Exam 1 Flashcards
Describe the shape/thickness of LOW SPEED airfoils
- Leading edge is thick, rounded and blunt
- Trailing edge is thin and sharp
- Greater camber on top of wing
The Area of Influence refers to:
The area AHEAD of, around and behind the airfoil where pressure waves affect airflow
As air pressure decreases, the temperature and density ____
decrease
How is static pressure applied to the airfoil?
Static pressure vectors will always act PERPENDICULARLY to the solid (airfoil)
When airflow is accelerated, STATIC pressure ______
decreases
When airflow is accelerated, DYNAMIC pressure ______
increases
The lift vector is always perpendicular to:
The CAMBER of the wing
What happens to drag if the speed is doubled?
Drag increases by the square of speed
If the surface area is cut in half, what happens to the drag?
Drag decreases in half, as well.
What causes wing vortex drag?
Lift/Drag
What is the actual measurement of a super sonic shock wave?
1/10,000th of an inch
Drag is always _____ to RW, and Lift is always _____
Drag is Parallel
Lift is Perpendicular
Where is lift anchored along the chord of a SUBsonic airfoil?
25% chord
Total Lift is always ______ to the RW/Flight Path
Perpendicular
The stagnation point is a region of high ____ pressure
STATIC
What is the Adverse Pressure Gradient
- Where air molecules begin to flow from Low to High pressure (vs the natural way H to L)
- 1/2mV^2 gives them enough velocity to bust through the higher pressure at the Adverse Pressure Gradient
The difference in chambers on the top and bottom of an airfoil causes what phenomenon?
- Air molecules going over the top of the wing will travel faster, creating lower static pressure
- Air molecules traveling under the wing will go slower, creating higher static pressure
The speed at which 1 air molecule will go supersonic (meaning a/c is entering transonic flight) is known as:
Mcrit (the critical mach number)
Does temperature affect the speed of sound?
Yes, as temperature increases the speed of sound increases.
True or False: Sound waves are Pressure Waves.
TRUE
What is the number one thing that affects the speed that sound waves travel?
TEMPERATURE
The ___ Mach is read on the dial in the cockpit and used for Mcrit
Local Mach (vs Flight Mach)
What is a shock wave?
- A sharp change in pressure caused by an object moving faster than the speed of sound
What is a bow wave?
The bunching up of sub-sonic air out in front of the wing!
In regards to a shock wave, when supersonic air pushes through to subsonic, the ____ and ____ _____ INCREASE
Temperature and STATIC pressure increase
What is Transonic flight? Give an example of a Transonic speed.
- When there is a mix of subsonic and supersonic airflow acting on the airfoil
- Mcrit (~M1.3)
True or False: You will encounter Compressability issues below Mcrit.
FALSE
- Compressability issues will only occur in Transonic flight (speeds at or above Mcrit)
True or False: Airflow behind/past the Bow Wave is ALWAYS subsonic
TRUE (think of picture)
What are the Mach ranges for Sub, Trans, Super and Hypersonic flight?
Subsonic: 0 - Mcrit (~0.75)
Transonic: Mcrit - 1.2
Supersonic: 1.2 - 5.0 (low super 1.2-3.0)
Hypersonic: 5.0+
In subsonic flight, what are the negative effects of compressability?
None, compressabilty does not exist at SUBsonic speeds.
What is the reason Supersonic flight is divided into Low-Super and High-Supersonic?
Because at Mach 3.0 THERMODYNAMICS come into play
True or False: The speed of sound is constant.
FALSE
- Temperature causes the speed of sound to vary
____ is Parallel to the Relative Wind
Drag
____ distribution changes with AOA, and is always Perpendicular to the Relative Wind
Lift
For subsonic airfoils, the Center of Pressure is located at ____ chord and the origin of the vector (aerodynamic force) remains constant there
25 percent chord
What is Bernoulli’s part in airflow? (Over a wing)
The velocity distribution over the wing (faster->slower) and beneath the wing (slowest) creates a downward deflection of airflow
What is Newton’s part in airflow? (Over a wing)
- Equal and opposite reactions law
- When upwash (positive) is out ahead of wing, then transitions to downwash, slicing down through the airfoil around 25% chord and ending beneath/behind (negative)
- Downward deflection of airflow = Upward deflection of the airFOIL
What kind of airflow is incompressible and the density does not change?
