In the future, aircraft will have to fly in a much more environmentally friendly manner in order to contribute to meeting the targets for limiting global warming. With the aim of testing new innovative engines, DLR has added a new research aircraft, a Do228-202, to its fleet. On this aircraft, electric engines powered by a hydrogen fuel cell are to be tested.
The new research aircraft has already transported many passengers and flown many miles for airlines over the course of its life. This has however been achieved with two conventional combustion engines. In its second life as a DLR test vehicle for electric flight called "Electric Flight Demonstrator", the aircraft is to take off for research with new, innovative propulsion systems and fuels.
For the conversion of one engine, the aircraft had to be heavily modified. Nevertheless, the following applies: Safety first! This is where aeroelasticity comes into play: Before the aircraft can start its new service, the researchers at the Institute for Aeroelasticity carried out what is known as a Ground Vibration Test (GVT) on the aircraft, which still had conventional engines. In order to achieve this, the aircraft was excited dynamically with electromagnetic shakers.
After all, anyone who has ever taxied down the runway in an airplane or flown through minor or major turbulence has certainly seen the wing tips vibrate or even felt first-handthe vibration in their bodies. This is completely normal and is due to the lightweight construction of aircraft. Nevertheless, before a new aircraft model or an aircraft that has been heavily modified, such as DLR's new research aircraft, is allowed to take off, this vibration behavior must be checked and evaluated on the ground before the first flight.
In order to be able to measure the vibrations on the ground, the specialists at the Institute for aeroelasticity instrument the aircraft with numerous acceleration sensors. Distributed across the aircraft, the sensors record the motion on the wings, fuselage, empennage and propulsion system.
In order for the aircraft to vibrate sufficiently on the ground, it is artificially excited sequentially on various components, such as the engines, wings, control surfaces, etc., with vibration exciters, also known as shakers. The principle of such a shaker is similar to that of a loudspeaker, except that instead of generating sound waves and transmitting them through the air, mechanical vibrations are coupled via a push-pull rod that is attached to the aircraft.
The vibrations caused by the excitation are then sent as electrical signals from the sensors to a measurement system via many cables and finally displayed and evaluated on a PC using special software (developed at DLR).
With the large amount of data measured during the ground vibration test in the form of natural frequencies, damping ratios and mode shapes, researchers are able to update the simulation models (finite element method, FEM for short) with the reality of the actual aircraft. With this validated simulation model, the vibration behavior of the aircraft in flight can be simulated as well as planned changes for flying with electric engines. The new concept can therefore be checked and validated early in the development phase.
Author: Julian Sinske, DLR Institute of Aeroelasticity, Department: Structural Dynamics and System Identification