How Aircraft Fly

How Aircraft Fly

Every day millions of people board jet aircraft headed for destinations around the world. It's puzzling how an aircraft made of metal, weighing several tonnes, can lift passengers miles into the sky and propel them safely at speeds over 500 knots. Aerodynamics coupled with engineering makes this possible!

A Simplified Introduction to Aerodynamics!

All aircraft in flight are constantly engaged in a tug of war between the opposing forces of lift versus weight and thrust versus drag. The downward force of gravity must be overcome by an upward-acting, aerodynamic force called lift, which is produced by the aircraft’s wings.

When an air stream passes quickly over a wing its aerodynamic shape and its angle to the flow (‘angle of incidence’ or ‘angle of attack’) causes some of the air to be deflected downwards. The rest goes up and over the wing, accelerating as it does so. The net result is that the pressure above the wing reduces slightly and the pressure below the wing increases slightly. This small pressure difference results in an upward force – lift. This effect, called the venturi principle, was discovered in the 1700s by a Swiss mathematician named Daniel Bernoulli. 

The forward speed of the aeroplane generates the flow of air over the wing that is necessary to produce lift. As the aircraft accelerates on its takeoff roll, it eventually a reaches speed at which the airflow over the wings produces sufficient lift to overcome the weight of the aircraft - and the aircraft takes off. The heavier the aircraft, the more thrust is needed. Pilots must always ensure that there is a sufficient length of runway to achieve a safe flying speed, taking into to account the wind speed and direction and the air temperature.

In light aeroplanes the engine-driven propeller generates the necessary thrust. This force opposes the drag, which has two main causes. The first is simple air resistance to the frontal area of the airplane, which increases with speed. The second is produced as a

by-product of lift and is known as ‘induced drag’. Most of this is generated at the wing tips where higher pressure air under the wing spills over to try to equalise the lower pressure over the wing, forming a swirling vortex of air as it does. Some airliners are now fitted with winglets to reduce the wingtip vortices and the induced drag.

During take off, thrust must overcome drag and lift must overcome weight for the aeroplane to become airborne. The heavier the take-off weight of an aircraft the more distance it needs to achieve the necessary flying speed.

The tug of war between the four forces: lift, gravity force or weight, thrust, and drag continues while an aeroplane is in flight. The weight pulls down on the plane opposing the lift created by air flowing over the wing. In level flight at constant speed, thrust exactly equals drag and lift exactly equals the weight or gravity force. For landings, thrust must be reduced below the level of drag and lift below the level of the gravity force or weight.

Constructing a Flying Machine

So now we know what makes an aircraft fly, how do we construct a flying machine? The major components of a light aeroplane today are much the same as they were at the time of World War I. Changes are largely due to improved engines, coupled with improved building techniques, modern materials and a better understanding of aerodynamics.

In this section we will discover the purpose of each of the aircraft’s main components, which are the:

·        fuselage;	·        engine and propeller;
·        tail assembly (or empennage);	·        wings;
·        landing gear (or undercarriage);	·        flying controls

All these components can be seen on the “exploded” view of a typical, high-wing, training aircraft, below. You can see that the fuselage is below the wings, which are braced by struts. Later, we will learn about the various techniques for building the components of the airframe (the fuselage, wings and tail assembly).

Fuselage

The fuselage forms the body of the aeroplane to which the wings, tail, engine and landing gear are attached. It contains a cabin or cockpit with seats for the pilot and passengers, plus the flight controls and instruments and, sometimes, a fuel tank. The illustration below shows the fuselage of a low-wing aircraft (in which the fuselage sits on top of the wings), complete with the tail assembly.

Tail Assembly

The tail assembly, or empennage, consists of a vertical and horizontal stabiliser, more usually known as the tail fin and tail planes, which have a similar internal structure to the wings. These keep the aircraft flying cleanly through the air, just like the flights on an arrow. The fin also counteracts the turning effect of the swirling air from the propeller (“prop wash”).

