Downloaded from https://www.everyspec.com WIL-STD-81OD 19 July 1983 ~ I-3.4.’i ture D?’oiil~. With a Standard Atmosphere Table the. corresponding ambient temperature’for each altitude used in the’mission analysis may be found. Corresponding stoic skin temperatures meY then be calculated from the ambient temperature when the aircraft velocity is known (see 1-2’.2.2”). Sometime? skin temperature data from instrumented flights is. available to provide a check for the calculated temperatures. For each pertinent,mission, a chart of expected skin temperature versus time is prepared. Figure 523,0-2 ie,an example of a temperature profile for a single mis9i0n.
The next step is to prepare a frequency distribution of ,storeskin temperatures for a standard day. One method of accomplishing this is to divide each mission” into a group of representative stabilized temperature levels (temPeritUre9 stabilized for a period of 3 minutes or more) and then determine the total mission duration for each level. ‘Thesemission temperature level durations ai’e then weighted’for the relative frequency of each mission (i.e., multiplied by the fraction of total mission operations time for the store that the individual mission type is used).
A composite distribution can now be generated by performing a sum of the weighted mission durations for each temperature level used. With a composite frequency distribution of temperature levels at hand, a composite mission ‘temperatureprofile may”be constructed by arranging the composite temperature level durations in segments ordered to simulate a typical mission or a few missions. Such a’standard composite mission temperature profile ig shown in’“figure523.0-3.
When the temperature profiles of the various missions are diesimilati,segments”may be separated into groups of similar characteristics and:the composite profile constructed to include a mission or mission phase requirements”for ,each of the groups. Total duration,. at each temperature level miet still reflect the distribution function calculated. The Standard composite missicn temperature profile must now be adjusted for climatic temperature variations since the standard atmosphere, based on the Ilstandardday., represents only an abstract climati! condition.
Temperaturea both higher and lower than ‘standard day” values are commonly met due to seasonal or geographical changes in mission operations. Preparation of temperature versus altitude information to cover all likely climatic mission situations would become too involved for practical consideration. Therefore, estimation of the overall temperature range to be encountered, and a fair judgment of the relative frequency of expected occurrence of temperatures within this range, is normally resorted to. Temperature extremes (frequency of occurrence,5% or less) may be omitted. A climatic atlas is helpful at this step. ,.,. .a .,, ,. 0 . ,. METHOD 523:0 19 July 1983 523.0-5