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 mold 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 mold was cut at the workshop of Mechanical Engineering at the North West University in Potchefstroom.

 

Johan Bosman, JS Chief Aerodynamicist, explains:

Whenever there is a difference in load on the wing, a vortex will form with excess drag as a result. Flap fences reduce the vortex size, because the pressure difference is “capped off”. The flap fences on the JS1 Revelation are fixed to the flaperons, not the wing root, as the load and pressure difference is higher on the flaperon.

There is however a trade-off for fitting flap fences, and in this case it is the added wetted surface area of the fences and interference drag. Recent oil flow experiments on the root with flap fences showed good flow lines, with no separation or vortices forming on the surface near the fences, which indicate no added detrimental effects.

Whether it is truly beneficial to add flap fences is still a good question, but a current CFD study might shed light upon the problem in the near future.

It is known that winglets can reduce induced drag by reducing the tip vortex. They can also increase aileron efficiency, giving better overall handling characteristics. However an incorrectly designed winglet can absolutely ruin the performance, especially at higher cruise speeds.

Designing winglets is probably the second biggest challenge faced by sailplane aerodynamicists. The basic design variables are:

  • Area
  • Aspect ratio
  • Toe-out angle
  • Sweep
  • Cant angle
  • Loading

The winglets for the JS1 Revelation were specifically tailored for the wing shape and tip airfoil. KKAERO was used to obtain the optimum induced efficiency factor for all configurations by adjusting toe out angles and sweep angles. Care was taken not to overload the winglet and create load peaks at the tip. For all operational angles of attack, the load on the tip region was kept within limits of the maximum airfoil lift coefficient, for safe handling characteristics.

Due to the polyhedral wing configuration of the JS1 Revelation, the last wing panel is already at an angle of 24 degrees which increase the angle between the winglet and wing. This reduces the detrimental 3D flow effects at the junction corner due to super positioning of adverse pressure gradients which causes separation.

Oil flow tests have also revealed no separation problems in the winglet junction area.

 

At low speeds, the winglets give around 5% performance increase compared to a wing without winglets. The cross-over speed, which is the speed where the presence of winglets becomes detrimental to overall performance, is above 220 km/h.

Johan Bosman, JS Chief Aerodynamicist, explains:

Interference effects at the root junction cause additional drag and the fuselage itself can cause a reduction in wing lift. Design of the wing-fuselage junction to avoid these adverse effects is extremely challenging.

However before the wing-fuselage junction itself was designed, the optimum wing angle was established by assuming standard up wash and downwash angles from the airfoil and aligning these with the fuselage streamlines. For high and low speeds the optimal angle is different. The wing setting angle is nearer the high speed optimum as it is not such a big influence at low speeds.

Not much information was available on wing-fuselage junction design for sailplanes at the time when the JS1 Revelation was developed.

Therefore a new approach to wing-fuselage design was taken in conjunction with Professor Krzysztof Kubrynski from Warsaw (who developed the panel method software KKAERO).

 

The aim was to minimize lift loss due to the fuselage and to maximize laminar flow area without flow separation problems at the operational angles of attack.

This was achieved by applying a positive twist to the root section around the flap hinge point, thus compensating for the lift loss by increasing the local angle of attack. With KKAERO and self written software to ensure that airfoil geometries are not corrupted by twisting, the optimum wing-fuselage junction for the JS1 revelation has been obtained. Several iterations on the root airfoil were done to ensure a separation-free root junction.

 

For maximizing laminar flow on the root area, the blending into laminar flow airfoils is made over a short distance and adequate fillets ensured no excess turbulent flow on the wings.

Johan Bosman, JS Chief Aerodynamicist, explains: The optimum wing planform has an elliptical lift distribution for minimum induced drag at low speeds, and a small wing area for reduced profile drag at high speed. The wings of the JS1 Revelation have an almost perfect lift distribution. We achieved this with the six tapered polyhedral sections along the wing span, with aerofoils based on the T12 aerofoil optimised at each spanwise position for the specific chord length and Reynolds number. The polyhedral also contributes to the handling qualities. We believe that while sailplanes must have excellent cruise performance at high speed, often competitions are won on weaker days and so the JS1 Revelation does not have the minimum possible wing area. With a wing area of 11.2 sq.m (120.56 sq.ft) and an aspect ratio of 28.8, the JS1 Revelation's wing provides outstanding climb performance and with the very thin aerofoil, also has exceptional high speed performance. We feel that the JS1 Revelation wing planform has the optimum balance between aerofoils and wing area, and with ideal low and high speed characteristics, it is optimised for the wide range of conditions encountered in competitions.