If high-quality equipment for rescue purposes has to be transported to crisis regions, the challenges are all the greater if there is little space available for protection. This is the case with a transport aircraft for which SMJ Sondermaschinenbau GmbH has developed an ingenious fastening system. The object being transported is a helicopter, which has to be transported like raw eggs inside a cargo plane with the rotor blades removed. The interior dimensions of the aircraft are only a few centimetres larger than the external dimensions of the helicopter body. In order to protect the overall structure efficiently, a landing protection system for securing the helicopter was developed to series maturity in close partnership with the client. This depends on special devices that transfer the occurring loads into the helicopter structure. Extensive calculations were necessary to ensure the load transfer to the individual structural points on the main rotor head and the cabin floor. In addition to secure attachment, the problem of overloading the structure also had to be avoided. Weather-related turbulence, rapid changes of direction and maneuvers during take-offs and landings lead to considerable force effects. As helicopters are built to save a lot of weight and are correspondingly fragile, simply lashing them down with chains and tensioning straps is not enough. This is mainly because the forces acting on the body during flight can correspond to a load multiple of over 4.6 g, i.e. more than four times its own weight. Without the rotor blades alone, this amounts to slightly more than 3 tons. Perfect protection is required to prevent structural damage, which is very difficult to detect during an inspection after air transportation. In this particular case, SMJ Sondermaschinenbau GmbH cooperated very closely with ACE Stoßdämpfer GmbH.

Part of the damping systems are customized ACE TUBUS safety dampers. © SMJ Sondermaschinenbau GmbH and ACE Stoßdämpfer GmbH

Tailor-made TUBUS dampers as the key to safe transportation
An exact design was required to determine the precise structural damper types. This involved determining the necessary energy absorption and force-displacement characteristics. It was taken into account that the helicopter skids allow a certain amount of deflection due to their design and can therefore absorb some of the energy in the event of a hard landing. However, this ability is not sufficient to prevent damage in the event of a rough landing or transportation. For example, if a load of 10,000 N is applied vertically to the helicopter, the skids deform by 30 mm. The greater the load, the more serious the deformation. Based on these values, the two TUBUS safety dampers selected had to have a special force-displacement characteristic. Accordingly, the dampers had to adopt a specific design, as they also had to fit into the design of the overall SMJ damping system.

The TUBUS TC-S series used here is already available as standard in a wide range of sizes and offers energy absorption in the range from 450 to 12,725 Nm © ACE Shock absorber GmbH

In line with these requirements, ACE manufactured customer-specific special models. Because the individual safety dampers had to be tested before delivery as proof of the characteristic curve, various loads were applied during development. The TUBUS safety dampers specially modified for the application in the course of this process enable the additionally required energy absorption of 124 kN and thus prevent damage to the helicopter structure in the event of an extreme landing or other shocks during flight.

With diameters between 64 and 176 mm, the machine elements from ACE are easy to integrate into designs and, as in this case, can be modified to measure. © ACE Shock absorber GmbH

The TUBUS TC-S series, specially modified for this application, was originally developed for use in crane systems and meets the international industry standards of the Occupational Safety and Health Administration (OSHA) and the Crane Manufacturers Association of America, Inc. (CMAA). The TC-S models are characterized by the so-called dual concept of two damping structural bodies connected in a single machine element, which achieves the required spring rate for crane systems. ACE has adapted the diameter especially for the case described here and provided each customized TUBUS with an extra-long stroke of 146 mm. As a result, the first air loading operations were carried out to the complete satisfaction of everyone involved.

Further information on TUBUS structural dampers can be found on the ACE homepage

or in the video on YouTube.