Principles of flight Flashcards
What are the four forces that act on an aircraft when in flight?
Lift , Drag , Weight , Thrust
Climb with power
- In a climb at constant velocity, weight which acts towards the centre of the earth.
- The component of W which acts against lift is WL
- Weight force also has a drag component represented by WD which acts against Thrust
- For the aircraft to be in constant velocity, all forces must be in equilibrium.
- L = WL
- T = D + WD
- In plain English, it means Thrust (T) must equal Drag (D) + the Drag component of Weight (WD)
Gliding
- In a glide descent, Thrust (T) is non-existent as engine is set to idle
- There is still a force counteracting Drag (D), otherwise the aircraft would be moving back up towards the sky which doesn’t make much physical sense
- The force that counteracts Drag is a component of Weight shown by WD
- For an aircraft to be gliding at constant velocity, all forces must be in equilibrium
- L = WL and;
- D = WD
Descending
- When an aircraft applies power during descent, Thrust (T) is increased
- When T is increased, more air flows over the wings of the aircraft.
- This will increase Lift (L).
- If the nose of the aircraft is held down to keep airspeed constant, the increase in T will result in the aircraft rising due to increased L.
Very important to know this when learning how to land an aircraft
Turning part 1
- When an aircraft turns, it rolls the wings. The angle at which it rolls is represented by [θ]
- This is also called the Angle of Bank (AoB)
- When the aircraft is banked, the component of Lift (LW) is less than total lift, so W > LW and the aircraft will descend.
- During a turn, back pressure must be applied to keep the aircraft level. Backpressure creates additional lift during the turn.
Turning part 2
- You can see from the image below that if the AoB increases, the following happens:
- LT which is the component of L that counteracts W is greatly diminished and it becomes more difficult to keep the aircraft level during the turn.
Turning part 3
- To keep an aircraft from losing altitude during a turn, back pressure must be applied. The amount of force due to this backpressure is known as the ‘load factor’ which you will explore later. During a level turn, this is the amount of extra weight a person will feel in the aircraft.
- LR can be worked out with the following formula:
cosθ=Lw/L_R
Stalling speed of an aircraft does what with weight
increases
In a turn , the apparent weight of an aircraft
increases
Therefore stalling speed
also increases
Load factor formula
new or apparent weight / real weight
new stall speed formula
= old stall speed * square root of load factor
stalling speed is the speed at which an aircraft stalls when its
landing gear and flaps are re-tracted , the throttle is closed and speed reduced in straight and level flight
Indicators that signal a low speed stall
Nose attitude is high
Air noise reduces
Airframe buffeting commences
Controls start to feel ‘sloppy’
Recover from a stalled spin , what method should be used
P.A.R.E
What are the steps
(P) ower idle,
(A) ilerons neutral,
(R) udder opposite the spin, and
[E] levator forward.
Recovering from a spiral dive , what are the steps
- reducing power (to idle),
- Identify that aircraft is in a spiral dive or a stalled spin,
- leveling the wings with ailerons,
gradually pulling up on the elevator while adding power if necessary.
Spiral dive often appears to pilot as spin but
at least one wing is stalled in a spin
In a spiral dive
neither wing is stalled
Manoeuvre merely a
diving steep turn which aero plane is flying tight descending circle around vertical axis
Spiral dive all controls
functioning
Factors affecting lateral stability
High wing aircraft - Pendulum effect
In a high wing winged aircraft
- CoG of the fuselage will counteract the direction of a roll bringing the plane back to wings level
In a low wing aircraft
- COG of fuselage will assist the plane in direction of the roll, will put the plane into a steeper bank
Dihedral wings
For an aircraft with dihedral wings:
When a roll is initiated:
- Due to sideslip of relative airflow due to the roll, the lower wing will experience a higher angle of attack
- The higher angle of attack will automatically produce more lift in the lower wing than the higher wing
- The lower wing will lift and will want to bring the aircraft back to level flight.
