## B. ENVELOPE PRESSURE CONTROL SYSTEM 

(1) General Description 

The maintenance of a predetermined pressure differ› ential between the gas in the envelope of a non-rigid air› ship and the surrounding atmosphere is the first basic requirement to the successful operation of this type of airship. 

The functioning of the envelope in assuming the various flight stresses, the proper suspension of the car, the effic› ient operation of the controls, all depend upon a closely held pressure differential. 

It is the function of the pressure control system to maintain this pressure differential within a certain range and to do this with a minimum loss of lifting gas. This result is accomplished by inflating the envelope partly with gas and partly with air, the air being contained in a forward and an aft ballonet, and by regulating the inflation of the air by means of an air system,without changes to the amount of gas in the envelope. Under extreme conditions, outside of the range of the air system, gas can be released auto› matically by meatls of two gas valves to prevent the internal pressure of the envelope from rising above a safe value. 

The pressure control system is also used to adjust the trim of the ship by regulating the relative inflation of the two ballonets. 

