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National Advisory Committee for Aeronautics, Technical Notes - Investigation at Low Speed of the Longitudinal Stability Characteristics of a 60° Sweptback Tapered Low Drag Wing

An investigation was made in the Langley 300 MPH 7— by 10—foot
tunnel to determine at low speed the longitudinal stability character-
istics of a 60° swept—back, tapered, low—drag wing of aspect ratio
2.55. Several modifications were made to this wing in an attempt to
improve its longitudinal stability characteristics.

The results show undesirably large changes in the longitudinal
stability characteristics of the 60° swept-back wing. The most
effective modification consisted in an alteration to the plan form
of the wing by extending the leading edge forward about half a chord
length over the outer 25 percent of the span. The maximum.lift coefL
ficient of the swept- -back wing was about the same as that of the
unswept wing, but the angle of attack for maximum lift of the. swept
wing was more than twice that of the straight wing. Decreasing the
aspect ratio i‘rom 2. 55 to 1 improved the longitudinal stability
characteristics of the wing, particularly in the range cf high lift
coefficient.

The results of testing the wing with a deflectable tip showed
little promise with regard to improvement of the longitudinal
stability characteristics, but deflecting the tip offered interesting
possibilities as a means of longitudinal and lateral control.

The problem of producing airplanes capable of flight speeds
equal to and greater than the speed of sound with a reasonable
expenditure of power has been studied by airplane designers for some
time. In order to solve this problem it is necessary to design an
airplane that does not exhibit a sharp drag rise near the epeod of
sound. Reference 1 proposes the use of highly swept wings as one
method of eliminating this sharp drag rise. The analysis of
reference 1 is based on the assumption that only the component of
the free—stream flow normal to the wing leading edge.affects the
pressure distribution over the wing, and thus the critical flight
Mach number will be increased by the ratiopof one over the cosine
of the angle of sweep. This analysis also indicates that the flow
affecting the forces and moments of the wing is subsonic so long as
the wing remains inside the Mach cone. Much information on the
stability and control of a swept wing to be used at. high Speeds can
therefore be obtaj.ned at relatively low speeds.