P4 Airflow Over Transonic Plane

Question 1

What is the M range of transonic flight?

Answer 1

Mcrit < Mfs < Mdet

Question 2

What is the definition of Mdet?
What is the definition of Mdd?
What causes Mdd?

Answer 2

Mdet: FREE STREAM MACH number when BOW SHOCK DETACHES from LEADING EDGE;
Mdd: FREE STREAM MACH number when CD begins to INCREASE SIGNIFICANTLY;
Second SHOCKWAVE on LOWER SURFACE and INCREASING Mfs which INCREASES shockwave INTENSITY and therefore INCREASES turbulent WAKE

Question 3

What are the conditions and simplified process of shockwave formation on an aerofoil?

Answer 3

When Mfs reaches Mcrit, ML over the MOST CAMBERED location reaches 1, the airflow becomes SONIC and MACH WAVES form;
When the LOCAL SPEED is GREATER than the SPEED of SOUND ie: Mfs INCREASES, the waves get STRONGER and ACCUMULATE to form NORMAL SHOCKWAVE due to the PRESSURE DISTURBANCE;
BEHIND the SHOCKWAVE is a TURBULENT WAKE, which is NOT STABLE, OSCILLATES and tends to SEPARATE from surface when Mfs INCREASES

Question 4

Describe the structure of a normal shockwave on an aerofoil?

Answer 4

Form on the UPPER and LOWER surface of the aerofoil;
Very THIN, but the ROOT is much THICKER called the LAMBDA FOOT;
The AFT side the LAMBDA FOOT is part of the NORMAL SHOCKWAVE and the FORWARD side is a SMALL OBLIQUE SHOCKWAVE;
The lambda FOOT is partially CAUSED by a PRESSURE LEAK where the pressure CHANGE is NOT SUDDEN through BOUNDARY LAYER where LOCAL SPEED can be LOWER than SPEED of SOUND, so the PRESSURE in the lambda FOOT is GREATER than pressure in FRONT of normal shockwave but SMALLER than pressure BEHIND normal shockwave

Question 5

What are the characteristics of the turbulent wake?

Answer 5

FORMS BEHIND normal shockwave in OSCILLATING MOTION;
HIGHER the MACH BEFORE shockwave, STRONGER the SHOCKWAVE, STRONGER the WAKE OSCILLATION;
Tends to SEPARATE at the REAR when SHOCKWAVE is WEAK, when SHOCKWAVE is STRONGER it SEPARATES closer to shockwave, can SEPARATE IMMEDIATELY AFTER shockwave if strong enough

Question 6

How does the shockwave move as Mfs increases from Mcrit to Mdet?
What are the conditions?
What is the speed of airflow once fully established in different locations?

Answer 6

MOVEMENT is CAUSED by CHANGE in MACH number;
LOCAL airflow becomes SONIC when Mfs reaches Mcrit and MACH WAVES form at MOST CAMBERED location;
Mfs INCREASES beyond Mcrit, MACH WAVES ACCUMULATE and get STRONGER forming a NORMAL shockwave on the UPPER SURFACE which moves REARWARD, lambda FOOT INCREASES in STRENGTH and SIZE, turbulent WAKE OSCILLATES MORE and SEPARATION POINT moves FORWARD;
For ASYMMETRICAL aerofoil, further INCREASE in Mfs will cause SHOCKWAVE to INCREASE in STRENGTH, turbulent WAKE forms IMMEDIATELY BEHIND and NORMAL shockwave forms on the LOWER SURFACE. However, for a SYMMETRICAL aerofoil at 0°AoA NORMAL shockwaves are formed on the UPPER and LOWER surface at the SAME INSTANT;
The shockwaves formed on SYMMETRICAL aerofoil at 0°AoA move in SYNC towards TRAILING EDGE. However, the LOWER shockwave on ASYMMETRICAL aerofoil moves FASTER and settles at TRAILING EDGE first, while UPPER shockwave settles there when Mfs is APPROXIMATELY 1;
When both UPPER and LOWER shockwaves are established at REAR the entire SURFACE is covered by SUPERSONIC flow and there is NO SEPARATION;
When Mfs EXCEEDS 1, BOW SHOCKWAVE forms at LEADING EDGE until a certain Mfs where it DETACHES (Mdet);
UPPER, LOWER, FREE STREAM and BEHIND OBLIQUE segments of BOW shockwave is SUPERSONIC. MACH number BEHIND NORMAL segment of BOE shockwave is SUBSONIC

Question 7

What is shock drag?

What are the types of shock drag?

