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National Advisory Committee for Aeronautics, Technical Notes - Tests of Airfoils Designed to Delay the Compressibility Burble

Development of airfoil sections,suitable for high-
speed applications has generally been difficult because‘ '
little was known'of the:flow phenomenon that occurs at
high Speeds. .A definite critical speed has been found T -~»
at.which serious detrimental flow changes occur that ~
lead to serious losses in lift and-large increases in ”
drag. This flow_phenomenon, called,the compressibility '
burble, was originally a propeller problemsbut, with the -
development of high-speed aircraft; serious consideration "y—
has to. be given to other parts of the- airplane. It is ~~~——w
important to realize, however, that the prepeller will ..l
continue to offer the most serious compressibility pi = _-‘~
problems for two reasons: first, because propeller— ‘ - ._
section speeds. are h—igher than the speed of. the airplane ' "s
and, second,- because structural requirements. lead_ to ‘~~~—
thick sections near the root.

Fundamental inveStigations of.high~speed air-flow
phenomena recently completed (references 1 to 5) have :
provided much_new information.‘ From practical considera—
tions an important conclusion of these investigations
has been the determination of the_critical speed, that
is, the speed at which the compressibility burble occurs.

The critical speed was shown to be the translational
velocity at which the sum of the translational velocity
and the maximum local induced velocity at the surface

of the airfoil‘or other body equals the local speed of
sound. Obviously, then, higher critical speeds can be
attained through the development of airfoils that have
minimum induced velocity for any given value_of the lift.

Presumably, the highest critical speed will be
attained by an airfoil that has uniform chordwise distri- —~
bution of induced velocity or, in other words, a_flat
pressure-distribution curve. All conventional airfoils
tend to have high negative pressures and correspondinglywa
high induced velocities near the nose, which gradually '
taper off to the air-stream conditions at the rear of
the airfoil.”

If the same lift coefficient can be obtained
by decreasing the induced velocity near the nosé and - ‘
increasing the induced velocity over the rear portion of
the airfoil, the critical speed will be increased by an -
amount proportional to the decrease obtained in the
maximum induced velocity. The ideal airfoil for any
given high-speed application is, then, that shape which
at its operating lift coefficient has uniform chordwise
distribution of induced velocity. Accordingly, an ana-
lytical search for such airfoils has been conducted by
members of the staff of the Langley memorial Aeronautical _
Laboratory‘and these airfoils have been investigated -
experimentally in the Langley ah-inch high—speed tunnel.