High Speed Aerodynamics

Compressibility Effects
Importance of the Speed of Sound
Realms of Flight
Supersonic Flow Patterns
V. High Speed Aerodynamics
High Speed Airfoils
Critical Mach Number
Aerodynamic Heating

V. High Speed Aerodynamics
Realms of Flight
High Speed Airfoils
Aerodynamic Heating

High Speed Aerodynamics
As WWII progressed, aircraft flew faster and faster. By mid-war, P51s, Spitfires, and other types were reaching speeds close to that of sound, especially in dives. Pilots began to report control difficulties and unexpected problems which experts determined were due to flying too close to the speed of sound.
Realms of Flight
High Speed Airfoils
Aerodynamic Heating
History and Introduction

In 1940, NACA commissioned the Bell Aircraft Company to build a special research aircraft for the purpose of exploring the speed range near and beyond the speed of sound. It was considered better to do the research using an actual aircraft because the USA had no wind tunnels capable of operating at supersonic speed. Of course, the Germans did.
Realms of Flight
High Speed Airfoils
Aerodynamic Heating

The research aircraft Bell built was designated the X1. Two operational aircraft were finally built, although they did not fly until the war was over.
Realms of Flight
High Speed Airfoils
Aerodynamic Heating

Realms of Flight
High Speed Airfoils
Aerodynamic Heating

By the time the war was over, the German research data had been captured and several of the questions the X1 was intended to answer were already known. Nevertheless, the X1 became the first aircraft to exceed the speed of sound in October 1947 when Chuck Yeager flew it to Mach 1.1 .
Realms of Flight
High Speed Airfoils
Aerodynamic Heating

Realms of Flight
High Speed Airfoils
Aerodynamic Heating
History and Introduction X1 movie

At subsonic speed (less than Mach 1) air acts as if it’s an incompressible fluid
Can change it’s velocity and pressure but not density
As long as it can still move, the space between molecules will effectively not change = density remains constant
Realms of Flight
High Speed Airfoils
Aerodynamic Heating

Realms of Flight
High Speed Airfoils
Aerodynamic Heating
Bernoulli’s Principle:
Subsonic:
In CONVERGING portion of venturi (Inlet)
Air must travel faster to get to other side at same time as bypass air
Velocity goes up
Pressure goes down

Subsonic:
In DIVERGING portion of venturi (Exit)
Pressure and Velocity return to original
Based on Total Energy being constant throughout
Pressure = Potential Energy
Velocity = Kinetic Energy
Realms of Flight
High Speed Airfoils
Aerodynamic Heating

At supersonic speed (more than Mach 1) air can be compressed and it’s density increased
Space between molecules can and is reduced
Realms of Flight
High Speed Airfoils
Aerodynamic Heating
Compressibility Effects Link to venturi simulation

Realms of Flight
High Speed Airfoils
Aerodynamic Heating
Compressibility Effects Link to venturi simulation
Opposite effects as compared to subsonic
Converging: Pressure goes up and Velocity goes down
Diverging: Pressure down, Velocity up

Sound is pressure disturbances radiating in all directions from the source
Travels @ 661.7 knots (1,116.9 fps, 761.5 mph) under Standard Conditions
Speed is directly proportional to temperature (goes down as temp. goes down)
Can be calculated (C S = Local Speed of Sound):
C S in kts = 29.04  o R
C S in fps = 49.022  o R
C S in mph = 33.42  o R
Realms of Flight
High Speed Airfoils
Aerodynamic Heating
Importance of the Speed of Sound Note: o R = o F + 460

Note that at 36,089 feet the temperature stabilizes at –69.7 and the Speed of Sound also stabilizes at 573.8 kts
Also note that above 80,000 feet the temp starts to rise and the Speed of Sound also rises
Realms of Flight
High Speed Airfoils
Aerodynamic Heating

In subsonic flight
Sound waves radiate from all points of airframe
Can go in all directions as are faster than aircraft
Realms of Flight
High Speed Airfoils
Aerodynamic Heating

In supersonic flight
Airplane faster so sound waves “pile up” at nose of airfoil
Creates Shock Wave(s) = changes in pressure and velocity of airflow
Realms of Flight
High Speed Airfoils
Aerodynamic Heating
Importance of the Speed of Sound Pressure Waves Link (Hide Shockwaves)

Mach # = ratio of True Airspeed of an aircraft to the Speed of Sound at that temperature
Subsonic:
.75 Mach or less
All airflow over aircraft is < Mach 1
Realms of Flight
High Speed Airfoils
Aerodynamic Heating
Realms of Flight

Transonic:
.75 to 1.2 Mach
Mixture of sonic airflow speeds
Some < Mach 1, some >= Mach 1
Aerodynamic center of lift moves to about 50% of chord
Shock waves form and move around = large changes in trim & stability (control buffeting)
Realms of Flight
High Speed Airfoils
Aerodynamic Heating
Realms of Flight

Supersonic:
1.2 to 5.0 Mach
All airflow is > Mach 1
Smooth & efficient flight
Realms of Flight
High Speed Airfoils
Aerodynamic Heating
Realms of Flight

