The principles of flight are the aerodynamics dealing with the motion of air and forces acting on an aircraft
Lift is the most apparent force, as its what we think of as giving an aircraft the ability to fly
Thrust provides a method with which to move the aircraft
Drag, and weight are those forces that act upon all aircraft in flight
Understanding how these forces work together and knowing how to control them with the use of power and flight controls are essential to flight
Lift:
Lift vs. Relative Wind
Instrument Flying Handbook, Angle of Attack and Relative Wind
Lift is the critical aerodynamic force that brings an aircraft to fly
The dynamic effect of the air moving across an airfoil produces lift
Common airfoils include not just the wings but the flaps/slats, and stabilizers too
Most see a lift vector as acting "up," but instead, it acts perpendicular to the relative wind and the lateral axis of the aircraft [Figure 1]
This means "up" is relative to the aircraft, and being in a turn or even upside down changes the direction the lift vector points (a key principle in understanding turn performance and aerobatics)
Lift concentrates from the Center of Lift/Pressure [Figure 2]
Lift vs. Relative Wind
In straight and level flight, to be effective, the total lift must overcome the total weight of the aircraft, comprised of the actual weight and the tail-down force used to control the aircraft's pitch attitude
It is important to note that lift has no reference to Earth
This means that when performing a loop, for example, the lift vector is still perpendicular to the relative wind, which would have the lift vector pointing toward the ground as the aircraft becomes inverted
Instrument Flying Handbook, Angle of Attack and Relative Wind
Bernoulli's principle demonstrates that as the velocity of a moving fluid (liquid or gas) increases, the pressure within the fluid decreases
A Venturi [Figure 3] demonstrates Bernoulli's principle: A1V1P1 = A2V2P2
A = Area, V = Velocity, and P = Pressure
Assuming area is constant, you get: V1P1 = V2P2
The formula shows that as the velocity of fluid (air) increases, its pressure must decrease
Relating this principle to an airfoil, we see a similar shape
The rounded upper surface increases the velocity of the air, which causes pressure to decrease
As pressure above the wing decreases, the relative pressure below it is higher, creating a pressure differential which we know as lift
Note: with regards to rotary-wing aircraft, lift and thrust are both in the vertical direction
Note: We say lift is created by air moving faster over the top of the wing, but more specifically, its the decreased pressure which causes lift
Bernoulli's Principle
Newton's Laws of Motion:
Newton's first law:
"Every object persists in its state of rest or uniform motion in a straight line unless compelled to change that state by forces impressed on it"
A body at rest tends to remain at rest, and a body in motion tends to remain moving at the same speed and in the same direction
This means that nothing starts or stops moving until some outside force causes it to do so
An aircraft at rest on the ramp remains at rest unless a force strong enough to overcome its inertia is applied
Once it is moving, its inertia keeps it moving, subject to the various other forces acting on it
These forces may add to its motion, slow it down, or change its direction
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"
When a body is acted upon by a constant force, its resulting acceleration is inversely proportional to the mass of the body and is directly proportional to the applied force
This takes into account the factors involved in overcoming Newton's First Law
It covers both changes in direction and speed, including starting up from rest (positive acceleration) and coming to a stop (negative acceleration or deceleration)
The equation F(force)=M(mass)A(acceleration) may express this law where the force is equal to the mass times the acceleration
Newton's Third Law:
"For every action, there is an equal and opposite reaction"
Newton's Third Law
In an airplane, the propeller moves and pushes back the air; consequently, the air pushes the propeller (and thus the airplane) in the opposite direction—forward
This principle applies whenever two things act upon each other [Figure 4]
The lift coefficient is a number that aerodynamicists use to model all of the complex dependencies of shape, inclination, and some flow conditions on lift
Air Density:
If air density decreases and the total lift must equal the total weight to remain in flight, it follows that another factor needs to increase
The factor usually increased is the airspeed or the Angle of Attack (AOA) because these are controlled directly by the pilot
Wing Area:
The lift varies directly with the wing area, provided there is no change in the wing's planform
If the wings have the same proportion and airfoil sections, a wing with a planform area of 200 square feet lifts twice as much at the same AOA as a wing with an area of 100 square feet
