Chapter 3: Wing and Empennage

The wing is one of the most important structures of an aircraft. Due to its special shape, it generates lift to overcome the weight of the aircraft, enabling flight.
The empennage (tail section) contains two of the three primary flight controls: the rudder and elevator. Along with the ailerons attached to the wing, these primary flight controls allow pilots to maneuver the aircraft in flight.
Secondary flight control: flaps, which are beneficial during take-off and landing.
Wings come in different configurations:
- Monoplane: a single wing is the norm for most aircraft.
- Biplane: two sets of wings (an aircraft with two wings).
- Wing attachment options: wings can be mounted on the top, middle, or lower portion of the fuselage.
- Braced (semi-cantilever) wings: external braces are used to share loads between wing and fuselage, common in high-wing aircraft.
- Full cantilever wings: no external braces; loads are carried internally between wing and fuselage without external bracing.

The wing has several key structural components. The main load-bearing component is the spar, which runs the length of the wing. A wing may have several spars, even on small aircraft.
A common design uses a main spar that runs along the length of the wing at the point of maximum thickness, and a smaller, lighter rear spar toward the wing's trailing edge to which the flaps and ailerons are attached.
Spars are complemented by ribs, which run perpendicular to the spar and provide the aerofoil shape.
The aerofoil shape (upper and lower surfaces) is critical for lift: typically a curved upper surface and a flatter lower surface. The aerofoil shape varies depending on the aircraft’s purpose.
The skin supports some loading (stressed skin design) and is usually a thin metal, such as an aluminum alloy.
Stringers run along the same direction as the spar and the skin is attached to them to help share the load.
In modern aircraft, fuel tanks are integrated into the wing structure (to be explored in Chapter 14).

The aerofoil shape is essential for generating lift; the curvature on the top and the flatter bottom create a pressure difference that produces lift as air flows over the wing.
The choice of aerofoil thickness varies: a thin aerofoil is generally better for high-speed flight, while a thicker aerofoil is better suited for slower, smaller aircraft.

The empennage is the main airframe structure at the rear of the aircraft and includes:
- Vertical stabilizer (fin) with the rudder.
- Horizontal stabilizer with the elevator.
Some aircraft have an all-moving horizontal stabilizer (stabilator) where the entire horizontal tail surface moves about a central pivot instead of having a separate elevator.
The empennage typically uses spars, ribs, and stressed skin similar to the wing to share and absorb loads.
Figure 3.3 (referenced) shows the typical components of the empennage.

The three primary motions of the aircraft are pitch, roll, and yaw.
These are controlled by three types of control surfaces attached to the wing and empennage:
- Ailerons: control roll; attached to the outboard section of the wing; move in opposite directions to each other.
- Rudder: controls yaw; a movable control surface fixed to the vertical stabilizer; controlled by the rudder pedals in the cockpit.
- Elevator: controls pitch; attached to the end of the horizontal tailplane (conventional tailplane).
All-moving tailplane (stabilator): an alternative design where the entire horizontal tail surface moves around a central pivot point; operates similarly to a traditional elevator in affecting pitch.
Figure 3.4 illustrates the position of primary flight controls and flaps.

In the stabilator design, the whole horizontal tail surface Move to adjust pitch, rather than moving only a separate elevator.
As with a conventional elevator, this adjusts lift on the horizontal tail surface to control pitch.

In small aircraft, primary flight controls are connected to the cockpit by cables and pulleys (and sometimes pushrods). This is known as a mechanical or manual flight control system.
The pilot does not require additional assistance to move the control surfaces.
The feel of the control column (stick force) depends on the deflection of the control surface and the aircraft’s speed.
In larger or faster aircraft, extra assistance may be required to move the flight controls, similar to power steering in a car.

Most aircraft have a range of secondary flight controls designed to improve performance.
The most common secondary control is the flap, which provides the best of both worlds: extra lift at low speed and no drag penalty at high speed once the flaps are retracted.
Flaps are located on the inner trailing edge of the wing (see Figure 3.4).
They are operated from the cockpit, either with a lever or an electrical switch.
Most flaps can be extended (lowered) in stages.
Take-off: a small flap setting is used to increase lift with a small drag penalty.
Landing: a larger flap setting is used to increase drag, enable a steeper approach, and lower landing speed.

There are four main types of flap:
- Plain flap: hinged to the back of the wing and pivots down when extended; increases lift but also greatly increases drag.
- Split flap: deflected from the lower surface of the wing; generates slightly more lift than the plain flap but creates more drag; when fully extended, both plain and split flaps produce high drag for little extra lift.
- Slotted flap: has a gap between the flap and the wing; significantly increases lift compared to plain or split flaps; air from below flows through the slot and over the upper surface of the flap, improving lifting capability.
- Fowler flap: a type of slotted flap that also increases the wing area; slides backward on tracks instead of rotating on a hinge; the first stage of a Fowler flap provides a significant lift increase with very little drag penalty.
All control surfaces (including flaps) are usually constructed similarly to the parent surface (wing) they are attached to, with spars, ribs, and skin. Because flaps are smaller, they can be built with lighter weight construction.

Aircraft consist of a large number of structures, both big and small, all carefully integrated with the rest of the aircraft.
Aircraft must be capable of operating safely on the ground despite their considerable weight; ground operations and weight considerations are discussed in the next chapters.
The chapter references figures (e.g., Figures 3.1–3.6) to illustrate wing designs, empennage components, and flap types.

The wing and empennage work together with primary and secondary flight controls to enable control of pitch, roll, and yaw during different flight phases: take-off, climb, cruise, descent, landing.
The choice of wing type (monoplane vs biplane), wing cantilever design, and empennage configuration affects structural loads, drag, stability, and control feel.
Flaps illustrate trade-offs between lift, drag, and wing area, and explain why aircraft adjust wing geometry for take-off versus landing.
The evolution from manual to powered control systems reflects a balance between pilot workload and control effectiveness on different aircraft types.

Chapter 14: Fuel tanks integrated into the wing structure.
General principles: load sharing between wing, fuselage, and empennage; use of stressed skin and internal spars/ribs to carry loads.
Ground handling and overall aircraft weight considerations are addressed in the next chapters.