value known as the "reversing speed," control becomes reversed and pulling up the elevators causes the airship to descend, although it turns the nose upward. The reason for this is that at low speeds (for most types about 15 miles per hour) the air forces are entirely unimportant in comparison with the static restoring moment due to the weight when the airship is inclined. Then if the elevators are pulled up, the momentary effect is to turn the nose upward, but the axis will incline only at a very small angle before the static restoring moment becomes equal to the moment due to the force on the elevators, and the inclina› tion will then cease to increase. If this angle of inclination is held to a small enough value, the dynamic force on the nose will be less than the downward force on the elevators. There will then be an excess of downward force and the airship will be thrust downward as a whole. This reversing speed offers a reason for not making the static stability excessive, since reversing speed increases as the center of gravity is lowered and the resulting difficulty in control becomes more serious where the static stability is large. 

b. The phenomenon of reverse control is especially apparent if the airship is trimmed quite nose heavy. Then any attempt on the part of the pilot to lift the nose at slow speeds is resisted by the static moment. The decrease in the dynamic thrust downward on the nose will be less than the gain in the down ward force on the elevator and the airship as a whole will descend. 

c. The particular situation just described is one of the most serious into which the airship can be brought. It is of most frequent occur› rence when a nose heavy airship is being brought to a landing and due to loss• of superheat becomes statically heavy. The airship will descend as a result of this heaviness and, if the speed is below reversing speed, application of the elevators at that speed will simply cause more rapid descent. 

d. The only recourse of the pilot in this situation, unless his airship is equipped with reversing propellers, is to throw ballast or materially increase his speed beyond the reversing limit as he raises the elevators. •when the airship is quite near the ground there may not be sufficient altitude to execute the latter maneuver without striking the ground with the tail. If his airship is equipped with reversing propellers, the pilot can cause the airship to ascend while the nose is down by merely reversing the direction of propeller rotation. 

e. There is one other situation in which reverse control occurs. The maximum dynamic lift on the hull occurs at an angle of attack of 10° or 11° for most. types of airships. If an airship is flying with this angle of attack and the elevators are raised so as to increase the angle 

