Analysis of total energy probes for sailplane application

Abstract

Sailplane manufacturers who strive to design and build the best competition sailplanes in the world try to outwit their competitors through improved gliding performance. Although significant effort is made to make the design as sleek as theoretically possible, the external sensors needed to operate the flight instruments diminish these efforts. The sensors cause the predominantly laminar boundary layer that forms on the aerodynamic surfaces of the sailplane where they are installed to prematurely transition to the turbulent boundary layer, creating parasitic drag. The dissertation aims to identify the possibility of reducing this drag using the total energy probe on the JS-1C Revelation sailplane manufactured by Jonker Sailplanes (JS-1C) as a baseline. Possible total energy probe designs, as well as other external sensors compatible with the JS-1C, were applied to different parts of the sailplane to determine the most optimum sensor selection and arrangement that would promote parasitic drag reduction. The combination of a total energy probe design applied to the sides of the fuselage as protrusions on the skin, along with the pitot-static probe installed on the tip of the horizontal tail plane, would induce nearly 80 % less drag than that of the current total energy probe. The design required further refinement to ensure uniform pressure drop changes during pitch manoeuvres and insensitivity to pitch and sideslip manoeuvres. This configuration, however, would rather benefit sailplanes in the design phase where the total energy probe design is built into the tooling without having to make modifications later. The popularity of electric variometers provides an alternative probe configuration, where the installation of two pitot-static probes on each tip of the horizontal tail plane would induce 90 % less drag than that of the current total energy probe. The pitot-static probe provides the best performance during pitch manoeuvres, while the installation of pitot-static probes on each tip should improve the sideslip capabilities thereof by measuring an average pressure. This configuration is also the least invasive method to accurately incorporate the probes into the building process of the sailplane, without having to make significant changes to existing tooling that would reduce tool life and risk diminishing an out-of-mould surface finish.