AVIT 421 Block 1 Jason Evans Spring 2024

What are standard atmospheric conditions?

14.7 psi, 15 degrees and 2 C per 1000 ft

Atmospheric pressure, temperature, and air density ______ with altitude increase

Decrease

When airflow is accelerated, the ____ pressure decreases

Static

When airflow is accelerated, the dynamic pressure _______

Increases

When subsonic flow is accelerated, total pressure _________

Stays the same (conservation of energy)

How does a subsonic airfoil produce lift?

The airflow over the top of the wing is faster than the bottom which results in a decrease in static pressure on the top of the wing. Also, airfoils deflect the air mass downwards resulting in an opposite reaction force.

What are the two main theories that describe how a subsonic airfoil produces lift?

Bernoulli's principle and Newton's Third Law

What is the primary cause of form drag?

Pressure differential between the front and back of an object, caused by flow seperation

What profile would have the least coefficient of drag assume that they all have the same frontal area?

Thin airfoil

What is a limitation to laminar flow?

Surface must be kept clean and free of blemishes

What happens to parasite drag of an object if speed is doubled?

Drag is increase by a factor of 4

What decreases induced drag?

Faster airspeed, better spanwise efficiency factor, higher aspect ratio, increases in air density

What decreases parasite drag?

Altitude increase, temperature increase, pressure decreases, slower airspeed

Doubling weight has which impact on induced drag?

Increases by a factor of 4

What is the most efficient airfoil?

Elliptical

How does ground effect reduce induced drag?

Reduction in downwash angle leads to a reduction in induced flow resulting in less induced drag

Flying an aircraft into ground effect with a constant angle of attack results in:

Lift coefficient increase, thrust requirement decrease

What three factors have the greatest impact on TOTAL drag (in addition to velocity)

Weight, Aircraft Configuration, Altitude (air density)

Re: the lift equation, tripling your velocity will increase your lift by a factor of

9

What is an aspect ratio?

Span divided by chord

What is an advantage of designing an aircraft with long slender wings?

Decreased induced drag compared to aircraft of the same weight with shorter wings

How does a stall progress on a swept-back wing (like an airliner)

Wing tip moving aft to front

How fast do pressure waves travel?

At the speed of sound

What is the main factor in determining the speed of sound?

Temperature of the air

The _____ the temperature, the speed of sound gets ______

higher, faster

What is a mach number?

The ratio of an airplane's TAS to the local speed of sound

It is generally advisable to fly at speeds ______ L/D Max

Higher than

For any wing section, Mcrit will always be _____ than Mach 1

Less

What is the transonic region?

0.75 M to 1.2 M

Why do transport category aircraft fly using/referencing Mach at altitude?

At higher TAS, relationship to maximum Mach is more important due to compressibility affects above Mcrit

As an airfoil accelerates above Mcrit, a shock wave forms on the ______ surface. As the air accelerates even more, a second shock wave appears on the ______ of the wing.

Upper, bottom

What region of speed provides unstable aerodynamic conditions?

Transonic

As an aircraft accelerates above Mcrit, the shockwaves....

Move back

What type of shockwave has supersonic flow on both sides of it?

Oblique

Airflow behind a _______ shockwave has subsonic air flow

Normal

What happens as air that passes through a normal shockwave?

Flow doesn't change direction, velocity decreases to subsonic, static pressure, density, and temperature increase, total energy decreases

Why is a bow wave sometimes termed a "detached" shockwave?

The forward stagnation region causes the bow wave to separate from the leading edge

What determines the size of an airplane's Mach Cone?

Speed and shape of the object

What is Whitcomb's area rule?

Reducing parasite drag by controlling the effective cross-sectional area of an entire aircraft

How are supersonic engine inlets designed?

Depending on design, they use diffusers to create oblique and/or normal shock waves to slow the airflow prior to reaching the engine compressor

Which of the following in NOT an incorrect theory of lift production by an aircraft’s wing in subsonic flight?

Newtons 3rd law

For the Magnus Effect to explain lift what must happen

Ball must be rotated to created localized differences in velocity to deflect flow in the opposite direction of lift

According to the lift equation, tripling your velocity will increase your lift by?

9

Aspect Ratio

Span divided by average chord

What is the primary cause of form drag?

Pressure differential between the front and back of an object, caused by flow separation

Which profile would you expect to have to lowest (least) Coefficient of Drag, assuming all object have the same frontal area?

Thin Airfoil

What is a limitation to a laminar flow airfoil?

The surface must be kept clean and free of blemishes

What happens to the parasite drag of an object when the speed is doubled?

Drag is increased by a factor of 4

Which variable has the largest impact in increasing parasite drag?

Velocity

How can we reduce induced drag?

Fly faster

Doubling weight has which impact on Induced Drag?

