Aircraft Flashcards
What are the four forces acting on an aircraft in level flight?
- Lift
- Weight
- Drag
- Thrust
What is the centre of gravity?
A theoretical balance point for the entire aircraft through which the weight of the aircraft and everything on board is considered to act.
It can be represented by a single downward vector.
What is the lift force created by?
The wings
Will the axis of lift force align with the centre of gravity?
Not necessarily
What is a couple?
Two or more forces acting in difference axes to cause a rotation
What is drag?
A force resisting the forward motion of the aircraft, acting opposite to the direction of travel.
When will thrust equal drag?
In straight and level unaccelerated flight.
In level flight what will the lift force be balancing?
Both weight and the balancing force
What are the three axes an aircraft can be manoeuvred around?
- Vertical or normal
- Lateral
- Longitudinal
Around which axis does an aircraft yaw?
Vertical
Around which axis does an aircraft roll?
Longitudinal
Around which axis does an aircraft pitch?
Lateral
Which flight control controls yaw?
Rudder
Which flight control controls roll?
Ailerons
Which flight control controls pitch?
Elevator
Why are wings usually mounted high or low?
So that the middle space of the cabin is not wasted
What are disadvantages of a tail wheel arrangement?
Can be difficult to land and taxi.
When on the ground, the cabin is on a slope - not ideal for transport aircraft
What is pressure?
Force acting on a given area
What unit is pressure measured in?
Pascals
What is static pressure?
Pressure exerted in all directions at a given point as a result of air molecules in the atmosphere pressing down due to gravity.
Is static pressure higher or lower at sea level than altitude?
Higher
What is dynamic pressure?
The pressure felt by a body as a result of relative movement through the atmosphere.
Dynamic pressure increases with increased relative speed as more air molecules per second press on the body.
Will dynamic pressure be higher or lower at a higher altitude?
Lower, due to the reduced number of molecules in the less dense atmosphere.
What is Bernoulli’s theorem?
Dynamic Pressure plus Static Pressure equals a constant.
What is Bernoulli’s theorem also known as?
The venturi effect
What happens to air as it passes over the cambered top surface of a wing?
Increases in speed, increasing dynamic pressure, and thus decreasing static pressure, creating a negative pressure or lift region.
What is the leading edge of an aerofoil?
The edge that first meets the relative airflow
What is the trailing edge of an aerofoil?
The rear of the aerofoil
What is the chord line of an aerofoil?
The line between the leading edge and trailing edge points
What is the camber line of an aerofoil?
A line following the midpoint between the upper and lower surface camber.
What is the maximum camber of an aerofoil?
The greatest distance between the chord line and the camber line.
Normally occurs about 30% of the chord back from the leading edge.
What is the maximum thickness of an aerofoil?
The greatest distance between the upper and lower surfaces
What is the boundary layer?
The thin layer of airflow closest to the aircraft skin.
What happens to the boundary layer as air passes the maximum camber point?
The boundary layer starts to lose energy and become turbulent.
What is the angle of the chord line to the relative airflow known as?
The angle of attack
What happens to the transition point with an increase of the angle of attack?
It moves forward
At what angle of attack does lift stop increasing?
Approx. 15 degrees
When is an aerofoil said to be stalled?
When the boundary layer loses energy and separates from the upper surface at the separation point causing a large area of turbulence over the aerofoil.
Lift reduces dramatically at this point.
What is the airflow drawn upwards towards the leading edge known as?
Upwash
What is downwash?
Airflow past the trailing edge that is forced downwards
When does a stall typically occur?
When an aircraft is flying too slowly, or at a higher speed if the aircraft is suddenly pitched past the angle of attack.
What is unique about symmetrical aerofoils?
They permit lift to be generated when a wing is inverted
What kind of aerofoil do most modern airliners use?
Laminar flow aerofoils.
These aerofoils have the point of maximum thickness and therefore the boundary layer separation point moved to around the middle of the chord. This increases the laminar flow area on the wing and reduces the turbulent airflow.
What does increasing the camber of a wing do?
Increases both lift and drag.
What is the total reaction?
The overall force produced as a result of the differences in pressure over the aerofoil surfaces.
What is the centre of pressure?
The point the total reaction force acts through.
What does relative airflow depend on?
