254 SYMPOSIUM ON AERONAUTICS. 



suspended below a model of the car by a suspension of cords. The 

 ratio of densities of hydrogen and water in air is about 900 and it 

 can be shown that if the scale of a model be ]/,q and that if the model 

 is made of the same material, the stresses at corresponding points 

 are equal and the model as it deforms under load remains similar to 

 the full size envelope as it would deform under corresponding 

 loads. 



21. Finally we have to consider the dynamical problem of driv- 

 ing the dirigible through the air at high speeds. As is well known 

 to students of hydrodynamics, an elongated body tends to place itself 

 broadside to the stream. Dirigibles of good form are essentially 

 unstable and it is necessary to fit fins at the tail end. It is not prac- 

 ticable or necessary to fit very large horizontal fins since the center 

 of gravity is usually below the center of buoyancy and hence 

 affords a statical righting couple against pitching. This statical 

 righting moment is supposed to overcome the tendency of the en- 

 velope to deviate from the trajectory. However, as speed is in- 

 creased the upsetting moment increases as the square of the speed, 

 while the statical righting moment of weight remains constant. Con- 

 sequently, there is some critical speed at which the dirigible becomes 

 unstable or even unmanageable. 



