Ground Vibration Tests and Flutter Analysis
Ground vibration tests (GVT) and flutter analysis are performed in conjunction with Leichtwerk AG (http://www.leichtwerk.de/eng), which continues the tradition started by Professor Dr. Norbert Niedbal and the DLR (German Aerospace Center) Goettingen, who have investigated over 400 aircraft models. Leichtwerk AG performs ground vibration tests internationally at the site of the aircraft manufacturer. Leichtwerk AG is approved as EASA Design Organisation for flutter analysis, which is of particular interest to manufacturers who plan to market their products in Europe.
Currently, Leichtwerk AG has the capability of testing airplanes (single or multiengine, gliders, piston engines and turboprops). The method used is the phase resonance method, which has an excellent track record of identifying flutter cases.
To satisfy 14CFR23.629 Flutter or, in the case of LSA, ASTM F2245-04, paragraph 4.6, each new design must demonstrate to be free of flutter to its maximum design speed Vd. A substantiation purely by analysis (based on a ground vibration test) or by flight test alone is not acceptable, because neither of the two methods can be regarded with full confidence by itself. All new aircraft and existing aircraft with certain modifications should be investigated for flutter. Modifications which can affect flutter are for example the addition of large masses outboard on the wing like tip tanks, control system changes, reduction in structural stiffness, installation of more powerful engines and increase in design speeds.
Before the start of a ground vibration test, the aircraft manufacturer must provide detailed information on the aircraft geometry, fuel tank volume and location, mass and CG’s of all components and design of the flight control system. Because some of the required data is more difficult to obtain from a finished airplane, it is best to document much of this during the construction process. Typically, a ground vibration test takes 3-5 days with two people for a small single engine airplane. The actual time will depend on the number of configurations and speed range to measure. A configuration is defined by the mass (large differences in wing mass due to fuel or ballast must be measured separately) and flap setting, for example. A large speed range means a higher frequency range will be investigated.
For the test setup, electromechanical exciters are attached to the airframe in different locations to get the vibrations started. Up to 64 accelerometers measure the response of the structure at different frequencies. The airframe modes and control surfaces modes (fixed and free) are measured separately. The test equipment records the results, which are used to calculate damping and critical speeds. All test equipment is provided by Leichtwerk AG.
The result will be a test report, showing plots of damping over airspeed for different frequencies. Based on these plots, it can be shown that a proper margin of damping exists at Vd and that there is no rapid reduction in damping as Vd is approached. These results are then validated by flight test. If low damping is forecast at speeds below Vd, Leichtwerk can make recommendations for design modifications (decrease of control surface mass or hinge moments or increase in structural stiffness). To minimize the chance of having to make design changes after the aircraft is finished, Leichtwerk AG also offers to perform preliminary numerical simulation of the vibrational characteristics. To cover production variations of flutter-relevant characteristics, tolerances can be given for example for inertia data of the control surfaces.
A ground vibration test is more reliable in identifying issues than pure analysis based on a structural model because it covers real-world issues like play in the control system, flexibility of mounting hard points etc.
In the US, Leichtwerk AG is represented by Sonja Englert. To obtain a list of required aircraft data for the GVT or schedule a ground vibration test in the US or countries other than Europe, contact Sonja Englert at 1-209-210-3174 (or 1-541-213-1349).