AERO-Elastic tests
Dr.Upadhya is one of the few specialists in the country in the multi-disciplinary field of Aeroelasticity. His major contributions to Aerospace research and development and industry, many of them as first in the country are listed below :
(i) Transonic buffet Testing of Launch Vehicle Models at NAL, Bangalore .
Transonic buffet is a serious problem for launch vehicles with bulbous nose and strap-on boosters. The problem is not amenable for easy mathematical modelling because of the complex nature of the unsteady, aerodynamic flow involving shack-boundary layer interaction and flow separation. Hence the problem was studied by aeroelastic modelling and testing in wind tunnel scaled aeroelastic models simulating the external aerodynamic shape and structural dynamic characteristics were designed and built. The model was supported internally on specially designed flexible spring—moments on a wing tunnel sting so as to simulate free-free dynamic behaviour of the vehicle. The model was instrumental and calibrated to measure dynamic bending moment distribution in the vehicle in its vibration modes due to aerodynamic buffet excitation in the wind tunnel. The wind tunnel tests in the transonic regime in the NAL 1.2 m transonic tunnel yielded valuable data on the dynamic structure to buffet and adequacy of structural design. Such models and tests were done for the first time in India for ASLV by Dr.Upadhya which were later adapted by NAL for PSLV and GSLV buffet testing. VSSC was very appreciative of the effort.
Earlier to this, Dr.Upadhya was involved as a member of the tests at NAL in the SLV-3 body divergence and fin-flutter modelling and testing.
Aeroelastic Design studies on LCA
Aeroelastic interactions played a major role in the design of LCA as it employs many advanced technologies such as carbon fibre composites in its airframe, digital fly-by-wire flight control systems, unstable and dynamic configurations etc. which cause interaction between aerodynamic, structural dynamics and control systems dynamics affecting loads, flutter, FCS stability and response etc. Dr.Upadhya ked a team of engineers which analysed these problems in a very detailed manner and gave valuable inputs for design and certification.
Maneuver Loads for structural design
Manueuver loads computations were done synthesising steady aerodynamics inputs from CFD Wind tunnel test group, Finite element model and mass data. Effort of aeroelastic differentiation on the air loads differentiation was taken into account. Hundreds of load cases were guaranteed correspond to cover points of load envelopes and during flight conditions out of the a few critical ones, from design point of view were selected for using new selection procedure based on weighting function approach for detail design/optimisation. Finally, elaborate design slate on aeroelastic efficiency on aerodynamic derivation were generated for use by control law design groups.
Aeroelasticty tailoring of CFC wing skins.
Aeroelastic losses in control effectiveness is very significant in combat aircraft. Attempts to decrease the losses leads to weight penalty. However, with composite skins, an opportunity to tailor the number of layups in the chosen films directions in the different regions of wing was available and this was effectively utilised in the structural optimisation process, thus meeting the weight targets and control requirement simultaneously.\
Tests Analysis.
Analysis of structure control aerodynamic interactions became very important in LCA due to the reasons mentioned above. An integral ASE model was developed using component models from structural FE vibration analysis, unsteady aerodynamic Actuator dynamics, FCS dynamics etc. Detailed response calculations at the motion sensor locations were done in the frequency domain, determining structural mode participation in the response. Extensive parametric studies were conducted to get full envelopes of the gains and phase response. This data was used by the control law team for preliminary design of NFs. Such analysis was used for extrapolation or read across of data for tests to internal configurations.
Testing full scale flight for vibrations and structural control coupling evaluation.
Analytical prediction methods for these aero-servo elastic interactions have been developed by the team at ADA . However, they are not adequate for airworthiness certification. Specialised tests on the full aircraft are called for to asses the structure control coupling characteristics and take remedial action to suppress this coupling. Such specialised tests are Ground vibration Test and Structural coupling Tests for generation of relevant design data and alter for certification. These tests were successfully conducted on LCA by a team of ADA Scientists led by Dr.A.R.Upadhya.
The Ground Vibration Test (GVT) was conducted to evolve the modal vibration characteristics of the aircraft which formed the basis for all interactions with FCS and unsteady aerodynamics. This involved exciting the aircraft with electro-dynamic shakers located on wings, fuselage and fin, and acquisition and analysis of response data on the aircraft at nearly 250 locations using accelerometers. A state-of-the-art multi-input multi-output (MIMO) GVT system was used for this purpose. A novel technique of suspending the aircraft on rubber bunge cords from a rigid suspension rig to simulate free flight condition was used. Perhaps, for the first time in aircraft development, random excitation technique was used in testing. This saved valuable test-time and avoided fatigue damage. It also made possible extraction of all the modal data from a single test. The test results were extensively compared with analytical predictions of the team, showing good correlation.
The Structural Coupling Test (SCT) was conducted in two phases. The first phase(SCT1) involved ‘Aircraft Identification’ wherein the coupling of structural vibration response with the aircraft motion response as measured by the FCS sensors was characterised. This test generated data for the design of notch filters to be implemented in the Flight control Computer(FCC) to suppress undesirable response. After implementation of the notch filters in the FCC, the second phase of the test (SCT2) was conducted to verify that adequate gain and phase margins were represented at the structural mode frequencies. Both these tests involved excitation of the primary control surface actuators through the FCC using sweep sine excitation, and measurement of the frequency response characteristics of the aircraft motion sensor responses or the actuators demand signal from the FCC. In order to overcome a likely situation of poor signal to noise ratio in the actuator demand signal due to the presence of the notch filters, an innovative technique of profiling the excitation signal amplitude was used in the tests.
The highlights of the work are:
1. Conducted for the first time in the country perhaps only second or third in the world.
2. Tests done on a flight standard aircraft
3. Extensive and efficient coordination with several design groups involving airframe, FCS on-board and ground based systems.
4. Development of the computer controlled Ground Check-out System(GCS) for definition, execution and control of tests and on-line data analysis and display.
5. Ensuring safety of aircraft during tests using a specially developed Loads Monitoring System and FCS Health Monitoring
6. Extensive post-test data analysis and synthesis and correlation with computational predictions.
7. Provided all the data for the design of notch filters in the FCC
8. Confirmed adequacy of FCS margins.
The tests were also most essential from airworthiness point of view. The Expert Review Committee which reviewed the procedures and results specially complimented the team for their excellence of work in a difficult and challenging technical area. The efforts of the team has resulted not only in establishing facility for such large scale tests bug has generated expertise to asses vibration characteristics and structure control interactions for any complex flight vehicle with confidence. The techniques and facilities developed such as the Aircraft Suspension System, Random Excitation Technique, the Ground Checkout System and SCT techniques are innovative and unique and are perhaps the best in the world.
http://www.nal.res.in/oldhome/pages/arupadhyacontriai.htm
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