Direction and attitude the aircraft is flying.
What happens to the centre of pressure as angle of attack is increased?
It moves forward
What is the lift equation?
Lift equals the coefficient of lift multiplied by half Rho or density, multiplied by velocity squared, multiplied by the wing area.
How can an aircraft recover from a stall?
Reduce the angle of attack.
What is density measured in?
kilograms per cubic metre
What is the international standard density of air?
1.225kg per cubic metre
What is speed?
Distance over time
How many metres is a nautical mile?
1852m
What is velocity?
The direction and rate of change of an object. i.e. speed and direction.
Velocity can be expressed as a vector or a line with an arrow head.
What is induced drag?
The drag produced as a result of producing lift and acts rearwards and is aligned to the relative airflow.
What produces induced drag?
Wing vortices and wing downwash.
When are wing vortices strongest?
When the wing has a high angle of attack such as during takeoff and landing
Explain wing vortices.
The air underneath the wing has an outward flow component and the air above the wing and inward flow component. The high pressure air curls around the wingtips and forms vortices which create a large amount of drag.
The high pressure air below the wing curls around the wingtips to the negative pressure region above the wing.
When viewed from behind the vortex on the left wing will revolve clockwise and the vortex on the right wing will revolve anticlockwise
How does induced drag behave in relation to the angle of attack?
It increases with the increased angle of attack.
Induced drag will increase significantly beyond the stall point.
How can wing tip vortices and induced drag be reduced?
- a high aspect ratio wing such as on gliders
- shaping or tapering the wing tips to reduce their size, or
- using winglets
What is the aspect ratio of a wing?
Wing span divided by chord.
What is the advantage of shorter wings?
Greater structural strength and greater manoeuvrability.
What is wake turbulence?
The spread of disturbed air caused by vortices from an aircraft.
What is parasite drag?
Parasite drag is the drag created by aircraft components that do not contribute towards lift.
Includes
- form drag
- skin friction
- interference drag
What is form drag caused by?
Form drag is caused by the frontal area and shape of the aircraft.
Form drag decreases dramatically when the shape is streamlined
What is skin friction caused by?
Skin friction drag is caused by the friction between an aircraft surface and the air flowing over it and increases with speed.
Skin friction slows the airflow closest to the skin.
The airflow speed progressively increases further away from the skin producing laminar or layered smooth airflow.
Any disruption to the aircraft skin such as an unpolished surface will disrupt the boundary layer flow.
What is interference drag caused by?
Interference drag is caused by the interaction of airflow between two or more aircraft components such as the wing and fuselage. By fitting fairings and blending surfaces the turbulence created at the junction of the aircraft components can be reduced.
How can interference drag be reduced?
By fitting fairings and blending surfaces.
What is the minimum drag speed for an aircraft?
The airspeed that coincides with the lowest total drag point on the total drag curve
What is required to obtain the best performance in flight?
Lift at its highest and drag at its lowest.
The best lift/drag ratio.
At what angle is the best lift/drag ratio typically found?
4 degrees
What is the angle of incidence?
The angle of the wing compared to the longitudinal axis of the aircraft
What does deflecting a control surface down do?
Effectively increases the angle of the chord line in relation to the relative airflow, increasing the camber of the aerofoil and generating more lift.
If the elevator is moved down, what happens to the aircraft?
An upwards force is created on the tail causing the nose to pitch downwards.
What will moving the rudder to the right make the aircraft’s nose do?
Yaw to the right
What is the rudder used for?
Correcting a yaw caused by turbulence or out-of-balance forces, and to prevent the aircraft from “slipping” or “skidding” or flying out-of-balance.
When the left aileron is up and the right aileron is down, which wing will lift and which will lower?
The left wing will lower, and the right wing will rise, resulting in leftward or anticlockwise roll.
True or false. Yaw and roll are always connected and one will always generate the other.
True
What are spoilers used for?
- Reduce airspeed
- Dump lift on landing
- Assist with roll
How is control effectiveness affected by airspeed?
- At low airspeeds, dynamic pressure is small and large control movements are required, during landing for example.
- As airspeed increases, controls become more effective and only small movements are required at high airspeeds
What is the purpose of trimming controls?
To remove the force the pilot needs to provide through the control column (or rudder pedals) to deflect the control surfaces.
