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National Advisory Committee for Aeronautics, Technical Notes - Notes on Longitudinal Stability and Balance

More or less complete studies of longitudinal stability
have now been made on five airplanes, - the $5743, D34, VE7, USAC-ll
and Martin Transport. In addition to these tests, numerous modi-
fications have been made in the design of the JN4H and the effect
on stability and balance investigated.

The tests "on the IDEA and on the JIMH as normally used are
described and discussed in Report No. ’10 of the National Advisory
Committee for Aeronautics, and the methods of making the tests and.
interpreting the results are also taken up in some detail in that
report. The conclusions drawn may, however, be summarized here.

The balance of an airplane. and its degree of nose-heavi-
ness or tail-heaviness at any speed and. throttle setting, can man—
ifestly be determined by measuring the force which the pilot must
apply at the anger and of the stick in steady flight. This force
can always be modified in either direction and in any desired. de-
gree by changing the stabilizer setting, and this means of correc-
tion should be chosen in preference to moving the 0.02. relative to
the wings unless there is special reason for adopting the latter
plan. The balance, as expressed by the force required on the stick,
is much affected by the weight and chord of the elevators. The
JN4H, for example, is very nose-heavy when normally rigged, but
this is not because the 0.0:. is too far forward. (it is, on the con-
trary, too far back) but because the static moment of the elevators
about the hinges, due to their own weight. is almorrally large,
and a large pull (about 9% lbs.) on the stick is required to hold
the flippers up, even “then there is no downward air load on them.
Machines with adjustable stabilizers can, of course, he made to bal-
ance at any speed by adjustment of the surface.

The stability with free controls can best be determined
hy measuring the force on the stick at a fixed throttle setting and.
a hunter of different air-speeds and plotting the forces as ordia
nstes (a pull on the stick being taken as positive) against the
air-speeds as abscissae. The necessary and sufficient condition
for stability with free controls at any speed is then that the curve
of forces shall have a negative slope at that speed. and the steeper
the negative slope the greater the stability.