Module 8 - basic aerodynamics

What is the composition of the atmosphere?

78 nitrogen

21 oxygen

1 other gases

This composition remains until 60000ft.

What happens to pressure with increased altitude ?

Pressure decreases.

What is the troposphere and its properties ?

- surface to 11km or 36000ft

- -6.5degree/km temp drop

- contains weather

What is the tropopause and its properties ?

- separating boundary layer between troposphere and stratosphere

What is the stratosphere and its properties ?

- from 11km to 50km

- -56.5 degree fixed

- dry and stable conditions

- contains the ozone layer

What is pressure and its relevancy to aerodynamics ?

• pressure decreases with altitude

• Is 1013 hpa or 14.7 psi at sea level

What is density and its relevance to aerodynamics ?

• air density gets lower with altitude

• Humid air is less dense

• 0.1225kg/m3 at sea level

What is temperature and its relevancy ?

• -6.5 degree per km decreases with altitude

• Plateaus at 56.5 degrees after 36000ft

What is the ISA?

• international standard atmosphere

• 1013 hpa

• 15 degrees

• 0.1225kgm3

• Is used to consistent performance comparisons and instrument calibration

What is stability ?

Stability is the aircraft’s ability to return o steady flight after a disturbance

What is manoeuvrability ?

How easily an aircraft changes direction.

What is control ability ?

Control ability relates to how well the aircraft responds to pilot inputs

What are the 3 axis of flight?

Vertical, longitudinal, lateral

What is lateral axis of flight ?

Lateral axis relates to the wingtip to wingtip, and so is longitudinal stability. It relates to pitch and is controlled by the elevators.

What is longitudinal axis of flight?

Longitudinal relates to nose to tail, relating to roll/bank being controlled by ailerons. It is relating to lateral stability.

What is the vertical axis of flight?

The vertical axis of flight is relating to top to bottom of the aircraft, relating to yaw being controlled by the rudder, and relates to directional stability.

What is the keel effect ?

The effect of the fuselage acting like a pendulum and stabilising the aircraft

What is a spiral dive ?

Where the directional stability is greater than the lateral, sending the aircraft into a spiral, can be corrected by pilot inputs

What is a Dutch roll?

Where the lateral is greater than directional, so the aircraft rolls, can be corrected by a yaw damper / rudder inputs.

What is passive vs active stability ?

Passive are the natural features stability the aircraft e.g. dihedral wings, keel effect etc

Active is the fly by wire inputs from the pilot

What is laminar flow?

Laminar flow is the orderly flow of stream lines

What is turbulent flow ?

Turbulent flow is a chaotic flow of streamlines, causing swirling formations in the aircraft’s wake.

The continuity equation?

Says the speed of airflow is inversely proportional to the cross sectional area of tube

What is Bernoulli’s theorem ?

Bernoulli’s states that as airspeed increases, the static pressure will decrease and vice versa.

What is Static pressure?

Static pressure is the pressure when a fluid isn’t in motion, and is acting equally in all directions like pascals.

What is dynamic pressure ?

Pressure created from the forward flowing motion of a fluid.

Total pressure is ?

Static + dynamic.

What is the stagnation point?

The point at which airspeed goes to 0, and the static pressure will equal the total pressure.

What is the boundary layer?

The boundary layer is the thin layer in contact with the surface, that goes to airspeed 0 basically, because of friction. The air right in contact is the no slip condition. The molecular level above this will slip slightly and so on, until it’s the same speed as the free flow. It is affected ie made smaller by increasing velocity, decreasing viscosity or increasing air density.

What is laminar flow? Advantages? Disadvantages?

Laminar flow is neat, orderly layers of flow. It occurs usually in stable conditions. It is good for efficiency as it has low drag, however it can be easily disturbed leading to issues with turbulent flow generation, flow separation all which can induce stall conditions. It is disturbed easily especially after the thickest part of the wing, when flowing from negative to positive / low to high pressure areas, and can separate and flow backwards causing drag.

What is turbulent flow? Advantages ? Disadvanatages?

Turbulent flow is messy chaotic airflow that is unpredictable compared to laminar, it has high drag but it has better ‘stickiness’ to the surface of the wing so can aid in stall recovery or prevention. Naturally can form after laminar flow breaks down.

What is relative airflow ?

The direction of airflow opposing the direction of the aircraft, not always necessarily toward the nose though.

What is upwash?

Upwash is air being pulled upward, toward the lower pressure side of the wing

What is downwash?

Downwash is the air behind the wing being pulled downward. It is a bi product of lift and it reduces efficiency

What is effective AOA?

Effective angle of attack is the angle between the chord line and the downwash airflow. Higher downwash reduces this effective aoa meaning a higher real angle of attack is needed to sustain same lift, so means more induced drag.

What are vortices ?

Vortices are circular patterns of air, that are created when low and high pressure air intersect over the sides of the wingtips.

What is air foil contamination

where ice or other contaminants like bird droppings collect on wings and flight control surfaces affecting aerodynamics, stability etc

name the 3 types of ice build up

frost, rime ice, clear ice.

what is frost?

frost forms in sub zero temps on the ground and in the air

it appears as a white, feathered, semi crystal coating

it can disrupt airflow, increase drag and reduce max lift coefficient

What is rime ice?

Rime ice is where droplets freeze on impact with a surface, form as opaque brittle droplets and against affect aerodynamics and increase weight.

What is clear ice?

Clear ice forms in concentrations of cooled droplets, and forms large clear sheets, it can affect flight controls, break of in chunks and damage aircraft and affect balance

What is the glide ratio?

glide ratio is the distance an aircraft travels forward, compared to how much altitude is lost.

