Low-speed tests of a model simulating the phenomenon of control-surface buzz Page: 3 of 18
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NACA RM L50F19
pressure impulses has been used as the .basis for determining the time
lag in some empirical theories of control-surface buzz.
An example of unstable single-degree-of-freedom oscillations
involving flow separation and occurring at low speeds has been cited by
Goethert in reference 2. In this case, an airfoil with a sharp leading
edge was found to be statically stable in pitch about an axis of 25 to
30 percent of the chord. When this airfoil was free to rotate, oscil-
lations occurred. In an oscillation of this kind, the lag in develop-
ment of aerodynamic hinge moments is attributed to flow separation,
inasmuch as the transmission of pressure impulses at the low speeds
involved is practically instantaneous.
The oscillations of the airfoils at low airspeed involving flow
separation were used in reference 2 to indicate the possible importance
of flow separation in the phenomenon of aileron buzz at transonic speeds.
A more direct indication of the effects of flow separation would be
obtained, however, if the oscillating system more closely resembled a
control surface. A brief discussion of transonic buzz is now presented
to show how this phenomenon might be simulated at low airspeed.
The flow over an airfoil at Mach numbers slightly above the criti-
cal Mach number is characterized by supersonic regions on the upper and
lower surfaces. These supersonic regions are terminated by shock waves.
Shadowgraph pictures of these shock waves and their action during
aileron buzz are given in reference 3. In the examples shown in refer-
ence 3, the shock waves were located at about 60 to 70 percent of the
chord in the Mach number range where aileron buzz occurred. The shock
waves cause separation of the boundary layer. Oscillation of the
aileron during buzz causes a chordwise oscillation of the shock waves
and, presumably, a corresponding variation in intensity of the shock
waves. Thus, a downward deflection of the flap creates a larger super-
sonic region on the upper surface and a more intense shock wave. This
more intense shock wave would be expected to increase the flow separa-
tion occurring on the upper surface, while a corresponding decrease in -
separation would occur on the lower surface. The changes . in separation
in conjunction with the flap motion are believed to be responsible for
the occurrence of buzz.
It was thought that a simulation of the effects of these shock
waves at low airspeed could be obtained by the use of spoilers. Chord-
wise motion of the spoilers to simulate the shock-wave motion did not
appear to be feasible, but changes in spoiler projection to simulate
changes in shock intensity could be readily obtained. This spoiler
motion was produced by attaching spoilers to a flap so that, for
example, downward deflection.of the flap resulted in an upward projec-
tion of the spoiler; this spoiler projection simulated a shock wave of
increased intensity on the upper surface.
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Phillips, William H. & Adams, James J. Low-speed tests of a model simulating the phenomenon of control-surface buzz, report, August 16, 1950; (digital.library.unt.edu/ark:/67531/metadc59818/m1/3/: accessed November 20, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.