While the Descent-Vanes primary function is to arrest an unpowered descent, in forward flight its' function is that of an ordinary wing. However, historically, only a handfull of wings have been identified as having the necessary slow & stable vertical descent characteristics. These include the 1896 Montgomery Glider (Left, top) and the 1933 Pou du Ciel,  (left, middle) both of which were tandem-winged. Besides its descent-arresting ability, the tandem planform provides greater stability and eliminates the yaw-induced "bobble" that can occur when torque-imbalances occur in the flying platforms' rotor. This "bobble" can be seen in this video (bottom left) of a single-descent-vaned test model. At about 1/3rd into the flight the models' tail momentarily interrupts the radio control signal, cutting-off the motor, but the model quickly regains  its equilibrium.