The RV Journal
What is it?: Oil Door Hidden Hinge
The level of ingenuity exhibited by RV builders has always delighted me, often by elevating some of the most overlooked components of our aircraft into examples of the "better mousetrap". This article is about just such a component, matter of fact, it even resembles in a minor way some sort of mousetrap.
The access door on our RV's through which we check the oil level during preflight is one of those areas of the plane where most builders merely use the supplied parts to complete the door in the least amount of time necessary, then move on to more exciting parts of the project. But there are ways the oil door can frustrate us, whether by mechanical breakdown or just a mental breakdown on the part of the embarrassed pilot who is required to shut down the plane after getting buckled in just so he can get out and latch the overlooked oil door.
The Hidden Hinge is a dandy little piece that will take your oil door to another level, not only mechanically but in a way that will most likely prevent you from ever doing the "Aw RATS!" oil door shuffle. The crafty builder can use the Hidden Hinge as the basis for fabricating a truly trick oil door if time is taken to conceal external latches and fasteners. But even those of us content with a more pedestrian installation can utilize the Hidden Hinge to achieve a nice upgrade from the standard hinge shipped in the kit.
In my case, the Hidden Hinge was a retrofit to the well-flown cowl of my 850-hour RV-6. The original hinge had already been replaced once after several eyes had broken off due to the incessant pounding of the slipstream, and since I had decided to spruce up the top of the cowl with a fresh coat of paint and repair minor wear and tear, it was deemed a great time to upgrade the oil door to the Hidden Hinge. Since I elected to retain the old-fashioned fasteners that were originally installed on the cowl, my Hidden Hinge installation isn't as slick as some of those on new aircraft. But all the advantages of the hinge are present even on my, ah, experienced cowl.
The heart of the Hidden Hinge is a uniquely shaped arm that allows the hinge to be mounted completely out of view to the underside of the cowl. Included is a spring that loads the hinge in such a manner that forces the oil door open when it is unlatched.
The hinge is four inches long and consists of four components; the specially curved arm, the hinge base, a spring, and the hinge pin. The hinge base and arm come already riveted together, and the hinge base is the high quality, extruded variety.
The only instructions included with the Hidden Hinge is a little sheet of paper suggesting the hinge be first mounted to the cowl, and contact info if further questions arrive. I think it is safe to assume that the RV builder who has progressed to this stage of the project should be able to quickly determine how to mount the hinge so it will function properly. Also stated on the sheet is an observation that some builders fab an oil door out of aluminum instead of using the original epoxy or polyester door. This is due to the hinge spring exerting a constant force on the door while trying to do its job of opening the door. If the glass door gets hot, it might bow or deform enough to detract from the clean lines of the installation. An aluminum door offers sufficient stiffness to avoid the possibility of the hinge spring deforming the door.
In my case, since the original hinge was mounted where the hinge pin protruded between the door and cowl, my old poly door, while I think stiff enough, was undersize and left a large gap where the old hinge had once resided. I had some leftover 0.050" aluminum in the shop so I spent a morning fabricating not only a new door but a new retaining flange to replace the old, beat up piece. Some builders will rivet the bits and pieces together and fill the rivets to achieve a flawless installation, but I used #4 screws to attach the hinge to the cowl so I could remove it when I repaint the cowl.
This photo shows the hinge holding the oil door open, and you can see how the curved arm allows the hinge to be mounted to the underside of the cowl. The spring is stiff enough to open the door with authority and it would take a rather bone-headed pilot to not see the door standing at attention if he neglected to latch it during preflight.
The geometry of the hinge is quite clever and the Hidden Hinge functions perfectly with no tweaking, bending, cussing, etc necessary. Please excuse the extra holes in my cowl, those are where the old hinge was attached and they will be filled prior to painting. I noticed there were no keepers included with the hinge to secure the pin, but after everything was installed I found the pin to be gripped so tightly by the force of the spring that I don't think it could ever exit the hinge. However, I went ahead and put a slight bend in each end of the pin to make sure it could not exit the hinge.
The Hidden Hinge is a dandy little item that really enhances the cowl installation, and has a high "Well, look at that!" quotient. The price is reasonable in that most of us would probably go through multiple iterations of the hinge before we were able to figure out the precise geometry needed to make the device work smoothly.
The Hidden Hinge is a nice piece, and I recommend it to any builder who wants to add an extra flair to their pride and joy.
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