Smooth Out Your Belly
October 2007
This month’s Shop Talk will walk the reader through the conversion of an older M20 to a one-piece belly that was stock on 1984 and newer aircraft.
If you have ever wondered what was involved time-wise and cost-wise in doing this conversion then read on and follow the pictures accompanying this article.
Prior to 1984 all midsize Mooneys (F, G, J and K) came with four removable sheets to gain access to the equipment installed under the floor. The main spar box was accessed through individual access panels as these two sheets on the belly were riveted on to the airframe. A structure was built to hold these belly sheets on with rivets, # 10 machine screws and #8 PK screws.
In 1984, Mooney developed a one-piece composite belly panel that when removed, exposes all the equipment from the flap motor to the aileron control links. This panel attaches with approximately 36 cam locks so it came off in a few minutes instead of spending a half an hour removing striped-out screws and pulling panels off.
Once completed, this conversion amounts to a weight savings of 3 to 4 pounds. Mooney never provided this in kit form so this article includes a complete parts list, as you have to purchase each part. Because the F and G models never came with the one-piece belly from the factory, this modification requires a 337 field approval. The J and K models only require a return-to-service logbook entry.
When Mooney developed this conversion the factory determined that the belly and the associated components that hold it to the airplane are non-structural and the belly is only considered a faring. You’ll see in the J and K parts manual that it is called out as a fairing. This is one reason that any antennas that were mounted on the old belly panels be relocated. The marker beacon antenna and possibly the ADF antenna can be mounted at the flap motor bay. You can locate the transponder antenna and DME antennas to the forward inspection panels that are right next to the gear doors. This will leave you with a clean antenna-free one-piece belly. Another compelling reason to relocate antennas is that the one-piece panel is composite material and therefore provides no ground plane.
Due to this conversion’s complexity we will list only its highlights. A bit of a warning here: if you are not a sheet-metal-savvy mechanic, do not attempt this job without qualified help, including an experienced gas welder. All bare aluminum surfaces should be zinc chromate primered before riveting.
1) Remove all the belly skins that cover the aileron control links to the flap motor.
2) Drill out and remove the rest of the belly skins located between the above areas.
3) Remove the structure that holds the belly skins to the airplane including the four ring root fairings.
4) Remove the bulkheads between the flap motor and the aileron control links. Drill out the rivets from both rows of the outboard seat rails.
5) Cut all the wiring and antenna coaxial cable. Disconnect and bleed dry the brake lines. Disconnect the trim. (Some advice: trim to full nose-up position before disconnecting. It will be easy to reconnect and will not have to be re-rigged; just verify that full travel, nose-up nose-down.) Remove the elevator tubes and the rudder tubes.
5) Install (rivet in) the new bulkhead located in front of the wing flap motor.
6) Modify the bulkhead located aft of the wing flap motor and install the cam lock plate assembly.
7) Fabricate two 2” x 2” metal plates out of .064 in. 4130 mild steel. These will be angles that will have a 2” X 2” flat area and locate these angles on the truss assembly just forward of the gear motor. These plates are in direct line with the other four center cam locks that hold the belly on. Locate their position on the truss and gas weld them into place (use the correct rod for 4130 steel).
8) Install two new half bulkheads at the wing spar on right and left sides. Replace the emergency gear spring strap. The old one will be too short.
9) Install new left and right side bulkheads for the new belly to attach to. These are the bulkheads that run fore and aft and have all the cam lock receptacles. Locate these to fit under the inboard row of the outboard seat rails. Use the existing holes in the seat rail to drill through the bulkhead.
10) Fit, trim and rivet the new wing root fairings in place.
11) Fabricate and run extension coax cables to the new antenna locations.
12) Once all the above is up in place and riveted together you can start to trim and fit the new belly. Locate all the 2700 series cam locks. Mark, drill and countersink the holes for the new panel.
13) Locate and cut a hole in the belly for the fuel strainer drain. The old belly skin may be used for a template.
14) Trim and refit the flap adjuster rod access covers.
15) Install the one-piece panel and all access covers and double check for fit. Paint.
16) Complete a new weight and balance computation for the new panel and any changes to antennas.
17) Make the return-to-service logbook entry.
18) Fly.
The above steps appear straightforward but you need to plan for one full week on your back under the airplane to complete this job; even more time if this is your first conversion. The retrofit of a one-piece belly on your pre-1984 Mooney is no easy task and it covers all types of work from welding to fabrication to avionics and electrical work.
After a gear-up landing, a one-piece belly is almost always installed. It’s the best way to go and most likely, insurance is paying for some of the repair. This conversion will cost about $5,500.00 for parts and $2,000.00 to $2,500.00 for labor.
There are some common problem areas during this conversion. Most of the pre-drilled holes on the four wing root sheets will not match anything on your aircraft. As mentioned, the flap hinge covers will have to be trimmed and the old holes will not match the new belly formers. The aft bulkhead holes for the screws that held on the flap motor access belly sheet will not match anything and all of these holes will need to be filled before re-painting the bottom of the aircraft. The forward belly skin that covers the aileron control links is cut wider than the new one-piece belly. This leaves a ⅛" - ¼" gap on each side between the belly and the airframe skin. This is not a hole in the airframe as the bulkhead former bridges the gap. Because it’s on the bottom of the plane most people don’t notice it. It would be nice if Mooney would leave ¼” more material on each side of the belly panel as it’s probably easier to trim material off than add to it.
As always, if you have a question about this article, you may contact me at my aircraft repair shop, 307-789-6866 or via e-mail. Until the next ShopTalk, enjoy flying your Mooney.
