Slope gliders have long been the fastest airplanes in model flight. “Fastest Model Ever Flown” in the August 1978 issue of MA is an account of someone flying a Slope glider to 242.9 mph. That was on the front side of the hill.
In the 1990s, Joe Wurts demonstrated practical Dynamic Soaring (DS) for model sailplanes—flying a pattern on the backside of Parker Mountain in Acton, California, and breaking this 1978 speed record.
As of the time of this writing, 11 RC pilots had flown faster than 400 mph. At the top of the list is Spencer Lisenby, age 42, from Calabasas, California. I asked Spencer some questions about his Slope Soaring career.
Dave Garwood: When and where did you first fly an RC glider? How about your first Slope flying and DS experiences?
Spencer Lisenby: My first attempts to fly were with the Zagi at Stone Mountain in Georgia. I soon learned of other pilots who were into RC Soaring and started meeting up with them whenever the wind would blow. Our typical meeting place was a small slope on a construction site on the side of the road. It wasn’t much, but it made for some really fun close-in combat while scratching for lift.
I remember being blown away by Billy Ott’s smooth flying and acrobatic moves. That’s when I learned about dual rates and exponential and started thinking about getting a computer radio. Billy told me about a friend who knew how to DS at the dam on Carters Lake.
I met John Kesler and he taught me all about Dynamic Soaring. My first challenge was to dive in and make it back out to the front side. Next, I tried to see how many consecutive laps I could make before the inevitable crash.
After hours of practice and almost as many hours spent repairing the Zagi and JW, I was able to string together some decent laps and build up some speed.
The DS zone at Carters Lake was usually turbulent because of its shallow backside, but it really hones the reflexes and forces you to learn quickly or pay the price of hitting the rocks or the steel guardrail.
On one occasion, turbulence forced an unscheduled landing of my Mini Blade deep into the lake behind the dam. It was wintertime and cold, but I didn’t think twice before stripping down to my skivvies and swimming out 100 yards to retrieve the still floating airplane. After a good drying out, everything worked fine again! We spent hours combating and DSing with our JWs, Sonics, and Zagis. Those were great times.
DG: When someone asks, how do you explain how DS works?
SL: I start by having them consider the airplane’s airspeed through a single shear crossing. Let’s say you’re going 100 mph (airspeed) on the backside and you cross the shear layer into a 20 mph headwind. Now you have 120 mph of airspeed. Return as efficiently as possible and repeat this process. Regardless of the airspeed before the shear crossing, you always come out with 20 mph more afterward. After they get this, we cover the second shear crossing where the tailwind is lost, resulting in the same airspeed gain.
DG: You say headwind when crossing the shear layer. That seems counterintuitive.
SL: The backside is the part of the ridge that is downwind and typically has little to no air movement. So coming out of this lee behind the ridge (heading upwind), you enter the 20 mph wind that is present on the front side. The airspeed will increase by 20 mph, so that is a headwind. You then make a 180° turn on the front side and begin going downwind.
Now you have a tailwind of 20 mph. As you cross the shear the second time, the glider again feels a 20 mph headwind as it goes from the 20 mph moving air into the still air. Each shear crossing presents a headwind for the glider. In between shear crossings, you transition from a headwind to no wind (on the backside) or from a headwind to a tailwind (on the front side).
DG: How would an interested RC Slope pilot begin in DS?
SL: I’d recommend reading up a little on the RCGroups DS forum and watching some slower DS videos on YouTube (where you can actually see what’s happening). Hopefully, then you can find someone local who can show you the ropes at a hill that has already been proven to be adequate for DS. This type of introduction to DS will save you a lot of time and potential frustration.
If you can’t find an experienced local to help, I recommend putting a post on the RCGroups DS forum and you will get all kinds of support. There is a ton of knowledge and experience on RCGroups, and everyone is eager to share.
You can learn to DS with plenty of different airplanes, but it’s hard to beat a 60-inch EPP JW from L2 Airframes. It can be built to a wide range of flying weights to suit your conditions and is a good balance of efficiency vs. durability. When out on the hill, fellow experienced DSers will be your best help. They can help evaluate your airplane’s airworthiness and demonstrate a good line to fly.
