how airplanes fly forces
DESCRIPTION
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 - PowerPoint PPT PresentationTRANSCRIPT
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