PHAK 5: Aerodynamics of Flight Flashcards
The four forces acting on an aircraft in straight-and-level,
unaccelerated flight are…
- Thrust
- Drag
- Lift
- Weight
What is thrust?
- Forward force produced by the powerplant/propeller or rotor.
- As a general rule, it acts parallel to the longitudinal axis. However, this is not always the case, as explained later.
What is drag?
- Rearward, retarding force caused by disruption of airflow by the wing, rotor, fuselage, and other protruding objects.
- As a general rule, drag opposes thrust and acts rearward parallel to the relative wind.
What is lift?
Force that is produced by the dynamic effect of the air acting on the airfoil, and acts perpendicular to the flight path through the CL and perpendicular to the lateral axis.
What is weight?
- Combined load of the aircraft itself, the crew, the fuel, and the cargo or baggage.
- Weight is a force that pulls the aircraft downward because of the force of gravity.
- It opposes lift and acts vertically downward through the aircraft’s CG.
In steady flight, what is the sum of thrust, drag, lift, and weight?
Zero
It does not mean the four forces are equal. It means the opposing forces are equal to, and thereby cancel, the effects of each other.
The sum of all upward components of forces (not just lift) equals the sum of all…
In straight, level, unaccelerated flight.
downward components of forces (not just weight)
The sum of all forward components of forces (not just thrust) equals the sum of all…
In straight, level, unaccelerated flight.
backward components of forces (not just drag)
In climbs and slow flight a portion of thrust is directed…
Upward and acts as if it were lift
In climbs and slow flight a portion of weight is directed…
Backward and acts as if it were drag
Relationship of forces acting on an aircraft.
What is 1?
Thrust
Relationship of forces acting on an aircraft.
What is 2?
Lift
Relationship of forces acting on an aircraft.
What is 3?
Drag
Relationship of forces acting on an aircraft.
What is 4?
Weight
Force vectors during a stabilized climb.
What is 1?
Flight path
Force vectors during a stabilized climb.
What is 2?
Lift
Force vectors during a stabilized climb.
What is 3?
Relative wind
Force vectors during a stabilized climb.
What is 4?
Thrust
Force vectors during a stabilized climb.
What is 5?
Center of lift (CL)
Force vectors during a stabilized climb.
What is 6?
Center of gravity (CG)
Force vectors during a stabilized climb.
What is 7?
Component of weight opposed to lift
Force vectors during a stabilized climb.
What is 8?
Rearward component of weight
Force vectors during a stabilized climb.
What is 9?
Drag
In glides, a portion of the weight vector is directed along the…
forward flight path and, therefore, acts as thrust.
Any time the flight path of the aircraft is not horizontal, lift, weight, thrust, and drag vectors…
must each be broken down into two components.
What does AOA stand for?
Angle of Attack
How is AOA defined?
The acute angle between the chord line of the airfoil and the direction of the relative wind.
If thrust is less than drag, what will happen to the aircraft’s speed?
It will decelerate.
If thrust is greater than drag, what will happen to the aircraft’s speed?
It will accelerate.
If thrust equals drag, what will happen to the aircraft’s speed?
It will remain constant.
Lift varies with the…(2)
- AOA
- Airspeed
What are the three categories for speed regimes of flight?
- Lowspeed flight
- Cruising flight
- High-speed flight
When the airspeed is low, the AOA must be relatively ____ if the balance between lift and weight is to be maintained.
high
What type of AOA is this?
Level cruise speed
What type of AOA is this?
Level high speed
What type of AOA is this?
Level low speed
What is CL‑MAX?
Critical Angle of Attack
When the aircraft reaches the maximum AOA, lift begins to diminish rapidly. This is the stalling AOA
What is CL?
Coefficient of Lift
What is the equation to find lift?
L = CL * (1/2) * ρ * V^2 * S
- L: Lift
- CL: Coefficient of Lift
- (1/2): The fraction 1/2, representing half
- ρ: Air density (can also be written as “rho” in plain text if special characters aren’t available)
- V^2: Velocity squared (using the caret ^ to denote an exponent)
- S: Wing surface area
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.
The above lift equation exemplifies this mathematically and supports that doubling of the airspeed will result in four times the lift. As a result, one can see that velocity is an
important component to the production of lift, which itself can be affected through varying AOA. When examining the equation, lift (L) is determined through the relationship of the air density (ρ), the airfoil velocity (V), the surface area of the wing (S) and the coefficient of lift (CL) for a given airfoil.
In order to maintain its lift at a higher altitude, an aircraft must fly at a greater…
true airspeed for any given AOA.
On a hot humid day, an aircraft must be flown at a ____ true airspeed for any given AOA than on a cool, dry day.
Greater
If the wings have the same proportion and airfoil sections, a wing with a planform area of 200 square feet lifts ____ at the same AOA as a wing with an area of 100 square feet.
twice as much
Two major aerodynamic factors from the pilot’s viewpoint are ______ and ______ because they can be controlled readily and accurately.
lift, airspeed
What is the equation to find drag?
D = CD * (1/2) * ρ * V^2 * S
- D: Drag force
- CD: Coefficient of drag
- (1/2): The fraction 1/2 (represents half in dynamic pressure)
- ρ: Air density (can also be written as “rho” in plain text if needed)
- V^2: Velocity squared (using ^ to denote an exponent)
- S: Reference area (e.g., wing area or frontal area)
What are the two basic types of drag?
- Parasite drag
- Induced drag
What is parasite drag?
It is the drag that is not associated with the production of lift. This 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.
What are the 3 types of parasite drag?
- Form drag
- Interference drag
- Skin friction
What is form drag?
The portion of parasite drag generated by the aircraft due to its shape and airflow around it.
Examples include the engine cowlings, antennas, and the aerodynamic shape of other components.
Which type of drag is easiest to reduce when designing an aircraft?
- Form drag.
- The solution is to streamline as many of the parts as possible.
What is interference drag?
Comes from the intersection of airstreams that creates eddy currents, turbulence, or restricts smooth airflow.
For example, the intersection of the wing and the fuselage at the wing root has significant interference drag. Air flowing around the fuselage collides with air flowing over the wing, merging into a current of air different from the two original currents.
When is the most interference drag observed?
When two surfaces meet at perpendicular angles.
What are some examples of interference drag?
- Wing-fuselage intersection
- Landing gear
- Tailplane-fuselage junction
- Engine nacelle and wing junction
- External fuel tanks, weapons, or pods
- Struts and braces (e.g., on high-wing aircraft)
- Wingtip devices (e.g., winglets)
What is skin friction drag?
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.
What is the free-stream velocity?
The speed of a fluid flow far from any object or boundary, where the flow is undisturbed by obstacles or viscosity effects.
How thick is the boundary layer?
About as wide as a playing card.
What are some ways to reduce skin friction drag? (4)
- Flush mount rivets.
- Remove any irregularities that may protrude above the wing surface.
- A smooth and glossy finish.
- Keep the surfaces of an aircraft clean and waxed.
What is induced drag?
A byproduct of lift caused by the downward deflection of airflow and wingtip vortices. It increases at low speeds and high angles of attack, making it most significant during takeoff, climb, and landing.
What happens to induced drag at higher airspeeds?
It’s reduced due to lower AOA.
What affect does the downwash over the top of the airfoil at the tip have on the lift vector?
- Bends it rearward. Therefore the lift is slightly aft of perpendicular to the relative wind, creating a rearward lift component.
- Induced drag.
What is gravity?
The pulling force that tends to draw all bodies to the center of the Earth.
What happens when the CG is forward of the CP?
There is a natural tendency for the aircraft to want to pitch nose down.
