# Torsional Balloon Flight Line Oscillations: Comparison of Modelling to Flight Data

## TL;DR

This study analyzes torsional oscillations of the EBEX2013 balloon payload's flight line, comparing a simple bifilar pendulum model's predictions with actual flight data to understand the oscillation behavior.

## Contribution

It applies a bifilar pendulum model to estimate the torsional stiffness of the flight line and compares it with observed data from the EBEX2013 flight.

## Key findings

- Bifilar model predicts a torsional period of 58 s, 25% shorter than observed.
- Model overestimates torsional stiffness by 45%.
- Simple model provides a coarse approximation useful for initial estimates.

## Abstract

During the EBEX2013 long duration flight the payload was free to rotate in azimuth. The observed azimuth motion consisted of a superposition of full rotations with a period of 10-30 minutes and oscillatory motion with an amplitude of tens of degrees, average period of 79 s, and period dispersion of 12 s. We interpret the full rotations as induced by slow rotations of the balloon and the shorter period oscillatory motion as due to torsional oscillations of the flight line. We derive the torsional stiffness of the flight line using the bifilar pendulum model and apply it to the flight line of the EBEX2013 payload. We find a torsional spring constant of 36 kg m$^2$/s$^2$ corresponding to a period of 58 s. We conclude that the bifilar model, which accounts for the geometry of the flight line but neglects all material properties, predicts a stiffness and period that are 45% larger and 25% shorter than those observed. It is useful to have a simple, easy to use, coarse approximation for the torsional constant of the flight line.