Lower Cowling Split

From Bondline

Contents 1 Introduction 2 Bob Steward's Approach 3 Gil Alexander's Approach 4 John Horton's Approach

Introduction

Requests for information on splitting the lower cowl on Tigers and Cheetahs was made on the GG and I'll post some pictures and an explanation of the means I used to repair lower cowls that had failed the spot welds and had cracking doublers.

Bob Steward's Approach

Here is the Tiger lower cowl with failed spot welds and cracked doublers after removing the cowl from the plane and taking the doublers out.

This is a Cheetah LH lower cowl from the outside with larger ramp being drilled and clecoed in place. Paint is off because of the amount of metal work that was needed with an English Wheel to smooth the ripples that had been filled with bondo during previous work by persons unknown (no log entry!).

Here is the inside of that same Cheetah lower cowl half with the ramp in place and the new doubler mounted, ready to attach the other side with screws.

Gil Alexander's Approach

Gil Alexander took a slightly different approach for a Tiger repair, since he didn't want to harm a new paint job. Gil left the doubler in place and added a 0.032 strip of aluminum 1 7/8 wide on the inside. Here's what he did:

• Removed the nut plate at the front on the nose gear boot mount, and clamped the strip in place.
• Drilled #30 holes from the outside for the mounting screws, roughly located where rivets from a previous repair were removed. Clecos were inserted as the holes were drilled.
• Drilled matched #19 holes for #8 screws for the aft screw at the nose gear boot mount and for the central retaining screw into the nose bowl.
• Removed the strip.
• Drilled all holes in the cowling and new strip #19 for #8 screws and nutplates riveted to the new strip.

Only now, after the cowling is completely drilled, is the original joining strip cut down the middle with a 2 inch cutting wheel on a Dremel tool. Working from the inside, this avoids harming the (new) exterior paint.

Only the forward 2 inches of the original joining strip need be removed. This ensures that the extra layer of the new joining strip does not create an extra layer at the center where the cowling is screwed to the nose bowl.

This technique ensures that the original alignment of the two halves of the cowling is precisely maintained, allowing accurate #19 holes (a slip fit for #8 screws) and fixed, not floating, nutplates to be used.

This photo shows the left side cowl and the center strip. Note that the center strip end that is shown at the bottom of the picture is actually the end that is attached to the nose bowl. The slight joggle in the strip can just be seen. This joggle keeps the same thicknesses of metal at the surfaces that are screwed to the nose bowl.

John Horton's Approach

To be honest this should be called the Bob Steward option B approach. I got the inspiration from the Bondline website and after several emails with Bob and assessments of my cowl I came up with a method that appears to be a combination of the Gil Alexander and Bob Steward methods.

First I split the left and right cowl by grinding off the spot welds and separating with a sharpened putty knife. The original doubler that surrounded the flexible boot for the nose strut was in good shape except for the top section with the nutplate for the forward screw. So I opted to leave the existing nose strut boot doubler and only cut out the top portion. I then fabricated 2 new exit ramps, a top doubler with nut plates (above the air filter opening) and a lower doubler with nut plates (below air filter opening). The nut plate location on the lower doubler is important as they can interfere with the carb heat mechanism if placed in the wrong locations.