A forward center of gravity increases stall speed because it induces a nose-down attitude, requiring additional downward force from the stabilizer to maintain the required en route cruise attitude.
Negative g force is the opposite to positive g force, representing the influence of the normal terrestrial environment above the force of gravity.
Exceeding the limiting weight of an aircraft results in reduced performance, increased takeoff and landing distances, decreased rate of climb and ceiling height, reduced range and endurance, increased stalling speed, decreased maneuverability, increased wear on tires and brakes, and reduced structural safety margins.
The center of gravity position moves due to changes in weight, which can occur from fuel burn, passenger movement, or high speeds.
A too aft position of the center of gravity can make the aircraft tail heavy, which may lead to insufficient turning moments generated by the horizontal tailplane, compromising stability and control during flight.
Weight is a factor of the stall speed; the heavier the aircraft, the higher its stall speed.
Spanwise airflow is created because a wing producing lift has a lower static pressure on the upper surface than on the lower surface, leading to air flowing around the wing tip to equalize pressure.
A swept wing reduces the effective chordwise velocity because it only responds to the velocity vector normal to the leading edge. This causes the wing to perceive that it is flying slower than its actual speed, thus delaying the onset of sonic airflow.
Fences and vortex generators direct the airflow over the wing’s upper surface perpendicular to the leading edge.
Coffin corner occurs at an aircraft’s absolute ceiling, where the speeds for Mach number buffet and prestall buffet coincide, making it difficult to distinguish between them, thus imposing a margin between the aircraft’s operating and absolute ceiling.
The speed margin in a jet aircraft is significant due to its relatively small margin between VMO/MMO and VDF/MDF, combined with low cruise drag and high engine power, leading to a distinct overspeed tendency.
Drag is the resistance to motion of an object (aircraft) through the air.
Speed stability is the behavior of the speed after a disturbance at a fixed power setting, defined as the ability of an aircraft’s speed to return naturally to its original speed after being disturbed.
An aft center of gravity allows the aircraft to achieve a nose-up attitude naturally, reducing drag and allowing for optimal thrust settings, thus maximizing range and performance.
The critical Mach number (M crit) is the speed at which the airflow over some part of the aircraft reaches the speed of sound.
Coefficient of lift (Cₗ) is the lifting ability of a particular wing, depending on the shape of the wing section and the angle of attack.
A decreased wing angle of attack results in lower induced drag, which enhances the aircraft's overall performance and range.
The center of gravity (C of G, CG) is the point through which the total weight of a body will act.
VIMD, or Minimum Induced Drag Speed, is the speed at which a jet aircraft experiences minimal drag. In jet aircraft, VIMD is higher due to the efficiency of swept wings against profile drag.
A swept wing stalls at the wing tip first due to higher aerodynamic loading, causing a pitch-up tendency as the stall progresses inboard.
Stalling characteristics can be improved by increasing the chamber at the tip and introducing washout or twist to lower the angle of incidence at the wing tips.
A forward center of gravity increases the stall speed due to the need for a balancing download from the horizontal tailplane, which effectively increases the aircraft's weight and reduces its maneuverability.
The glide range does not vary with weight if the aircraft is flown at its optimal angle of attack and speed for that weight, as the glide range is proportional to the lift-drag ratio.
A heavier aircraft flown at the correct angle of attack and speed will glide the same distance as a lighter aircraft but will have a higher airspeed.
Keeping the center of gravity within its limits ensures that the aircraft is not too nose heavy or tail heavy, allowing the horizontal tailplane to generate sufficient turning moments, maintaining pitch control, minimizing drag, and enhancing performance and maneuverability.
The horizontal tailplane helps balance the large lift-weight pitching moments caused by center of gravity movement, ensuring longitudinal stability and pitch controllability.
The center of pressure is a single point acting on the wing chord line at a right angle to the relative airflow, through which the wing’s lifting force is produced.
An aircraft stalls when the streamlined airflow over the wing’s upper surface breaks away when the critical angle of attack is exceeded, leading to turbulent airflow and a loss of lift.
A swept-wing aircraft has a wing-tip stalling tendency due to high local lift coefficient loading, which can lead to longitudinal instability if not corrected in the design.
To reduce spanwise airflow and thereby minimize its effects, increasing the effectiveness of the control surfaces, especially the ailerons.