This enables the aircraft to be flown “hands-off”.
What does a trim tab do?
Adds camber to the control surface moving the control surface to the side opposite to the overall trim force required.
The control surface and the stabiliser then creates the overall trim force.
Trim tabs are usually fitted to the elevators, rudder and ailerons.
What are the types of high lift device?
- Flaps
- Leading Edge Slats
What are the four types of flap?
- Plain flaps
- Split flap
- Slotted flap
- Fowler flap
What is a leading edge slat?
A small auxiliary aerofoil which is either mounted or extends ahead of the leading edge of the aerofoil with a carefully designed gap, or slot between them.
It operates by allowing the passage of air through the slot from the high pressure region below the wing to the low pressure region over it.
Energy is added to the boundary layer on the upper surface, and any tendency for separation of the flow is reduced.
As the angle of attack is increased the slot delays separation, the lift coefficient continues to increase and the stall is delayed.
What is a disadvantage of leading edge slats?
Since maximum lift is obtained at high angles of attack, the aircraft’s attitude at landing and take-off is very much nose up.
What is stability?
The ability of an aircraft to return, of its own accord, to its original attitude after being displaced.
How are stability and manoeuvrability related?
They tend to oppose each other.
The more stable an aircraft is the less manoeuvrable and vice versa.
What is static stability?
The tendency of an aircraft to return to its original attitude after being disturbed.
Positive - tends to return to original attitude
Neutral - will carry on, will not return to original attitude, will not further diverge
Negative - will tend to continue to diverge after being disturbed
What is dynamic stability?
Relates to the behaviour of the aircraft over time.
Positive - tends to return to original attitude
Neutral - will remain in disturbed attitude
Negative - will tend to continue to diverge from the disturbed attitude
Which axis does longitudinal stability act on?
The lateral axis
Which axis does lateral stability act on?
The longitudinal axis
Which axis does directional stability act on?
The vertical axis
If the wing root is lower than the tip of the wing what does it have?
Dihedral
If the wing root is higher than the tip of the wing what does it have?
Anhedral
At what angle will an aircraft generally not be able to recover if it becomes laterally unstable?
15 to 20 degrees
What wing factors contribute to lateral stability?
- Angle of wing
- Whether the wing is sweptback
- Combining high wing with low centre of gravity (pendulum effect)
What is V1 Decision Speed
The speed above which the aircraft has to continue its take-off should an emergency arise.
What is VR
Rotation Speed
The speed at which the pilot pulls back on the elevators to make the nosewheel leave the ground.
With a larger angle of attack, lift is increased to overcome weight and the aircraft leaves the ground.
What is V2?
The Initial Climb Speed
The speed at which the climb-out can be safely executed with one engine inoperative, in take-off configuration with adequate obstacle clearance.
The aircraft is flown away from the ground in the correct climb attitude.
What effect will taking off downslope have on takeoff distance?
Acceleration will increase and takeoff distance will decrease
How will higher temperatures affect takeoff?
Reduced density, resulting in reduced thrust and lift and thus increasing takeoff roll.
How will higher elevation affect takeoff?
Reduced density, resulting in reduced thrust and lift and thus increasing takeoff roll.
How will taking off into the wind affect takeoff?
By taking off into wind, there is an effective airspeed acting on the aircraft even while it is standing still. This being the case, the aircraft has less speed to accelerate through to reach flying speed and hence a shorter ground roll.
A headwind means that the Indicated airspeed needed for take-off will occur sooner, at a lower groundspeed. Takeoff Distance required decreases with a headwind.
In which direction will weight always act?
Downwards - perpendicular to the horizontal.
In a climb, what must thrust equal?
Drag + rearward component of weight.
What is the angle of climb?
The angle between the aircraft’s flight path and the horizontal.
When is the best angle of climb speed Vx generally used?
Obstacle clearance e.g. mountains
What is the rate of climb?
The height gained per unit of time.
Expressed in feet per minute
What is Best Rate of Climb speed Vy used for?
Gaining altitude in the least amount of time
What is the service ceiling?
The point at which the best rate of climb that the aircraft achieves is reduced to 100 feet per minute.
What is the absolute ceiling?
The point at which the best rate of climb that the aircraft achieves is reduced to 0 feet per minute.
