ATPL PRINCIPLES of FLIGHT

ATPL PRINCIPLES of FLIGHT. The primary requirements of an aircraft are as follows:  A wing to generate a lift force.  A fuselage to house the payload.  Tail surfaces to add stability.  Control surfaces to change the direction of flight and,  Engines to make it go forward. The process of lift generation is fairly straightforward and easy to understand. Over the years aircraft designers, aerodynamicists and structural engineers have refined the basics and by subtle changes of shape and configuration have made maximum use of the current understanding of the physical properties of air to produce aircraft best suited to a particular role. Aircraft come in different shapes and sizes, usually, each designed for a specific task. All aircraft share certain features, but to obtain the performance required by the operator the designer will configure each type of aeroplane in a specific way. As can be seen from the illustrations on the facing page, the position of the features shared by all types of aircraft - i.e. wings, fuselage, tail surfaces and engines varies from type to type. Why are wing plan shapes different? Why are wings mounted sometimes on top of the fuselage instead of the bottom? Why are wings mounted in that position and at that angle? Why is the horizontal stabiliser mounted sometimes high on top of the fin rather than on either side of the rear fuselage? Every feature has a purpose and is never included merely for reasons of style. 4 Chapter 1 Overview and Definitions An aeroplane, like all bodies, has mass. With the aircraft stationary on the ground it has only the force due to the acceleration of gravity acting upon it. This force, its WEIGHT, acts vertically downward at all times. W Figure 1.1 The Force of Weight The Force of Weight Before an aeroplane can leave the ground and fly the force of weight must be balanced by a force which acts upwards. This force is called LIFT. The lift force must be increased until it is the same as the aeroplane’s weight. W L Figure 1.2 The Forces of Weight and Lift The Forces of Weight and Lift 5 Overview and Definitions Chapter 1 To generate a lift force the aeroplane must be propelled forward through the air by a force called THRUST, provided by the engine(s). W L Figure 1.3 The Forces of Weight, Lift and The Forces of Weight, Lift and Thrust Thrust From the very moment the aeroplane begins to move, air resists its forward motion with a force called DRAG. W L Figure 1.4 The Forces of Weight, Lift, Thrust and Drag The Forces of Weight, Lift, Thrust and Drag 6 Chapter 1 Overview and Definitions When an aeroplane is moving there are four main forces acting upon it:- WEIGHT, LIFT, THRUST and DRAG. These are all closely interrelated. i.e.:- The greater the weight - the greater the lift requirement. The greater the lift - the greater the drag. The greater the drag - the greater the thrust required, and so on... Air has properties which change with altitude. Knowledge of these variables, together with their effect on an aeroplane, is a prerequisite for a full understanding of the principles of flight. The structural and aerodynamic design of an aeroplane is a masterpiece of compromise. An improvement in one area frequently leads to a loss of efficiency in another. An aeroplane does not ‘grip’ the air as a car does the road. An aeroplane is often not pointing in the same direction in which it is moving. 7 Overview and Definitions Chapter 1 GENERAL DEFINITIONS Mass Unit - Kilogram (kg) - ‘The quantity of matter in a body.’ The mass of a body is a measure of how difficult it is to start or stop. (a “body”, in this context, means a substance. Any substance; a gas, a liquid or a solid.)  The larger the mass, the greater the FORCE required to start or stop it in the same distance.  Mass has a big influence on the time and/or distance required to change the direction of a body. Force Unit - Newton (N) - ‘A push or a pull’. That which causes or tends to cause a change in motion of a body. There are four forces acting on an aircraft in flight - pushing or pulling in different directions. Weight Unit - Newton (N) - ‘The force due to gravity’. ( F = m x g ) Where (m) is the mass of the object and (g) is the acceleration due to the gravity constant, which has the value of 9.81 m/s2 . ( A 1 kg mass ‘weighs’ 9.81 newtons ) If the mass of a B737 is 60,000 kg and F = m x g it is necessary to generate: [60,000 kg x 9.81 m/s2 ] 588,600 N of lift force. Centre of Gravity (CG) The point through which the weight of an aircraft acts.  An aircraft in flight is said to rotate around its CG.  The CG of an aircraft must remain within certain forward and aft limits, for reasons of both stability and control. Work Unit - Joule (J) - A force is said to do work on a body when it moves the body in the direction in which the force is acting. The amount of work done on a body is the product of the force applied to the body and the distance moved by that force in the direction in which it is acting. If a force is exerted and no movement takes place, no work has been done.  Work = Force x Distance (through which the force is applied)  If a force of 10 Newton=s moves a body 2 metres along its line of action it does 20 Newton metres (Nm) of work. [10 N x 2 m = 20 Nm]  A Newton metre, the unit of work, is called a joule (J). 8 Chapter 1 Overview and Definitions Power Unit - Watt (W) - Power is simply the rate of doing work. (the time taken to do work)  Power (W) = Force (N) x Distance (m) Time (s)  If a force of 10 N moves a mass 2 metres in 5 seconds, then the power is 4 Joules per second. A Joule per second (J/s) is called a Watt (W), the unit of power. So the power used in this example is 4 Watts. Energy Unit - Joule (J) - Mass has energy if it has the ability to do work. The amount of energy a body possesses is measured by the amount of work it can do. The unit of energy will therefore be the same as those of work, joules. Kinetic Energy Unit - Joule (J) - ’The energy possessed by mass because of its motion’. ’A mass that is moving can do work in coming to rest’. KE = ½m V2 joules The kinetic energy of a 1 kg mass of air moving at 52 m/s (100 knots) is 1352 joules; it possesses 1352 joules of kinetic energy. [ 0.5 x 1 x 52 x 52 = 1352 J ] From the above example it can be seen that doubling the velocity will have a greater impact on the kinetic energy than doubling the mass. (velocity is squared) Newton’s First Law of Motion ’A body will remain at rest or in uniform motion in a straight line unless acted on by an external force’. To move a stationary object or to make a moving object change its direction a force must be applied. Inertia ‘The opposition which a body offers to a change in motion’. A property of all bodies. Inertia is a quality, but measured in terms of mass, which is a quantity.  The larger the mass, the greater the force required for the same result.  A large mass has a lot of inertia.  Inertia refers to both stationary and moving masses. Newton’s Second Law of Motion ’The acceleration of a body from a state of rest, or uniform motion in a straight line, is proportional to the applied force and inversely proportional to the mass’. Velocity Unit - Metres per second (m/s). - ‘Rate of change of displacement’ 9 Overview and Definitions Chapter 1 Acceleration Unit - Metres per second per second (m/s2 ) - ‘Rate of change of velocity’. A force of 1 newton acting on a mass of 1 kg will produce an acceleration of 1 m/s2 Acceleration = Force Mass  For the same mass; the bigger the force, the greater the acceleration.  For the same force; the larger the mass, the slower the acceleration.