Theory of Flight
Theory of Supersonic Flight
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Transonic Flight
● At low speed pressure and density effects are
less significant
● At high speed pressure and density change
becomes more significant (i.e. pressure and
density increase is significant)
● In transonic flight some airflow over the wing is
sonic, while some is supersonic
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Supersonic Flight
● When aircraft is supersonic, all parts of it are
considered to be above the speed of sound.
Hence aircraft is passing faster than pressure
wave
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Critical Mach Number (Mcr)
● On the upper chamber of the airfoil, airflow
accelerates
● Mcr is the airflow speed at which a point on the
upper surface of the airfoil becomes sonic when
the aircraft is still flying at M<1
● At this speed shock waves forms on the surface
of airfoil
● Higher Mcr allows the aircraft to fly at higher
speeds as it delays the formation of shock
waves (thin wing has higher Mcr than thick)
● Mcr is also the speed at which any small
disturbance may affect the stability,
controllability, lift and drag
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Critical Mach Number (Mcr)
● A supercritical airfoil is an airfoil designed
primarily to delay the formation of wave drag in
the transonic speed range. This is done by
flattening the upper surface of the wing.
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Adverse Transonic Effects
● The adverse effects of transonic region include
buffeting, shock waves, increase in drag,
decrease in lift and movement of the center of
pressure occur
● Compressibility Buffet is the violent vibration felt
in the aircraft due to shock wave formation on
wings or control surface
● Disadvantage is that if continued for long
structure damage or loss of control can occur
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Adverse Transonic Effects
● When flow speed reaches the critical mach
number a normal shock wave is formed on the
upper surface of the airfoil
● Flow downstream of the shock wave is
subsonic, hence velocity is decreased with
increase in pressure
● At this point turbulent wake and flow separation
point will be form which will reduce the lift thus
increase in drag which is known as Shock drag
● Shock drag is wave drag plus boundary layer
drag
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Adverse Transonic Effects
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Increase in velocity
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Bow wave
When an oblique shock is likely to form at an
angle which cannot remain on the surface. These
are termed bow shocks.
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Lift and Drag Variation in transonic region
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Aerodynamic Heating
● Aerodynamic heating is the heating of a solid
body produced by its high-speed passage
through air, whereby its kinetic energy is
converted to heat by skin friction on the surface
of the object at a rate that depends on the
viscosity and speed of the air
● Example concord aircraft
● Special ceramic tile ‘heat-sink’ insulation on the
structure of the Space Shuttle is for protection
against aerodynamic heating
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AREA RULE
● For high speed aircrafts (plus transonic region)
area rule is used for aircraft shape for minimum
drag which is defined as
● For the minimum drag at the connections,
(wing/fuselage), the variation of the aircraft’s
total cross-sectional area along its length,
should approximate that of an ideal shape
having minimum wave drag
● It means the cross-sectional area of aircraft
from nose to tail, conform to those of a simple
body of streamline shape
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AREA RULE
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FACTORS AFFECTING AIRFLOW IN ENGINE
INTAKES OF HIGH SPEED AIRCRAFT
● For M<1 intake is divergent duct
● For M>1 intake is convergent duct
● For supersonic speed of the aircraft both types
of ducts are required
● This is done by two ways
– moveable doors (which change the intake shape)
– bullet fairing (in which shock wave is formed before
entry in compressor)
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Intake Moveable doors
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Bullet fairing Intake
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EFFECTS OF SWEEPBACK ON Mcr
● Sweepback delays the production of the shock
wave as well as reduces the severity of the
shock stall
● There are two components of the airflow on
sweepback wing. Spanwise and chord wise
component
● Chordwise component produces shock wave
● Greater the sweepback greater will be Mcr
● Sweepback results in a thinner mean
aerodynamic chord which also raises the Mcr
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EFFECTS OF SWEEPBACK ON Mcr
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