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Customizing Wheel Pants
By Tony Bingelis (originally published in EAA Sport Aviation, December 1979)
IN BUMMING AROUND airports I find most fixed landing gear installations are dirty aerodynamically and would benefit, both aesthetically and performance-wise, by the addition of customized fairings to the wheel pants already installed.
All wheel pants installations have some sort of a strut or axle sticking out of them. Usually out of the inboard side or the top of the pants. Most of us realize that if a landing gear leg, strut or whatever doesn't intersect the wheel pants at approximately a 90 degree angle, that intersection will generate quite a bit of turbulence and drag. If this is so, it is hard to understand why more homebuilders don't remedy the deficiency by constructing and installing a fairing to enclose or alter the drag producing area.
Some landing gears are difficult to fair. A good example is the type of gear installed on the Emeraude, Jodel and similar designs. Their cantilever gear legs, with attached scissors, must be the dirtiest of all conventional installations . . . and the most difficult to streamline. The only way to effectively reduce the drag of such a gear is to enclose everything in a customized housing or fairing. European designs have long used large bulbous wheel pants to enclose all the drag producing elements.
The Zenith design utilizes a similar installation and I understand wheel pants designed for it may be obtained from Canada. For the most part, however, you will have to make your own customized version using stock wheel pants and tailoring them to suit your aircraft.
The original wheel pants installed on my latest Emeraude were the long pointed type used on some Cessnas. Their pointed noses complemented the large pointed propeller spinner installed on my aircraft and the sweeping lines of the pants matched the lines of the Emeraude's fuselage. Initially they looked pretty good to me but in time, with those ugly scissors sticking out in the open for everyone to see, I found the installation crude and offensive. So, some cosmetic surgery was in order. Although the treatment the wheel pants received is adequately detailed in the photos for experienced builders, some behind the scenes comments might be helpful to anyone who wants to try his hand in customizing his own wheel pants.
The Customizing Process
Almost any kind of foam is good enough for building up wheel pant fairings. I used a sheet foam manufactured for homebuilders (I mean house builders). It is a smooth textured material, pale blue in color and may be purchased in 4' x 8' 3/4" thick sheets. Thicker sheets are available too. This material is stocked by almost all lumber yards. It is inexpensive and easily worked.
To enable me to get started with the foam build-up and to provide a good reference surface to work from, I glued a slab of roughly shaped foam to the inboard side of the pants with a few dabs of 5 minute epoxy. I would caution you that because this foam dissolves in the presence of fumes or contact with volatile liquids (lacquer thinner and polyester resins, etc.), use an epoxy adhesive, white glue or any other adhesive which will not attack the foam. How can you tell if a glue is O. K. to use? Try a small sample. If the foam doesn't dissolve or erode away before your eyes when in contact with the glue, it will work. However, another problem you'll have with most glues (other than the 5 minute epoxy), should be pointed out. They set-up too slowly. Of course, they would be all right for overnight gluing and when time is not a factor.
With pants installed, glue additional pieces to the reference slab to fill out the profile needed. Use the glue sparingly and try not to apply the glue dabs to areas where the foam is to be sanded or cut away as it will leave hard spots. Those hard areas do not sand well and will make your contour finishing a bit more difficult.
After the gear assembly in the area to be faired has been enclosed with foam pieces fitted and glued in place, determine where to cut out and temporarily remove the foam build-up so that the entire assembly, wheel pants foam and all, may be removed from the aircraft for additional work.
When you get the wheel pants with all that foam build-up to your workbench, replace the cut-out hunk of foam by gluing it back in place. You should now have two wheel pants with a large foam mass affixed to each of them ready for shaping.
The foam may be shaped with a Surfoam file, a hacksaw blade, large butcher knife, coarse sandpaper or almost any other abrasive or cutting tool that you like to work with.
Developing the Fairing Contours
While viewing the foam build-up from the front, shape it into a smoothly curved contour that blends into the basic wheel pants. Keep in mind that a minimum frontal area is most desirable. Because the foam pieces were fitted around the gear legs with the wheel pants installed on the aircraft you are assured of adequate clearances for the scissors and other linkages. You can, therefore, cut the foam down to as thin as 1/4" wherever needed. When you have that side shaped to your satisfaction, turn the wheel pants sideways and shape the foam to blend into the profile view of the pants. The final shaping is that of rounding the foam and working it into a streamline shape. It could be that when you finish the rounding and fairing process the completed contours may not be what you consider to be an ideal shape for obtaining a smooth flow of air around the pants. In this case you could add a piece of foam to extend the contours or to modify them into the streamline shape you visualize as being appropriate. Instinct can stand you in good stead here. Naturally, you should avoid abrupt changes in shape and keep the contour lines flowing smoothly from front to rear. Later you can tuft the wheel pants and have someone take photos of them in flight to see how successful you were and whether you will need to alter their shape to improve their efficiency . . . that is, if obtaining minimum drag is important to you. For most of us with moderate performance aircraft, I think a striking ground appearance is satisfying enough.
