Ch 29 - Effects Of High Speed Flight

Question 1

Mcrit

Answer 1

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

Question 2

A weak normal shock Wave (just past Mach 1)

Answer 2

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

Question 3

Slightly stronger normal shock wave

Answer 3

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

Question 4

Normal Shockwave even faster

Answer 4

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

Question 5

Normal Shock Wave very intense

Answer 5

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

Question 6

Normal Shock wave with MFS now almost at M1

Answer 6

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

Question 7

Bow Waves

Answer 7

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

Question 8

Shock Stall

Answer 8

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

Question 9

Effect of Mach Number on CLMAX and the Stalling Angle

Answer 9

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

Question 10

Mach Tuck

Answer 10

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

Question 11

Mach Trimmer

Answer 11

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

Question 12

The Drag Divergence Mach Number

Answer 12

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

Question 13

Centre of Pressure Effects

Answer 13

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

Question 14

Stick Force Gradient

Answer 14

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

Question 15

Control Effectiveness

Answer 15

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.

Question 16

Buzz

Answer 16

When the shock waves moves across the control surface you get what’s called a Buzz

Buzz: High Frequency Oscillation

Question 17

Slow Speed Stall

Answer 17

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

Question 18

High Speed Stall

Answer 18

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

Question 19

Aerodynamic Ceiling

Answer 19

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

Question 20

How to offset the effect of Mcrit

Answer 20

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

Question 21

Thinner Wings aren’t practical for increasing the value of Mcrit because…

Answer 21

Structurally they are now hard to cope with the demand on them

Less space for fuel and flaps

Less space for the landing gear

Question 22

Super Critical Wings

Answer 22

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

Question 23

Sweep Back

Answer 23

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

Question 24

Vortex Generators

Answer 24

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

Question 25

Area Ruling

Answer 25

Reducing the cross sectional area of the aircraft which reduces the amount of wave drag