Airframe Form & Structure
NOTE: This document is for information only and refers to UK procedures only. Not to be taken as an authority.

Fuselage

The purpose of the fuselage is to form the main body of the aircraft in which the pilot and passengers can be carried and for other parts to be attached to. It can be constructed in one of three ways:

• Monocoque: In which the strength of the structure is provided by the skin or shell and is free from internal bracing, in much the same way as an eggshell derives its strength.
• Semi-monocoque: An aircraft structure in which the outer skin is inadequate to carry the primary stresses, and is reinforced by internal structures such as frames, formers, stringers or struts. The outer shell also offers the possibility of pressurizing the internal volume for high-altitude flight.
• Framed or Truss: In which the strength is provided by struts and braces.

Most modern aircraft use the semi-monocoque structure, and this explains why any damage to the aircraft skin must be examined by an engineer. The fuselage will often have vertical bulkheads and frames joining with horizontal stringers to provide strength.

Vintage aircraft were often constructed using the truss method.

Cessna 152: In the case of this aircraft, the fuselage is of the semi-monocoque type and of aluminium alloy construction, although some non-load bearing parts may be polymers. The metal skin is riveted to the internal framework.

Wings

The wings’ function is to generate the lift required for flight, and they are often used to house the fuel tanks. Wings will often contain one or more commonly two spars which run from wingtip to wingtip via the fuselage and form the strength of the structure. Attached to the spars at right angles are the ribs, which help to give the wing its aerodynamic cambered shape, and to spread the load from the skin onto the spars. During flight, stresses are transmitted first to the wing skin, then to the ribs, and finally to the spars. Spars also must carry loads distributed by the fuselage, landing gear and any engines.

Wings may be either of strut-braced or cantilever design, depending on whether an external brace is employed to help transmit loads from the wing to the fuselage. Cantilever wings must obtain their entire strength from their internal structural elements, and this construction is employed in most modern aircraft, external bracing being used only for some small, light aircraft, such as the single-engined Cessnas.

Strut-braced high wing monoplane Cessna 172

 
         Cantilever low-wing monoplane PA32                                               Bi-plane

The wings of the aircraft may be rigged at the top or bottom of the fuselage and result in a high-wing or low-wing construction. Aircraft may have more than one set of wings, such as in a bi- or tri- plane. Multiple-wing planes have the advantage of superior lift and relatively stronger construction, but the monoplane has lower drag.

In order to confer lateral stability to an aircraft, the wings may be angled upwards from the fuselage towards the wingtips. This is known as a dihedral. Some wings may be inclined the opposite way and this is called anhedral and allows aircraft to be very manoeuvrable, as is needed for military or aerobatic aircraft.

 
                           Dihedral Angle                                                         Aircraft showing a marked anhedral

Wings may have an angle of sweepback to help with lateral stability and to give improved characteristics at high speeds.

Various other components may be attached to the wings, such as:

Engines, landing gear, ailerons, flaps & slats, spoilers, lights, pitot-static heads

Cessna 152: This is a high wing monoplane, and the wings are of the semi-cantilever type, in which some of the loads are supported by internal structures and the remainder by a strut at each side. The wings have a 1º dihedral.

Tail Assembly

The tail usually consists of

• Vertical fin - to give vertical stability
• Rudder – to control yaw via the rudder pedals. On aircraft without pilot controllable rudder trim, a fixed rudder trim is often fitted to the trailing edge of the rudder, and can be adjusted only on the ground by an engineer. These are usually rigged so that in cruise, there is no residual yaw. Two vertical fin surfaces are used in some older aircraft, in which case a double rudder is used. The V-shaped tail combines the rudder and elevator functions in a single device.

       V Tail

• Horizontal stabiliser - to give longitudinal stability Some aircraft, such as the Piper Cherokee series, do not have a separate horizontal stabiliser and elevator, but a stabilator or all-flying tailplane which combines both features. Most large jets feature all-flying tailplanes.
• Elevator – to control pitch via the control column In order to prevent dangerous control flutter, rudders, ailerons and elevators are often fitted with mass balances or horn balances. These ensure the centre of gravity of the control surface is close to the hinge. Some aircraft with stabilators have an anti-balance tab built into the elevator, which moves in the same direction as the tailplane , and serves to increase control loads to prevent the pilot over-stressing the airframe.
• Trim tab(s) – to relieve control pressures

  
   Cessna 172 stabiliser with elevator, trim tab and horn balance                            PA28 stabilator with anti-balance tab

The tail may also house lights and radio aerials.

