I've heard the cowl hinges are prone to breakage. How can I avoid this?
This is indeed a problem on some RVs due to the high vibration in this area, It usually manifests itself as hinge eyes breaking off progressively from the ends. This is rarely a safety issue unless you let it go too far, but it is a pain in the neck regardless. If you have a flying plane and are experiencing a lot of hinge or fastener wear or breakage, the first thing to do is check the prop balance. Many RVers use harmonic balancers on their wood props with good results; metal/CS props can be dynamically balanced. All this costs money but it's usually worth it in the long run.
Beyond that, RV builders have used a wide variety of methods to prevent hinge breakage. One obvious solution is to just go with Camlocks or screws all the way around, but that has obvious aesthetic drawbacks. Most problems can be solved by using a combination of solutions in different areas. We'll look at these areas one at a time, starting at the front:
1. Front joint, directly behind the spinner bulkhead. The plans call out (or used to anyway) hinges here, but most people have gone to screws and nutplates. Most common is 3 or 4 #10 screws or 3/8" bolts. This works well, but be sure your nutplates are fastened to a good solid base, not just the fiberglass of the cowl. Use a piece of .063 AL, well roughed up (60-grit) on the back, drilled with bonding holes, then riveted AND bonded, with proseal, or cotton flox or milled glass slurry, to the inside of the cowl where the nutplates go.
2. Top/Bottom cowl hinges. The hinge eyes tend to break off mainly at the front where the curve is greatest, but can also do so at the back. One reason may be that the hinges are simply not properly manufactured for this sort of duty -- they have sharp 90 degree inside corners at the hinge eyes, and if looked at with a microscope, have micro-cracks right from the start. Simple solution: take a jewelers file or 400 grit sandpaper or a combination of both, and round off these inside corners before attaching the hinge to the cowl. Tedious, but probably worth the effort.
3. Top cowl-to-fuselage hinges. See 2, above. This is also an area where many people go with camlocks or screws. But smoothing the hinge eye corners will go a long way towards preventing this problem. Another issue here is the hinge pin itself. Vans specifies a reduced-diameter steel hinge pin here in order for the pin to be able to take the curve. But this smaller pin is more likely to move around and wear the hinge eyes from the inside. A neat solution is to get a normal diameter pin (steel, not AL) and use a belt-sander to narrow down the ends of the pins where they have to curve the most.
4. Side cowl-to-fuselage hinges. This area is probably the least prone to breakage, although the smoothing-the-corners trick is still a good idea.
5. Bottom cowl-to-fuselage hinges (except the -4). This area is the MOST prone to breakage. One solution is to go with stainless steel hinges, although reports from people who've done that indicate it is not a foolproof solution. Adding acowl -scoop center brace (for those models that use a scoop) will also help. Probably the most successful solution is to just toss the hinges and instead rivet some .063 AL flanges to the firewall and screw thecowl to them with #10 screws/nutplates. 3 screws per side is enough, and 4 is more than enough.
