Fixed Wing CPL Course
HORIZONTALLY OPPOSED ENGINES
The following classifications are used to identify these engines:
- O = Opposed Cylinders
- H = A horizontally mounted engine designed specifically for a helicopter
- V = A vertically mounted engine designed specifically for a helicopter
- I = Fuel Injected
- T = Turbocharged
- S = Supercharged
- 360 = Capacity in cubic inches
Hence an 0 360 is a 360 cubic inch engine, with opposed cylinders that's made to be mounted horizontally in either a helicopter or an aeroplane.
The HIO 360 is a 360 cubic inch, fuel-injected engine, with opposed cylinders that's specifically made to be mounted horizontally in a helicopter. This engine is fitted to the Hughes 300 C.
The TVO 435 is a 435 cubic inch, turbocharged engine, with opposed cylinders that's specifically made to be mounted vertically in a helicopter. This engine is fitted to some of the Bell 47 series.
Another engine is the O 320 that's fitted to the Robinson R22. It's a 320 cubic inch engine with opposed cylinders that was designed for use in an aeroplane, hence there's no 'H' or 'V' in its designation.
Regardless of what sort of engine is fitted, in order to maintain height and speed, the engine must be capable of producing enough power to overcome:
- Profile drag,
- Parasite drag,
- Induced drag, and
- Ancillaries such as generators, hydraulic pumps, air conditioners, tail rotors, etc.
In order to climb or increase speed (accelerate), the engine must be capable of producing additional power over and above these requirements.
This is the base upon which the engine is built. It's a two-piece casting, joined along the centre-line.
It contains the main bearings that support the crankshaft, the camshaft bearings, and it's fitted with the mounting points to attach other casings (accessory cases, reduction gearboxes, etc) to the crankcase. It also has the attachments to mount the engine into the airframe.
This, along with the compression ratio, ignition timing and fuel mixture, controls the pressure that's exerted on each piston during its power stroke.
Manifold pressure (MP or MAP) is sensed in the inlet manifold, after the carburettor.
The manifold pressure gauge should read ambient air pressure if the engine isn't running. This is 29.92" Hg at sea level on an ISA day, and decreases by approximately 1" Hg per 1,000' of altitude.
With the engine running at full throttle, the MP will be slightly less than ambient air pressure (around 1/2" to 1" Hg) due to the restriction to flow through both the air filter and the venturi. Therefore, the maximum MP that can be achieved in a carburetted engine will be slightly less than in a fuel injected engine that either doesn't have a venturi (Continental) or has a far less restrictive venturi (Lycoming).
Note that the engine doesn't know whether it's not producing full power because the throttle isn't fully open (a restriction to airflow) or the filter's dirty (a restriction to airflow).
BAKC-3.3.1 - BAKC-3.3.2(b)
This is a spontaneous explosive combustion (self-ignition) of the fuel air charge after normal ignition, which means it normally occurs late on the compression stroke, and opposes the piston's upward travel.
Whereas pre-ignition is generally confined to one or two cylinders where there are hot-spots, detonation usually is more likely to occur in all of the cylinders.
It occurs if the temperature of the unburned mixture between the converging flame fronts from each of the spark plugs reaches the spontaneous combustion temperature, causing this unburned mixture to explode. This increases the combustion pressure and temperature dramatically and causes peak pressure to occur before the piston reaches TDC.
This increase in both temperature and pressure while the piston is still moving up can cause severe mechanical damage to the engine.
It can be caused by :-
- Incorrect type or grade of fuel - the octane rating of the fuel is a measure of its resistance to detonation.
- High MP, particularly when combined with a low RPM.
- Lean mixtures at high power settings. and/or
- Excessive use of carburettor heat - the higher the induction temperature, the closer the mixture is to the spontaneous combustion temperature.
When increasing the power, the mixture must be enriched to avoid detonation, and this excess of fuel cools the charge and slows the flame front, thus reducing the combustion temperature.
Excessively rich or lean mixtures should be avoided, as either one will result in a decrease in power and engine life.
If either detonation or pre-ignition is suspected (vibration and/or an abnormally high CHT), there are three things a pilot should do :-
- Select full rich mixture.
- Reduce the MP.
- Eliminate carburettor heat.
