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16. Deceleration test.-a. Tests are 1nade frequently on full› sized airships to determine actual risistance of the airship at various speeds. In these tests the airship is brought to a certain velocity and then the motors are idled, the velocity being recorded against time as the airship decelerates. 

b. The general theory is that the resistance, or force causing de› celeration, is given by the equation : 

R=Mv a, where 

ex.= (deceleration in feet per second) z. 

Mv= the virtual mass of Lhe ship. 

The virtual mass of an airship is the mass of airship and contents plus the mass of air which is carried along with it. This latter is computed by the Munk formula : 

AMo=P1 , where r is the radius of largest cross section. 

c. Observing velocity at end of each second gives the rate of change of velocity, or deceleration, for each second and by interpola› tion for each air speed. Actually formulas are employed which in- . volve calculus and are beyond the scope of this manual. 

d. These deceleration .tests are quite valuable as a check against the resistance formulas developed in this section. They are however often complicated by poor instruments or faulty observation, render› ing it difficult to place a proper value on results so obtained. For the present more confidence is to be placed on the resistance formulas and the power requirement formulas which will be developed in the next section. 