Subsonic Air
Dynamic pressure + Static Pressure = ________
Total Pressure
Total Energy is constant but will change forms. The most common change will be from _____ Energy to _____ Energy during shockwave formations
Kinetic Energy -> THERMAL Energy
Explain the incorrect “Fred and Barney” lift theory.
The distance traveled across the upper chamber is greater, therefore air on top is moving faster in order to “meet back up” with air traveling across the bottom of the wing
Explain the incorrect “Venturi” lift theory.
The airfoil and airstream act as a Venturi that CONSTRICTS the flow ABOVE the wing…but not the airflow below
List the ranges for the different sonic speeds
Subsonic: 0 - Mcrit (.75) Transonic: Mcrit (.75) - 1.2 Supersonic Low: 1.2 - 3.0 Supersonic High: 3.0 - 5.0 Hypersonic: Faster than Mach 5.0
As temperature increases, the speed of sound:
Increases!
As air flows across a shockwave, what happens to the temperature, static pressure and velocity of the air?
- Temperature increases
- Static Pressure increases
- Velocity decreases
True or False: Airflow in-front of a Normal Shockwave is ALWAYS supersonic
TRUE
True or False: Airflow behind a Normal Shockwave is ALWAYS subsonic
TRUE
What are the compressibility affects in Transonic flight? (5)
- They are all Negative
1. Loss of trim/rudder effectiveness
2. Mach Tuck
3. Control Reversal
4. Buffeting/buzz
5. Aeroelastic Twisting
The Adverse Pressure Gradient is an area of high _____ Pressure
Static pressure
How do we get air molecules to flow from Low to High Static Pressure?
MOMENTUM
Expansion waves ______ airflow
Increase
Why is airflow affected way out in front of the airfoil? Is it affected in front at all speeds?
- Pressure Waves (in subsonic flight)
- In Transonic flight shockwaves will cut of Pressure Waves when formed on the airfoil
Mcrit varies inversely with _____ and ______
- Gross Weight
- Angle of Bank
Is it better to have a Low or High Mcrit?
Increased Mcrit = good
Decreased Mcrit = bad
What are the 3 kinds of shock waves?
- Normal
- Expansion
- Oblique “attached” and “detached” to leading edge due to sharp or blunt shape
Name the 3 main characteristics of Normal Shock-waves?
- Perpendicular to airflow
- Flow direction does not change
- Airflow behind is ALWAYS subsonic
Draw .95 mach airfoil and the airflow over it.
Draw it!
- Top shock-wave cannot attach to wing because at .95 the disturbance is so strong
For academic purposes, at .95 Mach (high transonic) is the shock-wave attached or detached from the wing? Why?
Detached due to the disturbed airflow
At .82 Mach is the shock-wave attached or detached from the wing?
Attached
At Mach 1.0 where is the disturbed airflow located? Why?
- Back behind the wing
- Oblique shock-waves have made it all the way past the airfoil
Bow wave is approx. 90 degrees with subsonic airflow behind it at ____ Mach
1.02
Pure supersonic flow is achieved at ____ Mach
1.3
How can you avoid the negative affects of a forward stagnation region/point?
Have a sharp/pointed leading edge (vs a blunt/rounded leading edge)
Is going from Subsonic to Supersonic a big deal?
No, it is a smooth transition
Is going from Supersonic to Subsonic a big deal? Why?
Yes, there are LARGE changes is pressure, temperature and density across the shock-wave
What are 2 “big” difference with Oblique Shock-waves that differ from Normal Shock-waves?
With Oblique:
- Slows airflow down
- Changes direction of the airflow
Who cares about Mcrit?
Engineers
Who cares about Mmo? (max operating speed)
Pilots
Why should pilots care about Mmo?
- It is negative to a/c controllability
- Compressability issues arise
What 4 main affects do Shock-waves have on airflow?
- Increased Density and Static Pressure
- Increased Temperature
- Decreased Velocity
- Decreased Kinetic Energy
How is Wave Drag impacted with airspeed?
- At Mcrit drag levels increase
- At Mach 1.0 drag levels decrease
What are “reversible” flight controls?
