HIGH SPEED FLIGHT & FLYING ABOVE SPEED OF SOUND MACH SHELL OIL CO. EDUCATIONAL FILM 45604

when an airplane moves forward the air begins to flow over its wings once it's moving fast enough this airflow over the wings lifts it off the ground in flight the airflow over the wings continues to support the airplane and keeps it up in the air provided it's moving forward fast enough this is how it happens as the airflow passes over the wings curved upper surface it speeds up temporarily and slows down again at the rear this speeding up causes a reduction in air pressure on the wings upper surface and this gives the aircraft its lift but when aircraft fly faster than about 400 miles an hour things begin to get complicated the speed at which sound travels through air comes into the story this film will describe some of these complications and show why aircraft intended to fly at speeds near or above that of sound have to be designed differently from low-speed aircraft let's first consider how sound travels slowing down the picture shows how the sound is produced the vibrating prongs give the air a succession of pushes and like pressure waves along a spring the sound waves travel on through the air sound waves are in fact one kind of pressure wave like ripples on a pond they spread out from their source in all directions not all pressure waves are audible but all small disturbances audible or not travel outward at the speed of sound watch this explosion a mile away it sound takes five seconds to reach us at sea level sound travels at about 760 miles an hour but the speed depends on the temperature of the air which drops as we climb upward in the stratosphere which begins at about 36,000 feet the speed of sound is only about 660 miles an hour because of the lower temperature now look at the wing again every point on it by traveling through the air sets up tiny disturbances which move outward as pressure waves of the speed of sound and parts of these disturbances reach the air ahead of the wing here smoke is used to show how the air flows lowering a flap affects the airflow well ahead of the wing this is because the tiny pressure waves warn the air ahead of the wings approach this warning allows the air to part smoothly round an aircraft provided its flying well below the speed of sound at higher speeds approaching the speed of sound the exact relation between the speed of the aircraft and the speed of sound becomes very important it's called the Mach number usually shortened to M this is how it is calculated this aircraft has flown six tenths of a mile in the time the sound wave traveled ten tenths of a mile it is flown at six tenths of the speed of sound in the same atmospheric conditions that is it is flown at Mach point six this aircraft is flying at Mach point 9 at high speeds it's essential for the pilot to know his Mach number and Mach meters are fitted to all high-speed aircraft what actually happens to the air and the aircraft when approaching the speed of sound the airflow speeds up as it passes over the wing and reaches the maximum speed at a certain point on the wing so the Mach number of the air at this point is greater than that of the aircraft as a whole as the aircraft speed increases so does the local Mach number at this point on the wing eventually just at this point on the wing the air reaches the speed of sound although the aircraft as a whole is still flying slower than sound the aircraft's Mach number when this happens is called its critical Mach number usually written M crit soon after this things can begin to happen this aircraft is buffeting violently aircraft vary in their behavior above the critical Mach number but until designers overcame them there were always some adverse effects they're caused by shock waves let's see what this means if a model wing is put into a high speed wind tunnel a special photographic technique known as schlieren makes visible the places where the pressure of the air changes as it passes around the wing in this picture wherever the color is green the speed and pressure of the air are almost constant blue has been chosen to show up the areas where pressure is falling fastest while red shows where the air is being slowed down and pressure is increasing now look at a diagram at the critical Mach number the airflow is moving across the wing at this point at exactly the speed of sound M equals one above the critical Mach number the point grows into an area in which the flow is faster than sound M is greater than one everywhere else the flow is slower than sound M is less than one the rear boundary of this area is formed by a shock wave as the speed is increased still further the shock wave grows larger what causes the shock wave think again of the tiny pressure waves radiating out at the speed of sound from every point on the wing the ones coming from points on the rear part of the wing such as this one meet airflow coming the opposite way and make less and less headway until they can't travel any further forward at all because the airflow itself is moving backwards as fast as they are moving forward at the speed of sound it's like trying to step off an escalator going the wrong way the little pressure waves pile up building into one big pressure wave the shock wave and here's the real thing in the wind tunnel the wing has just passed its critical Mach number and there's the shock wave read because it's a sudden sharp increase in pressure but shockwaves don't only form on wings wherever the air speeds up enough in its passage over the aircraft on cockpit canopy fuselage and stabilizer there too shockwaves will form the most striking effect of shockwaves is a sudden very sharp increase in the aircraft's drag this is because the shockwave heats up the air and compresses it and this absorbs a lot of extra energy which has to be continuously supplied by the engines the rate at which the drag increases is itself highest somewhere around Mach 1 although the actual drag goes on growing all the time this sudden increase in drag above the critical Mach number is what used to be meant by the sound barrier most of the other adverse effects of shockwaves arise because they cause the airflow to separate from the wing surface this produces a large wake of turbulent air behind the shock wave which alters the pressure around the wing this in turn may cause