Chap F: High Speed Flight Flashcards
what is transonic flight?
flight range between high subsonic and low supersonic
definition of mach number
ratio of true airspeed to local speed of sound
VTAS/a
a - local speed of sound
speed ranges in terms of M
low subsonic (incompressible): below M0.3
subsonic (compressible): M0.3 ≤ M ≤ M0.75
transonic: M0.75 ≤ M ≤ M1.2
supersonic: M1.2 ≤ M < M5.0
hypersonic: ≥ M5.0
what is sound created?
propagation of weak pressure waves
what happens when the sound emitter increases the speed?
pressure waves cannot outrace the soundwaves at M1.0 causing shock waves to occur
is maximum local speed greater or lower than flight speed?
greater than
some parts of the wing may experience M1.0 locally if flying at neat sonic speed
formation of incipient shock wave is due to?
rapid increase in pressure and density
definition of critical mach number
Mach number at which airflow at any part of the wing attains a velocity equal to local speed of sound
where is airflow fastest at?
maximum thickness
why is there a region of separated flow immediately aft of shock waves?
presence of large adverse pressure gradient
what is shock stall?
shock waves forms at the aft boundary of the supersonic flow causing the airflow to separate behind the shock waves, leading to loss of lift
difference between shock stall and high AOA stall
shock stall can occur at low AOA
how does shock stall happen?
flow after shock is turbulent and separated, causing region after shock to lose capability to produce pressure reduction for lift generation
behaviour of aircraft at shock stall
1) loss of lift
2) large increase in drag
3) rearward movement of CP
4) increased nose down pitching moments
reason for loss of lift at shock stall
due to boundary layer separation on upper surface
does lift coefficient increase or decrease after flow is fully supersonic?
decrease
what is transonic drag rise?
wave drag on highspeed aircraft increase substantially as flight speed approaches M1.0
what is the reason of transonic drag rise?
formation of shock waves around M0.8
how does drag coefficient increase/decrease in supersonic speed?
increase until bow wave attaches to the leading edge
decreases further once flow is fully supersonic
what is the force divergence mach number?
mach number where wave drag becomes very significant
where does CP move due to shock waves?
from 1/4 chord to 1/2 chord once flow is fully supersonic
how does movement of CP due to shock waves affect aircraft performance?
1) increase nose down pitching moment
2) increase longitudinal stability
3) more difficult to manoeuvre aircraft
what is buffeting?
result of unsteady flow caused by separation, shock waves move back and forth rapidly
what will buffeting cause?
structural failure if buffet frequency is the same w resonance frequency
where does the first shock wave appear first on the wing?
at the thickness part of the aerofoil on the upper surface as it is where velocity is sonic
what is tuck under effect?
shock waves appear at the wing root, resulting in flow separation and shock stall at the wing root
where does CP move during a shock stall? and what does it cause
rearwards and outwards, towards the wing tip
rearwards movement results in nose-down pitching moments
what is control reversal?
adverse effect on the controllability of aircraft (aircraft move in the opposite direction that the pilot chose)
when does control reversal occur?
when hinged trailing edge device type control surfaces deflect downwards resulting in CP moving aft
when amount of airflow over wing is large enough for forces generated by aileron twist the wing by itself
ways to raise critical mach number
1) slimness - use thin wing sections
2) sweepback - leading edge sweepback
3) area rule - coke bottle shape
4) boundary layer control
how does little mean camber raise Mcrit?
lesser curvature means reduced acceleration over upper surface of wing
disadvantages of thin wings
1) low CL max
2) higher stalling speed
3) higher take-off & landing speed
4) reduced fuel carrying capacity
what is a supercritical wing?
- flatter upper surface, rounder lower surface
- has downwards curve on trailing edge
how does supercritical wing increase Mcrit?
1) delay shock wave formation
2) reduce strength of shock wave over wing
3) lesser drag
4) more rigid but lighter
advantages and disadvantages of supercritical wing
adv:
1) thicker section
2) improved structural strength
3) more capacity for fuel
4) lighter wing structure
disadv:
increased pitching moment at low speed
thinner trailing edge (harder to install TE devices)
how does sweepback increase Mcrit?
