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National Advisory Committee for Aeronautics, Technical Notes - Free Flight Tunnel Investigation of Dynamic Longitudinal Stability as Influenced by the Static Stability Measured in Wind Tunnel Force Tests Under Conditions of Constant Thrust and Constant Power

An investigation has been conducted in the Langley free—flight
tunnel to determine the dynamic longitudinal stability as influenced by
the static stability measured in wind—tunnel force tests under conditions
of constant thrust and constant power. The amount of static stability
was varied in the flight tests by changing the power applied to a pusher
propeller with an offset thrust axis and by shifting the center of gravity
of the model. Force tests were made for the purpose of determiningthe
amount of static stability for all flight—test conditions.

The results of the investigation show that reductions in constant—
thrust static stability caused a decrease in the longitudinal steadiness
(the reaction of the airplane to disturbances at essentially constant
airspeed). When the constant—thrust static margin was reduced to zero,
the flight behavior became very poor. For a given value of constant—
thrust static margin, however, no reduction in longitudinal steadiness
was noticeable as the constant—power static margin was reduced. Even
with negative values of constant—power static margin good flight
behavior was obtained as long as the constant—thrust static margin was
adequate. These results are in agreement with previous studies which
indicated that the longitudinal steadiness of airplanes is affected to
a much greater extent by changes in constant—thrust static margin than
by changes in constant—power static margin.

In discussions of the power—on longitudinal stability of propeller—
driven airplanes confusion has sometimes resulted from the use of two
different methods of designating the amount of static stability of.an
airplane as determined from wind—tunnel force tests. As pointed out in
reference 1, these designations differ in that one refers to the static
stability measured at a constant—thrust condition which is interpreted
by the pilot in terms of control movement or control force required to
effect a given acceleration at a constant speed, whereas the other
refers to the static stability as measured at a constant—power condition
which is manifested as stick—force or stick—position stability.