Axes of Rotation and the Four Forces Acting in Flight
Turns, Loads, and Load Factors
An airfoil is a structure of body which produces a useful reaction to air movement. Airplane wings, helicopter rotor blades, and propellers are airfoils. See Figure 1-1.

Figure 1-1. A typical airfoil cross-section
The chord line is a straight reference line from the leading edge to the trailing edge of an airfoil. See Figure 1-2.

Figure 1-2. Chord line
Changing the shape of an airfoil (by lowering flaps, for example) will change the chord line. See Figure 1-3.

Figure 1-3. Changing shape of wing changes the chord line
In aerodynamics, relative wind is the wind felt by an airfoil. It is created by the movement of air past an airfoil, by the motion of an airfoil through the air, or by a combination of the two. Relative wind is parallel to and in the opposite direction of the flight path of the airfoil. See Figure 1-4.

Figure 1-4. Relative wind
The angle of attack is the angle between the chord line of the airfoil and the relative wind. The pilot can vary the angle of attack by manipulating aircraft controls. See Figure 1-5. When the angle of attack of a symmetrical airfoil is increased, the center of pressure movement is very limited.

Figure 1-5. Angle of attack
The angle of incidence is the angle between the wing chord line and the center line of the fuselage. The pilot has no control over the angle of incidence. See Figure 1-6.

Figure 1-6. Angle of incidence
An airplane has three axes of rotation: lateral, longitudinal, and vertical. See Figure 1-7.

Figure 1-7. Axes of rotation
The lateral axis is an imaginary line from wing tip to wing tip. The rotation about this axis is called pitch. Pitch is controlled by the elevators, and this type of rotation is referred to as longitudinal control or longitudinal stability. See Figure 1-8.

Figure 1-8. Effect of elevators
The longitudinal axis is an imaginary line from the nose to the tail. Rotation about the longitudinal axis is called roll. Roll is controlled by the ailerons, and this type of rotation is referred to as lateral control, or lateral stability. See Figure 1-9.

Figure 1-9. Effect of ailerons
The vertical axis is an imaginary line extending vertically through the intersection of the lateral and longitudinal axes. Rotation about the vertical axis is called yaw. Yaw is controlled by the rudder, and this type of rotation is referred to as directional control or directional stability. See Figure 1-10.

Figure 1-10. Effect of rudder
The center of gravity (CG) is the point at which an airplane would balance if suspended from that point. The three axes intersect at the center of gravity. Movement of the center of gravity can affect the stability of the airplane.
Four aerodynamic forces are considered basic because they act upon an aircraft during all flight maneuvers. The downward-acting force called weight is counteracted by the upward-acting force called lift. The rearward-acting force called drag is counteracted by the forward-acting force called thrust. See Figure 1-11.

Figure 1-11. Relationship of forces in flight
Air is a gas that can be compressed or expanded. When compressed, more air can occupy a given volume and air density is increased. When allowed to expand, air occupies a greater space and density is decreased. Temperature, atmospheric pressure, and humidity all affect air density. Air density has significant effects on an aircraft’s performance.
As the velocity of a fluid (gas or liquid) increases, its pressure decreases. This is known as Bernoulli’s principle. See Figure 1-12.

Figure 1-12. Flow of air through a constriction
Lift is the result of a pressure difference between the top and the bottom of the wing. A wing is designed to accelerate air over the top camber of the wing, thereby decreasing the pressure on the top and producing lift. See Figure 1-13.

Figure 1-13. Development of lift
Several factors are involved in the creation of lift: angle of attack, wing area and shape (planform), air velocity, and air density. All of these factors have an effect on the amount of lift produced at any given moment. The pilot can actively control the angle of attack and the airspeed, and increasing either of these will result in an increase in lift.
Weight is the force with which gravity attracts all bodies (masses) vertically toward the center of the Earth.
Thrust is the forward force produced by the propeller acting as an airfoil to displace a large mass of air rearward.
Drag, the force acting parallel to the flight path, resists the forward movement of an airplane through the air. Drag may be classified into two main types: parasite drag and induced drag.
Parasite drag is the resistance of the air produced by any part of an airplane that does not produce lift (antennae, landing gear, etc.). Parasite drag will increase as airspeed increases. If the airspeed of an airplane is doubled, parasite drag will be quadrupled.
Induced drag is a by-product of lift. In other words, this drag is generated as the wing develops lift. The high-pressure air beneath the wing trying to flow around and over the wing tips into the area of low pressure causes a vortex behind the wing tip. This vortex causes a spanwise flow and creates vortices along the trailing edge of the wing. As angle of attack is increased (up to the critical angle), lift will increase and so will the vortices and downwash. This downwash redirects the lift vector rearward, causing a rearward component of lift (induced drag). Induced drag will increase as airspeed decreases. See Figure 1-14.

