A PARAMETRIC STUDY OF THE TAKE-OFF PERFORMANCE OF GETOL AIRCRAFT 610118
The concept of using the air cushion ground effect principle to aid an aircraft's take-off and landing is relatively new. This allows the airplane to hover and accelerate to flying speed at a definite height, i. e., free of the ground. This study was initiated to determine whether such a procedure is feasible for the take-off of an airplane, and in so doing, to examine some dominating parameters. This study is limited in scope: the take-off consists of a constant height acceleration to flying speed, then a pull-up to clear an obstacle; a low aspect ratio wing is assumed, furthermore, all thrust is obtained from the lifting fan during take-off. In addition, stability considerations were neglected, augmentation is assumed unaffected by forward speed, the lift fans are operated at constant power and at only one pressure ratio, and the total ram drag was used. The above suppositions are not meant to infer an optimum configuration or an optimum take-off technique, rather, they appeared reasonable at the initiation of this study. The operating regime of the lifting fan is chosen so as to be relatively insensitive to inlet efficiency. The thrust of the fan itself is vectored so as to obtain a propulsion force. No other propulsive device is used.
The ground effect take-off of GETOL fan-in-wing aircraft was investigated for a range of major aircraft parameters believed to be representative of the V/STOL aircraft regime. The parameters studied were thrust-to-weight ratio (T/W), 0.6 to 1.1, wing loading (W/S), 40 to 100 lbs per sq ft, maximum lift coefficient (CLmax), 0.8 to 1.2, and ground effect thrust augmentation ratio (A), 1.1 to 1.9. The study illustrates the usefulness of ground effect take-off (GETO) and defines the limits of its applicability. For a given aircraft, defined by T/W ratio, W/S and CLmax, there is a specific augmentation (A) which results in the minimum take-off distance to clear a 50-foot obstacle. For a wing loading of 40, thrust-to-weight ratio of .90 and maximum lift coefficient of .8, the minimum ground effect take-off distance is 1630 feet and is accomplished at an augmentation ratio of 1.54. Further increases in augmentation will not significantly shorten the take-off distance, but augmentations less than 1.54 will result in significantly increased distances.
Engine ram drag and the ability to vector thrust are the limiting factors governing GETOL. The high ram drag of the fan-in-wing aircraft (because of high airflow) dictates low take-off speed. The ability to vector thrust for acceleration is limited by the losses due to vectoring and by fan operating characteristics.