Four Dynamic Forces on an Airplane

Four Dynamic Forces on an Airplane

• Lift is the upward acting force.
• Gravity (or weight) is the downward acting force.
• Thrust is the forward acting force.
• Drag is the backward acting force.

Flight Condition with Equal Opposing Forces

• In steady-state, straight-and-level, unaccelerated flight, the sum of opposing forces equals zero.
• Unbalanced forces cannot exist in steady, straight flight, following Newton's Third Law.
• The opposing forces being equal cancels each other out.

Airfoil Definition

• An airfoil is a device that gets a reaction from air moving over its surface- Lift.
• Examples of airfoils: wings, horizontal tail surfaces, vertical tail surfaces, and propellers.

Angle of Incidence

• Angle of incidence is formed by the longitudinal axis of the airplane and the chord of the wing.
• Measured by the angle at which the wing attaches to the fuselage.
• The angle of incidence is fixed and cannot be changed by the pilot.

Relative Wind

• Relative wind is the direction of airflow with respect to the wing.
• When a wing moves forward and downward, the relative wind moves backward and upward.
• The flight path and relative wind are always parallel but travel in opposite directions.

Angle of Attack

• Angle of attack is the angle between the wing chord line and the direction of the relative wind.
• It can be changed by the pilot.

Bernoulli's Principle

• Pressure of a fluid (liquid or gas) decreases where the fluid's speed increases.
• High-speed airflow has low pressure; low-speed airflow has high pressure.
• Aircraft airfoils are designed to increase airflow velocity above their surface, decreasing pressure.
• Simultaneously, the impact of air on the lower surface increases pressure below, resulting in lift.

Factors Affecting Lift and Drag

• Wing area affects lift and drag, acting on a wing are roughly proportional to the wing area.
• Pilots can change wing area using flaps (e.g., Fowler flaps).
• Increasing the upper curvature of an airfoil increases lift. Lowering an aileron or flap can accomplish this.
• Ice or frost on a wing can disrupt airflow, changing camber and disrupting lifting capability.
• Increasing the angle of attack increases both lift and drag, up to a certain point.
• Increasing air velocity over the wing increases lift and drag.
• Lift and drag vary directly with air density increases, lift and drag increase.

Torque Effect

• Torque effect involves Newton's Third Law: every action has an equal and opposite reaction.
• As internal engine parts/propeller revolve in one direction, an equal force tries to rotate the airplane oppositely.
• Torque is greatest at low airspeeds with high power settings and a high angle of attack.

Torque Reaction on the Ground and in Flight

• In flight, torque reaction acts around the longitudinal axis, causing the airplane to roll.
• Compensation includes rigging older airplanes for more lift on the downward-forced wing.
• Modern airplanes offset the engine to counteract torque.
• During takeoff roll, torque reaction induces a turning moment around the vertical axis.
• The left side of the airplane is forced down, increasing weight on the left main landing gear.
• Greater ground friction on the left tire causes a further turning moment to the left.

Factors Contributing to Torque Effect

• Torque reaction of the engine and propeller: every action has an equal/opposite reaction.
• Propeller rotation (from cockpit) to the right tends to roll/bank the airplane to the left.
• Gyroscopic effect: resultant action/deflection of a spinning object when force is applied to its rotational rim.
• If a propeller axis is tilted, the force is exerted 90° ahead in the direction of rotation/applied force.
• Corkscrewing effect of the propeller slipstream: high-speed propeller rotation results in corkscrewing rotation.
• At high propeller speeds/low forward speeds (takeoff), the slipstream strikes the vertical tail, yawing the airplane.
• Asymmetrical loading of the propeller (P-Factor): at a high angle of attack, the downward blade has a greater bite.
• Thrust is greater on the downward moving blade, yawing the airplane to the left.

Centrifugal Force

• Centrifugal force is the airplane's equal and opposite reaction to a change in direction.
• It acts equal and opposite to the horizontal component of lift.

Load Factor

• Load factor is the ratio of total load supported by airplane's wing to the airplane's actual weight.
• Expressed as the ratio of a given load to the pull of gravity (e.g., 3 Gs).

Importance of Load Factor to Pilots

• Prevents dangerous overload on aircraft structure.
• Increased load factor increases stalling speed, making stalls possible at seemingly safe flight speeds.

Situations Resulting in Maximum/Exceeded Load Factors

• Level turns: load factor increases after a bank reaches 45° or 50°.
  • Load factor in a 60° bank turn is 2 Gs; in an 80° bank turn, it is 5.76 Gs.
• Turbulence: Severe vertical gusts cause a sudden increase in angle of attack, resulting in large loads.
• Speed: Excess load depends on airplane speed.

