<DOC>
<DOCNO>
441
</DOCNO>
<TITLE>
evaluation of high angle-of-attack aerodynamic derivative
data and stall-flutter prediction techniques .
</TITLE>
<AUTHOR>
halfman,r.l., johnson,h.c. and haley,s.m.
</AUTHOR>
<BIBLIO>
naca tn.2533, 1951.
</BIBLIO>
<TEXT>
  the problem of stall flutter is approached in two ways .  first,
using the m.i.t.-naca airfoil oscillator, the aerodynamic reactions on
wings oscillating harmonically in pitch and translation in the stall
range have been measured, evaluated, and correlated where possible with
available published data, with the purpose of providing empirical information
where no aerodynamic theory exists .  the major effects of
reynolds number, airfoil shape, and reduced frequency on the aerodynamic
reactions have been reaffirmed .  no instances of negative damping were
observed in pure translatory motion and the ranges of negative damping
occurring in pure pitch had the same general trends noted by other
experimenters .  data on the time-average values in the stall range of
both lift and moment are presented for the first time .
  second, the results of numerous experimental observations of stall
flutter have been reviewed and the various known attempts at its prediction
have been examined, compared, and extended .  the sharp drop in
critical speed and change to a predominantly torsional oscillation
usually associated with the transition from classical to stall flutter
is apparently primarily but not entirely caused by the marked changes
in moment due to pitch .  fairly good stall-flutter predictions have
been reported only when adequate empirical data for this aerodynamic
reaction happened to be available for the desired airfoil shape,
reynolds number range, and reduced-frequency range .  a semiempirical
method of predicting the variations of moment in pitch with airfoil
shape, reduced frequency, initial angle of attack, and amplitude of
oscillation has been presented .
</TEXT>
</DOC>
