Aircraft exam Flashcards
What is Bernoulli’s principle?
Rise (fall) in pressure within a flowing fluid must always be accompanied by a decrease (increase) in the speed, and conversely, an increase (decrease) in the speed of the fluid results in a decrease (increase) in the pressure within the fluid.
Why the air going over the wing has a lower pressure
What are the four forces acting upon an aircraft in flight?
Lift
Weight
Thrust
Drag
What would be condsidered stable flight?
If an aircraft in flight will tend to return to level flight of its own accord without pilot interference it is stable.
What is lift?
Lift: To overcome weight, aeroplanes generate an opposing force called Lift, which is generated by the motion of the aircraft through the air and is an aerodynamic force. Argument between Bernoulli principle and Newtons Laws of Motion. Works through the centre of pressure of the wing.
What is weight?
Weight: Force generated by the gravitational attraction of the earth on the aircraft. Acts downwards towards the earth irrespective of the aircrafts attitude. Works through the centre of gravity of the aircraft.
What is thrust?
Thrust: Mechanical force provided by the aircrafts system of propulsion. Used to overcome drag. Works through the thrust line.
What is drag?
Drag: Aerodynamic force that opposes an aircrafts motion through the air, generated by every part of the aircraft: Profile drag- consequence of shape, increases as speed increases. Induced drag- consequence of lift, decreases as speed increases. Profile + Induced= Total Drag. Works through the drag line which is the exact opposite of the thrust line.
What is laminar flow?
Laminar Flow is the smooth, uninterrupted flow of air over the contour of the wings of an aircraft in flight.
It is most often found at the front of a streamlined body and is an important factor in flight.
If the smooth flow of air is interrupted, turbulence is created which results in a loss of lift and increased drag.
An Aerofoil designed for minimum drag and a smooth flow of the boundary layer is called a laminar aerofoil.
If the angle of attack of a wing is too high the laminar flow of air will separate from the wing, becoming turbulent, this is called the critical angle of attack and is the point where the wing will begin to stall.
How does an aircraft’s wing work?
Bernoullis principle
The curvature of the wing forces the air going over the top to speed up and therefore decrease in pressure whilst the opposite happens underneath the wing. The pressure underneath the wing then pushes the aircraft into the air.
What is an aerofoil?
A part of an aircraft designed to produce lift.
What is the chord line of a wing?
A line that dissects a wing from the leading edge through to the trailing edge.
What is the angle of incidence of a wing?
the angle at which the wing is attached to the fuselage.
What is a stall?
An aircraft is considered to be stalled when the critical angle of attack is reached and the laminar flow over the top of the wing seperates leading to a drop in the amount of lift generated.
What is the Angle of Attack of a wing?
The angle between the chord line and the flow of air. Has a large affect on amount of lift generated by a wing- greater angle= more lift (up to stall point where lift decreases because of flow separation).
How does a Piston Engine work, and what are its advantages and disadvantages?
Based on internal combustion engine using four stroke cycle: Intake- air and vaporised fuel drawn in Compression- fuel vapour and air compressed and ignited Combustion- fuel combusts and piston driven downwards Exhaust- exhaust gasses expelled.
How does a Jet Engine work?
Discharges a fast moving jet of hot expanded gases to generate thrust in accordance with Newtons Third Law (every action…). Uses the intake, compression, combustion and exhaust process. Turbojet- Generic term for simple turbine engines (100% air in/out) Turboshaft- similar to turbojet, gas turbine optimised to produce shaft power.
How does a Turboprop Engine work?
Type of gas turbine engine where the power produced is used to drive a propeller attached to the turbine shaft.
Variable pitch propellers- angle of attack of propeller blade can be altered to change thrust and drag profile in flight.
What are the three differences between aircraft and car engines?
- Aircraft engines are air cooled- saves weight and is safer
- Dual ignition systems- independent magnetos and spark plugs
- Fuel/Air mix- manually controlled.
How many operations is there in a four stroke engine?
5
- Induction
- Compression
- Ignition
- Power stroke
- Exhaust
What is a carburetor?
