F402 Bulkhead mod

I am buying a flying RV-4 and plan on making a panel upgrade. I want to add the Garmin GPS175 and the only area available is the very bottom right side of the panel. I know that there is an interference problem with the F402 bulkhead in that area and have read that some have modified the bulkhead to accommodate an avionics tray in this location. The photos of how this was done, I can't, however, find any pics showing how these mods were done.

Anyone have pics that they can share showing the mods (including how not to do it)?

I'm sure that my -4 is about to implode at any moment, however.... I removed 402C cross member 2 years ago when I did the first panel upgrade. I did upsize the panel thickness to .090 to add some rigidity. No side effects and no regrets.

That was a few revisions ago, here is the current set up.

Room on top!

Nick, looks like you have provisions to add another box above your Garmin displays. What did you have planned?

Nick, I'm reallllly hoping you didn't remove 402B as it's the vertical bulkheads on the fuse sides. In your picture, your 402B's (left and right) appear intact. F-402C is the horizontal member to provide rigidity to the canoe as the panel with all of the holes cut in it for instruments can't guarantee the same strength for the fuse.

I'm on the other side of the fence... I would caution against modifying 402C.

Have you ever sat in the last row of a long airliner and watched the overhead bins in turbulent air? The whole fuse flex's and moves quite a bit. I know RV's are much different airplanes, but some of that force is still happening continuously on your airplane. Every roll, every bank, every bump in the air. There's a lot of things trying to twist your fuselage.

In a perfect world, you might not ever need the extra strength. I'm sure most people who've modified it have airplanes that are just fine. And as with most things in the homebuilt world, there's nothing stopping you.
I look at it like drilling holes through the fuse bulkheads for cables and wire pass throughs, the fewer holes in the original structure, the better. I've seen a few crashed RV-4 fuselages, weather in person or pictures, and they've all crumpled or failed at the extra holes or where something was modified from the plans. My 402C is intact because you just never know what forces its there to withstand. I don't believe the instrument panel (with holes in it) to be structural. I'm just a dumb airline guy, not an engineer, but it's only held in with two bolts... I know once the forward turtle deck (avionics cover is installed) there is A LOT more rigidity added to the structure which a panel does play a major part in. But again, airline guy over here, not a structural engineer.

Attached is a picture of my panel. F-402C intact. The bottom of the panel is raised 1" for knee room and the Garmin GTX327 transponder on the lower left is sitting on the 402C. The rack is mounted to the top of F-402C. Still, plenty of panel space for an IFR panel. Just my $.02.

Mistype on my end. The F402C is what was removed. In fact, in the picture you can see the "B's". The way 'C' is constructed it handles very little anti-twist. Its almost purely for compression and tension. I built my panels from .090 with a .25x.5 extruded bar screwed to the bottom. They come out VERY stiff and while not in the exact location that 402C came from, ends up doing nearly the same job. Everyone has their own level of comfort, I'm all good with that, this is just another way to get the job done.

Bud K said:

Nick, looks like you have provisions to add another box above your Garmin displays. What did you have planned?

Click to expand...

Yup. That spot is for a Garmin 175 or 355. Its on the 'want' list, not the need list. Priorities right now are getting new Tanks built and new Paint before Osh.

Compromised 402-C crossmember

The plane that I am buying already has multiple holes in the 402-C bulkhead crossmember for various controls and buttons. The largest hole is for the old time 12vdc power (cigarette lighter). I might have to reinforce the existing channel or add some reinforcement across the fuse.

yes i modified 402C. no, the plane has not fallen apart yet. what I did was throw out the sheet metal 402C and used a piece of .063 x 3/4 x 3/4 angle instead. I figured that gave me even more stiffness than the sheet metal part and a lot less vertical height. however, it did cost me a couple of Oz. in weight.

bob burns
RV-4 N82RB