Inside the Mite: a study of wood deterioration, Part III

In Part Two, we discussed typical deterioration patterns seen in the Mite fuselage. Since that time, a few more points have emerged which merit mention.

The aging process can do bad things to the thin plywood used in the Mite for lightness and strength. Every Mite we have seen has some "ripple", concave and convex waves in the surface of the plywood. As racing sailors will tell you, such ripples create significant drag, reducing possible speed, even at cruise.

The Mite fuselage is particularly prone to rippling on the flat areas of the sides just aft of the cockpit, as seen in Photo 1. Addition of short longerons on each side between bulkheads will almost completely solve the problem, adding speed and improving appearance, at the cost of 3 or 4 ounces of weight..

Also on the fuselage, attach points usually rot out where screws are inserted to fasten wing and tail fairings, because the screws create entry points for moisture. Over time they also attract ambient moisture from the air.

Photo 2 shows how a small plywood doubler can be glued inside the fuselage to cover such holes, making it possible to fill the hole from the outside with epoxy and fine sawdust, creating a perfect base for a new screw to be inserted..

Photo 3 shows the "rotisserie" we use to comfortably work on all sides of the fuselage. In the photo, Bill Wegmann, our aeronautical engineer "consultant", is placing doubling plates to repair enlarged screw holes. Lockheed graciously allowed him to retire, so he is a very much a welcome member of our team.

Let's move on to consideration of that beautiful wing the Mooney brothers designed for us. It is strong, light and comparatively simple in design. But it is made of wood, and therefore it needs special care or it will deteriorate. The wing's dihedral insures that any water in the wing will run toward the lowest points on the trailing edges where the wing meets the fuselage. In moist climates or during extended rainy periods, condensation inside the wing cannot be avoided.

Rot generally occurs at the trailing edge flap attach points, as you can see in Photo 5. Pictured is the rotted plywood segment to which the flap is actually attached. These are critical attach points not readily observed by visual inspection.

Any up and down play in the flap attachments suggest this problem and requires immediate attention. They should be thoroughly inspected and replaced if needed when the wing is recovered. See Service Letters 16 and 17 for more detailed wing inspection instructions.

Over time, the presence of moisture and heat may cause the 1950's glues to degrade into crumbly grainy pieces in some of the hundreds of glue joints in the wing. However, the main spar and the rear short spar are protected from almost all moisture conditions because they are totally enclosed by plywood "webs" on both front and back center surfaces until you reach the outer third of each wing, where the spar structure is open. Regular inspection is essential to insure that weep holes in the bottom center wing are open, thereby preventing damage to spars. If they are closed, water gathers in the center wing areas, rotting both spars, delaminating the spar webs and plywood skins in that area. We have built new spars, but it is a lot of work, certainly to be avoided if possible.

Photo 6 reveals the wing center section, front and rear spar, and their relationship to the entire wing.

A considerable amount of rain water can enter between the fuselage and wing fairings, unless connections between wing and fuselage under the fairings has been sealed off with tape, and wing fairing edges have been sealed with silicone or other suitable sealers. With all these water entry points, it is really best to keep your beautiful baby in a hangar if at all possible.

Rippling is usually present on wing plywood surfaces, requiring leveling by applying lightweight bondo in low spots, leveling and sanding it smooth, followed by sealing with epoxy. If rippling is more severe on one wing, resulting loss of lift can cause chronic trim problems.