Quite apart from the improvement in the pilot's downward view, there were aerodynamic gains from the reduction in area of the lower wing, which on a biplane produces far less lift than the upper wing, yet causes considerable drag. This helped give sesquiplanes a superior climb rate. Unfortnately at very high speeds (at what would now be termed its VNE, or "velocity, never exceed") the lower wing was prone to flutter, an aerodynamic phenomenon not fully understood at the time. The single spar was behind the centre of lift, which at high speeds could cause the lower wing to twist, increasing the angle of attack until the wing stalled, at which point it would return to its normal position, repeating until the wing snapped or the pilot slowed the aircraft.