Subsonic And Transonic Flight
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Published: Fri, 05 May 2017
There are basically three speed ranges for the flight of aircrafts, namely; subsonic, transonic and supersonic. All the three speed ranges were taken taking the speed of sound in a medium as a reference point. In this analysis, more emphasis is given to the former two which are subsonic and transonic flight. Subsonic flight refers to flying the aircraft at speeds less than the speed of sound with no formation of shockwaves. This is the speed range where most of the commercial aircrafts operate. The ratio of the speed of the aircraft and the speed of the sound is known as the Mach number. In technical terms for subsonic flight, the aircraft is flying at mach numbers which are less than Mach 1.0 since the aircraft is travelling at relatively lower speeds than the speed of sound. There are no shockwaves for aircrafts flying at subsonic speeds. This is because the acoustical disturbances generated by the passage of a subsonic aircraft, and the sound waves suffer attenuation with distance from the aircraft due to spherical spreading (Crocker, 2007).
On the other hand, transonic flight deals with flying at higher subsonic speeds ranging from Mach 0.7 to approximately equal to Mach 1.0 (Aircraft Research Association, 2012). In transonic flight an aircraft flies at speeds less than the speed of sound, but with the presence of shockwaves. This leave a question of how is it possible that shockwaves form at speeds less than the speed of sound? But the answer to this question is explained by the movement of air around the aircraft. Far upstream from the aircraft, the motion of air is in uniform manner, all at the same speed. As the air come in contact with the aircraft, some regions of the air speeds up especially the air moving above the wings, creating a differential airspeed around the aircraft. These accelerate the air molecules and they end up moving very fast. These regions move faster than the speed of sound at transonic speeds, and these regions at all times end in a formation of a shockwave.
DESIGN FEATURES THAT DELAY TRANSONIC FLIGHT PROBLEMS
Different aircrafts are built in different ways, so the transonic region for a particular aircraft will depend on its design characteristics. There are many design features that can be employed in aircrafts to delay the transonic wave drag. In this analysis more emphasis will be on the use of thin aerofoils, use of low aspect ratio wings and the use of swept back wings.
THE USE OF THIN AEROFOILS
For transonic flow the wave drag rise is roughly proportional to the square of the thickness-chord ratio (NASA, No Date). This implies that when thinner aerofoil sections are used, the flow speeds around an aerofoil will be less than those for the thicker aerofoils, due to the minimum curvature of their upper and lower cambers. This give a clear indication that even at higher free-stream mach numbers, flight is possible before a sonic point appears and before the drag divergence Mach number. The drag divergence Mach number is the Mach number at which the aerodynamic drag increases rapidly as the Mach number continues to increase. It can be concluded that thinner aerofoils delay the drag divergence Mach number to a greater value.
Thin aerofoils have got some disadvantages associated with their use even though they are very useful in solving transonic flow problems. Firstly, in the subsonic range of speeds they tend to be inefficient in producing lift. Also, given that the wing is too thin, it can accommodate less structure such as the structural support and fuel tanks of which means for such structures as the fuel tank they have to be embedded maybe under the fuselage. Aircrafts with thinner aerofoils also face common landing accidents due to their landing speeds which are particularly high.
THE USE OF LOW ASPECT RATIO WINGS
The wings aspect ratio is another factor that control the critical Mach number and the transonic drag rise. Aspect ratio is the ratio of the wingspan to its mean chord length. When looking at the aircraft from above the aspect ratio refers to the measure of how long and slender the wing appears. It is linked to the wing plan form arrangement as opposed to its cross-sectional arrangement. In this case the wingspan is the straight line distance connecting the two wingtips.
Strike (2008, pp.45) clearly stated that high aspect ratio wings have an advantage in that they form low trailing edge vortices and thus reduce the induced drag. He further explain that induced drag is inversely proportional to aspect ratio, which implies wings with high aspect ratio produces low induced drag and the ones with low aspect ratios creates high induced drag. The picture below shows the configuration of both low aspect ratio wings, moderate aspect ratio wings and high aspect ratio wings.