The above information is the conventionally accepted concept that pre-ignition can lead to detonation, and it's the detonation that causes the damage.
Current technology of being able to measure pressures and temperatures within the cylinder many times per second has led to the discovery that detonation on its own rarely creates sufficient pressure to be destructive, but when it raises the temperature to a level where it creates an intense hot spot within the cylinder head (usually the tip of the spark plug, particularly fine-wire plugs) it causes early pre-ignition, and it's the incredible pressures created while the piston is still rising, along with the high temperatures that does the damage.
Detonation causes an increase in EGT and a decrease in CHT. This is because more of the heat energy within the combustion chamber leaves when the exhaust valve opens, but the additional heat heat is created after normal combustion, and therefore isn't in the combustion chamber long enough to increase the CHT.
Pre-ignition causes a decrease in EGT and an increase in CHT. This is because the extreme pressures created blast through the boundary layer and into the cylinder head, leaving less heat to pass into the exhaust pipe when the exhaust valve opens.
NOSE WHEEL STEERING
A nose-wheel increases directional stability whilst on the ground because the aircraft's C of G is located well forward of the main wheels, whereas with a tail wheeled aircraft, the C of G must be behind the main wheels.
# Helicopters with a wheeled undercarriage are fitted with a nose-wheel that's free to castor, and utilise tail rotor thrust and/or differential braking to steer the helicopter whilst on the ground.
Some light aircraft have steering rods attached to the nose wheel that are linked to the rudder pedals via a torque link, and some utilise differential braking to steer the aircraft whilst on the ground.
The outer cylinder of the oleo is attached to the aircraft structure, and the inner cylinder of the nose wheel assembly rotates within the outer cylinder as well as moving up and down. Those with nose wheel steering control the turning of the inner cylinder via the torque link, which is in turn controlled by the rudder pedals, and for those with a castoring nose wheel, the inner cylinder is free to turn.
On larger aircraft, hydraulic nose-wheel steering is required.
Regardless of the method of steering, nose-wheels with a steering mechanism require a means of dampening nose-wheel shimmy (vibration) at high ground speeds.
This is provided by one or more dampers, which are essentially a closed cylinder with a piston that moves as the nose wheel moves from side to side within the free-play in the steering system. As the piston is forced to move, fluid must move from one side of the piston to the other via a small orifice, thus dampening any shimmy of the nose-wheel.
Castoring nose wheels don't require a shimmy damper, but generally use a marstrand tyre, which is one that has a large groove in the middle of the contact area, leaving a contact area at each edge of the tyre only. This type of tyre resist shimmy as each of the contact areas resists the oscillation. These tyres must be changed before the wear is such that the centre section would make contact with the ground.#
RELAYS & SOLENOIDS
It's often necessary to open and close a circuit carrying a large current with a small switch in a remote location. A typical example is the starter button in the cockpit that activates the starter motor attached to the engine, and in this situation, the small switch in the cockpit is connected to a relay (an electromagnetically operated switch) near the starter motor with light wiring, and when the relay closes heavy contacts connect the battery to the starter motor.
Circuits that draw a lot of current, such as the battery or starter motor, are fitted with a relay that uses a light current to close a pair of heavy duty contacts and operate the starter motor (or other load).
Many large aircraft utilise a 200 Volt AC electrical system, and all of their electrical systems are operated by DC relays to ensure the high voltage current isn't being switched in the cockpit, where a faulty switch could give a nasty, if not fatal, electric shock to the operating crew member.
When the switch is actuated, a light current from the switch circuit flows through a coil wound around an iron core; the magnetic field created by the current flowing through the coil pulls the iron rod (the core) through the coil, which closes the heavy duty contacts on the supply circuit.
If the static tube is blocked all three pressure instruments will be affected.
If the aircraft climbs, the static pressure trapped in the line from the static vent will be higher than the actual static pressure, but the total pressure on the other side of the diaphragm will be representative of the true static pressure. Therefore the ASI will under-read.
In other words, the ASI is behaving IN THE REVERSE SENSE to an altimeter, in that it decreases its reading in a climb and increases its reading in a descent.
If the aircraft remains at the same height, the ASI will remain constant, but if it descends, the ASI will over-read, as the trapped static pressure will be lower than the actual static pressure.