Velocity:
The shape of the wing or rotor cannot be effective unless it continually keeps "attacking" new air
If an aircraft is to keep flying, the lift-producing airfoil must keep moving
In a helicopter or gyroplane, the rotation of the rotor blades creates the necessary lift
For other types of aircraft, such as airplanes, weight shift control, or gliders, air must be moving across the lifting surface by way of forward speed
The forward speed of the aircraft accomplishes this
Lift is proportional to the square of the aircraft's velocity, meaning that an airplane traveling at 200 knots has four times the lift as the same airplane traveling at 100 knots if the AOA and other factors remain constant
All other factors being constant, for every AOA, there is a corresponding airspeed required to maintain altitude in steady, unaccelerated flight (true only if maintaining level flight). Since an airfoil always stalls at the same AOA, the lift must increase if increasing weight. The only method of increasing lift is by increasing velocity if the AOA is held constant just short of the "critical," or stalling, AOA (assuming no flaps or other high lift devices). Lift and drag also vary directly with the density of the air. Density is affected by several factors: pressure, temperature, and humidity. At an altitude of 18,000 feet, the air density has one-half the air density at sea level. An aircraft must fly at a greater true airspeed for any given AoA to maintain its lift at a higher air density altitude. Warm air is less dense than cool air, and moist air is less dense than dry air. Thus, on a hot, humid day, an aircraft must be flown at a greater true airspeed for any given AOA than on a cool, dry day
Controlling Lift:
Pilot's can control lift principally with two factors:
Angle of Attack
Velocity/airspeed
Angle of Attack:
Airplane Flying Handbook, Coefficients of lift and drag at various angles of attack
AOA is fundamental to understanding many aspects of airplane performance, stability, and control
AoA is the acute angle measured between the relative wind or flight path and the chord of the airfoil [Figure 2]
Any time the control yoke or stick is moved fore or aft, the Angle of Attack, or AOA, is changed
As the AOA increases, lift increases (all other factors being equal)
When the aircraft reaches the maximum AOA, lift begins to diminish rapidly
This is the stalling AOA, known as CL-MAX (maximum CL)critical AOA
The CL increases until reaching the critical AOA, then decreases rapidly with any further increase in the AOA [Figure 6]
Lift created (or reduced in the case of negative AoA) is measured with the coefficient of lift, which relates to the AoA
Every airplane has an angle of attack where the maximum lift occurs (stall)
Velocity/Airspeed:
For instance, in straight-and-level flight, cruising along at a constant altitude, altitude is maintained by adjusting lift to match the aircraft's velocity or cruise airspeed while maintaining a state of equilibrium in which lift equals weight
In an approach to landing, when the pilot wishes to land as slowly as practical, it is necessary to increase AOA near maximum to maintain lift equal to the weight of the aircraft
An aircraft could not continue to travel in level flight at a constant altitude and maintain the same AOA if the velocity increases. The lift would increase, and the aircraft would climb due to the increased lift force or speed up. Therefore, to keep the aircraft straight and level (not accelerating upward) and in a state of equilibrium, lift must be kept constant as velocity is increased. This is normally accomplished by reducing the AOA by lowering the nose. Conversely, as the aircraft is slowed, the decreasing velocity requires increasing the AOA to maintain lift sufficient to maintain flight. There is a limit to how far the AOA can be increased, if a stall is to be avoided
Lift/Drag Ratio:
Airplane Flying Handbook, Drag vs. Speed
The lift-to-drag ratio (L/D) is the amount of lift generated by a wing or airfoil compared to its drag
A L/D ratio is an indication of airfoil efficiency
Aircraft with higher L/D ratios are more efficient than those with lower L/D ratios
In unaccelerated flight with the lift and drag data steady, the proportions of the coefficient of lift (CL) and coefficient of drag (CD) can be calculated for specific AOA [Figure 7]
The coefficient of lift is dimensionless and relates the lift generated by a lifting body, the dynamic pressure of the fluid flow around the body, and a reference area associated with the body
The coefficient of drag is dimensionless, used to quantify the drag of an object in a fluid environment, such as air, and is always associated with a particular surface area
The L/D ratio is determined by dividing the CL by the CD
This is the same as dividing the lift equation by the drag equation as all of the variables, aside from the coefficients, cancel out
