Principles of Flight Spitfire. Chapter 1 – Weight and Lift How is an aircraft, which is much heavier than the air it flies in, supported by the air? C130.

Principles of Principles of FlightFlight

Spitfire

Chapter 1 – Weight and Chapter 1 – Weight and LiftLift

How is an aircraft, which is much How is an aircraft, which is much heavier than the air it flies in, heavier than the air it flies in,

supported by the air?supported by the air?

C130

Sir Isaac Newton formulated laws to Sir Isaac Newton formulated laws to explain the movement of objects.explain the movement of objects.

Newton’s Third Law states that ‘ to Newton’s Third Law states that ‘ to every action there is an equal and every action there is an equal and opposite reaction’.opposite reaction’.

For example: Imagine a car weighing For example: Imagine a car weighing 10,000N parked on the road. The cars 10,000N parked on the road. The cars weight presses down on the road with a weight presses down on the road with a force of 10,000N – and from Newton’s force of 10,000N – and from Newton’s third law the road must be supporting third law the road must be supporting the car and pushing up with an equal the car and pushing up with an equal and opposite force of 10,000N.and opposite force of 10,000N.

How does this apply How does this apply to an aircraft flying?to an aircraft flying?

How can an aircraft How can an aircraft which might weigh which might weigh many tonnes be many tonnes be supported by such a supported by such a flimsy substance flimsy substance such as air?such as air?

One clue is that an One clue is that an aircraft can only aircraft can only stay airborne as long stay airborne as long as it is moving. as it is moving. (except a Harrier of (except a Harrier of course!)course!)

Air is a substanceAir is a substance Moving air can Moving air can

exert forces on exert forces on objects – e.g. in objects – e.g. in hurricaneshurricanes

The greater the The greater the speed, the greater speed, the greater the force it exerts.the force it exerts.

Conclusion: Air is Conclusion: Air is undoubtedly a very undoubtedly a very real substance, real substance, invisible as it is!invisible as it is!

Moving objects through Moving objects through airair

Is there a force if an object is moving Is there a force if an object is moving through stationary air?through stationary air?

Imagine cycling along a flat road on a Imagine cycling along a flat road on a perfectly calm day. As your speed perfectly calm day. As your speed increases, you will begin to feel the air increases, you will begin to feel the air on your face. The faster you pedal the on your face. The faster you pedal the greater the force of this apparent wind.greater the force of this apparent wind.

Whether you have air moving past a Whether you have air moving past a stationary object or an object moving stationary object or an object moving through stationary air, similar forces are through stationary air, similar forces are experienced.experienced.

Every object has Every object has weight.weight.

The weight of an The weight of an aircraft must be aircraft must be supported or lifted supported or lifted by a force.by a force.

How is this lifting How is this lifting force to be given to force to be given to an aircraft?an aircraft?

Try this: Hold two sheets of paper with Try this: Hold two sheets of paper with the edges vertical and about finger the edges vertical and about finger width apart. What will happen if you width apart. What will happen if you blow between the two pieces of paper?blow between the two pieces of paper?

What causes this to What causes this to happen?happen?

Wind Tunnel TestWind Tunnel Test The air goes in at point A, passes the The air goes in at point A, passes the

constriction at point B, and comes out at point C.constriction at point B, and comes out at point C. What are the speeds and pressures at points A, B What are the speeds and pressures at points A, B

and C?and C?

Bernoulli’s PrincipleBernoulli’s Principle

The same amount of air enters B as is leaving, The same amount of air enters B as is leaving, therefore the speed of air at B must increase therefore the speed of air at B must increase to pass through the narrowest part of the tube.to pass through the narrowest part of the tube.

The increased speed of the air at B, creates a The increased speed of the air at B, creates a streamlined flow (non-turbulent) which exerts streamlined flow (non-turbulent) which exerts less pressure on the tube.less pressure on the tube.

Conclusion: Air moving a increased speeds Conclusion: Air moving a increased speeds exerts less pressure or creates an area of exerts less pressure or creates an area of lower pressure.lower pressure.

Can you now Can you now explain why the explain why the two pieces of two pieces of paper move paper move together when you together when you blow air between blow air between them?them?

Proving the theoryProving the theory Try this: Hold a half sheet of A4 (halved lengthways) Try this: Hold a half sheet of A4 (halved lengthways)

such that the end between you fingers is horizontal, such that the end between you fingers is horizontal, but allow the rest of the paper to curve downwards but allow the rest of the paper to curve downwards under its own weight. Then blow along the curved top under its own weight. Then blow along the curved top of the paper only. of the paper only.

What happens?What happens?

Observation: The paper Observation: The paper rises in line with the air rises in line with the air flow.flow.

By speeding up the air By speeding up the air over top you have over top you have reduced the pressure reduced the pressure above the paper, so the above the paper, so the air pressure under the air pressure under the paper pushes it up.paper pushes it up.

What happens when you What happens when you blow harder?blow harder?

The harder you blow the The harder you blow the more it rises. more it rises.

LiftLift

The top surface of an aircraft’s wing is shaped The top surface of an aircraft’s wing is shaped such that air which flows between it and the such that air which flows between it and the undisturbed air a little way above the wing is, in undisturbed air a little way above the wing is, in effect, being forced through a constrictions.effect, being forced through a constrictions.

