[Courtesy of Pete Young, December 1998 - as published Pete's RCM Soaring Column]
Hobby Hangar's Windfree is a recreation of the classic sailplane which Mark Smith used to win Standard Class at the AMA Nationals in the early '70s. Flying against much more sophisticated ships with his original two function design with rudder and elevator controls only, Mark won the Nats from 1970 through 1974, a string of consecutive wins which has not been duplicated before or since by any other flier or design in the sailplane events. With the resurgent popularity of the Nostalgia class designs, the competitive heritage and affordable price of the Windfree makes it a very serious contender for '90s sailplane flying with standard sized radio equipment.
Looking at the Windfree's aerodynamic design, the fully tapered wing catches your attention right away. One thing for sure, the wing sports the highest aspect ratio around, computing out at 18.6 to 1 where values of 10 to 12 are more common. The two-piece wing is mounted to the fuselage by two 5/32" diameter music wire wing rods, thus eliminating the need for any hooks, rubber bands, or other untidy wing retainer details. The sheet balsa fuselage, although looking somewhat boxy by today's standards, provides plenty of room for two standard servos for the rudder and elevator. With the wing mounted rather solidly to the fuselage, installing spoilers shouldn't be difficult - spoilers are a legal modification to this design for Nostalgia class competition. Spoiler installation details are not provided in the plans or instructions but this shouldn't be a hard detail to add: the spoilers can be sections of 1/4" x 1 trailing edge stock opened by spoiler lines connected to a fuselage-mounted servo.
Close examination of the kit's contents showed good balsa, spruce, and ply stock, rolled plans, and diecut wing ribs. Our Windfree construction started with the wing panels as is our usual custom. Since the wing planforms are fully tapered, each wing panel's ribs change in geometry from the root to the tip; to avoid any possible confusion, it's good common sense to sort and number the ribs before you start gluing them in place. The wing panels, due to their almost 50" length, require the builder to splice the 3/32" x 1/4" spruce spars, 1/4" square leading edge, and 1/16" balsa sheet leading and trailing edge sheeting during construction to make up the proper lengths. Do a good job on the splicing as the wing panels take quite a bit of bending load during launches and flight.
The wing is a conventional D-tube structure with 1/16" sheet used for leading edge sheeting and trailing edges. Balsa inter-spar shear webs and capstrips complete the basic wing structure and the written instructions are fairly clear on the necessary building sequences. I did make one minor, but significant, change during assembly. The instructions ask the builder to epoxy the 3/16" OD brass wing rod tubes into the wings during the wing panels' final construction, but before the fuselage-mounted wing rods are available. If you epoxy the tubes into the wing panels at this time, there is absolutely no guarantee that the tubes' spacing or incidence alignment will be correct during final wing panel installation, so I deferred this critical step until later in the building sequence.
Fuselage construction is next and proceeded in a fairly straightforward manner. The 5/32" music wire wing rods are "J" bolted to a fuselage former and two ply doublers (F2-5) provide the proper incidence and alignments for the two wing rods; be careful when installing the F2-5s, it would be very easy to install either one upside down - my advice is to check the F2-5s against the plans, mark them for proper orientation, and double check their orientation before gluing them into place. Incidentally, the music wire rods are supplied pre-bent but were not at the angle shown on the plans; it was a simple matter to bend them to the proper angle but the discrepancy is curious.
Once the fuselage is framed up and the wire rods are initially installed with their "J" bolts,it's time to install the wing panels onto the wire rods at the proper dihedral, incidence angles, and sweepback. Step 20 warns you that "...this step is one of the most difficult, so take your time..." and they're absolutely right in saying that. After some thought, I first aligned and epoxied the "J" bolts so that the wing wires symmetrically exited the fuselage sides; then aligned the wing panels to the fuselage as per the written instructions. This is where I was really glad that I had not previously epoxied the wing rod tubes into the wing structure, as previously mentioned above, as it was now very easy to make minor adjustments to fit the wing panels snugly to the fuselage sides.
During this step, it became apparent that there were no tip dihedral measurements called out on the plans or in the instructions. There are instructions for the wing rod tubes to rest flush against the lower surface of the top spars, and there is an angular measurement for the wing rods themselves, but no specified tip "rise". I made sure that I followed the instructions and made sure that both sides were the same - for the record, I measured my final tip dihedral at 6" which works out to 7.1 degrees tip dihedral. I hope it's close to what the designer intended.
Turning to the tail surfaces, the "full flying" horizontal stabilizers are very lightweight structures to fabricate. Balsa cap strips are formed over a central spruce spar, meeting up at the leading and trailing edges to form a symmetrical stab airfoil section. The vertical stabilizer is 1/8" ply and balsa and very lightweight. Incidentally, I used Satellite City's cyano-acrylate products for construction, augmented with Elmer's wood glue for joints with high porosity, and had excellent results throughout.
The horizontal stabs are driven by an externally mounted ply bellcrank which is a pragmatic way to actuate a horizontal stabilizer located, as in this case, above the wing's downwash effects. What I did find a little distressing is that the plans show the linkage installations for the horizontal's bellcrank and the rudder both on the left rear side of the fuselage - this makes for a crowded and untidy rear fuselage back there with clear possibilities of interferences. It's a simple move to rearrange the linkages so that the rudder and horizontal's linkages exit the fuselage on opposite sides and I recommend you do the same.