Sweepback wings
For an aircraft with swept back wings, there are two things that will help lateral stability
- When the aircraft rolls, sideslip of relative airflow will mean the lower wing’s leading edge will be close to 90o to relative airflow compared to the upper wing which will have a far less direct angle of airflow to leading edge. The more direct airflow will lead to more lift created on the lower wing, bringing the aircraft back to wings level.
- Because of the sideslip the front section of the fuselage will block some airflow over the upper wing, reducing it’s effective span for creating lift. The lower wing will experience more lift (because of more span exposed to airflow) creating more lift bringing the wings back to level flight.
Factors that affecting directional stability
When an aircraft yaws in one direction, the forward wing opposite the direction of yaw offers more frontal area to the relative airflow, creating more drag. The backward wing, blocked by some of the fuselage experiences less frontal area to the airflow and experiences less drag. The wing with more drag has an automatic effect of yawing the aircraft back to centre
Size of the fin and position of center of gravity
The fin of an aircraft has an inbuilt ability to recover from a yawing disturbance about the normal axis. The moment of the vertical fin, due to large surface area and moment due to the large distance between it and the centre of gravity, acts to restore the nose to its original position
Factors affecting longitudinal stability
- The greatest contributor to restoring longitudinal stability is by the horizontal stabiliser. As an aircraft pitches upward, the airflow strikes the HS which are located behind the CoG. The airflow causes lift in the horizontal stabilisers, which lift the tail of the aircraft subsequently returning the nose of the aircraft to level (resisting the climb)… the same works in the opposite direction when the aircraft is descending.
Longitudinal dihedral
Longitudinal dihedral involves wings and horizontal stabilisers having different angle of incidences.
By mounting the horizontal stabilisers at a higher angle of incidence, any change of angle of attack in the wing will have an added effect by the HS.
Because of the position of the HS at the rear of the aircraft, any lift experienced by it will cause the tail to rise and nose of aircraft to fall, hence increasing longitudinal stability.
Tailplane moment
Because of the great length of the moment arm between the centre of gravity and the tailplane , the aerodynamic force produced by the tailplane need not be large for its turning effect to be powerful.
Position of center of gravity
Forward CoG leads to increased longitudinal stability
Rearward CoG leads to decreased longitudinal stability
Pilot can arrange any cargo or weights within the aircraft to change position of CoG to suit.
Static Stability
Static stability is the initial tendency of an aircraft to return to its original position when it’s disturbed.
Positive Static Stability
tends to return to its original attitude when it’s disturbed.
Neutral static stability
tends to stay in its new attitude when it’s disturbed.
Negative static stability
tends to continue moving away from its original attitude when it’s disturbed.
Dynamic Stability
Dynamic stability is how an airplane responds over time to a disturbance.
Positive Dynamic Stability
have oscillations that dampen out over time
Neutral dynamic stability
have oscillations that never dampen out
Negative dynamic stability
have oscillations that get worse over time
Servo tabs
On high speed aircraft , movement of the controls into the fast moving air stream requires so much effort from pilots that controls become exceptionally difficult to manipulate.
Servo tabs works in
opposite direction to the control surface
Anti servo
Some aircraft have light and responsive control surfaces that a small input by the pilot , particularly at higher speeds will produce an unnecessarily large response
Therefore anti servo tab works
in same direction as control surface is fitted
Causes of wheelbarrowing
Causes of wheelbarrowing:
- Only occurs in aircraft with tricycle undercarriage
- Landing too fast
Trying to force the front wheel onto the ground after landing causing too much load to be on front wheel and not enough on the main wheels (rear)
Problems associated
- Loss of directional control of aircraft
- Violent swerving as the aircraft pivots around the front wheel
- Can cause ground strike of wing in direction of the wheelbarrowing or collapse of main wheel undercarriage due to excessive side loading.
Ground looping , only occurs in aircraft with a
tailwheel undercarrige
Causes
Causes:
1. Caused when aircraft lands in crosswind and tailwheel touches down sidewards of the aircraft… then the pilot applies brakes.
Because tailwheel aircraft has CoG behind main wheels, the CoG and tailwheel can overtake the main wheels from the side, causing loss of directional control and wing strike.
