1.12 Model Based Characterization of the Movement of Hot Air Balloons

The hot-air balloon is the oldest aircraft and still fascinates young and old. This project aims to develop a detailed physical model of a hot-air balloon that describes its movement based on real weather data. The model will later be used in the aviation studies program to promote the understanding of the phenomena occurring during ballooning and as a planning and analysis tool for hotair balloon pilots. As a prerequisite for "virtual" balloon flights, unknown model parameters must first be determined using literature data and experiments. For the latter, the ambient conditions and the temperature of the balloon envelope were measured during a parallel balloon flight.

Contributors: Partner(s): Duration: J. Stoll, T. Hocker, S. Ehrat Air Ballonteam Stefan Zeberli GmbH 2020

The movement of a hot-air balloon results from its mass and the forces acting on the balloon, i.e. its weight, its buoyancy and the air resistance force, which depends on the winds prevailing at the respective position. The hot air temperature is decisive for the ascent and descent behaviour, which in turn depends on the energy supply via the burner and the heat losses via the balloon envelope. For this reason, during a parallel balloon flight with Lea and Stefan Zeberli, the ambient conditions and the temperature of the balloon envelope at different altitude levels were measured and the fuel consumption estimated. A thermal imaging camera is used for surface temperature measurement, as it measures large areas. However, measurements with this instrument are complicated because the emissivity of the balloon envelope, the measuring distance, the ambient temperature and the humidity all influence the accuracy of the measurement. Therefore, it is important to take measures to minimize the measurement errors when developing the measurement procedure. For example, a shielding device for the thermal imaging camera has been developed to protect it from radiation from the burner. Furthermore, it must be taken into account that, depending on its position, the proportion of thermal radiation emitted by the balloon and absorbed by the atmosphere must be calculated. Based on this proportion, the temperature of the balloon envelope is then corrected. Figure 1 shows a typical thermal image taken during a parallel balloon flight at an altitude of 1080 m. It can be seen that the temperature of the balloon envelope increases from bottom to top. Furthermore, it can be seen that the balloon envelope is made of different fabric strips with different emissivities, which lead to significant measurement errors in the thermal image.

Figure 2 Thermal image of the hot air balloon flight of Lea Zeberli at an altitude of 1080 m, temperature scale in °C.

For a complete model, numerous influencing variables of the atmosphere have to be taken into account. Specifically, wind speed, wind direction, temperature, humidity and air pressure have to be considered depending on the respective position. In this regard, extensive research on national and international weather models was carried out to obtain forecast and reanalysis data. It has proved to be difficult to obtain freely accessible weather data with sufficient local resolution. For 2020 it is planned to use the weather data for the validation of the model, for the analysis of real balloon flights and for the planning of future flights.