AIRFLOW VELOCITY EFFECTS ON LIGHTNING IGNITION OF AIRCRAFT FUEL VENT EFFLUX

Abstract : Studies of the effects of airflow, at aircraft holding velocities up to 250 knots, over the surface of an aircraft wingtip containing a recessed fuel vent showed that the probability of lightning ignition of the fuel vent efflux was greatly reduced by the airflow. From almost certain ignition with no airflow and the optimum fuel flow rate and mixture obtainable, the ignition frequency dropped to one in 34 shots with 50 knots airflow, one in 31 shots at 90 knots, none in 200 shots at 200 knots and none in 10 shots at 250 knots. Streamering at the recessed fuel vent studied, due to a five megavolt surge, did not produce ignition, even with no airflow. However, with swept strokes the ignition frequencies were greatly increased to six in 8 shots at 100 knots and two in 36 shots at 200 knots. This result demonstrates that a swept stroke could be most hazardous. The probability of swept strokes from the wing leading edge appears very low. Very high flame speeds, of the order of 1,000 feet per second, were measured. The lightning blast wave and the airstream effects appear to increase the flame speeds. Since present flame suppression devices and flame arresters are not effective against such very high flame front velocities, further study is also indicated.