Chap D: Elements of Aircraft Performance Flashcards
why cant aircraft fly above the troposphere?
1) Stratosphere has ozone layer, need ozone filter to fly in it
2) higher altitude means lower air pressure, leading to larger pressure difference between cabin and atmosphere (stronger fuselage)
3) higher altitude means higher speed, might reach critical mach number inducing high level of drag (shock waves)
4 forces in level flight and where do those act on
1) Lift - acts upwards, through CP
2) Weight - acts downwards through CG
3) Thrust - acts forwards
4) Drag - acts backwards
what does higher AOA result in for lift?
higher AOA means higher CL (from CL - AOA graph)
higher CL means more lift (direct proportional)
what are the conditions for equilibrium?
ΣFy = 0 (L = W)
ΣFx = 0 (T = D)
ΣM = 0 (nose up moments = nose down moments)
which point is infront of each other? CG or CP and why?
CG is infront to ensure nose-down pitch in engine failure
why is it difficult to balance the forces in flight?
lift acts through CP, changes with AOA
weight acts through CG, changes are fuel drain
effective thrust line affected by engine position and AOA
drag line determined by aircraft configuration
what is the use of horizontal stabiliser?
nose up pitching moment due to T & D is less than nose down pitching moment due to L & W
horizontal stabiliser is use to make up for the missing nose up pitching moment
loads on horizontal stabiliser
ΣFy = 0 [ L = W + LT]
ΣFx = 0 [ T = D]
ΣM = 0
what is the Coanda Effect?
airflow over upper surface of wing will bend with the curvature of the surface which the flow will leave the wing ‘shooting downwards’
why is downwash bad?
as it will reduce the AOA of the tail if immersed in it. (lesser AOA means lesser lift)
how much is downwash relative to AOA?
downwash is half of AOA
how to minimise downwash effects on tail?
placing tail plane as far away and as high enough compared to main wing
how does tail plane work?
it produces downward lift and hence nose up moments
is canard rigged higher or lower incidence angle of the wing? and why?
higher (larger force due to small moment arm from CG)
what can canard configuration act as well?
stall prevention device (loses lift and counter-balancing moment)
why is canard not widely used?
impact of airflow over the wing and airflow entering the engine
mess up airflow going over the wing and into the engine causing performance reduction
how does tailess aircraft work?
flaps or elevators act ailerons
why is fixed tail plane bad?
fixed incidence angle, weight is reduced as time goes so lesser lift is needed which results in lesser moment for tail to balance
how does adjustable tail-plane work?
has adjustable trailing edge devices to control pitch and trim during flight
can change camber as well
helps to keep fuselage parallel to flight path (minimise drag)
what is trimming?
keeping resultant pitching moment zero
where is adjustable tail-plane most common in?
low speed aircraft
types of tail plane
1) fixed tail plane
2) adjustable tail plane
3) slab tail plane
4) all flying tail plane
how does L/D ratio affect tail load?
the higher the L/D ratio, the larger the balancing tail load needs to be, meaning larger horizontal stabiliser
difference between AOA and incidence angle
AOA: angle between relative airflow and wing chord line
incidence angle: angle between wing chord line and fuselage centreline
what is incidence angle?
best lift to drag ratio angle of attack
define stall speed
minimum speed to sustain level flight
how to find velocity using lift formula?
W = L = 1/2 ρ S CL V2
rearrange formula to find V
difference between stalling speed and stalling angle
stalling speed: lowest speed to sustain level flight
stalling angle: AOA at which CL is maximum
airspeed wrt to CL
as airspeed increases, CL decreases exponentially
Dp & Di wrt to V during cruise?
Dp increases as V increases
Di decreases as V increases
when does Dp = Di occur?
when V is at min. drag speed
resultant total drag minimum occurs when? and what does it mean?
when Di = Dp
minimum drag speed leading to lesser fuel consumption
how does Di change when H changes? and how does air density affect Di?
as altitude increases, Di increases (VTAS remains constant)
Di is inversely proportional to density
how is Dp relative to frontal area?
as frontal area increases, Dp increases
how does altitude affect Dp?
as H increases, Dp decreases
how does ice/frost affect the aircraft?
formation of ice/frost may change wing profile, leading to Vstall changes
may create unintended roll
why is icing a hazard to flight?
changes aerofoil section profile, destroys smooth flow of air
ice may break mid-flight and strike engine intake/propeller
how does ice formation affect stall angle?
stall angle decreases
what is de-icing?
procedure where frost/ice/snow is removed in order to provide clean surface
difference between de-icing and anti-icing
de-icing: reactive, breaking up after allowing ice to form
anti-icing: proactive, preventing ice from forming
how to implement anti-icing
heating or chemical fluids to prevent ice from forming
how does snow/ice on runway affect aircraft performance?
snow/ice makes runway smoother, leading to less rolling friction,
a/c needs longer takeoff and landing distance
definition of maximum speed
Vmax is the speed which there is zero excess thrust available
definition of minimum speed
Vmin is approx = Vstal (based on aerodynamics)
common aircraft propulsion types
1) reciprocating engine
2) turbojet
3) turbofan
4) turboprop
when is turbojet good at?
high flight speed (higher thrust and efficiency)
thrust derivation from Newton 2nd law
F = ma + ṁV
where, ma is for solid body, ṁV is for fluid motion
F = T = ṁV (ma = 0, no solid body in propulsion)
ṁ - mass flow rate
V - flow speed
also can be written as T = ρAV2 or T = ṁ∆v
how to increase thrust by formula?