The tail planes carry the elevators, which enable the pilot to control the pitch attitude of the aircraft; the fin carries the rudder, which enables the pilot to control the aircraft in yaw – the effects of these primary controls are explained later.

Landing Gear

The landing gear (or undercarriage) supports the weight of the aeroplane when it is on the ground. The undercarriage of low-wing aircraft may be attached to either the wings or the fuselage.

Undercarriage may be of either the tricycle type, with a nose wheel, or the tail wheel type (aircraft with this type are known as “tail-draggers”). Almost all aeroplanes have brakes that act on the main wheels. Most tricycle landing gear aeroplanes are fitted with nose wheel steering connected to the rudder pedals. Tail-draggers may be steered by their tail wheel, which is connected to the rudder, by differential braking – the ability to brake the main wheels independently – or both.

The more advanced aeroplanes have a retractable landing gear that folds up into the fuselage or wings. Training aircraft generally have fixed landing gear, which means that it cannot be retracted. The landing gear must be able to absorb significant loads during a heavy landing. There are very many types of suspension in use, from sprung steel or composite (typically when the gear is fuselage-mounted), through various types of piston arrangements to lever and spring, where often the “spring” is a bundle of bungee cords.

The Engine

Most light aeroplane engines, like most cars, are powered by petrol and usually have four, six or, occasionally, eight cylinders. A popular configuration for the smaller aircraft is a three litre, air-cooled, “flat four”. This has two horizontally-opposed banks of cylinders containing pistons connected to a central crankshaft. Cooling air enters the cowling behind the propeller, passes down past the cylinders and out beneath the fuselage.

Aero-engines are generally of the very reliable “four stroke” design. The four-stroke engine is so called because each piston moves up and down four times in one complete power cycle, which means there is one power stroke per cylinder for every two revolutions of the crankshaft.

The power cycle often described as “suck, squeeze, bang, blow”. As the crankshaft turns over, each piston in turn descends (stroke 1), sucking fuel mixture into the cylinder. The piston then rises (stroke 2), compressing or squeezing the fuel mixture until it occupies about an eighth of its volume. At this point a spark ignites the compressed fuel mixture (with a bang!), which forces the piston down (stroke 3), providing the power to keep the engine turning (also known as the “power stroke”). As the piston comes back up the cylinder (stroke 4) it sweeps out the burned gasses, blowing them out of the exhaust.

The Propeller

A piston engine requires a propeller (or “airscrew”) to convert the power output of the engine into forward motion. Propellers may be made of wood, metal or composite (glass fibre). The 'turning effect' or torque produced by the engine is used to rotate the propeller, which usually has two blades but may have up to four. Each blade is shaped as an aerofoil section, like the wings. By much the same principles, when the blades rotate, air from in front of the propeller is propelled backwards at speed, generating the horizontal force that we call thrust.

The optimum rotational speed for a propeller is around 2,200 to 2,600 rpm (revolutions per minute). At this speed the tips of the blades are not very far below the speed of sound. Higher revving engines must be geared down to turn the propeller at the optimum speed.

Most light aircraft have propellers with a fixed pitch (the distance it would travel through a solid in one revolution). Some allow the pilot to vary the pitch of the propeller in flight. Fine pitch is used for take-off allowing the engine to achieve peak rpm (therefore maximum power). Coarse pitch is used in the cruise so the engine may be run at a lower rpm for the same air speed than the equivalent fixed pitch propeller. This gives either improved fuel economy (lower power for a given speed) or a faster cruise (more speed for a given power setting).

‘Complex’ light aircraft and commercial turbo-fan aircraft are fitted with constant speed propellers. A ‘governor’ in the hub of the propeller automatically adjusts the pitch of these propeller blades to maintain a constant engine rpm.

Wings

The wings are designed to generate lift and must support the fuselage in flight. They need to be able to carry several times the total weight of the aeroplane to accommodate the “g” forces caused by manoeuvres. Wings generally have one or more internal spars attached to the fuselage and extending outboard to the wingtips. The spars carry the major loads, which are upward bending where the lift is generated and downward bending where they support the fuselage and the wing fuel tanks, if fitted. High-wing aircraft usually have bracing struts, which carry some of this load, allowing the wings to be built with a lighter structure.

