Aircraft technical Basics: Introduction to Airplanes - Navy Training Courses Edition of 1944: Chapter 3 Structures

CHAPTER 3  STRUCTURES

WHAT THEY ARE

For purposes of discussion, an airplane is usually divided into five parts

FUSELAGE

WINGS

EMPENNAGE (TAIL ASSEMBLY)

LANDING GEAR

POWER PLANT

The FUSELAGE is the main body of an airplane. It is comparable to the body of an automobile.

The WINGS are the principal lifting surfaces. The wing assembly includes the following:

STRUTS - which are structure supports to carry either tension or compression loads.
BRACE WIRES (OR TIE RODS) - Which carry tension loads only.
AILERONS - which are control surfaces for banking an airplane.
TABS. FLAPS and SLOTS.

The EMPENNAGE (TAIL ASSEMBLY) is made up of a number of control surfaces called - the FIN, the RUDDER, TABS, STABILIZERS, and ELEVATORS. The fin is a stationary vertical surface for the purpose of increasing directional stability - that is, to hold an airplane on a straight course and prevent weaving from left to right when you DON'T want to change your direction of flight. The rudder is a flexible vertical surface, hinged to the rear of the fin, for the purpose of swinging an airplane from left to right when you DO want to change your direction of flight. The stabilizer is a stationary horizontal surface, projecting from either side of the tail, for the purpose of increasing longitudinal stability - that is, to keep an airplane from bobbing up and down like a porpoise. The elevators are flexible horizontal surfaces, hinged to the rear of the stabilizers, for the purpose of controlling upward and downward direction of flight.

The LANDING GEAR supports the weight of an air-plane on land or water. There are several different types of landing gear suitable for different types of airplanes. Some have WHEELS, some have FLOATS, some a combination of WHEELS - AND - FLOATS. Some have HULLS for landing gear. Landing shock, such as bounce during take-off, is eased by OLEO STRUTS - which act as shock absorbers for airplanes with wheels.

Figure 5.-Main parts of the airplane.

FUSELAGE

The fuselage, as you have seen, is the body of the airplane, to which all the other main parts are attached. It starts at the FIREWALL - a sheet of metal between the powerplant and the body - in most airplanes. The purpose of the firewall is to prevent gases, flames or burning fuel from entering the crew compartments. The fuselage itself may contain one or more sections. The pilot's compartment is the COCKPIT.

Other compartments provide space for passengers, ammunition, bombs, and so forth. There are two types of metal fuselage structures in common use - the TRUSS type, and the MONOCOCQUE. Monococque is a French word. It means "single shell."

Figure 6.-Pratt truss and Warren truss fuselage construction.

The truss-type fuselage is built up of LONGERONS (longitudinal members), vertical and diagonal BRACES, and the COVERING. This type of construction relies on the strength of the truss frame to carry the loads and stresses in flight. The Pratt-type truss consists of vertical braces with diagonal, adjustable wires or rods between the longerons to permit easy realining of the frame. The Warren-type truss has an all-welded frame, which is very rigid and requires no adjustment in the course of ordinary use. Figure 6 shows both of these truss types. Some fuselage construction combines these two types of trusses. In all cases the truss frames are covered with a light-weight streamlined framework, or fairing, over which the skin is applied.

The monococque fuselage, which you see in figure 7, is one in which the stresses are borne by the SKIN itself. The monococque is a variety of stressed-skin construction. It has certain advantages over truss-type construction, among them being greater strength per weight, more room in-side the fuselage, better streamlined form, and easier access for repairs.

Figure 7.-Monococque fuselage construction.

Some fuselages of this type are called F ULL MONOCOCQUE, because they are made by attaching thin sheets of metal to FRAMES - also called formers or bulkheads. They give the cross-sectional shape to the fuselage, which thus becomes a hollow tube made out of thin sheets of material. Full monococque structures are generally used only in light airplanes, or in those parts of an airplane where the stresses are not extreme. When more strength is needed, metal stringers are added, and the fuselage is then classified as SEMI-MONOCOCQUE.

