Chapter 3 External Loads Flashcards
external loads
applied loads like aerodynamics, inertia, engine thrust, etc.
external and internal forces are in equilibrium
internal loads
counter react to the external forces
external and internal forces are in equilibrium
design loads (aerospace context)
forces applied to the structure components to establish the strength safety level of the complete aerospace vehicle
4 aircraft load cases
manoeuver
dynamic: gust, turbulence, buffer
ground handling: landing impact, taxi, break, turn, tow, jack, hoist
special: crash, failure
depend on the aircraft type and planned usage described in the Structur Design Criteria (SDC)
5 aircraft load sources
aerodynamics: lift, drag, speed, altitude, airfoil, trimming conditions
inertia: mass distribution, centre of gravity, accelerations
engine: thrust
landing gear: aircraft mass, sink rate, type of landing gear
special loads: engine and control surface faults
aircraft loads analysis
sources -> load case -> trimming (equilibrium) -> load case categories
Mach number
quantity defined as the ratio of the local flow velocity to the local speed of sound
indicates the flow compressibility (air is compressible from around M = 0.3)
M < 0.8 subsonic
0.8 < M < 1.2 transonic
1.2 < M < 5.0 supersonic
5.0 < M < 10 hypersonic
10 < M < 25 high hypersonic
25 < M re-entry speeds
Reynold’s number
helps to predict the flow patterns in different fluid flow situations
represents the importance of viscosity
low R: laminar flow, low drag
high R: turbulent flow, high drag
optimal lift distribution
for rigid wings: elliptic
but wings are not rigid
infinite vs finite wing
lift and drag polars are determined for infinite wings
in finite wings - due to tip vortices the lift slope curve is reduced (downwash effect) and induced drag is generated
aircraft mass categories
75% non-structural mass (engine, fuel, equipment, painting, fairings, etc.)
25% structural mass (primary structure (skin, stringers, spars, ribs, frames, floors, joints, etc.), secondary structure (pad-ups, rivets, bolts, nuts, etc.))
aircraft mass configurations
- minimum take-off weight
- basic flight design mass
- maximum wing zero fuel mass
- maximum design mass
- minimum flying mass
- landing design mass
mass changes during operation
XYZ
X: roll
Y: pitch
Z: yaw
trimming
procedure of bringing all the applied external loads into equilibrium in order to fly a steady manoeuver
main contributors:
F_x -> thrust, drag, inertia
F_y -> rudder, lift, thrust, inertia
F_z -> lift, weight
M_x -> aileron, inertia
M_y -> elevator
M_z -> rudder
unknown trimming variables
- angle of attack (alfa)
- yaw angle (beta)
- tailplane angle of attack (alfa_T)
- rudder deflection (chi)
- deflection of elevator (eta)
- deflection of any other control surface (gamma)
structure design criteria (SDC)
define, among others, the maneuvers, speeds, useful load, aircraft design weights
include V-n diagram (flight envelope)
V-n diagram
flight envelope
V: velocity
n: load factor
shows the capabilities of a design in therms of airspeed and load factor
defined for each critical combination of altitude and weight
load factor n
n = lift / inertia force = L / mg
ratio of the lift of an aircraft to its weight
represents a global measure of the “load” to which the structure of the aircraft is subjected
limit load
maximum expected load during operation
no permanent structural deformation or damage allowed
NEVER DESIGN FOR LIMIT LOADING
ultimate load
limit load x safetu factor (1.5)
maximum structural load above which structural failure can occur
between limit load and ultimate load local damages and permanent deformations are allowed
gust
sudden, brief increase in speed of the wind
dynamic load case
the faster the aircraft the less severe the gust
sharp edge gust, ramped gust, 1-cos gust
different gust speeds at different altitudes
landing gear loads categories
- landing impact
- other ground handling (taxi, take-off, etc.)
landing conditions
- level landing conditions (three-point landing, two-point landing, one-point landing)
- taildown landing
sink rate
velocity of aircraft descend
design limits:
3.05 m/s at landing weight
1.83 m/s at maximum take-off weight
assumed friction coefficient between the tires and the ground
0.8
landing speed
at a standard day (15 C) at sea level:
V_landing = V_stall_speed + V_tail_wind = V x L_1
at a hot day (37.8 C) at the highest altitude the aircraft is allowed to land:
V_landing = 1.25 x V x L_2 + V_tail_wind
V_stall_speed is the slowest an aircraft is allowed to fly
V_tail_wind = 0 when aircraft is certified for tail winds of <19 km/h
V_tail_wind =/= 0 when aircraft is certified for tail winds >19 km/h
on a hot day …
… you need higher speed to achieve the same lift as on a standard day
standard day parameters
temperature: 15 C
density: 1.225 kg/m^3
pressure: 1 atm = 101.325 kPa
viscosity: 17.3 x 10 ^ (-6) Ns/m^2
two-point landing conditions
main wheels contact the ground with the nose wheel just clear of the ground
provides maximum loads at main landing gear
three-point landing conditions
nose and main wheels contact the ground simultaneously
provides maximum loads at the nose gear
inertia (bezwładność)
capacity of an object to resist changes in motion (Newton’s First Law)
force is required to initiate or terminate motion; in absence of force constant velocity is maintained
force acting against acceleration of a mass m
added in the context of non-inertial reference frames to account for the effects of acceleration of the frame itself
fictitious force
inertial reference frame
an object we assume is not moving and with respect to which we assess motion
e.g. earth
aerodynamic loads depend on …
… air density, air speed, structure geometry and orientation
aerodynamic loads
normal stress due to fluid pressure
shear stress due to skin friction (viscosity + surface roughness)
airspeeds
GS: ground speed - speed relative to the ground
IAS: indicated airspeed - uncorrected airspeed
CAS: calibrated airspeed - IAS corrected for installation and instrument errors
TAS: true airspeed - CAS corrected for deviations from standard atmosphere conditions and compressibility effects
EAS: equivalent airspeed - speed of equivalent dynamic pressure at sea level as TAS at currsent altitude
control surfaces
- ailerons: roll and increase drag on one of the wings and increase lift on the other
- elevator: pitch
- rudder: yaw
- spoiler: reduces lift
- flaps: raise max. lift coefficient
- slats: reduce stalling speed
- air brakes: increase drag
