HUNSAKER— ENGINEERING ASPECTS. 253 



meters height may have different charges above and below which 

 may cause sparks and consequent explosion of the leaking hydrogen. 

 There have been explosions for which no explanation is adequate. 

 Should high metal parts such as valves have a wire to the car as a 

 ground, or should we use a valve cord of non-conducting material? 



i6. The addition of radio on an airship for signaling introduces 

 another complication. The radio uses the car as a counterpoise and 

 has a trailing wire as antenna. It is possible that sparking between 

 car and envelope may be induced when sending unless precautions 

 are taken. The nature of the necessary precautions is at present not 

 clearly understood. 



17. The structural strength of the envelope of a nonrigid dirigible 

 is not yet a definite engineering problem. As you know a torpedo- 

 shaped elongated envelope inflated with hydrogen carries by means 

 of a set of cables a car containing passengers and power plant. 

 The buoyancy of the envelope is distributed from end to end of the 

 envelope, but the weight is largely concentrated in the short car. 

 Hence there are serious bending moments impressed on the envelope 

 which is held stiff" only by its pressure of inflation. The well-known 

 theory of an elastic membrane can be used to compute the stress in 

 the envelope at any point due to the inflation pressure. However, 

 the stresses due to these bending moments must also be considered, 

 and at a high velocity the suction of the stream line motion of the 

 external air tends to augment the eff"ective inflation pressure at points 

 near the maximum cross section. 



18. In addition to stresses due to inflation pressure, bending 

 moments and external pressures and suctions, we have our problem 

 confused from an engineer's point of view by having to deal with 

 balloon fabric of indefinite elasticity and strength. 



19. The strength of the fabric in warp and filler may be 

 measured, but when we use a doubled fabric in which the threads 

 cross at 45° the strength becomes more difficult to estimate. 

 Furthermore, the envelope under load deforms and parts severely 

 stressed may shirk their load. The exact calculation of the stress 

 in an envelope is not attempted. 



20. There is, however, a simple experimental method of study- 

 ing the problem. A model of the envelope filled with water is 