A device that provides a suitable mixture of air and fuel to the cylinder for ignition.
What happens to air as it enters a carburetor?
- It accelerates
- There is a drop in air pressure?
- This causes the fuel to be drawn through the main jet where is mixes with air in the venturi
How does carburetor icing occur?
Due to the venturi effect and bernoulli’s principle;
- The pressure decrease of the air accompanying the deceleration also causes an instant air temperture drop
- Latent heat is absorbed by the fuel as it vaporises into vapour, further cooling the air.
Can cause temp drops of 20°C
What should pilots do to prevent carburetor icing?
- If possible avoid areas where such icing is likely to occur
- Turn on the carburetor heating system
- During descents with engine at idle power, apply engine power frequently, so as to keep the engine warm
How does a propoller work?
Propeller: Means of converting engine power into a propulsive force, rotating blade accelerates a mass of air rearwards, aircraft moves forward in reaction. Aerofoil shape (like a wing). Outer edge of disc travelling faster, therefor blade is twisted in order to alter the angle of attack and give constant lift along whole blade. More blades= solidity increasing= less radius necessary.
What is the pitch of a propeller blade?
The angle made by the chord of the blade and its plane of rotation.
Much like the angle of attack of a wing, the higher the pitch the higher the propellers angle of attack and the more air it pushes back with each revolution.
What pitch is better for taking off and climbing?
A fine pitch
Which pitch is better for cruising?
Coarse
What is the term for a pitch that is a compromise between a fine and coarse pitch for a fixed propeller blade?
Standard
What is a variable pitch propeller?
A propeller that can vary its pitch dependent on need. They can even use a reverse pitch.
What are the advantages of piston engines and propellers?
Advantages:
- More protected and easier to access and maintain
- Lower temperatures and pressures
- RPM increases and decreases more rapidly than jet
- Cheaper to manufacture
- Shorter take off and landing runs
What are the disadvantages of a piston engine and propeller?
Disadvantages:
- Less economical on fuel= more fuel carried
- Vertical rather than linear movement of engine components, inefficient transfer of power and more wear and tear due to vibration
- Relatively expensive fuel
- Lower ceiling (10-12000ft) due to air density
- Noise, both onboard and ambient
What are the stages of a jet engine?
- Air intake
- Compressors
- Combustion chambers- mixed with fuel and ignited
- Turbine stages- extracts energy from the air leaving to drive the compressors
- Exhaust stage
Name the different types of jet engine
Low bypass Turbofan
High bypass Turbofan
Turboprop
Ramjet
Scramjet
Turboshaft (turboprop for a helicopter)
What are the two types of compressors found in a jet engine?
Axial
Centrifugal
How does a centrifugal compressor work?
Uses an impeller to accelerate air and a diffuser to produce the required pressure rise.
How does an axial flow compressor work?
Employs alternate rows of rotating blades to accelerate the air and stationary vanes to diffuse the air until the required pressure rise is reached.
What advantage does a centrifugal compressor have over an axial one?
Favoured for small engines due to its simplicity, ruggedness and ease of manufacture outweighing any disadvantages.
What advantages does an axial compressor have over centrifugal?
Can have long thin engine shapes
Extra pressure rises can be achieved with addition of other compressor stages.
What is a turbo fan engine?
An engine with a turbojet core which also uses the fans to acclerate air around this engine, the purpose of the turbines is mainly to turn the fans.
This helps to reduce engine noise and increase efficiency.
Bypass ratios can equal 26:1
What are the advantages of a turbofan engine?
- More efficient- thrust generated by core and the air being pushed around it
- Can use afterburner to reignite gases to increase thrust
- Can travel further for less fuel
What are the disadvantages of a turbofan engine?
High cost
Longer to accelerate than piston engines
What are the advantages of a turboprop engine?
Advantages:
- More efficient at lower levels
- Can operate on shorter runways
- Propeller can be feathered to minimize drag in event of engine failure
- More mechanically reliable, due to lower moving parts.
