11.1.2 High Speed Flight Flashcards
What is the speed of sound in dry air at 20°C (68°F)?
343.2m/s
In a stiff material, such as diamond, what is the speed of sound?
12,000m/s which is the maximum speed of sound in normal conditions
What are the three sound regions?
- subsonic
- transonic
- supersonic
In what sound region is the air said to be “incompressible”?
Subsonic region
What happens when an aircraft travels supersonic?
Shockwaves are produced
- Large pressure changes
- Changes in density
What are the Mach number zones for:
- subsonic
- transonic
- supersonic
Subsonic = Mach <0.8 Transonic = Mach 0.8 - 1.2 Supersonic = Mach 1.2 - 5.0
How do you find the Mach number?
The speed of the aircraft
———————————
The speed of sound
What would a Mach number of 0.7 indicate?
70% of the speed of sound
What is the critical Mach number?
The lowest forward aircraft speed before local Mach 1 can occur (over the wing)
It is the highest Mach number an aircraft can have without supersonic flow
When does a normal shock wave occur?
At Mach 1.2
What is the problem with a normal shockwave?
It increases the static pressure behind the shockwave and flow separation occurs as the air has little kinetic energy. This reduces the lift created
What is a bow wave?
A wave that forms at the leading edge when an aircraft exceeds the speed of sound
What is wave drag?
Drag caused by shockwaves
How can wave drag be reduced?
- By the use of vortex generators (the reduced air speed behind the oblique shockwave reduces the normal shockwave)
- The ‘area rule’
What is the disadvantage of using vortex generators?
It increases parasite drag slightly
What is the ‘area rule’ diagram?
The cross sectional areas of the wing are plotted against the body length of the aircraft to show a very smooth curve
Some aircraft have a waist fuselage. What is the advantage of this?
It reduces the drag and fulfils the area rule diagram of a smooth curve
How do you work out the profile thickness?
Actual thickness
———————
Chord length
Why do Aircraft have a thin wing profile rather than a thick one?
It reduces the shock wave created in transonic flight and also the wave drag of the profile
If a profile is 1.5m thick and the chord length is 10m, what is the overall thickness?
15% or 0.15
What is the purpose of a swept wing?
- reduced thickness as the chord length is longer
- increases the critical Mach number
What is the sweep angle on a modern aircraft?
30°
What is the purpose of a variable swept wing?
It improves the performance at both high and low speeds.
- high speeds = increased chord length by swept wings
- low speeds = decreased chord length
What is a transonic profile?
A profile which performs better in the transonic range - no flow separation behind the shockwave
- flatter upper surface (decelerating the supersonic airflow creating a much smaller shockwave)
- more curved leading edge (immediately accelerates to supersonic)
- thinner trailing edge
What is a wing with a transonic profile also known as?
A ‘rear loaded’ wing
What is the cruising Mach speed for most airliners?
Mach 0.8
What is the advantage of a transonic profile’s construction?
The entire outer skin is used as a torque box which allows for thinner and lighter material to be used
What is the advantage of a greater wing chord?
Greater fuel capacity
A transonic profile can have a smaller sweepback angle than a subsonic profile. What is the advantage to this?
- it improves the low speed characteristics
- it means simpler lift devices can be used
What is a disadvantage of a transonic profile?
The drag on a transonic profile compared to a conventional profile is greater at a mid Mach number to below the critical Mach number.
What is the ‘nose-down’ reaction called when an aircraft with a transonic profile stalls at the root as it passes the critical Mach number?
The ‘tuck-under’ effect
What control surface counteracts the ‘tuck-under’ effect and what is the automated system called?
The horizontal stabiliser.
The automated system that does this is called the Mach trim system
Why do shockwaves waste energy?
They convert useful energy into wasted heat energy
What happens to temperature, density, pressure and velocity to supersonic airflow when it passes through a shockwave?
Temperature = increases Density = increases Pressure = increases Velocity = decreases
What are the two main types of waves formed in supersonic airflow?
Shockwaves and expansion waves
What are the two types of SHOCK waves?
Normal shockwaves
Oblique shockwaves
What are normal shockwaves?
- It slows the velocity of the airflow down to subsonic
- A lot of energy is wasted in the form of heat
- The direction remains the same
What is an oblique shockwave?
- it consumes less energy than a normal shockwave
- the velocity of the airflow remains supersonic
- the direction of the airflow is sharply changed however
What is an expansion wave?
- supersonic airflow increases in velocity
- no change in the energy
- airflow follows the surface (as long as there is no flow separation)
What are the two supersonic profiles?
- double wedge
- circular arc
What is important about a double wedge profile in regards to the lift?
It produces zero net lift at a small angle of attack
Where is the centre of lift on a circular arc profile if the airflow is SUPERSONIC?
50% chord position
Where is the centre of lift on a circular arc profile if the airflow is SUBSONIC?
25% chord position
What must be done to the air entering the compressor section of a supersonic aircraft?
It must be slowed down to subsonic airflow in the most efficient way (with the minimum energy loss)
What type of supersonic jet engine compressor inlet is designed for speeds just in excess of the speed of sound?
A normal shock diffuser inlet
- creates a normal shock wave to slow the air down to subsonic
What is a single or multiple oblique shock inlet?
A jet engine inlet that employs external oblique shockwave(s) to slow the supersonic airflow before a normal shockwave occurs.
This reduces the energy loss
What is a variable supersonic inlet?
Similar to a multiple oblique shockwave inlet it has actuators that move panels to create 3 oblique shockwaves to slow the supersonic airflow down before the normal shockwave occurs. This in turn, ensures minimum energy loss.
What is aerodynamic heating?
The temperature rise that occurs when the Mach number increases. It increases slower at subsonic airflow but increases rapidly at supersonic airflow.
What happens to an aluminium alloy’s strength if the temperature rises up to 250°C?
It loses strength by up to 80%
What happens to stainless steel’s strength if the temperature rises up to 250°C?
It loses strength by up to 20%
What happens to a titanium alloy’s strength if the temperature rises up to 250°C?
It loses strength by up to 45%
