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Aircraft Mechanics

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

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  1. Aircraft Mechanics Alan Altschuler (Mr. A)

  2. 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

  3. 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

  4. 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

  5. Lift curve • Bernoulli • Angle of attack • Camber • Bernoulli • NACA airfoil

  6. 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) • 2-bladed props Principle axes

  7. Inertia • Linear • Angular • Issue with Most Helicopters • Banking a bicycle • Car • Tilting on an unbanked roadway • Roadway banking • Center of Gravity

  8. Ground Configuration

  9. 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

  10. Lifting Surfaces • Wing • Bi-plane wing structures • Winglet • Horizontal Stabilizer (Tail) • Vertical Stabilizer (Tail) • Canard • X-29 • Fuselage • F-14 body lift

  11. Ground Configuration - parked Air Configuration – steady level flight

  12. Control Surfaces • Elevators • Ailerons • Rudders • Pictures • Spoilers • Used less often • Speed brakes • Elevons (Space Shuttle) • “Flying” Horizontal Stabilizers (F-14) • Aka stabilators

  13. 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 • USN – Barricade1 • USN – Barricade2

  14. Level Turn • Increased g’s are need to maintain altitude AND turn simultaneously

  15. 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

  16. 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

  17. 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

  18. 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.

  19. “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.

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