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Air Extractor

Johan Bosman, Chief Aerodynamicist at Jonker Sailplanes, and his team have used Computational Fluid Dynamic (CFD) methods to successfully develop and optimise a significantly improved type of cockpit air extractor. The unique aspects of the JS Louvred Air Extractor are the subject of a worldwide patent application.

It is known that sailplane cockpit ventilation is affected by internal flow resistance within the fuselage; with the consequence that air entering a cockpit generally cannot exit sufficiently at the rear of the tail-boom. The inherent flow restriction within the fuselage results in cockpit pressurization, causing air to escape between the canopy frame and the cockpit edge.

This air leakage trips the laminar boundary layer on the canopy and fuselage to turbulent flow, thus increasing profile drag. Various attempts have been made at efficient cockpit ventilation devices, but correctly guiding the ventilation air out of the cockpit and into the free-flow outside the fuselage requires particular attention to detail.

Simply adding a collector funnel and hole in the fuselage may achieve some beneficial effect. However the opening location, size, and geometry details are crucial to successfully allowing the ventilation flow to re-enter the boundary flow over the fuselage with minimal disturbance (i.e. without increasing drag and generating noise).

Unlike some air extractors, the JS Louvred Air Extractor opening is situated at the optimum fuselage location in the region of most negative pressure, on the upper fuselage approximately aligned with the negative pressure peak of the wings.

As Dick Butler, recognised forerunner in extractor experimentation, successfully discovered, this means the extractor sucks air out of the cockpit, instead of requiring a slight positive pressure to blow air out of the cockpit.

To minimise boundary layer disturbances downstream of the ventilation exit location, the JS Louvred Air Extractor utilises a new louvre type design with centre aerofoil. The patent-pending design effectively directs the extracted airflow tangential to the fuselage surface which is crucial for minimising drag and avoiding any separation bubbles.

The size and geometry of the extractor lower the pressure level inside the cockpit to ambient levels and allow the ventilation air to seamlessly re-enter the external flow outside the fuselage.

The JS Louvred Air Extractor properly depressurises the cockpit (as validated by in-flight pressure measurements with and without the extractor) and prevents air from escaping from the canopy perimeter.

Recent calculations indicate that a leaking canopy can increase total drag by as much as 5%, equivalent to 2.5 points on the glide angle. Observations during side-by-side flights between JS1 Revelation sailplanes fitted with and without the new extractor indicate a perceptible performance improvement.
Cockpit noise levels are also generally lower with the JS Air Extractor. And of course, the JS Louvred Air Extractor increases pilot comfort due to improved cooling airflow in the vicinity of the pilot’s head.

All new production JS1 Revelation sailplanes can be fitted with the JS Louvred Air Extractor as an optional extra and prior production sailplanes may be retrofitted. The JS Louvred Air Extractor is the subject of a worldwide patent application to protect the innovative aspects of the design.

The mould for the extractor is very complicated and it was designed and drawn with SolidWorks. The machining tool paths were generated with the CAM software suite ”VISI 17” from Vero-International, provided by Software Development (Pty) Ltd. The mould was cut at the workshop of Mechanical Engineering at the North West University in Potchefstroom.