Material test stand for long-term tests of aircraft
Hänchen test actuator in the test stand

30 aircraft types in 40 years

Tests with Hänchen test actuators

Simulation not only on the computer

Hydraulic core technology

"For the dynamic testing of material fatigue on aircraft, hydraulics is the core technology for load simulation. Because control, measurement and fluid technology work hand in hand with us," says Woithe. "The computers have to make specifications in real time, which are then controlled with the help of PLCs via control circuits with force measuring cells in a target-actual comparison. In addition to the control system, they also serve to prevent overloads. High-quality test actuators are a basic requirement in order to guarantee a realistic test sequence here. In 27 years together with Hänchen, we have repeatedly redefined the limits of what is possible and still achieved a good price-performance ratio with high adherence to schedules." The test actuators from Ostfildern were successful due to their strengths such as particularly low friction, optimum tightness, excellent response, low wear, extreme piston speed, low breakaway torque, fatigue strength and long service life.

Two and a half lives in 18 months

For safety reasons, more than two and a half aircraft lives are experimentally verified. In order to test the material fatigue, all flight phases are simulated: These are take-off and landing as well as all flight phases in which the Airbus is subjected to load changes, i.e. vertical and horizontal gusts as well as flight manoeuvres. In this way, even a long transatlantic flight in good weather conditions can be summarised in a simulation program of a quarter or half an hour. For the categories of short, medium and long-haul flights, a series of typical flights from standard flights to hard extreme flights were defined. They consist of load data for the airframe with the assignment to an altitude profile. This is because, depending on the simulated altitude, the cabin is placed under increased internal pressure via a compressor system and two air tanks in order to simulate the pressure difference between the cabin and the environment in accordance with the respective flight altitude. A flight-by-flight program sequence is created from these flight types, which comprises more than 1,000 flights. It is repeated until the specified number of total flights has been reached. Constant visual monitoring by inspectors, extensive inspections of the entire test structure lasting for days and periodic measurements of 3,600 strain gauges and 80 deformation transducers ensure that any damage that occurs is found immediately. Since the aircraft is designed to be damage-tolerant, cracks are observed in their development after they occur until the critical length is reached. A repair or parts replacement is then carried out. A sophisticated monitoring system ensures that unintentionally high loads, in particular, are not accidentally applied.

The Servofloat^® floating gap seal

The accuracy requirements make it necessary to avoid disturbing forces such as stick-slip effects of the cylinders. For example, very low restoring forces occur at the wing tips, but they must be moved at up to 670 mm/s at the same time. The wings are moved up to 2.9 m upwards and up to 1.2 m downwards from the zero position. However, disturbing vibrations of the soft structures can easily occur if the pistons and piston rods of the hydraulic cylinders are not as free-moving as possible. Here, tolerances of only 3 percent of the cylinder's nominal load are accepted; in practice, these are below 2 percent. Disturbing vibrations would lead to unwanted load deviations and falsify the test results. The hydraulic cylinders with the floating gap seal patented by Hänchen are therefore used in structural tests on aircraft because they always have the same friction, regardless of the pressure. In these cylinders, a steel bushing is deformed by a throttle gap, creating a non-contact sealing gap of just a few 1/100 mm. A prerequisite for this technology is a production accuracy in the range of a few μm, as otherwise the leakage would lead to high hydraulic losses. This cylinder series 320 in Servofloat^® design brings a cost advantage of around 30% compared to cylinders with hydrostatically mounted piston rod guide. Due to their very low friction, Hänchen test actuators offer the possibility of very high positioning and repeat accuracy, are stick-slip-free and are equally suitable for extremely slow and fast movements. However, the stability was also a decisive factor in the selection of the Hänchen hydraulic cylinders. After all, the test runs 24 hours a day, seven days a week.