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Aeronaut Schempp-Hirth SHK
I’m one of those scale modelers who is always looking for something unique that no one else has. It seems as though everyone owns at least one modern glass ship in their fleet of scale model sailplanes, such as from the ubiquitous Schleicher or Glaser-Dirks line, and I didn’t want to bring yet another one out to the next aerotow event. I did, however, want something sleek and sexy, with the appeal of a glass slipper but the character of a ‘woodie’. Then one day I heard about the 4-meter Schempp-Hirth SHK from Aero-naut Modellbau in Germany, and I knew instantly that I had found what I was looking for. Here was an elegant and pert-looking sailplane in nearly quarter-scale, with a tall and slender V-tail and a sleek canopy that looked like something off a Maserati sports car. Not only that, it turns out the full-size Schempp-Hirth SHK was one of the last all-wood sailplanes to be manufactured, before the advent of the all-glass sailplanes, so it had the aura of a classic. It appeared to be perfect for aerotowing and winch-launching, or even slope flying. Not only that, but it was available at an affordable price from one of my favorite US distributors, Hobby Lobby International in Tennessee. I made the call and got one sent out to me straightaway.
The kit is very complete, with an extensive parts list in the instruction booklet. Among the items included in the kit is a beautiful white gel-coated fiberglass one-piece fuselage, with a matching tail cone fairing that stovepipes onto the aft end with a perfect fit. The fuselage is stout and light, and the wall thickness is just right for the job. I could tell there would be no need for additional internal reinforcement. The wings are obechi-over-foam, with the ailerons already cut out and faced with balsa, ready for cover and hinging. The servo wells and spoiler channels are pre-routed, and the wing roots are precision-bored to receive the joiner rod sleeves. Schempp-Hirth style double-decker fence spoilers are also furnished. The tail feathers are balsa sheeted foam, with the pivot wire hole bored and sleeves pre-installed. Two canopies are included, one clear and one blue-tinted, in case the builder prefers a blue-tint canopy. I decided it would make a good replacement canopy if I ever damage the clear one. There is also a nifty vac-formed seat tray and seat back, along with a separate instrument panel console, and a vac-formed frame for the canopy. With a full-body pilot installed, no part of the radio equipment can be seen from the outside. Nice! There is also a half-size plan included, with expanded drawings showing some of the assembly details that require particular attention, such as the tail bellcrank actuators and the vee-tail alignment information. The instructions are in German, but an English version is available over the internet, and the translation is nearly flawless. The Build
Construction begins with the die-cut lite-ply tail box. A ply former is glued into position in the aft end of the fuselage and a triangular cross-section box extends aft from this former. Installed on this box are the cast-aluminum torque tube bellcranks. These clever pieces have slots in the output bellcrank to provide the ruddervator actuator pin some free play as the bellcrank rotates the ruddervator. The slot is just long enough to allow the full range of translocation during rotation, so care must be taken to locate the bellcrank axis the correct distance from the ruddervator pivot axis. The ruddervator pivot wires are installed to the box and former assembly, and care must be taken here to ensure good alignment of the 92o vee angle. I used all my favorite visual alignment techniques to good avail, and got everything set up just right. The tail fairing is then slotted for clearance so that it can be slipped past the pivot rods and tailwheel strut. The tail feathers feature brass teardrop-shaped counterweights, which extend forward from the leading edge out near the tips. These are screwed into 2-56 threaded sockets buried in the tip blocks. They seemed rather vulnerable to me, and sure enough, one of them broke off on my first outing with the SHK, so I have subsequently replaced the threaded sockets with larger diameter brass tubing. Now, the counterweights merely plug into place, and are more solidly mounted. The ruddervators are retained with small grub screws in the root ribs, which engage with a groove in the pivot wire socket. I thoughtfully found a hiding place inside the fuselage for the required Allen wrench, so that it will always be with the airplane wherever I go. The wings are largely pre-fabbed, so that all that really needs to be done is to glue and shape the balsa tip blocks, install the servos in their wells, and install the spoilers in the spoiler grooves. The joiner rod sleeves are brass tubing, which must be capped on one end with ply or balsa, buttered up with glue, and inserted into the precision-bored holes. The kit supplies a small container of polyurethane glue for this task, but I felt more confident using epoxy. With the center brass sleeve glued into the pre-formed wing root holes in the fuselage, the wings can then be installed with the steel joiner rod, and checked for proper alignment while the epoxy sets. I found no need to make any adjustments to either wing. There is an index mark on the fuselage root to indicate the location for the alignment pins. I was quite satisfied with the results, and the incidence meter gave proof that everything was in good alignment. At this point, the wings can be covered. I chose to use white Oracover on the top surfaces, and red on the bottoms, for better visibility in the hazy summer skies. The pushrod outer sleeves were pre-installed inside the fuselage. I found that the only place the ruddervator servos could go was just behind the seat tray. This location worked well, for accessibility and for clearance with the seat back. I chose to install a retract wheel in my SHK. The fuselage is scribed for the gear doors, so it is only a matter of cutting them out, installing a retract wheel assembly with partial ply formers, and hinging the doors. I attached the seat back to the seat tray with a flexible hinge, so that it could be swung forward for access to the retract wheel, should that be necessary. The SHK is made to an odd scale, at a 1-to-4.25 scale ratio, but I found that a fifth-scale retract wheel diameter of 70mm looked just right. I built a wood box in the nose to accept the receiver battery, per my usual practice, and installed a wire-over-hole aerotow release underneath the battery. I poured off 11 ounces of lead, into a simple mold which I made by wrapping the nose of the fuselage with aluminum foil and pressing it into a blob of clay. The lead slug that resulted from this process was then glued inside the nose of the fuselage with silicone adhesive. In this manner, the CG location of 90mm aft of the wing leading edge was achieved. All told, the build process consumed approximately 40 hours of bench time, during which no difficulties were encountered. The kit seemed well considered and well made in every respect, and the all-up weight of 168 oz was quite reasonable for a sailplane of this size. Flying the SHK
My first outing with the SHK was in the light lift conditions of western Canada, at the annual aerotow event in Mission, B.C. Even though the skies were mostly damp and grey for all three days, and the lift was never outstanding, I was able to achieve a good handful of extended flights of well over a half hour duration, which speaks well to the ability of the SHK to work weak thermals. My first flight with the SHK was a non-event, with only minimal trimming required. Coming off a high tow, I had oodles of time to get used to the handling and assess the program mixes. I experimented with the tail throws, to see if any differential was needed, and soon determined that full rudder input caused no noticeable pitch changes. The SHK is a well-balanced airplane! I also decided that I had programmed just the right amount of aileron differential, but that I needed a bit more aileron throw. In addition, I needed to add a bit of up-elevator with full spoiler input. There were no trim changes when the wheel was extended for landing.
I have since been able to fly the SHK in extreme lift conditions, and over the full range of airspeeds and through turbulence and crosswinds. It has proven to be a strong and capable machine, with all the performance of a glass slipper, and all the grace and beauty of a wooden classic. The Aeronaut SHK is definitely a keeper!
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On the second flight, and all subsequent flights, everything was dialed in perfectly, and I was able to enjoy the smooth and predictable flying characteristics of the SHK. I was impressed with its almost liquid handling and easy responsiveness. Thermal turns are readily ‘locked in’, and the SHK can climb with the best of them. The stall is quite tame, and there are no surprise tip stalls lurking in the radical maneuvers. Landings are easy, with no need to flare, due to the pronounced ground effect, and with an impressively scale-looking roll-out.