The aircraft can no longer climb.
How must forces be interacting for an aircraft to fly straight and level?
Lift equals Weight plus the Balancing Force
Thrust equals Drag
Total sum of forces equals zero.
No acceleration is taking place.
How can an aircraft be flown straight and level at different speeds?
By adjusting the angle of attack.
Increasing the angle of attack will increase the lift produced.
To fly slower the angle of attack will be reduced.
True or false. An aircrafts weight will reduce during flight?
True.
An aircraft will burn fuel during flight, reducing its weight.
What force is applied to allow an aircraft to turn?
A centripetal force is applied to the centre of a turn
Is there acceleration occurring when an aircraft turns?
Yes, as direction is changing.
Is the lift force required in a turn greater or less than that in straight and level flight?
Greater. Lift must both balance the weight and provide the horizontal turning force.
How is radius of turn affected by airspeed for the same angle of bank?
Radius of turn will increase with airspeed.
How are turning force and lift required affected by bank angle?
Increasing the bank angle increases the turning force and lift required.
What is the rate of turn?
The heading change achieved per unit of time
What is a rate one turn?
A turn with a turn rate of 180 degrees per minute
When is a rate one turn used?
During holding procedures
Which controls are involved in a turn?
- Ailerons to roll the aircraft
- Rudder may be needed to balance the turn. For larger aircraft rudder is generally only used at low speeds.
- Elevator is used to increase the angle of attack to provide extra lift.
- Throttle to increase thrust to overcome the extra induced drag produced by the increase in angle of attack and maintain a constant Indicated airspeed.
What is the load factor?
Lift divided by weight
Or
Wing loading in a turn divided by wing loading in straight and level flight
When do spins occur?
When the aircraft is at the point of stall either during straight and level flight or while turning and one wing drops.
How can a pilot recover from a spin?
- Centre the controls. Because the wing is stalled ailerons will be ineffective.
- Close the throttle
- Apply full opposite rudder to the direction of the yaw to stop the turning
- Apply forward pressure until rotation stops to unstall the wing
- Recover from the dive.
How can rate of descent be increased?
Reducing power
Using spoilers and speed brakes
Increasing drag
Extending flaps
Lowering the undercarriage
Will a headwind or tailwind affect rate of descent?
No. Flying into a headwind or tailwind will not impact rate of descent or Indicated Airspeed.
It will, however, affect glide range and groundspeed.
What is Vref
Final Approach Speed
Equals 1.3 times the stall speed of the aircraft in the landing configuration at maximum weight.
What are thrust reversers used for?
To reduce rearward thrust to zero and to provide a braking effect on landing.
Their use also reduces wear on wheel brakes.
How much will a 2% slope increase landing roll?
Approx. 20%
How will landing in areas of decreased air density or high temperature affect landing roll?
Landing roll will be increased as true air speed will be greater.
What are the standard flight instruments?
Gyroscopic Instruments
- Airspeed Indicator
- Turn Coordinator
- Heading Indicator
Magnetic Compass Instruments
- Magnetic Compass
Pressure Instruments
- Altimeter
- Airspeed Indicator
- Vertical Speed Indicator
What is the magnetic compass?
The primary source of directional information to which the Heading Indicator is aligned.
When won’t the magnetic compass be accurate?
During turbulence, turning, accelerating, climbing and descending
What is a gyroscope?
A spinning wheel which is free to move on one or more planes.
Gyroscopes maintain their alignment in space.
The faster and heavier the gyro the greater resistance to change
What does the heading indicator do?
Provides a stable heading reference and can be aligned with the magnetic compass using the adjustment knob.
Uses a gyro mounted on a gimbal. The gyro is spun using either an electric motor or a high speed air source. A vacuum system in the aircraft creates the high speed jet of air which is directed onto the circumference of the gyro to spin it. As the aircraft moves the gyro maintains alignment moving the heading indicator.
How often will the heading indicator lose alignment and thus need to be aligned with the magnetic compass?
Every 15 to 20 minutes.
What is the attitude indicator also known as?
The artificial horizon
What does the attitude indicator do?
Represents the theoretical position of the horizon and pitch attitude of the aircraft.
What does the turn coordinator do?