Stringers are light-weight reinforcing members extending through the bulkheads and riveted directly to the skin. They serve as longitudinal stiffeners. Since the modern, high-speed airplane develops great stresses in flight and must be well streamlined, most of the fuselage construction is now of the semimonococque type. Some of the big airplanes are monococques of the REINFORCED SHELL type, however. That is, they have extra bracing for the skin at points of highly concentrated load.

WINGS

Remember how you build kites? If you make the diamond-shaped kind - with two crossed sticks for braces - the construction is fundamentally like that of an airplane wing.

The long stick - running from the top corner to the bottom - serves the same purpose as the RIBS in a wing. The crosspiece, running from side to side, is like a SPAR. A kite's cover material - probably an old newspaper - is stretched over the sticks and held in position by them. The airplane wing's covering is stretched over ITS structural ribs and spars and held in the proper airfoil shape in just the same way. A glance at figure 8 will show you a typical example of wing construction.

SPARS - or beams, as some people call them - provide the main strength of the wing. The lift and other forces which act upon the wing take effect first on the covering. From the covering they are transmitted to the ribs, and by the ribs to the spars. Thus, in the long run, all the load on the wing is taken by the spars.

There are quite a number of types of wing construction. Wing structures may be made of wood, and covered with fabric or plywood. Some are made of metal and covered with fabric.

Others are made of metal and covered with metal. Wings covered with fabric usually have two spars-one near the front or leading edge and one about two-thirds the distance toward the rear or trailing edge. Metal-covered wings, however, sometimes have as many as five spars.

Some spars are made of wood, others of metal. Wooden spars usually have a solid cross-section, and are either I-beam sections, or box-shaped sections. Most metal spars are made of aluminum alloy or of steel. Metal spars may be shaped like the letter I in cross section, or may be made like a truss, such as is used to hold up the roof of a building.

Figure 8. - Spars, ribs, and nose fairing of a wing.

You might compare the spar of an airplane wing to your own backbone. Without it you couldn't stand upright. The ribs in a wing also do much the same sort of job as your own ribs do for your body. The ribs that give the wing its shape are called FORM RIBS. Airplane wings also have other short ribs called NOSE RIBS along the leading edge, so that this front section will hold its shape in flight. There is usually a stiff layer of plywood or metal under the outside cover of an airplane wing at its nose, to help the nose ribs keep the leading edge in proper contour. Form and nose ribs are almost always made either of wood or aluminum alloy.

When an airplane goes at a high rate of speed there is generally a great deal of pressure on the wing's leading edge. In order to keep fabric-or wood-covered wings rigid in a fore-and-aft direction, it is necessary to put braces between the spars. The system of braces provided for this purpose is known as the DRAG TRUSS. The drag truss is composed of COMPRESSION RIBS, which are sturdy ribs that often do not have the contour of the wing. In wings of metal construction, compression ribs are frequently just round steel tubes. In wooden wings, compression ribs are generally made of plywood or spruce timber. The compression rib at the inner end of the wing is referred to as the ROOT or BUTT RIB.

Compression ribs are used to resist the tension of bracing wires or struts inside the structure of the wing. Wire braces for this purpose are known as DRAG and ANTIDRAG WIRES, and are more commonly used than are internal bracing struts. Metal-covered wings usually have no drag trusses. The metal covering itself keeps the wing from losing its shape or being pushed backward by the force of the air. This method of building wings is known as STRESSED-SKIN construction - a term you saw before in the discussion of the fuselage.

The wings of some airplanes have FLAPS hinged near their trailing edges. These serve as a sort of air brake. They are not in any sense control surfaces. DON'T confuse them with ailerons. Flaps reduce the airspeed of an airplane in landing, and sometimes are used to provide additional lift for take-offs. There are several different types of flaps, but they all operate on the same principle and vary only in design and efficiency.

Some wings have air passages through them in the form of SLOTS. The reason for having slots is to smooth out the flow of air over the wing when it is at a high angle of attack. In general, an airplane with slotted wings will not stall as quickly as an airplane with the same shape of wing airfoil but without slots.