Under these conditions, the altimeter reading will remain fixed and the VSI will read zero.
If both the pitot and static sources are blocked, the IAS will remain at the speed indicated where the blockage occurred.
Direct Reading Compasses have only one moving part, therefore they're extremely reliable. In most light aircraft, they're the only means of determining directional information.
Apart from regular compass swings by maintenance engineers, the only time a compass is likely to need attention is if the fluid becomes excessively discoloured or a bubble is visible, indicating the fluid is leaking and the level has dropped.
This fluid dampens oscillations of the needle, and by both its buoyancy and lubrication properties, reduces the friction on the pivot assembly. This means the compass will react faster to a change of heading (increased sensitivity).
NOTE :- Some questions use the term 'increase aperiodicity', which, in plain English, means 'increase dampening' or stabilise quickly after a disturbance.
It's important to note that the compass card remains stationary and the aircraft turns around it. In this illustration, the aircraft is on a heading of North, but if it turned 30o to the left, the body of the compass, along with the lubber line, would move around the card and the lubber line would be over the figure 33 (330o M).
The magnetic direction is expressed as a three-figure group representing the number of degrees you've turned clockwise from North. A heading of 030 means you are heading 30o away from North in a clockwise direction. A heading of 330 means you are heading 330o away from North in a clockwise direction.
Remember, if you are turning onto a new heading with a greater number (going from 300o to 330o), you must turn clockwise.
The needle of a magnetic compass that's unaffected by extraneous objects, aligns itself with the earth's magnetic field in both the horizontal and vertical plane. These lines of force are parallel to the Earth's surface at the Equator, and progressively tilt downwards as they get closer to the poles, at which time they're aligned at right angles to the Earth's surface, and therefore, depending on the latitude (the distance from the Poles) the magnet may not be parallel to the earth's surface. This is called Compass Dip or Magnetic Dip
To minimise compass dip, the compass card, with the magnets attached, is suspended from above the card (pendulous suspension), and when the card is parallel to the Earth's surface (as it is at the Equator), the C of G of the compass card is directly below the pivot point. A collar at the suspension point prevents the compass falling apart when inverted (either on the shelf or in flight).
To improve sensitivity, there are usually 2 magnets side by side.
The closer the compass is to the Equator, the more horizontal the compass card is, and the closer the C of G of the compass card is to being directly below the pivot point. Because the magnets are below the point of suspension, whenever the C of G of the compass card moves away from a point directly under the pivot point, compass errors can occur when the aircraft changes speed or direction.
Compass dip is at a minimum at the equator, and increases as you move closer to the magnetic poles, and the compass needle aligns itself with the magnetic lines surrounding the Earth. This causes the centre of gravity of the needle and card assembly to move away from the pivot point as the compass card moves away from the horizontal plane.
This causes turning errors, acceleration errors and deceleration errors. These errors increase as the latitude increases and the compass card moves further away from the horizontal plane. These errors occur in both hemispheres, but with opposite effects.
Because of these errors, the compass should only be read when the aircraft is flying straight & level at a constant speed. In other words, the compass is accurate.
The closer you get to the magnetic poles, the less accurate a magnetic compass is. This is because the Earth's magnetic field is becoming more and more vertical, which means the horizontal component is becoming weaker and weaker.
In the two sorties I did to the Antarctic in the early 70's, I found the deviation to be around 60o, but very stable, making the compass quite reliable. As we didn't have DG's (and GPS wasn't invented then), we managed to find every location we needed just by using the compass.
As the compass needle aligns itself with the magnetic field around it, it's important that you don't place anything near the compass that might influence the Earth's magnetic field and cause an error in the compass. Headsets are one of the worst items to place near a compass as they have magnets in the small speakers within the earpieces.
#An Artificial Horizon (AH) or Attitude Indicator (AI) is an earth gyro with a vertical spin axis (A-A), which keeps the rotor's plane of rotation (B-B) horizontal. It's used to give the pilot pitch and roll information, primarily used when the real horizon is obscured.
A miniature aircraft is attached to the case in the centre of the AH, with its wings parallel to the lateral axis of the aircraft. A beam bar behind the miniature aircraft is connected to the gyro mechanism and a pendulous unit ensures it remains parallel to the Earth's surface to represent the horizon. There's usually provision for altering the vertical position of the miniature aircraft to provide a basis for subsequent attitude control.
