Basic Aerodynamics/ Principles of Flight Flashcards
What is a vector?
- Magnitude: size of sth (scalar)
- Direction
*represented by an arrow = 4 forces
The atmosphere is composed of ? (%)
Which element is heavier?
The atmosphere is composed of
78 percent nitrogen,
21 percent oxygen, and
1 percent other gases, such as argon or helium.
The heavier elements, such as oxygen, settle to the surface of the Earth, while the lighter elements are lifted up to the region of higher altitude.
What is the altitude that most of the oxygen contained? (ft.)
Most of the atmosphere’s oxygen is contained below 35,000 feet altitude.
What is the Standard Sea Level Pressure?
1013 mb (milibars)
What is the Sea Level Pressure (in Hg)
29.92 Hg (Inches of Mercury)
What is Fluid?
Gasses, Air, Liquid
Properties of fluid
Hints: F,D
- F: Ability to flow → fills in a container
- D: Deformed
Fluids generally do not resist deformed when even the smallest stress is applied, or they resist it only slightly 流體通常也不會抵抗變形，或只能輕微抵抗變形
Viscosity will cause?
Hints 2
F: flowing
cause Property of a fluid resist flowing*
A: adhere
Individual molecules of fluid tend to adhere/stick to each other
→ determine how much a fluid resist flow
→ All fluid are viscous and resistance to flow
High viscosity of fluids -> ?
Low viscosity of fluids -> ?
Is Air High/Low viscosity fluid? Does it flow easily?
High-viscosity fluids are THICK and resist flow
Low-viscosity fluids are THIN and flow easily
Air has a low viscosity and flows easily
What is Friction?
Friction exists between any two materials that contact each other.
Friction is the resistance that one surface or object encounters when moving over another.
Keywords: resist, moving, contact
What situation will accelerate friction?
A rough surface ramp impedes the flow of the fluid due to resistance from the surface (friction)
The surface roughness causes resistance and slows the velocity of the air flowing over the wing
Please explain boundary layer
Hins: Friction → Surface
- Molecules of air pass over the surface of the wing and actually adhere (stick, or cling) to the surface because of friction.
- Air molecules near the surface of the wing RESIST MOTION and have a relative velocity near zero.
The roughness of the surface impedes their motion. The layer of molecules that adhere to the wing surface is referred to as the boundary layer.
What is Drag?
Hins: viscosity / Friction → Drag
Once the boundary layer of the air adheres to the wing by friction,
further resistance to the airflow is caused by the viscosity, the tendency of the air to stick to itself.
When these two forces act together to resist airflow over a wing → Drag
Characteristic of Pressure
Hins: P,U,D
P: Perpendicular
The force applied in a perpendicular direction to the surface of an object
U: Uniformly
completely immersed in a fluid will feel pressure uniformly around the entire surface of the object.
D: Direction
If the pressure on one surface of the object becomes less than the pressure exerted on the other surfaces, the object will move in the direction of the lower pressure.
Why atmospheric pressure is important?
- weather changes
- helps to lift an aircraft
- Flight Instrument
These instruments are the altimeter, airspeed indicator, vertical speed indicator, and manifold pressure gauge
Standard Condition at sea level, what is the average weight of the atmosphere?
Hints: in psi, mb
14.70 pounds per square inch (psi) of surface
1013.2 millibars (MB).
The thickness of the atmosphere is limited → Attitude ↑, Air ↓
The weight at 18,000 ft is one-half of sea level
The standard atmosphere at sea level?
Hints: Temp and pressure
surface temperature: 59 °F or 15 °C
surface pressure: 29.92 inches of mercury (“Hg) or 1,013.2 mb.
Standard temperature lapse rate? up to how many feet?
temp decrease at the rate of 3.5 °F or 2 °C per thousand feet up to 36,000 feet
(approximately –65 °F or –55 °C.)
Above this point (36,000ft) the temperature is considered constant up to 80,000 feet
Standard pressure lapse rate? up to how many feet?
when pressure decreases at a rate of approximately 1 “Hg per 1,000 feet of altitude gain to 10,000 feet.
What is pressure altitude?
Hints Height & Weight
It is the height above a standard datum plane (SDP), where the weight of the atmosphere is 29.92 “Hg as measured by a barometer
Please describe how the barometer works. give an example of a barometer
altimeter
It is a sensitive barometer calibrated to indicate altitude in the standard atmosphere. 經過校準可指示標準大氣中的高度
If the altimeter is set for 29.92 “Hg SDP: the altitude = the pressure altitude.
How is pressure altitude important to flying?
