FLUTTER TESTING Flutter flight testing is extremely hazardous and requires strict adherence to safety procedures. A proper build up approach with good modeling and simulation and a solid preliminary ground and laboratory tests analysis is essential. The following chapter will explain the essential steps in aircraft flutter testing.
2 Preliminary Ground Tests Because mathematical modeling of the aerodynamics and structural properties of an aircraft are so subject to uncertainties, many ground tests are done with models and full scale articles prior to the beginning of flight testing. The results of such tests are used to improve the computer models and refine the estimates used in planning the flight test.
2.1 Wind Tunnel Tests An important preliminary test conducted prior to flutter flight test of an aircraft is a wind tunnel flutter program. In the wind tunnel, an elastically and mass distribution-scaled model of the aircraft (Figure 1) is subjected to an airstream that has also been scaled to the same dimension (i.e., reduced frequency number) as the aircraft. Only a partial model (e.g., tail surfaces removed) may be used in an effort to isolate effects. The scale model of the entire aircraft may be flown either free or on bungees (Figure 2) at speeds corresponding to the envelope of the real aircraft, and the flutter mechanisms verified as much as possible. It is common for the scale airspeed (occasionally produced with a gas other than air) to be taken beyond the normal envelope to verify the flutter margin. Definitely checking for the critical mode by observing the flutter (hopefully without destroying the model) is also done. Testing and data reduction methods are much the same as for full scale flight tests. General tunnel turbulence, upwind oscillating vanes, and external inputs (tapping the surface with a rod) are common excitation means. Mass, stiffness and shape changes can be made relatively easily and the effects tested on the model before being incorporated in the actual design. The wind tunnel data are always subject to errors involved in accurately modeling the aircraft and the airflow. Corrections to the wind tunnel data for tunnel effects and compressibility are typically performed, to include blockage effects (velocity errors).
2.2 Ground Vibration Tests One of the typical essential preliminary tests conducted on an aircraft is the ground vibration test (GVT). This test may be performed at an early stage on the wind tunnel model discussed in the previous section to ensure that it adequately replicates the desired stiffnesses. A full-scale GVT is mandatory for new designs
and for any substantial changes to an existing aircraft. This test is used to verify and update the flutter model as well
as provide a means of identifying modes from frequencies found in flight test data. Details of the GVT are contained in the next chapter.
3 Flutter Flight Tests Flutter flight tests of aircraft are performed to verify the existence of the flutter margin of safety estimated by analysis. Flight near the estimated flutter speed entails a risk of accidentally encountering the flutter sooner than anticipated, with the potential for a catastrophic structural failure. For this reason, the flight envelope is cleared for flutter in an incremental fashion from a considerably lower subcritical airspeed. The frequency and damping of the vehicles response to an excitation are measured at successively increasing increments of airspeed. The frequencies permit an identification of the modes of the response by comparison with ground test and analytical data. If sufficient instrumentation is installed, actual mode shapes can be determined to supplement the frequencies in performing modal identification. This is done by comparing phase (Figure 3) and magnitude of responses from the transducers across the structure. For example, wingtips responses from opposite sides of the aircraft that are in phase suggest a symmetric mode while out-of-phase suggest and antisymmetric mode. Responses at the leading and trailing edge along the same chordline that are out of phase would suggest torsion. The damping provides a quantitative measure of how near each airspeed point is to flutter for each mode examined.
Figure 3 Diagram of Phase Relationship of Two Single-DOF Modes
The structural response is typically measured with accelerometers or strain gages (Chapter 11). Pressure transducers and velocity sensors have also been used. Anything that shows the dynamic response of the structure from which frequency and damping can be determined may be suitable. Strain gages, of course, cannot be used at extremities of surfaces where the strain goes to zero. Yet, they have the advantage of not measuring rigid body aircraft dynamics (pitch acceleration, for example) which the accelerometers will tend to show. Likewise, accelerometers cannot be used at the root or base of a surface where no acceleration occurs. A combination of accelerometers and strain gages has usually offered the best flutter instrumentation.
