Marc Masquelier

How Airplanes FlyForces

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

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

Weight

DragThrust

Lift

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

Weight

DragThrust

Lift

Lift

• Mostly created by the wings• Lift = CL * q * S– Where q = dynamic pressure = 1/2*ρ*V2

Lift

Pressure distribution on upper surfaceNet Lift

Flow accelerates here

Lift

Higher AOA higher lift … until the wing stalls

LiftStall

AOA is controlled by the pilot!

Lift – Airfoil

• Angle of Attack “AOA” or “α”– 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

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

Weight

DragThrust

Lift

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

Weight

DragThrust

Lift

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

Weight

DragThrust

Lift

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

Lift

Induced Drag

Net Force

Induced Drag

Low angle of attackLow induced drag

What can you say about these two flight conditions?

Airflow

High angle of attackHigh 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

Vent

ral F

ins

LANTIRN Pods

Lift versus Drag

• Function of aircraft configuration

Cl –

Lift

Coe

ffici

ent

Cd – Drag Coefficient

Zero lift line

Best L/D

“approaching stall”Best L

/D

Add a bunch of dragL/D reduces slower max range speed More thrust required

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

Lift

Coeffi

cien

t - C

L

Angle of Attack - α

Flaps and Slats

Lift

Coeffi

cien

t - C

L

Angle of Attack - α

Lift

Coeffi

cien

t - C

L

Angle of Attack - α

Flap

Slat

Flap

Wing Fence

Recommended Reading

• Stick and Rudder by Wolfgang Langewiesche