aero engineering 315 lesson 20 supersonic flow part ii
TRANSCRIPT
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Aero Engineering 315
Lesson 20
Supersonic Flow Part II
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Supersonic Flow Objectives Define speed of sound
Calculate speed of sound in air Calculate Mach Number Describe the different “Mach” regimes Describe how lift and drag curves vary with
Mach number Define critical Mach number and drag
divergence Mach number Describe how A.C. moves with increasing
Mach Describe ways to increase critical Mach Describe ways to minimize wave drag
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As an aircraft approaches Mach 1, the flow at some point on the aircraft will reach Mach 1 before the freestream velocity (V∞) does, since the flow accelerates over the aircraft’s surface. The freestream Mach number (M∞) where this occurs is called the critical Mach number. Mcrit is always less than 1.
Critical Mach number
MM=1
MM=1
= Mcrit
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MM < 1
TerminatingShock
SeparatedWake
As Mach number increases beyond Mcrit, shock waves form on the wings, fuselage and other surfaces. The shock moves aft as Mach continues to increase.
Normal Shock
M > 1
Shock-induced separation- Huge increase in drag- Significant loss of lift
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Transonic Region
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Normal Shock Parameters
Po1 > Po2
V1 > V2
P1 < P2
T1 < T2
1 < 2
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Bow Shock
When the freestream Mach number > 1, bow shocks form on bodies with blunt leading edges
M > 1
M > 1
Bow Shock
M < 1
Oblique Shock
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M > 1
Oblique Shock
When the freestream Mach number > 1, oblique shocks form on bodies with sharp leading edges. M > 1 everywhere in the flowfield.
M > 1 M > 1
M > 1
Oblique Shocks
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Mach Sweep Video
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Effect of Mach on Lift
TransonicSubsonic
Mach Number, M
Supersonic
1.0 2.0
Lif
t Coe
ffic
ient
Cur
ve S
lope
, CL
Theoretical
TransonicSubsonic
Mach Number, M
Supersonic
1.0 2.0
Lif
t Coe
ffic
ient
Cur
ve S
lope
, CL
Theoretical
2
)0(
1
M
CC ML
L
Prandtl-Glauert( 0.3 < M < 0.7 )
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Mach Effects
NACA 0012 – 3 degrees AOA
Mach
M = 0.6 = MCR
Cl
Local Mach = 1.0Mach
Shock moves to TE
Prandtl-Glauert
Shock Stall
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M
C Do
Profile Drag
Wave Drag
MCRIT
Effect of Mach on Drag
M=1
MDrag Divergence
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Mach Effects
NACA 0012 – 3 degrees AOA
Mach
M=0.8
Shock Separated Flow
Shock Wave
Cd MCR
MDD
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Effect of Mach on A.C.
When the Mach number > 1, the aerodynamic center of all aerodynamic surfaces moves from approximately the quarter chord point (c/4) to the half chord (c/2) point.
c/4Subsonic
c/2 Supersonic
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Mach Effects
NACA 0012 – 3 degrees AOA
Xac
Trailing Edge Shock
Detached Bow Shock
M=1.6
Mach
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Ways to Increase MCRIT
In order to fly faster before shock waves form, we need to increase MCRIT. This requires that the flow accelerate less over the aircraft.
The drawback of these methods is that less lift is generated, so high Mach aircraft must have effective high lift devices.
Thin wings (F-16) Less camber (T-38) Swept wings (F-15) Sharp, slender leading edges and fuselage (F-104)
Supercritical airfoil (C-17)
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Wing Sweep
Anderson, J. D., Introduction to Flight, 4th Edition, page 300
Wing sweep increases the critical Mach number by decreasing the velocity component the airfoil “sees”
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Supercritical Wings
Anderson, J. D., Introduction to Flight, 4th Edition, page 274
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Stretching MCRIT
Boeing 707 – Mach .80 727 – Mach .82 767 – Mach .80 747 – Mach .85 777 – Mach .84 Sonic Cruiser – Mach .95-.98
Airbus A320 – Mach .82 A380 – Mach .89
Boeing Sonic Cruiser
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Minimizing Wave Drag Blended wing-body
Eliminates sharp corners which cause shock waves to form Area-ruled (coke bottle) fuselage
Smooth transition of aircraft cross-sectional area (T-38) Tailplane offset above or below wing
Gets tailplane out of separated, turbulent wake from shock and reduces vibration (F-104)
Sharp, slender wing leading edges and fuselage Causes oblique shocks to form, which produce less wave
drag than bow shocks (F-106, F-104 etc) Variable geometry wings (F-111, B-1)
Wings swept increases MCRIT and allows aircraft to fly faster Wings unswept increases Aspect Ratio and improves lift
characteristics for slower flight conditions such as takeoff and landing
Penalties in weight, cost, and reliability
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Blended wing-body
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Area Rule
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Tailplane Offset & Sharp, Slender Shapes
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Variable Geometry Wings
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Next Lesson (21)… Prior to class
Review 3.5.1 – 3.5.8, 3.5.10, and 4.1- 4.6 Review/rework problems 16 - 25
In Class Review for GR#2