If you can process verbal input on the fly, a good coach can have you flying nice lines in no time. When you are ready to give it a go, don’t be timid. Get as high as you can on the front side and dive in with as much speed as possible. Go deep to get under the turbulent shear layer and stay as low as you are comfortable with.
Diving in with insufficient speed and flying a high line is a surefire formula for trouble. First and foremost, plan ahead for a safe place for yourself and anyone else on the hill to stand before diving in.
DG: What encouraged you to pursue DS speed records?
SL: After getting the hang of the Dynamic Soaring (DS) basics at Carters Lake, I couldn’t help wanting to go to California and experience the real deal that I had read about in the magazines. I really wanted to reach a speed above 150 mph so I could make it onto the speed list that Nathan Woods used to maintain on sloperacing.com. I finally broke down and flew to Parker Mountain to give it a try. Little did I know that summertime wasn’t the best time to DS at Parker. After a couple days of nonstop flying, I’d had a blast, but still hadn’t broken 150 mph. It was great to visit the birthplace of RC DS, but I went home slightly disappointed about not reaching my goal.
One afternoon, I flew my F3F Extreme to exactly 151 mph on radar and finally made it onto Nathan’s speed list. As I savored the moment, it occurred to me that I was hooked!
DG: What encouraged you to pursue glider design?
SL: I majored in mechanical engineering at Vanderbilt University, so I’ve always been interested in flight and all things mechanical. At the time, when I started DSing molded airplanes, it became clear that most so-called DS airplanes were really just F3F or sport gliders built with a heavier DS layup.
I and others had gone through many models only to find that they had structural limitations or lacked the desired performance or efficiency. As speeds increased, it was clear that all existing gliders were designed for a lower speed range than was now possible. The time was right to explore the potential of a DS-specific design.
DG: What characteristics are important in DS glider design?
SL: This is a question that you could ask several people and get valid, but different, answers. All DS gliders are a result of compromises. My designs tend to lean more toward all-out performance than things such as ease of construction or economics.
Durability is also a major consideration. Typical DS sites have unforgiving landing zones with strong, turbulent winds. Nearly any design has to make some compromises for the sake of being able to land consistently without damage.
In the air, DS airplanes are subjected to a lot of abuse such as large accelerations upward of 90 Gs and strong, turbulent shear crossings. Structurally, these influences favor low-aspect-ratio wings with thick airfoils and large cross-section fuselages.
Aerodynamically optimizing the airframe tends to go against the structural criteria. Higher-aspect-ratio wings are necessary to decrease induced drag. Thinner airfoils can reach higher speeds before encountering Mach drag rise problems. Smaller cross-section fuselages are desirable to reduce parasitic drag. These are just some of the compromises that must be dealt with on a DS glider design.
DG: What are your personal DS glider-design objectives, and how do you prioritize or resolve conflicting objectives?
SL: My primary design objective has always been to fly faster. Some days this means flying faster in light to medium wind conditions. Other days this means flying faster in all-out ballistic conditions. The Holy Grail would be a DS airplane that performs equally well in all conditions.
Although some things such as ballast can be changed to suit specific wind conditions, other components such as airfoils must be chosen with a specific design goal in mind. I concentrate first on the ultimate performance in ballistic conditions, then second on the medium wind performance. If everything still shows good, stable behavior at slow speeds, then I call it a winner.
DG: What will we see next from your design work?
SL: At high speeds, transonic drag rise problems arise. Drag from the wing increases dramatically because of the formation of shockwaves on the upper wing surface. At fast speeds, portions of our wings are can reach high velocities because of the acceleration of air over the upper surfaces. Either supercritical airfoils and/or swept wings are required to increase speed.
I am currently working on a conventional airframe with a swept wing. I want to explore the aerodynamic and structural issues that arise with this approach. There are plenty of great aircraft with swept wings, but none that combine the need for transonic efficiency with the ability to pull 90 G turns. I hope some valuable insight will come from this effort.
