How an airplane flies

[Music] Lara's argent atlas the herring go weighing a little over two pounds supported in flight by a wing of two feet span exerting a lifting force exactly equal to its weight Boeing 747 the jumbo jet weighing over 700,000 pounds supported in flight by a wing spanning a hundred and ninety five feet exerting a lifting force in level flight which is also exactly equal to its weight [Music] [Music] in determining just how an airplane flies and the forces acting on it its natural behavior and the ways in which we can control it let us go to the grassroots of aviation and from a considerable range take a typical light airplane as our example the Cessna 150 aerobat it weighs 1,600 pounds fully loaded and is supported in flight by wing spanning just over thirty two and a half feet this wing like every other exerts a lifting force in level flight exactly equal to the weight of the airplane [Music] to understand how an airplane flies we need to know something about the air it flies in what are its properties and how it behaves this envelope of air that surrounds our planet has certain physical characteristics and patterns of behavior though it's invisible it's made up of molecules like everything else it has density and pressure both greatest at sea level where the pressure average is nearly 15 pounds per square inch but further from sea level we climb the lower the pressure becomes until if 20,000 feet it is only about seven pounds per square inch and at 60,000 feet only about one pound per square inch and as the pressure falls so does the density and the temperature but that fifteen pounds per square inch at sea level is only an average in fact the pressure in any given place changes all the time and as it changes the air circulates around the areas of high and low pressure and the wind blows now we realize that air is a fluid seeking its own level like any other fluid and we can feel the force of its moving molecules as they rush past us and pull at everything in their path moving air is a source of energy has been harnessed for centuries with ever-increasing skill and understanding [Music] the airplane harness is the same source of energy by the opposite process by moving through the air let us look at the airplane for a moment it's basically a simple and functional machine wings set at right angles to the direction of flight support a body or fuselage to carry the load smaller wings and a vertical fin of the tail end of the fuselage provides stability in control in the nose an engine and propeller provide the motive power but it is the wings moving through the air that lift and support the airplane how to find out we must first make air visible we can do this in a smoke tunnel having first turned it on its side so that we can relate it to horizontal motion in the tunnel we can put various solid bodies in the path of the smoke filaments change their angle to the moving airflow and see what happens first let's take a cylinder notice that the air flows closely around the frontal curve of its circumference clinging to it until suddenly it separates it can't hold on any longer and becomes unstable and disturbed it's moving too fast to streamline behind the cylinder and maintain contact with it so it has to let go now let's take a flat plate lying horizontally to the Airstream the flat plate disturbs it hardly at all but turn the plate across the airflow and see what happens [Applause] turn it back again and we can see that even at the slightest angle to the horizontal the airflow over the upper surface tends to break up next a curved plate this base um what better the airflow is smoother but it soon breaks away when the plate is rotated through an angle to the airflow and at a smaller angle the airflow breaks away underneath where the plate is concave but if we look at the wing itself we find that it's neither a flat plate nor a curved plate but a combination of the two it has thickness the underside is almost flat the leading edge is blunt and rounded while the trailing edge is sharp the upper surface is curved or cambered and the wing is in fact attached to the fuselage with a slight angle to the horizontal cut a section through the wing and it looks like this if we set it in the smoke tunnel we can see how smoothly the air flows around its contours what's more we can increase the angle to the airflow by quite a large amount without it breaking away we call this variable angle - the airflow incidence or angle of attack but the really important thing is what happens to the airflow as it flows over and under the wing here is the airflow again this time in slow motion and with an irregular smoke flow notice how the smoke accelerates as it moves over the upper surface of the wing while the smoke flow below the wing tends to slow down this is just what happens in flight and it is this phenomenon that causes the wing to lift for when a fluid in motion increases its velocity its static pressure which acts at right angles to the direction of flow decreases and when the velocity is reduced its static pressure increases so that it's total sum of energy always remains the same to prove it for ourselves we need another kind of wing model the surfaces of this one are perforated by a number of pin holes each pin hole is connected separately by an air tight tube to a manometer tube manometers measure pressure when we drink through a