Cessna 152: Has a conventional horizontal stabiliser and an elevator incorporating an elevator trim tab and a horn balance. Rudder trim is by a fixed tab which can only be adjusted on the ground.

Flight Controls & Flaps

The main flight controls are the

• Ailerons – located on the outer part of the wing to control roll about the longitudinal axis. They move in opposition to each other i.e. one up and the other down. Aircraft are usually fitted with differential ailerons to control adverse yaw. In this type, the up-going aileron goes up further than the down-going aileron goes down. Frise ailerons perform a similar function by having a lip protruding into the airstream when the aileron is deflected upwards.

• Elevator – Attached to the tail to control pitch about the lateral axis.
• Rudder – to control yaw via the rudder pedals
• Flaps - Situated inboard of the ailerons, they work together to provide increased lift, drag and wing area for take off and landing

  
        Cessna Flaps 0 degrees (up)                      Cessna Flaps 40 degrees (fully down)

Flaps can be manually, hydraulically or electrically operated and be of several types depending on their construction:

• Plain flap: Bends down to increase camber but not area.

• Split flap: Splits to increase camber but not area

• Fowler flap: Moves rearwards as well as down to increase area as well as camber
• Slotted flap: Opens a gap between the wing and flap to re-energise the boundary layer.
• Slotted Fowler flap: Combines the advantages of the above into a single flap

Cessna 152: Has differential ailerons(up 20º down 15º)with mass balancing; a conventional rudder; and slotted fowler flaps operated electrically. The flaps can be set at 10º for take off from a soft surface and at 20º or 30º for landing.

Landing Gear

The function of the landing gear is to support the weight of the aircraft on the ground and to allow steering and braking. Landing gear can be arranged in two ways:

• Tailwheel – Consisting of two main wheels towards the front and a small castoring tailwheel under the tail. Examples DC-3 Spitfire
• Tri-cycle or nose-wheel - Consisting of a steerable nosewheel and two main wheels further back. Most modern aircraft are configured in this way since it leads to greater directional stability.

  
                 Nosewheel aircraft                                                               Tailwheel aircraft

The aircraft main wheels must be strong enough to withstand the stresses of landing, and so are usually of robust design. They are attached to the fuselage of wing by means of struts or braces. An oleo-pneumatic unit is attached to absorb any forces and to dampen any oscillations. This consists of a piston within a cylinder filled with air or inert gas and lubricated with oil. The correct extension should be checked before flight. On landing it compresses and in the air it extends. Tyres will be fitted to the wheels and these should be checked for deep cuts and that they are correctly inflated.

Creep marks may be painted on the tyre and the wheel rim, to show any tendency for the tyre to rotate about the hub. As long as these marks overlap, then the tyre may be accepted.

Creep Marks

Aircraft fitted with nosewheel can sometimes experience nosewheel shimmy during take-off – an unpleasant vibration of the nosewheel from side to side. In order to prevent this, a shimmy damper is often fitted, which works in a similar manner to the oleo-pneumatic unit.

A torque link is also fitted to maintain correct nosewheel alignment. Braking is usually controlled from the tops of the rudder pedals, and a separate parking brake is often fitted.

Brakes are usually of the disc type and when used differentially may be used to reduce turning circles.

Landing gear on more advanced aircraft is retractable to reduce drag during flight and so allow higher cruising speeds. These aircraft usually have backup mechanisms for lowering the gear in case of a failure as well as devices to warn the pilot if the gear is not in the correct position for landing. Both tailwheel and retractable gear aircraft require differences training to be undertaken.

Cessna 152: Is of the nosewheel type with the main wheels attached to the fuselage by means of a tubular steel leg which absorbs landing stresses. The steerable nosewheel is fitted with a shimmy damper. Braking is hydraulically actuated disc braking.

15Jun06 ©T&FA