Alan Altschuler Mr A Forces and Moments on Aircraft Forces L ift Drag Thrust Inertia Gravitational weight Linear and Rotational Momentum Ground Support Weight SpeedUp Catapult SlowDown ID: 205393
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Slide1
Aircraft Mechanics
Alan Altschuler (Mr. A)Slide2
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
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 rightSlide3
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 systemsSlide4
Ground Configuration
All axis-systems aligned
Z
b,a,i
X
b,a,i
Air Configuration
All axis-systems
mis
-aligned here
Z
b
X
b
Z
i
X
i
Z
a
X
a
airflow
Aligned with airframe
Aligned with airflow
Aligned
with earthSlide5
Lift curve
Bernoulli
Angle of attack
Camber
Bernoulli
NACA airfoilSlide6
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 enginesE-2 tail (Hawkeye)
Single main-rotor helicoptersA-10 nose wheel (Warthog/Thunderbolt)2-bladed props
Principle axesSlide7
Inertia
Linear
Angular
Issue with Most
Helicopters
Banking a bicycle
Car
Tilting on an unbanked roadwayRoadway bankingCenter of GravitySlide8
Ground ConfigurationSlide9
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/dtIn non-calculus F = DP/Dt = m Dv/Dt + v Dm/DtDv/Dt = aF = ma + v Dm/Dt the second part is the “rocket” part, exhausted gas quickly from the bodySlide10
Lifting Surfaces
Wing
Bi-plane
wing structures
Winglet
Horizontal Stabilizer (Tail)
Vertical Stabilizer (Tail)
Canard X-29FuselageF-14 body liftSlide11
Ground Configuration - parked
Air Configuration – steady level flightSlide12
Control Surfaces
Elevators
Ailerons
Rudders
Pictures
Spoilers
Used less often
Speed brakesElevons (Space Shuttle)“Flying” Horizontal Stabilizers (
F-14)Aka stabilators Slide13
Landing Gear(Alighting Gear)
Main Gear
Metering Pins
Nose Gear
Tail Gear (
DC3
)
Parachute/Drag Chute (Space Shuttle)Tail Hook (Navy)
Tow Bar (Navy)EmergenciesUSAF – Tail HookUSN – Barricade1USN – Barricade2Slide14
Level Turn
Increased g’s are need to maintain altitude AND turn simultaneouslySlide15
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 altitudeSlide16
Aero-elastics
Static
Lift
Roll
Pitch
Divergence
Dynamic
FlutterExample1Example2Non-aero effectShimmie
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 upInbdnear fuselageab
airstreamelasticaxisa & b are chord-linesperpendicular to elastic axis,b deflects up more than a,producing an elastic
increment of negativeangle of attack in the airstreamSlide17
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 distributionSlide18
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.Slide19
“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.