Ground Roll (Taxi) and Flight Tests
A special experimental aircraft is equipped with the capability of making realtime measurements of the amount of fuel, airspeed, Mach number, altitude, load factors, and control surface deflections. The aircraft is also instrumented as in the ground – vibration tests with strain gages and accelerometers. Special actuators are included to operate the ailerons and elevators over a range of frequencies. A sweep of actuation frequencies is first conducted to identify important modes. In addition, at certain frequencies, responses to step and impulse commands are measured.
Ground Roll (Taxi) Measurements. Aircraft ground roll (taxi) provides the first insight into an airplane’s aeroelastic response. The relatively rough runway excites the airplane’s structural modes. In addition, here we conduct a sweep of frequencies and FFT-PSD analyses of measurements and determine whether results match with analysis predictions of aeroelastic behavior at near zero speeds. If they do not, we must stop the test to correct and/or adjust the computational model until agreement is found, and then flutter predictions are reevaluated. Only when results do agree, may we then proceed to take off.
Flight Tests. In this step, we take off and fly at the lowest speed at low altitude and in level flight. We measure the airplane’s response to air turbulence and conduct FFT and PSD analyses. We determine whether these results match our analytical predictions for the tested speed. If they do not, we must again stop the test to correct and/or adjust the computational model until we find agreement. When they do agree, we then may proceed to activate the actuators for an impulse command. Next, we determine whether there is sufficient damping. If there is, we conduct a sweep of frequencies and conduct FFT and PSD analyses of frequencies and damping. If these results match the analysis, then we may (cautiously!) activate actuators at the calculated flutter frequency and conduct FFT and PSD analyses of the response. When the damping measurements match theoretical predictions, then we activate actuators for a step command. Next, we determine whether the response matches the analysis. When it does, we then collect and store data in the form of U-g-g’ diagrams. Only if and when there is reasonable agreement with analyses we proceed cautiously to perform maneuvers at various load factors at the same speed and altitude. During each maneuver, we activate the actuators for an impulse command to see whether there is sufficient damping. If there is, we move on to increase the load factor until the complete set of specified load factors within the flight envelope is tested.
If our computed predictions are in agreement with the results obtained at any stage, only then is it safe to go to a higher speed (e. g., 25 knots faster) at the same altitude. At this point we repeat all of the steps, collecting and storing data in the form of U-g-g’ diagrams. We systematically and cautiously increase the speed up to its maximum, checking at every increment to ensure that our analysis is valid. Similarly, we systematically increase altitude to its maximum and repeat the regimen. We stop (i. e., reduce speed to the previous safe speed) immediately whenever any one of the following happens:
1. A modal damping coefficient g decreases below the level of damping required by regulations (5% in a civil aircraft).
2. Oscillations in at least one measurement diverge and grow beyond preapproved limits.
3. The dominant frequency deviates from its predicted value.
Thus, it is observed that the analysis of airplane flutter is strongly based on theoretical studies. The theory is the work tool for analysis and decisions about critical configurations and flight conditions. Ground vibration experiments are used to tune the structural dynamics analysis to yield accurate structural modes, and wind tunnel experiments to tune the unsteady aerodynamics code. Flutter flight tests are extremely dangerous. Real-time measurements and various actuation techniques are used to estimate the damping of the airplane at various flight altitudes, speeds, and load factors and move from one point to another with much caution. Analysis, ground experiments and flight tests always go together to provide full clearance for flight without flutter problems.