What happens when the CP is forward of the CG?
A nose up pitching moment is created.
What is an aircraft’s CG?
Center of Gravity
Considered as a point at which all the weight of the aircraft is concentrated. If the aircraft were supported at its exact CG, it would balance in any attitude.
Where is an aircraft’s weight concentrated?
The center of gravity.
Where is an aircraft’s lift concentrated?
The center of pressure.
What is a greater AOA’s effect on wingtip vortices and induced drag?
- Wingtip vortices increase.
- Induced drag increases.
During what phases of flight do aircraft produce the strongest wingtip vortices?
- Takeoff
- Climb
- Landing
The intensity or strength of wingtip vortices is ______ proportional to the weight of the aircraft and ______ proportional to the wingspan and speed of the aircraft.
directly, inversely
Wingtip vortices are greatest when the generating aircraft is
Heavy
Clean
Slow
When taking off, how can you minimize the chances of flying through an aircraft’s wake turbulence:
Rotate prior to the point at which the preceding aircraft rotated when taking off behind another aircraft.
When landing, how can you minimize the chances of flying through an aircraft’s wake turbulence:
Approach the runway above a preceding aircraft’s path when landing behind another aircraft and touch down after the point at which the other aircraft wheels contacted the runway.
When in the air, how can you minimize the chances of flying through an aircraft’s wake turbulence:
- Avoid flying through another aircraft’s flight path.
- Avoid following another aircraft on a similar flight path at an altitude within 1,000 feet.
How far should pilots of small aircraft avoid a hovering helicopter by?
At least three rotor disc diameters to avoid the effects of down wash.
How long is the margin of safety to avoid wake turbulence if a pilot is unsure of the other aircraft’s takeoff or landing point?
Approximately 3 minutes.
When the vortices of larger aircraft sink close to the ground (within 100 to 200 feet), they tend to move laterally over the ground at a speed of…
2 or 3 knots
A crosswind will decrease the lateral movement of the upwind vortex and increase the movement of the downwind vortex. Thus a light wind with a cross runway component of 1 to 5 knots could result in the upwind vortex…
Remaining in the touchdown zone for a period of time and hasten the drift of the downwind vortex toward another runway.
In the majority of cases, ground effect causes an ______ in the local pressure at the static source and produces a ______ indication of airspeed and altitude.
increase, lower
What is ground effect?
The reduction of induced drag and increased lift that occurs when an aircraft flies close to the ground (typically within one wingspan height). It results from disrupted wingtip vortices and altered airflow patterns.
An aircraft leaving ground effect will:
- Require an increase in AOA to maintain the same CL
- Experience an increase in induced drag and thrust required
- Experience a decrease in stability and a nose-up change in moment
- Experience a reduction in static source pressure and increase in indicated airspeed
An aircraft entering ground effect will:
- Require a decrease in AOA to maintain the same CL
- Experience a decrease in induced drag and thrust required
- Experience an increase in stability and a nose-up change in moment
- Experience an increase in static source pressure and decrease in indicated airspeed
The longitudinal axis
- Roll
- Extends through the aircraft from nose to tail
- Controlled by the ailerons
The vertical axis
- Yaw
- Passes through the aircraft vertically
- Controlled by the rudder
The lateral axis
- Pitch
- Passes parallel to a line from wingtip to wingtip
- Controlled by the elevators
Where do all the aircraft’s axes intersect?
The center of gravity
Moment
The product of the weight of an item multiplied by its arm. Moments are expressed in pound-inches (lb-in). Total moment is the weight of the airplane multiplied by the distance between the datum and the CG.
Moment arm
The distance from a datum to the applied force.
Datum (Reference Datum)
An imaginary vertical plane or line from which all measurements of arm are taken. The datum is established by the manufacturer. Once the datum has been selected, all moment arms and the location of CG range are measured from this point.
What does MAC stand for?
Mean aerodynamic chord
What is the mean aerodynamic chord?
The average distance from the leading edge to the trailing edge of the wing.
What is aircraft stability?
The inherent quality of an airplane to correct for conditions that may disturb its equilibrium, and to return or to continue on the original flight path. It is primarily an airplane design characteristic.
What are the two types of stability?
- Static
- Dynamic
What is static stability?
The initial tendency an aircraft displays when disturbed from a state of equilibrium.
What are the 3 sub types of static stability?
- Positive
- Neutral
- Negative
What is positive static stability?
The initial tendency to return to a state of equilibrium when disturbed from that state.
What is neutral static stability?
The initial tendency of an aircraft to remain in a new condition after its equilibrium has been disturbed.
What is negative static stability?
The initial tendency of an aircraft to continue away from the original state of equilibrium after being disturbed.
What is dynamic stability?
The property of an aircraft that causes it, when disturbed from straight-and-level flight, to develop forces or moments that restore the original condition over time, often through dampened oscillations.
What are the 3 sub types of dynamic stability?
- Positive
- Neutral
- Negative
What is positive dynamic stability?
Over time, the motion of the displaced object decreases in amplitude and, because it is positive, the object displaced returns toward the equilibrium state.
What is neutral dynamic stability?
Once displaced, the displaced object neither decreases not increases in amplitude.
A worn automobile shock absorber exhibits this tendency.
What is negative dynamic stability?
Over time, the motion of the displaced object increases and becomes more divergent.
What two areas of an aircraft are significantly affected by stability?
- Maneuverability
- Controllability
What is maneuverability?
Ability of an aircraft to change directions along a flight path and withstand the stresses imposed upon it.
What governs an aircraft’s maneuverability? (5)
- Weight
- Inertia
- Size and location of flight controls
- Structural strength
- Powerplant
What is controllability?
A measure of the response of an aircraft relative to the pilot’s flight control inputs.
The capability of an aircraft to respond to the pilot’s control, especially with regard to flight path and attitude. It is the quality of the aircraft’s response to the pilot’s control application when maneuvering the aircraft, regardless of its stability characteristics.
Which axis of stability is considered to be the most affected by certain variables in various flight conditions?
Longitudinal stability about the lateral axis.
Longitudinal Stability
Static longitudinal stability, or instability in an aircraft, is dependent upon which three factors?
- Location of the wing with respoect to CG.
- Location of the horizontal tail surfaces with respoect to the CG.
- Area or size of the tail surfaces.
Longitudinal Stability
The CL tends to move ______ with an increase in AOA and to move ______ with a decrease in AOA.
forward, aft
CL is also known as what?
CP
Center of Pressure
Longitudinal Stability
Why is a slight downward force on the horizontal stabilizer necessary in most aircraft?
To counteract the “nose-heavy” design caused by the wing’s center of lift (CL) being behind the center of gravity (CG), keeping the aircraft balanced.
Longitudinal Stability
How does the horizontal stabilizer balance the aircraft?
By being set at a slight negative angle of attack, it creates a downward force (at point T) to counterbalance the heavy nose (at the CG).
Longitudinal Stability
What effect does aircraft speed have on downwash over the horizontal stabilzer?
- Cruise speed - Balanced tail load
- Low speed - Lesser downward tail load
- High speed - Greater downward tail load
Longitudinal Stability
What is the effect of lesser downwash on the horizontal stabilzer at lower speeds?
Lowering of the nose.
Longitudinal Stability
How does thrust line position affect longitudinal stability under power?
- Below center of gravity: Nose up attitude
- Through center of gravity: Straight and level
- Above center of gravity: Nose down attitude
Longitudinal Stability
How do power adjustments affect longitudinal stability?
- Cruise power: Straight and level
- Idle power: Nose down attitude
- Full power: Nose up attitude
Longitudinal Stability
What is longitudinal stability?
