Performance and Limitations

Forces of Flight: PHAK 5-(2-8)

Lift: is the upward force created by the difference of pressure of the air that passes over and under the wing.
- Bernoulli’s Principle: as the velocity increases, pressure decreases, creating an area of low pressure above the wing and an area of high pressure under the wing. High pressure seeks low pressure creating an upward movement.
- Newton’s Third Law: For every action there is an equal and opposite reaction.
Weight: the force of gravity which acts vertically through the CG of the plane toward the center of the earth.
Thrust: the forward-acting force which opposes drag and propels the engine.
Parasite Drag: opposes thrust and acts rearward parallel to the relative wind.
- Form drag: due to the shape of an object obstructing airflow.
- Interference drag: intersecting airstreams between aircraft components.
- Skin Friction drag: microscopic surface roughness impending smooth airflow over surfaces.
Induced drag: is the aerodynamic byproduct of lift. It is caused by the high-pressure air beneath the wing spilling over the wingtip to the low-pressure air above, forming vortices that spiral behind the aircraft. These vortices create resistance, which we call induced drag. Induced drag increases as angle of attack increases and is most pronounced at lower airspeeds, higher aircraft weights, and during climb-out or approach when the wing is working harder to produce lift.

Pressure and Density Altitude: PHAK 4-4

Pressure and density altitude are crucial in aviation because they directly impact aircraft performance, especially during takeoff and landing, by affecting lift, thrust, and engine power output, ultimately influencing takeoff distance and climb rate.

PA=(29.92-Current Altimeter)x1000+Field Elevation

DA=(Current Temperature-Standard Temperature)x120+PA

Note: remember that for every thousand feet of elevation, the standard temperature decreases by 2ºC. For example, at 4,000 ft, the standard temperature is about 7ºC.

V Speeds

Vso: stall speed in landing configuration
Vs1: stall speed in clean configuration
Vr: rotation speed (changeable)
Vx: best angle of climb
Vy: best rate of climb
Vfe: maximum flaps extended speed
Va: maneuvering speed (changeable)
- Maneuvering speed, or Va, is the maximum speed at which full or abrupt control inputs can be made without causing structural damage to the aircraft. It is the speed below which the aircraft is designed to stall before it breaks under aerodynamic stress. At higher weights, Va increases because more lift is required, which increases the aerodynamic load the structure can handle before a stall occurs. At lower weights, less lift is needed, and the aircraft can stall at a lower speed, so Va decreases.
Vno: maximum structural cruising speed
Vne: never exceed speed
Vg: best glide speed (changeable)
Approach speed

Vr=√(Takeoff Weight/MGW)Published Vr
Va=√(Landing Weight/MGW)Published Va
Vg=√(Takeoff Weight/MGW)Published Vg

Load Factor

Load Factor: the actual load supported by the wings divided by the total weight. It is important because of the possible overload that can be applied on the aircraft. Increased load factor increases the stall speed making it possible to stall at a seemingly safe speed.

Normal Category:

Positive Load Factor: 3.8G
Negative Load Factors: -1.5G

Center of Gravity

Center of Gravity (CG): is the point at which the aircraft would balance if it were suspended at that point.

Forward CG:

M.I.L.F.

More induced drag
Increased stall speed
Lower cruising speed
Favorable stall recovery

AFT CG

(opposite of FWD CG)

Adverse yaw

Adverse yaw is the tendency of the nose to yaw in the opposite direction of a turn.

Cause: When rolling into a turn, the upward-deflected aileron on the descending wing creates more drag than the downward aileron on the rising wing. This pulls the nose away from the turn.

How to Correct:

Use coordinated rudder input in the direction of the turn
Keep the ball centered with proper rudder usage

Stalls and spins

A stall occurs when the critical angle of attack is exceeded, causing airflow separation over the wing and a sudden loss of lift.

