Here is a guest column by Joe Chovan, an engineer, model airplane designer, and longtime Slope Soaring friend of mine.
Dave “Old Buzzard” Thornburg wrote in his Old Buzzard’s Soaring Book, “The slopers’ motto is, it doesn’t have to fly, as long as it turns.”
Some may recognize that this correlates with, “Anything can fly with enough power and a way to control it.” Of course, this describes one end of the spectrum in our Slope Soaring community, claiming many of the slowest and fastest RC aircraft on Earth.
Whisper-quiet, in-your-face Ultrabatics and blistering 500 mph Dynamic Soaring aircraft share one important trait: they don’t use motors for propulsion, yet both play within Dave Thornburg’s “river of air.” How can they do that?
To perform Slope aerobatics, we convert potential energy to kinetic while extracting energy from available lift. In essence, we fly down through rising air to propel us forward, choosing a path that prepares us for the next maneuver. That primary requirement directs Slope Soaring performance challenges.
There are many factors, and all designs are compromises. We choose traits and design sailplanes to fulfill them. We use terms such as energy retention, penetration, aspect ratio, and efficiency more than designers of powered airplanes, who normally only speak of wingspan or wing loading, and possibly tapered, rectangular, or perhaps sweep when describing the wing planform.
Sal Defrancesco, of Northeast Sailplane Products, said he could reliably predict the characteristics of a designer’s home slope by studying his or her sailplane. A large hill with a vertical slope presents less headwind and more vertical lift, so penetration may not be as important for a sailplane designed for a flatter hill with higher wind. An alpine location will have periods of strong lift then sink, making long, efficient, possibly reconfigurable, wings desirable.
A rocky terrain will entice fliers to choose EPP foam or other durable materials compared with an airplane-friendly grassy landscape. Vast expanses of low and gently rolling pasture beckon for a sailplane with long “legs” to explore lift at great distances, compared with a small or bowl-shaped hill that invites a nimble acrobat to dance.
So what are airplane design parameters and how do they apply? Here are the ones I consider the most important.
Wingspan: The Chief Discriminator
There is a reason sailplanes have long wings. A wing providing lift has pressure on one side, causing air to spill off the end. You may have seen wingtip vortices produced by full-scale sailplanes in moist conditions. This “tip loss” wastes some lift, and shorter wings suffer a larger percentage than longer ones.
The aspect ratio describes the wingspan/chord dimensions, and a larger ratio generally characterizes a more efficient wing. However, long wings have higher moments of inertia, so rolling prowess favors shorter wings. If the lift is light or changing, choose a sailplane with a greater wingspan. For maximum speed and turning performance, wingspan is king.
Wing Area, Thickness, and Planform
Bigger flies better. The more air molecules we can harness in our flight pursuits, the better. Larger surfaces let us turn and control faster. We don’t want thick wings because thicker is draggier, but if wings are too thin, they can exhibit poor turn and stall characteristics. The price of being large is drag, which is normally not good for sailplanes. We seek to minimize all of these dimensions, but need to consider where to best utilize area and volume.
Perhaps the worst trait any wing can have is a propensity to tip stall, which is when the wingtip loses lift before the root. This causes a snap, and often a spin, with altitude loss. Highly swept and tapered wings are the most likely to tip stall.
An elliptical wing planform has the lowest induced drag. Many efficient sailplanes utilize a single or multi-tapered wing planform and minimal sweep, with thickness decreasing from root to tip. This approximates a portion of the ideal elliptical wing planform with an easy-to-build, structurally stiff, minimal drag compromise.
Wing Loading: Speed and Weight
Any powered aircraft flier will tell you that weight is the enemy. The worst thing you can do to a powered airplane is add weight, but we Slope Soarers may trample this maxim. Our mass, speed, and altitude define our stored energy, and knowing our energy state enables options at each moment when Slope flying. In general, the more wind, the heavier a sailplane needs to be to punch through that wind, and the faster, higher, and farther it may venture.
Wing loadings of 40 ounces per square foot or more may be flown on a windy, large hill. Things happen faster and you must plan your moves well in advance, but your freedom is directly proportional to the weight you carry. Conversely, in light lift we seek to minimize loading to stay on top of the lift, rather than penetrate it.
The Importance of Airfoil
Unless you are competing, airfoil is the least important parameter. Even a Carl Goldberg Gentle Lady with a flat bottom airfoil can be strengthened and loaded to fly fast if needed. Wings can be damaged, repaired, warped, painted, and generally mistreated in ways that may perturb the pristine airfoils originally molded into them, and still fly well. Even careful builders admit repeatability is difficult to achieve on some critical airfoils.
In general, as we wish to minimize drag, I’ll pay homage to the venerable RG 14 and SD7037 airfoils to represent fast and slow extremes. These are relatively thin at roughly 9% maximum thickness/chord length. Because they are semisymmetrical, they generate lift from forward motion.
Build for Durability
Slope flights can last for several hours, delaying the inevitable. For faster sailplanes, why not build strength into the weight of the model in key high-stress areas? Add extra fiberglass to the wing roots and fuselage fore and aft of the wing. Choose thick basswood for the leading edges. Make vulnerable tips and edges from plywood or other laminate.
If you are flying in a rocky area, consider sailplanes that bounce. EPP foam is an amazing medium and can rival even fiberglass/carbon composite for looks and performance. Add weight as needed to balance, and strive to keep weight out of the tail and control surfaces to avoid problems with inherently flexible designs.
If wind is soft, choose a lightweight sailplane that won’t be damaged—no matter how it lands. Even settling into a tree may not be so bad if the sailplane can support itself in the branches that cradle it.
A transparent thermal glider might be the perfect tool to explore a light-wind day on a hill that collects thermals. Its landing could be a beauty to behold, even among the thickets.
The Importance of ‘Full Quiver’
Like wind or kite surfers who carry multiple sails to maximize opportunities, a wise Slope Soarer will bring multiple sailplanes to the hill—especially if it is far from home. It should be obvious by now that choices are manifold, and having the right tool for the job lends fuel to the passion for exploration. So many sailplanes, so little time …