High Altitude Operations Flashcards
Why can modern aircraft fly higher than previous high speed aircraft?
Lighter weight materials that are also high strength and higher engine thrust from high bypass turbofan engines. (Less fuel burn , better range )
Indicated airspeed is derived how?
Difference between total and static pressure = dynamic pressure or airspeed.
What is calibrated airspeed?
IAS corrected for instrument /installation errors and positional error (static source position)
ICET, it’s a pretty cool drink 😎
What is equivalent airspeed ?
Calibrated airspeed corrected for compressibility error. (At high speeds, stagnation pressure recovered in the pitot tube is not representative of the free stream dynamic pressure due to compressibility magnification).
What is True airspeed?
Equivalent airspeed corrected for Density altitude.
Define Mach number ?
Ratio of TAS to the LSS. Measured in decimals of 1.00
Define Mcrit?
The free stream Mach number at which local sonic flow commences due to the air speeding up over the surface.
Operating at / above Mcrit does what ?
- increase drag (wave drag )
- decrease lift
- change pitching moment
- cause buffet
Explain changeover altitude and what is it when flying the PC24?
When the limiting speed of the aircraft changes from Vmo (concerned with airframe structural limits and flutter) to Mmo (compressibility and flutter limits).
PC24 = 28000ft
What are the 3 types of shockwave ?
- Normal shock (perpendicular to wing) with compressibility
- Oblique shock (at an angle) with compressibility
- Expansion wave (no shock )
Describe a normal shockwave ?
Perpendicular to the wing on upper and lower surface of the wing
Increase in pressure and density behind the shockwave
Large increase in wave drag as the boundary layer separates and the air becomes turbulent.
Describe an oblique shockwave ?
Associated with supersonic flight, supersonic airflow in front of and behind the shockwave
Pressure and density behind the shockwave still increases, airflow changes direction to follow the wing surface.
Describe an expansion wave?
Supersonic flow throughout
Increased velocity behind the wave hence decreased pressure and density behind the wave
Follows the surface of the wing.
What is centre of pressure ?
The point along the wing chord that lift acts the most. Occurs usually between 23-27% chord
What is wave drag?
As a normal shockwave forms, airflow behind the wave separates and becomes turbulent.
Large amounts of heat is generated when the shock wave is created, this needs to be overcome with thrust which presents as drag
What is Mach tuck?
Is a nose down pitching moment caused by the centre of pressure moving aft as the shockwave forms . Beyond Mmo the wave moves further aft compounding the effect.
How do we counter Mach tuck ?
- operate below Mmo
- have a movable horizontal stabiliser
- incorporate a Mach trimmer
What is high speed buffet?
• an all you can eat feast at high speed above Mmo
What actually is high speed buffet ?
As the shockwave forms and speed is further increased , boundary airflow hits the shockwave and it separates. Can be induced in a high alt/ speed bank where lift /boundary flow is sped up with with loading.
What is the load factor if flying twice Vs and pitching for max lift ?
4g
Load factor is proportional to the square of speed.
So if flying 3 times Vs, load factor is 9.
What is the most critical gust load ?
Vertical gust loads , due to the fact they change angle of attack , hence lift , hence load factor.
What gust loads must an aircraft be certified to?
+/- 30ft/sec (18kts ) at Maximum level flight speed
What is better for gust loads, high aspect or low aspect ratio?
A swept low aspect ratio wing (due to the lower lift curve)
What is roll reversal / aileron reversal?
At high speeds, aileron input causes high torsional forces on the wing causing a reduced AoA on the downward going aileron side. This causes the aircraft to roll opposite to that intended. Can be defined as a speed. Above that speed structural damage can occur.
What is put in place to alleviate aileron reversal?
Mid span control device , either a flaperon or roll spoiler.
Ailerons are commonly locked out at high speed
What is divergence ?
When aerodynamic force over powers torsional stiffness. Can lead to structural failure. Short span, sweep back and taper all increase divergence speed.
What is flutter ?
A combination of aerodynamic forces , inertia and elastic properties of a surface induced by subjecting a structure to a force frequency near its natural frequency which can lead to unstable oscillations. Occurs most commonly at wingtips.
How is wing loading measured ?
Lb/ft2 or Kg/m2
An aircraft with a high mass and small wings has what kind of wing loading ?
High
What are the benefits of lower wing loading ?
More lift, slower takeoff and landing speeds , and hence shorter distances
What is the wing loading of a B747 Vs a bird ? (Kg/m2)
730kg/m2 Vs 1-20kg/m2
What is Dutch roll?
An out of phase yaw and roll oscillation induced by disturbances from equilibrium.
Presents when there is a stronger positive lateral stability and weaker positive directional stability
Why does Dutch roll occur in transonic aircraft ?
It has strong lateral stability due to its large dihedral effect in comparison to static directional stability.
When is Dutch roll dangerous ?
When oscillatory stability is negative and left uncorrected.
What is used to correct /prevent Dutch roll?
Yaw Dampners. Or if correcting manually use brief aileron input only.
Define an aircraft upset ?
- Pitch attitude >25 up
- Pitch attitude <10 down
- Bank angle >45
- inappropriate airspeeds
What are the main 2 causes of fatalities?
- Loss of control in flight
2. CFIT
What conditions could cause a high altitude wake turbulence event?
- following too close
- crossing paths or reciprocal tracks with 1000ft separating
- generally high or super aircraft Vs medium wake cat
What procedure can be used to mitigate high alt wake encounters?
SLOP - strategic lateral offset procedure up to 2nm to the right of cleared route
The formula for thrust is?
Thrust = mass flow x (inlet velocity - exhaust velocity)
• thrust measured in lbs or newtons
• mass flow thorough the engine itself measured in slugs per second
• velocity measured in ft/sec or m/sec
Do jet engines have higher or lower propulsive efficiency compared to turboprop or piston engines?
Lower. However they make up for it with size and weight of engine Vs thrust.
What does the engine inlet do?
Reduces velocity and increases pressure
What does the turbofan do?
First stage of compression and mainly to bypass a large of volume of air around the engine for thrust and cooling.
What do the compressors do?
Compresses the air through stages (axial or centrifugal) and reduces volume
What does the combustion chamber do?
Mixes fuel and the compressed air then ignites it to produce heat and pressure and an increase in volume.
What does the turbine do?
Extracts the energy out of the gas flow with further expansion, increase volume and decrease in pressure.
Usually 3 stages to power the compressor, 1st high speed compressor, 2nd low speed compressor, 3rd the fan
PC24 - Single HP turbine powers single centrifugal HP compressor
2 LP turbines power the fan and 3 LP axial compressors
What does the tailpipe/exhaust do?
Mixes all the mass flow and directs it rearwards. Convergent duct
Where does maximum range occur?
Where the maximum proportion between the square root of the lift coefficient and drag coefficient occurs. i.e. tangent line on thrust required vs velocity graph
How does altitude effect range (nil wind)?
Increased range at higher alts. Higher TAS, lower fuel flow for given thrust required
How does aircraft weight effect range?
Higher weights mean higher thrust required to maintain the desired speed, so range is less. Speed must be reduced as fuel is burnt off to maintain best range.
How can best range be achieved constantly?
Get a block level for climb and remain at optimum altitude for fuel burn off.
How does temperature effect range?
Increased range as temperature decreases. Decreased temp reduces specific fuel consumption
How does wind effect range?
Headwind will reduce range, vice versa.
Must fly faster in a headwind to counter wind and slower in a tailwind.
When should a change in airspeed be made to counter wind for best range?
When wind velocity exceeds 25% of the zero wind cruise speed.
