project pattern
H
Redux
istory is full of instances of cycli- cal behavior. While some are sim- ply trends reflective of nothing more than aesthetic value, others are iterative improvements or the result of changing priorities. A prime example of this with respect to Pattern planes is the use of anhedral in the stabilizer. While quite pop- ular in the era of the Curare, anhedral dis- appeared for nearly 20 years, and is once again quite common.
The pros/cons of anhedral stabilizers (a long topic for another time) are debated to- day as they were in the time of the Curare. A big change in the evolution of Pattern de- signs that will not be cyclical (in my opinion) is the disappearance of retractable landing gear.
Initially thought to be a design enhance- ment, the touted benefits of retracts were the reduction of drag and shift of both mass and drag closer to the center of the plane. “Common” wisdom was that Pattern planes simply could not achieve the same degree of aerobatic trim with fixed gear. And without a doubt, retracts added an element of wow factor and helped preserve the clean lines of a Pattern plane. On the down side, retracts added sub- stantial
weight, complexity, and unless
meticulously set up and maintained, could be a substantial reliability issue. And as far as the touted benefits, the drag reduction was a myth for the average installation. The large holes in the wing and fuselage for the struts and wheels were large sources of drag and compromised the efficiency of the wing airfoil.
Retract installations using tightly fitted gear doors could result in decreased drag (versus fixed gear), but at the expense of more weight and complexity. Additionally, if a gear leg did not retract fully, or a door “floated” during high G maneuvers, asym- metric drag and lift could result causing trim problems in both yaw and roll. My father and I flew a number of fixed gear Pattern air- planes (Dirty Birdy, Deception, Curare) in the mid to late 1980s very successfully in competition, but succumbed to the peer pres- sure for the aesthetics of retracts in the late 1980s.
by dave lockhart with algirdas ungulaitis You can reach Dave Lockhart via e-mail at
davel322@comcast.net
The revolution to the fixed gear of today’s Pattern aircraft was started by Australian F3A Champion Peter Goldsmith in the mid- 1990s with his conventional fixed gear Car- rera. Peter had been afforded access to a state of the art wind tunnel at an Australian university, and found a cleanly installed set of airfoiled struts and panted wheels were of lower drag than a typical retract setup. Peter’s appearance at the US Nats (1996 to recall) sparked the revolution to fixed gear in the US. Among the early adopters were Hanson Models with the EMC2, which flew extremely well with either retracts or fixed gear. In 1999, I flew a fixed gear Hyde- out, and that year I would say about 10–15% of planes had fixed gear. By the early 2000s, very few were building new planes with re- tracts, and fixed gear was the majority on the flight line.
While wheel pants are an integral part of a low drag fixed gear setup, and essential to maintaining the good looks of a Pattern plane, they are often absent due to breakage or a maintenance nightmare. Even the strongest and highest quality wheel pants have little chance of staying attached when used on grass fields if they are not properly mounted. A little extra time spent on proper installation will ensure many many trouble free flights. The method presented herein is the basic installation method I have used for many years, and while it is very lightweight, it has proven to be quite reliable, even on the less than ideal grass fields from which I do 95% of my flying. The method I use is notable for several reasons; the wheel pant is supported at both the inboard and outboard edges, the wheel pant can rotate under heavy load (preferred to breaking), and little more than the wheel axle is used to mount the wheel pant. The wheel pants supplied with Yuri are, in my opinion, close to the ideal ratio of weight and strength. They are molded of traditional fiberglass, which is good materi- al to flex and absorb the bumps incurred on grass fields. They also have a molded pocket/ recess for the gear leg, which helps reduce the likelihood of the wheel pant rotating on the gear leg. Designed for 65 millimeter (2.5-
inch) wheels, I found I was able to squeeze in 2.75-inch wheels (70 mm). My preferred wheel is the DuBro Super Light; I find them to be a very good combination of lightweight, stiff sidewall (unlikely to snag the edges of the wheel pants), with reasonably good wear characteristics.
Starting with the outer edge of the wheel pant, I added a small patch (~1-inch square) of carbon fiber cloth and then a smaller square (~ ½ inch) of 1⁄16-inch balsa (soaked with thin CA). The outer edge of the wheel pant was then drilled to be a slip fit for the head of an 8–32 socket head bolt. The inner edge of the wheel pant is drilled to be a slip fit for the threaded portion of an 8–32 bolt. The gear legs also need to be drilled to be a slip fit for the threaded portion of the 8–32 bolt. I generally drill the wheel pants with a tapered round file to avoid splintering the thin fiberglass with a drill bit. When drilling the gear legs, use a very sharp drill bit and light pressure to avoid fracturing the gear legs.
The holes I drilled were very close to the bottom edges of the wheel pants and gear legs; doing so allowed the use of the 2.75- inch wheels, and left a greater portion (about 40%) of the wheel exposed below the wheel pant. While not as aerodynamic, the bigger, more exposed wheel reduces stress- es on the wheel pant and gear legs on rough grass fields. Locating the holes slightly higher and using a 2.5-inch wheel will re- sult in approximately 25% of the wheel be- ing exposed. Two-inch partially threaded socket head bolts are used for the axles. While 2 inches is more than long enough, this is the length that has approximately ¾ inch of unthread- ed shoulder, the area on which the wheel will spin. The hubs of the wheels are drilled out slightly, just enough to spin freely on the shoulder of the 8–32 axles. Assembly/attachment of the wheel pant,
wheel, and axle happens in one process (multiple steps):
1. The 8–32 bolt is inserted into the outer edge of the wheel pant. 2. 2–3 #8 nylon washers are put on the
bolt.* 3. The wheel is inserted into the wheel
PHOTOGRAPHY: DAVE LOCKHART
The axle assembly, used to mount both the wheel and wheel pant (above left), consists of an 8–32 allen head steel bolt, nylon washer spacers, 8–32 nut, and 8–
48
32 nylon locknut. Accurate location of the holes on both sides of the wheel pant (above right) is critical to getting proper alignment and clearance for the wheel.
JUNE 2013
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