The lift and drag equations are as follows (L = Lift in pounds; D = Drag; CL = coefficient of lift; ρ = density (expressed in slugs per cubic feet); V = velocity (in feet per second); q = dynamic pressure per square foot (q = 1⁄2 ρv2); S = the area of the lifting body (in square feet); and CD = Ratio of drag pressure to dynamic pressure)
Typically at low AOA, the coefficient of drag is low and small changes in AOA create only slight changes in the coefficient of drag
At high AOA, small changes in the AOA cause significant changes in drag
The shape of an airfoil and changes in the AOA affect the production of lift
Notice in [Figure 6] that the coefficient of lift curve (red) reaches its maximum for this particular wing section at 20° AOA and then rapidly decreases
20° AOA is, therefore, the critical angle of attack
The coefficient of drag curve (orange) increases very rapidly from 14° AOA and completely overcomes the lift curve at 21° AOA
The lift/drag ratio (green) reaches its maximum at 6° AOA, meaning that at this angle, the most lift is obtained for the least amount of drag
Note that the maximum lift/drag ratio (L/DMAX) occurs at one specific CL and AOA
If the aircraft operates in steady flight at L/DMAX, the total drag is at a minimum
Any AOA lower or higher than that for L/DMAX reduces the L/D and consequently increases the total drag for a given aircraft's lift
[Figure 7] depicts the L/DMAX by the lowest portion of the blue line labeled "total drag
The configuration of an aircraft has a great effect on the L/D
Pilot Handbook of Aeronautical Knowledge, Typical Airfoil Section
Pilot Handbook of Aeronautical Knowledge, Airfoil Designs
Airfoil Design:
Airfoil construction takes advantage of the air's response to Newton's and Bernoulli's principles
Air acts in various ways when submitted to different pressures and velocities: a positive pressure lifting action from the air mass below the wing and a negative pressure lifting action from lowered pressure above the wing
If all the lift required were obtained merely from the deflection of air by the lower surface of the wing, an aircraft would only need a flat wing like a kite
However, the balance of the lift needed to support the aircraft comes from the flow of air above the wing
Herein lies the key to flight
It is neither accurate nor useful to assign specific values to the percentage of lift generated by an airfoil's upper surface versus that generated by the lower surface
These are not constant values. They vary, not only with flight conditions but also with different wing designs
Different airfoils have different flight characteristics
No one airfoil can satisfy every flight requirement
The weight, speed, and purpose of each aircraft dictate the shape of its airfoil
The most efficient airfoil for producing the greatest lift is one that has a concave or "scooped out" lower surface
As a fixed design, this airfoil type sacrifices too much speed while producing lift and is not suitable for high-speed flight
Advancements in engineering have made it possible for today's high-speed jets to take advantage of the concave airfoil's high lift characteristics
Leading-edge (Kreuger) flaps and trailing edge (Fowler) flaps when extended from the basic wing structure, literally change the airfoil shape into the classic concave form, thereby generating much greater lift during slow flight conditions
On the other hand, an airfoil that is perfectly streamlined and offers little wind resistance sometimes does not have enough lifting power to take the airplane off the ground. Thus, modern airplanes have airfoils that strike a medium between extremes in design. The shape varies according to the needs of the airplane
[Figure 8] shows some of the more common airfoil designs
Airfoil Construction:
By looking at the cross-section of a wing, one can see several prominent characteristics of design [Figure 8/9]
Notice that there is a difference in the curvatures (called cambers) of the upper and lower surfaces of the airfoil
The camber of the upper surface is more pronounced than that of the lower surface, which is usually somewhat flat
The two extremities of the airfoil profile also differ in appearance as the rounded end, which faces forward in flight, is called the leading edge; the other end, the trailing edge, is relatively narrow and tapered
Chord Line:
A straight line connecting the extremities of the leading and trailing edges denotes the Chord Line
The Chord line is a reference line often used in discussing the airfoil
The distance from this chord line to the upper and lower surfaces of the wing denotes the upper and lower magnitude of camber at any point
Another reference line, drawn from the leading edge to the trailing edge, is the mean camber line
This mean line is equidistant at all points from the upper and lower surfaces
High Pressure Below:
A certain amount of lift is generated by pressure conditions underneath the airfoil
Because of air flows underneath the airfoil, a positive pressure results, particularly at higher angles of attack
There is another aspect to this airflow to consider:
At a point close to the leading edge, the airflow nearly stops (stagnation point) and then gradually increases in speed