Answer 7

DRAG caused by SHOCKWAVES;

WAVE DRAG and BOUNDARY SEPARATION DRAG

Question 8

What is wave/energy drag?

Why does it occur?

Answer 8

The RESISTANCE which the AIRFLOW must OVERCOME to PASS the SHOCKWAVE;
The formation of a shockwave requires energy as PRESSURE, DENSITY and TEMPERATURE all INCREASE after it;
The ENERGY is provided by the KINETIC energy of the AIRFLOW where the HIGHER the MACH number, the STRONGER the shockwave and the more KINETIC energy is required to pass the RESISTANCE/WAVE DRAG of the shockwave

Question 9

What is boundary separation drag?

Why does it occur?

Answer 9

The THICKNESS of the turbulent BOUNDARY LAYER INCREASES after the shockwave due to turbulent WAKE DETACHING from surface BEHIND shockwave when Mfs INCREASES affecting the AERODYNAMIC FORCES;
The VORTICES in the separating WAKE cause a DECREASE in the PRESSURE REGION where there is HIGH PRESSURE at the FRONT and LOW PRESSURE at the REAR;
The PRESSURE DIFFERENCE exerts a FORCE on the aircraft in the form of a DRAG

Question 10

What is the Mach buffet?

How does it form?

Answer 10

The RANDOM FORCED VIBRATION/HIGH FREQUENCY INSTABILITY caused by shockwave OSCILLATIONS;
When Mfs EXCEEDS Mcrit, shockwaves form on UPPER and LOWER surface of an aerofoil;
With INCREASING Mfs shockwave INTENSITY INCREASES and WAKE SEPARATES CLOSER towards shockwave;
The turbulent WAKE produces VORTICES which DETACH with a certain FREQUENCY and causes NOISE, shockwave OSCILLATION, and an INCREASE in CD;
Further INCREASE in Mfs DETACHMENT becomes very INTENSE and CLOSE to shockwave, the aeroplane will experience random VIBRATION with LOUD NOISE called MACH BUFFET

Question 11

Explain the formation of a shock stall and describe the behaviours of an aircraft that might be experienced?

Answer 11

When Mfs EXCEEDS Mcrit, shockwaves form on UPPER and LOWER surface of an aerofoil;
With INCREASING Mfs shockwave INTENSITY INCREASES and WAKE SEPARATES CLOSER towards shockwave;
The turbulent WAKE produces VORTICES which DETACH with a certain FREQUENCY and causes NOISE, shockwave OSCILLATION, and an INCREASE in CD;
Further INCREASE in Mfs DETACHMENT becomes very INTENSE and CLOSE to shockwave, the aeroplane will experience random VIBRATION with LOUD NOISE called MACH BUFFET;
If INTENSITY INCREASES further AFTER BUFFET the aerofoil will FAIL to PRODUCE LIFT called SHOCK STALL there will also be a significant INCREASE in CD

Question 12

What are the general symptoms of shock stall?

How can shock stall be avoided?

Answer 12

NOSE HEAVY, BUFFETING/VIBRATION of AILERONS, PITCHING and YAWING OSCILLATIONS (Porpoising/Snaking/Dutch roll);
Get into HIGH TEMPERATURE;
SLOW DOWN SPEED;
Considerable CHANGE of LONGITUDINAL TRIIM;
POWER CONTROL or automatic STABILITY augmentation SYSTEMS

Question 13

What is a bow shockwave?

How do the Mach numbers around a bow shock vary?

Answer 13

A BOW shockwave is a shockwave that forms at the LEADING EDGE of an aerofoil and can DETACH when Mfs reaches Mdet;
It is made of 3 SECTIONS: 2 OBLIQUE shockwave and 1 NORMAL shockwave;
Behind the OBLIQUE section airflow remains SUPERSONIC because…
Behind the NORMAL section the airflow becomes SUBSONIC because at the LEADING EDGE the VELOCITY of the airflow is 0 causing an INCREASE in TEMPERATURE due to the ENERGY EQUATION (T1 + v^2/2Cp = To);
This causes an INCREASE in SPEED of SOUND and since SPEED is SAME as Mfs MACH NUMBER DECREASES (v/a = M)

Question 14

How does CoP change with the movement of a shockwave when Mfs increases from Mcrit to Mdet?