Banking an airplane is accomplished by use of the AILERONS. These, you recall, are movable surfaces hinged to the trailing edges of the wings. The hinges are attached either to the rear spar or to a false spar near the trailing edge. When an aileron is moved DOWN, the LIFT on that wing is increased and the wing rises. The cables or mechanisms which control the ailerons are hooked up together so that when ONE aileron moves DOWN the other moves up. Move the control stick to the right, and the left aileron goes down as the right aileron moves up, thus banking the airplane to the right. Reverse the motion of the control stick and the airplane banks to the left.

EMPENNAGE (TAIL ASSEMBLY)

When you drive your car down the street, you steer it merely by turning the steering wheel. Directing an airplane through the sky, however, is quite a different proposition. You have probably discovered that an airplane is fitted with a number of flight controls. Each of these controls is a separate "steering wheel" in itself. Except for the ailerons, which are attached to the wings, most of an airplane's steering surfaces are parts of the EMPENNAGE.

The empennage, or tail assembly, is made up of parts built in much the same way as wings. Fins, stabilizers, rudders, and elevators can be made of wood, steel tubing, or aluminum alloy, and covered either with fabric or metal. MOVABLE SURFACES of the empennage (meaning the elevators and rudder) are also provided with TABS in most airplanes, as in figure 9. Some tabs can be adjusted only when the airplane is on the ground. Others may be controlled from the cockpit while the airplane is in flight.

Figure 9.-Trim tabs.

Movable control surfaces are BALANCED, as in figure 10, to keep them from fluttering during flight. There are two ways of balancing such control surfaces. A STATIC balance is a weight located at some position forward of the hinge to which the control is fastened. This weight is often a small streamlined shape attached to the end of an arm projecting from the forward end of the control surface. Large airplanes generally have AERO-DYNAMICALLY balanced control surfaces. This type of balancing is made possible by locating the hinge somewhat BACK from the control surface's leading edge. Thus, the air striking the portion forward of the hinge tends to assist in moving the surface in the direction you wish.

Figure 10.-Static and aerodynamic balancing of control surfaces.

LANDING GEAR

The landing gear of an airplane can be a complete pain in the neck a great part of the time. It adds weight to an airplane in flight, cuts down the flying speed, and presents you with just one more thing to get out of kilter. But, if that little old landing gear wasn't there WHEN YOU NEEDED IT, you  wouldn't have to worry about keeping that date with the blonde on your next liberty. Thereafter you'd just be a vital statistic.

Even the birds have trouble with their landing gear. You'll notice that almost all birds tuck their feet up flat against their bodies to keep them out of the way during flight. Wheels and floats, like birds' feet, are useful only during take-off and landing, or when moving about on land or water. The main types of airplane landing gear are pictured in figure 11.

Landplanes, as you know, are equipped with wheel-type landing gear. The gear includes the chassis, shock absorber unit, wheels and brakes, tires, metal fairings, tail wheel or tail skid assembly, and all of the necessary fittings to attach the gear to the airplane. Many modern types of air-planes have retractable landing gear which folds up into the wings or fuselage after the take-off.

Figure 11.-Various types of airplane landing gear.

Landing gear which makes it possible for some types of airplanes to alight on either land or water is called AMPHIBIAN gear. Such gear is a combination of retractable wheels and a hull or floats. Amphibian-equipped airplanes are very flexible as far as landing and take-off adaptability is concerned. But, as you'd imagine, the gear itself is somewhat cumbersome. This fact restricts the use of amphibian airplanes to special situations.

As a general rule, aviation people call float-equipped craft SEAPLANES. Strictly speaking, ALL, airplanes designed to alight on the water are sea-planes, but water-based planes that use their hulls as landing gear are usually referred to as FLYING BOATS. The differentiation between flying boats and seaplanes is merely a matter of usage among aviators.

An airplane equipped with ONE main float is called a SINGLE FLOAT seaplane. Such airplanes also have small floats at each wing tip to prevent them from capsizing. These wing-tip floats do not contribute materially to the support, or buoyancy, of the airplane. If the plane is equipped with TWO large floats, which share equally in supporting it, the craft is a TWIN FLOAT seaplane and wing-tip floats are unnecessary. In a twin float seaplane the horizontal distance between the keels of the floats is called the TREAD. Frequently these floats are braced by a bar running from one to the other. Such a brace is called a SPREADER BAR.