What are the disadvantages of a turboprop engine?
Disadvantages:
- Propellers lose efficiency at higher altitudes
- Vibration levels can cause slight passenger discomfort
- en-route weather can cause problems
- additional passenger discomfort due to operating altitudes
- older generation turboprops are slow.
What shape of propeller blades are used for higher flight speeds?
Scimitar shaped
What are the two types of drag?
-
Parasite Drag
a. Form Drag
-Form Drag
-Skin Friction
b. Interference Drage - Induced Drag
What is form drag?
A type of parasite drag
Results when airflow seperates from the surface and the streamlined flow becomes disturbed and increases drag.
Point at which streamline flow seperates from the surface is known as the seperation point.
Can be postponed by streamlining the aircraft.
What is skin friction drag?
Type of parasite drag
Results from friction between the aircraft and the air.
Is dependent on;
- Surface area of aircraft
- Whether the boundary layer airflow near the surface is laminar or turbulent
- Roughness of surface
- Airspeed
- Aerofoil thickness.
What is interference drag?
Type of parasite drag
Drag created at the junction of aircraft surfaces caused airflow interference. This creates additional drag
Fairings and streamlining can reduce this.
What is induced drag?
Drag caused by the generation of lift.
Some of the high pressure air will ‘spill’ over the edge of the wings (at the edges and the wing tips) causing a flow of air towards and away from the fuselage producing wingtip vortices.
This is most pronounced during phases of flight where lift generation is at its highest, eg. take-off and landing.
What wing shape is most efficient at combating induced drag?
Long narrow wings.
Does an aircraft in a clean configuration have more or less induced drag?
More as the vortices are undisturbed.
What aircraft configuration has the greatest effect on dispersing wake vortices and induced drag?
Landing configuration with wheels and flaps down
This is due to these interrupting the smooth airflow over the wing which upsets the generation of the wake vortices.
What can be attached to an aircraft wingtip to resuce wing tip vortices?
Winglets.
What are the types of devices that can be used to modify lift and drag?
Trailing edge devices
Leading edge devices
Spoilers/Airbrakes
What is a FLAP?
A type of trailing edge device that modifies drag and lift.
It works by increasing the camber (or curvature) of the wing which increases the amount of lift generated by that wing.
They also produce extra drag but this can be useful when an aircraft is landing as it would reduce the landing roll.
What happens to an a/c’s take off or landing roll when it has deployed its Flaps?
It decreases, so less runway is needed for a take off and the landing roll is much shorter.
What is a slot?
A fixed leading edge device that works like a fixed SLAT.
It increases the maximum angle of attack a wing by around 20° allowing it to generate more lift by using a higher angle of attack
What is a SLAT?
A retractable leading edge device
It increases the maximum angle of attack a wing by around 20° allowing it to generate more lift by using a higher angle of attack
What unfortunate side effect do flaps and slats produce?
The cause the a/c to pitch up causing an unstable condition and higher drag.
What are spoilers/airbrakes?
Small hinged plates on the top portion of wings.
When deployed they disturb the airflow over the wing reducing lift and increasing drag.
They are used to slow down aircraft on landing roll and ensure it remains on the runway and does not lift off again.
What problems are associated with landing during the failure of high lift devices?
- Landing approach will have to be made at a shallower angle
- Land at higher speed leading to;
- Possible brake fires
- Possible tire bursts
- Runway overrun
- Overheating of brakes
What problems are associated with take off during the failure of high lift devices?
- Shallow take of angle, leading to less obstacle clearance
- Slow rate of climb
- Premature retraction (failure to climb or loss of height)
- Full stall close to ground- usuall fatal.
What behaviour will an a/c exhibit when stable?
Aircraft tends to return to its original condition after being disturbed
What behaviour will an a/c exhibit when neutrally stable?
Aircraft will assume a new position
What behaviour will an a/c exhibit when unstable?
Aircraft will continue to diverge from its original position.
What are the three axes and how is an aircraft controlled through them?
Vertical- Yaw
Longitudinal- Roll
Lateral- Pitch