Figure 1-14. Drag curve diagram
During unaccelerated (straight-and-level) flight, the four aerodynamic forces which act on an airplane are said to be in equilibrium, or: lift = weight, and thrust = drag.
The lift-to-drag (L/D) ratio is the lift required for level flight (weight) divided by the drag produced at the airspeed and angle of attack required to produce that lift. The L/D ratio for a particular angle of attack is equal to the power-off glide ratio.
Problem:
Refer to FAA Figure 3. If an airplane glides at an angle of attack of 10°, how much altitude will it lose in 1 mile?
Solution:
L/DMAX occurs at the angle of attack that gives maximum glide performance and maximum range in a propeller driven aircraft. At an airspeed slower (or at a higher angle of attack) than needed for L/DMAX, the glide distance will be reduced due to the increase in induced drag.
FAA Figure 5 is a VG diagram which plots load factor against indicated airspeed and shows the pilot the limits within which the aircraft will safely handle structural loads. Point C is maneuvering speed (VA). Any plotted combination of load factor and airspeed which falls in the shaded area may result in structural damage.
VNO, the maximum speed for normal operations, is shown by the vertical line from point D to point G on the VG diagrams, and is marked as the upper limit of the green arc on the airspeed indicator. The red line on the airspeed indicator is represented by the line from point E to point F. The line connecting points C, D, and E represents the limit load factor above which structural damage may occur.
Stability is the inherent ability of an airplane to return, or not return, to its original flight condition after being disturbed by an outside force, such as rough air.
Positive static stability is the initial tendency of an aircraft to return to its original position. See Figure 1-15.

Figure 1-15. Static stability
Positive dynamic stability is the tendency of an oscillating airplane (with positive static stability) to return to its original position relative to time. See Figure 1-16.

Figure 1-16. Positive static stability relative to dynamic stability
Aircraft design normally assures that the aircraft will be stable in pitch. The pilot can adversely affect this longitudinal stability by allowing the center of gravity (CG) to move forward or aft of specified CG limits through improper loading procedures. One undesirable flight characteristic a pilot might experience in an airplane loaded with the CG located behind the aft CG limit would be the inability to recover from a stalled condition.
The location of the CG with respect to the center of lift (CL) will determine the longitudinal stability of an airplane. See Figure 1-17.

Figure 1-17. Effects of CG on aircraft stability
An airplane will be less stable at all airspeeds if it is loaded to the most aft CG. An advantage of an airplane said to be inherently stable is that it will require less effort to control.
Changes in pitch can also be experienced with changes in power setting (except in T-tail airplanes). When power is reduced, there is a corresponding reduction in downwash on the tail, which results in the nose “pitching” down.
When an airplane is banking into a turn, a portion of the lift developed is diverted into a horizontal component of lift. It is this horizontal (sideward) force that forces the airplane from straight-and-level flight and causes it to turn. The reduced vertical lift component results in a loss of altitude unless total lift is increased by increasing the angle of attack, increasing airspeed or both.
In aerodynamics, load is the force, or imposed stress, that must be supported by an airplane structure in flight. The loads imposed on the wings in flight are stated in terms of load factor.
In straight-and-level flight, the wings of an airplane support a load equal to the sum of the weight of the airplane plus its contents. This particular load factor is equal to “one G,” where “G” refers to the pull of gravity.
However, a force which acts toward the outside of the curve, called centrifugal force, is generated any time an airplane is flying a curved path (turns, climbs, or descents).
Whenever the airplane is flying in a curved flight path with a positive load, the load that the wings must support will be equal to the weight of the airplane plus the load imposed by centrifugal force; therefore, it can be said that turns increase the load factor on an airplane.
As the angle of bank of a turn increases, the load factor increases, as shown in Figure 1-18.

Figure 1-18. Increase in load on wings as angle of bank increases
The amount of excess load that can be imposed on the wing of an airplane depends on the speed of the airplane. An example of this would be a change in direction made at high speed with forceful control movement, which results in a high load factor being imposed.
An increased load factor (weight) will cause an airplane to stall at a higher airspeed, as shown in Figure 1-19.

Figure 1-19. Effect of angle of bank on stall speed
Some conditions that increase the weight (load) of an aircraft are: overloading the airplane, too steep an angle of bank, turbulence and abrupt movement of the controls.
Because different types of operations require different maneuvers (and therefore varying bank angles and load factors), aircraft are separated into categories determined by the loads that their wing structures can support:
|
Category |
Positive Limit Load |
|
Normal (nonacrobatic) (N) |
3.8 times gross weight |
|
Utility (normal operations and limited acrobatic maneuvers) |
4.4 times gross weight |
|
Acrobatic (A) |
6.0 times gross weight |
The limit loads should not be exceeded in actual operation, even though a safety factor of 50% above limit loads is incorporated into the strength of an airplane.
As the angle of attack is increased (to increase lift), the air will no longer flow smoothly over the upper wing surface, but instead will become turbulent or “burble” near the trailing edge. A further increase in the angle of attack will cause the turbulent area to expand forward. At an angle of attack of approximately 18° to 20° (for most wings), turbulence over the upper wing surface decreases lift so drastically that flight cannot be sustained and the wing stalls. See Figure 1-20.

