Author: Dave Garwood
Edition: Model Aviation - 2006/08
Page Numbers: 124,125,126,128
,
,
,

Radio Control Slope Soaring - 2006/08

Dave Garwood [[email protected]]

A flight report of the Thorn from Plane Insane Models

There is a new concept in high-performance sailplane design and kit production, brought to you by a maker you may not have heard of: Plane Insane Models in Albany, New York.

The Thorn is a 60-inch-span fast slope soarer, made entirely from solid wood by modern computer-controlled machining processes. The model has been in production since January 2006 and, as of this writing, has flown in California, Colorado, Massachusetts, Michigan, New York, and Australia.

I built an early production kit of the Thorn and flew it. This column contains a brief flight report and an interview with designer and maker Andreas Mergner.

Also included in this column:

  • An interview with Thorn designer Andreas Mergner

The Thorn kit consists of solid-wood parts, machined and sanded:

  • Fuselage
  • Wing halves
  • Canopy
  • Fin
  • Horizontal stabilizer
  • Ailerons and elevator
  • Bag of hardware and printed instructions

My model took 7.5 hours to build, 4.25 hours to finish, and 3.5 hours to install the radio gear. It weighs 59 ounces, for a wing loading of 25 ounces per square foot.

Its first flight exceeded an hour in duration, flown by Joe Chovan and myself over Lake Ontario near Syracuse, New York, from a 115-foot escarpment in 15–20 mph wind blowing into the hill approximately 10° from perpendicular. Many thanks to Joe for flying for the camera.

The Thorn was in its element and was impressive in the air. It penetrates extremely well in slightly cross and gusty conditions. It displays smooth movements with no sign of twitchiness or overcontrol.

My version is moderately quick in axial rolls, with more than one per second. It does not love inverted flight. It does love the Cuban Eight maneuver. It handles stall turns well, but an intentional forward stall was dramatic; it fell 20–30 feet before recovering. It is good to fly this speedster like the heavy-wing-loading sailplane it is and avoid stalls. This is not a trainer, but if you fancy the fast, heavy stuff, the Thorn will bring a smile to your face.

A full kit review will follow in a future issue of MA.

Interview with Andreas Mergner (Plane Insane Models)

DG: Response from Thorn early adopters as reported on RC Groups has been phenomenal. Anyway one looks at it, you have a cutting-edge sailplane here. This must make you feel great as a designer. Please tell us about your design philosophy.

AM: I try to take into account performance, durability, ease of building, and value in the design. I also try to keep things as simple as I can. My goal was to get within 90–95% of the performance of composites, and from the feedback I have received, it looks like I met and exceeded my goal. I also really enjoy experimenting with materials and construction techniques. I like to think of my design as using relatively light wood, a thin airfoil, and high-aspect-ratio wings to reduce the overall weight.

DG: Stiffness is a major advantage of wood—an essential characteristic of high-performance model sailplanes. What less desirable characteristics of wood have you had to deal with, and how have you overcome the problems?

AM: Actually, wood is not as stiff as most composites out there, but there just happens to be a lot of it in the airplane in the right places. So the wings are stiff under flight loads, but flexible during higher crash impacts.

One disadvantage to wood is warping due to water-content changes. For this reason and a few others, I use poplar wood, which is very stable and not as prone to warping. Sealing the wood with paint, stain, or covering will also reduce the risk of warping. All that being said, I haven't had a problem with warping in my prototype over the last couple of years. High weight is the other major disadvantage.

DG: Model parts have been made by die-cutting, machining, molding, and hand-shaping. Your manufacturing process is revolutionary for a model kit maker. "Computer controlled machining" is mentioned on your Web site. Please tell us generally how computer-controlled part-shaping is done in general and how it is used to make the parts for Thorn kits.

AM: Computer-controlled machining has been used for a long time and is generally referred to as CNC, or Computer Numerical Control. Only recently has it become cheaper and easier to buy a machine or make one yourself, which is what I did.

The process of making a part using CNC is done in a few steps. First you design a model of the part you want to make using CAD (Computer Aided Design) software. This model is then put into other software called CAM (Computer Aided Machining). The CAM software is used to produce a tool path that the router will take to cut out the part on the machine.

Finally, the cutting path is put into the controller (software or hardware) for the machine, which moves the router along the path using motors. There are typically three motors in the machine that control movement of the router: one for the X (left to right), Y (front to back), and Z (up and down) axes.

My general production process is to secure a piece of stock wood in the machine and start the cutting path. The router first takes off the bulk of the wood with faster, coarser movements. I then change bits and the router makes smaller, finer movements to get a final shape on the part. Usually I will need to do this for both sides of the part. After the machining, I sand the parts by hand using a random orbital sander to get a nice, smooth finish.

DG: I've seen others of your designs in the air—your smooth-finish EPP wing, your EPP One Design Racer, and your scale Hurricane—but none of these has caught on with fliers like the Thorn. Please say a little about the origin and development of the design for the Thorn in particular.

AM: I used to live and fly slope in Tucson, Arizona. One flier built a solid-balsa wing for a fuselage of an old model called the Tempest by Scott Metze. It flew like the business and was fast, retained energy well, and was efficient. It had a single-taper wing and was sanded and carved by hand using templates on the root and tip. This was probably nine years ago. I always had it in the back of my mind that I would build my own but didn't get around to it until recently.

I thought that a solid-hardwood sloper would be a good middle ground between EPP and composites in terms of performance, durability, repairability, and value. I ended up making a proof-of-concept model out of cheap lumber. I shaped it using an electric hand planer and a belt sander along with some templates. It was ugly and very rough, but it flew okay enough to pursue the next step to a machined design.

The machined design took a lot of effort to get something that would actually work. I made about 20 versions before coming up with one that I would actually make and fly. My first prototype flew great right from the start, which was quite a relief. I continue to refine the design even today.

DG: Could you discuss the considerations behind selection of solid wood as a sailplane construction material? At first it seems like it might be too heavy, then one realizes we do add quite a bit of weight to our sailplanes when we want them to go fast.

AM: I am not the first one to say that weight is good on a slope airplane. I think since traditionally most slopers start out learning to fly thermal duration, they get used to construction that is very weight-conscious. This is great on the light-lift and wind days, but as the lift and wind increase, things change.

More speed actually makes it safer for you to fly in greater wind up to a point. Instead of having to fly straight ahead into the wind the majority of the time, you can move to any point in the sky with ease.

I think there is also a feeling that an airplane that is twice as heavy will need twice as much lift/wind, which is not the case. It will require less than 40% more lift/wind since it is related to the square root of the weight. The Thorn does not require as much lift to fly as you'd think.

DG: What's next for Thorn development? What other new ideas might we see coming from Plane Insane Models?

AM: I have more ideas for cool products down the road. Pretty much all of them will be made from solid wood.

I recently made a 100-inch Thorn prototype, which flies great but will be going through more testing before release. It flies in lighter lift than the 60-inch version but should be just as fast. It has very effective camber and crow due to the four-servo wing.

I have a prototype European-style flying wing (very fast) that I am hoping will work out. It's tentatively named the Decapitator. I also have an idea for a wingeron/pitcheron-type airplane that may be a bit more durable than usual.

I have a mailing list on my Web site that people can join if they are interested in finding out about new products as they become available.

Thanks, Andreas, for bringing these unusual and high-performance airplanes to market.

I have flown the Thorn 60-inch-span and 100-inch-span versions, and both are highly impressive in design, in quality of parts supplied, and in flight performance.

You can contact Andreas Mergner at Plane Insane Models:

MA

Transcribed from original scans by AI. Minor OCR errors may remain.