I believe I solved the mystery of the Zp38 Boom failure!!!

I have to say overall there is a weak spot in the boom build that has been around since conception - weather or not you have an issue with it will depend on how you build the bird.

There are several ways to build this plane and if you choose to build the removable turbocharger / wing cover not to plans you will have this weak spot.

If you look at the original photos above of the cracking you will notice they are spreading down and to the rear - this is a sign of stress fracture not vibration although vibration will exaggerate any weakness.

More importantly note in the picture the top of the turbocharger/wing cover. You will see it is removable with a spring loaded release pin and Im sure there are dowels or another pin on the front to also hold it in place.

This is a design modification - Plans are plans and fliers are fliers - we all make modifications to make assembly at the field easier.

There are two stresses in Pitch to deal with (upward and downward) Yaw stress on the boom (side to side) is minimal. Anyway the upward stress loading is transferred from the rear of the boom through the turbocharger / wing cover directly to the firewall through the dowels. (this is good enough)

Here is the problem - downward stress on the boom has no support (as the top of the boom has no bracing and is only sliding on the hatch release pin.) Majico presto you have weak spot



I had a quick assembly modification and also had the stress fracture (although much smaller)

Since I had already built, covered, painted and detailed the plane I did not want to completely tear it apart.

I opted to install thin ply plating on the "inside" of the boom using a combination of screws and epoxy. The ply runs approx. 3 inches past the bottom corner in both directions and on both sides of the boom.

On the top of the boom I installed a mounting block/post. This post is a solid block (the same density as you would use to mount gear, solid and strong)

The post is mounted to the top of the boom with epoxy and extends approx 2 inches back into the boom and 2 inches beyond the end. I then strengthened the bottom of the turbocharger / wing cover and drilled a hole in the top for a mounting screw.

This top block spreads the downward stress load away from the boom and along the entire top of the wing cover from front to back.

The combination of these three steps spread all the forces of downward stress (across the two internal side plates and the top post) immediately eliminating the stress fracturing.

This was a quick fix, added minimal weight and I now have 20+ flights on the original G38’s and now several more on new DA50’s.

I’m posting this so everyone out there knows it’s a real issue – there are other posts on the subject and if you know anyone who has been successful with twins and actually fly’s the ZP38 they will tell you about this as well.

Glassing the booms will fix this and the available Glass booms will probably be ok - at least no one is talking about an issue with those - But I say better safe than sorry.


PS - I horse this thing around with no fear

http://www.youtube.com/watch?v=tjV8wDclKxc