By the way, I will mention here that there are scant details provided for installing the radio equipment or linkages. This is an annoying oversight which I hope Hobby Hangar corrects in the future. There is a note suggesting that pushrods be installed before the fuselage top sheeting is installed; I didn't do this and subsequently had some difficulty fitting solid pushrods back to the rear of the fuselage later on during construction, so fit your pushrods as early as you can. I also ran into difficulty during pushrod fitting as the vertical fin's 1/8" ply core, sandwiched between the rear fuselage sides, gets in the way - the ply core requires trimming away to provide clearances back there for the rudder and elevator linkages, so do it earlier rather than later.
That just about covers the out-of-the-ordinary items during construction. The fuselage takes a bit of sanding to get the desired contours, so break out the sandpaper and use some old-fashioned "elbow grease" when doing this task. To get an accurate leading edge contour on the wings, I drew a reference line which marked the "nose" of the airfoil, then carefully planed and sanded the leading edge to get an even airfoil contour. After careful fine sanding of the entire structure, it was time to apply the final finish. I used Monokote on the wings' and horizontal stabs' open structure, and Black Baron polyester film on the wings' sheeted D-tubes, prepping the balsa beforehand with Coverite's Balsarite to provide better adhesion. The decorative vinyl graphics were provided by Don McColgan from Comp-U-Cut Vinyl Products (976 W. Foothill Blvd, #328, Claremont CA 91711). Be sure to add the recommended 3/16" to 1/4" washout to the wing panels to provide a smoother stall response; the actual washout values are not as important as being sure that both wing panels are washed-out the same amount.
Finishing the fuselage presented a different set of challenges. You will note the large lightening holes in the rear fuselage which I made by using a Dremel motor tool and sanding drum. While I was first tempted to cover the fuselage with a low-temp iron-on covering, I didn't particularly care to cover the extreme rear of the fuselage, including the vertical stabilizer, in this way. So I covered the rear fuselage with silkspan and nitrate dope, also using the nitrate to fill the balsa grain on the remainder of the fuselage. After fine sanding and a sprayed primer coat, the entire fuselage was finally spray-painted with Black Baron's Century 21 white paint for an easy-to-apply and durable finish.
My choice of radio equipment was two standard sized Airtronic servos, #94102's, installed side by side up in the nose, with a 7 channel Airtronics receiver just behind the servos. I used carbon tube pushrods with wire end linkages, and here found that snaking the assembled pushrod back into the fuselage was really a chore. Gee, maybe next time I'll follow the instruction to install the pushrods before sheeting the top of the fuselage - I recommend you do that even if I didn't.
Since the written instructions didn't list any control throws, I used end-point adjustment control with my Airtronics Stylus transmitter to set control surface deflections for the horizontal stabs and the rudder. With a 450 mah battery pack and 2 1/2 ounces of lead weights for nose ballast, my Windfree weighed in at 37 ounces ready to fly, yielding a wing loading of 9.9 ounces per square foot. The box top lists "25 ounces and up" for a flying weight which seems a trifle optimistic to me. I'm sure that I could have knocked off a few ounces here and there, but not 12 ounces, at least not using the supplied materials.
The first flights with the Windfree were made on a chilly winter day with just a moderate breeze. Hand glides produced a stable floating glide, confirming that the center of gravity and stab incidence was satisfactory for first flights, so without further ado I put the Windfree up for several launches on a high start launch system. Incidentally, the instructions have a caution to not stress the Windfree with "pedal to the metal" launches on electric winches, and I heartily concur with that - keep in mind that the Windfree's heyday was back in the '70s when contests were flown using high-starts and less energetic electric winches than those available today. On my fairly strong Pinnacle high start, the Windfree really zips up to altitude due to its light weight but is stable and easily controllable. There is some wing flex, as expected, but nothing alarming and I would feel very comfortable launching it - carefully! - on an electric winch system. Once the tow-ring has been dropped, the Windfree is really in its element, wheeling and soaring in absolutely superb fashion. With its extremely low moments of inertia due to lightweight wingtips and fuselage extremities, the airplane is very responsive to both rudder and elevator inputs, a real plus when flying in turbulent air conditions. With its high aspect ratio and 100" span, this sailplane has an extremely good glide and readily cruised the flying area in search of rising air. Handling performance in weak winter thermals was excellent, absolutely fun to fly. Overall, I found the Windfree to be a real pleasure to fly and worked several small thermals, chased several soaring seagulls, and in general just had a good time flying the plane.
Overall I found the Windfree to be an excellent flying machine and there's no question that it has extremely good flying qualities as evidenced by Mark Smith's Nats-winning performances. The discrepant items noted above are easily corrected and I recommend the Hobby Hangar Windfree as an excellent value in all-wood built-up sailplanes. The quality of the kitting is excellent and at the suggested retail price, the Windfree is a real bargain for a 100" sailplane. Build one and perhaps we'll meet flying Nostalgia at the Nats!