- Pitching
- Stability about the lateral axis. A desirable characteristic of an airplane whereby it tends to return to its trimmed angle of attack after displacement.
Lateral Stability
What is lateral stability?
- Rolling
- The stability about the longitudinal axis of an aircraft. Rolling stability or the ability of an airplane to return to level flight due to a disturbance that causes one of the wings to drop.
Lateral Stability
Which 4 main design factors make an aircraft laterally stable?
- Dihedral
- Sweepback
- Keep effect
- Weight distribution
Lateral Stability
What is dihedral?
The positive acute angle between the lateral axis of an airplane and a line through the center of a wing or horizontal stabilizer. Dihedral contributes to the lateral stability of an airplane.
Lateral Stability
How does a dihedral wing act in a gust of wind from the side?
When a dihedral wing encounters a gust of wind from the side, the aircraft rolls slightly into the wind, causing a sideslip. The lower wing, which is more into the wind, experiences a greater angle of attack (AOA), generating more lift. This increased lift counteracts the rolling moment caused by the gust, stabilizing the aircraft.
Lateral Stability
What is a sweptback wing?
A wing in which the leading edge slopes backward.
Lateral Stability
How does a sweptback wing help with lateral stability?
When a disturbance causes an aircraft with sweepback to slip or drop a wing, the low wing presents its leading edge at an angle that is more perpendicular to the relative airflow. As a result, the low wing acquires more lift, rises, and the aircraft is restored to its original flight attitude.
Lateral Stability
Roughly estimated, how much effective dihedral is provided by 10° of sweepback?
About 1°
Lateral Stability
How much effective dihedral can a high wing configuration provide over a low wing?
About 5°
Lateral Stability
What is the keel effect?
The stabilizing tendency of a high-wing aircraft caused by the wings being attached in a high position on the fuselage, making the fuselage behave like a keel exerting a steadying influence on the aircraft laterally about the longitudinal axis.
Lateral Stability
Laterally stable aircraft are constructed so that the greater portion of the keel area is above the ______.
CG
Directional Stability
What is directional stability in an aircraft?
Stability about the vertical axis (yaw) that helps keep the aircraft’s nose pointed into the relative wind, primarily influenced by the vertical fin and fuselage design.
Which axial stability is the most easily achieved?
Directional, yawing stability.
Directional Stability
What are the two prime contributors to directional stability?
The area of the vertical fin and the sides of the fuselage aft of the CG.
Directional Stability
How is positive directional stability is achieved in design?
By making the side surface aft of the CG greater than ahead of the CG.
Directional Stability
How does the vertical fin contribute to directional stability?
It acts like the feather on an arrow or a weather vane, creating a restoring force that resists yaw and aligns the aircraft with the relative wind.
Directional Stability
How does wing sweepback improve directional stability?
Sweepback moves the center of pressure aft relative to the center of gravity, improving stability by correcting aerodynamic imbalances.
Directional Stability
How does drag help correct yaw in an aircraft?
When the aircraft yaws, the forward wing creates more drag due to a longer perpendicular leading edge. This drag pulls the wing back, helping return the aircraft to its original path.
Directional Stability
What happens when an aircraft skids sideways during yaw?
The vertical fin creates a restoring force by generating pressure on the side opposite to the yaw, reducing the skidding motion and stabilizing the aircraft.
What are free directional oscillations AKA dutch roll?
A coupled lateral and directional oscillation involving roll and yaw that is usually dynamically stable but oscillatory, causing the nose to trace a figure-eight pattern on the horizon.
What causes Dutch roll in an aircraft?
Dutch roll occurs when lateral (roll) and directional (yaw) oscillations, often triggered by a sideslip, are out of phase with each other.
How do modern aircraft counteract Dutch roll?
Aircraft prone to Dutch roll, like high-speed swept-wing designs, are equipped with gyro-stabilized yaw dampers to suppress the oscillations.
Why are modern aircraft designed with spiral instability instead of Dutch roll tendencies?
Spiral instability is easier to control and less disruptive than the oscillatory nature of Dutch roll.
What is spiral instability in an aircraft?
It occurs when strong directional stability overpowers lateral stability, leading to an increasing bank angle and a downward spiral if not corrected by the pilot.
What causes spiral instability in an aircraft?
A disturbance (e.g., a gust of air) causes a sideslip, where strong directional stability yaws the aircraft into the relative wind while weak dihedral fails to restore lateral balance, resulting in overbanking.
What are the risks of spiral instability if not corrected?
It can lead to rapid airspeed buildup, excessive load factors, structural failure, and potentially fatal crashes.
What must pilots avoid when recovering from spiral instability?
Avoid excessive back elevator force, which increases the load factor and tightens the turn, worsening the spiral.
How is spiral instability mitigated in modern aircraft?
With control devices like wing levelers and pilot intervention to counteract the gradual divergence of the spiral motion.
Effect of Wing Planform
What is wing planform?
The shape of the wing as viewed from directly above and deals with airflow in three dimensions.
Effect of Wing Planform
What are the 3 dimensions of wing planform?
- Aspect ratio
- Taper ratio
- Sweepback
Effect of Wing Planform
What is wing aspect ratio?
The ratio of wing span to wing chord.
Effect of Wing Planform
How does aspect ratio affect wing performance?
- High aspect ratio: Less drag, better climb performance, but heavier wings.
- Low aspect ratio: More drag, higher stall speeds, and better strength for high-speed flight.
Effect of Wing Planform
What is wing taper ratio?
A decrease from wing root to wingtip in wing chord or wing thickness.
Effect of Wing Planform
How does tapering the wing affect performance?
Tapering reduces drag, increases lift, and saves structural weight, particularly beneficial at high speeds.
Effect of Wing Planform
What is wing sweepback?
The rearward slant of a wing, horizontal tail, or other airfoil surface.
Effect of Wing Planform
What is the purpose of sweepback?
Sweepback reduces drag at high speeds and improves aerodynamic cleanness but requires more precise flying techniques, especially at low speeds.
Effect of Wing Planform
What is the primary factor in determining the three dimensional characteristics of the ordinary wing and its lift/drag ratio?
Aspect ratio
Effect of Wing Planform
What type of wing planform is this?
Elliptical Wing
Effect of Wing Planform
What type of wing planform is this?
High Taper Wing
Effect of Wing Planform
What type of wing planform is this?
Moderate Taper Wing
Effect of Wing Planform
What type of wing planform is this?
Pointed Tip Wing
Effect of Wing Planform
What type of wing planform is this?
Regular Wing
Effect of Wing Planform
What type of wing planform is this?
Sweepback Wing
Effect of Wing Planform
What are the two main ways a designer can change the planform of a wing?
- Aspect ratio
- Taper ratio
Effect of Wing Planform
What are the advantages and disadvantages of elliptical wings?
- Advantage: Minimum induced drag and high lift coefficients.
- Disadvantage: Poor stall warning and aileron effectiveness, and harder to construct.
Effect of Wing Planform
Why are rectangular wings favored for training aircraft?
They provide good stall warning, stable flight, and aileron effectiveness, making them forgiving and cost-effective.
Aerodynamic Forces in Flight Maneuvers
What happens to lift during a turn?
Lift is divided into two components: the vertical component opposes weight, and the horizontal component (centripetal force) pulls the aircraft into the turn.
Aerodynamic Forces in Flight Maneuvers
How does bank angle affect lift during a turn?
As the bank angle increases, the vertical lift decreases, requiring an increase in angle of attack (AOA) to maintain altitude.
Aerodynamic Forces in Flight Maneuvers
What is the role of the rudder in a turn?
