How Airplanes Fly Forces Marc Masquelier

Your Ideas… What is an airplane? What are wings? A heavier-than-air aircraft kept aloft by the upward thrust exerted by the air passing over its wings Airfoils attached transversely to the fuselage of an aircraft that provide lift For many forces on an airplane, wing area (S) is a major reference number Airplane - a heavier-than-air aircraft kept aloft by the upward thrust exerted by the air passing over its wings Wing - airfoils attached transversely to the fuselage of an aircraft that provide lift

Some Terminology Knots  1 kt = 1.15 mph Angle of Attack  AOA, or alpha, or α The angle of the wind relative to the wing AOA

Forces Lift Weight Thrust Drag Lift Thrust Drag Weight

Before We Start on Forces We need to understand Pressure

Pressure Two types of pressure Static (surrounding air) Dynamic (speed) Total pressure = static + dynamic pressure

Pressure Total Pressure = Static Pressure (p) + Dynamic Pressure (1/2*ρ*V2) = constant for a given flight condition Flow accelerates over the top – static pressure decreases Flow remains constant – static pressure stays constant

Forces Lift Weight Thrust Drag Lift Thrust Drag Weight

Lift Mostly created by the wings Lift = CL * q * S Where q = dynamic pressure = 1/2*ρ*V2 Wings keep airplanes aloft by pushing the air down Every action has an equal and opposite reaction

Lift Net Lift Pressure distribution on upper surface Flow accelerates here

Lift Higher AOA  higher lift … until the wing stalls

Lift Stall AOA is controlled by the pilot!

Lift – Airfoil Angle of Attack “AOA” or “α” Lift Coefficient Relative wind The angle where the wing meets the air Lift Coefficient Lift = CL * q * S, or if you turn it around: CL = Lift / (q * S) Is a function of angle of attack (as shown on last chart)

Lift – Wing Aspect Ratio Tradeoffs Show CL alpha curve Affected by camber (more drag, higher CL/a), thickness (thicker = more drag, benign stall)

Net Lift So now we have: Lift = CL * q * S, where CL = function of wing design, AOA q = dynamic pressure = 1/2*ρ*V2 S = wing area And recall that AOA is controlled by the pilot So you get more lift by flying faster, or increasing AOA (until you stall)

Forces Lift Weight Thrust Drag Lift Thrust Drag Weight

Weight What contributes to weight? Can it change?

Weight Counteracts lift (generally) 1 lb extra on an airplane requires 8 lb extra other “stuff” to support it (stronger structure, bigger wing, extra electrical power, more cooling, more powerful engine, more gas…) Additional weight means aircraft stalls at a higher speed  higher approach/landing speed  longer runway/bigger brakes/harder on gear higher AOA required to maneuver  less stall margin  less maneuverable higher AOA at a given speed  more drag  more thrust required  more fuel consumption Aircraft designer’s #1 enemy

Forces Lift Weight Thrust Drag Lift Thrust Drag Weight

Thrust Generally provided by jet or prop Pushes the airplane forward Generally directed along aircraft waterline A function of throttle position and airspeed Props – max thrust when stationary – good for low-speed applications Jets – max thrust when moving – better for high-speed applications

Propellers “Rotating wings” Push the air backwards Reaction is … Usually powered by a gasoline engine similar to a car engine, or a gas turbine

Jet Engines Smash the air down (compressor) Toss in some fuel Ignite (combustor) Make the burning air do some work (turbine) Expand and accelerate the hot gases out the back (nozzle)

Forces Lift Weight Thrust Drag Lift Thrust Drag Weight

Drag What is drag? What contributes?

This May Have Some Extra Drag…

This One Also

Drag LOTS of sources of drag Drag due to lift (induced drag, typically the biggest drag source) Flight controls Fuselage External stores Sensor packages

Induced Drag Induced Drag Lift Net Force

What can you say about these Induced Drag Airflow Low angle of attack Low induced drag What can you say about these two flight conditions? High angle of attack High induced drag Airflow

Net Drag Drag = CD * q * S CD is a composite of all drag sources Can be a function of AOA “drag counts” – 1 drag count = 0.0001 CD q = dynamic pressure = 1/2*ρ*V2 And remember S = aircraft wing area (ft2)

Another Note about Drag Putting something external on an airplane is just like selling a house… How you condition the airflow is a Big Deal Flat plates are ugly – unless parallel to the airstream Fairings are important

A Quick Side Story LANTIRN Pods Ventral Fins

Lift versus Drag Function of aircraft configuration Best L/D Best L/D Add a bunch of drag L/D reduces  slower max range speed  More thrust required Best L/D “approaching stall” Cl – Lift Coefficient Zero lift line Cd – Drag Coefficient

Summary Weight and drag are overcome by lift and thrust Weight increases wreak havoc on aircraft performance Adding stuff on the outside of the airplane must be carefully done to minimize drag and turbulence Aircraft design is always a compromise between vehicle performance and onboard systems (weapons/ sensors/ avionics/ fuel/ cargo) Best if requirements are known from the start

Stall White Board

Flaps and Slats Flap Slat Lift Coefficient - CL Angle of Attack - α

Flap

Wing Fence

Recommended Reading Stick and Rudder by Wolfgang Langewiesche