Wings also carry two important control surfaces, the ailerons and the flaps. Ailerons are hinged control surfaces located at the ends of the wings that enable the pilot to bank, or roll, the aircraft into a turn. Locating them outboard of the wing provides optimum leverage to help the plane bank, and keeps them out of the disturbed airflow from the propeller.

Simple aircraft often don’t have flaps. If fitted they are found on the inner trailing edges of each wing. They are lowered together to increase the lifting ability of the wing for take-off and deployed further during landing to decrease the stalling speed and increase drag. 

Airframe Construction Techniques

There are four basic methods of constructing airframes: semi-monocoque or “stressed skin”, “wood and fabric”, “rag and tube” and composite (glass fibre).

The semi-monocoque construction technique, favoured by modern training aircraft such as Cessnas and Pipers, consists of a light, pressed-metal-sheet framework covered by a metal skin (usually aluminium) that carries much of the stress. The outer sheeting is fixed to the framework by many thousands of aluminium and steel rivets. The famous Spitfire of World War II was one of the first aircraft to be built using this technique.

“Wood and fabric” aircraft are built with synthetic fabric stretched over a light wooden structure, a technique that dates from the very earliest flying machines. Plywood box sections are used where loads are carried, such as the wing spar and fuselage. All wooden components are glued in place. The fabric on the wings is glued and stitched to the wing ribs.

“Rag and tube” aircraft again employ synthetic fabric but stretched over a welded, tubular steel “skeleton” that makes up the fuselage. The wings are generally wood and fabric or semi-monocoque.

Composite is the most modern building material. Composite aircraft are usually assembled, like an Airfix kit, from component halves. These are built up from several layers of resin-impregnated glass and carbon fibre cloth, in special moulds. Foam cores are used in sealed areas, like wings and tail planes, to impart additional strength. The moulding technique enables aircraft to be built with elegant, compound curves.

Many aeroplane kit manufacturers are now using tubular steel fuselages but clad with lightweight, non-load-bearing glass fibre panels. This technique is simpler than a composite aircraft to build and is lighter in weight but still gives an elegant, smooth and rounded finish.

Controlling a Flying Machine

So we know what makes an aeroplane fly, what are its main components and what techniques are used to build its airframe. But how does the pilot control it so we can take-off, steer a course to our destination and land it safely? The answer is that all aircraft are fitted with a number of control devices, the primary flying controls, that the pilot operates from inside the cockpit.

Primary flying controls

Whenever we need an aircraft in flight to change its horizontal or vertical position, we need to rotate it about one or more of three axes, which are imaginary lines that pass through the airplane’s centre of gravity. The axes of an airplane can be imagined as axles around which the airplane turns, much like the axle around which a wheel rotates. The primary flying controls let the pilot control the aircraft movement around these axes.

The main flight controls (elevator, ailerons and rudder) are operated from the cockpit, usually via an internal system of cables and pulleys.

Elevator

The elevator is a hinged control surface located on the trailing edge of the tail plane that controls pitch—movement about the aeroplane’s lateral axis. The elevator moves up and down when you apply back or forward pressure on the control yoke or stick. Moving the elevator causes the aircraft’s attitude to change, pitching the nose up or down. Without changing the power, when the nose pitches down, the aeroplane descends and gains speed. When the nose pitches up, the aeroplane climbs and slows down.

With the nose pitched down, unless the power from the engine is reduced, the aeroplane will accelerate until it can no longer overcome the increasing wind resistance and it reaches its terminal velocity, although most aircraft will break up due to the enormous load caused by the wind resistance before terminal velocity is reached. With the nose pitched up, unless the power from the engine is increased, the aircraft will climb and decelerate until it is no longer going fast enough for the wings to generate lift. At this point, the aircraft stalls and starts to fall from the sky – until it gains the necessary airspeed for the wings to produce lift again.