Winglets are aerodynamic efficient surfaces located at the wing tips designed to reduce induced drag by preventing the intermixing of spanwise airflow from the upper and lower surfaces, which creates induced drag vortices.
Induced drag is caused by creating lift with a high angle of attack, which exposes more of the wing surface to the airflow.
The heavier the aircraft, the greater its rate of descent, as a heavy aircraft flies at a higher airspeed for a given angle of attack, increasing its rate of descent.
A Mach trimmer is a system that artificially corrects for Mach tuck above the aircraft’s M crit by sensing the aircraft’s speed and signaling a proportional upward movement of the elevator or variable-incidence stabilizer to maintain the aircraft’s pitch attitude throughout its speed range.
Momentum of a body is the product of the mass of the body and its velocity, enabling it to maintain its previous direction and magnitude for a time after an opposing force has been applied.
Less elevator availability for pitch control reduces the aircraft's maneuverability, making it harder to recover from a pitch-up stall attitude.
1. Creates wing-tip vortices. 2. Reduced aileron (wing control surface) efficiency. 3. Reversed spanwise airflow increases disturbed airflow on the wing’s upper surface at the tip, contributing to a wing-tip stall.
The advantages of a swept wing for jet aircraft include enabling high Mach cruise speeds and providing stability in turbulence. This design allows the aircraft to maximize the potential of its jet engines and reduces profile drag.
The factors affecting lift include configuration (flap setting), speed of airflow over the wing, angle of attack, and air density.
High lift devices are mechanisms like trailing edge flaps, leading edge flaps, and slots that increase the lift force produced by the wings.
Washout is a decrease in the angle of incidence from the wing root to the tip, compensating for early stall due to higher loading at the tips.
A speed margin is the difference between the aircraft’s normal maximum permitted operating speed and its higher certified testing speed, ensuring structural integrity and adequate handling qualities.
Lift is the phenomenon generated by an aerofoil due to pressure differences above and below the aerofoil, resulting in an upward lift force.
The formula for lift is ½ R V² S Cₗ, where R is half the value of air density, V² is airflow velocity squared, S is wing plan area, and Cₗ is the coefficient of lift.
Wing-tip vortices are created by spanwise airflow over the upper and lower surfaces of a wing that meet at the wing tips, inducing turbulence and drag due to pressure equalization at the tips.
Design considerations for a high-speed wing include thin, minimal-chamber, swept wings, with a focus on the requirements for economical high-speed performance, airfield performance, and structural integrity.
Speed instability occurs below minimum drag speed (VIMD) as the aircraft slides up the jet drag curve, where required power increases faster than lift, leading to a degradation in lift-drag ratio.
Maneuverability margin/envelope is contained by its upper and lower speed limits, defined either by the aircraft’s stall speed (VS) at the bottom and VDF/MDF speed at the top, or between 1.2/3 VS for safe operation above stall and VMO/MMO at the top.
Below Minimum Drag Speed (VIMD), speed is not stable, and a decrease in speed leads to an increase in drag, causing further decreases in speed. This steep increase in the drag curve is very noticeable.
When lift and weight are in equilibrium, an aircraft will maintain a steady, level attitude.
In a banked turn, lift is lost due to the effective reduction in wing span, requiring increased speed and/or angle of attack to restore lift.
Dihedral is the upward inclination of a wing from the root to the tip.
A stabilizer is an aerodynamic surface, typically located on the tail of an aircraft, that provides stability and control by balancing the aircraft's pitch and maintaining a desired angle of attack.
The heavier the aircraft, the earlier is its required descent point because it has greater momentum, necessitating a shallower rate of descent to check its momentum.
Positive g force is the influence of the force of gravity on the normal terrestrial environment, perceived as the normal weight of any body, equivalent to 1 g.
The lift force is the main force generated to balance the aircraft’s gross weight, enabling the aircraft to maintain level flight.
The drag curve on a jet aircraft is similar to that of a piston aircraft, consisting of induced drag, profile drag, and a VIMD speed, but with differences in the speed-to-drag relationship due to swept wings designed for high cruise speeds, resulting in poorer lift capabilities at low speeds.
The component arm is the distance from the datum to the point at which the weight of a component acts. A longer arm increases the moment created by the weight.
The stall speed increases with an increase in the aircraft’s actual or effective weight because more lift must be produced, and stall occurs at a constant angle of attack, requiring increased speed.