Operational Categories for Aircraft

• Maximum safe load factors (limit load factors) specified for airplanes:
  • Normal Utility (mild aerobatics including spins) +4.4 to -1.76
  • Aerobatic +6.0 to -3.00

Effect of Increased Load Factor on Stalling Speed

• Stalling speed increases as load factor increases.
• Any airplane can stall at any airspeed within structural/pilot limits.
• Direct relationship between load factor/stalling characteristics.
• Stalling speed increases in proportion to the square root of the load factor.

Maneuvering Speed

• The maximum speed at which the limit load can be imposed (gusts or full deflection) without structural damage.
• Allows moving a single flight control one time, to its full deflection, for one axis of rotation (pitch, roll or yaw) without risking damage
• Allows aircraft to stall before exceeding the limit load.
• Operating at or below maneuvering speed doesn't provide structural protection against multiple full control inputs.

Effect of Weight on Maneuvering Speed

• Maneuvering speed increases with an increase in weight and decreases with a decrease in weight.
• An aircraft operating at a reduced weight is more vulnerable to rapid accelerations.
• Design limit load factors could be exceeded if a reduction in maneuvering speed is not accomplished.
• Aircraft operating at or near gross weight are less likely to exceed design limit load factors

Loss-Of-Control-Inflight (LOC-I)

• Significant deviation of aircraft from intended flight path, often resulting in an airplane upset.
• Situations that increase the risk:
  • Uncoordinated flight
  • Equipment malfunctions
  • Pilot complacency/distraction
  • Turbulence
  • Poor risk management

Cause of an Airplane Stall

• Every stall is caused by an excessive angle of attack.
• Each airplane has a specific angle of attack where airflow separates from the wing, causing a stall.
• The critical angle of attack varies from 16° to 20° depending on the airplane's design.

Spin Definition

• A spin in a small airplane or glider is a controlled/uncontrolled maneuver.
• Aircraft descends in a helical path at an angle of attack greater than the critical angle.
• Spins result from aggravated stalls in a slip or skid. A spin cannot occur if a stall does not occur.

Causes of a Spin

• Primary cause: exceeding critical angle of attack while applying excessive/insufficient rudder, and to a lesser extent, aileron.

Conditions for a Spin

• A stall/spin can occur in any phase of flight.
• Most likely in these scenarios:
  • Engine failure on takeoff during climbout: Stalling while trying to return to runway
  • Cross-control turn from base to final : Stalling when overshooting final with a crosswind.
  • Engine failure on approach: Stalling while trying to reach the runway.
  • Go-around with full nose-up trim: Stalling due to uncoordinated rudder use
  • Go-around with improper flap retraction: Rapid sink rate followed by stall due to back pressure.

Spin Recovery Procedure

• Close the throttle (if not already done).
• Neutralize the ailerons.
• Apply full opposite rudder.
• Move elevator control forward to approximately neutral position.
• Neutralize rudder when spinning stops.
• Apply aft elevator pressure to return to level flight.
• Remember PARE: Power-Reduce, Ailerons Neutral, Rudder-Apply Opposite, Elevator-Apply positive (forward)

Adverse Yaw

• When turning an airplane (left instance) the downward deflected aileron on the right produces more lift on the right wing.
• The right aileron also produces more drag, so the opposite left aileron has less lift/ drag.
• The extra drag attempts trying to pull/veer the airplane's nose in the direction of the raised wing (right).
• This undesired veering during the turn is adverse yaw.

Ground Effect

• Improved performance Experienced when aircraft is operating near the ground.
• Change in 3-D flow pattern due to airflow restriction.
• Reduces wing's upwash, downwash, and wingtip vortices.
• Wing must be close to the ground.

Adverse Effects of Ground Effect

• During landing: Excess speed (40% less drag) increases floating distance. The pilot may run out of runway.
• During takeoff: reduced drag, the aircraft seems capable of takeoff well below recommended speed.
• Aircraft rising out of ground effect loses speed, greater induced drag results in little to marginal climb performance

Weight and Balance Definitions

• Empty weight: Weight of airframe, engines, and all permanently installed equipment including unusable fuel (either undrainable oil or full reservoir of oil).
• Gross weight: Maximum allowable weight of both the airplane and its contents.
• Useful load: Weight of pilot, copilot, passengers, baggage, usable fuel, and drainable oil.
• Arm: Horizontal distance in inches from reference datum line to the item's center of gravity.
• Moment: Product of an item's weight multiplied by its arm, expressed in pound-inches.
• Center of gravity: Point aircraft would balance if suspended, expressed in inches from datum.
• Datum: Imaginary vertical plane/line from which all arm measurements are taken, established by the manufacturer.

