Formsurface cover-up not working

I have a program where i am making a helicopter move across the screen using the formsurface. I have a timer, and every tick it moves in the direction that i click using the arrows(verticalmove and horizontalmove). At the beginning of the timer sub, i print my helicopter on the screen the color of the background, then the points move over, then print it out again the color the helicopter should be. For some reason, this isn't working. It never gets "erased." Any help is appreciated!
[Dim helicopterpoints() As Point = {New Point(x, y), etc......}
formsurface.FillPolygon(Brushes.SkyBlue, helicopterpoints)
x += horizontalmove
y += verticalmove
formsurface.FillPolygon(Brushes.Black, helicopterpoints)]