TM 1-410. Technical Manual, Airplane Structures 1941: Section 3 - Engine Mounts

SECTION III: ENGINE MOUNTS

FIGURE 12.-Nacelle complete with engine mount and engine

8. General.-Engine mounts are designed to meet particular conditions of installation such as location on airplane, methods of attachment, and size, type, and characteristics of the engine they are intended to support. Although they vary widely in their appearance and in arrangement of their members, the basic features of their construction are similar and are accepted commonly as standard practice. They are usually constructed as a single unit which may be detached quickly and easily from the remaining structure and in many cases are removed from the airplane and replaced with the engine and its accessories as a complete assembly or "power unit." This not only speeds up engine changes but also facilitates maintenance and overhaul.

9. Structure.-a. A primary consideration in design of engine mounts is to render the engine and its equipment accessible for maintenance and inspection. A framework construction of welded chromemolybdenum steel tubing is well adapted to this purpose and is used extensively. Forgings of chrome-nickel-molybdenum are used for

FIGURE 13-Mount, radial engine.

the more highly stressed fittings these being accurately machined at points of rigid connection to the engine or other structures. After construction or major repair, engine mounts are subjected to a normalizing process to remove strains due to forging, bending, and welding. Typical engine mounts of the above construction for use with a radial and an inline engine are shown in figures 13 and 14, respectively.

b. Although the exact location depends entirely on design of the airplane, points of attachment of the engine mount are either at or just forward of a flametight, fireproof bulkhead which separates the engine compartment from the rest of the structure. Vibrations originating in the engine will be transmitted to the engine mount and through it to the airplane structure unless provision is made to damp them out, and use of some one of the standard devices for that purpose is mandatory on service equipment. These are usually rubber cushions or pads installed where the engine is bolted to the mount. In some cases additional units are installed between mount and airplane structure. The primary purpose of these vibration-absorbing elements is to produce a natural period of vibration of the assembly below that of the engine at the lowest possible cruising r. p. in. The maximum absorbing characteristics are obtained when the engine mount vibration-absorber bolts are so tightened that the engine is restrained from any fore and aft motion, but is permitted to move or rotate within certain limits, in a torsional direction.

FIGURE 14-Mount, inline engine.

Tightening the bolts excessively reduces the flexibility of the mount and tends to bring period of resonance of the engine mount into synchronism with or above that of the engine at minimum cruising r. p. m. which is highly undesirable. When limits to which these bolts are to be tightened are specified in the handbook of instructions, they must be adhered to exactly.

10. Maintenance.-Cracked, bent, or broken members of these structures constitute a highly dangerous condition and without exception must be replaced or repaired by activities authorized to do such work before the airplane is permitted to be flown. In general, cracks are most likely to occur at the welded joints, and small cracks particularly may be difficult to discern through the protective coatings. This is especially so if the structure is not kept thoroughly clean and special care must be exercised in making inspections at these points. Mounting clamps and bolts if not properly tightened will allow movement of the mount with consequent rapid wear of the bolts, elongation of bolt holes, and a resultant serious vibration. Protective coatings if damaged should be retouched promptly to prevent rustings of exposed steel surf aces.