The research

The day after The Finding, I was flying to Wichita for business and I remember all sorts of questions were going through my mind. How am I going to fix this? How long I'm going to be grounded? But mainly: Why is my fuselage cracked?? I left home that morning with the firm intention of coming back two weeks later with answers to my questions.

Here's a little summary of what I found by talking to Pitts gurus in Wichita and on forums.

•  There is an old, well known problem with spring gears mounted on metal tube fuselages: if the attachment points are rigid in torsion, the flexing of the spring gear induce torsional loads on the longerons which will fail with fatigue, one day or another. The Grove Aircraft company in California has a website that illustrate this well:

• One way to prevent these torsional loads to be transferred to the fuselage structure is to install radius plates instead of flat plates in between which the gear is clamped. This allows some flexing along the X axis. As the gear flexes, the line of contact will follow the radius and therefore only forces will be transferred to the fuse, not moments.

 

I found the following references regarding the installation of radius plates on different aircraft:

Grove Aircraft Radius Plates

Christen Eagle Service Letter No. 364

I also learned that the Pitts Model 12 and the Steen AeroLab S1 are designed with radius plates.

• The heavy duty installation is to create a hinge like the Extra 300. However this is something quite difficult to retrofit on an already existing fuselage:

Source: NickyP on biplaneforum.com

• With a spring gear installation, the fuselage needs additional strength compared to the original bungee gear fuselage. The diagonal member that go from the back of the gear to the top engine mount attachment point is not part of the original Pitts plans and is specific to the spring gear installation. In addition to that, Steen adds a sleeve over the forward portion of the bottom longeron to provide additional strength at the gear location. See Biplane Forum threads here and here.

• A spring gear on a Pitts typically requires the installation of a "butterfly plate" i.e. an aluminium sheet that is bolted to the center of the gear as well as on the gear attachment points. This plate is to prevent lateral movements of the gear (along the Y axis) relative to the fuselage. See biplane forum thread here.

Now, what do I have on my airplane? The diagonal member is there; it's the one cracked! For the radius plates, I did not know until I came back from my trip. But even if I didn't, how come the crack appeared near the top longeron? I had to come back home to answer these questions...