was always vertical in the earth axis frame. One other aspect to consider was that the magnitude and location of the centre of buoyancy, and the drag from the balloon would vary as the balloon inflated and deflated. These characteristics characteristics were easy to add in using J2 Builder as the key points are automatically provided, and these can be quickly defined as dependent upon other user-defined parameters.
Analysing the Behaviour The first stage of analysis was to investigate the effects of inflating and deflating the balloon in-flight to see how the stability is impacted. With the model defined, it was simply a question of trimming the aircraft over a range of airspeeds with the balloon retracted, and then creating a manoeuvre with J2 Freedom whereby the pilot eased back on the throttle, and inflated the balloon. It was then
possible to run several scenarios to see how different initial airspeeds impact on the behaviour of the aircraft. What can be seen is that the aircraft experiences a pitch up as the balloon is deflated, the more violent pitch up is experienced at higher initial velocities, leading to a recommendation that the aircraft is slowed down to 45knots prior to inflation. When it came to Deflating the balloon, the opposite effect was found. That is if the aircraft was flying too slowly when the balloon started to deflate, the aircraft would be unable to increase its velocity sufficiently to be able to return to conventional flight. As such it was recommended that the aircraft be accelerated, with the balloon deployed, to a minimum of 25knots prior to commencing the deflation.
Trade Studies One of the great benefits of the J2 Universal Tool-Kit is the ability to very quickly perform trade studies through the use of Delta Models. With a Delta Model the user simply selects an original aircraft, and then adds in the increments where necessary to the characteristics. One area where this proved useful is to investigate the possibility of increased drag from the balloon. Here we perform the same analysis as before. With the increase drag the pitch up is even more violent, resulting in the aircraft t oscillating uncontrollably.
Conclusions The first objective of the analysis was to assess the feasibility of the concept of using a helium balloon to enable long term loiter for an aerial platform. This approach had not been attempted previously due to the fact that the analysis capability has not been available until the J2 Universal Tool-Kit. By utilising the power of the J2 Universal Tool-Kit, it was possible to: Construct a Model Add in the Balloon Effects Model the deployment, and retraction of the balloon along with investigating manoeuvring the aircraft. Perform trade studies to assess the impact of any assumptions and design changes. Recommend further stages of investigation. All this work was possible without the need to develop complex scripting models or bespoke solutions to investigate a one of scenario, and meant that the work was completed within 10 man days.
The conclusions drawn were that the project was indeed feasible, based upon various assumptions made regarding the speed of deployment and retraction, and the integrity of the balloon profile during these stages. Further investigations can now take place in order to generate more detailed and complex models, and the development of an Automatic Flight Control System (AFCS) all within the J2 Universal Tool- Kit, by continually building on the work already performed. All cases are configuration managed and version controlled so it is possible to track the progress of the analysis as well as the development of the model right through to flight matching at latter stages of development. Company Background J2 Aircraft Dynamics are the leading aircraft design software company. They develop sophisticated aerospace analysis tools by building upon our unique combination of aircraft design knowledge and extensive software skills to create the fully integrated tool-kit. Using their software, companies are able to eliminate the unknown leading to significant cost and time savings that will result in increased profitability.