P4 Transonic Flight and Wings

Question 1

Explain the differences between speed limits determined by power required/power available diagram and a transonic limitation diagram?

Answer 1

POWER REQUIRED/POWER AVAILABLE: THEORETICAL LIMITS are the INTERCEPTS however, the LOWER limit is usually caused by the AERODYNAMIC LIMIT of BOUNDARY layer SEPARATION DESTROYING the LIFT and the UPPER limit caused by an OPERATIONAL LIMIT of Vne;
TRANSONIC: LIMITS are determined by point where aeroplane CANNOT produce EFFECTIVE AERODYNAMIC FORCES to maintain flight;
LOWER LIMIT is at LOW SPEED, HIGH AoA caused by BOUNDARY layer SEPARATION over the aerodynamic surface DESTROYING the LIFT;
UPPER LIMIT is at HIGH SPEED, LOW AoA where aeroplane starts MACH STALL caused by turbulent WAKE SEPARATION BEHIND shockwave which results in complete LOSS of LIFT and significant DRAG INCREASE

Question 2

Describe the characteristics of coffins corner?

Can it be escaped?

Answer 2

SPEED LIMITS are determined by point where aeroplane CANNOT produce EFFECTIVE AERODYNAMIC FORCES to maintain flight;
LOWER LIMIT is at LOW SPEED, HIGH AoA caused by BOUNDARY layer SEPARATION over the aerodynamic surface DESTROYING the LIFT;
UPPER LIMIT is at HIGH SPEED, LOW AoA where aeroplane starts MACH STALL caused by turbulent WAKE SEPARATION BEHIND shockwave which results in complete LOSS of LIFT and significant DRAG INCREASE;
These 2 LIMITS INTERCEPT at an ALTITUDE called COFFINS CORNER, at this altitude a SPEED INCREASE or DECREASE will result in a form of STALL;
Using LIMITED MANOEUVRES it may be escaped

Question 3

What is crossover altitude?

What factors will change crossover altitude?

Answer 3

The OPERATIONAL ALTITUDE where a specified IAS/CAS and MACH represent the SAME TAS, here the PILOT switches from KNOTS to MACH;
Changes with CLIMB PROFILE, TAS (DENSITY), MACH (TEMPERATURE)

Question 4

What is the difference between coffins corner and crossover altitude?

Answer 4

COFFINS CORNER is a AERODYNAMIC LIMITATION which varies which CHANGES with LOAD FACTOR and WEIGHT;
CROSSOVER ALTITUDE is an OPERATIONAL region which does NOT vary with LOAD FACTOR or WEIGHT, it does CHANGE with CLIMB PROFILE

Question 5

What is the buffet boundary?

What is buffet margin?

Answer 5

When BOUNDARY LAYER starts to SEPARATE, VORTICES DETACH at a certain FREQUENCY which causes VIBRATION or OSCILLATION and the aircraft begins to BUFFET. The AIRSPEED where this occurs is the BUFFET BOUNDARY which runs almost PARALLEL to airspeed LIMITS;
The speed RANGE between LOW speed BUFFET and HIGH speed BUFFET is the BUFFET MARGIN

Question 6

What factors affect buffet boundary?

Answer 6

ALTITUDE INCREASE causes MARGIN DECREASE;
WEIGHT INCREASE causes INCREASE Vs and MARGIN DECREASE;
LOAD FACTOR INCREASE causes INCREASE Vs and MARGIN DECREASE

Question 7

What are the problems related to longitudinal control in transonic flight?

Answer 7

FEELS HEAVY to move elevator due to shockwave SETTING on ELEVATOR and SUDDEN INCREASE in PRESSURE;
Does NOT respond EFFECTIVELY due to a CHANGED PRESSURE DISTRIBUTION so it CANNOT PRODUCE EFFICIENT LIFT;
ELEVATOR VIBRATES/BUZZES NOISILY/BUFFETS due to WAKE SEPARATION behind shockwave;
ADVERSE CONTROL movement due to PRESSURE CHANGES over the elevator

Question 8

What are the methods to deal with longitudinal control problems in transonic flight?