- increases all stability
- delays onset of shock stall
sweepback advantages and disadvantages
adv:
- higher Mcrit
- improved stabilities
disadv:
- wing tip stall due to twisting
- possible span-wise flow breakaway near wing tip
- possible loss of aileron control
what is the area rule?
reduce volume of body when there is wing, tail surface, etc to avoid discontinuities in cross-sectional area distribution from nose to tail
area rule advantages
1) reduced drag
2) pitch instabilites removed
3) higher acceleration
area rule disadvantages
1) reduced cross sectional area
2) conflict with:
landing gear storage
passenger seating
headroom
lesser fuel tank area
ways for boundary layer control in high speed flight
1) vortex generators
2) wing fences/stall wedges
3) LE notches/dogtooth
what are vortex generators and what do they do?
small airfoils that are attached to the surface of the wing to catch airflow at high AOA
re-energise boundary layer thus preventing flow separation
what do flow separation create at high speed fight?
loss of lift and increased drag
what are wing fences?
fin-like vertical surfaces attached to the upper surface of the wing
what do wing fences help with?
minimise span-wise flow separation & reduce wing tip stalling
how does transonic speed affect control systems?
control reversal likely to happen
lose control over ailerons and elevators when shockwave disrupt airflow
control surfaces at the back has no effect on flow ahead of shock wave
where is shock wave most likely to form?
at hinge line (between fixed and control surfaces)
how to prevent stabilisers from promoting formation of shock waves?
adjust trim or aircraft using slab or all moving tail plane
one way to have good control at high speed?
fuel transfer to front/back of aircraft
low speed - more forward CG
high speed - more aft CG
definition of supersonic flight using Mach number
M ≥ M1.2
mach angle relative to mach number
higher the mach number, small the mach angle = smaller mach cone
definition of compressive flow
hitting against a sharp corner (shock waves) formed by mach lines
converging-diverging duct for compressive flow properties
converging:
P increase, V decrease, density increasing
diverging:
P decrease, V increase, density decreasing
opposite of incompressible flow (for P & V)
definition of expansive flow
flow going into divergent duct (bigger space, convex corner) forming expansion waves
what do expansion waves do? are they gradual or shock change?
increase velocity
decrease: pressure, density, temp
gradual change
what corner creates shockwaves? and how does the airflow change its properties?
concave corner
tighter space, so increase pressure, decrease velocity
expansion wave effect on energy
no change as there is no shock
typical section for high speed flight?
slender and with minimum camber
pressure coefficient arrow direction on wing
-ve Cp point outwards
+ve Cp point inwards
flat plate for supersonic flight
ideal pressure distribution but has no depth (no structural)
types of section for supersonic flight
1) flat plate
2) double wedge
3) hexagonal wedge
4) circular arc
double wedge section description
sharp LE and TE, max thickness at mid-chord
hexagonal section description
upgrade from double wedge:
improved aerodynamic qualities
increased structural integrity
but cannot handle low speed flight
law of forbidden signals meaning
disturbance originating from a point on wing can only affect area behind it
supersonic wing planform
low AR swept wings
cause of aerodynamic heating
caused by fluid friction due to high speed of airflow over it
friction between air molecules and surface slows it down
aerodynamic heating process
- airflow come to rest at various stagnation points (considerable change of energy)
- total energy is constant (Bernoulli’s eqn)
- KE decrease means pressure energy increase
- pressure increase means temp increase
where is aerodynamic heating lowest and highest?
lowest at low speed (negligible)
highest at high speed and in troposphere where density is greatest
effect of aerodynamic heating on aircraft
strength of metal drops as temperature increases
ways to minimise aerodynamic heating on aircraft
parts of aircraft subjected to high aerodynamic heating can be made with other type of high heat resistance materials (Titanium, stainless steel)
actual surface temperature formula
t = T + ΔT
similarities & differences of shock and conventional stall
similarities:
loss of lift
flow separation
differences:
shock is in high speed, low AOA
conventional is low speed, high AOA
what happens when airflow through oblique shock waves?
increase in temp and static pressure, decrease in velocity and change in direction
why do supersonic aircraft have sharp leading edge?
enables the bow wave to fully develop and attach to the LE therefore minimising the size of subsonic patch
