McFarlane's stiffener does not deform the skins

Original stiffener deforms the skins, resulting in crack inducing stress in the corrugations and around the rivet heads.

New Improved Fit to Reduce Skin Stress
The new McFarlane stiffeners are accurately shaped to the same angle that the top and bottom skins come together to completely eliminate the bending stress due to riveting the skins together. This prevents stress concentrations in the skin corrugations and near the rivets, making cracks less likely. McFarlane has improved the stiffener angle as compared to other brands of triangle stiffeners.

New Streamline Shape
The new McFarlane design smoothly transitions from the skin surface to a blunt trailing edge. It maintains the flap's natural airfoil shape by eliminating the unsightly bulb and resultant drag found on common trailing edge stiffeners. 

What is the purpose of a flap trailing edge stiffener?

• Reinforce the flap trailing edge to make it more resilient to hangar rash.
• Ensure a straight trailing edge. Without a stiffener, the trailing edge tends to have a "wavy" appearance where the top and bottom skin rivet together.
• Reduce stress in the flap skins and prevent cracks around the rivets. Without a stiffener of the proper shape, the skin must abruptly bend where the top and bottom skins rivet together.
• Provide a blunt surface to protect the foreheads of unsuspecting pilots and passengers.

Instructions for Continued Airworthiness