3.1 Excitation Several methods can be used to excite the aircraft structure to permit the individual response modes to be observed and their frequencies and dampings obtained. Some methods are more suitable than others for certain frequency ranges, nature of the modes sought to be excited, and amplitude or amount of energy to be put into the modes. Experience will also help in selecting the best method to use. Whatever the method employed, it is important that the excitation be applied as close to the surface of interest as possible. For example, an impulse at the tip of the wing may not have much chance of sufficiently exciting a high frequency horizontal tail mode for adequate analysis. The position of the excitation source relative to the node lines of the significant modes is also critical. Applying an impulse directly to the wing elastic axis has no chance of producing any torsional displacement. However, an impulse off of the elastic axis will produce both torsional and bending excitations. An impulse close to the node line will not produce as much displacement as the same amplitude of impulse farther from the node line by virtue of the longer moment arm.
3.1.1 Pulse and Raps Pulses, or raps, are sudden control impulses made to produce sudden control surface movements that can excite up to 10-Hz modes very well, and occasionally
to 30 Hz not so well. The objective is to produce an impulse that closely approximates a step input since such an input ideally contains the highest frequency content possible. Therefore, the sharper the input, the better the data. Longitudinal stick raps, lateral stick raps, and rudder kicks are the most common pulses used. The choice of pulse direction depends upon the modes sought to be excited.
For pulse excitation, the pilot will stabilize the aircraft on condition and make the rap after a call like Ready, Ready, Now. The stick pulse can be made by striking the stick with the palm of the hand or with a mallet. For irreversible control systems above a certain level of input abruptness, the flight control system or control surface actuator may attenuate the input and give no additional frequency content. Only trial inputs at a benign flight condition will demonstrate the best input to expect. Aircraft with automatic flight controls can be programmed to make these pulses without pilot intervention.
Sometimes, a rapid stick movement, with the stick held in the usual manner, (a singlet) is sufficient excitation. More energy may be generated by a doublet, which is two such inputs in rapid succession in opposite directions. However, the singlet and doublet can only be done at a low frequency compatible with human limitations about 4 Hz for stick inputs and less for rudder pedal inputs. A control oscillation is a variation of the doublet but with some specified time between the two inputs. An example may be a 1-inch aft stick deflection, hold for 3 seconds, and a return to neutral. A time to produce the control input, a ramp-up time such as 1 second for the 1-inch stick deflection, may be specified to ensure that the mode is adequately excited. The oscillation is tailored to excite a specific mode, such as a 3-Hz fuselage first vertical bending mode for an elevator input.
The pilot may deflect the control surface and then fly hands-off to allow the aircraft oscillations to naturally damp out (stick-free pulse), or the pilot may arrest the stick as it returns to the neutral position (stick-fixed pulse), depending upon which technique yields the best response from the aircraft. A variation is to hold the stick as the rap is made. For a large aircraft a rap may not produce enough control deflection for sufficient excitation. In this case a rapid, full-deflection displacement of the control followed by a release is useful.
A problem with all such inputs is the tendency to excite aircraft rigid body modes with very dominant acceleration amplitudes. When close to the elastic mode, these can confound the modal analysis.
It may be desirable to make several raps and to average either the time histories or fast Fourier transformations ([FFTs] of the raps together to remove noise and other
corrupting influences. The principal deficiencies of the pulse technique are the non-selectivity of the frequencies to be excited and the generally poor energy content above about 10 Hz.
3.1.2 Pyrotechnic The so-called bonker or thruster is a very small pyrotechnic charge that is typically placed externally near the trailing edge of a control surface and detonated electrically. The tiny explosion produces a sharp pulse excitation that comes very close to simulating the ideal step input. A series of such charges may be placed along the surface trailing edge so that