Ch 29 - Effects Of High Speed Flight Flashcards
Mcrit
The highest speed an AC can travel at with no supersonic flow around the aerofoil
Will have sonic - where the local flow first reaches Mach 1
No shock waves are present
Any faster than this and a normal shock wave will be created
In modern airliners = ~0.78/0.80
A weak normal shock Wave (just past Mach 1)
Shock wave is 0.0025mm thick
The static pressure reduces ahead of the wave but increases suddenly after the wave resulting in an irregular pressure distribution.
A strong adverse pressure gradient forms at the base of the shock wave
Speed before the normal shock wave will increase slightly over Mach 1, the speed behind the shock wave will decrease by the same factor.
Cl and Cd slightly increases
Slightly stronger normal shock wave
Upper surface has intensified and moved back slightly, Cl reaches its maximum value just as separation occurs at the base of the shock wave. This causes CD to increase rapidly and the Cp moves slightly further back
Normal Shockwave even faster
The upper surface shock wave intensifies and moves further aft, a normal shock wave forms on the lower surface causing the Cl to drop rapidly whilst because of separation, the CD increases rapidly with the CP moving slightly aft
Normal Shock Wave very intense
The lower and upper normal shock waves have intensified rapidly, lower shock wave has moved rapidly to the trailing edge of the wing, CL drops to its lowest value in the transonic region while CD continues to rise
The CP moves forward now because the lower shock wave is now aft of the upper
Normal Shock wave with MFS now almost at M1
Both shock waves are now at the trailing edge, CL increase as the extremely low static pressure moves aft with the normal shock to cover more of the upper surface.
The normal shock waves are now at their most intense, CD reaches its greatest value, CP moves aft towards the trailing edge and now sits at the mid chord position
Bow Waves
Formed just after MFS1.0
A central normal shock wave on the leading edge extends out into an oblique shockwave
Almost all the flow around the aerofoil is now supersonic minus the trailing edge shock wave
Shock waves on the trailing edge are now oblique and CD steadily falls
Rhomboidal lift distribution around the aerofoil
Shock Stall
Shock stall is defined as the point when the lift coefficient, as a function of Mach number, reaches its maximum value at a Mach number, just above Mcrit
Usually at the lower end of the transonic region
It can occur at any AOA and only occurs at high speed
Effect of Mach Number on CLMAX and the Stalling Angle
Because compressibility causes steeper upwash at the leading edge; The Cl for a given AoA increases and the stalling angle of attack and CLMAX decreases
This is why the CAS stall speed is faster at higher altitude
Mach Tuck
When the normal shock waves shoot to the trailing edge of the aerofoil
This causes a nose down pitching moment as the CP is now further aft
Compensated for by a Mach trimmer
Mach Trimmer
Fitted to aircrafts that plan to operate in the transonic region
Automatically adjusts the stabiliser for you so that the pitching moment doesn’t effect the AC significantly
Anticipates the movement coming and so trims constantly preparing for it
If unserviceable, just reduce cruise speed
The Drag Divergence Mach Number
The Mach number at which the CD rises rapidly. It provides the permanent Mach number limit for jet transport aeroplanes.
Mdrag-divergence is; faster than Mcrit, Just past the shock stall, caused by flow separation
Centre of Pressure Effects
The CP moves from ~25% of the chord at Mcrit to ~50% of the Chord at Mfs1.0
CP movement is not progressive, its rate and direction change throughout the range
Stick Force Gradient
Stick force gradient may start to reduce in Aircraft that aren’t designed for high speed flight. They may become neutral and then unstable as the speed increases
Control Effectiveness
As the speed increases, the Control effectiveness also increases.
The controls (which are situated at the back of Aerofoils are most effective just at Mcrit
They reduce past Mcrit due to seperation and can reduce to pretty much nothing (controls become ineffective)
To reduce the decrease in control, you can use all moving tailplanes.
Buzz
When the shock waves moves across the control surface you get what’s called a Buzz
Buzz: High Frequency Oscillation
Slow Speed Stall
When the air has to use more energy to move over the wings, the stall speeds increase as the air is less able to stay attached to the aerofoil (separates)
Amount of work done by the air is effected by: AoA Thickness of aerofoil Camber LF - How hard you are working the wing
At altitude, the slow speed stall speed increases as alt. Increases because the air has to work harder due to compressibility, therefore there is more energy loss. Stall speed increases
High Speed Stall
More energy used with altitude so the high speed stall gets slower
At a high enough altitude, the slow speed stall and the high speed stall will equal the same
Aerodynamic Ceiling
The highest altitude at which the AC can get to without stalling
Where the slow speed and the high speed stall speed will be the same value.
Known as coffin corner, hemmed in by your stall speed, the higher you go .
Will reduce any time you make the wing work harder; Increasing the LF, increasing the Mass and moving the CG fwd
How to offset the effect of Mcrit
To travel fastest, we want to increase the value of Mcrit as high as we can and we have design features that help us do this;
Thinner wings; not practical for many reasons (less fuel for one)
Flatter top surface known as Super Critical Wings
Sweep Back
Thinner Wings aren’t practical for increasing the value of Mcrit because…
Structurally they are now hard to cope with the demand on them
Less space for fuel and flaps
Less space for the landing gear
Super Critical Wings
Used to reduce the effect of Mcrit
Positively and negatively Cambered
Increased Thickness:Chord Ratio
Delays Mcrit which is what we want
Has a larger leading edge radius
Sweep Back
Decreases the effective Thickness:Chord Ratio by altering the effective flow velocity
Increases Mcrit
This provides a smaller drag hump to get over
But has a less defined stalling AoA
And is still prone to tip stalls
Vortex Generators
Placed on the wind surface usually just behind the position where the normal shock wave normally forms
They re-energise the airflow which delays separation delaying the onset of high speed buffet and drag divergence.
Ensuring aileron effectiveness
Area Ruling
Reducing the cross sectional area of the aircraft which reduces the amount of wave drag
