What makes aerodynamics so crucial
by Hendrik Thielemann
Aerodynamic factors greatly influence the characteristics of an aircraft, and what is even more exciting is understanding how these factors affect flight. How fast does the plane climb? What is the range and the cruising speed? How does the aircraft behave in crosswind conditions? How do we improve agility, speed and performance? RUAG’s aerodynamics experts investigate these and other questions at their wind tunnels in Emmen, Switzerland, where they study many different aircraft models, including the Dornier 228. The results from wind tunnel tests provide performance and aerodynamic characteristics and structural load data for the development of new aircraft or modifications.
“We’re standing right under the measuring section here. From here you can move the models with a platform into the wind tunnel. This particular tunnel is a closed return type, where the wind comes from the left side and goes out on the right side, then is routed around the back and comes into the test section again.” Andreas Hauser is in his element when explaining the Large Wind Tunnel Emmen (LWTE) to visitors. As Manager of the Department of Aerodynamics, Andreas is responsible for the entire wind tunnel facility and its organization. “In the large wind tunnel, we usually support aviation industry projects relating to the development of new aircraft or the further development of existing aircraft,” summarizes Andreas.
The positioning of the control surfaces relative to the core wing has to match precisely the full-scale aircraft configuration.
All major western aircraft manufacturers have run test campaigns at the Large Wind Tunnel Emmen. In addition to aviation testing, RUAG also performs tests for the automotive industry, where environmental conditions such as precipitation can be added to the testing of prototype cars.
The Large Wind Tunnel Emmen can accommodate aircraft models with a wingspan of up to five meters. Two counter-rotating turbines with a diameter of 8.5 m ensure wind speeds of up to 68 m/s (250km/h), making the tunnel particularly suitable for testing configuration changes on aircraft, including the testing of aircraft propellers – integrated on the model or isolated in their full size. The engineers support their customers throughout the test campaign – from the design of the wind tunnel models, to manufacturing, to the implementation of instrumentation and force-measuring sensors.
“A large part of our work is experimental,” Andreas continues. “We conduct experiments with scaled models in the wind tunnel and can then project the results to the originals. We also simulate the aerodynamics of the aircraft in computer models. This process is called computational fluid dynamics (CFD) and is often used during the initial design phase or to analyze very specific flow characteristics on the aircraft.”
A series of fluorescent tufts on the aircraft surface helps the engineer to visualize interesting flow patterns.
Some time ago, the engineers in Emmen were visited by a familiar flying friend: the Dornier 228, which is manufactured by RUAG. The aircraft has proven itself as a robust and versatile workhorse in almost 30 years of operation. It has set standards in terms of aerodynamics, efficiency, flight behavior and short takeoff and landing (STOL) capability. So why did RUAG run a test campaign with the Dornier 228?
Investment in the future of the Dornier 228
“The test campaign was an investment in the future of the Dornier 228,” explains Andreas Hauser. “We needed the test results as a reference to investigate forthcoming configurations of the Dornier 228, which have not yet been integrated and tested in the current model.”
Much is already known about the aerodynamics of the Dornier 228. There are computer models, extensive measurements from flight operations and older wind tunnel data from the development of the aircraft. These come from tests carried out in the former Dornier’s in-house wind tunnel. “The data from back then no longer meets today’s requirements,” says Andreas. This is scarcely any wonder, since wind tunnel technology has evolved: instead of simple wooden models, detailed aircraft models with precisely manufactured metal surfaces are used today. In addition, a multitude of sophisticated sensors allows more precise measurements, which in turn allow the aircraft to be optimized in terms of performance and efficiency. Furthermore, the Dornier 228 is no longer the same aircraft it was 30 years ago: the fuselage has been extended by around 1.5 m, and the four-bladed propellers of the turboprop engines have been replaced by new five-bladed airscrews.
The Large Wind Tunnel Emmen allows the investigation of large-scale aircraft models with high geometrical accuracy, giving engineers good confidence in the acquired results.
“The Dornier 228 stands out from an aerodynamic perspective: a highly flexible aircraft which is optimized for short to medium distance commuter flights, long loiter time and heavy lift cargo haul on unprepared airstrips. All these characteristics stem from a very well designed aerodynamic shape,” Andreas summarizes.
Above all, aerodynamic studies of the Dornier 228 have led to the design of a highly unique wing. Dornier engineers invented this wing as part of a program sponsored by the German Research Ministry. In the process, they further developed a wing structure that aerospace specialists from NASA had come up with. While a conventional wing has a main and a secondary spar and ribs, stringers and a sheet metal skin – all riveted together – the Dornier 228 wing structure is a box formed from four integrally milled alloy panels.
The wing contributes strongly to the outstanding aerodynamic properties of the aircraft. When the landing flaps are retracted, its characteristic shape is responsible for low air resistance, which allows high cruising speeds and a long range. The airfoil cross-section also ensures a high glide ratio. This allows long dwell times at low speeds when needed for patrol or surveillance flights.
In landing and take-off configuration with flaps extended and drooped ailerons, together with a powerful propeller-induced wake, the wing of the Dornier 228 produces tremendous lift. This enables safe operation into short and unpaved runways in remote areas or at airports with a high density altitude – that is, in very hot climates or at high altitudes.
With the five-blade propeller, the aerodynamic properties have improved even further. The primary reason for introducing the propeller was to lower the propeller noise acting on the immediate surroundings as well as on the fuselage and the cabin interior so as to provide a more comfortable flight. As a beneficial side effect, the five-bladed propeller thrust was slightly increased, boosting performance.
Aerodynamics also play a key role in the pilot-friendly flight characteristics of the Dornier 228. “The aircraft’s control surfaces are aerodynamically optimized so that the high-energy airflow allows full controllability even at high angles of attack. The large rudder is crucial for good controllability of the aircraft in strong crosswinds and in single-engine emergency operation at low airspeeds,” explains Andreas.
Special mission aircraft
In addition to commuter flights, special missions are the Dornier 228’s most important field of operation. It is in the nature of things that the aircraft must be adapted to the requirements of such missions – this can mean external elements being attached to the fuselage, which can also affect the aerodynamics of the plane.
ometimes it is “only” a question of installing a radar antenna, a camera or an infrared sensor on the fuselage. Other times, the Dornier 228 needs to be configured to drop parachutists or relief supplies over disaster areas. And other times the aircraft serves as a research platform – for example, to test the use of electric engines, as planned by the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR), or to gather information about permafrost melting or the loss of trees in Africa’s Kruger Park.
The effects of such modifications on aerodynamics, and thus on flight behavior, must be evaluated before a modified configuration can be certified. Engineers and technicians base their investigations on three pillars: computer models, wind tunnel tests and in-flight measurements. “In the special mission segment, the requirements are manifold. Many customers want to adapt the configuration of the aircraft to their mission demands, and with our aerodynamic tests we have been able to create a basis for implementing changes efficiently and reliably,” explains Andreas.
Following the test campaign, RUAG engineers compared the results with the existing data from previous flight tests. They also used the measurement results to further improve the existing computer models. “This was, of course, a bit of reverse engineering,” says Andreas. When developing a new aircraft, the procedure is normally the opposite: At the beginning there are the computer simulations, then the wind tunnel measurements and finally the test flights. The Dornier 228, on the other hand, is a tried-and-tested aircraft, so the priorities were different: “The goal was to create a database that helps the operators to optimally adapt the aircraft to their requirements, for commuter or special mission purposes,” summarizes Andreas.
Investigating the aerodynamic influence of the landing gear on stability and control behavior.