AOP
In Remembrance 2023
One of the differences listed by the Army in descriptions of the various models of L-4 is that the L-4H differed from the B model in the method of attaching the windows. It does not specify what exactly that difference was, but it is pretty well accepted that it was the change from screws and nuts to rivets pressed in from the outside into tinnerman flat speed nuts on the inside. H and J models had them.
When I installed the glass on mine, I used screws and nylock nuts. Installing tinnermans and rivets when replacing damaged glass is a real bear of a bad job, especially at the rear of the desk. However, when I restored my upper door window this past spring, I used the rivets and tinnermans. I just finished the annual in time to go to the big annual WWII Weekend at Reading, Pa airport this past weekend.
I was flying merrily on my way, being buffeted around and slowed down by the 30 knot headwind. I timed myself when I reached a local airport and found that instead of the usual 28 minutes, I was at the 50 minute point. Definitely not the time to press on with 2/3 of the trip yet to go. I dropped down and got more gas, and went to takeoff to finish the trip. Just after I broke ground, I heard a big bang (Any bang in a flying airplane is a big one!). I looked out to my right and saw that the leading edge of the glass in my upper window was bent up and flapping violently on the bottom surface of the wing. I had about 2000 feet of runway ahead, so I just cut power and landed. After I taxied in, I looked and saw that all of the leading edge rivets had let go and the leading edge attachment strip was gone. 8 rivets had blown away. I spent the next hour finding screws, washers and nuts to replace the missing rivets.
Those rivets only had about 3
flying hours on them before they departed the airplane. I consider myself lucky that they didn't decide to take the rest of the day off 30 minutes before when I was at around 5000 feet. I recommend very strongly to anyone restoring an L-4 to never use rivets and tinnermans to attach anything to anything. It seems to me that the problem is that today's tinnermans are fitted to a specific size of screw and not a rivet. The difference is that the rivets are not the correct size and the speed nut will not grip the shaft securely. The constant vibration they are subjected to in flight is enough to shake them loose in very short order.
When I installed the glass on mine, I used screws and nylock nuts. Installing tinnermans and rivets when replacing damaged glass is a real bear of a bad job, especially at the rear of the desk. However, when I restored my upper door window this past spring, I used the rivets and tinnermans. I just finished the annual in time to go to the big annual WWII Weekend at Reading, Pa airport this past weekend.
I was flying merrily on my way, being buffeted around and slowed down by the 30 knot headwind. I timed myself when I reached a local airport and found that instead of the usual 28 minutes, I was at the 50 minute point. Definitely not the time to press on with 2/3 of the trip yet to go. I dropped down and got more gas, and went to takeoff to finish the trip. Just after I broke ground, I heard a big bang (Any bang in a flying airplane is a big one!). I looked out to my right and saw that the leading edge of the glass in my upper window was bent up and flapping violently on the bottom surface of the wing. I had about 2000 feet of runway ahead, so I just cut power and landed. After I taxied in, I looked and saw that all of the leading edge rivets had let go and the leading edge attachment strip was gone. 8 rivets had blown away. I spent the next hour finding screws, washers and nuts to replace the missing rivets.
Those rivets only had about 3
flying hours on them before they departed the airplane. I consider myself lucky that they didn't decide to take the rest of the day off 30 minutes before when I was at around 5000 feet. I recommend very strongly to anyone restoring an L-4 to never use rivets and tinnermans to attach anything to anything. It seems to me that the problem is that today's tinnermans are fitted to a specific size of screw and not a rivet. The difference is that the rivets are not the correct size and the speed nut will not grip the shaft securely. The constant vibration they are subjected to in flight is enough to shake them loose in very short order.
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