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 ConfigurationAll axis-systems aligned

Zb,a,i

Xb,a,i

Air ConfigurationAll axis-systems mis-aligned here

Zb

Xb

Zi

Xi

Za

Xa

airflow

Aligned with airframeAligned with airflow

Alignedwith earth

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)– 2-bladed props

Principle axes

Inertia

• Linear• Angular– Issue with Most Helicopters– Banking a bicycle– Car• Tilting on an unbanked roadway• Roadway banking

• Center of Gravity

Ground Configuration

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– USN – Barricade1– USN – Barricade2

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

Aero-elastics• 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)

Outbd tipdeflects up

Inbdnear fuselage

ab

airstream

elasticaxis

•a & b are chord-linesperpendicular to elastic axis,•b deflects up more than a,•producing an elasticincrement of negativeangle of attack in the airstream

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.