NPFC - MIL-HDBK-17 P2
PLASTICS FOR AEROSPACE VEHICLES - PART II TRANSPARENT GLAZING MATERIALS
|Publication Date:||8 June 1977|
This handbook has been prepared to provide information for the selection of transparent glazing materials and the factors to be considered in their installation and use in aircraft.
In the last decade, aircraft glazing configurations have become increasingly complex, to the extent of not only performing as structural members but also as protective coverings in a wide variety of situations. Structural loading in the flight profiles of high-performance aircraft involves not only differentials between cockpit pressurization and outer Aerodynamic air loading, but also transient thermal conditions which momentarily exceeds the deformation temperatures of some of the glazing materials.
Adding the protective features usually increases the transparency thickness and weight beyond that required for structural integrity. If a special coating is required, an additional transparent coating or sheet is often necessary to protect the active coating from damage. In situations where differences of thermal expansion coefficients are critical between two rigid materials, a separating elastic interlayer is added as a further complexity. With one exception, any of the additional protective features will act negatively toward light transmission and optical integrity.
The design engineer has to carefully evaluate the structural and protective requirements of the aircraft transparency, keeping in mind that the final design must meet certain optical requirements which can be very critical in areas used for landing or gun-sighting operations and less critical in general viewing areas.
This handbook contains information on materials currently being used or considered for aircraft glazing. The format of the document provides for continuous revisions which can be accomplished by adding to or removing information from under the various topical headings. Because of the many materials now available and the more complex design parameters, the data are organized in the progression the engineer would probably follow to finalize the transparent enclosure design.
The data on the mechanical, thermal, optical, and other properties of transparent plastics and glass have been selected from a number of specifications and reports. Sufficient test data were not available on all materials for establishing design allowables. Therefore, some material property data are only representative values and should be considered as such. Because most configurations are complex, the designer should prepare test specimens of the final design configuration and conduct confirmation tests of all critical design factors.
The format starts with available materials and sources available for transparent enclosures and includes both glazing and supplementary materials for the design and manufacture of composite constructions. Design considerations include allowable strength values and a discussion of some of the more critical properties that influence design. The majority of the technical data is presented in Chapter 4, which treats each type of monolithic transparent sheet as a separate entity and is grouped in two parts, with military specified materials divided from those which are being considered for aircraft use.
Supplementary materials follow, headed by discussions of transparent interlayer and coating properties. The opaque cloth laminates and adhesives used in edge attachments are the final material properties needed for all-plastic configurations.
Glass data becomes a separate topic because of the great difference in properties from those of plastic. Thermal and chemical tempering are mentioned in the process of explaining monolithic properties. The inherent brittleness of glass and its relatively low tensile strength usually relegate it to a sandwich construction with a flexible interlayer. A few designs have incorporated glass-plastic configurations.
Edge attachment designs have become more and more complex as the demands on enclosure requirements have increased. Because most edge attachments are developed for a particular aircraft, it becomes impractical to specify a certain design for a given set of parameters. Discussions ensue on problematical areas and are supplemented with sketches of edge attachments which have been used or proposed as a guide.
The final chapter explains environmental factors which, for the most part, are transient but can cause concern or actual physical damage if they are not compensated for in the design of the transparency.
Where available, applicable references will be found in the Bibliography; they are listed in the sequence in which they are mentioned or apply. Some of the references are considered valuable background material and are included in the bibliography, although their content may not be specifically referenced in the text.
The mention of any trade name or proprietary product in this handbook does not constitute an endorsement by a Government agency, nor does it indicate that the products will necessarily meet Government specifications.