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.
