4 Forces of Flight Forces Acting on an

4 Forces of Flight

Forces Acting on an Aircraft • Thrust • Drag • Lift • Weight (or Load) *All forces are in equilibrium during unaccelerated flight* © 2015 Coast Flight Training. All Rights Reserved.

Thrust • Moves the airplane through the air • The forward force produced by a powerplant • In equilibrium Thrust = Drag • Controlled by pilot via Throttle © 2015 Coast Flight Training. All Rights Reserved.

Drag • 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 © 2015 Coast Flight Training. All Rights Reserved.

Lift • Result of the dynamic effect of air acting across the airfoil • Keeps the airplane flying • Can be controlled by the pilot (to an extent) © 2015 Coast Flight Training. All Rights Reserved.

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 © 2015 Coast Flight Training. All Rights Reserved.

Newton’s Laws • 1 st Law: Object at rest, remains at rest, unless outside forces act upon • 2 nd Law: Force = Mass x Acceleration • 3 rd Law: Every action has an equal and opposite reaction © 2015 Coast Flight Training. All Rights Reserved.

Bernoulli’s Principle • As the velocity of a fluid increases, its overall pressure decreases • Think of the following illustration of a tube: © 2015 Coast Flight Training. All Rights Reserved.

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 © 2015 Coast Flight Training. All Rights Reserved.

Newton’s Third Law • As air hits wing, wing must go upward while air goes downward © 2015 Coast Flight Training. All Rights Reserved.

Labeling the Airfoil • The Airfoil interacts with the fluid by pushing some upward and some downward © 2015 Coast Flight Training. All Rights Reserved.

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 © 2015 Coast Flight Training. All Rights Reserved.

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 Ao. A, the airfoil will “stall” © 2015 Coast Flight Training. All Rights Reserved.

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 Ao. A • Stalls will always occur regardless of airspeed © 2015 Coast Flight Training. All Rights Reserved.

I REPEAT YOU CAN STALL AT ANY AIRSPEED! © 2015 Coast Flight Training. All Rights Reserved.

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 Ao. A beyond its critical value • The pilot will stall until he reduces the Ao. A and generates airflow over the wings © 2015 Coast Flight Training. All Rights Reserved.

F-16 Stalls at 400 Knots • https: //www. youtube. com/watch? v=Rv 9 YC-ga. NYo © 2015 Coast Flight Training. All Rights Reserved.

Stall Strips • Two metal strips attached to leading edge • Disrupt airflow at high Ao. A • This causes the wing area behind them to stall before the wingtips © 2015 Coast Flight Training. All Rights Reserved.

Pilot Control of Lift • Change Angle of Attack • Increase Speed • High-Lift Devices • Flaps are used to increase the efficiency of the wing and to decrease stall speed © 2015 Coast Flight Training. All Rights Reserved.

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 © 2015 Coast Flight Training. All Rights Reserved.

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 3 rd © 2015 Coast Flight Training. All Rights Reserved.

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? © 2015 Coast Flight Training. All Rights Reserved.

Drag in Context • We said earlier that drag opposes thrust (or power) • Comes in the form of Parasitic or Induced © 2015 Coast Flight Training. All Rights Reserved.

Parasite Drag • Caused by the aircraft surface which deflects or interferes with the smooth airflow around the aircraft • Divided into three types: • Form • Interference • Skin-friction Drag © 2015 Coast Flight Training. All Rights Reserved.

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 wind © 2015 Coast Flight Training. All Rights Reserved.

Interference Drag • When airflow around one part of the airflow interacts with the airflow around adjacent parts • Example: Wings join the fuselage © 2015 Coast Flight Training. All Rights Reserved.

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 © 2015 Coast Flight Training. All Rights Reserved.

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 © 2015 Coast Flight Training. All Rights Reserved.

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 © 2015 Coast Flight Training. All Rights Reserved.

Total Drag © 2015 Coast Flight Training. All Rights Reserved.

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 © 2015 Coast Flight Training. All Rights Reserved.

References • Pilot’s Handbook of Aeronautical Knowledge © 2015 Coast Flight Training. All Rights Reserved.