NASA (No Date) claims that an aircraft with an aspect ratio less than about four will experience a considerable increase in the critical Mach number. This become useful at high transonic speeds as the drag divergence Mach number is delayed as the critical Mach number is increase which means an aircraft can fly at high speeds before shockwaves form. Low aspect ratio wings are used at transonic speeds as there are structurally strong since the distance from the wing tip to the fuselage is not that big allowing agile manoeuvrability at high speeds. Nevertheless, the main disadvantage with low aspect ratio wings is the difficulties they face at subsonic speeds because of the higher induced drag.
THE USE OF SWEPT WINGS
Wing sweep as one of the aircraft design features has an important role in delaying the transonic flight problems. Most importantly swept wings can delay and reduce the effects of compressibility. The idea of swept wings in supersonic flight was put forward by Adolf Busemann in 1935 (NASA, No Date), the idea which most of the best aerodynamists didnââ‚¬â„¢t agree with at first. In transonic flow, swept wings delay the formation of shock waves to a far much higher Mach number. This reduces the wave drag over all these mach numbers. Shockwaves depend on the span component that is 900 to the trailing edge, and this is a far much smaller component than actual airspeed in swept wings.
Aircraft wings can be swept both forward and backwards. The disadvantage of forward sweep is its ability to lose stability and handling characteristics at low speeds. In backward sweep, the wing experiences early separation and stalling at the wing tip sections resulting in ailerons loosing roll control effectiveness. The figure below shows swept wings on a two-seat F-15E strike eagle.
With swept wings, the major disadvantage is the span-wise flow along the wings and for sweepback the boundary layer will thicken towards the wing tips and towards the root for sweep-forward. This span-wise flow can be reduced in a number of ways. Firstly, stall fences can be used at wing tips. These are parallel plates to the axis of symmetry of the aircraft. Stall fences helps to prevent the build-up of a strong boundary layer over the ailerons, allowing effective functioning of the ailerons. Wing twist can also be another solution to the span-wise flow condition.
EFFECTS OF ICE AND SNOW ON AIRCRAFTS
The build-up of ice and snow on an aircraft can have catastrophic effects on the ground and in flight. This ice and snow can form on the parts of the aircraft during flight where the aircraft will be flying on very harsh weather conditions which can allow the freezing of water molecules at high altitudes since the temperatures are always very low. Ice can also form on an aircraft on the ground during taxing or when the aircraft is not in use and is not housed on a hangar. On cold and rainy day, rainwater can freeze on the upper surfaces of the wings and if not removed, this ice can have many different effects on the aircraft. In this section the effects of snow on the aircraft is going to be analysed fully.
Ice is one of the foreign object debris which when left on the aircraft can pose a very critical threat to the aerodynamics and performance of an aircraft. It can also hinder the performance of the pilot who is controlling the aircraft. Considering the aerodynamics effects of ice, it can reshape the surfaces of the lift producing parts of the airplane thus the wings and the tail. This changes the aerodynamics of these parts completely such that more drag is produced and less lift. This increases fuel consumption. Wind tunnel and flight tests have been carried and it showed that frost, ice and snow on the upper surface of a wing can reduce lift by as much as 30% and increase drag by as much as 40% (CAA, 2000).
The amount of the lift produced depends on the angle of attack thus the angle between the aerofoil chord line and the relative airflow. As the angle of attack is increased, the wing generates more and more lift until a certain angle where air cannot flow over the upper surface and the wing experiences aerodynamic stall. The point where stall commences has to do with the contour of the aerofoil. If the surface is contaminated with ice and snow it will be slightly rougher and this reduces the lift and alters the point at which stall takes place. Borrell (2009) claims that for scheduled air carriersââ‚¬â„¢ commercial passenger airlines inclusive icing has been a major factor in 9.5% of fatal air carrier accidents.