The covering, or skin, of most seaplane floats is made of aluminum alloy sheet. The rear portion of the float is called the STERN, the front portion is called the BOW, and the vertical member at the front of the bow is known as the STEM - just as in a ship. Most floats have soft bumpers on their noses to prevent damage when striking a dock or other solid object. The top of the float is known as the DECK. The stringer, or longitudinal member that attaches the deck to the sides is called a DECK CLAMP. Inside and outside views of a float are shown in figure 12.

Figure 1 2.--Exterior and interior views of a seaplane float.

You will usually find one or more MOORING CLEATS attached through the deck to internal structural members of the float. These cleats, which are often in the shape of rings, are used for mooring or tying the plane to a fixed object, and also for the attachment of lines for towing. The deck is also provided with removable portions called HANDHOLES. These are used for inspection, pumping out water, and for access to the interior when making minor repairs.

A seaplane float has watertight interior bulk-heads, which divide the float into several compartments. Thus, if there should be a leak, water will enter only one small section of the float, and the seaplane won't sink. Most floats are also equipped with a rudder, attached to the stern post and used for steering the seaplane while it's on the water.

There's a step about halfway between the bow and stern on the bottom of a float. That step helps more to break the float away from the grasp of the water when the water is smooth than when it's choppy, as smooth water hangs on to a float for dear life.

The part of the float in front of the step is called the FOREBODY, and the part behind the step is the AFTERBODY. That section of the bottom which is back of the step is known as the AFTER BOTTOM; the portion in front is known as the PLANING BOTTOM. Some extra-large floats have two steps - a main step near the bow, and an after step near the stern. On these big floats the section between the two steps is referred to as the MIDDLE BODY. The ridge, or corner, where the bottom meets the side is known as the CHINE.

The long strength member at the bottom of the float, running from stem to stern, is the KEEL. It's usually an angle of strong metal to which the two halves of the bottom are riveted. Pretty generally, there'll be an extra strip of metal fastened over the outside of the keel. It's known as the RUBBING STRIP, and saves the keel from wear and tear. This protective strip can be replaced with a new one, if and when it gets worn out.

When a seaplane is on the water, carrying its normal load, a part of the float, or floats, will be under water. The amount of this normal submersion is marked by the LOAD WATER LINE (often abbreviated L. W. L.). The distance from the load water line to the deck is known as the FREE-BOARD, and the distance from the L. W. L. to the lowest underwater point of the float is the DRAFT.

Figure 13.-The parts of a flying-boat hull.

The curved members that give the float its curved cross-sectional shape are the FRAMES. These frames are attached at right angles to the lengthwise braces. The SKIN, in turn, is fastened to the frames. Supporting the skin between the frames are fore-and-aft strips called STRINGERS, the one at the top center having its own private monicker - the DORSAL STRINGER. Other reinforcement braces, inside or outside the float, are usually referred to as STIFFENERS.

In a flying boat, that part of the fuselage which carries the crew, equipment, fuel, and the rest of the useful load also serves as the landing gear, and is called the hull, as you see in figure 13. Although built as an integral part of the flying boat's fuselage, the hull is very similar in construction to a float, and its parts carry the same names.

Some hulls have additional strip-like watertight compartments running along their sides to give them extra buoyancy in the water. Such compartments are called SPONSONS. They are seldom used on floats. Also, since the crew must be able to move back and forth through the length of the hull, there are transverse doors which can be closed if some portion of the hull is damaged.

You will notice that most flying boats have wing-tip floats, much like those on float seaplanes. But some large flying boats use SEA WINGS - sometimes called STUB WINGS - instead of wing-tip floats. Sea wings are attached to the hull at the water line near the chine, and they not only steady the plane in the water, but add to the planing surface of the hull bottom during take-off. Sea wings should NOT be confused with sponsons.