Realms of Flight
High Speed Airfoils
Aerodynamic Heating
Realms of Flight
Hypersonic:
> 5.0 Mach

Realms of Flight
High Speed Airfoils
Aerodynamic Heating
Realms of Flight
Hypersonic:
> 5.0 Mach
Leads to Aerodynamic Heating & other flight problems

Shock waves form when the aircraft (or parts of it) are exceeding Mach 1 in speed
Sound waves slower than aircraft speed
Air “piles up” in Compression Wave = large changes in pressure, velocity, and density of airflow
Realms of Flight
High Speed Airfoils
Aerodynamic Heating
Supersonic Flow Patterns (Shock Waves)

Realms of Flight
High Speed Airfoils
Aerodynamic Heating
Supersonic Flow Patterns (Shock Waves) F18 flyby movie F14 flyby movie

3 kinds of Shock Waves:
Oblique
Normal
Expansion
Realms of Flight
High Speed Airfoils
Aerodynamic Heating
Supersonic Flow Patterns (Shock Waves) Pressure Waves Link Show Shockwaves

Oblique Shock Waves
Realms of Flight
High Speed Airfoils
Aerodynamic Heating
Forms on sharp edges with surface changing into the direction of the airflow
commonly on Leading and Trailing Edges of airfoil
Shock wave forms Oblique Angle with surface and “points” downstream in airflow
Angle gets less as speed goes up
“ flattens out”

Oblique Shock Waves
Realms of Flight
High Speed Airfoils
Aerodynamic Heating
@ wave:
Flow direction is deflected along surface
Velocity decreases but STAYS ABOVE MACH 1
Pressure increases
Temperature increases
Density increases
Total energy goes down
Energy loss comes from loss of temperature (Heat of Compression) to atmosphere

Normal Shock Waves
Realms of Flight
High Speed Airfoils
Aerodynamic Heating
Forms in front of or on blunt object or one with large included angle

Normal Shock Waves
Realms of Flight
High Speed Airfoils
Aerodynamic Heating
In front:
Molecules pile up and create wave detached and in front of the object
= “Bow Wave”

Normal Shock Waves
Realms of Flight
High Speed Airfoils
Aerodynamic Heating
On:
Attaches to curved portion where airflow speed is just above Mach 1
In Transonic range = form on upper airfoil surface first (at about .75 Mach) then lower surface (at about .85 Mach)
NOTE: the indicated Mach numbers vary depending on the airfoil

Normal Shock Waves
@ wave:
Flow direction doesn’t change
Velocity decreases TO BELOW MACH 1
Subsonic speed inversely proportional to Mach # entering wave (higher C S going in = lower subsonic speed coming out)
Large Pressure, Density, and Temperature increase
Large Total Energy decrease (from temp. radiation into air)

Expansion Shock Waves
Realms of Flight
High Speed Airfoils
Aerodynamic Heating
Form where surface turns away from airflow direction
Middle of airfoil
Velocity increases
Pressure and Density both decrease
Temperature remains relatively constant = little or no Total Energy change (is not a compression wave)

Subsonic Airfoils
Realms of Flight
High Speed Airfoils
Aerodynamic Heating
High Speed Airfoils and Critical Mach Number
Below about .75 Mach = all flow is subsonic

Subsonic Airfoils
Realms of Flight
High Speed Airfoils
Aerodynamic Heating
In Transonic speed range:
At some airspeed ( = Critical Mach Number or M CR ) some airflow will reach or exceed Mach 1

Subsonic Airfoils
Realms of Flight
High Speed Airfoils
Aerodynamic Heating
Normal shock wave forms on upper surface where max airfoil thickness is ( = greatest velocity)
Causes large increase in drag

Subsonic Airfoils
Realms of Flight
High Speed Airfoils
Aerodynamic Heating
Pressure rise through shock wave reduces strength of low pressure area on wing
Low pressure usually “sucks” air against airfoil for smooth flow
Loss of low pressure = air isn’t sucked down smoothly and SHOCK INDUCED SEPARATION occurs on upper surface
= loss of lift and reduced control effectiveness from turbulence

Subsonic Airfoils
Realms of Flight
High Speed Airfoils
Aerodynamic Heating
As speed increases closer to Mach 1:
Normal wave forms on lower surface and upper wave moves further back towards trailing edge
= shock induced separation on bottom, too

Subsonic Airfoils
Realms of Flight
High Speed Airfoils
Aerodynamic Heating
At Mach 1:
Top and bottom normal waves have moved to trailing edge and form oblique waves there
Entire surfaces have supersonic flow = smooth flow
Leading edge now has normal wave forming in front of it
Known as BOW WAVE
Like wave at bow of boat

Subsonic Airfoils
Realms of Flight
High Speed Airfoils
Aerodynamic Heating
At Mach 1:
Area behind bow wave is relatively slow and called STAGNATION AREA
Large energy loss through normal wave creates significant drag
Shockwave Link Normal and Oblique Shockwaves

