Aerodynamics

Question 1

What do winglets do?

Answer 1

Advantages

1. Reduce Induced Drag
2. Reduce wingtip Vortices
3. Increased aspect ratio of the wing
4. Move the vortice away from the lifting surface(increase lift)
5. Reduced parking space required
6. Advertising

Disadvantages

1. Reduce ground effect
2. Interference drag
3. Additional wing structure may be required
4. If poorly designed, they may flutter

Question 2

Why do 777-300ER have raked wingtips?

Answer 2

1. The Boeing 777-300ER is a Ultra long haul aircraft that get reduced benefit from induced drag reductions
2. The Boeing 777-300ER is unlikely to go into regional airports with tight parking so can have a true high aspect ratio wing.
3. Winglets increase lift and reduce drag(great for short sectors)/Raked wingtips just reduce (more) drag.
4. Raked wingtips separate the wing tip vortices from the trailing edge, reducing induced drag

Question 3

When is induced drag greatest?

Answer 3

1. Induced drag is a function of Angle of attack - Hence induced drag is greatest at the stall speed, with flaps and slats extended.
2. Not to be confused with wake turbulence which is greatest at the highest angle of attack in the clean configuration(1 big vortice rather then multiple smaller onces)

Question 4

What are the two types of drag?

Answer 4

1. Induced,

2. Parasite(Form, interference & skin friction)

Question 5

What is the critical engine of a jet on the ground?

Answer 5

The most into wind engine

Question 6

What is the critical engine of a Jet when airborne?

Answer 6

None?

Question 7

How is Vmca Calculated?

Answer 7

1. MTOW
2. Most rearward CofG
3. Sealevel @ ISA
4. Landing gear up
5. Flaps retracted.
6. Engine UN-feathered.

Question 8

Why do high speed aircraft have swept wings?

Answer 8

Advantages

1. Leading edge to trailing edge distance of airflow is longer then the airflow perpendicular to leading edge
2. Therefor less effective camber and acceleration
3. Therefor aircraft can fly faster before airflow goes supersonic
4. Therefor Delays the occurrence of Mcrit

Disadvantages

1. High spanwise flow
2. tip to tip span is shorter = lower aspect ratio = more induced drag
3. Tip stalls if uncorrected
4. torque on fuselage as life is behind pivot point
5. wing spars meet at a angle

Question 9

Explain spoilers and how they operate?

Answer 9

Ailerons
1 . Assist the Ailerons allowing them to be smaller so they can fit flap onto the wing.
2. lower the twisting moment on the wing and prevent aileron reversal
3. No adverse aileron yaw.

Drag device
1. Aircraft is low drag so needs a high drag device to help with control
2.

They “spoil” normally laminar airflow on the upper surface, causing premature separation, therefore increase in drag and reduce the lift.

They are also used to assist the ailerons in controlling roll, at high speeds they are used progessively less as speed is increased, this is achieved through “Blow down” and not supplying the spoilers with enough force to overcome high airloads.

Question 10

What is the TAS of your aircraft if you have a IAS of 100Knts?

Answer 10

approx 110Knts

Question 11

What is the TAS of a Jet doing 250knts IAS?

Answer 11

approx 450Knts

Question 12

Explain a stall

Answer 12

When the aircraft cannot produce enough lift to maintain flight

Question 13

If a 777 had a engine failure would Vmin speed be higher or lower?

Answer 13

Lower (draw the 2 drag curves)

Question 14

What is Mach tuck?

Answer 14

Rearward movement of the centre of pressure due to compression of supersonic flow, causing a pitching down moment

Question 15

What is a Mach trimmer?

Answer 15

A feature of an aircraft which anticipates Mcrit and automatically creates a pitching up moment to counter mach tuck

Question 16

Explain leading edge slats?

Answer 16

1. Detach forwards and downwards from leading edge
2. Enables aircraft to operate at higher AOA
3. Changes the camber of the wing, creating more lift at slower speeds.
4. Best of both worlds for supercritical aerofoils. Increases camber for low speed, tucks away for high speed

Question 17

How do Krueger flaps change the lift formula?

Answer 17

1. Increase wing camber, increasing the coefficient of lift

2. Increase the surface area of the wing

Question 18

Explain slots?

Answer 18

1. spanwise gap on leading edge of wing
2. channels airflow to promote laminar flow over highly curved surface
3. good for high lift coefficients at low speeds

Question 19

How was the 777 wing designed?

Answer 19

Fully computer designed for maximum efficiency, thence testing in a wind tunnel and in flight.

Question 20

What is Mcrit?

Answer 20

Lowest mach number at which airflow on some point of the aircraft reaches supersonic

Question 21

Define IAS

Answer 21

Total pressure - static pressure = Dynamic pressure calibrated to ISA sea level density.

Question 22

Define CAS

Answer 22

IAS calibrated for instrument and position errors

Question 23

Define TAS

Answer 23

The speed of the aircraft relative to the air mass in which it is flying.

Question 24

What will have greater vortices a 777 or a 747? Why?

Answer 24

1. The 747 has a higher max landing weight
2. Both aircraft have two flaps
3. The 777 has raked wingtip/747 has winglets

Question 25

Speed of sound formula

Answer 25

38.94 * (square root temp in kelvin)

Question 26

Rotating tyre hydro planning formula

Answer 26

9 * square root of tyre pressure

Question 27

How do vortex generators work?

Answer 27

Vortex generators pull high energy air into the boundary layer by creating a mini vortices which helps with delaying separation. Essentially the wing can operate at a higher angle of attack before flow separation causes a stall.

They can also be fitted to transonic aircraft to keep control surfaces effective at high speeds.