Shows the direction of turn and the ball indicates if the aircraft is slipping or skidding in a turn.
Where is the pitot tube normally mounted?
Mounted protruding in front of the wing, so that it is not affected by the pressure changes near the wing.
Aligned to relative airflow
What does the pitot do?
Feels the total pressure (i.e. static plus dynamic)
Where is the static port mounted?
Normally on the fuselage at a point least affected by pressure changes.
Which instrument is connected to both static and dynamic pressure sources?
Airspeed indicator
Which instruments are connected to the static pressure line only?
Altimeter
Vertical Speed Indicator
What does the airspeed indicator do?
Shows the airspeed in knots
What is Vne?
The never exceed velocity. If exceeded, structural damage is likely.
What does the green arc on an airspeed indicator indicate?
Normal operating area.
What does the yellow arc on an airspeed indicator indicate?
Caution range.
Flying at these speeds in turbulent air may overstress the aircraft.
What does the white arc on an airspeed indicator indicate?
Flap operating range
What does the lowest speed in the white arc on an airspeed indicator indicate?
Stalling speed with gears down and flaps down.
How is the airspeed indicator calibrated?
The airspeed indicator is calibrated for sea level international standard atmosphere conditions and measures the dynamic pressure.
What is true airspeed?
The actual airspeed that an aircraft travels through the air.
How is groundspeed calculated?
True airspeed adjusted for headwind or tailwind component.
What does the altimeter do?
The altimeter will display the vertical distance above whatever pressure datum has been set on the subscale.
What does the vertical speed indicator do?
Shows the rate of ascent or descent in feet per minute.
What does the automatic direction finder (ADF) do?
Shows the bearing to a Non Directional Beacon (NDB).
The pilot tunes to the frequency of the required NDB and the needle will point to the direction of the NDB.
A morse code identification sound will also be transmitted.
What does the VOR indicator do?
Tunes to a VOR station and indicates relative position to a ground station.
The VOR indicator shows radials from the VOR station and whether the aircraft is tracking to and from the VOR.
What does the Radio Magnetic Indicator (RMI) do?
This is a combination of heading indicator, ADF and VOR indicator.
The RMI’s compass card rotates with the aircraft’s heading simplifying the process of determining the bearing to an NDB.
What does distance measuring equipment (DME) do?
Provides the direct distance between the aircraft and the beacon being used. The Slant distance is measured which will be longer than the horizontal distance.
When the aircraft is directly overhead the ground station, the altitude is measured.
What is the electronic instrument which displays seperate dial instruments in a single display known as?
Electronic Flight Instrument System (EFIS) display.
What is work?
The force exerted moving an object a certain distance. Work is a transfer of energy. It is the product of force and displacement.
What is the formula for work?
W=FxD
What unit is work measured in?
Joules.
One joule = one Newton metre
What is power?
The amount of work done over a given time interval.
What is the formula for power?
P=Work/Time
What units are power measured in?
Watts
1 Watt = 1 Joule per second
Horsepower
1 Horsepower = 745 Watts
What is the power to weight ratio?
The ratio of an aircraft’s power to its weight
What kind of engines do aircraft use?
Piston
Turbine
— Turbojet
— Turboprop
— Turbofan
How are piston engines generally cooled?
Air flowing over the cylinders.
Oil from the oil sump.
What kind of cycle do aircraft piston engines use?
Four stroke.
- Intake
- Compression
- Combustion
- Exhaust
What stages does a jet engine consist of?
- Compressor
- Combustion
- Turbine
What occurs in the combustion stage of a jet engine?
- Compressed air enters
- Fuel is added
- The mixture is ignited causing the pressure to increase and the gas to expand rapidly
What kind of engine do modern aircraft use?
TurboFAN.
Uses a large fan to bypass the core turbojet engine.
What are the advantages of a turboFAN engine over turbojet and why?
- Quieter
- More efficient
This is because it moves a large volume of air at a slower velocity, rather than a small volume at high velocity.
What percentage of thrust from a turboFAN engine comes from the bypass fan?
Approx. 80%
How do turbofan engines slow the aircraft on landing?
By redirecting the bypass air forwards to create reverse thrust.
How do turboprop engines work?
Similar principle to turboFAN, but using a propeller rather than a bypass fan/duct.