The tailplane moment is calculated as the stick force, which is equal to the arm multiplied by the weight, affecting the control responsiveness and stability of the aircraft.
Profile drag, also known as zero-lift drag, is comprised of form (pressure) drag, skin-friction drag, and interference drag, and it increases directly with speed.
Rate of climb/descent is the vertical component of the velocity of an aircraft and determines the time it will take to either climb or descend from a given height, usually expressed in feet per minute.
Poor lift qualities result in higher stall speeds for the swept-wing aircraft.
An aircraft’s stall speed is affected by weight, altitude, wing design/lift, configuration, and propeller engine power.
Lift is caused by a pressure difference above and below the wing, and the size of the difference determines the amount of lift produced.
The purpose of a Mach trimmer is to automatically compensate for Mach tuck, which is a form of longitudinal instability occurring above M crit.
For an aircraft to climb, lift must exceed the weight of the aircraft.
A maximum lift-drag ratio, obtained by flying at its optimal angle of attack and corresponding minimum drag speed (VIMD), produces an aircraft’s maximum glide range.
The mean chord line is the wing area divided by the wing span, sometimes referred to as the standard mean chord.
The center of gravity range relates to the furthest forward and aft center of gravity positions along the aircraft’s longitudinal axis, inside which the aircraft is permitted to fly, ensuring sufficient lift from the horizontal tailplane to maintain longitudinal stability and manageable pitch control.
The forward position of the center of gravity ensures that the aircraft is not too nose heavy, allowing the horizontal tailplane to generate a sufficient turning moment to overcome natural longitudinal stability and maintaining effective pitch control, particularly at low speeds.
The swept wing has poor lift qualities because the sweep-back design reduces the lift capabilities of the wing.
Induced drag is the drag that results from the generation of lift and is greatest at lower speeds due to high angles of attack required to maintain necessary lift. It reduces as speed increases because higher speeds allow for lower angles of incidence, leading to smaller wing-tip vortices.
To optimize the lift design on a swept wing, one needs to examine and develop the lift design areas of the clean wing and add high lift devices to ensure adequate airfield performance.
Characteristics include initial Mach buffet due to shock waves, increased drag leading to changes in stick force, a nose-down change in attitude (Mach tuck), and a possible loss of control.
A different wing design is necessary because straight-winged aircraft experience sonic disturbed airflow, leading to a loss of lift at relatively low speeds. The swept-wing design delays airflow over the wing from becoming supersonic, facilitating higher cruise speeds.
A center of gravity forward of the center of pitch results in a nose-down pitching moment, while a center of gravity aft of the center of pitch causes a nose-up pitching moment.
An aerofoil is a body that produces a large lift force compared to its drag when set at a small angle to a moving airstream, such as aircraft wings, tailplanes, rudders, and propellers.
If speed is disturbed, it is stable if it returns to its original speed due to drag changes; it is unstable if disturbances lead to further speed divergence.
Jet aircraft need a large center of gravity range because their center of gravity position can change significantly with large changes in weight during flight.
Fuel burn decreases the aircraft's weight, causing a shift in weight distribution and thus moving the center of gravity as fuel is consumed during flight.
Coffin Corner is the altitude limit of an aircraft where it is constrained by both low-speed buffet and high-speed buffet, as the stall IAS and Mach critical (M crit) values are equal, restricting the attainable altitude.
1. Create aircraft drag (induced drag because the vortices induce a downward velocity in the airflow over the wing, causing a change in the direction of the lift force). 2. Vortices create turbulence, affecting the safety of other aircraft within approximately 1000 ft below or behind. 3. Downwash affects the direction of the relative airflow over the tailplane, impacting the aircraft's longitudinal stability.
Longitudinal stability refers to the aircraft's ability to maintain its flight attitude and resist pitching movements, which is influenced by the position of the center of gravity and the aircraft's design.
If the center of gravity is outside its forward limit, the aircraft will be nose heavy, resulting in increased longitudinal stability, reduced pitch control due to high stick forces, and a need for a balancing download from the horizontal tailplane.
In straight-winged aircraft, as speed increases, profile drag increases due to the design not being optimized for high speeds. Conversely, induced drag is high at low speeds due to the increased angle of attack needed to achieve lift.
