TM 1-320  9-10  AIRSHIP AERODYNAMICS  9. Coeftioient of skin friction.-a. In the case of a flat plate at  right angles to the air stream the resistance is almost entirely due to  the pressure difference in front of and behind the _plate.  This is not  however the case with most solids.  In  general, resistance may be  divided into two parts:  b.  When a solid passes through the air it carries along with it a very  thin layer of air, the exterior surface of which forms a plane of air  cleavage. The resistance of the air particles  to  shear on this plane is  called skin friction.  c.  The value of the skin friction on an airship hull, as determined  empirically by Zahm and others, is given by the formula:  ( 1) Pressure difference.  ( 2) Skin friction.  R ,=0.0035pS 0 • $^{98 }$vue  R r= 0.00309pS  vu 5  23  b.  Since the volume varies as the cube of a linear dimension, while  the cross-sectional area and surface area both vary only as the square,  The pressure-difference resistance is least for a very long and slender  form.  In fact, the greater the fineness ratio, the less will be the pres› sure-difference resistance.  An increase in fineness ratio, however, leads  to an increase in surface area and so  to an increase in skin friction.  It  is necessary therefore to compromise on a moderate fineness ratio, as  a very long and slender form would have so high a skin friction as to  more than counterbalance the gain by reduction of the pressure› difference resistance.  A fl_neness ratio of 4 to 1 is very good for a small  nonrigid, but for large rigids it has been found advisable to increase  this ratio to 6 or 7 to 1.  Recently an airship had been designed whose  hull has a much smaller fineness ratio than the conventional designs.  This airship has a capacity of  200,000 cubic feet and a fineness ratio of  2.82, noticeably shorter than any ships recently constructed.  A model  of this ship was tested in the wind tunnel of the Washington Navy  Yard and was found  to  have the lowest resistance coefficient -of any  model ever tested there.  where  S  is the total surface area.  A somewhat more convenient for› mula is-- 10. Resistance of streamlined body.-a. As mentioned before,  the total resistance is composed of resistanc:A-e- (1) Caused by pressure difference.  (2) Due to skin friction. 