THE AEROPLANE

DEFINITION

An aeroplane is an aircraft that is heavier than air that is kept aloft by lift derived form air flowing over fixed wing surfaces and is driven by propellers or jet engines.

PRINCIPLES OF FLIGHT

The motion of air over the wing as it moves forward through the air is what generates the lift on the wing.

Due to the shape of the wing the air flowing over the top of the wing must move faster to reach the slower moving air on the underside of the wing at the trailing edge. This creates a low pressure area on the top of the wing which gives rise to the upward lift force on the wind.

The lift increases with the effective area of the wing and the speed of the wing through the air. The latter relationship is given by the Benoulli equation:

P_dynamic = P₀ - v²/2 - g h

P_dynamic is the dynamic pressure
P₀ is the static pressure
v is the velocity of the air
g is the downward acceleration of gravity
h is the height above a fixed reference level (eg the ground)

The 'g h' term is negligible between the upper and lower wing surfaces.

We can use the above equation to computer the pressure on the top and bottom of the wing (P_u and P_l respective. The difference between these gives the pressure difference δP = P_l - P_u. The upward force on the wing with effective area A is then given by

F_upward = δP A

Of course the lift force is not the only force acting on the wing. There is a gravitational force or the weight of the wing, a force which moves the wing through the air due to the engine and a drag force. These are shown for the whole aeroplane below:

AIRCRAFT AXES AND MOTIONS

Unlike ground vehicles the aeroplane can move in three dimensions. The axes about which this 3D motion can occur is illustrated below.

The longitudinal axis runs through the length of the aeroplane and motion about this axis is called ROLL. At right angles to this in the normally horizontal plane is the lateral axis. This goes through the central axis of the wings. Motion about this axis is called PITCH. The final axis is the vertical axis which goes through the center of mass (or gravity) of the plane. Motion about this axis is called YAW.

These motions are controlled by moving various surfaces on the wings and the tail of the aircraft.

The two ailerons, one on each wing, move in opposite directions. When one moves up (from hinges on the inboard side) the other moves down. This produces motion about the roll axis. The other two axes are controlled by moveable surfaces on the tail assembly. The rudder which moves from side to side produces yaw motion about the vertical axis. The two elevators, one on each side, move together up and down producing a pitch motion about the lateral axis.

All three control surfaces have small trim tabs that can be moved independently of the surface they are on to relieve pressure on the pilot controls and provide what in effect is a neutral position for each control. This relieves control pressure on the pilot controls to make his job easier for long flights.

Large aircraft have a lot more control surfaces than their smaller cousins. These include inboard and outboard ailerons, leading and trailing edge flaps, slats, air brakes, and spoilers.

The flaps are used to provide additional lift (but also drag), usually for the descent/approach phase of the flight.

PILOT CONTROLS

Inside the cockpit are the controls that the pilot uses to move the control surfaces of the aeroplane. A view inside a light aircraft cockpit is shown below.

By convention the pilot-in-charge sits in the left hand seat. The main control is the control yoke (in high performance aircraft this is normally replaced by a control stick). This has two motions. A back and forth motion controls the elevator and the roll of the pitch of the aircraft. A rotational motion (just like a car steering wheel) controls the ailerons and thus the roll about the longitudinal axis.

The two rudder pedals on which the feet rest control the rudder and thus control the yaw. When the left pedal is pushed down the right pedal moves up and vice versa. The rudder pedals also act as a brake when taxiing on the ground. To activate the brakes the pilot moves his feet to the top of the pedals and pushes down to stop the aircraft ground movement. There is no reverse ground motion - the pilot must disembark and manually push the aircraft back into a park position. Or be towed into position for a large aircraft.

The instrument panel has a number of gauges that show the attitude of the aircraft, the airspeed, engine RPM, fuel status, navigation and engine temperature. The 'flying' instruments as normally placed in front of the pilot whereas the engine instruments are to the right.

Duplicate controls allow an instructor pilot to sit in the right seat and teach the student pilot in the left seat and take over control of the aircraft if the situation warrants.

Radios for communication and navigation are normally fitted in the centre or just to right of centre. This will normally include a transponder for location of the aircraft by a secondary ground radar.

In this aircraft the throttle control is located beneath the radios. To the left of this is control for carburetor heating (to prevent icing) and to the right is a fuel mixture control. If the aircraft has a variable pitch propeller there will also be a propeller pitch control. All of these controls are pushed in or pulled out. The throttle is pushed forward to increase the engine RPM and pulled back to decrease it.

Below the instrument panel is the switch panel which hosts a variety of switches, including power and light switches, the engine start switch and various others.

AIRCRAFT TYPES

Many different types of aircraft have been manufactured and these may be classified in different ways.

For light single engined aircraft the fuselage may be above the wing or below the wing.

The aircraft on the above left is a Cessna C-172 which has the wings above the fuselage. To the right is a Piper Warrior PA-28 which has the wings below the fuselage.

Aircraft may also be classified according the number of engines they have. These may be single-engined, twin engined or multi-engined aircraft. The two shown above are both twins, but quite different types of twins. On the left is a standard twin with both engines mounted on the wings. This is a Cessna C-310. On the right is a Cessna C-337 with two engines but in-line along the longitudinal axis. One engine is at the front of the fuselage as in a single-engined aircraft, but the other is behind the fuselage, in line with the first.

The two aircraft above are both powered by jet engines and have no propellers. The one on the left is a Cessna Citation executive business jet with two jet engines, mounted on the fuselage just ahead of the tail assembly. The image on the right is a Northrop T-38 twin jet engine trainer. It has two seats, for trainer and student. NASA uses these high performance jets to give astronauts experience with a high-powered jet aircraft, since they are capable of supersonic flight.

Australian Space Academy