The best glide ratio occurs under the best lift to drag ratio, as those forces still occur.

weight doesn’t affect the GR, but it will affect the time.

Name some factors affecting aircraft performance?

• range

• speed

• payload

• rate of climb

• cruise speed and alt

• fuel economy

• manoeuvrability

What is state steady flight?

Where at cruise all forces are balanced, thrust = drag, weight = lift, moments are balanced etc

What is happening in an aircraft turn?

Bank is induced by ailerons, this increases the horizontal forces. during banking less lift is opposing the wight, so altitude is lost and nose points downward, to correct this nose is brought up which increase AOA, and causes increased drag or a stall.

What is a slipping turn?

A slipping turn occurs when the turn isn’t strong enough for the bank, this means that the horizontal lift is greater than the centrifugal, so the aircraft will turn into the bank too much

What is a skidding turn?

A skidding turn is when the turn isnt strong enough for the bank, so the centrifugal forces are greater and the nose will point outisde the turn

What is load factor?

Load factor is basically g force, and is expressed in g’s. For example 3G, the aircraft acts 3x as heavy as it would in straight and level flight.

How does load factor change with banking?

When banking more lift is required as altitude is lost, more bank means higher load factor, leading for more lift required. E.g. a 60 degree bank is 2g, but an 80 degree bank is 5.76g.

What are the 3 main types of structural limits?

Redline airspeed

Ultimate limit

Structural limit

What is structural limits?

The load factor/ max g’s that the aircraft can be repeatedly subjected to.

What is ultimate limits?

Ultimate limits are upward of 150% above structural limits, and cannot be exceed or deformation will occur.

What is redline airspeed?

Redline airspeed is the max airspeed that can occur before structural damage and danger.

What is the fight envelope ?

Flight envelope outlines the max g’s that the aircraft can pull, at different airspeeds, before stall or structural damage occurs. With higher speeds you can pull higher G’s because more lift can be produced.

What are the 4 main types of trailing edge flaps?

Plain, slotted, fowler, split

What are the main types of leading edge flaps?

Leading edge flaps, cuffs, slots and slats

What is boundary layer control?

Boundary layer control, is prevented flow separation to produce more lift and reduce drag and prevent stall. Can be done with devices like vortex generators which mix fast flow with boundary layer air, or winglets to reduce vortices.

Clear up - what is the mean camber line?

Line running equidistant to top and bottom of the airfoil, it shows the camber with increased curve

Clear up - what is wash in and wash out?

Wash out wings mean the tips have lower aoa than the roots, allows for more control ability during a stall a the ailerons stay energied, washin is where aoa is higher at the roots but this is usually undesirable

Clear up - what is interference drag?

Interference drag is where airflow is disrupted by components meeting, this can be reduced by smooth transitions, or fairings.

Clear up - plain flaps vs split flaps

Plain - simple hange flap whole unit moves down, produces some lift and drag, but less efficient

Split flap - better for landing, as more drag can be produced.

Clear up - induced drag and airspeed relationship ?

As airspeed increases, the induced drag decreases, the relationship is inversely proportional.

Clear up - relationship between temperature and speed of sound?

With lowest temperatures, the speed of sound get lower, directly proportional relationship

Clear up - CP movements ?

With a lower aoa, the centre of pressure will move aft, with a higher angle of attack the cp will move forward, and when critical aoa or stall is reached the cp will rapidly moved aft as flow separates. This can cause pitch instability, nose down.

Clear up - what is form drag?

Form drag also known as pressure drag, result of the resistance from airflow separation around an object, can be reduced by streamlining.

Clear up - what is the linienthal diagram?

Also called polar diagram, plots the CL against the CD

Clear up - what are swept wings?

Sweep back - leading edge goes backwards, improves the lateral stability, usually stalls at wing tips, but can use wing fences, or washout to stop this

Sweep forward - more complex but allows better low speed handling,

Clear up - what is wing loading ?

Wing loading is aircraft weight / wing area

High wing loading suggests a heavy aircraft, with high stalling speeds

Low wing loading is a lighter aircraft, with decreased stall speeds and more lift at lower speeds like gliders

Clear up - what is coefficient of lift

How effective a wing is at generating lift based on its aoa, shape, flaps etc. factors in real life conditions.

Clear up - what is geometric vs aerodynamic twist ?

Geometric twist - physical washout twist in the wing, reducing the aoi from root to tip

Aerodynamic twist - not a physical twist, but reduction in camber as you go from root to tip to get washout.

Clear up - lower vs higher stall speeds

Lower stall speeds - better, so aircraft can get away with flying slower before stalling, better for taking off and landing, easier stall recovery

Higher stall speeds - aircraft stalls easier, longer take off and landing, suggest heavier aircraft.

Clear up - AOA and AOI symbols

AOA - alpha

AOI - gamma

Clear up - CP and CG relatonship

CP before CG is bad, nose up pitch

CG before CP - good, nose down pitch, encourages stall recovery

Clear up - what is side slip

Aircraft moving sideways, without nose into relative airflow, due to rudder input with a lack of aileron

Clear up - static vs dynamic stability

Static - initial reaction, positive negative neutral

Dynamic - reaction over time, positive negative neutral

Clear up - longitudinal lateral directional stability

Longitudinal - pitch control, horizontal stab, axis will be lateral

Lateral - roll control, ailerons, axis will be long (axis is opposite to stability)

Directional - yaw control, rudder, axis will be vertical

Clear up - what is AR

AR is the net product of lift and drag, like triangle method for lift and drag

Clear up - flow separation

Flow separation is when laminar flow becomes turbulent, and the boundary layer loses energy and sepersges from the surface, meaning that lift will decrease and drag will increase rapidly, aoa gets higher the transition points starting moving forward.