straw we suck in to reduce the pressure in the straw and this causes the liquid to rise up inside it if we alternately blow and suck on a manometer tube the same thing happens and so if we put the wings section into a wind tunnel any changes in pressure acting at right angles to the wing surface will be shown on the manometer tubes each tube is attached alternately to the upper and lower surfaces here is the wing section with wool Tufts to show us the air flow now let's superimpose it on the manometers and see what happens the manometer is attached to the upper surface rise indicating a considerable drop in pressure while those attached to the lower surface fall indicating a rise in pressure and it is this difference between the pressures over the upper and lower surfaces that gives the wing its lift in fact the pressure pattern looks like this with the pressure at any given point acting at right angles to the surface at that point the pressure difference is greatest where the curvature is greatest least where the surface flattens out the resultant lift force acts through a point we call the center of pressure and at right angles to the relative airflow and so we have a cambered wing flying at a small angle to the horizontal producing a resultant lift force exactly equal to the weight of the airplane in steady level flight lift increases with speed we prove it every time we take off it also increases with an increase in the angle of attack let's follow that takeoff through in the wind tunnel see how the lift builds up with the speed in fact it increases as the square of speed then as we increase the angle of attack at liftoff there is a sudden almost muscular increase in lift to help us off the ground but the extent to which we can increase the angle of attack to the relative airflow is strictly limited if we increase the angle by more than a certain amount depending on the type of airplane this happens the airflow breaks up over the upper surface of the wing the pressure pattern becomes unstable lift decreases sharply the wing has stalled this can happen at any speed once the wing has reached the critical angle thus we can see that the pressure difference that gives us lift can only be maintained if the airflow over the wing remains smooth and undisturbed the stall can be demonstrated in the air the angle of attack is increased until it becomes critical lift is suddenly reduced the airplane drops pitching nose down and quite a lot of height is lost the four normal controlled flight can be resumed this can only happen when the relative airflow over the wing regains its normal angle and lift is restored the stall can also occur in level flight at normal flying speeds the angle of the wing to the relative airflow can be maintained whether the airplane is in level flight on a descending flight path or climbing always provided that the speed is within normal limits but if the speed is allowed to decrease in level flight lifts will also decrease then if we try to keep the airplane level it will begin to sink when this happens the relative air flow gradually changes if the critical angle of attack is reached the airplane will stall and will drop momentarily out of control [Music] the speed at which the airplane stalls will depend on its weight and will increase with altitude as the air becomes less dense nevertheless we must be able to fly slowly and in particular to land slowly to achieve this we can change the characteristics of the wing by fitting trailing edge flaps these flaps slide out from the wing and curve downward as they do so this increases the camber of the wing over part of its span the upper surface of the wing now has even greater curvature and the air flow therefore moves even faster while the obstruction caused by the flap below the wing tends to divert more air over the upper surface where the pressure is now even lower in this way more lift is gained without any increase in flying speed with the flap fully deployed lift is still maintained over the wing itself but behind the flap the airflow breaks up slowing the airplane down by increasing another force acting on it the force of drag this enables us to approach the field at a steeper angle than we otherwise could which gives the pilot a better view of the landing area as well as a lower landing speed lift is the force that supports the weight of the airplane lift is generated by moving a precisely shaped and angled wing forward through the air causing pressure changes to take place around it the force that moves the wing through the air is that of thrust the force that resists its motion is that of drag thrust the principle is that of Newton's third law of motion to every action there is an equal and opposite reaction when we swim we push water backward to move forward in consequence action reaction equal and opposite to sustain this balance of action and reaction over greater ranges of speed and endurance we need more than human muscle we need mechanical power to push a boat forward through the water we must use an engine-driven propeller to push the water backward it's the same with the airplane the jet engine moves the airplane forward by pushing backward a stream of hot gases with great velocity [Music] the light airplane uses a gentler method an engine-driven propeller pushes air backward the airplane moves forward action and reaction equal and opposite gathering motion