Answer 14

Mfs reaches Mcrit and SHOCKWAVE can form on UPPER surface;
Due to INCREASING LOCAL AIRSPEED, PRESSURE DECREASES before the shockwave while PRESSURE INCREASES after, causing CoP to move towards LEADING EDGE;
Once CoP reaches it MOST FORWARD position it moves REARWARD with the shockwaves because with a more REARWARD shockwave the wing is able to produce MORE LIFT as TURBULENT WAKE SEPARATES further AFT;
Another shockwave forms on the LOWER surface and both CoP and the shockwave move REARWARD with the INCREASE of Mfs;
As Mfs INCREASES toward Mdet, both shockwaves SETTLE at TRAILING EDGE and CoP will SETTLE at APPROXIMATELY 50% of the CHORD;
CoP will REMAIN here even if Mfs INCREASES further

Question 15

How does CL change with the movement of a shockwave when Mfs increases from Mcrit to Mdet?

Answer 15

Mfs reaches Mcrit and SHOCKWAVE can form on UPPER surface;
Due to INCREASING LOCAL AIRSPEED, PRESSURE DECREASES which creates pressure DIFFERENTIAL between UPPER and LOWER SURFACE so CL INCREASES;
As Mfs INCREASES further, the SHOCKWAVE gets STRONGER, turbulent WAKE SEPARATES CLOSER and PRESSURE INCREASES BEHIND shockwave REDUCING the LIFTING capacity and CL;
When the LOWER shockwave forms CL DECREASES RAPIDLY due to second WAKE SEPARATION further REDUCING LIFTING capacity;
As Mfs INCREASES shockwaves move towards TRAILING EDGE and WAKE SEPARATION occurs more beyond TRAILING EDGE therefore the INCREASED pressure BEHIND shockwave DECREASES and DECREASED pressure FORWARD of shockwave INCREASES which allows CL to INCREASE and RECOVER;
As Mfs approaches Mdet both shockwaves SETTLE at TRAILING EDGE, and WAKE SEPARATION is entirely beyond TRAILING EDGE, only SUPERSONIC airflow is surrounds the aerofoil so CL begins to DECREASE because of the SIMILAR PRESSURE on UPPER and LOWER surface

Question 16

How does CD change with the movement of a shockwave when Mfs increases from Mcrit to Mdet?

Answer 16

Mfs reaches Mcrit and SHOCKWAVE can form on UPPER surface due to this and potentially WAKE SEPARATION, CD INCREASES;
As Mfs INCREASES so does CD and at the Mfs when a second SHOCKWAVE forms on the LOWER surface CD INCREASES SIGNIFICANTLY also known as the Mdd or Mdr;
Mfs INCREASES further, both shockwaves begin to move REARWARD and WAKE SEPARATION occurs CLOSER to the shockwave continuing the INCREASE in CD;
CD PEAKS and begins to DECREASE once the WAKE SEPARATION begins to occur BEYOND the TRAILING EDGE;
CD remains relatively CONSTANT with INCREASE in Mfs as there are NO SHOCKWAVES or SEPARATION on the surfaces

Question 17

Describe what the general trends are for the changes in CoP, CL and CD with an increase in Mfs?

Answer 17

CoP moves REARWARD when BOTH shockwaves move towards TRAILING EDGE, and will SETTLE near the CENTRAL part of aerofoil;
CL DECREASES AFTER initially INCREASING just after Mfs reaches Mcrit due to the INCREASE in PRESSURE BEHIND UPPER surface shockwave. The STRONGER the shockwave, the CLOSER WAKE SEPARATES to shockwave causing CL to DECREASE;
CD INCREASES when shockwaves FORM and SIGNIFICANTLY CHANGES when WAKE SEPARATION occurs;
CL and CD can RECOVER when shockwaves reach AREA near TRAILING EDGE

Question 19

What is a sonic boom?

How does it form?

Answer 19

A STRIKE of sudden PRESSURE CHANGE;
When flight reaches SONIC speed and BREAKS SOUND BARRIER a shockwave forms attached to aircraft;
The shockwave PROPAGATES in the air and experiences a SUDDEN CHANGE in PRESSURE;
The CHANGE STRIKES every object along its PATH including EARS of observers which is received as a HIGH INTENSITY SOUND

Question 20

Explain the movement of shockwaves on control surfaces?

What effect may this have on what the pilot feels?

Answer 20

Shockwaves form at the HINGE AREA of a control surface FIRST and an INCREASE in Mfs will cause it to move REARWARDS;
When in a LOCAL SUPERSONIC airflow the control surface may be INEFFECTIVE or have ADVERSE MOVEMENTS;
Since there is an INCREASE in PRESSURE BEHIND the shockwave the controls FEEL HEAVY and AERODYNAMIC FORCES exerted may become TOO GREAT for the PILOT to CONTROL the surface;
Turbulent WAKE SEPARATION also causes VIBRATION of CONTROL SURFACE which can introduce INSTABILITY