The general aviation market has recently been introduced to the “very small” turbofan engine (i.e. 2000 pounds thrust and less). Simply scaling down larger conventional turbofan engines, however, presents difficulties due mainly to the disproportionate scaling of certain factors, such as strength-to-weight and tolerances.
The engine case, such as that depicted in FIGS. 1 and 2, is subjected to asymmetric loading relative to the engine mounts, caused by loads exerted through the bearings, such as engine thrust, foreign object impacts and blade-off events, and caused by inertia loads caused by the engine weight which of course must be supported. These asymmetric loads result in bending moments and shears which must be transmitted through the engine case to the engine mounts. The prior art generally relies on thick walled structures, such as cast engine case components (such as 202 204, 206, 208, 210, 211 in FIG. 1), to react these bending moments in plate bending. Plate bending, however, requires thicker-walled casings to resist and carry bending forces without failure. In very small engines, however, thick casing become a significant component in overall engine weight.
An alternate approach is shown in U.S. Pat. No. 4,132,069, which provides an integrally-webbed structure for transferring loads, and in particular bending, through an engine and nacelle structure, so that nacelle loads can be passed to the engine. The scheme, however, adds components to the engine, which reduces reliability, and increases weight and cost. Improvement in engine case technology is therefore desired.