The relative movement between the gyro stabilised beam bar and the instrument case, with the miniature aircraft attached to it, displays pitch and roll information on the face of the instrument.
When the aircraft raises its nose, a guide pin moves the horizon bar down (miniature aeroplane above the horizon bar).
When the aircraft lowers its nose, the guide pin moves the horizon bar up (miniature aeroplane below the horizon bar).
When the aircraft rolls, the instrument case rotates around the stabilised beam bar.
Because the miniature aircraft is attached to the instrument case, its right wing is below the horizon bar when the aircraft is banked to the right and above the horizon when the aircraft is banked to the left; it moves below the horizon bar when the aircraft pitches nose down and is above the horizon bar when the aircraft pitches nose up.
A pointer indicates the angle of bank on a graduated scale that's attached to the instrument case.
In air-driven instruments, the spin axis of the gyro is maintained in the vertical position by a pendulous unit that alters the force of air on the buckets of the rotor from various ports in the instrument case.
This creates an additional force on one side of the rotor which precesses the gyro's rotor back to the horizontal position and, when this has been achieved, the reaction forces from the various air jets around the perimeter of the case are once again in balance.
Electrically driven gyros are also kept erect by a gravity-sensing system.
Vertical gyros have two inherent errors ;-
- They indicate a climbing starboard turn during a straight and level acceleration. This is caused by the inertia of the gravity sensing devices, and is far more noticeable with air driven gyros.
- They have a turning error, which, if uncorrected, shows up as incorrect readings of both bank and pitch. Modern instruments are corrected so that turning error is zero for a rate 1 turn at a given speed. The errors for other speeds and other rates of turn are minimal.
Transport wander in an Earth Gyro is eliminated by the erection mechanism that keeps the spin axis aligned with the gravitation pull towards the centre of the Earth.
The amount of water vapour air can hold increases with temperature, and Relative Humidity (RH) is a measure of the amount of water vapour contained in a sample of air expressed as a percentage of what it could hold at its saturation level at the same temperature and pressure. It's an indication of how close to saturation that parcel of air is. If the air is retaining half of the water vapour that it could hold at that pressure and temperature, its Relative Humidity is 50%.
The Relative Humidity of the air is generally higher on a cool morning when the cool air is closer to saturation, and lower in a warm afternoon when the warm air could hold more water vapour.
If the Relative Humidity of a parcel of air reached 100%, you'd see fog or a cloud forming, but if its RH was only 99%, you wouldn't see a cloud.
This can be likened to dripping water onto a sponge. The sponge absorbs the water until it's saturated, and then the water would start to drip out of the sponge. If it took 1 litre of water before the sponge was saturated, when you'd poured half a litre of water onto the sponge, it would be 50% saturated, but if the sponge was able to absorb 2 litres of water, it would only be 25% saturated.
If the temperature increases, the air can hold more water vapour without forming condensation, therefore, the relative humidity will decrease if the temperature increases, even though there's still the same amount of water in a given volume of air.
If the temperature decreases, the air isn't capable of holding as much water vapour, therefore it's closer to being saturated and its relative humidity is increased, even though there's still the same amount of water in a given volume of air.
All thunderstorms have the potential to produce damage from lightning, heavy rain, strong winds, down-bursts, hail and even tornadoes, however most severe thunderstorms form ahead of cold fronts.
A frontal thunderstorm is one that develops as the result of interaction between two air masses. If a cold front is moving rapidly, the warm air at the frontal boundary is forced to rise rapidly up and over the wedge of cold air, which often leads to the development of Cumulo-nimbus clouds. If this rising warm air is moist and very unstable, a frontal thunderstorm is likely to form, and the faster the front is moving, and/or the colder the advancing air is, the greater the intensity of the thunderstorm.
Thunderstorms can develop very quickly along the edge of the front, and a line of thunderstorms is more dangerous than single thunderstorms.
These types of storms present the greatest problems to aviation because the cells are more numerous and move rapidly. The cloud bases are usually quite low and the chance of encountering strong down draughts and turbulence underneath them is extremely high.
Although frontal thunderstorms generally form at the boundary of a cold front, they can sometimes form at the boundary of a warm front.
