Lecture 10 Flashcards by Morgan A.

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

How well did you know this?

Not at all

Perfectly

What are the two primary variables related to the operation of a fluid power system?

Pressure
Volume

How well did you know this?

Not at all

Perfectly

As airflow approaches sonic speeds, air becomes a _____.

compressible fluid

How well did you know this?

Not at all

Perfectly

As air is compressed, what is released from the air?

Water

How well did you know this?

Not at all

Perfectly

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

How well did you know this?

Not at all

Perfectly

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

How well did you know this?

Not at all

Perfectly

What is the speed of sound on a normal day?

760 mph

How well did you know this?

Not at all

Perfectly

What is the speed of sound at a higher altitude?

Why?

Lower than 760 mph

Cold molecules move slower

How well did you know this?

Not at all

Perfectly

At speeds in excess of Mach 1, when do you hear the aircraft?

Long after the aircraft has passed

How well did you know this?

Not at all

Perfectly

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

How well did you know this?

Not at all

Perfectly

When does a sonic boom happen?

As an aircraft crosses the speed of sound

How well did you know this?

Not at all

Perfectly

How do you calculate Mach number?

True speed of the aircraft / speed of sound

How well did you know this?

Not at all

Perfectly

What is a Mach number?

The speed of the aircraft in relation to the speed of sound

How well did you know this?

Not at all

Perfectly

What is subsonic?

Any region below the transonic region or below the critical Mach number

How well did you know this?

Not at all

Perfectly

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

How well did you know this?

Not at all

Perfectly

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