Basic Weight and Balance Equation

• Weight x Arm = Moment
  • Transposed: Weight = Moment/Arm, Arm = Moment/Weight,
• WAM (Weight × Arm = Moment)

Shifting Weight in an Airplane

• Total aircraft weight remains consistent if weight is simply shifted.
• This changes the total moments related to the direction/ amount of weight being moved.
• Moving weight forward decreases moments, moving weight aft increases moments.
• Moment change is relative to the amount of weight being in motion.

Formula for Weight Shift

Weight shifted / Total weight = Change in CG / Distance weight is shifted

Adverse Performance Characteristics of an Overloaded Aircraft

• Higher takeoff speed
• Longer takeoff run
• Reduced rate and angle of climb
• Lower maximum altitude
• Shorter range
• Reduced cruising speed
• Reduced maneuverability
• Higher stalling speed
• Higher landing speed
• Longer landing roll
• Excessive weight on the nosewheel

Effects of a Forward CG

• Higher stall speed
• Slower cruise speed
• Improved Stability
• Greater back elevator pressure required
• Longer Take off roll, high approach speed, problems with landing flares

Effects of a Rearward CG

• Lower stall speed
• Higher cruise speed
• Stall / Spin recovery becomes more difficult
• Instability

Standard Weights for Calculations

• Crew: 190 lbs
• Passengers: 190 lbs
• AvGas: 6 lbs/U.S. gal
• Jet A: 6 lbs/U.S. gal
• Oil: 7.5 lbs/U.S. gal
• Water: 8.35 lbs/U.S. gal

Aircraft Weight and Balance Records

• Changes of fixed equipment may drastically change the weight of the aircraft. Aircraft can be overloaded by radios/ instruments, so pilots / A&P mechanics should ensure the correct CG is maintained.

Aircraft Performance Elements

• Takeoff and landing distance
• Rate of climb
• Ceiling
• Payload
• Range
• Speed
• Fuel economy
• Maneuverability
• Stability

Factors Affecting Aircraft Performance

• Air density (density altitude)
• Surface wind
• Runway surface
• Upslope or downslope of runway
• Weight

Effect of Wind on Aircraft Performance

• Takeoff: Headwind decreases ground speed and increases climb angle. Tailwind increases ground speed, decreasing climb angle.
• Landing: Wind effect mirrors takeoff.
• Cruise: Headwind decreases ground speed, increases fuel requirement, and the opposite is true for a tail wind.

Effect of Weight on Takeoff and Landing

• Increased weight:
  • Higher liftoff speed
  • Slower acceleration
  • Increased retarding force
  • Longer takeoff distance
• Higher weight requires greater speed at landing angle of attack and lift coefficient, increasing distance.

Effect of Density Altitude on Takeoff and Landing

• Increased density altitude:
  • Increased takeoff distance
  • Reduced climb rate
  • Increased TAS on approach and landing
  • Increased landing roll distance

Density Altitude Definition

• pressure altitude corrected for nonstandard temperature.
• Air at each level has a specific density, linking standard pressure, altitude, and density.
• DA is the vertical distance above sea level where a certain density is located.

Air Density Effects on Aircraft Performance

• Lift produced by the wings
• Power output of the engine
• Propeller efficiency
• Drag forces

Factors Affecting Air Density

• Altitude: Density decreases at higher altitudes.
• Temperature: Density decreases as temperature increases.
• Humidity: Density decreases as humidity increases.

Effects of Temperature, Altitude, & Humidity on DA

• DA increases when:
  • Air temperature is high
  • Altitude is high
  • Humidity is high
• DA decreases when:
  • Air temperature is low
  • Altitude is low
  • Humidity is low

Aircraft Speeds

• Vso: Stall speed in landing configuration
• Vsi: Stall speed in specified configuration
• Vy: Best rate-of-climb speed
• Vx: Best angle-of-climb speed
• VLE: Max landing gear extension speed
• VLO: Max landing gear operating speed
• VFE: Max flap extension speed
• VA: Maneuvering speed
• VNO: Normal operating speed
• VNE: Never exceed speed

Best Glide vs. Minimum Sink Speed

• Best glide speed: Configuration providing greatest forward distance for altitude loss.
• Minimum sink speed: Used to maximize time in the air.

Glide Distance Rule

• 1.5 NM per 1,000 feet (Cessna 152s and 172s).

Flight Planning

• Time, fuel, and distance-to-climb should be determined by the chart.
• Cruise performance charts give TAS, fuel consumption, endurance, and range.
• Pressure altitude and temperature determine engine speed, airspeed, and fuel numbers.

Factors Affecting Fuel Consumption

• Engine condition
• Propeller RPM
• Mixture
• Horsepower for flight