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National Advisory Committee for Aeronautics, Technical Notes - Preliminary Report on the Flat Top Lift Curve as a Factor in Control at Low Speed

This report, which is the first of a general airplane safe—
ty program, is concerned with the importance of the flat-top
lift curve as a factor contributing to safety and control at
low speed. An analysis of existing airfoil data indicated def~
inite relations between the shape of the lift curve and certain
section dimensions. A section1(NACA 84), designed according
to these empirical relations, Was tested and found to have the
desired flatntop lift curve combined, however, with low aerody—
namic efficiency and high moment coefficients. The shape of the
lift curve at maximum lift appears to be of sufficient impor—
tance to justify additional investigation with the view of de~
veloping a section having satisfactory efficiency and moment
characteristics.

The most critical feature of airplane flight to—day is the
making of a safe and comfortable contact with ground or water
on landing. The skill required in landing under all possible
conditions must be reduced if safety in civil aviation is to be
assured. Surprisingly little work has been done to date on
this vital matter.

The chief factor to be considered in the problem is the
nature of the air reactions upon;the airplane as it assumes a
' landing attitude on approaching the ground. The most important
aerodynamic item is the manner in which the lift varies with
changes in.angle of attack of the wing system at low speeds or
large angles of attack. Safety demands that there be no abrupt
changes in lift at these angles. In other words, it is highly
desirable that the lift curve should have a flat top instead of
the relatively sharp peak generally characteristic of airfoils
in use at present.

This requirement calls for a study of the aerodynamics of
airfoils at large angles of attack. During the past two years
such a study has been made at the Langley Memorial_Aeronautical
Laboratory as a part of a general airplane safety_program under—
taken by the staff of the atmospheric wind tunnel.