Aircraft Mechanics

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# Aircraft Mechanics - PowerPoint PPT Presentation

Aircraft Mechanics. Alan Altschuler (Mr. A). Forces and Moments on Aircraft. Forces L ift Drag /Thrust Inertia Gravitational (weight) Linear and Rotational Momentum Ground Support Weight Speed-Up Catapult Slow-Down Arrestment Parachute (Drag-chute) Pressures (self-balancing) Cabin

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### Aircraft Mechanics

Alan Altschuler (Mr. A)

Forces and Moments on Aircraft
• Forces
• Lift
• Drag /Thrust
• Inertia
• Gravitational (weight)
• Linear and Rotational Momentum
• Ground
• Support Weight
• Speed-Up
• Catapult
• Slow-Down
• Arrestment
• Parachute (Drag-chute)
• Pressures (self-balancing)
• Cabin
• Hydraulic and others (bleed air)
• Moments
• Pitch – Nose up and down
• Roll – Left wing tip up with right down, and vice versa
• Yaw – Nose left and right
Axis Systems on Aircraft
• Local aircraft (x=aft, y=outboard (usually left), z=up – relative to aircraft (structure) – usually called Body Axes
• Thrust is “mostly” negative local x
• Air stream (x=streamwise flow (positive impinging on aircraft nose)
• Wing angle of attack is “mostly” angle between local x and air stream x from pitch (also provided via roll velocity)
• Vertical Tail angle of attack is “mostly” angle between local x airstream x from yaw (also provided via roll velocity)
• Lift is positive airstream z
• Drag is positive airstream x
• Inertial (z=up relative to earth)
• Weight is always inertial negative z
• Direction Cosines
• A method to rotate vectors among axis systems

Ground Configuration

All axis-systems aligned

Zb,a,i

Xb,a,i

Air Configuration

All axis-systems mis-aligned here

airflow

Zb

Za

Zi

Aligned

with earth

Aligned with airframe

Aligned with airflow

Xb

Xi

Xa

Lift curve
• Bernoulli
• Angle of attack
• Camber
• Bernoulli
• NACA airfoil
Roll – Yaw Coupling
• Due to asymmetry in the aircraft y-z and x-y planes
• Can have roll-pitch and yaw-pitch coupling, but symmetry about aircraft x-z centerplane would make these couplings zero.
• Interesting cases of asymmetry
• Single-engine aircraft
• Multi-engine aircraft without counter-rotating engines
• E-2 tail (Hawkeye)
• Single main-rotor helicopters
• A-10 nose wheel (Warthog/Thunderbolt)

Principle axes

Inertia
• Linear
• Angular
• Issue with Most Helicopters
• Banking a bicycle
• Car
• Tilting on an unbanked roadway
• Center of Gravity
Newton’s Second Law
• F = ma ?
• Not really
• Actually, force equals the change in momentum over time
• Momentum P = mv
• In calculus F = dP/dt = d/dt (mv)

= m dv/dt + v dm/dt

• In non-calculus F = DP/Dt = m Dv/Dt + v Dm/Dt
• Dv/Dt = a
• F = ma + v Dm/Dt
• the second part is the “rocket” part, exhausted gas quickly from the body
Lifting Surfaces
• Wing
• Bi-plane wing structures
• Winglet
• Horizontal Stabilizer (Tail)
• Vertical Stabilizer (Tail)
• Canard
• X-29
• Fuselage
• F-14 body lift

Ground Configuration - parked

Air Configuration – steady level flight

Control Surfaces
• Elevators
• Ailerons
• Rudders
• Pictures
• Spoilers
• Used less often
• Speed brakes
• Elevons (Space Shuttle)
• “Flying” Horizontal Stabilizers (F-14)
• Aka stabilators
Landing Gear(Alighting Gear)
• Main Gear
• Metering Pins
• Nose Gear
• Tail Gear (DC3)
• Parachute/Drag Chute (Space Shuttle)
• Tail Hook (Navy)
• Tow Bar (Navy)
• Emergencies
• USAF – Tail Hook
Level Turn
• Increased g’s are need to maintain altitude AND turn simultaneously
Coordinated Turn
• 1-g in the body axis-z direction is generated for passenger comfort.
• Combined rolling, pulling up when banked, and turning produced a “coordinated turn”.
• The aircraft actually accelerates down in the inertial-z axis and loses altitude

Inbd

near

fuselage

Aero-elastics

airstream

• Static
• Lift
• Roll
• Pitch
• Divergence
• Dynamic
• Flutter
• Example1
• Example2
• Non-aero effect
• Shimmie
• Unswept wings (whose chordlines align with the airstream) do not have a negative increment of angle of attack (gliders)
• Swept wings provide greater area with less frontal area (less drag)
• a & b are chord-lines
• perpendicular to elastic axis,
• b deflects up more than a,
• producing an elastic
• increment of negative
• angle of attack in the airstream

elastic

axis

Outbd tip

deflects up

a

b

Sonic Effects
• Sub-sonic
• Center of pressure at approx 25% chord
• Super-sonic
• Center of pressure at approx 40% chord
• Wing sweep (F-14)

Pressure distribution

Ground Effects

Case Study

• US Navy wanted to perform a test of the E-2C Landing Gear at the Maximum Sink Speed
• Sink speed is the vertical component of the landing velocity.
• This is no fun for the pilot, whose back can be injured.
• It is very difficult to do due to ground effects.
• In real life the Navy is concerned with aircraft carrier combined pitch, roll and vertical deck motions when the aircraft lands.
“Clean” Upper Wing Air Passage

Case Study

• Often airframe structural analysts are asked to review issues regarding parts manufactured out-of-tolerance.
• These parts may be scrapped, reworked or used as-is. Sometimes new parts are added, often called “doublers”
• Aerodynamicists rarely permit violation of the upper cover’s Outer Mole Line (OML), the shape of the wing on the top, but care little about the lower cover’s OML.
• Typically, no doublers are allowed on the upper cover.