High Speed Flight Part 1

aircrafts designed to approach the speed of sounds look different from low-speed aircraft to find out why let's first consider how sound itself travels through air 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 these sound waves travel on through the air the air is alternately a compressed and rarefied and as each wave passes there is a brief disturbance sound waves are pressure waves like ripples on a pond they spread out their source in all directions at the same speed all small disturbances in the air where the audible or not travel outwards at this same speed the speed of sound what is this speed here is a convenient source of sound at sea level let's measure the time it takes for the sound of the explosion to travel one mile the distance between these two forts the flash is coming now one two three four the sound took just under five seconds to travel the mile that is at sea level the speed of sound is about 760 miles per hour the exact speed depends effectively on only one factor the temperature of the air the higher the temperature the faster the sound travels on a really hot day it may reach over 800 miles per hour at sea level but temperature falls with altitude until the stratosphere is reached at about 36,000 feet above this height the temperature remains constant at approximately 60 degrees centigrade below zero so the speed of sound is lower only 660 miles per hour what has the speed of sound to do with high speed flying to find out let's consider first a single point sending out small pressure waves continuously each wave travels out words that have sound now suppose the point itself is moving if its speed is less than the speed of sound the pressure waves still travel out ahead but if the point is traveling at the speed of sound the pressure waves cannot travel out ahead of it or the point is traveling as fast as they are if the points travels faster than sand that is at supersonic speed this happens but we're not going to deal with this case here this is the kind of way in which the speed of sound effects high speed aircraft and therefore at high speeds the exact relation between the speed of an aircraft and the speed of sound is important but remember the speed of sound varies with temperature and therefore with altitude it's considered a bit lower in the stratosphere than at sea level the ratio of an aircraft's true airspeed to the speed of sound where it is flying is called the aircraft's Mach number after the 19th century Austrian physicist Ernst Mach it is usually shortened to M at high speeds it is essential for the pilot to know the Mach number and math meters are fitted to all high speed aircraft this is how an aircraft Mach number is calculated the aircraft has flown 6/10 of a mile in the time that a sound wave has traveled 10 tenths of a mile it has flown at six tenths of the speed of sound in the same atmospheric conditions so it's Mach number is 0.6 this aircraft is flying at mark 0.9 aircraft flying up the same true airspeed but a different heights will have different Mach numbers or the speed of sound is different in the two cases the Mach numbers at which an aircraft is intended to operate have a great influence on its design an aircraft is much more complicated than the point source of pressure waves we saw earlier so the behavior of the air is more complicated to to find out about it let us consider the airflow around a wing this wing section is symmetrical like most modern wings it is in a typical flying attitude the air is slowed down for nose to form what is called the stagnation region it speeds up as it passes around the curvature of the wing it slows down again towards the trailing edge these changes of speed cause changes in the air pressure the yellow regions show reduced and the green increased pressure all these variations in pressure together produce lift and drag now each point on the wing acts like a point source here we're only showing a few such points each sends out pressure waves which travel at the speed of sound and reach the air ahead of the wing we can use smoke to show how the air flows the influence of the pressure waves traveling ahead can be seen from the way the streamlines are deflected well ahead of the wing lowering a flap changes the entire flow pattern around the wing and affects the airflow ahead we can see this better with a single streamlined market position well ahead even as a distance the streamline changes direction the effect of the pressure waves on the air ahead is of great importance it smoothes the flow past an aircraft flying well below the speed of sound but what happens when approaching the speed of sound the airflow speeds up as it passes over the wing and reaches it maximum speed at a certain point Mach number here will always be greater than that of the aircraft as a whole called the flight Mach number as the flight Mach number increases so does the local Mach number at the maximum speed-up point eventually though the aircraft as a whole is flying at less than the speed of sound just at this point on the wing the air is moving at the speed of sound the flight Mach number when this happens is called the critical Mach number of the aircraft usually written M crit for any wing section M crit will always be less than one aerodynamically the critical Mach number is very important for the aircraft has reached the speed at which it meets mix air flow part subsonic that is less from the speed of sound part supersonic greater than the speed of sound it is the beginning of the transonic speed leg from the behavior of the aircraft the pilot has no way of telling that he has reached the critical Mach number but soon after it has been exceeded things begin to happen the NARC meter on the left has been altered since the actual critical Mach number of this type of aircraft has not yet been released in this picture it is 0.9 and soon after M fit is exceeded the aircraft starts to bucket violet aircraft vary greatly in their behavior above the critical Mach number some show violent instability while others specially designed for transonic flight may be little disturbed to find our flight problems of rise above the critical Mach number let's use a high-speed wind tunnel this one's fitted with special optical equipment to show in colors regions where the density of the air is changing these effects can be photographed solid objects like this nozzle appear silhouetted against a colored background when the compressed air jet is turned on other colors appear we have chosen red to show regions where density is increasing and blue regions where it is decreasing a symmetrical wing section designed for high speeds is put in the tunnel at low speeds the air behaves as if it were incompressible whatever pressure changes there are are so slight as to cause no color change but as speed increases the air does begin to show signs of compressibility the colors show regions of increasing and decreasing density the red area at the leading edge is the stagnation region where the air is being slowed down and becoming denser immediately behind are two blue areas where the rate of speeding up is greatest causing a reduction in density this diagram will remind us of what's happening as the flight Mach number increases a flow at the