At some point near the trailing edge, it again reaches a velocity equal to that on the upper surface
In conformance with Bernoulli's principle, positive upward pressure created where the airflow slowed beneath the airflow (i.e., the fluid speed decreases, the pressure must increase)
Since the pressure differential between the upper and lower surface of the airfoil increases, total lift increases
Low Pressure Above:
With an airfoil in the shape of a teardrop, the speed and the pressure changes of the air passing over the top and bottom would be the same on both sides
If the 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
This increased velocity reduces the pressure above the airfoil
Applying Bernoulli's Principle of Pressure, the airspeed increase across the top of an airfoil produces a pressure drop. This lowered pressure is a component of total lift. The pressure difference between the upper and lower surface of a wing alone does not account for the total lift force produced
The downward backward flow from the top surface of an airfoil creates a downwash
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
Pressure Distribution:
As air flows along the surface of a wing at different AOAs, there are regions along the surface where the pressure is negative or less than atmospheric and 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 [Figure 10]
The average pressure variation for any given AOA is called the center of pressure (CP). The aerodynamic force acts through this CP. 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, this CP travel is very important since it affects the position of the air loads imposed on the wing structure in both low and high AOA conditions. Changes in the CP govern an airplane's aerodynamic balance and controllability
Pilot Handbook of Aeronautical Knowledge, Pressure distribution on an airfoil and CP changes with AOA
Airfoil Behavior:
The lift production is much more complex than a simple differential pressure between upper and lower airfoil surfaces. Many lifting airfoils do not have an upper surface longer than the bottom, as in symmetrical airfoils. These are seen in high-speed aircraft with symmetrical wings or symmetrical rotor blades for many helicopters whose upper and lower surfaces are identical. In both examples, the only difference is the airfoil relationship with the oncoming airstream (angle). A paper airplane, which is simply a flat plate, has a bottom and top shape and length. Yet, these airfoils do produce lift, and "flow turning" is partly (or fully) responsible for creating lift
As an airfoil moves through the air, the airfoil is inclined against the airflow, producing a different flow caused by the airfoil's relationship to the oncoming air. Think of a hand placed outside the car window at high speed. If the hand inclines in one direction or another, the hand will move upward or downward. Deflection causes the air to turn about the object within the air stream. The velocity about the object changes in both magnitude and direction, in turn resulting in a measurable velocity force and direction
Wingtip Vortices & Lift:
Pilot Handbook of Aeronautical Knowledge Tip Vortex
While the biggest consideration for producing lift involves the air flowing over and under the wing, there is a third dimension to consider
Consider the tip of the airfoil also has an aerodynamic effect
To equalize pressure, the high-pressure area on the bottom of an airfoil pushes around the tip to the low-pressure area on the top [Figure 11]
This action creates a rotating flow called a tip vortex or wingtip vortices
This downwash extends back to the trailing edge of the airfoil, reducing lift for the affected portion of the airfoil
Manufacturers have developed different methods to counteract this action
Winglets can be added to the tip of an airfoil to reduce this flow (essentially decrease induced drag)
The winglets act as a dam preventing the vortex from forming
Winglets can be on the top or bottom of the airfoil
Another method of countering the flow is to taper the airfoil tip, reducing the pressure differential and smoothing the airflow around the tip
Weight is simply the force of gravity on the aircraft, which acts vertically through the center of gravity
It is the combined load of the aircraft itself, the crew, the fuel, and the cargo or baggage
Weight varies based on load, passengers, and fuel
A Load is essentially the back pressure on the control stick required, the G-loading, which an aircraft experiences
Passengers and fuel are more obvious
Opposing lift, as an aircraft is descending
Weight vs. Lift:
Weight has a definite relationship to lift
This relationship is simple but important in understanding the aerodynamics of flying
Lift is the upward force on the wing acting perpendicular to the relative wind and perpendicular to the aircraft's lateral axis
Lift is required to counteract the aircraft's weight
In stabilized level flight, when the lift force is equal to the weight force, the aircraft is in a state of equilibrium, and neither accelerates upward or downward