Controls with cables and pulleys
What are “irreversible” flight controls?
Hydraulically operated controls
Control reversal is due to _____
Wing-twisting
Name 6 Compressability issues
- Uncontrollable aileron roll
- Aileron buzz
- Wings twisting
- Control reversal
- Air-frame buffeting
- Mach Tuck
What does Mach Tuck result in? Why?
- Causes nose of a/c to pitch down uncontrollably because the aerodynamic center of lift slides aft
- This causes a pith down because the horizontal stabilizer is in the wash of aerodynamically dead air
What is a Mach Tuck solution?
A trimable horizontal stabilizer!
1 degree of Stabilator movement is equal to how many degrees of Elevator movement?
4 degrees of Elevator movement
How will the Buffet Boundary (altitude you can fly) be affected as a/c gross weight INCREASES?
Buffet boundary will decrease with increased weight
What is Area Rule? What did it result in?
- Method invented by Whitcomb to deal with TOTAL DRAG
- Looks at a/c GLOBALLY (as one whole airfoil)
- Coke bottle shape fuselage smoothing a/c front to back
- F-102 was the 1st prototype
How do we determine the shape of the Mach Cone?
- The speed and shape of the object
ex: sharper edge = smaller cone
What are 2 advantages of Swept Wings?
- Delays the onset of Mcrit (transonic flight) meaning you can get by with a low-speed airfoil if wing is kept inside the shock cone
- Allows higher cruise speed bc of #1
True or False: Thicker airfoils are stronger.
TRUE
What are the advantages of having a low-speed (subsonic) airfoil? (5)
- stronger and can hold more gas*
1. Allows a larger aspect ratio a/c
2. Allows Hi lift devices (to aid low speed flight)
3. Lower TO/LD speeds
4. Lower TO/LD distances
5. Allows use of spoilers (to aid in low speed maneuvers)
What are 2 advantages of Super-critical Wings?
- Delays onset of Mcrit
2. Shock-waves are weaker
What is Vmo
- The IAS for max operating speed
- Low Altitudes
- Has to do with STRUCTURAL limits of a/c
What is Mmo
- Mach Number for max operating speed
- High Altitudes
- Concerned with aerodynamic compressability affects
What does Coffin Corner refer to?
The distance between the low speed buffet and (Mmo) high speed buffet (gets smaller and smaller as you go up in altitude)
True for False: Full supersonic flight is way more stable
True!!
If supersonic flight is more stable, why do commercial airliners not fly at those speeds?
- Its nore expensive because drag is increased, which increases fuel burn!!
What are 2 ideal supersonic airfoils?
- Biconvex
- Double Wedge
(both have sharp leading edges)
What are characteristics of airflow when it goes AROUND a corner?
Supersonic Airflow:
- Accelerates
- Creates “Expansion Fans”
- NO disturbed airflow (compressability = good)
Subsonic Airflow:
- Becomes disturbed
What are characteristics of airflow when it goes INTO a corner?
Supersonic Airflow:
- NO disturbed airflow
4 characteristics of a Blunt Leading Edge (know for exam)
- Large forward stagnation point
- Large area of slow moving air
- Bow wave cannot attach to wing
- High drag (no desirable)
3 characteristics of a Sharp Leading Edge (know for exam)
- Virtually no forward stagnation point
- Bow wave CAN attach to wing
- Much lower drag (desirable)
Where is the center of lift located on SUPERsonic airfoils?
At 50% chord
On supersonic airfoils with a slight AOA, identify:
- Weak shockwave
- Strong shockwave
- Area of fastest airflow/lowest pressure
- Area of slowest airflow/highest pressure
Top-left: Weak shockwave/fast airflow
Top-right: FASTEST airflow/LOWEST pressure
Bottom-left: Strong shockwave/HIGHEST pressure
Bottom-right: Slowed airflow because of stronger shockwave out infront
What is the point of Supersonic Diffusers?
- Slows airflow to SUBsonic speeds before it hits the compressors on a gas turbine engine
- Design forces airflow to produce an oblique shock wave, or several weak shock waves
Does airflow have to be subsonic before it hits the compressor?
Yes!
Do RAM jets require a compressor/turbine?
No!