The rudder aligns the nose with the relative wind, preventing yaw and ensuring the nose and tail track the same path.
Aerodynamic Forces in Flight Maneuvers
What is the difference between a slipping and a skidding turn?
- Slipping Turn: The rate of turn (ROT) is too low for the bank angle; the aircraft yaws to the outside.
- Skidding Turn: The ROT is too high for the bank angle; the aircraft is pulled toward the outside of the turn.
Aerodynamic Forces in Flight Maneuvers
How does airspeed affect the turn radius?
Higher airspeed increases the turn radius, requiring a greater bank angle to maintain a given rate of turn.
Aerodynamic Forces in Flight Maneuvers
Why does induced drag increase in a turn?
Induced drag increases because the angle of attack (AOA) must be increased to maintain the vertical lift component, causing additional drag.
Aerodynamic Forces in Flight Maneuvers
What balances centrifugal force in a turn?
The horizontal component of lift balances centrifugal force, keeping the aircraft stable in a turn.
Aerodynamic Forces in Flight Maneuvers
What does ROT stand for?
Rate of Turn
Aerodynamic Forces in Flight Maneuvers
How does lift in a steady climb compare to level flight?
In a steady climb, lift is approximately the same as in level flight at the same airspeed after the climb path is stabilized.
Aerodynamic Forces in Flight Maneuvers
Why does total drag increase in a climb?
A portion of the aircraft’s weight acts rearward along with drag, increasing total effective drag.
Aerodynamic Forces in Flight Maneuvers
What happens to airspeed if no additional power is applied in a climb?
Airspeed decreases because thrust is insufficient to counteract the increased drag and rearward weight component.
Aerodynamic Forces in Flight Maneuvers
How does thrust required in a climb differ from level flight?
Thrust in a climb must equal drag plus a portion of weight proportional to the climb angle.
Aerodynamic Forces in Flight Maneuvers
What is an aircraft’s absolute ceiling?
The altitude where excess thrust is zero, preventing further climb.
Aerodynamic Forces in Flight Maneuvers
What happens to lift when entering a descent?
The angle of attack (AOA) decreases momentarily, reducing lift, which allows weight to exceed lift and initiate the descent.
Aerodynamic Forces in Flight Maneuvers
What must be done to maintain the same airspeed during a descent?
Power must be reduced to counteract the increased forward weight component along the flight path.
Aerodynamic Forces in Flight Maneuvers
How does the weight component affect a descent?
As the angle of descent increases, the forward component of weight increases, aiding the descent.
Stalls
What causes an aircraft stall?
A stall occurs when the critical angle of attack (AOA) is exceeded, causing a rapid decrease in lift due to airflow separation over the wing.
Stalls
What three flight situations is the critical AOA most often exceeded?
- Low speed
- High speed
- Turning
Stalls
How can operating with a CG that exceeds the rear limit affect stalls?
This can make a stall unrecoverable because the nose down elevator force can’t counteract the excess weight of the aft CG.
Stalls
Does a wing stop producing lift during a stall?
No, the wing still produces some lift but not enough to sustain level flight.
Stalls
At what angle does a stall occur?
The stall occurs at a specific critical AOA, typically between 16° and 20°, regardless of airspeed, weight, or load factor.
Stalls
Why do some wings stall at the root first?
To maintain aileron effectiveness at the wingtips, ensuring better controllability during the stall.
Stalls
Does stalling speed change in different situations?
Yes, stalling speed increases with higher load factors, such as in a turn or with added weight, but the critical AOA remains constant.
Stalls
How does the aircraft recover from a stall?
The nose pitches down, reducing the AOA, increasing airspeed, and restoring smooth airflow over the wings.
Stalls
How does ice affect stalls?
Ice disrupts smooth airflow, causing stalls at lower AOAs and reducing lift by up to 25%, while also increasing drag and weight.
Stalls
Can a stall occur at high speed?
Yes, pulling back sharply on the elevator at high speed can abruptly increase the AOA beyond the critical value, causing a stall.
Stalls
How does ice affect lift?
As little as 0.8 mm of ice on the wing surface can reduce lift by 25% due to disrupted airflow.
Stalls
How does ice affect the critical angle of attack (AOA)?
Ice causes the boundary layer to separate at a lower AOA, making stalls occur earlier than expected.
Stalls
What are the combined effects of ice on an aircraft?
Ice increases weight, reduces lift, and increases drag, significantly degrading aircraft performance.
Stalls
Why are small aircraft more vulnerable to icing?
They fly at lower altitudes where icing is more common and often lack de-icing systems like those on jet aircraft.
Stalls
When and where can icing occur?
- Altitudes of up to 18,000’ and sometimes higher.
- Icing can occur in clouds when temperatures are below freezing and super-cooled droplets freeze on the aircraft.
Super-cooled droplets are still liquid even though the temperature is below 32 °F or 0 °C.
Angle of Attack Indicators
What is the primary purpose of an AOA indicator?
To provide situational awareness about the margin between the current AOA and the critical AOA, helping prevent stalls and loss of control (LOC).
Angle of Attack Indicators
What are the benefits of AOA indicators in general aviation?
Increased situational awareness, better energy management, improved stall margin awareness, and enhanced flight efficiency.
Angle of Attack Indicators
Why is AOA better than airspeed for avoiding stalls?
The critical AOA is constant for a given configuration, whereas stall speed varies with weight, bank angle, and other factors.
Angle of Attack Indicators
What is required for effective use of an AOA indicator?
Comprehensive training on AOA concepts and the specific operating characteristics and limitations of the installed system.
Angle of Attack Indicators
How has the FAA streamlined AOA indicator installations?
In 2014, the FAA allowed non-required AOA systems to be installed as minor alterations under specific guidelines.
Angle of Attack Indicators
What are some limitations of AOA indicators? (5)
- Calibration techniques
- Unheated probes
- Indicator type
- Flap settings
- Wing contamination
Angle of Attack Indicators
How do AOA indicators help reduce LOC accidents?
By providing real-time AOA feedback, they help pilots avoid stalls and maintain control during maneuvering flight, where LOC accidents are most common.
Angle of Attack Indicators
What 5 things do not change the critical AOA?
- Weight
- Bank Angle
- Temperature
- Density altitude
- Center of gravity
Basic Propeller Principles
What is the primary function of a propeller?
A propeller transforms the engine’s rotary power into forward thrust by acting as a rotating airfoil.
Basic Propeller Principles
What is the blade angle of a propeller?
The angle between the propeller blade’s chord line and the plane of rotation, influencing thrust and efficiency.
Basic Propeller Principles
What is a fixed-pitch propeller optimized for?
Fixed-pitch propellers are optimized for a specific flight condition, such as takeoff, climb, or cruise, and cannot be adjusted in flight.
Basic Propeller Principles
How does a constant-speed propeller work?
It adjusts blade angle automatically to maintain an efficient angle of attack (AOA) across different flight conditions.
Basic Propeller Principles
What determines propeller efficiency?
The ratio of thrust horsepower to brake horsepower, typically ranging from 50% to 87%, depending on slippage.
Basic Propeller Principles
Why are propeller blades twisted?
To maintain a consistent angle of attack along the blade, as the outer parts travel faster than the inner parts.
Basic Propeller Principles
What is the difference between geometric and effective pitch?
Geometric pitch is the theoretical distance the propeller would advance in one revolution without slippage, while effective pitch includes actual slippage in the air.
Basic Propeller Principles
How does a propeller generate thrust?
Thrust is created by the blade’s cambered shape and angle of attack, causing lower pressure in front of the blade and higher pressure behind.
