Lecture 10 Flashcards
To create lift, Bernoulli’s Principle assumes that air is _____.*
Bernoulli’s principle does not apply at _____, so you must understand _____.
not compressible
supersonic speeds
compressible aerodynamics
What are the two primary variables related to the operation of a fluid power system?
Pressure
Volume
As airflow approaches sonic speeds, air becomes a _____.
compressible fluid
As air is compressed, what is released from the air?
Water
Why are pressure waves visible when an aircraft reaches the speed of sound?
Air gets compressed to the point where all water is squeezed out
Moisture becomes visible
Why does air get compressed at supersonic speeds?
The air is moving as fast as it can, and it can not get out of the way of the aircraft, so the air gets compressed
What is the speed of sound on a normal day?
760 mph
What is the speed of sound at a higher altitude?
Why?
Lower than 760 mph
Cold molecules move slower
At speeds in excess of Mach 1, when do you hear the aircraft?
Long after the aircraft has passed
What is a sonic boom?
What is it caused by?*
A loud sound kind of like an explosion
Sounds made from pressure waves or shock waves from supersonic objects
Shock waves created by any object that travels through air faster than the speed of sound
When does a sonic boom happen?
As an aircraft crosses the speed of sound
How do you calculate Mach number?
True speed of the aircraft / speed of sound
What is a Mach number?
The speed of the aircraft in relation to the speed of sound
What is subsonic?
Any region below the transonic region or below the critical Mach number
What is transonic?
Begins with the first evidence of sonic or supersonic flow anywhere on the aircraft
Ends when all flows around the aircraft are supersonic
Creates the most drag on a high speed aircraft
What is sonic?
Right at the speed of sound
What is supersonic?
Any speed faster than the speed of sound
What is hypersonic?
Any speed above Mach 5
Plasma effects begin to affect airflows
What are the five sonic regions?
Subsonic
Transonic
Sonic
Supersonic
Hypersonic
_____ causes the speed of the air on the aircraft to increase.
Camber (increases the velocity)
What is the critical Mach number?
The speed the aircraft is flying at when some air flowing over the aircraft is supersonic and some is subsonic (transonic region)
Drag _____ quickly as the aircraft hits the critical Mach number, and then _____ quickly as the aircraft becomes fully supersonic, and then the drag begins to _____ again.
rises
drops
rise
What is super cruise?
A supersonic cruising speed that has the same drag as when the aircraft was still flying subsonic
What is a shock wave?
Airplane catches up to its own pressure waves creating a shock wave
The air no longer has enough energy to get out of the way of the aircraft, the air molecules get shoved together and compressed
The air compresses, builds up and sticks to the wing
What are the two types of shock waves?
Normal
Oblique
List some characteristics of normal shock waves
Perpendicular to the flow direction
Airflow slows from supersonic to subsonic
Airflow direction through it does not change
Static air pressure, temp and density increase behind it
Absorbs energy from the air and causes tremendous drag
List some characteristics of oblique shock waves
Inclined to the flow direction
Airflow slows going across it from high to low supersonic speeds
Airflow direction changes going across it
Static air pressure, temp and density increase as the air passes across it
Consumes some of the energy from the air and creates some drag
How is lift made at supersonic speed?
Lift is generated only by the angle of attack instead of the camber
Lift is generated by the pressure differences
Pressure difference is generated by shocks at the leading edge
Causes a greater pressure under the airfoil and therefore creates lift
What is a supercritical wing?
What is beneficial about this design?
Long flat upper camber and a downward turn near the trailing edge
The long flat upper camber keeps the airspeeds across the wing lower
The lowest pressure forces are also concentrated around the highest camber
Pressure forces do not fall as far
Smaller pressure differentials are spread out across a bigger area so the total lift produced is the same
_____ are weaker at high altitudes since there are _____ molecules.
Shock waves
fewer air (air is less dense)
What on an aircraft can produce large shockwaves?*
Large and sudden changes in cross-sectional areas
How are large shockwaves prevented on supersonic aircraft?
Larger overall cross-sectional area, but a smaller wing aspect ratio to create a smoother transition from wings to fuselage cross-section
Airliner would have large shockwaves
Fighters, like F-22, are designed with the above features to prevent large shockwaves
Who was the Whitcomb area rule named after?
NACA engineer Richard Whitcomb
What is the Whitcomb area rule?
What does it do?
A design procedure used to reduce an aircraft’s drag at transonic speeds
Looks at cross-sections of an aircraft to help smooth out area transitions
Why is wing sweep used at subsonic speeds?
High subsonic speeds?
Fully supersonic speeds?
Subsonic: yaw stability
High subsonic: raise the Critical Mach Number
Fully supersonic: keep the wing tips out of the shock wave
What can happen if a shock wave comes in contact with the wing tips?
Airflow over the wing tips and ailerons can be disrupted
Why can highly swept wings be bad?
Very poor low speed take off and landing performance
Tremendous weight penalty
