The primary issue in the Russian certification of the Gulfstream IV-SP was the ability of the jet’s landing gear to withstand the rough runways which are common in that country.
Conventional methods of validating this capability, developing custom dynamics software or physical testing, would have taken at least six to twelve months. It took only three months to simulate take-offs, landings, and taxiing on rough runways using LMS DADS.
The Right Product
Gulfstream selected DADS to simulate and animate the landing gear systems. The software allows the development of mechanisms without simplifying assumptions that reduce the accuracy of specialized landing gear code. DADS can also incorporate flexibility of components, and closed and open loop systems needed to simulate the performance of the oleo-pneumatic strut.
The Design
The Gulfstream IV-SP’s landing gear consists of an oleo-pneumatic strut. Two chambers, one filled with oil and the other with air, are connected by a bulkhead with a small orifice. The gear is arranged so that when it contacts the ground, oil is forced through the hole into the chamber where it compresses the air. The resistance of the oil generates a damping force that is proportional to the square of the strut’s velocity. The compression of the air by the oil generates another force that helps to keep the gear extended while the plane is on the ground. The oil plays the same role as an automobile shock absorber while the air is comparable to the springs.
The Simulation
Computer simulation of a landing gear is complex because of the geometric nonlinearities that arise when the geometry changes with strut translation during takeoff and landing. The aircraft’s main gear and nose gear were analyzed during landing and taxi simulations. The main landing gear model was constructed to simulate a drop test. This consisted of eight bodies representing the ground, aircraft, post, piston, cylinder, arm, inboard wheel, and outboard wheel. Eight joints connect the bodies together: bracket (aircraft to post), revolute (post to arm), revolute (arm to inboard wheel), revolute (arm to outboard wheel), spherical (arm to piston), translational (ground to aircraft), translational (cylinder to piston), and universal (post to cylinder). This combination of bodies and restraints gives the system four relative degrees of freedom. One is the vertical translation of the aircraft, another is the motion of the arm and piston relative to the post and cylinder, and the other two are the rotation of the wheels. The oleo-pneumatic characteristics of the strut were modeled using DADS control elements. The control system measures the magnitude and velocity of the strut displacement, calculates oil velocity through the orifice and pressure drop across it, and determines the force due to the oil flow. The ideal gas law with heat flow involved is used to determine the force from the air pressure. The total strut force is the summation of the force due to oil flow, the force due to air pressure, and a constant force due to seal friction. This model was validated by plotting simulation results against physical measurements generated during drop tests a good correlation was found. The next step was enhancing the initial model to include flexibility of key components.
The flexible body in DADS was created by importing a set of mode shapes from an MSC/Nastran finite element analysis of the part. The flexibility of the main structural post and trailing arm in the main landing gear were included. Each modal coordinate value represents the contribution of that particular mode shape to the overall part deformation. Drop test simulations with the flexible bodies added to the model showed excellent correlation with physical measurements. The next step was to add aerodynamic loads, variable thrust, and braking. The resulting model could handle realistic landing, takeoff, and taxiing scenarios. Roughness data from representative Russian airports were reduced to power spectral density functions that described the runway as a function of roughness frequency. The simulations were re-run using this data as input to the model.
The Results
Engineers examined the results to determine response of the structure and critical components under both maximum design and repeated load conditions. The simulation results showed that the aircraft could manage all but the roughest runways without ill effects. By using DADS the need for physical testing was reduced saving Gulfstream thousands of dollars. Due to simulation, the Russian certification process proceeded more quickly than expected, shortening the time for Gulfstream to penetrate the rapidly growing Russian marketing by three to six months.