Hints: 2
- Determine aircraft performance
- Assigning flight levels
As atmospheric pressure changes, the SDP may be below, at, or above sea level.
Pressure altitude is important as a basis for determining aeroplane performance, as well as for assigning flight levels to airplanes operating at or above 18,000 feet.
Please list out different kinds of altitude and what are the differences?
Pressure altitude = SDP (standard datum pressure, 1013.2 hg)
Density altitude = operate in a nonstandard atmosphere + used for correlating aerodynamic performance
How low density of air affect on aircraft performance?
Hints 3
as air becomes less dense, it reduces:
* Power because the engine takes in less air
* Thrust because a propeller is less efficient in thin air
* Lift because the thin air exerts less force on the airfoils
Aircraft performance will decrease
What is density altitude?
Hints: 3
- used for correlating aerodynamic performance in the nonstandard atmosphere
- the vertical distance above sea level in the standard atmosphere
- is determined by first finding pressure altitude, and then correcting desnity altitude for nonstandard temperature variations.
Please explain how less dense air affects on density altitude.
- decrease in air density (less atmospheric pressure) -> High density altitude
What factors will air density be affected?
Hint: 3
affected by changes in altitude, temperature, and humidity
Which situation will indicate of low-density attitude?
Hints: 3 (air density)
- Lower elevations, 海拔 (geographic areas or locations that are situated at relatively low altitudes or heights above sea level.)
- high atmospheric pressure,
- low temperatures, and low humidity
are more indicative of low-density altitude.
Effect of Pressure on Density
Low pressure → ?
Pressure ↓ , Mass ↓ , Density ↓
air is compressed, a greater amount of air can occupy a given volume.
the air expands and occupies a greater space
At a lower pressure:
- the original column of air contains a smaller mass of air.
- The density is decreased because density is directly proportional to pressure.
- if the pressure is lowered, the density is lowered.
Effect of Temperature on Density
Increase the temp →?
Decrease the temp →?
Hints: consider mass
Increasing the temperature of a substance (air become big, mass ↓)
→ decreases its density
decreasing the temperature
→ increases the density
- the density of air varies inversely with temperature.
Is air lighter than water vapour?
Moist air lighter or dry air?
Water vapour is lighter than air consequently, moist air is lighter than dry air.
Effect of Humidity (Moisture) on Density
Water in Air ↑ (lighter) , Air density ↓
↑ density altitude (reduction in air density) , ↓ a/c performance
As the water content of the air increases, the air becomes less dense,
increasing density altitude and decreasing performance.
What is relative humidity?
the amount of water vapor contained in the atmosphere and is expressed as a percentage of the maximum amount of water vapor the air can hold
How does temp varies humidity?
(Density)
WARM: *less dense
Warm air holds more water vapor,
warm and moist
(both qualities tend to lighten the air)
COLD: *more dense
cold air holds less
old and dry (both qualities making it heavier
the dry air that contains no water vapour has a relative humidity of 0%
Density altitude will be lower or higher if there is no humidity?
Lower → a/c performance will be better
Why a/c can fly?
Hints: force
Lift from wing > Gravity
opposite direction
Newton’s first law:
Every object persists in its state of rest or uniform motion in a straight line
unless it is compelled to change that state by forces impressed on it. (external force)
forces may add to its motion,
slow it down, or change its direction.
Body at rest → remains at rest
Body in motion →remains in motion
Newton’s Second Law:
Force is equal to the change in momentum per change in time. For a constant mass, force equals mass times acceleration.
Force = mass (weight) x acceleration
Weight = ma = mg *gravitational acceleration)
Newton’s third laws
For every action, there is an equal and opposite reaction.
E.g. the propeller moves and pushes back the air; consequently, the air pushes the propeller (and thus the airplane) in the opposite direction—forward
Please explain Bernoulli’s Principle
Hints: Velocity -> pressure
as the velocity of a moving fluid (liquid or gas) increases, the pressure within the fluid decreases.
This principle explains what happens to air passing over the curved top of the airplane wing.
Please explain the application of venturi tube in terms of Bernoulli’s Principle.
The venturi tube has an air inlet that narrows to a throat (constricted point)
and an outlet section that increases in diameter toward the rear.
The diameter of the outlet is the same as that of the inlet.
The mass of air entering the tube must exactly equal the mass exiting the tube.
At the constriction, the speed must increase to allow the same amount of air to pass in the same amount of time as in all other parts of the tube.
When the air speeds up, the pressure also decreases.
Past the constriction, the airflow slows and the pressure increases.