Causes:

Excessive pitch-up attitude
Improper configuration (e.g., flaps, trim)
Slow airspeed with high AOA
Aggressive or uncoordinated control inputs

Spin Recovery – Use the PARE Method:

P – Power idle

Cut the throttle to reduce asymmetric thrust and slow the spin

A – Ailerons neutral

Prevents worsening the spin or entering a flat spin

R – Rudder opposite the spin

Apply full rudder against the direction of rotation

E – Elevator forward

Briskly push forward to break the stall and restore airflow over the wings

Once rotation stops:

Neutralize the rudder
Ease out of the dive and recover to level flight

Gust Factor

Calculating the gust factor will help you know how much to increase your approach speed by. To calculate gust factor, subtract the wind speed from the gust speed and divide the difference by two.

Example:

18014K24G

24-14=10

10÷2=5

Gust Factor=5

In this case, your approach speed will need to be increased from 66 kts to 71 kts. However, pitching for 66 kts is generally a safe and accepted speed.

Turning Tendencies: PHAK 5-(30-33)

Torque: Involving Newton's Third Law of Physics - for every action, there is an equal and opposite reaction. As applied to the aircraft, when the propeller of engine revolves to one direction, an equal force is trying to rotate the aircraft in the opposite direction.
Spiraling Slipstream (Corkscrew Effect): The high-speed rotation of an aircraft gives a spiraling rotation to the slipstream. At high and low speeds, the corkscrew is very compact and exerts a strong sideward force on the vertical stabilizer to the right causing the nose to yaw to the left.
Gyroscopic Procession: The resultant action, or deflection of a spinning rotor when a deflecting force is applied to its rim. When a force is applied, the resulting force takes effect 90º ahead of and in the direction of rotation.

Note: Two fundamental properties of gyroscopic action is rigidity in space and precession.

Asymmetrical Loading (P-Factor): When the aircraft is flying with a high AOA, the "bite" of the downward moving blade is greater than the "bite" of the upward moving blade. This moves the center of thrust to the right of the prop disc area causing a yawing moment toward the left. The downswinging blade has more lift and tends to pull (yaw) the aircraft's nose to the left.

Stability: PHAK 5-(14-15)

Static Stability: the aircraft’s initial tendency after a disturbance.

Positive Static Stability
- The aircraft initially returns to its original position.
Neutral Static Stability
- The aircraft stays in its new position after disturbance.
Negative Static Stability
- The aircraft moves further away from its original position.

Dynamic Stability: the aircraft’s response over time after being disturbed.

Positive Dynamic Stability
- The oscillations diminish over time, returning to level flight.
Neutral Dynamic Stability
- Oscillations remain constant in size over time.
Negative Dynamic Stability
- Oscillations grow larger with each cycle.

Airspeed and Altitudes: PHAK 8-(6-9)

Airspeeds

IAS: indicated airspeed. The direct instrument reading obtained from the ASI, uncorrected for variations. Used to determine aircraft performance by the manufacturer.
CAS: calibrated airspeed. IAS corrected for installation error and instrument error.
TAS: true airspeed. CAS corrected for altitude and nonstandard temperature.
GS: ground speed. The actual speed of the airplane over the ground. TAS adjusted for wind.

Altitudes

Indicated altitude: read directly from the altimeter.
True altitude: the vertical distance above sea level (MSL)
Absolute altitude: the vertical distance above terrain (AGL)
Pressure altitude: the altitude above the standard datum plane (29.92”Hg, 15ºC)
Density altitude: pressure altitude corrected for nonstandard temperature.

Ground Effect

Ground effect is a phenomenon that occurs when an aircraft is flying close to the ground, usually within one wingspan of the surface. As the airplane gets closer to the ground, the airflow around the wing becomes restricted, particularly below the wing, which reduces wingtip vortices and vertical airflow deflection. This results in decreased induced drag and a slight increase in lift efficiency.

Turbulence

Convective (Thunderstorm) Turbulence: Strong updrafts and downdrafts in and around thunderstorms or cumulonimbus clouds.
Mechanical Turbulence: Wind flowing over obstacles like buildings, mountains, trees, and rough terrain.
Clear Air Turbulence (CAT): Occurs at high altitudes, typically above 15,000 ft, near jet streams.
Wake Turbulence: Wingtip vortices created by aircraft generating lift—strongest behind large, heavy, slow, and clean aircraft.
Mountain Wave Turbulence: Strong winds (≥25–30 knots) blowing perpendicular to mountain ranges.
Frontal Turbulence: Caused by wind shear along a weather front, especially cold fronts.

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