1518. Unique Weight and Balance Aspects of V/STOL Airplane Design

Paper

Abstract

This paper is directed to supersonic Fighter/Attack airplanes designed to Navy requirements and
ground rules. It deals entirely with Lift/Cruise (vectored thrust) propulsion systems. Vertical/
Short Takeoff and Landing (V/STOL) airplane design presents many design considerations different from
those of Conventional Takeoff and Landing (CTOL) airplanes. Weight and balance differences between
V/STOL and CTOL will be discussed; but no attempt will be made to evaluate any weight or performance
penalties between the two concepts.
Some of the considerations which affect weight are larger, heavier propulsion systems for V/STOL.
To develop the thrust required to reach supersonic speeds, the engines must further grow in size and
weight. The alternative to this is to augment the thrust of today V/STOL engine. Burning the fan
bypass air can achieve significant thrust increases without increasing engine size and add a relatively
small weight penalty to incorporate burners in the cold air nozzles. This augmentation process is
known as Fan Stream Burning (FSB). An additional flight control system is required for vertical flight
in the vectored thrust design. Vertical or other landings eliminate catapult, arresting and high lift
device requirements. Lower sink rates reduce landing gear structure weight; and brake weight is reduced
because of slower landing speeds.
A major consideration in the V/STOL design is the vertical thrust center. The vertical thrust center
must react through the airplane c.g. during the vertical flight mode. Particular attention is given to
alignment of the vertical thrust center with airplane center of gravity, with consideration being given
to optimum stability margins relative to the airplane neutral point. These factors dictate engine
location relative to the wing.
It is therefore imperative that accurate center of gravity envelopes be designed to fit both the
vertical mode and the normal flight control envelope. Because of this the weight engineer exerts a
strong influence on the airplane design from the initial configuration layout to completion of the flight
article.