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How to Install Wheel Pants
By Tony Bingelis (originally published in EAA Sport Aviation, August 1975)
IF YOU ADD wheel pants (speed fairings) for good looks, you will undoubtedly come closer to being satisfied than if you add them to increase the speed of the airplane. On the other hand, properly done, you might just be pleasantly surprised and find that you have accomplished both objectives.
Some builders test and fly a new homebuilt without wheel pants, and this is a good practice. No use introducing more unknown elements than necessary in the early test stages. A poor wheel pants installation can unexpectedly begin to shake in a most distracting manner, or worse . . . blow off. Fortunately, such cases are the exception; however, to play it safe, you might review the following for helpful details.
One avowed purpose of speed fairings is to reduce drag. Wheel pants are, therefore, designed to be as small as possible in cross section in order to minimize drag. It is a great help that commercially available wheel pants are ordinarily of excellent quality and design.
You may, nevertheless, find that the wheel opening in some of these designs is much larger than needed. A large opening causes drag and reduces the effectiveness of the speed fairing. If you are interested in speed as well as in looks, reduce the wheel openings to the minimum practical. And what might that be? I don't really know, but certainly, the usual one inch to fist-sized gap between the tire and the wheel pants is much too much. Even some of the factory built beauties now have reduced the tire/pants clearance to approximately ½". Such close clearances may be functional for aircraft operating from good paved runways but what about those using unpaved surfaces and primitive strips?
Sooner or later, the aircraft will have to roll through muddy areas. Under such conditions, wheels pick up mud rapidly and before an airplane rolls many feet, the wheel-to-pants clearance disappears and the pants become packed solid. The airplane can't move any further. If you will ordinarily be operating where poor runway conditions are common, you might be better off without the fancy pants. One good bit of insurance for the "turf strip set" is to make and install mud scrapers directly behind the tires. Mud scrapers are a useful precaution for any wheel pants installation. You should, however, make sure that there is adequate clearance (about 3/8") between the scraper and the tire. If you make a set of mud scrapers, file the attachment holes in the scrapers into elongated slots to provide the adjustment range needed.
You may have heard that some builders install a bulkhead (partition) immediately behind the tire, sealing off the entire aft end of the wheel pants. The theory is that this will prevent the rear portion of the wheel pants from becoming loaded with mud and dirt. The value of this practice is debatable when weighed against the work and extra cost involved. Besides, as I see it, it would, in effect, only take less mud to pack and jam the wheel pants sooner.
Whether you buy a pair of speed fairings or make them yourself, all you will have before you, to begin with, are the two empty shells with not a single visible clue as to a means of support. Like many other things involved in the building of an airplane, the process of fitting the wheel pants can be difficult for anyone who has not had the opportunity to see how it is done. Perhaps, the drawings in Figures 1, 2 and 3 can serve the same visual purpose.
POSITIONING THE WHEEL PANTS
If the ready made wheel pants do not have some sort of an index mark or a small hole to identify the recommended location for the centerline of the axle, you will have to determine this for yourself.
Examine the wheel pants and you will see that the size of the wheel opening in the pants definitely limits the fore and aft placement of the pants over the tires. To a similar degree, the size of the tire limits how far the wheel will penetrate the pants before it rubs someplace. With these limitations, you will zero in on the approximate location for the axle's centerline. Except for racing aircraft, most wheel pants are installed so that a small amount of the wheel rim is visible beneath the pants. More often than not, as much as 1/2" of the wheel's rim shows. All in all, wheel pants should not cover so much of the wheels and tires that, when the aircraft is at rest on the ground, it would be impossible to slide any sort of wheel chocks under the wheel pants . . . especially behind the wheels. I guess most pilots know that tires have a mysterious inclination to get a bit low once in awhile. If this happens, you may find that the airplane has settled and the pants are jammed against wheel chocks which can't be removed. This phenomenon usually portends damaged wheel pants.
