The aircraft had two-axis flight controls. With a standard control stick fore-and-aft movement controlling the front wing’s angle of attack, increasing and decreasing the lift of the wing. Because the front wing was located forward of the center of gravity, this allowed the nose to pitch up and down. Side-to-side movement of the stick controlled the large rudder. This produced a rolling motion because the wings both had a substantial dihedral, through yaw-roll coupling. The rudder had to be quite large not only to produce adequate roll but also because the fuselage was very short, reducing the leverage of the rudder.
Because the Flying Flea was a two axis aircraft, it could not be taken off or landed in substantial crosswinds. This was not a big issue because at that time aircraft were usually flown from large open fields allowing all take offs and landings into the wind! Mignet claimed that anyone who could build a packing crate and drive a car could fly a Flying Flea!
Despite the initial popularity of the design, thanks in no small part to the passionate enthusiasm of Mignet himself, the original HM.14 revealed design flaws that could lead to an unrecoverable and often fatal dive under certain conditions. When the front wing was put in a high angle of attack for climbing, the flow of air deflected by the front wing radically increased the flow of air over the upper surface of the rear wing greatly increasing the rear wing’s lift and causing the nose of the aircraft to drop. Instinctively the pilot would pull back more on the control stick which aggravated the situation.
By extensive wind tunnel testing this condition was corrected and corrections made to the design. Unfortunately, the wave of bad publicity created by the crashes dogged Mignet for the rest of his life and continues to be associated with the design today despite the fact that the basic Mignet configuration has proven to be safe in hundreds of successful homebuilt aircraft and factory built microlights.
Source: Excerpts from en.wikipedia.org