Fiber Glassing Methods
Now that the foam fairings are shaped to your satisfaction you are ready to overlay them with fiber glass. You have two basic alternatives. One, you may lay fiber glass directly over the foam covered areas using epoxy resin making sure that the cloth extends at least two inches beyond the foam line. After the fiber glass is cured you can pour lacquer thinner or just about any other volatile liquid into the fairings to dissolve the foam. This will leave only the fiber glass shell permanently attached to the wheel pants. This method is the quickest and although it leaves you with a rough inner surface, this is of no consequence. All that is needed now is just a bit of outside sanding and fairing of the fiber glass edges.
The second method requires more extensive preparation and work. Because polyester resin will be used (it's cheaper and sets up faster), this type of foam has to be protected from its fumes and from direct contact with the resin. A light coating of patching plaster over the entire foam covered area will provide the necessary protection for the foam against the resin.
After the plaster layer is sanded smooth, lay a strip of masking tape around the border of the foam to delineate a smoothly curved outline for the fiber glass lay-up. This edging will aid in your trimming the glass shell later.
Proceed from here as you would with any fiber glass work, waxing everything heavily to insure easy removal of the fiber glass fairing after the lay-up has cured. Use whatever methods you are familiar with.
Do not make the mistake of using too many layers of cloth. Remember when you are permanently attaching the fairings later, an additional layer or two will be added around their periphery.
A maximum of three layers of cloth should suffice for all areas of the fairing except those that must be reinforced. Actually, two layers of cloth is plenty wherever the curvature is pronounced.
Before removing the fiber glass shells from the form, drill small holes around the outer boundary of the fairing so that they may be used to obtain exact realignment during gluing.
Using a felt pen and a flexible straight edge, mark the area of the fairing to be cut out to provide access to the gear legs for easy installation, maintenance and inspection. Use straight lines as much as possible although this is not mandatory . . . simply easier than working with curved joints. Refer to the photos for ideas.
A hand held hacksaw blade may be used to make the cut-out. An electric jigsaw with a metal cutting blade is also effective, but be careful!
After the opening has been cut out you might want to cut away some areas of the original wheel pants. Or you can cut a few lightening holes with a large diameter (2") hole saw. Be sure not to weaken the wheel pants attachment points. This lightening process is feasible because after the new fairing sections have been glued to the wheel pants the entire unit will become extremely strong and rigid and parts of the original wheel pants will be redundant to the new customized shape.
Attach 1'/4" wide aluminum strips to the underside around the cut-out in the fairing to form an edge for fasteners. These strips may be pop-riveted in place. If pop-rivets are used, their heads may be covered with a piece of glass cloth to hide them and provide a smoother surface.
Place the fairing cover piece in position and drill the attachment holes for it. Drill through the fiber glass shell and the metal edging under it. Install a few cleco fasteners or screws in each hole as soon as it is drilled. Next, install nut plates in the metal strip to permit easy installation and removal of the access panel.
Now all you have to do is finish sand the newly faired pants to the degree of perfection you want.
This is the time also to correct a deficiency common to many stock wheel pants designed for production aircraft . . . that tremendous opening they have in them for the wheels. This cavernous opening is a high drag producer and may be the reason why many builders realize little or no increase in airspeed after wheel pants have been added. If your wheel openings are excessive- greater than 1/2" in front or in back of the tires - you should fabricate small metal or fiber glass plates to reduce the opening around the tires. These plates or covers may be installed with nut plates or permanently glued in place.
Tire Valve Access
A totally enclosed wheel pants installation must have access to the valve stems so that the tires might be aired whenever necessary. A small spring-loaded door can be installed to provide this access. Take care in locating this opening properly otherwise the tire's valve stem may still be inaccessible to an air hose.
A Cosmetic Treatment
Even the best fiber glass work seems to have tiny indentations or pits in its finished surface. These show up even after you have carefully prepared the surface and have given it a surface primer treatment. These can be hidden by taking a bit of wet primer on your finger and rubbing it across these offensive areas. After this cosmetic treatment dries, resand the area. Keep repeating the process until you think you have gotten rid of all of them.
Large fairings such as these will naturally weigh more than the basic wheel pants originally installed. You can expect the 3 pound wheel pants to double in weight. Mine weighed 6 pounds each. I think I could have reduced them to 5 pounds with a bit of extra lightening effort. Of course, some landing gears require only small fairings and the resultant increase in weight will be much smaller.
My own modified wheel pants turned out to look something like a couple of dolphins. I like the effect from any angle although at least one or two of my friends don't seem overwhelmed. Still, I like them and I’m sure you'll like yours too.