Answer 8

THIN TAILPLANE/SHARP LEADING EDGE: INCREASES Mcrit of elevator, DELAYING the formation of shockwave;
MACH TRIM: CORRECTS ADVERSE STICK FORCE and eliminates CONFUSION;
ALL MOVEABLE SLABS: Slabs NOT AFFECTED by shockwave are STILL OPERATIONAL while others carry shockwave;
ADJUSTABLE and POWER OPERATED TAILPLANE to overcome EXTRA FORCES due to shockwave

Question 9

Explain what Mach tuck is and how it occurs?

Answer 9

The NOSE-DOWN pitching MOMENT or NOSE HEAVY FEELING of a TRANSONIC aeroplane;
As Mfs INCREASES SHOCKWAVES formed over aerofoil move REARWARD and CoP also moves REARWARD;
This INCREASES the ARM between CoG and CoP which INCREASES NOSE-DOWN pitching MOMENT;
The INCREASE in Mfs also means turbulent WAKE SEPARATES EARLIER, REDUCING the DOWNWASH over the TAILPLANE, so it will produce LESS restoring NOSE-UP pitching MOMENT;
The NOSE-DOWN causes the aeroplane to ACCELERATE which INCREASES Mfs and makes control DIFFICULTIES WORSE

Question 10

Explain what adverse stick force is and how it occurs?

Answer 10

When a PUSH/PULL FORCE on the central CONTROL COLUMN at a HIGHER Mfs in TRANSONIC region which results in a PULL/PUSH FORCE due to the SIGNIFICANT PRESSURE INCREASE BEHIND a shockwave formed on the ELEVATOR, particularly if there is TURBULENT SEPARATION, causing different PRESSURE DISTRIBUTIONS and a resultant NEGATIVE LIFT

Question 11

What are the problems related to lateral control in transonic flight when shockwaves form on top of wings?

Answer 11

AILERONS CANNOT operate EFFECTIVELY as located BEHIND shockwave - no AERODYNAMIC FORCES to produce ROLL MOMENTS;
AILERON FLUTTERS and VIBRATES when turbulent WAKE SEPARATES BEHIND shockwave;
Aircraft in ROLL DISTURBANCE due to VIBRATIONS varying LIFT on aerofoil RANDOMLY;
GREATER FORCES produced by DEFLECTING AILERON causing a TWIST of wing about LATERAL AXIS which CHANGES AoA in OPPOSITE DIRECTION called AILERON REVERSAL - this can be caused by BOUNDARY layer SEPARATION by turbulent WAKE

Question 12

What are the methods to deal with lateral control problems in transonic flight?

Answer 12

VORTEX GENERATORS: UPSTREAM of AILERONS to DELAY SHOCKWAVE formation and REENERGISE AIRFLOW over control surface to DELAY BOUNDARY layer SEPARATION by turbulent WAKE;
SMALL OUTBOARD AILERONS: Operate in the AREAS of LESS turbulent WAKE;
LARGE INBOARD SPOILERS: DISRUPTS AIRFLOW to REDUCE LIFT of down going wing to ROLL in the DESIRED DIRECTION WITHOUT AILERON REVERSAL;
POWER GENERATED CONTROLS: Located inboard to ensure STIFFNESS of WING

Question 13

What are the problems related to directional control in transonic flight when shockwaves form at the hinge of the rudder?

Answer 13

RUDDER CANNOT operate EFFECTIVELY as located BEHIND shockwave - no AERODYNAMIC FORCES to produce YAW MOMENTS;
As Mfs INCREASES and shockwave moves REARWARD, rudder FEELS HEAVY to move;
Turbulent WAKE and INCREASE of PRESSURE behind shockwave cause OSCILLATION and aeroplane YAWS in OPPOSITE DIRECTION to intended. The OSCILLATION may LEAD to DUTCH ROLL

Question 14

What are the methods to deal with directional control problems in transonic flight?