Figure 1-20. Flow of air over wing at various angles of attack
The angle at which a stall occurs is called the critical angle of attack. An airplane can stall at any airspeed or any attitude, but will always stall at the same critical angle of attack. The indicated airspeed at which a given airplane will stall in a particular configuration, however, will remain the same regardless of altitude. Because air density decreases with an increase in altitude, the airplane has to be flown faster at higher altitudes to cause the same pressure difference between pitot impact pressure and static pressure. The recovery from a stall in any airplane becomes progressively more difficult as its center of gravity moves aft.
An aircraft will spin only after it has stalled, and will continue to spin as long as the outside wing continues to provide more lift than the inside wing, and the aircraft remains stalled.
Extending the flaps increases the wing camber and the angle of attack of a wing. This increases wing lift and also increases induced drag. The increased lift enables the pilot to make steeper approaches to a landing without an increase in airspeed. Spoilers, unlike flaps, do not change the wing camber. Their primary purpose is to decrease or “spoil” lift (increase drag) of the wing. See Figure 1-21.

Figure 1-21. Use of flaps increases lift and drag
It is desirable for a wing to stall in the root area before it stalls near the tips. This provides a warning of an impending stall and allows the ailerons to be effective during the stall. A rectangular wing stalls in the root area first. The stall begins near the tip and progresses inboard on a highly tapered wing or a wing with sweepback. See Figure 1-22.

Figure 1-22. Wing shapes
Aspect ratio is the ratio of the span of an aircraft wing to its mean, or average, chord. Generally speaking, the higher the aspect ratio, the more efficient the wing. But for practical purposes, structural considerations normally limit the aspect ratio for all except high-performance sailplanes. A high-aspect-ratio wing has a low stall speed, and at a constant air velocity and high angle of attack has less drag than a low-aspect-ratio wing.
In an airplane of standard configuration there is an inherent tendency for the airplane to turn to the left. This tendency, called torque, is a combination of the following four forces:
Spiraling slipstream is the only one addressed in this test. A spiraling slipstream is the reaction of the air to a rotating propeller, which forces the air to spiral in a clockwise direction around the fuselage. This tends to rotate the airplane right around the longitudinal axis.
Ground effect occurs when flying within one wingspan or less above the surface. The airflow around the wing and wing tips is modified and the resulting pattern reduces the downwash which reduces the induced drag. These changes can result in an aircraft either becoming airborne before reaching recommended takeoff speed or floating during an approach to land. See Figure 1-23.

Figure 1-23. Ground effect phenomenon
An airplane leaving ground effect after takeoff will require an increase in angle of attack to maintain the same lift coefficient, which in turn will cause an increase in induced drag and therefore, require increased thrust.
All aircraft leave behind two types of wake turbulence: prop or jet blast, and wing-tip vortices.
Prop or jet blast could be hazardous to light aircraft on the ground behind large aircraft that are either taxiing or running-up their engines. In the air, prop or jet blast dissipates rapidly.
Wing-tip vortices are a by-product of lift. When a wing is flown at a positive angle of attack, a pressure differential is created between the upper and lower wing surfaces, and the pressure above the wing will be lower than the pressure below the wing. In attempting to equalize the pressure, air moves outward, upward, and around the wing tip, setting up a vortex which trails behind each wing. See Figure 1-24.

Figure 1-24. Wing-tip vortices
The strength of a vortex is governed by the weight, speed, and the shape of the wing of the generating aircraft. Maximum vortex strength occurs when the generating aircraft is heavy, clean, and slow.
Vortices generated by large aircraft in flight tend to sink below the flight path of the generating aircraft. A pilot should fly at or above the larger aircraft’s flight path in order to avoid the wake turbulence created by the wing-tip vortices. See Figure 1-25.

Figure 1-25. Vortices in cruise flight
Close to the ground, vortices tend to move laterally. A crosswind will tend to hold the upwind vortex over the landing runway, while a tailwind may move the vortices of a preceding aircraft forward into the touchdown zone.
To avoid wake turbulence when landing, a pilot should note the point where a preceding large aircraft touched down and then land past that point. See Figure 1-26.

Figure 1-26. Touchdown and wake end
On takeoff, a pilot should lift off prior to reaching the rotation point of a preceding large aircraft; the flight path should then remain upwind and above the preceding aircraft’s flight path. See Figure 1-27.

Figure 1-27. Rotation and wake beginning
[10-2024]