Material Selection
During this project a material selection was done on the flexures in order to ensure that with upgraded load capabilities the flexures would not fail. This was accomplished by calculating the aerodynamic forces which may act on different models and two-dimensional airfoil sections. The following equations were used in order to calculate these forces.
L = ½ * ρ * V^2 * S * Cl
L is the lift force acting on the model
Ρ is the density of the air in the wind tunnel
V is the velocity of airflow
S is the cross sectional area of the lifting surfaces
Cl is the lift coefficient for the lifting surfaces
The equation used for the maximum drag loading is the same as the lift only with the replacement of the lift coefficient with the drag coefficient. The maximum values possible are used in the above equations in order to determine the force being applied to the flexures and then to the load cells.
D = ½ * ρ * V^2 * S * Cd
D is the Drag force acting on the model
Cd is the drag coefficient for the entire model
The current material being used for the flexures is 17-4 PH precipitation hardening stainless steel. We found that the maximum force created by the lift would be approximately 1150 lbs. The yield strength of the current material is 42000 psi. With an average thickness of .375 inches in diameter the material will not yield even under the maximum loading. However, the current load cells are not designed to take the new lift loads that are applied because of increases in Cl due to advances in the aerospace industry.
Fmax = Yield Strength * Area
Area = PI * r ^ 2
Yield Strength is the material yielding point from testing
Area is the area of the flexures cross section
r is the radius of the flexure cross section
The next flexure problem to be considered is the sections where the flexures have been turned down to avoid transference of moments and side forces to the load cell. It has been determined that the minimum thickness of this section for the lifting load cell should be at least .0274 square inches.