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ShockWatch

posted Sep 17, 2016, 6:23 AM by Rohit Bhaskar
You might often wonder why in flight aerodynamics we tend to differentiate the flight speed characteristics in terms of subsonic and supersonic flow and what are the hindrances in going from subsonic to sonic flow... Well the answer to that is shock wave formation.

Shock waves are the major hindrances in sonic and supersonic flight. In general, disturbances in air travel at the speed of sound. If you consider a normal subsonic flight you will note that the the air ahead of the aircraft senses its presence and in 'layman' terms gets out of the way. However when the aircraft travels at mach 1, the air ahead doesn't get the time to get out of the way because before it can sense its presence the aircraft rams into the air. This causes massive changes in local properties of the air which includes increase in temperature, pressure and density. One can imagine the plane to be ramming into wall of obstruction. This whole phenomenon constitutes SHOCK WAVE formation. Shock waves not only give rise to huge drag to the aircraft but also cause the flow over the wings to detach thus causing in lift condition.

Shock waves in general travel at speeds greater than speed of sound. The speed of shock waves generally depends upon the amplitude of the waves. These waves in spite of being highly energetic cover lesser distance as compared to sound waves because they dissipate energy very quickly in the form of heat. When an airplane crosses the sound barrier you may often note liquid condensation along with it. This is again mainly because of the rise in density of the air arising out of shock wave formation .

CONDENSATION EFFECTS


Once an aircraft has reached the drag divergence mach number, there is tremendous increase in drag resistance offered to plane and a large amount of thrust is required to overcome it. However it is interesting to note that the wave drag at sonic flight is greater than that at supersonic flight .The reason for this is that in sonic flight there is flow resistance (drag) as a result of flow not being attached to the aircraft. However at higher speeds the flow again begins to attach itself to the aircraft, hence we see that lesser total drag is offered in this case.

Shock waves for flat thin bodies is normal to its area. These are called normal shock waves. The shock waves tend to turn around corner edges and angles giving rise to oblique shock waves which is most commonly seen in an aircraft. Now if the corner edges and angles are large then shock waves which tend to follow the curves detach. Such shock waves are called bow shock wave. Bow shock waves in general offer greater drag as compared to oblique shock wave. It is formed in case of blunt objects traveling at supersonic speeds. Bow shock waves have their application in space shuttle design wherein the shuttles are given a blunt shape so that they experience high drag and are slowed to a low enough speed for safe landing .

BOW SHOCK WAVE


Transonic flights are also to a great extent affected by shock waves. Although the aircraft is traveling at speeds less than mach 1 there is local acceleration at points on the aircraft particularly the airfoil where the velocity crosses the sonic barrier. At these places very high wave drag is experienced and lift production reduces significantly. However the SUB CRITICAL AIRFOIL has been hugely successful for attaining high efficiencies of aircraft in transonic range. This airfoil has characteristics such that the maximum chord thickness is pushed aft so that the point at which shock waves are produced are pushed backwards and thus a major part of the airfoil is still available for lift. The other characteristics include thinner sections and less total drag .

Generally when an aircraft crosses the sonic barrier the huge opposition of drag is overcome by pitching nose down and going ahead. Now as mach 1 is crossed the center of pressure of wings moves ahead (due to flow separation) and the aircraft pitches further nose down (this is known as mach tuck) and ultimately loses control and crashes. However with Carnard configuration of the aircraft the plane can be stabilized again .

Now some history on supersonic flight .

  • The first aircraft to cross the sound barrier was the Bell X-1.The first SST(supersonic transport) aircraft were the TU-144 and CONCORDE.
  • The TU-144 has a very interesting story.It is named after TUPOLOV, the great Russian aerodynamic expert and also its inventor. The development of TU-144 had begun well before its first successful flight in 1960's as an SST.Originally its development was aimed towards coming up with the world's first fighter jet. It was encouraged by the RED ARMY during WW II and the project handed over to Tupolov. However a number of factors caused a delay in its development (mainly Tupolov being imprisoned several times during this period ;) ). Also the innumerable failures of the TU-144 during flight testing prompted the then impatient government to scrap the project. However, post war the government changed and Tupolov along with his son continued on his endeavors and came up with the perfect plane years later.
Currently a number of hypersonic aircraft are in development, one noticeably being the NASA's X-15 which has three stages of flight. Most of the supersonic and hyper crafts run on ram and scram jets, which give high efficiency only at high speeds and very low efficiency at low speeds.


References:


  • introduction to aerodynamics of flight
  • introduction to flight

Contributed by:
Siddharth Tiwari
stiwari98 [at] gmail [dot] com
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