Basic Propeller Principles
Why is a low blade angle used during takeoff?
It allows high engine RPM, maximizing thrust at low airspeeds for efficient acceleration.
Basic Propeller Principles
How does propeller slippage affect efficiency?
Greater slippage reduces efficiency by increasing the difference between geometric and effective pitch.
Basic Propeller Principles
What is the most efficient AOA for a propeller?
Varying from +2° to +4°.
Basic Propeller Principles
A propeller designated as a “74–48” would be…
- 74 inches in length
- Have a pitch of 48 inches
Basic Propeller Principles
Usually 1° to 4° provides the most efficient lift/drag ratio, but in flight the propeller AOA of a fixed pitch propeller varies—normally from…
0° to 15°
Torque and P-Factor
What are the four elements that cause the left turning tendency of the airplane?
- Torque reaction from engine and propeller
- Corkscrewing effect of the slipstream
- Gyroscopic action of the propeller
- Asymmetric loading of the propeller (P-factor)
Torque and P-Factor: Torque Reaction
What is torque reaction in an aircraft?
It is the tendency of an aircraft to rotate in the opposite direction of the engine and propeller due to Newton’s Third Law of Motion.
Torque and P-Factor: Torque Reaction
How does torque reaction affect an aircraft in flight?
It causes a rolling tendency around the longitudinal axis, typically counteracted by design features like engine offset or rigging.
Torque and P-Factor: Torque Reaction
How does torque reaction affect the aircraft during the takeoff roll?
It increases weight and drag on the left landing gear (for clockwise-turning propellers), creating a yawing moment to the left.
Torque and P-Factor: Torque Reaction
What factors influence the magnitude of torque reaction?
Engine size and horsepower, propeller size and RPM, aircraft size, and ground surface conditions.
Torque and P-Factor: Torque Reaction
How is torque reaction corrected during the takeoff roll?
By using rudder or rudder trim to counteract the yawing moment.
Torque and P-Factor: Corkscrew Effect
What is the corkscrew effect in aviation?
It is the spiraling rotation of the propeller’s slipstream that exerts sideward force on the vertical tail, causing a yawing moment around the vertical axis.
Torque and P-Factor: Corkscrew Effect
When is the corkscrew effect most prominent?
During high propeller speeds and low forward speeds, such as during takeoff or power-on stalls.
Torque and P-Factor: Corkscrew Effect
What are the two moments caused by the corkscrew effect?
A rolling moment around the longitudinal axis (typically to the right) and a yawing moment around the vertical axis.
Torque and P-Factor: Corkscrew Effect
How does the corkscrew effect interact with torque reaction?
The rolling moment from the corkscrew effect (right) can counteract the yawing moment from torque reaction (left), but the pilot must apply corrective action as needed.
Torque and P-Factor: Gyroscopic Action
What is gyroscopic precession?
The deflection of a spinning rotor when a force is applied, with the resulting force acting 90° ahead of the applied force in the direction of rotation.
Torque and P-Factor: Gyroscopic Action
Why does a propeller behave like a gyroscope?
A spinning propeller has gyroscopic properties, including precession, which can cause pitching, yawing, or a combination of both when forces act on the propeller’s plane of rotation.
Torque and P-Factor: Gyroscopic Action
Why is gyroscopic action more prominent in tailwheel aircraft?
Raising the tail during takeoff applies a force to the top of the propeller’s rotation, causing a yawing moment to the left due to precession.
Torque and P-Factor: Gyroscopic Action
How are yawing and pitching moments related in gyroscopic action?
Any yawing motion causes a pitching moment, and any pitching motion causes a yawing moment due to gyroscopic precession.
Torque and P-Factor: Gyroscopic Action
How do pilots correct for gyroscopic action?
By properly using the elevator and rudder to counteract undesired yawing or pitching moments caused by precession.
Torque and P-Factor: Asymmetric Loading (P-Factor)
What is P-Factor (asymmetric loading)?
The tendency for an aircraft to yaw left due to the downward-moving propeller blade creating more thrust than the upward-moving blade at high angles of attack (AOA).
Torque and P-Factor: Asymmetric Loading (P-Factor)
What causes P-Factor?
The combination of the propeller blade’s rotational velocity and the aircraft’s forward velocity results in higher resultant velocity on the downward-moving blade, increasing thrust on that side.
Torque and P-Factor: Asymmetric Loading (P-Factor)
How does P-Factor affect the center of thrust?
The center of thrust shifts to the right of the propeller disc area (from the pilot’s perspective), causing the aircraft to yaw left.
Torque and P-Factor: Asymmetric Loading (P-Factor)
How can P-Factor be visualized?
Imagine a vertically mounted propeller; the blade moving into the airflow generates more lift than the blade retreating with the airflow, shifting thrust toward the forward-moving blade.
Torque and P-Factor: Asymmetric Loading (P-Factor)
How do pilots correct for P-Factor?
By applying right rudder to counteract the left yaw caused by the asymmetrical thrust distribution.
Load Factors
What is a load factor in aviation?
The proportion between lift and weight, expressed in Gs, representing the force acting on the aircraft’s structure during acceleration or maneuvering.
Load Factors
What does 3 Gs mean for an aircraft?
A load factor of 3 Gs means the total force on the aircraft’s structure is three times its weight, and the pilot experiences three times their body weight in force.
Load Factors
Why are load factors important for pilots?
They help pilots avoid dangerous overloads on the aircraft’s structure and understand how increased load factors raise stalling speeds.
Load Factors
Why are load factors a concern in modern aircraft?
Higher speeds increase the potential for larger load factors, making structural integrity a critical design and operational consideration.
Load Factors
How do load factors affect stall speed?
Increased load factors raise the stall speed, making stalls possible at higher-than-expected flight speeds.
Load Factors: Load Factors in Aircraft Design
What is a limit load factor?
The maximum load factor an aircraft can withstand during normal operation without structural damage.
Load Factors: Load Factors in Aircraft Design
What is the “factor of safety” in aircraft design?
Aircraft must withstand 1.5 times the limit load factor without structural failure to account for unexpected conditions.
Load Factors: Load Factors in Aircraft Design
What are the limit load factors for different aircraft categories?
- Normal: +3.8 to -1.52
- Utility: +4.4 to -1.76
- Acrobatic: +6.0 to -3.00
Load Factors: Load Factors in Aircraft Design
How are gust load factors accounted for in aircraft design?
Gust load factors are consistent across general aviation aircraft to ensure safety during rough air, with pilots expected to reduce speed in turbulence.
Load Factors: Load Factors in Aircraft Design
How do maneuvering load factors differ by category?
Aircraft in more severe operational categories (e.g., acrobatic) are designed to handle higher load factors for demanding maneuvers.
Load Factors: Load Factors in Aircraft Design
How do load factors for older aircraft differ?
Older designs lack specific category placards but generally align with utility category standards for aircraft under 4,000 pounds.
Load Factors: Load Factors in Aircraft Design
How should pilots approach load factors during flight?
Pilots must avoid willfully exceeding load limits and use the safety factor as a reserve for unexpected conditions.
Load Factors: Load Factors in Steep Turns
What forces contribute to the load factor in a coordinated turn?
Centrifugal force and the aircraft’s weight.
Load Factors: Load Factors in Steep Turns
What is the load factor in a coordinated 60° banked turn?
2 Gs, meaning the wings must produce twice the lift to maintain altitude.
Load Factors: Load Factors in Steep Turns
How does load factor change as bank angle increases?
The load factor increases rapidly beyond 45° to 50°, exceeding 5.76 Gs at an 80° bank.
