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The unsteady lift of an oscillating airfoil encountering a sinusoidal streamwise gust is experimentally investigated. The sinusoidal streamwise gust is generated by a multiple-fan actively controlled wind tunnel. A two-dimensional airfoil with a NACA0015 profile oscillates in the wind tunnel with a pitch motion frequency of 
$f_v=1\ \textrm {Hz}$ while the sinusoidal streamwise gust has a different oscillating frequency of 
$f_{\beta }=1.7\ \textrm {Hz}$. The non-dimensional unsteady lift coefficients determined from surface pressure show the same trends as Greenberg's prediction. Through spectral analysis, the sum frequency 
$f_{sum}=f_{\beta }+f_v$ and the difference frequency 
$f_{diff}=|f_{\beta }-f_v|$, proposed by Greenberg, are firstly observed in the experiment. The experimental results are compared with Greenberg's theory in the case of a small amplitude of gust velocity variation 
$\sigma =0.2$. The results of all cases indicate that the experimental results agree generally well with Greenberg's prediction, while there is a small deviation.
