“Fly low, fly fast, turn left” was the motto of the Reno Air Races in 2013 when, for the 50th time, pilots from around the world competed in various classes above the Nevada Desert. Not only are pilots required but also the engineers who put a great development effort behind the scenes for these amazing air racing machines. Race results from recent years show clearly just how tough the fight for the podium is. True to the motto “After the race is before the race”, a whole year of intensive preparations is a prerequisite to make the aircraft even faster for the coming year. The competition has turned to cutting edge techniques like artificial shark skin to squeeze more aerodynamic efficiency out of the airplane and gain the necessary edge to win.
Partnership in the Reno Air Races
Bionic Surface Technologies GmbH (BST), at home in the picturesque southern Austrian city of Graz, is a company that specializes in high-tech computational analysis. BST uses computational fluid dynamics (CFD) tools and has developed micro-structured surfaces called riblets to reduce friction flow on aerodynamics surfaces such as the wings of airplanes. Riblets have been called shark skin since scientists noticed 50 years ago that nature had made sharks more efficient through water than theory predicted possible. Since 2009, BST has successfully cooperated with the Swiss Air Racing Team, using both CFD and riblets to win at the Reno Air Races. This good cooperation and excellent aerodynamic technology have contributed to the top placing each year for the Cassutt “Scarlett Screamer” flown by Swiss pilot Vito Wyprächtiger.
Using CFD to more efficiency
Since the partnership began, the aerodynamics of the Cassutt racing aircraft have been continuously improved. The primary task of BST is to modify the external shape of the racing aircraft so that a circuit around Reno’s pylon course is flown in the shortest possible time. This requires an analytical approach with suitable CFD simulation programs. Using advanced simulation technology and proprietary algorithms, the aircraft shape is optimized with the pilot in mind. This results in efficient development, as time-consuming wind tunnel testing with scale models are not required and even small effects in the design can be evaluated in a short time. BST’s flow simulations also provide operational parameters relevant for the race and thus give advanced indications of expected performance.
The starting point for the flow analysis was provided by a full-scale 3D scan of the existing aircraft. With this data, a baseline calculation was performed and compared to the experience of the pilot. This was followed by a series of geometry modifications which gradually and incrementally improved the aircraft’s performance. Modifications were made over the entirety of the aircraft, as shown in series of images below. The result is more speed with improved flight characteristics and a modified aircraft that has only slight resemblance to the original Cassutt.
Application of riblets on the Scarlett Screamer
BST’s second area of expertise is the use of micro-structured surfaces (aka “rib lets” or “shark skin”) it has developed in-house to reduce drag. The beneficial effects of micro-structured surfaces are widely known in the aerospace industry. BST has successfully created such riblets specifically for this application. A 4% improvement in aerodynamic efficiency has been achieved with specially-designed riblets, resulting in a 1.58% increase in the top speed of the Scarlett Screamer, a significant amount in a tight race. Peter Leitl, Chief Technical Officer of BST, developed a proprietary algorithm to determine in advance the best geometry of the riblets and the appropriate areas on the aircraft to achieve maximum. This algorithm was integrated into the existing CFD simulation program and verified by various laboratory tests and real applications. BST can thus interpret – without costly and time-consuming wind tunnel testing – the benefits, the exact riblet geometry and the ideal areas for their use. This is especially useful for applications where actual tests are expensive (aerospace, wind energy, etc.) and to determine whether riblets might bring benefits over the lifetime of the system under consideration.
Presentation of the results – the customer must understand!
Any computations gain importance to the customer when they are presented using advanced visualization tools. In the case of BST’s work for the Reno Air Races, it is important that the pilot will not only sees details of proposed modifications of his aircraft, but also how the airflow will be influenced. In other applications, for instance complex industrial flow problems, chemical processes, etc., visualization is even more important as test results might be impossible, unsafe, or impractically expensive. For this purpose BST has found the product EnSight from CEI to be an ideal means to post process its CFD simulations.
With EnSight it is possible to present highly complex results in a simpler, more comprehensible form, allowing BST to create greater value for its customers.
As an additional service BST provides its clients with three-dimensional CFD results viewable in the freeware tool “EnLiten”, also from CEI. The customer can easily see his application in three dimensions and draw conclusions from the analysis. This provides a significant advantage over printed reports with mere captioned pictures, where a unique representation for complex flow situations is often not possible.
As shown here, flow simulations and the effective presentation of their results are ideal means to improve existing products such as the Scarlett Screamer Reno Racer and to develop new products such as riblets. As they say in Reno, “Gentlemen, we have a race!”.
Article by Kent Misegades, former President of CEI.