Subsonic Airfoils
Realms of Flight
High Speed Airfoils
Aerodynamic Heating
Above Mach 1:
Stagnation area gets smaller but energy loss increases
Greater change in velocity and pressure through normal wave
= greater drag (proportional to speed)

Subsonic Airfoils
To increase Critical Mach Number (or delay Shock Induced Separation occurrence):
Can use Vortex Generators or Swept Wings

Subsonic Airfoils
Realms of Flight
High Speed Airfoils
Aerodynamic Heating
Vortex Generators
Small, low Aspect Ratio airfoils mounted in pairs at 90 o on wing surfaces
Produces strong vortex around end = pulls air back onto wing surface
= delays airflow separation
Pair mounting = pairs work together for greater effectiveness

Subsonic Airfoils
Realms of Flight
High Speed Airfoils
Aerodynamic Heating
Swept Wings
Swept wings change the THICKNESS RATIO of the wing (also known as Fineness Ratio)
Ratio of max. thickness of airfoil to chord
Lower Thickness Ratio = thinner wing = less velocity change across wing = higher M CR

Subsonic Airfoils
Realms of Flight
High Speed Airfoils
Aerodynamic Heating
Swept Wings
Unfortunately, this also causes more flow of air in a spanwise direction = stronger wingtip vortices = increased drag overall
Also, aft swept wings generally cause the tip to stall first (= potentially loose aileron control)

Subsonic Airfoils
Realms of Flight
High Speed Airfoils
Aerodynamic Heating
Swept Wings
Forward swept wings (I.e. Grumman X-29) have same M CR advantages as aft swept + roots stall first
= can do more extreme high angle of attack maneuvers without losing control
But needs computer to control as is inherently very unstable

Subsonic Airfoils
Realms of Flight
High Speed Airfoils
Aerodynamic Heating
Swept Wings
Also, at higher speeds the wing will twist due to aerodynamic loading and will tend to wash in
Increase angle of incidence = increased lift and drag = increased loads to failure
Composite materials make wings light and stiff enough to withstand these forces

Subsonic Airfoils
Realms of Flight
High Speed Airfoils
Aerodynamic Heating
Movable Hor. Stab.
Horizontal Stablizer will also tend to loose effectiveness at Critical Mach Number
Can keep it effective by making it’s Angle of Incidence adjustable
F86 used this to allow supersonic speeds while maintaining control
Was first one to do so

Supersonic Airfoils
Realms of Flight
High Speed Airfoils
Aerodynamic Heating
To eliminate Shock Induced Separation in Transonic range = airfoils with max. thickness @ 50% of chord
Double Wedge or Biconvex

Supersonic Airfoils
Realms of Flight
High Speed Airfoils
Aerodynamic Heating
Thin airfoil = no normal shock forms
Sharp edges form obliques with tiny bow wave and expansion wave in middle

Supersonic Airfoils
Realms of Flight
High Speed Airfoils
Aerodynamic Heating
All lead to practically zero subsonic airflow (= zero turbulence and drag from energy change)
Problem: not good subsonic characteristics

Supersonic Aircraft Configuration
Realms of Flight
High Speed Airfoils
Aerodynamic Heating
Wing Planform
Low Aspect Ratio & tapered
Good strength and weight
Sweptback
For transonic and low supersonic
Straight
For high supersonic (Mach 2 +)

Realms of Flight
High Speed Airfoils
Aerodynamic Heating
Airfoil
Low Thickness (Fineness) Ratio
Gradual change in thickness (airfoil shape)
Max. thickness @ 50% of chord
Sharp leading and trailing edges

Realms of Flight
High Speed Airfoils
Aerodynamic Heating
Fuselage & Nacelles
Long
Slender
Fuselage “Wasp Waisted”
Decreases overall Form and Interference Drags from interaction of shock waves on various structure areas

Realms of Flight
High Speed Airfoils
Aerodynamic Heating
Tail Surfaces
Planform and airfoil similar to wings
Generally all movable horizontal surfaces
Powered irreversible systems

Realms of Flight
High Speed Airfoils
Aerodynamic Heating
Aerodynamic Heating
As speeds increase above Mach 1, the Stagnation Area sees great temperature rises
Faster = greater temp rise
Less affect at higher altitudes

Realms of Flight
High Speed Airfoils
Aerodynamic Heating
Aerodynamic Heating
Increased temps = decreased metal strengths
As we near Hypersonic speeds the temps are so high that normal materials will melt
Research today is in this area with ceramics and composite materials

Realms of Flight
High Speed Airfoils
Aerodynamic Heating
Future?
Lockheed is working on this design slated to fly by 2010
Supposed to disperse the shock waves to allow supersonic flight over populated areas

Realms of Flight
High Speed Airfoils
Aerodynamic Heating
Future?
Also, right now
“Quiet Spike”

Realms of Flight
High Speed Airfoils
Aerodynamic Heating
Future? Flew at Mach 9.6 Last year X43 Overview X43 flights

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High Speed Aerodynamics

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