Zlin Z-526 AS

Model Airplane News, Jan 2002 by van Mourik, Dick

CONSTRUCTION

20-percent precision-scale aerobat

I have always been fascinated by airplanes. Of course, the fact that my father built and flew model aircraft contributed to this fascination. A book that my father owned, "Great Planes" by James Gilbert, included a photograph of two Zlin Z-526s flying in close mirror formation. From the first moment I saw this picture, I was captivated, and this older type of Zlin still ranks among my most favorite aircraft.

In 1993, 1 found the address of the Otrokovice Zlin factory in the Czech Republic. I wrote to the company and acquired a very detailed drawing and many pictures of the 526 types. I decided to construct the 526 AS and made several visits to the factory for valuable scale documentation.

CONSTRUCTION

The fuselage cross-sections are very tapered toward the bottom, which doesn't allow the fuselage to be made with the common fuselage box structure using half formers. I decided to split the fuselage structure lengthwise along the bottom of the canopy line, which enabled the whole thing to be built inverted. The basic structure is nothing more than a plywood box in which the tank, radio and the beechwood engine rails all fit. You could use commercial mounts, but you might have trouble mounting them, as the engine thrust line is very high and the nose is narrow.

The rest of the fuselage is then built around the main structure the old-fashioned way-with formers and stringers added, followed by strip planking.

The vertical fin and horizontal stabilizer are constructed with 3/32-inch ribs and sheeted with 1/16-inch balsa. The rudder and elevators are built around 1/16-inch balsa core sheets with 1/8-inch balsa half ribs glued to each side. Plywood cores are shown on the pictures but I have since changed them to balsa because the thin plywood trailing edges (TEs) tended to warp.

* Wings. The Zlin's wing section is a bit odd! It changes from a thick, nearly symmetrical section at the root to a fully under-cambered section at the tip. Additionally, it has a generous 6 degrees of washout. The root incidence of 4.5 degrees makes things even more interesting, although it doesn't cause much trouble during construction. The wings must be built on a simple jig to ensure the correct amount of washout. See the detailed section sketch on page I of the plan.

The AS version of the Z-526 has no flaps, which eliminates the need for a third longeron and also saves weight. The large amount of washout requires the longerons to be built up in two layers, which is also shown on the plan. The retractable landing gear is mounted on two 1/4-inch aircraft-grade ply plates that are glued between two plywood ribs. The wing is sheeted with 5/64inch balsa, and the ailerons are cut out of the wing later. Each aileron is hinged with a 2mm carbon-fiber rod, which is inserted through three Robart Hinge Point hinges.

The retractable landing gear for my model is air driven and was custom designed for the model. Details for the gear are shown on the plan. The only deviation from scale is at the pivot point, which is slightly higher for ease of construction. This is the only deviation from the full-size machine.

* Detailing. The cockpit interior is a real standout and was made using commonly employed materials such as scrap balsa, plastic stock and hard foam. The lever knobs were taken from a plastic pearl necklace. Perhaps the most vital point to remember when creating an interior is to do a proper painting job; it will really bring things to life. You'll also want to carefully mask the inside surface of the canopy, add some framing and spray the whole thing in gray or light green. Nothing kills the look of a cockpit interior more than being able to see the glue that was used to attached the framing.

The seat was made from annealed brass rod and covered with some thin glass cloth, which was formed over a piece of blue foam using the lost-mold method. Behind this, I installed a small first-aid kit, which took only about an hour to make, but for some reason it really interested the onlookers. The canopy-sliding mechanism is very easy to create; it's just a piece of 1/16-inch square, H-shaped brass profile running in a square 3/32-inch tube in which a slot was milled.

Another eye-catching item on Zlin aircraft is the Cessna-like aileron stiffeners, which I made from plastic strips from a Goldberg Cessna kit. This material is very easy to use; the only snag is that you'll have to order a hundred lengths at a time.

The fuel indicators in the wing were made from pieces of different colored plastic card, covered by a small transparent blister cover.

* Covering and finishing. The wing is covered in thin glass cloth and epoxy resin. To avoid the use of primer, I applied the cloth with white pigmented epoxy resin. The fabric-covered part of the fuselage was finished with Polytex, which is available from Nelson Hobby Specialties and really is tremendous stuff. The rear turtle deck was covered with tissue and dope, which still is one of the finest methods for finishing a scale model. I painted the model with Hobby Poxy, mixed in the appropriate colors.