Vertical takeoff lift augmentation - The 'sculptured deck' concept

Experimental results are presented which demonstrate that the lift of a hovering VTOL aircraft can be in- creased greatly by appropriately contouring the ground surface under the vehicle. The augmented lift is created by redirecting the jet exhaust flow so that it impinges on the aircraft underbody. One configuration of the ground plane is shown to augment the total upward force on the vehicle by a factor of 2.4 and at the same time increases the roll, pitch, and yaw stability of the aircraft. The measurements were taken from a particualr VTOL model and two types of ground plane, but the idea can be generalized to include arbitrary aircraft planform, any number of jets, any shape jet exit, and a wide assortment of ground plane topographies. The high degree of design flexibility intrinsic to the concept may be sufficient to offset the serious problems of underbody heating, hot-gas reingestion, and the necessity of a prepared takeoff site. HE net aerodynamically induced lift on a VTOL aircraft in proximity to the ground is determined by the competition between two opposing propulsion-induced phenomena: the "suckdown effect" and the "fountain ef- fect." The suckdown effect is caused by jet entrainment of ambient fluid, which tends to lower the static pressure on the aircraft underbody and results in a net downward force on the vehicle. The fountain effect occurs only with multiple jet con- figurations and is characterized by a centrally located region of upwash which impinges on the aircraft underbody, in- creases static pressure, and results in a net upward force on the vehicle. The net strength of both the suckdown effect and the fountain effect is strongly dependent upon the elevation above the ground plane and the spacing of the jets relative to the aircraft planform. In practice, the strength of the suck- down effect is considerably greater than that of the fountain effect, resulting in as much as 15% loss of total thrust. This propulsion-induced loss of thrust during takeoff and landing is a very serious problem to those involved with VTOL design because it significantly decreases mission radius. Margason1 estimates that a 3% loss of thrust during hover, when trans- lated into a reduction of fuel capacity, results in a 10% reduc- tion of design range. Consequently, any modification to the system which tends to increase fountain impulse and/or decrease suckdown force is of interest to the VTOL com- munity. The theoretical prediction of a VTOL ground-effect flowfield is an exceedingly complex task, and we know of no published accounts that treat the multiple jet problem suc- cessfully. However, much experimental work has been done. Margason1 examines the entire spectrum of ground-effect problems, including the suckdown effect, the fountain effect, hot-gas reingestion, and cross-winds, all with single and/or multiple jets. He presents data that show between 1.5% and 3.0% loss of thrust for a single jet in absence of the ground. This is consistent with the theoretical analysis of an axisym- metric jet issuing from an infinite plane by Wignanski. 2 Margason also presents data which show that the impulse carried by the fountain upwash can be as great as 10% of the total thrust of the vehicle. Davenport and Spreemann3 demonstrate that increasing the number of jet exits provides the most favorable thrust characteristics in proximity to the ground, with the annular jet performing best of all. A recent and general discussion of propulsion-induced effects on VTOL flight, with special emphasis on the transition flight regime, is presented by Winston et al.4 Finally, modifications of the aircraft underbody for the purpose of capturing the fountain impulse can improve hover performance noticeably. The "strake-box" concept currently is being used on the Harrier AV-8B and results in a significant increase of payload capability.5 Since we knew of no attempts in the literature at ground plane modification as a means of combating the suckdown ef- fect or augmenting fountain strength, we decided to in- vestigate this possibility. The basic idea of the "sculptured deck" concept is to modify the topography of the ground plane so that it turns the jet exhaust flow through 180° and causes it to impinge on the aircraft underbody. This objective can be realized in different ways (Figs. 1 and 2). The exhaust flow can be turned inward to impinge on an area internal to the jet thrustors, forming a stronger-than-usual fountain (Fig. 1), or it can be turned outward to impinge on an area external to the primary sources of thrust (Fig. 2), thus forming an ex- ternal fountain and partially eliminating the secondary en- trained flow which causes the suckdown effect. By placing a skirt around the periphery of the vehicle, the return flow can be redirected toward the ground, imparting additional lift to the craft. An upper limit on the peformance of the "sculptured deck" can be determined by idealizing the jet exhaust flow as a well- ordered volley of, say, machine-gun bullets. Assuming that the cupped array and aircraft underbody can turn this im- pinging volley without frictional loss, then the arrangements