Waco YKS-7 Design and Construction

The Waco YKS-7 shares many design and construction features with earlier Waco models. Its fuselage has welded steel tubing and the wings have wooden spars. The fuselage and wings are fabric covered, over wooden ribs, formers and longerons.

In the complex system of Waco model designations used in the 30’s, a series of letters were used to identify different models. The first letter indicates the engine, the second indicates the wing and fuselage design, and the third indicates the model series (sometimes referred to as the "drawing identification"). For this airplane, The Y designates a Jacobs L-4 225 hp. engine, the type installed at the factory and later replaced in EAA’s airplane. The K indicates a specific fuselage and wing design, in this case the wing employs a Clark Y airfoil. The S denotes this airplane is a “standard” as opposed to a “custom” series airplane. The digit 7 indicates that this airplane is the 7th model of the series.

Because the YKS-7 was intended for business cross-country travel, passenger comfort and increased cruise speed were major design considerations. Its fully enclosed cabin and heating system were major advances from open-cockpit predecessors. Speed increases came from use of a 225 hp. Jacobs engine and through reduction of aerodynamic drag. Features for drag reduction included a fully cowled engine, main wheel fairings and elimination of flying wires between the wings. The inter-wing “N” struts and aileron connection struts are all aligned to minimize profile drag.

The Waco YKS-7 wings are each attached to the fuselage at two points, the main spar and rear spar. The upper wing is forward of the lower wing, as with most biplane designs. The unusual wing bracing design of the YKS-7 was taken from the 1933 Cabin Waco model UIC. With this design, the inter-wing struts transfer the lift and weight of the upper wing to the lower wing. Both wings are braced with a single, streamlined metal strut running from the lower wing to the upper wing root at the fuselage attachment point. Note that this bracing is the opposite of the bracing used in most Cessna high-wing aircraft. Lifting forces are carried as compression forces on the YKS-7 diagonal strut and landing forces are carried as tension, the opposite of many other strut-braced designs. Wing bracing using an oval-profile, diagonal, metal strut is a design still used in some current day aircraft.

Control System

The Waco YKS-7 controls copied features from earlier Waco designs. A single control wheel is attached to a throw-over yoke, allowing the plane to be flown from either front seat. Ailerons are fitted to all four wings. They are actuated by pushrods and innovative bell-cranks in the lower wings. A strut links upper and lower wing ailerons. Wing flaps (called “air brakes”) were an optional feature, but rarely installed. The rudder and elevator are controlled with cables. The elevator trim control is a hand crank on the cabin ceiling. It uses cables to drive a jackscrew, moving the front end of the horizontal stabilizer, a design used on many later planes. There is no aileron or rudder trim.

The Waco Cockpit

Fuel system

Fuel is carried in two 35-gallon wing tanks, one in each upper wing root, with fuel filler caps on the top of the wings. Fuel is gravity fed through a fuel selector valve to the engine. Glass sight gauges project below each upper wing, showing an indicator connected to a float inside each tank. Optional 15-gallon tanks can also be installed in the lower wings.

Electrical system

The YKS-7 uses a generator and battery to drive its electrical system. It has an electric starter for the engine and provision for interior and navigation lights. The engine has two independent igintion systems, but only one uses a magneto. The other ignition system is an automotive type, using battery power, an ignition coil, and a distributor. A single electrical switch controlled the engine ignition systems and also served as an electrical master switch; a separate master switch was later installed. An interesting option for night flying was a set of paraflares. Because so few landing areas were lighted, the aircraft had to provide its own area lighting for night landings. Three phosphorus flares were mounted in the left side of the fuselage and could be individually controlled from a panel on the pilot’s left side.

Flare Tubes located on the left side of the Waco

After being launched from the side of the plane, each flare would illuminate a potential landing area while drifting down with its small parachute. If the first flare showed an unsafe landing area, the pilot could fly somewhere else and use one of the two remaining flares. Fortunately, modern airport lighting has eliminated the need for such a potentially hazardous system.

Landing Gear

The YKS-7 is a conventional gear (tailwheel) design. Hydraulic oleo struts on the main landing gear legs were introduced on later models to provide softer landings. This was a major improvement, allowing safe and comfortable landings on rough fields. The brakes were a mechanical design, controlled by a “Johnson bar” which, when correctly adjusted, applied both brakes simultaneously. The brakes were among the features changed in EAA’s YKS-7 for increased safety.