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WINGGRID AERODYNAMICS BOOST PERFORMANCE

Life is never easy for revolutionaries - this does not only apply to politics but also to technology. The Swiss engineer Ulrich La Roche could tell a story about the difficulties in getting new ways of thinking accepted, "When I presented the 'Winggrid concept' for the first time, it was met with a pitying smile". Empirically handicapped aerodynamics specialists and flight mechanics found La Roche's idea too absurd.

After all the design principle that La Roche wanted to apply to aircraft aerodynamics has been very successful in nature: The finger feathers at the wingtips of many birds reduce the induced drag by up to 50 percent. This allows the African bird "Gyps Rupelli", for example, with its almost rectangular wing to achieve a glide number of 20 with a wing aspect ration of around eight, while the Albatross with its slim wing has a wing aspect ratio of 20 and achieves a glide number of only 25.

Dr. Ulrich La Roche noticed this connection when reading a book by Professor W. Nachtigall at the end of the 80s. This book dealt with bird flight under aerodynamic-scientific aspects. After this he decided to get to the bottom of this question. His aim was to improve the distribution of lift across the wingspan. In 1992 about 30 different half wings were created for the "Little Eagle Owl", a reknown aircraft model. La Roche found out that a wing equipped with winggrid, which supplies the same lift and has the same drag as the original wing of the "Little Eagle Owl", only needs 64 percent of the original wingspan, or, in other words, would generate an increased lift of 245 percent when having the orginal wingspan with winggrid.

In 1993 and 1994 windtunnel tests followed in Emmen as well as at the Technical College Brugg, some of these tests "caused hilarity" as La Roche remarks. However, La Roche was able to demonstrate that the "Little Eagle Owl" was able to fly with a winggrid-equipped wing. On one side it had the factory produced standard wing and a markedly smaller wing on the other, which was fitted with an end piece resembling the wing feathers of birds, the so-called winggrid.

There are five criteria for the successful functioning of this attachment, which caused violent disputes in the following years with international Patent Offices. Since Lilienthal there had been 120 patent applications for this type of wing. La Roche stated the following for his Winggrid patent:
1) The size of the winggrid must not be more than 50 percent of the entire wingspan.
2) The winggrid must have little wings, which are aranged at a certain distance to each other.
3) The plane, in which the winggrids are positioned, must be angled to the main wing.
4) The little wings must be parallel to each other.
5) The winggrid must supply the same lift per cm wingspan as the main wing if the distribution of lift is at right angles.

If these points were adhered to it would be possible to reduce the wingspan of an aircraft by half. The flight performance and properties would remain the same. The aerodynamic efficiency of the winggrid is based on a strong interference between the individual little wings. These do not have any effect separately, but work as an aerodynamic grid and can only cause an increase in lift when combined.

An important effect when using winggrid at the tip of the wing is a significant reduction of the induced drag - a force, which works on any wing when lift is being generated. Since the pressure on the upper surfaces of the wing is lower than on the lower side, a balancing airflow is being created at the end of the wings. This combines with the air stream around the aircraft in the flight direction and forms vortexes, which slip off the end of the wings. The kinetic energy, which is being created, causes the induced drag.

Well-known measures to combat this negative aerodynamic force are an increased wingspan and the tip cones or discs, which have the task of limiting the balancing airflow at the wing tips.

The winggrid tackles the induced drag more basically. According to a publication by the scientists Spreiter and Sacks in 1951 only two parameters are important for the value of induced drag: The thickness of the so called dead core of the vortex and the distance these vortexes have far behind the wings.

The thicker the dead vortex-core (similar to the eye in a hurricane), the lower the energy in the vortex and thus induced drag The bigger the distance of the two wing tips, (measured far behind the aircraft), the lower the induced drag. It is only indirectly significant where and how the vortex leaves the wing tips. What is important is the position of the center of the departing vortex distribution.

The rectangular lift distribution of a wing with winggrid causes the distance of the vortexes behind the aircraft to be as big as the wing span, (far behind), and many parallel little wings result in a thick vortex core.

In 1996 La Roche reached a decisive step in putting his findings into practice, when Prof. Dr.- Ing. Hans-Reinhard Meyer-Piening from the Swiss Technical University Zurich committed himself with his Institute for Light Construction and Cable Car Technology to the Winggrid project. Furthermore the EEF, (Development Group for Aircraft Construction), looked after the full-scale tests for ground and flight tests. On 1 September 1997 at the military airport in the Swiss town of Emmen a manned aircraft, which was fitted with winggrind technology, took off.

The scientists chose the Prometheus to be the test aircraft. It is a motor glider, which is powered by two Microturbo jet engines (TSR 18-048 with 0.9 kN thrust each). This aircraft originally had a wingspan of 23 meters and realised a maximum glide number of 35. Fitted with winggrids at her wingtips, the wingspan was reduced to 12 meters. Calculations led the scientists to expect a glide number of 15 to 17. In fact the winggrid version achieved a glide number of 25, which corresponds with a reduction of the induced drag for Prometheus of about 50 percent.

As this and further test flights showed, flight characteristics did not change drastically. The aircraft stayed very stable despite the markedly reduced wingspan, and what was new was the positive behaviour when approaching stall.

The effect of the winggrid on the stall speed, i. e. the speed in which the air stream separate from the wings, is benevolent, because the air stream at the winggrid stops a lot more slowly than it does at the main wing. The stall speed of the original version of Prometheus was around 95km/h, while during flight tests the pilots of the winggrid version indicated first signs of a stall at 115 to 130km/h.

Winggrid has also a positive effect on the co-ordination of the rudders. The aileron of Prometheus' conventional version needs some correction with the rudder however the version with the unconventional winggrids attached to the wing tips does not require this.

Admittedly the shape of the winggrid might seem very odd for conservative aviators, however, it looks like the aerodynamical benefit might soon fing a commercial application. In April the French company Dyn Aéro of Christophe Robin introduced the concept of a motor glider with winggrid on the basis of the ultralight aircraft MCR 01 during the AERO '99 exhibition in Friedrichshafen, Germany. With the following data this aircraft will point the way for motor gliders as far as space and performance is concerned. It has a wingspan of only 9.8 meters, a cruise speed of 240 km/h and a glide number of 30.

According to Christophe Robin a few more months will pass before the first winggrid aircraft go into series production. As La Roche reported after the AERO, Dyn Aéro has signed a promotional contract with the French government and intends to begin with the comparative measuring of three MCR-01 variants by autumn of 1999. Among them will be a conventional MCR 01, one version as a winggrid motor glider and a third variation with special wingtips, which were developed by ONERA (Organisation National Aéronautique). We are already eager to know whether the envisaged specifications can be realised. Professor Meyer-Piening from the ETH Zurich predicts far-reaching possibilities for winggrid technology, "Aircraft like the planned Airbus A3XX with a wingspan of 86 meters will need a lot of space. Just imagine the possibility of cutting their wingspan by half".

From page 86 of FLUG REVUE 7/1999