Even after establishing the desired position for the centerline of the axle, one question remains. Should the wheel pants be centered over the tire (as viewed from the front) or should they be moved inboard so that they will also cover the brake installation? Pilot builders who are habitually heavy brake users should perhaps cut away some of the inboard portion of the pants to leave the brake housing exposed for better cooling. Positioning the inboard edge of the pants so that they are flush with the brake housing will still afford some drag reductions and a reasonable amount of cooling air for the brakes.
Positioning some styles of wheel pants inboard of the centerline of the tires might permit enclosing the entire brake assembly and its drag producing clutter. The easiest way to do this, however, is to center the pants over the tires and fabricate a separate bowl-like fairing or fillet to cover the clutter around the brake area.
INSTALLATION OF DOUBLERS
Fiberglass wheel pants will require doubler reinforcements at each of the attachment points to help distribute the loads around the bolt holes. Reinforcement plates are made of aluminum sheet and are riveted or epoxied to the inside surfaces of the pants. If you use rivets in place of, or in addition to epoxy adhesive, the rivet heads will show on the outside of the pants unless you go to a lot more work to cover the rivet heads and refinish the pants. If you intend to forego the rivets, prepare the doubler to assure better bonding by drilling a number of random holes through the metal plate. These holes will permit the epoxy to seep through for added reliability of the bonded joint.
A small 2" x 3" rectangular plate, or a similar sized aluminum disc doubler is adequate reinforcement for the outboard attachment point at the axle.
On the inboard side, a larger plate must be bonded to the inner surface of the pants. A half moon sort of a doubler is effective as it permits the cutting away of a section of the wheel pants from around the brake housing, if desired. This type of doubler provides greater leeway in adjusting the ground angle for the pants without first having to know the exact attachment bolt locations.
Attaching wheel pants to spring-type landing gears (Cessna, Bede 4, Scooter, Sonerai, etc.) is much easier than it is to some whip-type gears (Tailwind, Sidewinder, T-18, etc.). Somewhere in between in difficulty, are the scissors type and the welded type tripod styles. A lot also depends on the type of wheel and brake installation you have. Most landing gears are not difficult to equip with wheel pants if there is a plate, or disc, or bracket welded to the axle for the attachment of the brake torque plates. In most instances, the wheel pants are attached to a bracket that is held in place by the same bolts that secure the brake torque plates to the axle or gear leg.
A three point attachment for the pants usually provides the most rigid and effective means. This arrangement requires one attachment point to be located at the outboard end of the axle, and the other two attachment points extending from the inboard side of the gear leg. The inboard attachment points are located on a bracket tailored to fit the gear and style of pants used. An offset type of bracket may be necessary to obtain the desired positioning of the speed fairings over the tires. See Figure 2.
Special sleeve-like axle nuts are in use for attaching the outboard sides of the pants. These are standard hardware units for some production aircraft installations. Their cost is high and, as a consequence, they are not commonly available to most of us. A machinist could easily make his own. But for the rest of us, alternate or modified versions of the attachment are easier to make. For installation of the pants, the hole for the axle attachment is drilled through the pants first. Then, slip the pants over the wheel insert and tighten that axle attachment bolt. This will permit you to set the wheel pants to the proper ground angle.
Before you drill the attachment holes in the inboard sides of the speed fairings, be sure that both wheel pants are rotated the same amount. Back away and eyeball both wheel pants from either wing tip to be absolutely certain your measurements were accurate. When both are in alignment with each other, mark and drill each of the two inboard attachment holes.
One final consideration is how to provide for the airing of the tires without removing the wheel pants? Some designs in which speed obviously is not the greatest consideration afford access to the valve stems by means of the rather cavernous opening around the wheels.
Often the wheel protrudes from the pants so far that access to the tire valve is possible. Tight fitting speed fairings, however, require some special accommodation for the chore of airing the tires. Perhaps a small removable plate in each of the wheel pants could provide this access to the tire valves.
Some popular 1975 model aircraft now feature neat little access doors built into the speed fairings. They are rectangular in shape and are hinged on piano hinges. Opening and closing of these mini-doors is by means of push button latches. The periodic ritual of removing the speed fairings simply to air the tires is not considered a fun thing . . . ask anybody.