T = ṁV
increasing ṁ or V or both
propeller efficiency relative to H
as H increases, density decreases, so propeller eff. decreases
KE wasted formula
KElost = 1/2 (Vj - V)2
Vj - jet exhaust speed
V - speed of air coming in to engine
how does jet engine perform better at high altitude?
as H increases, temp decreases, making it cooler to fly in (can help with cooling of engine)
Power available formula during cruise
PA = TVTAS = DV = PR
propulsion efficiency formula
ηp = (useful power avail.)/(total power generated)
ηp = (2V)/(V - Vj)
how does propeller aircraft create thrust?
engine transmit torque to propeller
propeller converts torque to thrust
why is propeller blade twisted?
to meet relative wind at most efficient AOA
jet engine working principle
75% of energy used for compression of air
25% left used for exhaust gas (thrust)
jet engine points
drag same regardless of altitude (can fly faster)
T due to small ṁ with large ∆v
how is turbulent air good for jet engine?
turbulent air promotes mixtures of fuel-air
better burning efficiency
turboprop points
fast acceleration to take-off
95% of thrust developed by propeller, remaining 5% by jet exhuast
turbofan points
combining high thrust from turbojet and high efficiency from propeller
large volume of air bypass combustors
fan bypass air can be used to cool down jet core
efficiencies of turboprop/fan/jet
propeller good for low speed
jet good for high speed
turbofan combines both
how is independent concentric spinning spool useful?
allows for independent control of spool, leading to better efficiency
definition of range
maximum distance an aircraft can fly for a given quantity of fuel (usually assumed full tank)
what does range depends on?
1) engine performance
2) type of powerplant
3) fuel consumption
4) amount of fuel carried
5) L/D ratio
range equation
R = (V/ct) * (L/D) * ln(W0/W1)
ct - thrust specific fuel consumption
W0 - gross weight of a/c
W1 - empty weight of a/c
how to achieve max range?
- carry as much fuel as possible
- fly at L/D max (D is minimum)
definition of endurance
maximum duration an a/c may stay airborne for a given quantity of fuel
endurance equation
E = (1/ct) * (L/D) * ln(W0/W1)
ct - thrust specific fuel consumption
W0 - gross weight of a/c
W1 - empty weight of a/c
conditions for range and endurance depends on?
1) weight (also amt of fuel carried)
2) altitude
3) thrust provided type
how to obtain max range & endurance for jet aircraft?
- fly at max:
for range: CL1/2/CD
for endurance: max L/D ratio - fly at high altitude (for range)
- lowest ct
- highest possible gross weight to empty ratio (as much fuel as possible)
definition of maximum endurance speed
speed when power required is least
definition of maximum range speed
speed for which thrust required is least
explain ground effect
when a/c flies close to the ground, CDi from wing tip vortices is reduced due to the reduction in induced downwash angle
benefit of ground effect
induced drag is significantly reduced
ground effect relative to wing’s height above ground
GE tends to 1 when further away from ground
rolling friction formula
R = µr * NormalForce
why is AOA for take-off significantly less than stalling angle?
1) reduce both Di and Dp by reducing lift
2) avoid stall due to over rotation during LO
3) avoid tail striking ground (too steep)
landed speed definition
stalling speed for touching down
why does power in propeller decrease with altitude?
higher altitude has lower density, lower mass flow
Lift and Thrust forces formula during climb
L = Wcosy
T = D + Wsiny
speed for best rate of climb is when?
when there is greatest reserve of power
rate of climb & climb angle symbol
Vc (rate of climb)
γ (climb angle)
how does weight affect Vstall, drag, and Prequired?
weight increase means:
Vstall increase
Drag increase
Prequired increase
definition of absolute ceiling
height when there is no power reserve available for climb
definition of service ceiling
height where climb velocity falls below 30m/min
how many degrees of freedom does an aircraft have?
6
3 translational, 3 rotational
definition of load factor
ratio of wing loading during a manoeuvre to that for straight and level flight
(lift to weight ratio during manoeuvre)
what is a coordinated bank turn?
a turn where a/c does not lose altitude or speed
why is inverted flight dangerous?
- a/c may not be able to upright itself
- engine may shut down due to fuel supply interrupted
how does glider generate velocity for flight?
by trading PE for KE
it loses altitude to gain speed
types of rising air to increase glider flight time
thermals - rising columns of hot air
ridge lift - wind deflected upwards
wave lift
what are thermals?
columns of rising air created by heating of surface by the sun
what is ridge lift
wind blowing against mountains, hills
endurance for a glider
AOA that is max L/D ratio
methods to lower stalling speed of aircraft
using lift devices (slots, slats, flaps) by changing shape & area of aerofoil
when do we need to lower stalling speed?
during take-off and landing, as the aircraft is at a high AOA
why do aircraft with canard wings less likely to stall?
higher angle of incidence than the main wing, meaning higher AOA, it will stall before the main wing, removing the counter-balancing moment created by it
why is lfit due to wing slightly higher than weight during cruise?
as L = W + LT
lift created by horizontal stabiliser is downwards along with weight
AOA wrt airspeed graph
as airspeed increases, AOA decreases exponentially