Load Factors: Load Factors in Steep Turns
Why can’t a 90° banked turn maintain constant altitude?
A 90° bank would require infinite lift, which is mathematically and physically impossible.
Load Factors: Load Factors in Steep Turns
What is the practical maximum bank angle for general aviation aircraft in coordinated turns?
60°, as additional bank angles approach structural limits.
Load Factors: Load Factors and Stalling Speeds
How does load factor affect stalling speed?
Stalling speed increases with the square root of the load factor; e.g., at 4 Gs, a stall occurs at double the normal stalling speed.
Load Factors: Load Factors and Stalling Speeds
What are the risks of high load factors in flight?
They can cause inadvertent stalls, especially in steep turns or abrupt maneuvers, and impose tremendous structural loads during high-speed stalls.
Load Factors: Load Factors and Stalling Speeds
What is design maneuvering speed (VA)?
The maximum speed at which you can apply full control deflection in one axis (pitch, roll, or yaw) without risking structural damage, in smooth air.
Load Factors: Load Factors and Stalling Speeds
How is VA estimated in older aircraft?
Approximately 1.7 times the normal stalling speed; e.g., an airplane with a 60-knot stall speed has a VA of about 102 knots.
Load Factors: Load Factors and Stalling Speeds
What load factor is imposed at a stall speed 1.7 times higher than normal?
2.89 Gs, as the load factor is the square of the speed increase ratio (1.7 × 1.7 = 2.89).
Load Factors: Load Factors and Stalling Speeds
How can pilots estimate load factors without instruments?
By developing a feel for seat pressure during maneuvers, as accelerometers are uncommon in general aviation training aircraft.
Load Factors: Load Factors and Stalling Speeds
What types of accidents can be prevented by understanding load factors and VA?
- Stalls from steep turns or excessive maneuvering near the ground.
- Structural failures from violent maneuvers or loss of control.
Load Factors: Load Factors and Flight Maneuvers
What is the load factor during straight, unaccelerated flight?
1 G
Load Factors: Load Factors and Flight Maneuvers
How does load factor change with bank angle?
Load factors increase significantly beyond a 45° bank and reach approximately 3 Gs at a 72° bank.
Load Factors: Load Factors and Flight Maneuvers
How do load factors affect stall recovery?
Abrupt pull-ups after a stall recovery can impose high load factors, potentially causing structural stress or secondary stalls.
Load Factors: Load Factors and Flight Maneuvers
What load factor is typical during a stabilized spin?
Slightly above 1 G due to low airspeeds and pivoting motion.
Load Factors: Load Factors and Flight Maneuvers
What causes high-speed stalls?
Added load factors, such as abrupt elevator inputs, can induce stalls at higher airspeeds, imposing significant structural loads.
Load Factors: Load Factors and Flight Maneuvers
How can load factors be minimized in chandelles and lazy eights?
Perform smooth pull-ups with moderate load factors (less than 2 Gs) to improve performance and altitude gain.
Load Factors: Load Factors and Flight Maneuvers
Why is maneuvering speed important in rough air?
Staying at or below maneuvering speed ensures the aircraft stalls before exceeding structural load limits caused by gusts.
Load Factors: Load Factors and Flight Maneuvers
What is the difference between limit load and ultimate load?
- Limit Load: Maximum force the aircraft can sustain without permanent deformation.
- Ultimate Load: Force beyond the limit load that causes structural failure.
Load Factors: Load Factors and Flight Maneuvers
How should pilots manage speed in rough air?
Reduce airspeed to the design maneuvering speed to avoid exceeding load limits from gusts.
Load Factors: Vg Diagram
What does a Vg diagram represent?
The Vg diagram shows the relationship between airspeed (velocity) and load factor (G loads), defining the aircraft’s operational envelope.
Load Factors: Vg Diagram
What are the key points on a Vg diagram?
- Maximum positive and negative lift capability.
- Limit load factors (positive and negative).
- Maneuvering speed (Va).
- Redline speed (VNE).
Load Factors: Vg Diagram
What happens if the aircraft exceeds the line of maximum lift capability?
The aircraft stalls and cannot generate higher aerodynamic load factors.
Load Factors: Vg Diagram
Why is maneuvering speed important? (VA)
Below maneuvering speed, the aircraft cannot generate damaging flight loads, even during gusts or abrupt maneuvers.
Load Factors: Vg Diagram
What is the significance of the redline speed? (VNE)
It is the maximum airspeed the aircraft can safely fly; exceeding it risks structural damage or failure.
Load Factors: Vg Diagram
What is the structural envelope of an aircraft?
The combination of airspeeds and load factors that prevent structural damage and ensure the aircraft’s service life.
Load Factors: Vg Diagram
How does negative lift differ from positive lift on the Vg diagram?
Higher speeds are required to produce the same magnitude of negative load factors compared to positive load factors.
Load Factors: Vg Diagram
Why must pilots understand the Vg diagram?
To ensure safe operation within the aircraft’s structural envelope and avoid maneuvers or gusts that exceed limits.
Load Factors: Rate of Turn
What is rate of turn (ROT)?
The number of degrees of heading change per second an aircraft makes, expressed in degrees per second.
Load Factors: Rate of Turn
How is ROT calculated?
ROT = (1,091 × tangent of bank angle) ÷ true airspeed (TAS).
Load Factors: Rate of Turn
How does airspeed affect rate of turn?
At a constant bank angle, increasing airspeed decreases the ROT, and decreasing airspeed increases the ROT.
Load Factors: Rate of Turn
How does bank angle affect ROT?
At a constant airspeed, increasing the bank angle increases the ROT, and decreasing the bank angle reduces the ROT.
Load Factors: Rate of Turn
How long does it take to complete a 360° turn at 120 knots TAS with a 30° bank?
68.6 seconds (ROT = 5.25°/second)
Load Factors: Rate of Turn
Why is understanding ROT important?
It allows pilots to calculate the time and distance needed to complete a turn, aiding in navigation and maneuver planning.
Load Factors: Radius of Turn
What is the radius of turn?
The horizontal distance from the center of the turn to the aircraft’s flight path, determined by airspeed and bank angle.
Load Factors: Radius of Turn
What is the formula for the radius of turn?
R = (V^2) / (11.26 × tan(bank angle)),
where R is the radius, V is the true airspeed, and the bank angle is in degrees.
Load Factors: Radius of Turn
How does airspeed affect the radius of turn?
Doubling airspeed quadruples the turn radius, requiring a steeper bank angle to maintain the same radius.
Load Factors: Radius of Turn
What is the radius of a 360° turn at 120 knots with a 30° bank?
Using the formula, the radius is approximately 2,214 feet.
Load Factors: Radius of Turn
How are ROT and radius of turn related?
A higher ROT results in a tighter turn radius, while a lower ROT increases the turn radius.
Load Factors: Radius of Turn
Why is understanding the radius of turn important?
It helps pilots safely navigate tight spaces, like canyons, and avoid obstacles during turns.
Weight and Balance
Why is weight and balance data important for pilots?
It ensures the aircraft is within safe weight and center of gravity (CG) limits, which affect stability, controllability, and performance.
Weight and Balance
When should weight and balance data be updated?
After equipment changes, modifications, or any changes in load distribution.
Weight and Balance
What can happen if the CG is outside the allowable range?
It can cause instability, difficulty in stall and spin recovery, and reduced aircraft performance.
Weight and Balance
Why is it critical to compute weight and balance for each flight?
Passenger weights, baggage, and fuel loads vary and can impact CG and exceed maximum gross weight.
Weight and Balance
Why are aircraft certified for weight and balance?
- To ensure structural integrity and performance.