- After aircraft has landed but is still rolling at speed on runway, if pilot incorrectly applies brakes to one wheel only, can cause tail of aircraft to swing out. CoG of aircraft and tail wheel can overtake main wheels from the side causing wing strike.
Anhedral
- In summary, anhedral wings make the aircraft less laterally stable, or makes it easier for the pilot to roll the aircraft.
- For a heavy lift cargo plane, most of the weight is located in fuselage, creating a pendulum effect to resist rolling motions. They may be so fuselage heavy that the aircraft cannot roll or bank, hence the anhedral wings help to maintain the aircraft in a roll.
Spoilers or lift dumpers
Surfaces hinged to rise on the upper surface of the wing to reduce lift and create drag.
If spoilers are deployed simultaneously, the loss of lift is symmetrical and aircraft decelerates and sinks
If deployed individually, can be used as ailerons or aids for rolling the aircraft.
Speed brakes
Are surfaces which can be mounted on any part of the aircraft such that , when deployed will create drag and cause a rapid deceleration of the airplane
Stabilators
Are a combination of horizontal stabilizer and elevator
Tailerons
Are stabilators in which two parts can move together to pitch the aircraft or independently to roll the aircraft
Ruddervators
Are a combination of rudder and elevators
Elevons
Combination of elevators and ailerons which extends across the trailing edge of delta winged aircraft which have no elevators or stabilators
Flaperons
Combine flaps with ailerons
They can also be
lowered to function as a dedicated set of flaps.
Advantage of using flaperons instead of dedicated ailerons and flaps is a reduction in weight.
Canard
Is a small wing mounted either side of the nose of the aircraft
Fixed canard to be used safely
it must be set at an angle of incidence that is greater than the wings of the aircraft
In a stall
the fixed canard at the front of the aircraft must stall first, which pitches the nose down and helps with recovery.
In an aircraft where a fixed canard is placed in front of the wing and C of G of the aircraft
if the wing stalls before the canard, the aircraft’s CoG will drop the wing and aircraft down pitching the nose up and further stalling the aircraft which can lead to an unrecoverable position.
Vortex generators
Vortex generators are small plates about an inch deep standing on edge in a row spanwise along the wing. They are placed at an angle of attack and (like a wing airfoil section) generate vortices. These tend to prevent or delay the breakaway of the boundary layer by re-energizing it.
Flaps, the different types
- Plain Flap - The rear portion of the wing aerofoil rotates downwards on a simple hinge arrangement mounted at the front of the flap.
- Split Flap - The rear portion of the lower surface of the wing aerofoil hinges downwards from the leading edge of the flap, while the upper surface remains immobile.
- Slotted Flap - Similar to a Plain Flap but incorporates a gap between the flap and the wing to force high pressure air from below the wing over the upper surface of the flap. This helps reduce boundary layer separation and allows the airflow over the flap to remain laminar.
- Fowler Flap - A split flap that slides rearwards level for a distance prior to hinging downwards. It thereby first increases chord (and wing surface area) and then increases camber. This produces a flap which can optimise both takeoff (partial extension for optimal lift) and landing (full extension for optimal lift and drag) performance. This type of flap or one of its variations is found on most large aircraft.
- Double Slotted Fowler Flap - This design improves the performance of the Fowler flap by incorporating the boundary layer energising features of the slotted flap.
Benefits of flaps
- You can produce more lift, giving you lower take off and landing speeds
- You can produce more drag, allowing a steeper descent angle without increasing your airspeed on landing
- You can reduce the length of your take off and landing roll
Slots
The slot permitted the wing to stall at angles up to 10 degrees greater than their normal stalling angle of attack and allowed the aeroplane to operate at much lower airspeeds before the onset of a stall
Slot downside
In aerodynamics, everything comes with a penalty. In a slot’s case, it’s drag, capping your airplane’s cruise speed and efficiency. Since slots are always open, the drag is always there
Slats
Slats are extendable, high lift devices on the leading edge of the wings of some fixed wing aircraft.
Purpose of slats
- Their purpose is to increase lift during low speed operations such as take off, initial climb, approach and landing.
- They accomplish this by increasing both the surface area and the camber of the wing by deploying outwards and drooping downwards from the leading edge.