What kinds of aircraft utilise turboprop engines?
Slower aircraft that fly at medium altitudes.
True or false. Propellers are more efficient at higher speeds.
False.
Propellers are more efficient at lower speeds.
What is maximum range?
The greatest distance that can be achieved from the fuel available.
For propeller aircraft this occurs at the minimum drag or best L/D ratio.
What is Maximum Endurance?
The greatest time the aircraft can fly with the available fuel, taking no account of distance travelled.
Occurs at minimum power speed.
Which is faster? Maximum Range Speed or Maximum Endurance Speed?
Maximum Range Speed
What are the main components of a helicopter?
- Main rotor
- Rotor head
- Rotor blades
- Tail rotor
- Fixed vertical stabiliser
- Fixed horizontal stabiliser
What is the purpose of a helicopter’s tail rotor?
Counteracts the torque generated by the main rotor by producing a thrust in the opposite direction to the torque.
What are the characteristics of helicopter blades?
- Symmetrical aerofoil
- Designed to flex upwards
What are the main helicopter controls?
- Control stick - pitch and roll via main rotor
- Tail rotor pedals - control tail rotor thrust and yaw
- Collective lever - controls the angle of the rotor blades, thus the amount of thrust to main rotor
Is a helicopter rotor more efficient when hovering or when moving forward?
When moving forward, as the rotor is cutting into clean, undisturbed, air.
What can a helicopter pilot do if the engine fails?
Perform an autorotation.
- As the helicopter descends the pilot will lower the collective to reduce rotor blade drag and energy will be stored in the rotor.
- As the helicopter nears the ground the collective lever will be pulled increasing the rotor blade angle of attack converting the energy into lift and slowing the helicopter for a gentler landing.
What kind of waves are sounds waves?
Pressure waves
What kinds of waves does an aircraft generate?
Pressure/Sound which propagate in all directions
What happens to the waves in front of an aircraft when the aircraft is moving?
The aircraft will tend to compress them.
When an aircraft travels at the speed of sound what happens?
The aircraft will catch up with the sound waves it creates and a single compression wave will be created.
This is also called a shockwave.
What occurs as an aircraft’s speed continues to increase past the speed of sound?
The shockwave becomes a cone around the aircraft and intensifies.
What happens to airflow over an aircraft as the aircraft approaches the speed of sound?
Airflow over some parts of the aircraft will reach the speed of sound
A shockwave will form most likely on the wing.
The airflow ahead of the shockwave will be supersonic and the airflow behind the wave subsonic. The centre of pressure will move causing changes in pitch trim.
What is a mach number?
A ratio of true airspeed to the local speed of sound.
Mach Number = TAS/Local Speed of Sound
When an aircraft is flying at the speed of sound what will its mach number be?
1.0
What is the critical mach number (Mcrit)?
The lowest Mach number at which the airflow over some point of the aircraft reaches the speed of sound, but does not exceed it.
This is usually on the upper surface of the wing but may form on the fuselage or canopy.
List the categories of high speed flight, and provide the mach number ranges which they refer to.
Subsonic - Below 0.70
Transonic - Between 0.70 and 1.20
Supersonic - Between 1.20 and 5.0
Hypersonic - Above 5.0
When flying in the transonic range, where airflow over some parts of the aircraft are supersonic, and others are subsonic, what will the aircraft experience?
- Rise in drag
- Buffet
- Trim and stability changes
- Decrease in control surface effectiveness
- Loss of lift due to shock wave formation and
- Rolling moments due to uneven shockwave formation
Above what altitude will an EFIS display airspeed in Mach Number rather than Knots?
Above 25000ft
How are swept wings beneficial to aircraft operating at high speeds?
They reduce the airspeed component perpendicular to the leading edge and thus increase the Critical Mach Number.
How does temperature affect the speed of sound?
Lower temperatures lead to a lower speed of sound.
How does altitude affect the speed of sound?
At higher altitudes the speed of sound is reduced.
What will an aircraft experience as it reaches the critical mach number?
High speed buffeting.
What is coffin corner?
The small speed range available for aircraft to operate at in higher altitudes without either stalling or experiencing high speed buffeting.
How does altitude affect stall speed?
At higher altitudes, the stall speed will increase due to the reduced air density.