The air flows over the wing at an increased speed-The air flows over the wing at an increased speed-and therefore at a reduced pressure-compared and therefore at a reduced pressure-compared with the surrounding atmospherewith the surrounding atmosphere

The resulting pressure difference between the air The resulting pressure difference between the air above and below the wing tends to lift it up.above and below the wing tends to lift it up.

This is not the only thing that contributes to lift. In real This is not the only thing that contributes to lift. In real life, the airflow rarely approaches a wing as shown life, the airflow rarely approaches a wing as shown above.above.

In most flight conditions, the wing is inclined to the In most flight conditions, the wing is inclined to the airflow at a slight positive angle so that it deflects airflow at a slight positive angle so that it deflects some of the airflow downwards.some of the airflow downwards.

This results in an upward force, though not normally as This results in an upward force, though not normally as much as the lift generated by the top surface of the much as the lift generated by the top surface of the wingwing

Distribution of liftDistribution of lift There are pressure forces acting all over There are pressure forces acting all over

the wing- and there can be lift forces all the wing- and there can be lift forces all over the wingover the wing

The length of each arrow indicates the The length of each arrow indicates the amount of lift at that point on the wing’s amount of lift at that point on the wing’s surface. surface.

Pressure patterns nearA wing

Note that:Note that: Lift is not distributed evenly around the wingLift is not distributed evenly around the wing The top surface generates more lift than the The top surface generates more lift than the

bottom surface (sometimes as much as 80% of the bottom surface (sometimes as much as 80% of the total!)total!)

The greatest amount of lift on the top surface The greatest amount of lift on the top surface occurs where the surface is curved the most.occurs where the surface is curved the most.

The greatest effect, on both top and bottom The greatest effect, on both top and bottom surfaces, is nearer the front edge of the wing than surfaces, is nearer the front edge of the wing than the rear (1/3 of the way from the front)the rear (1/3 of the way from the front)

All lift forces act at 90° to the direction of the All lift forces act at 90° to the direction of the airflow.airflow.

Centre of PressureCentre of Pressure

We normally add all the forces We normally add all the forces together and represent them by a together and represent them by a single straight line.single straight line.

This force is drawn from the point at This force is drawn from the point at which all the forces balance.which all the forces balance.

This is the point which all the lift can This is the point which all the lift can be said to act and it is called the be said to act and it is called the ‘centre of pressure’.‘centre of pressure’.

How Lift VariesHow Lift Varies Several factors affect the amount Several factors affect the amount

of lift produced by a wingof lift produced by a wing A) Air speed – greater airspeed A) Air speed – greater airspeed

creates greater lift (doubling the creates greater lift (doubling the airspeed, quadruples the lift!)airspeed, quadruples the lift!)

B) Angle of attack- This is the angle between B) Angle of attack- This is the angle between the chord line of the wing and the the chord line of the wing and the oncoming air. A pilot can alter this by oncoming air. A pilot can alter this by altering the pitch attitude of the aircraft- altering the pitch attitude of the aircraft- by easing forward or pulling back on the by easing forward or pulling back on the controls. The lift will increase until the controls. The lift will increase until the angle reaches about 15°. Beyond this point, angle reaches about 15°. Beyond this point, the lift rapidly decreases and the wing has the lift rapidly decreases and the wing has stalledstalled

C) Air Density – If air becomes thinner C) Air Density – If air becomes thinner or less dense ( at increased height or less dense ( at increased height temperature or humidity), the amount temperature or humidity), the amount of lift is reduced. To calculate the take of lift is reduced. To calculate the take off run the pilot would use tables which off run the pilot would use tables which allow for engine performance, weight, allow for engine performance, weight, wind speed and direction, and of course wind speed and direction, and of course the air temperature, humidity and the air temperature, humidity and density.density.

D) D) Wing shape and Wing shape and area-area- The shape of the The shape of the wing section and the wing section and the plan area of the wing plan area of the wing are calculated by the are calculated by the aircraft designer to aircraft designer to suit the aircraft’s role suit the aircraft’s role and required and required performance.performance.

A A high-lift sectionhigh-lift section would be used where would be used where good lift at low speeds good lift at low speeds is all-importantis all-important

General Purpose General Purpose sectionsection would suit a would suit a moderately fast moderately fast aircraft, not intended aircraft, not intended to approach the to approach the speed of sound.speed of sound.

More lift for any More lift for any given wing section given wing section and airspeed can be and airspeed can be found by found by increasing increasing the plan areathe plan area of the of the wing- that is, by wing- that is, by making the wing making the wing bigger.bigger.

Unfortunately a bigger Unfortunately a bigger wing is heavier and wing is heavier and also more resistant to also more resistant to rapid manoeuvres- not rapid manoeuvres- not acceptable for air acceptable for air combat aircraft.combat aircraft.

Hinged flapsHinged flaps fitted to fitted to most light aircraft most light aircraft have the effect of have the effect of increasing the camber increasing the camber of the wing when they of the wing when they are lowered. are lowered.

More advanced More advanced Fowler Fowler flapsflaps slide backwards slide backwards to increase the wing to increase the wing area, as well as area, as well as hinging downwardshinging downwards

Lift and Weight in Straight Lift and Weight in Straight and Level Flightand Level Flight

The lift force equals The lift force equals the force of gravity the force of gravity acting on the acting on the aircraft (its weight)aircraft (its weight)

If the lift is greater If the lift is greater than the weight, an than the weight, an aircraft will climbaircraft will climb

If the weight is If the weight is more than the lift, more than the lift, it will descend.it will descend.

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