Full Scale Fatigue Testing of military aircrafts at NAL

Full Scale fatigue testing at NAL

After successful completion of the test programme of Gnat Fighter Aircraft using full scale fatigue test rig developed by NAL, the Structural integrity Group(SIG) has taken the major task of developing a computerised full scale fatigue test rig under the project of development of fatigue test facilities. This has felt necessary in view of the practical difficulties experienced during the use of the existing six actuator full scale fatigue test rig. Sufficient clear heights for rigging of loading trees was an important consideration in the design of the new test rig. The floor area of the test rig was arrived at by examining the plan form dimensions of number of fighter air-crafts in IAF. It was decided that the test rig should be designed as a "self reacting frame work in order to avoid heavy foundation work. Another requirement was that there should be enough free entry space so that air frames of most of the fighter aircrafts can be easily moved into the test rig. The facility has been conceived Initially as a 24 actuator system in which 16 actuators are assigned to the wings, 5 actuators to the fuselage and 3 actuators to the empennage. The system will be enlarged subsequently to include more number of actuators . The test rig itself has been designed for a maximum operational load of 50,000 lbs with the maximum structural deflection restricted to 20 mm at design load
The SIG has taken up several FSFT activities for life extension of fighter aircrafts .
The recent life extension studies was done for Mig-21 Bis aircraft for IAF and all the loads experienced by the aircraft in the sevice conditions are simulated on the ground to extend its life from 2400 hrs of flying which is the designed life of MIG 21and now IAF can heave a sigh of relief - the life of the MiG-21 Bis has been enhanced. The entire fleet of the MiG-21 Bis, tests by National Aerospace Laboratories (NAL) have revealed, can now fly an additional 1,000 hours or effectively for another 10 years.
With the entire MiG-21 Bis fleet of 150 aircraft approaching its maximum life-span of 2,400 hours as per original certification by Russia, IAF went for the life-enhancement test at NAL.
It flew in a MiG-2The results come after NAL's successful completion of full-scale fatigue testing (FSFT) on the MiG-21 Bis airframe C-2090. With this a major IAF project on the total technical life enhancement (TTLE) of the MiG-21 Bis fleet has come to an end.
the aircraft had completed 2,400 hours and had no fatigue cracks. The question before IAF was - how much longer could the aircraft fly? IAF requisitioned NAL to extend MiG-21 Bis life from 2,400 hours to 4,000 hours - an additional 1,600 hours. The aircraft, however, experienced cracks and break-up after around 1,000 hours of flying.
"There was no question of further testing as the aircraft had reached its limits. But it became evident that its life could be enhanced by 1,000 flying hours," said NAL official Dr.P K Dash.
Dash further said: "What loads the aircraft experiences in flight in a whole year, we simulate on the ground in one day, checking for fatigue.
When fatigue shows up, you know that is the point up to which the aircraft can fly."The key issue in preserving structural integrity against fatigue failure, Dash said, was to get precise answers to where and when fatigue cracks would appeared in the airframe, which, if undetected in time could lead to catastrophic structural failure.