If lift becomes less than weight, the vertical speed will decrease
When the lift is greater than weight, the vertical speed will increase
Thrust:
Thrust is the forward acting force that opposes drag and propels the airplane forward
It is through excesses or deficits of thrust that accelerations and decelerations can occur
The aircraft will continue to speed up/slow down until thrust again equals drag, at which point the airspeed will stabilize
In powered aircraft, thrust is achieved through the powerplant, be it a propeller, rotor, or turbine
With a glider, thrust is created through the conversion of potential energy (altitude) to kinetic energy (airspeed) by pitching toward the ground
Newton's second law: When a body is acted upon by a constant force, its resulting acceleration is inversely proportional to the mass of the body and is directly proportional to the applied force
This law may be expressed by F = MA (Force equals Mass times Acceleration), for example, speeding up, slowing down, entering climbs or descents, and turning
Acts parallel to the center of thrust to overcome drag, F = MA
Note: with regards to rotary-wing aircraft, lift and thrust are both in the vertical direction
Propeller & rotor driven aircraft are generally rated in horsepower
Turbine driven aircraft are generally rated in pounds
Thrust During Acceleration:
Increasing engine power increases thrust (now exceeding drag), thereby accelerating the aircraft
As long as the thrust continues to be greater than the drag, the aircraft continues to accelerate
When drag equals thrust, the aircraft flies at a constant airspeed
Thrust during Deceleration:
When reducing engine power, lessoning thrust, thereby decelerating the aircraft
As long as the thrust is less than the drag, the aircraft continues to decelerate
To a point, as the aircraft slows down, the drag force will also decrease
The aircraft will continue to slow down until thrust again equals drag, at which point the airspeed will stabilize
Straight-and-level flight:
Pilot Handbook of Aeronautical Knowledge Angle of Attack at Various Speeds
The pilot coordinates AOA and thrust in all speed regimes if the aircraft is to be held in level flight
Remember, (for a given airfoil shape) lift varies with the AOA and airspeed
Therefore, a large AOA at low airspeeds produces an equal amount of lift at high airspeeds with a low AOA
The speed regimes of flight can be grouped in three categories:
Low-Speed Flight:
When the airspeed is low, the AOA must be relatively high if the balance between lift and weight is to be maintained [Figure 12]
If thrust decreases and airspeed decreases, the lift will become less than weight, and the aircraft will start to descend
To maintain level flight, the pilot can increase the AOA an amount that generates a lift force again equal to the weight of the aircraft
While the aircraft will be flying more slowly, it will still maintain level flight
Cruising Flight
Straight-and-level flight in the slow-speed regime provides some interesting conditions relative to the equilibrium of forces
With the aircraft in a nose-high attitude, there is a vertical component of thrust that helps support it
For one thing, wing loading tends to be less than would be expected
In level flight, when thrust increases, the aircraft speeds up, and the lift increases
The aircraft will start to climb unless the AOA is decreased just enough to maintain the relationship between lift and weight
The timing of this decrease in AOA needs to be coordinated with the increase in thrust and airspeed. Otherwise, if the AOA decreases too fast, the aircraft will descend, and if the AOA decreases too slowly, the aircraft will climb
High-Speed Flight
As the airspeed varies due to thrust, the AOA must also vary to maintain level flight
At very high speeds and level flight, it is even possible to have a slightly negative AOA
As thrust reduces and airspeed decreases, the AOA must increase to maintain altitude
If speed decreases enough, the required AOA will increase to the critical AOA
Any further increase in the AOA will result in the wing stalling
Therefore, extra vigilance is required at reduced thrust settings and low speeds so as not to exceed the critical angle of attack
If equipped with an AOA indicator, it should be referenced to help monitor the proximity to the critical AOA
Aircraft like the V-22 and F-35, pivot the engines or vector the exhaust, to change the direction of the thrust rather than changing the AOA
Drag:
Drag Curves
Drag is the rearward, resisting force caused by disruption of airflow
Drag is the net aerodynamic force parallel to the relative wind
Drag is always a by-product of lift and thrust
Always a by-product of lift
There are two basic types of drag (induced and parasite), with total drag being a combination of the two [Figure 13]
Induced Drag:
In level flight, the aerodynamic properties of a wing or rotor produce a required lift, but this can be obtained only at the expense of a certain penalty
