Skandanavisk KZ.III

Written by Gene Smith Find the entire feature in the October 2010 issue. Read an abridged summary and photos relating to the article.

History: The design was begun in 1943 Denmark under Nazi occupation. Development was allowed to proceed because the KZ.III was intended for use by the Danish Air Ambulance Service. The aircraft’s maiden flight was in 1944. Production continued until 1947. Of the 64 aircraft built, three were delivered to the Ambulance Service. The aircraft had excellent STOL performance. The civilian version modeled was equipped with 100 hp Cirrus Minor engine.
The Model: The original model was built roughly 12 years ago and has been campaigned indoors. I built the flying ambulance version last year using the traced drawing. There is just no way around it, lighter models fly better. You can use 1/16 square balsa to frame the fuselage, but 1/20 will be lighter and is what I recommend. When framing the rear of the fuselage, use a temporary spacer at the back of the stabilizer slot. The fuselage tapers to a point at the rear and the slot will tend to close without the spacer. Remove the spacer when you install the stabilizer. Be sure to allow a little “wiggle room” for the front of the stab in case you need to shim it up or down for flight trimming. The plan was traced from my original drawing. The front of the stab had to be lowered 1/16 inch on the original model. On the second model I built the front of the stab slot 1/16 inch lower than shown on the plan and the model flew well from the first test flights. There is one other change you should note. The dihedral breaks are at the ribs over the cabin frame, not in the center of the wing.

There is only one center brace in the side window. The other, wider vertical shown on the plan is actually a vertical support that comes up from the wing strut cross brace to the wing.
I made the mistake of putting that vertical support on the side window instead of the wing strut on my original model because I couldn’t tell where it belonged on the small side view I was using. When I later saw pictures of the plane on the Web, I realized the mistake. You can see the correct framing of the window and the vertical brace off the wing strut in one of the pictures with this article.
The landing gear is .025 music wire. It attaches to the front of B3. The landing gear fairing attaches to the front of the landing gear wire and is not glued to the fuselage. This allows the structure to flex back slightly to absorb landing shock. Note the groove in the balsa landing gear fairing starts near the back edge at the top of the fairing and angles forward toward the front of the fairing at the bottom.
The spinner on the original model is balsa, turned on a Dremel tool. The second model is intended for outdoor flying. A balsa spinner was turned, then used as a form to mold a plastic cap for the spinner.
The prop blades were installed in the balsa spinner. The front of the balsa spinner was removed to allow room for a freewheeler ramp. Once the prop shaft was installed and the freewheeler installed, the molded plastic cap was glued on the balsa spinner base.
I did not use a half rib at the wing’s center but instead ran the top center fuselage stringer over the wing TE and fuselage cross brace, ending the stringer at the wing spar.
Scrap was used to build the wing spar up to the level of the top of the ribs. Refer to the photos to clarify this area. The front braces are added after the wing is mounted. Dihedral is 5/16 inches for each panel measured at the last rib.
Before covering the model, I attached the tissue to a frame and pre-painted it with a very light coat of Floquil/nitrate dope mix. Because the top center stringer on the fuselage attaches to the wing spar, I covered the bottom and sides of the fuselage
The top fuselage stringer was mounted using small balsa supports over the fuselage cross braces. Once the top stringer was attached, the braces at the front of the wing mid section were added. Then the top of the fuselage can be covered, followed by the top of the wing.
The windshield can be made from .005 clear plastic or use the clear plastic wrapping from a DVD. Using the pattern on the plan, make copy from light paper, like tracing paper. Try this pattern against your model and make adjustments as needed for a good fit. Apply the windshield with white tacky-type glue sold by craft stores.
Make the markings to simulate the door and add the color side trim to the fuselage before mounting the wing struts. The wing struts are 1/32 x 3/32 balsa. The struts can be strengthened and finished at the same time if you cut a strip of finished tissue to the length of the strut and 5/16 inches wide. Apply glue stick to the tissue and the strut and wrap the strut in the tissue. The same technique was used for the landing gear braces.

The flight control surfaces were drawn with a fine tip Sharpie pen. Flap hinges and the aileron counterbalance weight were simulated with gray construction paper. Their legs were left a little long. Slits were cut in the tissue and the hinges and counterbalances were glued into the slits. The hinges and flap counterbalance locations are shown on the plan. They are on the underside of the wing. The next-to-last outboard fixture is the counterbalance.
The original model is yellow overall with light blue registration and trim. The wing slots were simulated with black domestic tissue cut into strips and attached with white glue on the original. All of the tissue covering on the model was preshrunk on a frame and lightly sprayed with Floquil before applying it to the model.
I elected to build a second model using the beautiful tracing done by Robert Sweitzer. This model used an air ambulance scheme I found on the Internet. It was equipped with a free-wheeling prop for outdoor flying.
This was my first attempt at applying the markings to the tissue before putting the tissue on the model. Tom Hallman uses this technique with great success. While my finished product isn’t of Tom’s quality, the technique did work well.
I anticipated flying the first model to the right and built 1/16-inch washin in the right wing. This proved to be too much and it induced a stalling right climb which would dampen as the power decreased. After removing almost all the washin, flights have been much better.
Start with the CG at the back edge of the spar and adjust as needed. My model now balances 1/8 inch to the rear of the spar.
To date the model has only been flown indoors under a 38-foot ceiling. A 20-inch loop of .080 Tan II wound to 1800 turns and .6 ounces of torque gave a respectable flight of 1:42.
The air ambulance version has been flown outdoors with a 16-inch loop of .125 rubber. Trimming is not complete at this time but the model flew safely right off the board.
Enjoy your Danish!