loss of lift and some aircraft may lose height in others one wing may drop the aircraft may pitch or porpoises it's called or undergo other weaving movements such as sneaking and Dutch roll the turbulent airflow from behind the shock waves may hit the stabilizer or other parts of the aircraft and cause buffeting or the controls may be effective here slowed up about 40 times is a schlieren picture of what may happen to an aileron as a result of shockwaves how are their craft designers overcome these shock wave troubles one way is to put them off to still higher speeds by actually raising the critical Mach number of an aircraft the designer does this in two ways first by using relatively thin wings wings sections whose thickness is small compared with their width from front to rear the thinner the wing the less the air is speeded up while traveling over it and so the higher the critical Mach number the second method is to sweep the wing back suppose this unswept wing has a critical Mach number of 0.8 if we sweep the wing back only that part of the airflow at right angles to the wing is speeded up as it passes over the wing so the critical Mach number is raised in this case to 0.98 high-speed jet airliners use thin wings and sweep back to obtain high critical Mach numbers by cruising at high speeds but still below the critical Mach number they avoid shock waves altogether and the troubles that come with them but wings can't be made too thin or too highly swept back or low speed flight and landing would suffer and what about aircraft which have to fly much faster than sound designers of such aircraft have had to accept the presence of shock waves and find ways of overcoming their effects above the critical Mach number speeds are grouped into two ranges the transonic range covers speeds at which the airflow is mixed parts subsonic parts supersonic ears subsonic airflow is shown in dark green and supersonic in light green remember that wherever there are shockwaves on the aircraft there are supersonic regions in front of them the transonic range shown here in this wind tunnel picture begins at the critical Mach number and extends through Mach 1 the speed of sound to about Mach 1.3 depending on the design of the aircraft speeds higher than this come into the supersonic range here shockwaves are still present but the airflow is now supersonic everywhere even behind them for transonic flight thin wings and sweet back are again used for by lowering the speed of the air as it passes over the wing they reduce the severity of the shockwaves compare the characteristics of thicker and thinner wings in the wind tunnel at the same speeds streamlining an aircraft's cross-section helps to reduce the huge increase in drag the aircraft shape is modified so that the cross section area pushing through the air changes smoothly from nose to tail this usually means giving the fuselage a waist to compensate for the wings and flaring it out again behind above Mach 1 sharp leading edges and pointed nose helped to reduce the drag caused by another shock wave which appears in front the bow wave this is rather like a bow wave on water with the sharp leading edge the bow wave attaches itself more like this one from a motorboat buffeting is reduced by putting the stabilizer above the wings or below them so that it misses the turbulent air coming from the shock waves on the wings control of the aircraft with shock waves is also helped by an all moving stabilizer instead of the normal fixed stabilizer with elevators on the end powered controls are also essential shockwaves exert such huge forces on all the control surfaces but no pilot could control the aircraft manually all these improvements in design have conquered shock wave troubles and helped to make transonic flight safe and relatively simple in supersonic flight the air flowing past every part of the aircraft is supersonic and behaves quite differently from subsonic airflow here is a section of a supersonic wing the sudden corner on it couldn't be used on a subsonic wing for as the smoke shows here subsonic air flow becomes turbulent and confused past the corner but supersonic flows surprisingly enough can turn the same corner easily as shown in this diagram as it does so the air expands through the region shown here in blue and pressure is reduced a sloping shock wave coming from the opposite kind of corner swings supersonic air flows smoothly the other way the corner may be at the leading edge of a wing so supersonic aircraft can use sharp corners as for instance the double wedge wing section the changes of pressure caused by the shockwave and blue areas give the wing its lift here's a wind tunnel picture nearly twice the speed of sound another shape formed by two arcs of circles is the biconvex which like the double wedge has sharply pointed front and rear edges the trouble about these winged sections is that they give very poor lift at low speeds they can be used on missiles such as this one with double wedge wings but for manned aircraft which have to take off and land it's not so simple and very long runways are needed and sometimes even extra braking devices today the problem is met for most supersonic aircraft by once again sweeping the wings back at a bigger angle than for a subsonic or transonic aircraft the wings can then have a subsonic section rounded leading edges smooth curved surfaces and no sharp corners but there have been a few supersonic aircraft today with straight unswept wings using the supersonic biconvex type of wing section and at speeds around Mach 2 twice the speed of sound opinions are still divided about straight or highly swept wings or the more favored Delta for the future new ideas are on the drawing board vertical takeoff and landing to overcome some of the low-speed difficulties already military aircraft exist which will change shape during flight from straight wind at lower speeds to highly swept at supersonic speeds shapes like these may one day fly passengers at speeds much faster than sound

Channel: PeriscopeFilm

Views: 23,534

Keywords: Periscope Film, Stock Footage, 4K, HD, 2K, Mach 1, Mach 2, Compressibility, High speed flight, transonic flight, Shell Oil Company, Early British jets, Bristol 188, sound barrier, sound, sound waves

Id: 4_hASWgu8EY

Length: 20min 47sec (1247 seconds)

Published: Wed Aug 31 2016