That penalty, induced drag, is inherent whenever an airfoil is producing lift
as AOA increases, induced drag increases proportionally
To state this another way—the lower the airspeed, the greater the AOA required to produce lift equal to the aircraft's weight and, therefore, the greater induced drag. The amount of induced drag varies inversely with the square of the airspeed
Wingtip Vortices & Drag:
Pilot Handbook of Aeronautical Knowledge, Wingtip Vortex From Crop Duster
Pilot Handbook of Aeronautical Knowledge, The difference in wingtip vortex size at altitude versus near the ground
Pilot Handbook of Aeronautical Knowledge, The difference in downwash at altitude versus near the ground
An airfoil (wing or rotor blade) produces the lift force by using the energy of the free airstream. Whenever an airfoil is producing lift, the pressure on the lower surface is greater than that on the upper surface (Bernoulli's Principle). As a result, the air tends to flow from the high-pressure area below the tip upward to the upper surface's low-pressure area. There is a tendency for these pressures to equalize in the vicinity of the tips, resulting in a lateral flow outward from the underside to the upper surface. This lateral flow imparts a rotational velocity to the air at the tips, creating vortices that trail behind the airfoil
When the aircraft is viewed from the tail, these vortices circulate counterclockwise about the right tip and clockwise about the left tip. [Figure 14] As the air (and vortices) roll off the back of your wing, they angle down, which is known as downwash. [Figure 15] shows the difference in downwash at altitude versus near the ground. Bearing in mind the direction of rotation of these vortices, it can be seen that they induce an upward flow of air beyond the tip and a downwash flow behind the wing's trailing edge. This induced downwash has nothing in common with the downwash that is necessary to produce lift. It is, in fact, the source of induced drag. Downwash points the relative wind downward, so the more downwash you have, the more your relative wind points downward. That's important for one very good reason: lift is always perpendicular to the relative wind. In [Figure 16], you can see that your lift vector is more vertical, opposing gravity when you have less downwash. And when you have more downwash, your lift vector points back more, causing induced drag. On top of that, it takes energy for your wings to create downwash and vortices, and that energy creates drag
The greater the size and strength of the vortices and the consequent downwash component on the airfoil's net airflow, the greater the induced drag effect becomes. This downwash over the top of the airfoil at the tip has the same effect as bending the lift vector rearward; therefore, the lift is slightly aft of perpendicular to the relative wind, creating a rearward lift component. This is induced drag
Parasite Drag:
Parasite drag is comprised of all the forces that work to slow an aircraft's movement
As the term parasite implies, it is the drag that is not associated with the production of lift
Parasite drag includes the displacement of the air by the aircraft, turbulence generated in the airstream, or a hindrance of air moving over the surface of the aircraft and airfoil
There are three types of parasite drag: form drag, interference drag, and skin friction
Profile/Form Drag:
Form Drag
Interference Drag
Form drag is the portion of parasite drag generated by the aircraft and components (antennas, wheels, etc.) due to its shape and airflow around it
Turbulent wake caused by the separation of airflow (burbling) created by the shape of the aircraft [Figure 17]
When the air has to separate to move around a moving aircraft and its components, it eventually rejoins after passing the body
Newer aircraft are generally made with consideration to this by fairings along the fuselage so that turbulence and form drag is reduced [Figure 18]
Interference Drag:
Generated by the collision of air-streams creating eddy currents, turbulence, or restrictions to smooth flow
For example, landing gear meeting the fuselage
The most interference drag is created when two surfaces meet at perpendicular angles
If a jet fighter carries two identical wing tanks, the overall drag is greater than the sum of the individual tanks because both of these generate interference drag
Fairings and distance between lifting surfaces and external components (such as radar antennas hung from wings) reduce interference drag. [Figure 5-8]
Learn more about the effects of interference drag here
Skin Friction Drag:
Skin friction drag is the aerodynamic resistance due to the contact of moving air with the surface of an aircraft
Every surface, no matter how apparently smooth, has a rough, ragged surface when viewed under a microscope
The air molecules, which come in direct contact with the surface of the wing, are virtually motionless
Each layer of molecules above the surface moves slightly faster until the molecules are moving at the velocity of the air mass moving around the aircraft
