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SUBCRITICAL AEROELASTIC DIVERGENCE OF A WING MODEL Aero Engineering Laboratory AOE 4154 Lab Instructor: Dr. Roger Simpson Lab TA: Scott Burger Lab Section: Friday 3:55 – 5:20 Date Performed: September 4, 2009 Student Name: Craig Sossi Student Number: 904512344 Honor Pledge: By submitting this document I pledge that I have neither given nor received unauthorized aid. Craig Sossi Page | 1
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The purpose of this experiment is to investigate the validity of divergence theory through the use of subcritical wind tunnel testing. A wing model and open jet wind tunnel are used to obtain data which can then be compiled into Divergence Southwell Graphs and divergence pressure can be determined. Two cases are run for this experiment so that the relation between spring stiffness and divergence pressure can be observed. INTRODUCTION Aeroelasticity is the study of the interaction between inertial, elastic and aerodynamic forces on structures. Since airplanes are not completely rigid, they deform while experiencing forces such as a lift and drag or sudden impulses such as turbulence. The structures deform and as a result may create larger forces or twisting forces on other parts of the aircraft. There are two major categories of aeroelasticity: steady, or static and dynamic. Static effects may include divergence or control reversal. Aeroelastic divergence occurs when a force is imposed on a structure and the structure deforms such that the experienced force is actually greater than the imposed force, and reversal occurs when the deformation in a particular lifting surface causes a control surface to act in the opposite direction from normal operation, such as an elevator deflecting up and causing the aircraft to pitch down. Dynamic aeroelastic effects may include such phenomena as flutter, or the coupling of structural vibration and aerodynamic effects to create periodic oscillations in flight. A basic wing structural calculation may only account for forces for one particular scenario in flight to determine the minimum required strength. A factor of safety would then be implemented and the structure deemed “safe.” In theory the structure will not fail, but if in flight the aeroelastic effects generate forces greater than what was calculated by the engineer, the structure will fail. This is why studying and accounting for aeroelastic effects is important for the structural design phase of an aircraft. This experiment investigates the static aeroelastic effects in what is known as the “subcritical” range of a wing model meaning that the actual divergence dynamic pressure of the model will be determined analytically. Since aeroelastic divergence can result in large forces, testing the model at its divergence dynamic pressure could result in a costly failure of the wing model. The use of a symmetrical wing and spring creates an elastic angle of attack which simulates the twisting of a real wing in flight.
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