The elevator is often fitted with a “trim tab”. This is a small flap that the pilot adjusts to “trim” the aircraft to fly at a particular attitude and power (throttle) setting. “Trimming” makes the aircraft easier to fly by removing any forward or rearward load on the elevator control. Some aircraft achieve the same effect by using an adjustable spring mechanism. 

Ailerons

Ailerons are hinged control surfaces located at the ends of the wings that control bank, or roll, about the aeroplane’s longitudinal axis. The pilot controls the ailerons by turning the control yoke or moving the stick left and right. The ailerons move in opposite directions. When rolling to the left, the left aileron moves up, so the airflow pushes the left wing down; simultaneously the right aileron moves down, so the airflow pushes the right wing up.

 Rudder

The rudder is a hinged control surface attached to the trailing edge of the tail fin. The pilot operates the rudder by pressing pedals in the cockpit. The rudder controls yaw movement about an aeroplane’s vertical axis. Unlike the rudder of a boat, it is not the primary means of turning the aeroplane (unless it is on the ground!). It is used to maintain co-ordinated flight during turns, which is a fancy way of saying “ensures that it flies cleanly into and out of turns by stopping any tendency to yaw”. On the ground, the pilot uses the rudder to steer when taxiing and to keep the aircraft running in a straight line when landing or taking off.

The Centre of Gravity

Before moving on it is worth considering the significance of the centre of gravity (CofG), as we have seen above that all the axes act through it. All aircraft, including massive airliners, must be loaded with passengers, fuel and baggage, so that the CofG relative to the longitudinal axis stays within the limits specified in the Pilots Operating Handbook (see later). A pilot must calculate this position based on the weight and position of every passenger, the weight of the fuel and which tanks are filled and the weight and position of any baggage.

Imagine an aircraft suspended by a string attached at its optimum design CofG. If it is loaded forward of this point then the nose will want to pitch down. If it is too nose heavy the elevator will not be able to pitch the nose up sufficiently at slow speeds to stop the aircraft from flying into the ground when trying to land. If it is heavily loaded too far aft of the design CofG, the nose will want to pitch up and the imbalance will overcome the stabilising effect of the tail plane, making the aircraft uncontrollable.

The Instrument Panel

All aircraft have an array of instruments that enable the pilot to operate the aircraft at its optimum efficiency and maximum safety. In an average panel there are flight and navigation instruments, flight controls, engine gauges and controls, radios and other “avionics” (electronic devices), such as Radar transponders and radio navigation aids.

An aircraft’s instrument panel can seem like a dauntingly confusing jumble of dials and switches! Pilots are taught to understand how to interpret and respond correctly to the readings from the instruments, especially when flying “blind” (in cloud or at night). Despite the large number of instruments and avionics, there are relatively few that are vital for flying in good weather in daylight (in VMC or Visual Meteorological Conditions). Most enhance safety or provide “redundancy”, which means that if one fails, the pilot can usually determine sufficient information to keep flying from the other instruments and gauges. Some are needed only for flying “blind” (also called IMC or Instrument Meteorological Conditions).

The essential instruments for VMC are:

Air speed indicator–	Reports speed through the air (not the same as speed over the ground!)
Altimeter –	Displays height of the aircraft over the ground or above sea level, depending on how the pilot sets it
Compass –	Indicates the direction the aircraft is pointing (its “heading”) - not the same as the direction it is actually travelling (its “track”)!

Other instruments useful in VMC but essential for IMC are:

Turn & slip –	Indicates the angle of bank and the degree of yaw
Artificial Horizon	Indicates the angle of bank and pitch attitude
Direction indicator -	As compass but a lot steadier so much easier to read
Vertical speed indicator -	Indicates whether and at what rate the aircraft is climbing or descending

The Pilots Operating Handbook

The pilot must be fully familiar with the Pilots Operating Handbook (POH), which provides information for a particular aircraft. It details all the instruments, performance graphs (see examples below), thrust, weight, balance, centre of gravity calculations, warnings, normal and emergency operating procedures.