Please list out 5 parts related to airfoil
- Camber
difference in the curvatures (called cambers) of the upper and lower surfaces of the airfoil
- Chord line
straight line drawn through the profile connecting the extremities of the leading and trailing edges.
- Trailing edge
- Leading edge
- Mean camber line
line is equidistant at all points from the upper and lower surfaces.
How An airfoil is constructed in such a way that its shape takes 2 advantage of the air’s response to certain physical laws?
a positive pressure lifting action from the air mass below the wing,
a negative pressure lifting action from lowered pressure above the wing.
What form of a wing is constructed in that it causes a lift force greater than the weight of the aircraft, the aircraft will fly?
(Hints: Flat, angle, deflection)
air stream strikes the relatively flat lower surface of a wing or rotor blade when inclined at a small angle to its direction of motion,
- the air is forced to rebound downward, causing an upward reaction in positive lift.
the air stream striking the upper curved section of the leading edge is deflected upward.
Reason: An airfoil is shaped to cause an action on the air, and forces air downward, which provides an equal reaction from the air, forcing the airfoil upward.
The design of airfoil: how to increase pressure (upper surface)
airfoil were then inclined so the airflow strikes it at an angle,
the air moving over the upper surface would be forced to move faster than the air moving along the bottom of the airfoil.
(i.e., as the fluid speed decreases, the pressure must increase).
This increased velocity reduces the pressure above the airfoil.
Applying Bernoulli’s Principle of Pressure, the increase in the speed of the air across the top of an airfoil produces a drop in pressure.
Explain downwash with one of the newton law
The downward backward flow from the top surface of an airfoil creates a downwash. a wing shaped to cause an action on the air, and forcing it downward, provides an equal reaction from the air, forcing the wing upward
Extra(This downwash meets the flow from the bottom of the airfoil at the trailing edge.)
Applying Newton’s third law, the reaction of this downward backward flow results in an upward forward force on the airfoil.
Which position generates higher pressure underneath of the wing?
a positive pressure results, particularly at higher angles of attack
(As more air particles hit the upper surface)
Which point close to the leading edge, the airflow is virtually stopped and then gradually increases speed?
stagnation point
Extra: At some point near the trailing edge, it again reaches a velocity equal to that on the upper surface. (Join back tgt)
Please explain Bernoulli’s Principle and Newton’s Laws are in operation to generate lift
Applying Newton’s third law, the reaction of this downward backward flow results in an upward forward force on the airfoil. (Deflection)
the pressure differential between the upper and lower surface of the airfoil increases, total lift increases.
(i.e., as the fluid speed decreases, the pressure must increase).
Pressure Distribution
The surface of a wing at different angles of attack (AOA)
- there are regions along the surface where the pressure is negative, or less than atmospheric,
- regions where the pressure is positive, or greater than atmospheric.
This negative pressure on the upper surface creates a relatively larger force on the wing than is caused by the positive pressure resulting from the air striking the lower wing surface.
(Check ch4 pg 8)
The average of the pressure variation for any given AOA is referred to as the______?
Please explain what’s that?
center of pressure (CP).
Aerodynamic force acts through this CP.
A higher AOA -> CP moves_____?
A lower AOA -> CP moves_____?
At high angles of attack, the CP moves forward, while at low angles of attack the CP moves aft.
In the design of wing structures, why CP travel is very important?
Hints: 2
since it affects the position of:
- air loads imposed on the wing structure in both low and high AOA conditions.
- airplane’s aerodynamic balance and controllability are governed by changes in the CP.
What caused by deflection and resulting in change of magnitude and direction?
Hints: airfoil _____ against airflow
As an airfoil moves through air, the airfoil is inclined against the airflow, producing a different flow caused by the airfoil’s relationship to the oncoming air.
It causes the air to turn about the object within the air stream.
As a result of this change, the velocity about the object changes in both magnitude and direction, in turn resulting in a measurable velocity force and direction.
What will cause tip vortex? What will happen to the aircraft?
The high- pressure area on the bottom of an airfoil pushes around the tip to the low-pressure area on the top.
The vortex flows behind the airfoil creating a downwash that extends back to the trailing edge of the airfoil.
This downwash results in an overall reduction in lift for the affected portion of the airfoil.
How to overcome tip vortex?
- Winglets can be added to the tip of an airfoil to reduce this flow.
- The winglets act as a dam preventing the vortex from forming.
- Winglets can be on the top or bottom of the airfoil.
For reference: taper the airfoil tip, reducing the pressure differential and smoothing the airflow around the tip.