This speed is called the free-stream velocity
The area between the wing and the free-stream velocity level is about as wide as a playing card and is called the boundary layer
At the top of the boundary layer, the molecules increase velocity and move at the same speed as the molecules outside the boundary layer
The actual speed at which the molecules move depends upon the shape of the wing, the viscosity (stickiness) of the air, and its compressibility (how much it can be compacted)
The airflow outside of the boundary layer reacts to the shape of the edge of the boundary layer
The boundary layer gives any object an "effective" shape that is usually slightly different from the physical shape
The boundary layer may also separate from the body, thus creating an effective shape much different from the physical shape of the object
This change in the physical shape of the boundary layer causes a dramatic decrease in lift and an increase in drag
When this happens, the airfoil has stalled
To reduce the effect of skin friction drag, aircraft designers utilize flush mount rivets and remove any irregularities that may protrude above the wing surface
Also, a smooth and glossy finish aids in the transition of air across the surface of the wing
Since dirt on an aircraft disrupts the free flow of air and increases drag, keep the surfaces of an aircraft clean and waxed
Drag can be caused intentionally through the use of speed brakes, spoilers, or dive brakes
Additionally, normal procedures such as lowering flaps can increase drag
Parasite drag increases as the square of the airspeed (V^2)
Thus, as airspeed decreases to near the stalling speed in a steady-state, the total drag becomes greater due mainly to the exponential rise in induced drag. Similarly, as the aircraft reaches its never-exceed speed (VNE), the total drag increases rapidly due to the sharp increase of parasite drag
Interaction Between the Four Forces of Flight:
The four forces of flight do not exist in isolation as each influences the other
While lift opposes weight, and drag opposes thrust, this is only true in level flight
For example, a pitch up requires thrust to not only overcome drag but also weight
Ground Effect:
Reduction of induced drag during takeoffs and landings
Caused by a reduction of wingtip vortices
It occurs at about a wingspan above the ground
Up-wash and Down-wash decrease
Down-wash can hit the ground and pushes the wing from below, forming what feels like a cushion
Causes floating if fast on approach
More noticeable in a low-wing aircraft
Ground Effect:
Increases lift while decreasing drag (induced)
The opposite is true when leaving ground effect
Critical Definitions:
Rotor Blade: spinning "wings" which allow for lift on helicopters or "rotor-craft"
Stabilizer: a control surface other than the wings which provide stabilizing qualities
Chord: Chord line longitudinal length (length as viewed from the side)
Chord Line: The chord line is the straight line intersecting the leading and trailing edges of the airfoil
Angle of Incidence (AoI): angle formed by the airfoil chord and the longitudinal axis of the aircraft, which is designed into the aircraft and cannot be changed by the pilot
Attitude: relationship of the aircraft's nose with the horizon
Flight Path: The course or track along which the aircraft is flying or is intended to be flown
Lift: A component of the total aerodynamic force on an airfoil and acts perpendicular to the relative wind
Center of Gravity: The average weight across an aircraft through which gravity is considered to act
Weight: Opposes lift via gravity
Thrust: Forward force which propels the airplane
Drag: Retarding force which limits speed
Conclusion:
The principles of flight are those basic characteristics that act upon an aircraft
Although simplified as thrust, lift, weight, and drag, we know that there are more upward forces than lift, and there are more downward forces than just weight
Although the pilot can only have limited control of some of these factors, principally, lift is affected by wing design, angle of attack, velocity, weight and loading, air temperature, and humidity
Both Bernoulli's Principle and Newton's Laws are in operation whenever an airfoil generates lift
You can see the four forces of flight are inter-related
To achieve flight, we must overcome drag, and resist gravity
To maintain a constant airspeed, thrust and drag must remain equal, just as lift and weight must be equal to maintain a constant altitude
Although AoA and velocity allows a pilot to manipulate lift, other factors are slightly under pilot control, such as air density (as a pilot could change altitude)
A balanced aircraft is a happy aircraft (fuel burn, efficiency, etc.)
In un-accelerated, level flight, the four forces are in equilibrium
Equilibrium is defined as lift equaling down-force (weight+tail downforce), and thrust equaling drag, but by changing these forces, we can affect climbs, descents, and other maneuvers