The Center of Pressure is not a fixed point but depends on the distribution of pressure along the chord, which changes with the angle of attack; it moves forward with a greater angle of attack and backward with a lower angle.
Anhedral is the downward inclination of a wing from the root to the tip.
Compressibility is the effect of air being compressed onto a surface, resulting in an increase in density and dynamic pressure above its expected value, directly associated with high speeds. It leads to compressibility error on dynamic pressure readings and a disturbed pressure pattern on the wing, causing shock-wave/drag effects at the critical Mach number.
Mach number (MN) is a true airspeed indication expressed as a percentage relative to the local speed of sound.
A change in thrust in straight and level flight can lead to a pitching tendency of the aircraft, such as a nose-up pitch when thrust is increased on an aircraft with engines mounted under the wing.
M crit is the aircraft’s Mach speed at which the airflow over a wing becomes sonic, indicating the point where subsonic aircraft experience a rapid rise in drag and loss of lift.
The pitching moment associated with the thrust-drag couple occurs when thrust and drag do not act through the same point, creating a moment that results in a nose-up or nose-down pitch, depending on whether thrust is above or below the drag line.
Mach tuck is the nose-down pitching moment an aircraft experiences as it passes its critical Mach number (M crit), resulting from the rearward movement of the center of pressure behind the center of gravity.
The center of pressure moves rearward on a swept wing due to shock waves occurring toward the leading edge and loss of lift inboard, resulting in predominant lift from the outboard part of the wing.
The mean chamberline is a line from the leading edge to the trailing edge that is equidistant on the upper and lower surfaces of an aerofoil.
Angle of attack is the angle between the chord line of an aerofoil and the relative airflow.
High-drag devices are components that increase the drag penalty on an aircraft, such as trailing edge flaps in a high-drag/low-lift position, spoilers, landing gear, reverse thrust, and braking parachutes.
The critical Mach number (M crit) is the speed at which airflow over the wing first becomes sonic. In swept-wing designs, M crit is increased because the wing is sensitive to the airflow's airspeed vector normal to the leading edge, allowing for higher speeds before reaching sonic conditions.
Minimum drag speed (VIMD) is the speed at which induced drag and profile drag values are equal, representing the lowest total drag penalty and the best lift-drag ratio for maximum aerodynamic efficiency and endurance.
The center of gravity moment is the turning effect of a weight around the datum, calculated as the product of the weight and the arm: Moment = weight × arm.
Wing slots are the main design feature that delays or suppresses stall speed by reenergizing the airflow to prevent it from separating over the wing.
Direct lift control refers to the function of the elevator and stabilizer, which create an upward or downward balancing force that controls the direct lift force from the wings, determining the aircraft's attitude around the lateral axis.
Lift-weight pitching moments occur when the forces of lift and weight do not act through the same point, creating a moment that causes either a nose-up or nose-down pitch depending on their relative positions to the center of gravity.
The main differences are: 1) A flatter total drag curve due to reduced profile and induced drag at higher speeds; 2) Minimal changes in flying qualities around VIMD, leading to less speed stability compared to piston aircraft; 3) A higher VIMD, as swept wings are more efficient against profile drag, resulting in a higher minimum drag speed.
The drag curve for a piston-engined propeller aircraft is characterized by a steep profile drag curve at high speeds and a well-defined induced drag curve at low speeds, with a notable increase in drag below the minimum drag speed (VIMD).
The pitching moment of the lift-weight couple is balanced by extra forces provided by the horizontal tailplane when it is not perfectly balanced.
When the center of gravity is outside its aft limit, the aircraft becomes tail heavy, leading to longitudinal instability, increased pitch control responsiveness, and a requirement for a balancing upload from the horizontal tailplane.
The two major types of drag are profile drag and induced drag.
Aspect Ratio is the ratio of the wing’s span to its geometric chord; a high aspect ratio indicates high lift (typical of gliders), while a low aspect ratio signifies lower lift but allows for higher speeds.
The chord line is a straight line from the leading edge to the trailing edge of an aerofoil.
The angle of incidence is the angle between the aerofoil’s chord line and the aircraft’s longitudinal datum, which is fixed for a wing but may be variable for a tailplane.
The forces acting on an aircraft in flight include drag, thrust, lift, and weight.
When thrust and drag are in equilibrium, an aircraft will maintain a steady speed.
For an aircraft to accelerate, thrust must exceed the value of drag.