until the airflow over the wing is moving fast enough to lift us off the ground [Music] to create and sustain thrust the air must be accelerated by the propeller so that it moves faster than the airflow caused by the forward motion of the airplane we can see this in the smoke tunnel with an electrically driven propeller at rest the air flows round it switch it on and the air in front of the propeller is drawn in towards the blades and then accelerated backward at a speed greater than the forward motion of the airplane it sets up a twisting wash in its wake just like the propeller of a boat only a lot faster thrust can be varied by controlling the power output of the engine if we increase thrust we increase speed but only up to a point for as speed increases so does another force acting on the airplane the force of drag on days like this we can see drag at work on any highway eddying air swirling behind the moving vehicles wasting their horsepower and their fuel by holding them back producing a chaotic pressure pattern like this drag can be measured take a disk of given area and put it in the mouth of an open wind tunnel mount the disk on a sprung slide attached to the indicator arm of a calibrated gauge the airspeed in the tunnel is measured by an airspeed indicator notice first that as the speed increases so does the drag in fact the drag increases as the square of speed with the airspeed indicators steady at a hundred miles an hour the drag on the disc records a value of five on the gauge if we mount a disc double the area of the first the drag gauge records a value of ten of the same airspeed double the drive finally if we mount a disk only half the area of the first and a quarter of the area of the second we get a reading of only two and a half at the same airspeed so the amount of drag depends on both the frontal area of the disk and on the speed of the air flow however airplanes nowadays can present very large frontal areas to the airflow and somehow manage to fly around at over 600 miles an hour so there must be an answer and there is streamlining if we add a bullet-shaped nose to the medium-sized disk the drag reading at a hundred miles an hour is reduced from five to just over one add a streamline tail and it goes down to less than one the drag arising from the shape and size of an airplane is known as form drag or normal pressure drag and if the shape is sufficiently streamlined the size doesn't matter so much one obvious way of streamlining an airplane is to give it retractable landing gear and eliminate struts the results of cleaning up the form in this way are often worthwhile both in terms of performance and fuel economy but if the gear has to be fixed then the answer is to streamline it along with the rest of the airplane for surface friction this is caused by molecules of air clinging to the whole surface area and combining with pressure drag to hold the airplane back we experience both these forms of drag in water as we move about we can feel the pressure resisting us and at the same time the water molecules in contact with our skin tend to move with us and try to pull the surrounding water along with them even when we streamline ourselves and apply thrust we can still feel the water clinging to us as we move in a stream of air it's this clinging layer of molecules in contact with objects in its path but tries to move them and carry them away if we pour a light easily flowing liquid over the leading edge of an airplanes wing and then take off we can see that as the airplane accelerates the liquid tends to ripple as the molecules in contact with the surface of the wing try to hold on to those immediately above them as the outer layers gradually let go we are left with a thin slow-moving film of liquid at the surface itself it is this clinging of the so-called boundary layer to the surfaces of the airplane but enables the pressure changes around the wing to give us lift pressure changes but also produce drag lift and drag are thus inseparable but the drag component could be reduced both by a clean design and by keeping the airplane clean and polished thus increasing its efficiency a third source of drag increases in inverse proportion to the speed of the airplane and its greatest when the speed is lowest we call this induced drag or trailing vortex and it arises from lift as a result of the pressure difference between the upper and lower surfaces of the wing air tends to spill over the wingtips from the area of higher pressure below the wing into that of lower pressure above the wing the two air streams meeting just above the wingtip form vortices a corkscrew pattern of twisting air that produces a considerable amount of drag at whatever speed the airplane is flying however the sum total of drag is equal to the thrust being exerted by the propeller waterskis can help us understand this principle the skis give us lift and drag the motorboat provides thrust when the two are accelerating thrust will be greater than drive until drag increases as the square of increase in speed and the two forces become equal they will remain in equilibrium for as long as the speed remains constant then the speed is reduced thrust becomes less than drag and the balance is lost on an airplane flying in steady level flight the total drag force is always equal and opposite to the force of thrust in just the same way as lift exactly equals weight