Investigation of the dynamics of passenger airplane landing
Central Aerohydrodynamic Institute named after N.E. Zhukovsky, TsAGI, 1, Zhukovsky str., Zhukovsky, Moscow Region, 140180, Russia
The main purposes of the operation of the complex for endurance bench testing of the full-scale attack airplane structure, which provides an extension of the design service life of the aircraft fleet, are the following:
— experimental verification of the remaining service life of the aircraft’s airframe and landing gear structure;
— location of the elements and units, which are important from the point of view of fatigue conditions, within the structure of the airframe and landing gear during the recreation of the operational spectrum of the variable loads;
— determination of the duration of fatigue cracks evolution and residual strength of the damaged airframe structure;
— determination of durability after overhaul.
The endurance tests of the two-seat combat trainer attack airplane were carried out according to these goals wherever possible. Airframe fatigue tests were carried out for the following types of cyclic loads: A) in-flight loads; B) takeoff and landing loads; C) ground loads. Additionally the following fatigue tests were performed: rudder control system linkage tests; elevator control system linkage tests; aileron control system linkage tests; tests of undercarriage doors in closed position; tests of landing gear up locks; tests of the main and nose landing gear legs by applying the loads that emerge during their retraction and extension.
After the fatigue tests were finished, the residual strength of the airframe structure, which was damaged and contained fatigue cracks, was tested.
The tests were performed by using a full-scale serially manufactured two-seat combat trainer attack airplane. The following airframe components were subjected to simultaneous fatigue tests: wings with leading-edge slats and trailing-edge flaps; fuselage; horizontal tail; elevators; engine mount fittings; mount fitting of brake parachute lock. Horizontal loads were balanced by the loads, which were applied to the brake parachute lock and main undercarriage legs. Variable loading was carried out by a flight cycle program block, which corresponded to 100 flight hours or 90 flights. At the same time the fuel tanks of the wing and fuselage were stressed with excessive pressure. The program block consists of 5 flight cycle types, which differed from one another in terms of maximum and minimum overloads in maneuver configuration. Each flight cycle included two different loading modes: in-flight configuration (FC) mode and maneuvering configuration (MC) mode. MC mode loadings were carried out with the extended high-lift devices of the wing (δslats=9º, δflaps=20º). Retraction and extension of the high-lift devices were performed after the appropriate overloads of ny=2,5 were attained. In FC mode the high-lift devices were retracted and loaded as an integral part of the wing.
A specialized test site was created to properly perform all of the required endurance bench tests.
The testing facility included the following main systems and components:
— airframe loading system;
— fuel tanks pressurization system;
— oil pump station and hydraulic system;
— automated system of multi-channel electro-hydraulic loading;
— information and measurement instrumentation system.
Strain-gauge sensor bridges were mounted at the right half of the wing to control the variable loadings during the fatigue tests. The sensors were placed according to the same layout as the one, which was used during the measurements of the real in-flight loads. The sensor bridges were calibrated during the application of the step-stress loads. After that their readings were recorded during the application of cyclic loadings. At the same time the dynamometer readings were measured and used to calculate the bending moments within the wing sections where the sensor bridges were mounted. The comparison of the sensor bridges readings and calculated bending moments allowed the research team to estimate the amount of the error.
The tests of the airframe structure residual strength were initiated due to multiple structural damages. During these tests the strain-gauge sensors were mounted on the undamaged ribs in the zones with fatigue crackings. These sensors were used to trace the step-by-step sequence of fractures of the fitting attachment during the residual strength tests.
The endurance tests have allowed the research team to increase the durability of the transverse joint of the central wing box lower panels and the detachable wing half considerably. In particular the following measures should be taken:
— the tests should be carried out with certain loosening (to ¼ of the turn) of the tightness of the joint bolts, which surround the fractured rib of the fitting attachment of the central wing box and detachable wing half;
— it is necessary to break corners around the flange holes with the improved surface smoothness, which are intended for fitting bolt mounting, to increase the durability;
— it is necessary to use the tension-control bolts in rib holes, which are intended for installation of covering hinge bracket.
The presented complex for fatigue and survivability tests of attack airplane structure has confirmed the possibility of the substantial increase of airframe and landing gear structure life on the condition that the airframes of the operated airplanes are subjected to the appropriate modifications.
Keywords:дефектоскопия, дифрактометр, нагружающее устройство, напряжение, объект испытаний, переменное нагружение, ресурсный стенд, тензометрия, усталость, фрактография
- Scherban K. S. – «Endurance tests of full–scale aircraft structure», Moscow, Fizmatlit, 2009, 236 p.