TM 1-410. Technical Manual, Airplane Structures 1941: Section 13 - Cowling and Fairing

SECTION XIII: COWLING AND FAIRING.

51. General.-Cowling and fairing are the many relatively small sheet metal units of aluminum or aluminum alloy attached to the structure of an airplane or engine mount for various purposes, but which are not usually considered a part of the basic structure of the airplane.

52. Cowling.-a. Cowling consists of detachable sections used to cover certain portions of an airplane structure such as engine mounts, nacelles, landing gear, and other places to which a ready means of access is necessary. The individual sections are of a size and weight convenient for one man to handle. They do not fasten one to the other, but are independent units so far as their assembly and disassembly are concerned.

b. Since the engine section is required to be fireproofed and the engine readily accessible for maintenance, it is usually completely sheathed with detachable metal cowling. Engine compartment cowling is provided with air inlets and outlets to permit a circulation of air about the engine and accessories. This is to assist in cooling and to prevent accumulation of gases. Engine ring cowling, shown in figure 61, is the cowling that encircles a radial engine. It is of relatively heavy sheet aluminum alloy and is usually supported by pads secured to the inside of the cowl which rest on the valve cover caps of the engine when the cowl is installed. It is made in two or more sections having turnbuckle type fasteners provided on the inside at the ends where the sections meet. It is important that these fasteners be tightened carefully to the correct extent with regard to the temperature of the engine.

FIGURE 61-Engine ring cowling.

 Excessive tightening of the turnbuckles when the engine is cold is likely to produce failure of the fasteners or the fastener brackets when the engine becomes heated and expands. If not tightened sufficiently when mounted on a hot engine, vibration of the cowling will result in possible serious injury to both engine and cowling. In either case the cowl should fit snugly but not too tight, and especially in the case of a cold engine turnbuckles should be tightened by hand only, and must always be safetied. Bottom cowling is shaped so that it is self-draining with the airplane in taxying position.

c. Cowling is attached to frames so spaced as to support the sections along their edges and at reinforced areas. Attachment is made by means of flush type (Dzus) fasteners which consist of a stud, grommet, and a stud anchor, shown in figure 62. The stud is a bullet

FIGURE 62.-Flush fastener (Dzus).

shaped body with a relatively large, flat head. In the body near the head is a shallow groove. The grommet is pressed around the. cowling, expanding into the groove to give a positive anchorage of the stud to the cowling. The nose or protruding end of the stud body has a spiraling slot which makes a one-quarter turn. At the base of this slot is a recess into which the stud anchor locks. The stud anchor is a steel wire coiled at each end similar to the coil spring of a bedmattress, permitting a limited range of movement in and out. It is fitted on supports at the point of cowling attachment. The stud is pressed against the anchor matching the groove, and turned until the anchor locks in the recess at the end of the slot, thus holding the cowling snug against the cowling support.

53. Fairing.-Fairing is generally used to refine or streamline contour of the airplane, although it may be used to protect. some minor piece of equipment and in some cases simply to improve appearance of the airplane. The units may be made in sections or stamped in one piece. Fairing pieces are sometimes attached the same as cowling, but are generally bolted or screwed into place and not removed except for inspections or repair.

54. Maintenance.-Care must be exercised in handling all cowling or fairing when off the airplane so that it is not bent or distorted. Many pieces, especially long strips or large sections, are not sufficiently rigid to support their shape unaided. On the other hand, the smaller parts, although formed so that they are rigid as a unit when installed, are constructed of light gage material and may be damaged easily. Cracks often open up and become evident after fairing is off the airplane. These should be patched or the fairing replaced before reassembly. The finish of cowling or fairing must be kept intact and, if marred during assembly, should be touched up as soon as practicable. All fasteners must be kept in working condition and replaced at the first sign of looseness or excessive wear. Where motion is necessary between pieces of installed cowling, chafing strips of fabric or fiber are used. These should be renewed whenever wear might permit metal parts to rub together.