Increases by a factor of 4

Due to wing tip vortices, a finite wing will produce _________ lift when compared to a theoretical infinite wing (no wing tip vortices) when both are flying at the same angle of attack.

Less

When considering the wing’s span efficiency factor (e), which type of wing design is the most efficient?

Elliptical

How does ground effect reduce drag?

Reduction in downwash angle leads to a reduction in induced flow, resulting in less induced drag

Flying an aircraft into ground effect with a constant angle of attack results in:

Lift coefficient increases, thrust required decreases

Increasing airspeed always increases Total Drag. (T/F)

False

In addition to velocity, which three elements have the greatest impact on TOTAL drag as discussed in the lecture?

Weight, configuration, altitude (air density)

The following are disadvantages of designing aircraft with long, slender wings EXCEPT?

Increased induced drag compared to an aircraft of the same weight with shorter wings

Extending flaps has what effect on an aircraft?

Changes the camber line and Increases the Coefficient of Lift value for the same angle of attack

What happens to the lift when an airfoil is stalled?

Lift it decreased (but not to 0)

A stall only occurs because an aircraft decreases airspeed causing the wing to exceed critical angle of attack. (T/F)

False (all dependent on AoA)

Critical angle of attack is a function of the ____.

Design of the airfoil

For sweptback wings, like those on most regional airliners, how does a stall progress?

From the wing tip moving aft to front

Pressure waves travel outward _________

At the speed of sound

With regard to L/D MAX it is generally advisable to fly at speeds:

Above L/D max

For any wing section, Mcrit will always be less than Mach 1.

True

The Transonic regime is approximated by the approximate speed range:

.75 to 1.2

Why do transport category aircraft fly using/referencing MACH at altitude?

At higher TAS, the relationship to maximum MACH is more important due to compressibility effects above MCRIT

The ___________ speed range results in aerodynamic conditions that are particularly unstable.

Transonic

As an airfoil accelerates above Mcrit, a shock wave forms on the ______ surface. As the air accelerates even more, a second shock wave appears on the ______ of the wing.

Upper, bottom

As the aircraft accelerates above Mcrit, both shock waves _____________.

Start to move back

An _____shockwave usually has supersonic flow on both sides of it.

Oblique

The airflow velocity just behind a normal shock wave is always subsonic.

True

What happens to the air that passes through a normal shock wave?

Flow doesn’t change direction; velocity decreases to subsonic; static pressure, density and temperature increase; total energy decreases

Parasite “wave” drag coefficient actually decreases once the aircraft velocity exceeds Mach 1, then remains steady until about Mach 5. (T/F)

True

Airflow through expansion waves exhibits what characteristics?

Speed increases due to bending around a corner, decrease in pressure and density, no change in total energy (no shock)

Oblique shock waves can be _______.

Attached or detacted

Which compressibility affect(s) appear(s) during low speed, sub-sonic flight?

None!

Increasing your bank angle will ____________ your MCRIT speed.

Decrease

Decreasing your gross weight will ____________ your MCRIT speed.

Increase

How can the aerospace community avoid MACH tuck?

Don’t exceed maximum aircraft MACH speeds and Design aircraft with trimmable horizontal stabilizers

What impact did the development of supercritical airfoils have on high speed flight?

Moved shockwave formation aft on the wing and Weakened shock wave

Supercritical airfoils are/were used in which types of aircraft?

Military, Airliners, Business jets, Space Shuttle

What do vortex generators do

Reenergize the boundary layer air through induced drag, ultimately delaying flow separation

Swept wings are used because

They increase the MACH Critical Airspeed because the wing only “sees” an angle of the airstream velocity vector

Why is a bow wave sometimes termed a “Detached” shockwave?

The forward stagnation region causes the bow wave to separate from the leading edge.

What determines the size of an airplane’s Mach Cone?

Speed and shape of the object

Refer to the following illustration (shown below). This double wedge airfoil has a small positive angle of attack. Identify the location of the strongest oblique shockwave.

Option B

Richard Whitcomb discovered the aerodynamic concept of “area rule”. Area rule pertains to ___________________________________.

Reducing parasite drag by controlling the effective cross-sectional area of an entire aircraft.

Refer to the following illustration (below) of an airfoil optimized for supersonic flight. This airfoil is experiencing supersonic flight. Which vector below correctly identifies the Lift vector (without the drag from lift component included)?

Option C

What is an additional benefit of highly swept wings in supersonic flight?

Wing and tail can be “shielded” from high-speed Mach by attached oblique shock waves, wings/tail effectively “see lower” speeds, and allows the use of supercritical airfoils

For supersonic flight, swept wings always produce a lower wing coefficient of drag than straight wings. (T/F)

False

How are supersonic engine inlets designed?

Depending on design, they use diffusers to create oblique and/or normal shock waves to slow the airflow prior to reaching the engine compressor