- To maintain safe flight characteristics, especially during stalls and spins.
Weight and Balance
Why don’t weight-shift control aircraft and balloons require weight and balance calculations?
Their suspended load design makes it difficult to exceed CG limits, but pilots must still avoid overloading.
Weight and Balance: Effect on Flight Performance
How does overloading affect takeoff performance?
It increases takeoff distance, reduces climb capability, and may prevent clearing obstacles.
Weight and Balance: Effect on Flight Performance
How does overloading affect landing performance?
It increases touchdown speed and extends the landing roll.
Weight and Balance: Effect on Flight Performance
What are the adverse effects of overloading during flight?
Slower climbs, reduced cruise speed, increased fuel consumption, engine wear, and reduced range.
Weight and Balance: Effect on Flight Performance
Why must weight distribution remain within CG limits?
To maintain stability, controllability, and prevent structural damage or failure.
Weight and Balance: Effect on Flight Performance
Why can’t some aircraft carry full fuel, baggage, and passengers simultaneously?
To remain within approved weight and balance limits, as exceeding them can compromise safety and performance.
Weight and Balance: Effect on Flight Performance
Where can weight and balance data be found for an aircraft?
In the FAA-approved Airplane Flight Manual (AFM) or Pilot’s Operating Handbook (POH).
Weight and Balance: Effect of Weight on Aircraft Structure
What load factor must normal category aircraft withstand?
A load factor of 3.8 Gs, which is 3.8 times the aircraft’s approved gross weight.
Weight and Balance: Effect of Weight on Aircraft Structure
How does overloading affect structural stress?
A 100-pound overload imposes an additional load of 380 pounds on the structure at 3.8 Gs, increasing cumulative stress and metallic fatigue.
Weight and Balance: Effect of Weight on Aircraft Structure
What is a common consequence of habitual overloading?
Cumulative structural stress that may remain undetected during inspections, leading to failure during normal operations.
Weight and Balance: Effect of Weight on Aircraft Structure
How does increased weight affect load during maneuvers?
In a 3 G maneuver, every 100 pounds of excess weight adds 300 pounds of structural load.
Weight and Balance: Effect of Weight on Aircraft Structure
Why should pilots adhere to maximum gross weight limits?
While the aircraft may perform beyond these limits, it imposes loads on the structure that exceed its design capabilities.
Weight and Balance: Effect of Weight on Aircraft Structure
Why are compartments like baggage areas placarded with weight limits?
They are designed to support only a specific load, regardless of the aircraft’s overall weight and balance status.
Weight and Balance: Effect of Weight on Aircraft Structure
16 gallons of fuel is approximately how many lbs?
100 lbs
Weight and Balance: Effect of Weight on Stability and Controllability
How does overloading affect an aircraft’s stability?
Overloading can make stability unsatisfactory, even in aircraft normally stable when properly loaded.
Weight and Balance: Effect of Weight on Stability and Controllability
What factor has the most direct effect on stability?
The distribution of weight, especially the location of the center of gravity (CG).
Weight and Balance: Effect of Weight on Stability and Controllability
How does CG location affect tail load?
- A forward CG increases the tail’s downward load, requiring more lift and reducing performance.
- An aft CG decreases the tail’s load, which can destabilize the aircraft.
Weight and Balance: Effect of Weight on Stability and Controllability
Are certificated aircraft stable when overloaded?
No, many aircraft become unstable and uncontrollable when the gross weight exceeds the certified limit.
Weight and Balance: Effect of Load Distribution
How does CG position affect wing loading?
A forward CG increases wing loading and drag, while an aft CG reduces wing loading and drag, improving cruise efficiency.
Weight and Balance: Effect of Load Distribution
How does CG location affect stability?
A forward CG improves stability but increases elevator control force. An aft CG reduces stability and can lead to neutral or unstable flight.
Weight and Balance: Effect of Load Distribution
How does CG position influence stalling speed?
A forward CG increases the stalling speed due to higher wing loading.
Weight and Balance: Effect of Load Distribution
What risks are associated with an aft CG?
Increased difficulty in stall recovery, risk of flat spins, and reduced controllability in turns and landings.
Weight and Balance: Effect of Load Distribution
What are the effects of a forward CG?
Increased elevator control force, higher stall speed, and potential taxi and landing difficulties, especially in tailwheel aircraft.
Weight and Balance: Effect of Load Distribution
How does load distribution affect control forces?
Longer moment arms (e.g., from wingtip tanks or aft cargo) increase the control forces required for maneuvering and stability.
Weight and Balance: Effect of Load Distribution
Summarize the effects of CG position.
- Forward CG: Stable, higher stall speed, harder to maneuver.
- Aft CG: Unstable, lower stall speed, improved cruise speed but reduced controllability.
High Speed Flight: Subsonic Versus Supersonic Flow
How is air treated in subsonic aerodynamics?
Air is considered incompressible, with constant density, and follows ideal-fluid principles like Bernoulli’s principle and continuity.
High Speed Flight: Subsonic Versus Supersonic Flow
What happens to air at speeds approaching the speed of sound?
Air becomes compressible, leading to density changes and the onset of compressibility effects like shock waves and drag increase.
High Speed Flight: Subsonic Versus Supersonic Flow
How can an aircraft experience both supersonic and subsonic flow?
Air accelerated over a wing’s upper surface can reach supersonic speeds even if the aircraft itself is flying at subsonic speeds.
High Speed Flight: Subsonic Versus Supersonic Flow
What are the effects of compressibility at high speeds?
Shock wave formation, increased drag, buffeting, and control difficulties.
High Speed Flight: Subsonic Versus Supersonic Flow
What marks the onset of compressibility effects?
When airflow over a part of the aircraft, such as the wing’s maximum camber, reaches sonic speeds.
High Speed Flight: Speed Ranges
How does the speed of sound vary with temperature?
At 15 °C, the speed of sound at sea level is 661 knots, and at –55 °C (40,000 feet), it decreases to 574 knots.
High Speed Flight: Speed Ranges
What is a Mach number?
The ratio of the true airspeed of the aircraft to the speed of sound in the same atmospheric conditions.
High Speed Flight: Speed Ranges
What are the Mach number ranges for speed regimes?
- Subsonic: Below 0.75
- Transonic: 0.75 to 1.20
- Supersonic: 1.20 to 5.00
- Hypersonic: Above 5.00
High Speed Flight: Speed Ranges
What is the critical Mach number?
The speed at which airflow over a part of the aircraft first reaches Mach 1.0, marking the boundary between subsonic and transonic flight.
High Speed Flight: Speed Ranges
What occurs at drag divergence?
Sharp drag increase, shock waves, buffet, trim and stability changes, and decreased control surface effectiveness.
High Speed Flight: Speed Ranges
What are VMO and MMO?
- VMO: Maximum operating speed in knots calibrated airspeed (KCAS), critical at lower altitudes for structural loads.
- MMO: Maximum operating Mach number, critical at higher altitudes for compressibility effects.
High Speed Flight: Speed Ranges
How does MMO affect speed limits at high altitudes?
At higher altitudes, MMO limits are reached before VMO due to reduced speed of sound, making MMO the governing limit.
High Speed Flight: Speed Ranges
At what Mach number are jet aircraft typically most efficient?
Near their critical Mach number, before encountering drag divergence.
High Speed Flight: Mach Number Versus Airspeed
What is the Mach number?
The ratio of true airspeed (TAS) to the speed of sound at the current atmospheric conditions.
High Speed Flight: Mach Number Versus Airspeed
How does stall speed in KCAS vary with altitude?