Answer 14

YAW DAMPERS: REDUCES directional OSCILLATION and WEAKENS/ELIMINATES DUTCH ROLL;
POWERED CONTROL SURFACE: INCREASES EFFECTIVENESS of rudder in conventional fin and rudder combination;
ALL MOVABLE SLAB FIN/RUDDER: Slabs NOT AFFECTED are still OPERATIONAL

Question 15

Why do designers try to increase Mcrit for transonic aircraft?
What features can they utilise to achieve this?

Answer 15

As Mfs INCREASES and reaches Mcrit, a SHOCKWAVE is able to FORM which causes DRAG INCREASE, LIFT DECREASE, STALL, STABILITY/CONTROL DIFFICULTIES. A HIGHER Mcrit will DELAY these aspects and make flight more ECONOMICAL;
SLIMNESS;
FLAT LEADING EDGE;
SWEEPBACK;
VORTEX GENERATORS

Question 16

How does a slim aerofoil achieve a higher Mcrit?

Answer 16

The 1-D COMPRESSIBLE airflow principle shows the AIRSPEED inside a CON-DIV NOZZLE INCREASES proportionally with CHANGE in AREA;
The UPPER surface of a wing forms half a CON-DIV NOZZLE where BEFORE THROAT SPEED INCREASES with DECREASE in AREA;
If aerofoil is THICK (DECREASED AREA), SPEED INCREASES LARGELY from V1 to a which means FREE STREAM and therefore Mcrit will be LOWER;
If aerofoil is THIN (INCREASED AREA), SPEED INCREASES SLOWLY from V1 to a which means FREE STREAM and therefore Mcrit will be GREATER

Question 17

How does a flat leading edge achieve a higher Mcrit?

Answer 17

Has a SMALL CHANGE in AREA therefore a SMALL CHANGE SPEED which means Vfs and Mcrit will be GREATER

Question 18

How does a sweepback wing achieve a higher Mcrit?

Answer 18

TRIGONOMETRY rules show that CHORD WISE flow SPEED is V for a STRAIGHT wing and Vcos for a SWEEPBACK wing meaning the CHORD WISE SPEED is GREATER for a STRAIGHT WING;
This means the Mfs for a SWEEPBACK wing will be GREATER by the time CHORD WISE SPEED reaches SONIC

Question 18

How does vortex generators achieve a higher Mcrit?

Answer 18

Located near LEADING EDGE and produce streets of MICROSCOPIC VORTICES in BOUNDARY LAYER to transfer more KINETIC ENERGY which DELAYS/PREVENTS BOUNDARY layer SEPARATION STALL;
The AERODYNAMIC DIFFICULTIES are therefore WEAKENED/DELAYED resulting in FASTER FLIGHT ie: INCREASED Mcrit

Question 19

Explain the characteristics of a supercritical aerofoil, include CL, CD, Mdd, Mcrit, and stall?

Answer 19

FLATTENED UPPER surface at LEADING EDGE and modest T/C ensures INCREASE in SPEED is GENTLE, INCREASING Mcrit but since it is THICKER, CL is not as LOW;
The gentle SPEED INCREASES Mdd and DELAYS shockwave and INTENSITY, which DELAYS the INCREASE in SHOCK DRAG allowing FASTER flight;
CAMBER LINE reflects UPWARD at the REAR which IMPROVES LIFT production and DELAYS LOWER surface SHOCKWAVE;
If used, WING SPAN and SWEEP ANGLE may be REDUCES as it REDUCES SHCOK DRAG, INCREASES CL and operates more EFFICIENTLY

Question 21

Explain the characteristics of a low thickness to chord ratio aerofoil, include CL, CD, Mdd, Mcrit, and stall?

Answer 21

If aerofoil is THIN (INCREASED AREA), SPEED INCREASES SLOWLY from V1 to a which means FREE STREAM and therefore Mcrit will be GREATER while PRESSURE DECREASES SLOWLY;
SHOCKWAVES will therefore be DELAYED and shockwave INTENSITY will be LESS which means LOW SHOCK DRAG and HIGHER Mdd;
Since Mach INCREASES SLOWLY WAKE SEPARATION is LESS violent and aircraft can travel FASTER with BETTER STABILITY;
Due to SLOW PRESSURE CHANGE CL is LOW especially at LOW SPEEDS

Question 22

Explain the advantageous characteristics of a sweepback wings?

