Wind Tunnel Balances Assure Safe Airplane Designs

Algor Around the World: Germany

Designing an airplane is a long and complicated process. The safety of the public must be assured while keeping the design within the realm of possibility. In order to understand the forces that act on a plane in flight, engineers have devised tests and measuring instruments, including those used in aerodynamic testing of scale models in a wind tunnel. A device called a balance is used to measure the aerodynamic forces to which the model is subjected.

After years of research, balances are achieving new standards of accuracy and resolution, thanks in part to Algor design and analysis software. Klaus Hufnagel and Junnai Zhai at the Technical University of Darmstadt, Germany, use Algor to develop balances that precisely measure the forces acting on models in a wind tunnel.

Junnai Zhai and Klaus Hufnagel at the Technical University of Darmstadt, Germany, use Algor software to design and optimize wind tunnel balances.

In the last two years, two balances for the European Transonic Wind Tunnel have been constructed based on designs analyzed and optimized with Algor: the W617, a balance for models of transport planes, and the W621, a balance for models of combat planes. Some of Mr. Hufnagel's and Mr. Zhai's balances were also used to test models of the Airbus A320, A340 and others.

The Wind Tunnel System

As shown in this diagram, the model is fixed on the balance and the balance is supported by a sting. The balance must separate the force into six components and then measure each individually. These forces are axial, side and normal force, and rolling, pitching and yawing moment.

The axial force is the most important, since reducing it has great economic and ecological significance. However, axial force is often much smaller than other aerodynamic forces and is therefore difficult to measure.

The balance contains a number of strain gauges. Ideally, one gauge will register only one of the six components of the force. However, the difficulty is making sure that other components of the force do not interfere. This could be done easily, if space were not so limited within the model.

Optimizing the Balances

The engineers at Darmstadt must optimize a balance for low interference while keeping within the constraints of space. The balance must also be optimized for low temperatures, high load capability and high stiffness.

Some of the forces are very large, thus, the capacity to withstand the forces must be sufficient. Stiffness is important because, if the balance is not stiff enough, vibration and deformation can cause a misleading measurement.

Temperature effect is a limiting factor for the improvement of balance accuracy in any conventional wind tunnel. However, Mr. Hufnagel and Mr. Zhai design balances which are used in cryogenic wind tunnels operating in the 200° Kelvin range. In these tunnels, the temperature is changed during the testing. Thus, the influence of temperature is much greater.

Analyses

Linear stress analyses of each of the balances enabled Mr. Hufnagel and Mr. Zhai to determine the structural reasons for interference and optimize for increased stiffness.

		Linear stress analysis was used to optimize the W621, a balance used to test models of combat planes.
		A model of a transport airplane model, sting and W617 balance was used in this heat transfer analysis.

From the heat transfer analyses, Mr. Hufnagel and Mr. Zhai reduced the effects of the temperature changes on the gauges. For these analyses, the whole system, including the wind tunnel, is modeled. The model of each balance is often simplified to keep the entire model to a workable size.

Results

In designs for the W617 transport balance, stiffness was raised by 12%, the maximum stress was reduced by 30% and the interference on axial force was reduced by 70%. Using a different alloy reduced the temperature gradient by 24%.

Construction of the W621 combat plane balance was more difficult because the loads were considerably higher and the allowable space was more limited.

"Here, Algor demonstrated its advantage in helping engineers to solve difficult structural problems" said Mr. Zhai. "After the optimization, the stiffness of the W621 balance was raised by 5%, the maximum stress reduced by 17% and the interference on the axial force reduced by 54%."

About Algor

As teachers, Mr. Hufnagel and Mr. Zhai appreciate that Algor is easy to learn and use. Their students have designed numerous measuring instruments including a five-component balance that measures the force acting on the wheel of a car and a high precision force-measuring box that can be used as a calibration machine.

"The 'Displaced' function in Superview is a very useful tool," said Mr. Zhai. "It provides direct information about the deformation of the structure. From that display, we can conclude how to improve the design. Using the 'Precision' function has also aided me in improving our designs.

"We originally chose Algor over other FEA packages because of its powerful modeling and graphical results visualization capabilities. Our decision has proved to be sound. Algor has demonstrated its advantages in complex engineering situations such as these."