4 Forces of Flight & Stability

Forces Acting on an Aircraft

• Thrust• Drag• Lift• Weight (or Load)

Thrust

• Moves the airplane through the air• The forward force produced by a powerplant• In equilibrium Thrust = Drag• Controlled by pilot via Throttle

• Opposes the force of Thrust• Parallel and in the opposite direction of the relative wind• Controlled by pilot by “dirtying the airplane”• Comes in two types:• Parasitic Drag• Induced Drag

• Result of the dynamic effect of air acting across the airfoil• Keeps the airplane flying• Can be controlled by the pilot (to an extent)

Weight (Load)

• Force of gravity acting upon the mass of the entire system• Doesn’t always oppose lift• Load is the opposition of lift• We’ll come back to this

Newton’s Laws

• 1st Law: Object at rest, remains at rest, unless outside forces act upon• 2nd Law: Force = Mass x Acceleration• 3rd Law: Every action has an equal and opposite reaction

Bernoulli’s Principle

• As the velocity of a fluid increases, its overall pressure decreases• Think of the following illustration of a tube:

Bernoulli’s Principle and the Airfoil

• The airfoil is a cut off portion of Bernoulli's tube• An airfoil is any surface that provides aerodynamic force when

interacting with a stream of air

Newton’s Third Law

• As air hits wing, wing must go upward while air goes downward

Labeling the Airfoil

• The Airfoil interacts with the fluid by pushing some upward and some downward

Angle of Incidence

• The wing is mounted on the airplane in such a way that the chord line at the wing tip is more aligned with the longitudinal axis than the root• Some people refer to this as twist

Angle of Attack

• An increase of the angle of attack will lead to more lift being generated• That is, until the airfoil reaches its “Critical Angle of Attack”• Any increase further than the Critical AoA, the airfoil will “stall”

• A stall is caused by the separation of airflow from the wing’s upper surface• The result is the loss in production of lift with a exponentially

proportional increase in drag• An airfoil will ALWAYS stall when reaching its Critical AoA• Stalls will always occur regardless of airspeed

I REPEAT

AIRSPEED HAS NOTHING TO DO WITH A STALL

Stall vs. Airspeed

• Airspeed is just a reference to the stall• Consider a pilot going downward to the earth:• If the pilot decided to pull back on the yoke to regain level flight too quickly• He will increase the AoA beyond its critical value• The pilot will stall until he reduces the AoA and generates airflow over the

F-16 Stalls at 400 Knots

• https://www.youtube.com/watch?v=Rv9YC-gaNYo

Stall Strips

• Two metal strips attached to leading edge• Disrupt airflow at high AoA• This causes the wing area behind them to stall before the wingtips

Pilot Control of Lift

• Change Angle of Attack• Increase Speed• High-Lift Devices• Flaps• Flaps are used to increase the efficiency of the wing and to decrease stall

Drag and Lift

• The two are directly proportional• Any change in lift results in a proportional change in drag• Likewise any change in drag results in a change of lift• The Aeronautical Engineer must determine the best use of this

tradeoff

Flaps and Stalls

• Increase lift (and drag)• Increases the wing’s camber• Changes the average chord line• Both of these together change the angle of attack

• Also remember Newton’s 3rd

Center of Gravity

• The place where all of the weight is concentrated on the system• Imagine a string holding up the airplane• Does it stay straight and level?• Does it flip forward or backward?• Does it roll?

Drag in Context

• We said earlier that drag opposes thrust (or power)• Comes in the form of Parasitic or Induced

Parasite Drag

• Caused by the aircraft surface which deflects or interferes with th smooth airflow around the aircraft• Divided into three types:• Form• Interference• Skin-friction Drag

Form Drag

• Results from the turbulent wake from the separation of the surface of a structure• Determined by the size and shape of the structure protruding into the

Interference Drag

• When airflow around one part of the airflow interacts with the airflow around adjacent parts• Example: Wings join the fuselage

Skin Friction Drag

• Caused by the roughness of the surfaces of the airplane• Even though they appear clean, they may be rough under a

microscope• Cirrus wing vs. Archer wing

Induced Drag

• Generated by the airflow circulation around the wing as it creates lift• “Drag due to lift”• High pressure underneath joins low pressure above• Causes a spiraling vortex which trails behind each wing

Drag Proportionality

• Induced drag is inversely proportional to the square of the speed• Parasite drag is directly proportional to the square of speed• Combine Induced and Parasite – Total Drag• The place where the lowest amount of total drag occurs is known as

L/D Max• L/D Max is the lowest drag, but the greatest lift – resulting in the

speed of best glide

Ground Effect

• The result of the earth’s surface altering the airflow patterns about the airplane• Airplane may become airborne before reaching its recommended

takeoff speed• Induced drag decreases and excess speed in the flare may cause

floating• Happens within one wingspan of the surface

References

• Pilot’s Handbook of Aeronautical Knowledge

4 Forces of Flight & Stability

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