During ground activities aircraft contamination with ice and snow lead to potential risks during takeoff and subsequent flight. These hazards are mostly sourced by the hindered flight controls or instruments and the deteriorated aerodynamic performance. If an aircraft has engines mounted at the rear of the fuselage, clear ice that has formed on the wings may become loose during flight due to flexing of the wings and may be ingested by the engines causing a possible engine blow or failure. In most cases engine failure is the result of icing within the engine fan blades or is because ice that has formed on the engine inlet has been ingested. Icing on the propeller blades can also cause a dangerous imbalance. It is vital therefore, that no aircraft should take-off before the pilot has ascertained that all the surfaces of an aircraft are ice/snow free.
Ice can also cause an uncontrollable rolling and pitching motion on an aircraft and recovery might be impossible in cases where there is too much ice. These may lead to the airplane stalling at much higher speeds and lower angles of attack than normal. The other effect is that it can cause antennas to vibrate so severely that they break end up breaking. These can result in a communication barrier between the pilots on the aircraft with air traffic controllers on the ground or other pilots. These on its own has an upper hand of causing an aircraft crash.
As ice forms on the windshield, the pilotââ‚¬â„¢s visibility may be lost leading to the pilot controlling the airplane on imagination which in most cases is a very dangerous threat. According to Brandon (2000), the weight of25mm of ice on a small general aviation aircraft would be about 30-40kg, which shows how much a little ice can have on the weight of the whole airplane, so there is no such thing as a little ice.
SOLVING ICING PROBLEMS
Aircraft icing problems can be overcome in a numerous ways which can be classified under the two sub topics of de-icing and anti-icing. De-icing are measures that are put in place to get rid of the ice that have already formed on the aircraft structure. This procedure of de-icing can be done with mechanical or pneumatic tools or with employment of warmed de-icing fluids. Mechanically, de-icer boots are fitted in sections along the leading edges of the wing, and horizontal and vertical stabilisers. The boots are made from natural rubber and fabrics made of rubbers between which are inflatable tubes closed at the ends. The tubes are then connected to the air supply. When in operation air is pushed in to the boots using the tubes hence increasing the pressure and as a result the bond between ice and the aircraft weakens and ice falls off.
For de-icing using fluids, the liquid is applied along the centreline of the upper part of the fuselage, and then over the sides. The problem with allowing the aircraft structures to ice and then rely on de-icing is that some will be invisible to the human eye and sometimes left on the aircraft resulting in catastrophic effects during flight.
On the other hand, anti-icing is a preventive way of not allowing the build-up of ice on the surfaces of the aircraft structures. Glycols act an important role in this operation because of their non volatile characteristics, non-toxic, non-corrosive and having low freezing points. Anti-icing is usually performed before takeoff as the liquid is usually effective for 10-20 minutes (RIA Novosti, 2013). Anti-icing systems on flight are usually turned on before approaching an icing zone. This systems are typically, carburettor heating, fuel vent heat, pitot heat and windshield heat. Anti-icing on the wings is done by spraying the fluid from the leading edges to the trailing edges of the wing. Anti-icing as a preventive measure is the method of the two since the user is assured of no ice formation on the aircraft hence minimum or no adverse effects of ice and snow build-up on aircraft structures.
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Brandon, J. (2009) Icing Conditions in Flight. Available at: http://www.pilotfriend.com/av-weather/meteo/thnder.htm (Accessed: 01 March 2013).
Borrell, B. (2009) How Does Ice Cause an Airplane to Crash. Available at: http://www.scientificamerican.com/article.cfm?id=ice-flight-3407 (Accessed: 01 March 2013).
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RIA Novosti (2013) De-icing of Airplanes. Available at: http://www.en.rian.ru/infographics/20110114/162128519.html (Accessed: 12 March 2013).
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