point of maximum speed up on the wing reaches Mach 1 the speed of sound the wing has reached its critical Mach number and when this is exceeded a sudden sharp region of increasing density forms on the wing just behind the point of maximum speed up this is a shock wave it is a sudden jump in the pressure of the air it grows and moves back as the Mach number increases shock waves can even be seen with the naked eye on certain atmospheric conditions watch them spilling back from the nose of this missile there this time we're going to hold the picture still for few seconds better see a shockwave form again in the wind tunnel a diagram shows what happens at the critical Mach number that is a point on the wing where M equals 1 at higher speeds the point grows into an area in which the flow is supersonic that is where m is greater than 1 outside this area the flow is still subsonic M is less than 1 the rear boundary of the area is the shockwave itself as the aircraft accelerates so the area of supersonic flow increases and the shock wave moves back growing larger and stronger a shock wave at right angles to the airstream is the means by which the airflow suddenly decelerates from supersonic to subsonic speed how our shock waves formed on the rear part of the wing every point such as this one sends out innumerable tiny pressure waves at the speed of sound in the forward direction these waves need airflow in the opposite direction and make less and less progress until they reach a stage where they can't travel any further forward because the airflow itself is moving backwards supersonically it's like trying to step off an escalator going the wrong way the pressure waves constantly pile up here and this is the shock wave a shock wave is a very narrow region about one ten thousandth of an inch thick across this region the supersonic airflow is violently reduced to subsonic speed much of the airs energy of movement or kinetic energy is dissipated as heat the temperature of the air rises suddenly as it passes through the shock wave there is also a sudden rise in pressure the energy wasted as heat in the shockwave must be continuously supplied by the engines otherwise the aircraft would decelerate so as the aircraft approaches the speed of sound it meets an additional kind of drag called wave drag wave drag is a large proportion of total drag of transonic speeds as the speed of airflow is increased still further the region of supersonic flow goes on growing logic and a second supersonic region starts to form on the lower surface of the wing with another shock wave you at speeds approaching the speed of sound the most important result of the shock wave is because the airflow to separate from the wind surface this is called shock induced separation it produces a large turbulent wake which alters the pressure distribution lift is produced and the turbulence creates thread in this aircraft the turbulence strikes the tail plane causes violent buffeting and so limits its speed other aircraft experienced such serious troubles as sudden loss of stability and reduced effectiveness of the controls we can't see separate flush when the airflow is smooth and flap when shock induced separation occurs but shockwave don't only appear on wings speed up of the air flow occurs around the canopy and many other parts of an aircraft wherever it's great enough their shock waves will fall shockwaves caused a vast increase in drag and may cause serious control troubles one way of avoiding these troubles is to put them off to still higher speeds by raising the aircraft's critical Mach number there are two chief design methods for doing this the first is to use relatively thin wings that is wing sections with maximum thickness small compared with width or cord thin wings in this sense can of course be very deep the thinner the wing the less the air is speeded up and so the critical Mach number is raised these two wings of the same general shape and at the same angle of incidence the tunnel speed is the same for each as it increases the shock wave forms on the picker wing at a Mach number of 0.8 but not a sign yet on the Phenom here it comes at last at a Mach number of 0.9 but wings can't be made to thin or the landing speed would be too high the other important way of raising the critical Mach number is by the use of sweet bear Wonder stamp is let's look from above with the flow over a wing the contour of the wing section determines the amount the air is speeded up now sweep the wing back we can represent the velocity the airflow ahead of the wing by an arrow of a certain length we can consider this velocity is made up of two smaller components a yellow component at right angles to the leading edge and a red component parallel to the leading edge the red component can be considered to flow along the span and is not speeded up by the wing section but the yellow component flows across the section and is speeded up right it is therefore only this component which affects the wings critical Mach number so the maximum speed reached over the swept wing will be less than it would be over a similar straight wing in particular if the flow over the straight wing has reached the speed of sound at the same flight speed the flow over the swept wing will still be subsonic the swept wing has therefore a higher M crit the greater the sweep back the more the critical Mach number is raised for instance suppose the straight wing has a critical Mach number of 0.8 by sweeping the wing back to 35° the M crit is raised to 0.98 but this is only the theoretical result for an infinitely long wing in practice the critical Mach number would only be raised to about 0.9 Sweetback black thin wings brings its own problems at low speeds such as tip stalling so designers must compromise between high speed and low speed performance and aircraft designs to fly near the speed of sound use in wings together with sweep back to obtain a high critical Mach number various arrangements have been adopted all very different in appearance if the swept wing is too thin to contain the engines they can be mounted externally in pods alternatively the engines can be buried in the winged roots which are more highly swept than the remainder of the wing a special kind of sweet back is the Crescent whip here the speed back is reduced in stages to avoid tip stalling the DeltaWing combines a high degree of sweet back and grapes trick the deltas large wing area makes it very maneuverable and gives it a good performance at high altitudes the simple Delta shape can have many variations some deltas have a tell plan to improve maneuverability at the expense of slight extra drag today aircraft are being designed to fly at speeds far above their critical Mach numbers problems of extra drag and loss of control caused by shock waves are being overcome but for many years ahead airliners will cruise below their critical Mach numbers in this way they will avoid shockwaves altogether and attain long-range and economy of operation at the same time designers will aim to make critical Mach numbers as high as possible thus permitting passenger flight at speeds approaching the speed of sound you

Channel: davypassane

Views: 66,348

Keywords:

Id: LSmqsg0DbTY

Length: 26min 32sec (1592 seconds)

Published: Tue Dec 24 2013