Design Criteria
Stress Analysis
Material Selection
Allowable Strength
Structural Joints
Pressure Systems
Electrical Equipment
Model Support Systems



Stress Analysis


Stress Analysis Overview

The stress analysis must include, but is not limited to, the following elements:

An analysis showing that all models, including mountings and emergency restraint systems, are statically and dynamically stable and free from divergence throughout the model test envelope (refer to Model Support Systems).

Aerodynamic derivatives used in the analysis, their source, and a discussion of the consideration given to effects of Reynolds number, Mach number, surface condition, etc.

Source and range of mass and inertia parameters, including cross-coupling terms such as Ixz and support-system stiffness coefficients.

Parametric variations of significant design variables; i.e., tension-to-weight ratio, center-of-gravity location, pulley locations, etc., to establish sensitivity.

Stresses or Loads

Allowable stresses are the lessor of the material ultimate stress divided by a safety factor of 4, or the material yield stress divided by a safety factor of 3.

The stress analysis is to show that allowable stresses or loads are not exceeded for the worst load case, including but not limited to:

Dynamic factors that could result from separated flows in wakes, on model surfaces or components, etc.

Thermal stresses due to factors such as cold or preheated air used in some propulsion tests

Stress concentration factors

Wind tunnel starting loads

Maximum operating loads

Forces and Moments

Each detailed analysis section should contain a sketch showing forces and moments acting on the part and a statement of:

Section properties
Type and heat treat condition of the material
Pertinent drawing number

General Equations

In all calculations, the general equations and their source must be given before substituting numerical values.

Air-Loaded Surfaces

Give shear and moment distribution diagrams resulting from worst-case pressure distribution.

Section Properties

Define section properties of the structural member for shear, axial load, bending, and torsion at an adequate number of stations to facilitate a check on the location of the designated critical sections.

Air Loads

All parts with lifting surfaces (such as vertical stabilizers, pylons, and struts) that are designed for operating only at zero angle of attack must be checked for air loads of ±2 degrees.

Static Test Instead of Stress Analysis

Static tests may be accepted in lieu of a stress analysis under the following conditions:

If the load on the component in question can be directly and continuously monitored, the stress tests will be carried to twice the predicted operating load, and measured deflections must not indicate a permanent deformation. These tests must be witnessed by facility personnel.

If the load on the component in question cannot be directly and continuously monitored (for example slats, ailerons, elevators, rudders, flaps), the static test must be carried to three times the predicted load without permanent set.

Following static testing, nondestructive inspection techniques are required to validate the structural integrity of the component.

Gauged Components with Stress Analysis

If the load on the component in question can be directly and continuously monitored, a safety factor of three (3) or greater, calculated using the allowable tensile stress (Ftu), is required in the stress analysis.

Reduced Requirements

If the model safety factors cannot be met (4.0 for ultimate and 3.0 for yield), contact the Test Manager to discuss the possibility of reducing (waiving) these requirements. Examples include tests of actual flight components, dynamically similar models, or aeroelastic models. Compensation for the safety factor reduction could include additional instrumentation, closely monitoring critical areas, provision of safety catches, or special proof loadings.

The provisions of this paragraph can only be implemented by waiver approval. See also Model Acceptance Criteria.

Previously Tested Components

Stress analysis must be submitted for all components to be tested in Ames facilities, even if they have been tested at Ames before. The customer must revise previous stress reports of previously tested models to incorporate any new worst-case loads for each component.