Aircraft charging using ion emission for lightning strike mitigation : an experimental study

Abstract

Lighting strikes are a problem for aircraft flying in large external electric fields. In most cases, the strike is triggered by the aircraft; as it flies through an electric field, it becomes polarized, and on areas with small radius of curvature, the electric field is magnified. This can result in bidirectional leaders which extend from opposite polarity aircraft extremities. These can connect to oppositely charged regions in a cloud or the ground, resulting in a lightning strike. Current methods to avoid lightning are limited to avoiding thunderstorm regions, as recommended by weather radar or conversations between pilots and the ground. Methods to treat the symptom of a strike have been relatively successful; a mesh placed under the skin of the aircraft can distribute the current and heat of the localized strike. However, there are currently no active measures to prevent the strike from happening.

The Boeing Lightning Strike team at MIT has recently proposed an active system that exploits the physics of how a lightning arc is triggered from an aircraft in flight based on net charge control of the vehicle. The objective of this thesis is to prove the feasibility of controlling the net charge of an aircraft in flight by using ion emission from its surface. Different strategies to control the net charge of a flying isolated body were explored and analyzed. The first strategy tested was based on using charge emission from an electrospray source. A passive flow and forced flow configuration were tested, however it was shown that there were numerous difficulties associated with running the electrosprays in atmospheric pressure. To overcome the limitations of the electrospray source, a second strategy was tested based on a controlled corona discharge, which is known to have increasing current emission with increasing wind speed.

The first experiment was setup in the Wright Brothers Wind Tunnel; sharp tips were used to generate a corona discharge and a metallic sphere was used to simulate the aircraft. Significant electrical potential saturation was observed on the sphere, and it is likely this was due to the filamentary streamer corona regime which produces both positive and negative ions. Thus a new experiment was designed; a thin wire was used to generate a glow corona, which produces predominantly positive ions, and this was attached using GlO (a fiberglass composite material) to a metallically coated airfoil. Charging of much higher magnitudes was observed, indicating the glow corona regime is critically important in optimizing the potential of the airfoil. Charge control of an airfoil (Chord 0.2 m, Span 1 m) at 40 m/s was demonstrated to a level of -42 kV.

For an object of a given characteristic size, a certain amount of charge is required to satisfy the optimal charge condition, where negative and positive leader strikes are both equally likely or unlikely. The achieved potential of -42 kV is the order of magnitude required for this size airfoil based on the theoretical estimates, and these tests also showed a trend of linear potential variation with wind speed.