Balanced Field Length Calculation for a Learjet 35A/36A with Under-Wing Stores on a Wet Runway

Autor: Florian Ehrig



The Learjet 35A/36A is a twin-engine business jet. In a special configuration, it can be fitted with under-wing stores, a configuration for which no takeoff performance data on wet runways is currently available. This report outlines the creation of a numerical takeoff performance simulation for this specific aircraft on wet runways. The results shall be used to set up takeoff performance charts that can be used in daily flight operations. To obtain Balanced Field Lengths and Decision Speeds according to EASA CS-25 certification specifications, the aircraft acceleration, takeoff and braking performance was determined. A comprehensive aircraft parameter estimation has been performed, permitting to consider the forces acting on the aircraft in various takeoff phases accurately in their dependency of time and speed. A focus of the parameter investigation was put on the precipitation drag acting on the aircraft due to the wet runway conditions. A specific geometry-based investigation of the factors determining the amount of spray drag acting on the Learjet 35A/36A airframe with under-wing stores was performed. This permitted a conclusion on the additional drag due to water impingement on the aircraft in the special takeoff configuration. The results of the simulation were set in relation with the existing aircraft performance data and a simplified calculation method. It was found that the simulation produces results of high accuracy and the results show consistent behavior with a variation in input parameters.