Answer 22

POSITIVE STABILITY in ANY DIRECTION;
HIGHER Mcrit, the GREATER the SWEEP ANGLE the HIGHER the Mcrit;
STALL AoA is HIGHER than STRAIGHT WING;
Mdd is GREATER which DELAYS the SHOCKWAVE and WAKE SEPARATION BEHIND shockwave which means LOW SHOCK DRAG;
LOW CD allowing relatively HGIH ECONOMICAL cruising SPEEDS

Question 23

Explain the disadvantageous characteristics of sweepback wings?

Answer 23

Due to VISCOSITY of AIR it will CHANGE DIRECTION over a swept WINGTIP and depart FREE STREAM. THE EFFECTIVE SWEEP ANGLE at wingtip is SMALLER so EFFECTIVE Mcrit is REDUCED from theoretical Mcrit;
THICKNESS of wing is NOT UNIFORM and DECREASES from ROOT to TIP which COMPRESSES AIR toward ROOT, compression WAVES may form and have the possibility to form a SHOCKWAVE which INCREASES DRAG;
Since there is SPAN WISE flow the BOUNDARY LAYER THICKENS toward WINGTIP which encourages large WINGTIP VORTEX which can start at LEADING EDGE called RAM HORN vortex. It induces MORE DOWNWASH leading to HIGHER INDUCED DRAG;
THICK WINGTIP BOUNDARY LAYER also makes TIP STALL easier, when this occurs CoP moves FORWARD which DECREASES the RESTORING PITCHING MOMENT. Can be REDUCED through WASHOUT;
SHOCKWAVE WAKE SEPARATION can SHADOW T-TAIL in addition to causing a LOSS of LIFT over the MAINPLANE which introduces INSTABILITY in PITCHING. When SHADOWING occurs the tailplane CANNOT work EFFECTIVELY as a RESTORING PITCHING MOMENT which creates a DEEP STALL. Can be AVOIDED with HIGH T-TAIL

Question 24

What devices are used to delay the shock stall?

Answer 24

WING FENCES;
VORTEX GENERATORS;
ANTI-SHOCK BODIES

Question 25

How do wing fences assist in transonic flight?

Answer 25

Located close to WINGTIP area to PREVENT large WINGTIP VORTEX which would develop into RAM-HORN VORTEX that causes HIGH INDUCED DRAG and WINGTIP STALL;
They also INTERRUPT REAR SHOCKWAVE over SWEEP BACK wings DEVELOPING WIDELY and REDUCE a the WAKE separation BEHIND shockwave

Question 26

How do vortex generators assist in transonic flight?

Answer 26

Located near LEADING EDGE and produce streets of MICROSCOPIC VORTICES in BOUNDARY LAYER to transfer more KINETIC ENERGY which DELAYS/PREVENTS BOUNDARY layer SEPARATION STALL;
When Mfs exceeds Mcrit the generators DE-STABILISE formation of incident NORMAL shockwave, it DELAYS the formation of SHOCKWAVE, WAKE SEPARATION and SHOCK STALL

Question 27

How do anti-shock bodies assist in transonic flight?

Answer 27

Located on LEADING or TRAILING EDGE starting from point of MAX THICKNESS extending BEYOND trailing edge;
REDUCES INTERFERENCE of flow streams from different regions so IMPROVES BUFFET behaviour and REDUCES INTERFERENCE DRAG;
Local AIRFLOW CAUSED by ANTI-SHOCKS INTERRUPTS SHOCKWAVE when moving towards TRAILING EDGE as Mfs INCREASES, therefore REDUCING Mach BUFFET, DELAYING SHOCK STALL, and REDUCING transonic WAVE DRAG

Question 28

What is the area rule?

What is the purpose of applying it in high speed aeroplane design?

Answer 28

Shaping the CROSS-SECTIONAL AREA of the aeroplane to SMOOTHEN out the TRANSITION from NOSE to TAIL specifically immediately IN FRONT and BEHIND the WINGS and TAIL;
To MINIMISE TRANSONIC DRAG RISE in transonic flight