Chapter 3 External Loads

Question 1

external loads

Answer 1

applied loads like aerodynamics, inertia, engine thrust, etc.

external and internal forces are in equilibrium

Question 2

internal loads

Answer 2

counter react to the external forces

external and internal forces are in equilibrium

Question 3

design loads (aerospace context)

Answer 3

forces applied to the structure components to establish the strength safety level of the complete aerospace vehicle

Question 4

4 aircraft load cases

Answer 4

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)

Question 5

5 aircraft load sources

Answer 5

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

Question 6

aircraft loads analysis

Answer 6

sources -> load case -> trimming (equilibrium) -> load case categories

Question 7

Mach number

Answer 7

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

Question 8

Reynold’s number

Answer 8

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

Question 9

optimal lift distribution

Answer 9

for rigid wings: elliptic

but wings are not rigid

Question 10

infinite vs finite wing

Answer 10

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

Question 11

aircraft mass categories

Answer 11

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.))

Question 12

aircraft mass configurations

Answer 12

1. minimum take-off weight
2. basic flight design mass
3. maximum wing zero fuel mass
4. maximum design mass
5. minimum flying mass
6. landing design mass

mass changes during operation

Question 13

XYZ

Answer 13

X: roll
Y: pitch
Z: yaw

Question 14

trimming

Answer 14

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

Question 15

unknown trimming variables

Answer 15

1. angle of attack (alfa)
2. yaw angle (beta)
3. tailplane angle of attack (alfa_T)
4. rudder deflection (chi)
5. deflection of elevator (eta)
6. deflection of any other control surface (gamma)

Question 16

structure design criteria (SDC)

Answer 16

define, among others, the maneuvers, speeds, useful load, aircraft design weights

include V-n diagram (flight envelope)

Question 17

V-n diagram

Answer 17

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

Question 18

load factor n

Answer 18

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

Question 19

limit load

Answer 19

maximum expected load during operation

no permanent structural deformation or damage allowed

NEVER DESIGN FOR LIMIT LOADING

Question 20

ultimate load

Answer 20

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

Question 21

gust

Answer 21

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

Question 22

landing gear loads categories

Answer 22

1. landing impact
2. other ground handling (taxi, take-off, etc.)

Question 23

landing conditions

Answer 23

1. level landing conditions (three-point landing, two-point landing, one-point landing)
2. taildown landing

Question 24

sink rate

Answer 24

velocity of aircraft descend

design limits:
3.05 m/s at landing weight
1.83 m/s at maximum take-off weight

Question 25

assumed friction coefficient between the tires and the ground

Answer 25

0.8

Question 26

landing speed

Answer 26

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

Question 27

on a hot day ...

Answer 27

... you need higher speed to achieve the same lift as on a standard day

Question 28

standard day parameters

Answer 28

temperature: 15 C density: 1.225 kg/m^3 pressure: 1 atm = 101.325 kPa viscosity: 17.3 x 10 ^ (-6) Ns/m^2

Question 29

two-point landing conditions

Answer 29

main wheels contact the ground with the nose wheel just clear of the ground provides maximum loads at main landing gear

Question 30

three-point landing conditions

Answer 30

nose and main wheels contact the ground simultaneously provides maximum loads at the nose gear

Question 31

inertia (bezwładność)

Answer 31

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

Question 32

inertial reference frame

Answer 32

an object we assume is not moving and with respect to which we assess motion e.g. earth

Question 33

aerodynamic loads depend on ...

Answer 33

... air density, air speed, structure geometry and orientation

Question 34

aerodynamic loads

Answer 34

normal stress due to fluid pressure shear stress due to skin friction (viscosity + surface roughness)

Question 35

airspeeds

Answer 35

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

Question 36

control surfaces

Answer 36

1. ailerons: roll and increase drag on one of the wings and increase lift on the other 2. elevator: pitch 3. rudder: yaw 4. spoiler: reduces lift 5. flaps: raise max. lift coefficient 6. slats: reduce stalling speed 7. air brakes: increase drag