Stall speed in KCAS remains constant, but true airspeed (TAS) and Mach number increase with altitude due to lower air density.
High Speed Flight: Mach Number Versus Airspeed
How does the speed of sound change with altitude?
It decreases as temperature drops, going from 661 knots at sea level to 574 knots at FL 380.
High Speed Flight: Mach Number Versus Airspeed
What happens when KCAS is held constant during a climb?
TAS and Mach number increase with altitude as air density decreases.
High Speed Flight: Mach Number Versus Airspeed
What happens to KCAS and TAS during a constant Mach climb?
Both KCAS and TAS decrease with altitude as the speed of sound drops with temperature.
High Speed Flight: Mach Number Versus Airspeed
What is the “coffin corner”?
The altitude where stall speed in Mach equals the maximum Mach operating speed (MMO), leaving no margin to slow down or speed up.
High Speed Flight: Mach Number Versus Airspeed
Why does climbing at constant Mach reduce KCAS?
Because the speed of sound decreases with altitude, lowering the equivalent calibrated airspeed (KCAS) for the same Mach number.
High Speed Flight: Boundary Layer
What are the two types of boundary layer flow?
- Laminar flow: Smooth and orderly.
- Turbulent flow: Chaotic and mixed.
High Speed Flight: Boundary Layer
What is a laminar boundary layer?
A smooth, orderly flow with low skin friction drag, but less stable than turbulent flow.
High Speed Flight: Boundary Layer
What is a turbulent boundary layer?
A chaotic flow with swirls or eddies that creates more drag but is more stable and resists separation better than laminar flow.
High Speed Flight: Boundary Layer
Where does the boundary layer transition from laminar to turbulent?
It transitions at a point further back from the leading edge of the wing, increasing in thickness and energy.
High Speed Flight: Boundary Layer
What is boundary layer separation?
The point where airflow detaches from the surface of an airfoil, causing high drag and loss of lift.
High Speed Flight: Boundary Layer
How does angle of attack (AOA) affect boundary layer separation?
As AOA increases, the separation point moves forward on the wing.
High Speed Flight: Boundary Layer
What is the purpose of vortex generators?
To delay or prevent boundary layer separation by increasing the energy of the boundary layer using vortices that mix low and high-energy airflow.
High Speed Flight: Boundary Layer
How do vortex generators help in transonic flight?
They delay shock wave-induced separation by creating higher surface velocities, requiring a stronger shock wave to cause separation.
High Speed Flight: Shock Waves
What is a shock wave in aviation?
A boundary between undisturbed air and compressed air, formed when a supersonic airstream slows to subsonic speed without changing direction.
High Speed Flight: Shock Waves
What happens to airflow behind a normal shock wave?
- Air slows to subsonic speed.
- Static pressure and density increase.
- Total energy (dynamic + static pressure) decreases.
High Speed Flight: Shock Waves
What is wave drag?
The increase in drag due to shock wave formation and airflow separation, peaking sharply beyond the critical Mach number.
High Speed Flight: Shock Waves
What are the effects of shock wave formation on an aircraft?
- Increased drag.
- Airflow separation.
- Buffeting (Mach buffet).
- Trim and stability changes.
- Reduced control effectiveness.
High Speed Flight: Shock Waves
What is Mach tuck?
A diving moment caused by the center of pressure moving aft due to shock wave formation and airflow separation.
High Speed Flight: Shock Waves
Why do turbine-powered aircraft often have T-tail configurations?
To position the horizontal stabilizer away from the turbulent wake of the wing, maintaining pitch control effectiveness.
High Speed Flight: Shock Waves
What happens when speed exceeds critical Mach by 10%?
Wave drag increases sharply, requiring significantly more thrust to maintain speed.
High Speed Flight: Sweepback
What is the purpose of wing sweepback?
To delay shock wave formation by reducing the airflow component perpendicular to the wing’s leading edge, thereby improving transonic aerodynamic performance.
High Speed Flight: Sweepback
What are the advantages of wing sweepback?
- Increases critical Mach number.
- Delays onset of compressibility effects.
- Reduces the magnitude of drag, lift, and moment coefficient changes.
High Speed Flight: Sweepback
What is the force divergence Mach number?
The Mach number where drag increases sharply, usually 5–10% above the critical Mach number.
High Speed Flight: Sweepback
Why do swept wings tend to stall at the tips?
Spanwise boundary layer flow toward the wingtips causes separation near the leading edge, leading to tip stalls.
High Speed Flight: Sweepback
How does a wingtip stall affect swept wing aircraft?
It moves the center of lift forward, causing a nose-up pitch that can worsen the stall.
High Speed Flight: Sweepback
Why are T-tail aircraft prone to deep stalls?
The tail stays effective during a stall, allowing the wing to enter a deeper stall at higher AOAs, potentially burying the tail in the wing’s wake and losing elevator effectiveness.
High Speed Flight: Sweepback
What is the purpose of a stick pusher and stick shaker?
- Stick Pusher: Automatically prevents a stall by pitching the nose down near stall speed.
- Stick Shaker: Provides tactile stall warning 5–7% above stall speed.
High Speed Flight: Mach Buffet Boundaries
What is Mach buffet?
A phenomenon caused by shock waves forming over the wing, occurring at both high-speed (near MMO) and low-speed (high AOA) conditions.
High Speed Flight: Mach Buffet Boundaries
When does high-speed Mach buffet occur?
When the aircraft approaches MMO, or excessive lift demand causes airflow over the wing to exceed the speed of sound.
High Speed Flight: Mach Buffet Boundaries
When does low-speed Mach buffet occur?
At high AOA due to slow speeds for the aircraft’s weight and altitude, causing localized airflow over the wing to reach supersonic speeds.
High Speed Flight: Mach Buffet Boundaries
What conditions increase the chances of Mach buffet?
- High altitudes: Thinner air requires higher AOA for lift.
- Heavy weights: Greater lift demand increases AOA.
- G loading: Higher G forces (e.g., from turns or turbulence) increase AOA.
High Speed Flight: Mach Buffet Boundaries
How does angle of attack (AOA) affect Mach buffet?
AOA increases airflow velocity over the wing, potentially inducing Mach buffet at high or low-speed boundaries.
High Speed Flight: High Speed Flight Controls
What are primary and secondary flight controls?
- Primary: Ailerons, elevator, rudder (control pitch, roll, and yaw).
- Secondary: Tabs, flaps, spoilers, slats (enhance control and lift).
High Speed Flight: High Speed Flight Controls
What is the primary function of spoilers on high-speed aircraft?
To reduce lift, act as speed brakes, and improve braking performance by transferring weight to the wheels upon landing.
High Speed Flight: High Speed Flight Controls
How do jet aircraft manage roll control with limited aileron space?
By using spoilers in conjunction with ailerons; spoilers reduce lift on the descending wing for additional roll control.
High Speed Flight: High Speed Flight Controls
Why are leading-edge devices like slats and slots used?
To delay airflow separation, improve low-speed performance, and increase CL-MAX, enabling lower stall speeds.
High Speed Flight: High Speed Flight Controls
What is a variable incidence horizontal stabilizer, and why is it used?
It adjusts the horizontal stabilizer’s angle to handle pitch trim changes, minimizing drag and preserving elevator range.
High Speed Flight: High Speed Flight Controls
Why are hydraulic or electrical systems used for flight controls in jet aircraft?
The forces required to move control surfaces at high speeds are too great for manual operation; powered systems assist the pilot.
High Speed Flight: High Speed Flight Controls
How can control surfaces be moved if hydraulic or electrical systems fail?
By manually adjusting control tabs, which create aerodynamic forces that move the control surface.
