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## AIR CORPS 

approach altitude the speed of the airship is reduced to 15 miles per hour plus the wind velocity. This is an excellent approach speed. 

(2) The pilot now wishes to check behavior of the airship at this speed. Controls are placed in neutral and if the trim is correct the airship will maintain constant altitude in a manner described in b above. If it does not, it is necessary to adjust further the trim to effect that result. The principle is exactly the same whether the air. ship is statically light or heavy. During remainde~ of the approach •controls are useu to overcome gusts or changes in static conditions, care being taken to observe principles of reverse control should the speed fall below reversing speed or should the airship be placed in danger by loss of static lift. 

g. When the airship arrives within 50 to.200 yards of the landing party it is brought to landing height. This depends on type of air› ship and length of handling guys used. Large nonrigids usually land about 60 feet off the ground, rigids at a much higher altitude, while the motorized observation balloon must be landed at an altitude of 25 feet or less. In this conn.ection it should be borne in mind that the lower a statically h~avy landing can be made the smaller the drop after aerodynamic control ceases. In that case,• also, care should be t aken to level the airship by use of elevators as it falls into the hands of the party, as otherwise the tail would be injured. 

h. The landing described above is the usual type of landing. The description is not at all complete since it omits nearly all the static principles involved. The other types of landings, such as turn land› ings, will not be discussed, since the dynamic principles involved therein are similar to those already explained. 

## SECTION VI AERODYNAMIC STRESS 

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38. Assumption as to condition of maximum stress.-a. For airships designed prior to the World War the air speeds were quite slow. The aerodynamic forces acting on these airships were conse- 

