Peter White, age 14, of Haggerstown, Md., for his question:

WHAT IS A CONVERTIPLANE?

A convertiplane is an aircraft designed to fly horizontally, and also to land and to take off vertically. This type of aircraft can also hover or remain motionless in the air.

Convertiplanes are also known as VTOLs, or "vertical take off and landing planes," or STOLs, "short take off and landing." The term V/STOL, meaning "vertical or short take off and landing," includes all types of convertiplanes.

Active experiments on convertiplanes started more than 50 years ago in 137 with models that were tested by Gerald Herick of New York City and E. Burke Wilford of Philadelphia.

In 155, the United States Department of Defense reported testing the first truly successful convertiplane, the McDonald XV 1. Since then, many airplane manufacturers have built numerous experimental convertiplanes with steadily better performance.

In 142, convertiplanes were not available at any price. In 152, they cost twice as much as a conventional plane. Today, they cost about 25 percent more than ordinary aircraft.

Convertiplanes have great commercial and military possibilities because they need only a small landing space compared to that for conventional aircraft. With convertiplanes, commercial airlines could operate from small landing areas in the heart of a city or even from building tops.

One of the problems of military aircraft operations is the difficulty of building and maintaining suitable airfields. Military convertiplanes, however, could operate in any area where there was room enough to land or where a small landing field could be built.

A variety of systems have been developed for achieving both horizontal and vertical flight. Basically, however, there are two types. One has two separate propulsion systems: one that can be used for vertical flight and the other for horizontal flight. A second type of convertiplane would use only one propulsion system for both horizontal and vertical flight.

The Sikorsky Stowed Rotor VTOL is a type of convertiplane that has one propulsion system for vertical flight and another propulsion system for horizontal flight. This aircraft covers the helicopter rotors it uses for vertical flight when it reaches speeds over 150 m.p.h.

The convertiplane that uses only one propulsion system for both horizontal and vertical flight often looks more like a conventional aircraft. It has jet, propeller or ducted fan engines mounted in the fuselage or on the aircraft's wings.

This type of airplane can operate by tilting or rotating the wings, engines or the entire craft to fly in the desired direction.

Or the aircraft may use vanes or flaps to deflect the jet thrust or air blast from the engines downward for vertical flight. The Ryan XV 5A is an example of this type of aircraft. It has a ducted fan mounted on each wing.