Frontal thunderstorms shouldn't be mistaken for a cold stream thunderstorm, which develops in the cold air behind the front.
If the aircraft is East of the station and the pilot turns the Omni-Bearing Selector (OBS) to 090, the indicator would be centred, showing the aircraft is on that radial, and the TO - FROM flag would show FROM, meaning that by keeping the indicator centred whilst holding a heading of 090o ± the wind correction angle, the aircraft would remain on a track of 090° FROM the station.
If the aircraft drifts to the left of the track (shown here on the 083 radial) the indicator will show that the 090 radial is 7o to the right and the aircraft must turn right (toward the CDI) to regain the correct track.
If it was a helicopter hovering on the 090 radial with the 090 radial selected, and it turned 360o whilst remaining in the same position, the indicator would remain centred, and the TO - FROM flag would continue to show FROM, indicating the aircraft was on the 090 radial from the station, regardless of its heading (or its track if it was crossing that radial).
If, at any time during this spot turn, the helicopter pilot turned the OBS to '270', the TO - FROM flag would show TO, showing that by keeping the indicator centred, the helicopter would remain on a track of 270° to the station, regardless of whether it was travelling sideways or backwards, in other words, the aircraft's heading is irrelevant.
Using the 1:60 rule, if an aircraft is 60 nm from the station with a deflection of 3o, it's 3 nm off track.
WHY WE NEED TWO EYES
Each eye sees an object from a slightly different angle and passes these distorted images to the brain, where the two images are processed and the blind spot from each eye is replaced with that portion of the image from the other eye, thus allowing us to 'see' the object in a true perspective (stereoscopic vision), and the brain then perceives this image in three dimensions.
If we only had one eye, we wouldn't get any depth perception, and everything would look flat.
Depth perception is based on binocular vision at close distances, and the rules of proportion and perspective for objects that are further away.
Stereoscopic vision is the ability to focus both eyes on a single object.
If a person's brain is effected by alcohol or fatigue, it may not be able to coordinate the muscles controlling each eye, which results in each eye presenting the brain with a slightly different image, which results in what's commonly called double vision.
For the eardrum to function normally, it must have an equal pressure on each side. A narrow passageway called the Eustachian tube achieves this by connecting the middle ear to atmospheric pressure via the pharynx (the cavity that connects the nose and mouth to the lungs and the stomach). The Eustachian tube also allows fluid to drain down the throat.
A flap valve in the Eustachian tube opens when the pressure in the middle ear is greater than atmospheric pressure (as in a climb) but closes when the atmospheric pressure is greater than the pressure in the middle ear (as in a descent).
If an aircraft enters a very slight bank angle at a rate undetectable to the sense of balance, the angle of bank will generally increase until there's a noticeable loss of altitude.
Noticing the decrease in altitude but still believing the aircraft is in level flight, the pilot may pull back on the controls and perhaps add power in an attempt to regain the lost altitude.
Unless the bank attitude is corrected first, this manoeuvre will usually put the aircraft into a spiral dive.
Once the spiral has stabilised, the sensory mechanism in the motion sensing system ceases to be stimulated and they can suffer an illusion of turning in the opposite direction and increase the bank angle whilst trying to stop the turning motion of the aircraft.
Under these circumstances, it's highly unlikely appropriate corrective action will be taken. Rather, the spiral will continue to tighten until it becomes non-recoverable.
This unfortunate situation occurs all too often when pilots without instrument training or with limited instrumentation mistakenly believe they can maintain orientation in clouds.
A stall is the result of an excessive angle of attack, and although it can occur at various airspeeds as the result of any number of manoeuvres, it always occurs at the same angle of attack for a given aerofoil.
The angle at which this occurs is called the stalling angle and the airspeed at which this occurs is called the stalling speed.
The stalling angle is constant for a given aeroplane.
The speed at which this angle is reached varies with changes in weight, power, load factor, flap configuration etc. The stall speed quoted in the flight manual is for a power-off stall at maximum AUW.
In a turn, the nose must be raised (increased angle of attack) in order to maintain height, therefore the stalling angle will be reached earlier (at a higher airspeed).
A wing will stall when the air flowing over it is disrupted, and separates from the surface.
If an aircraft is fitted with a conventional tailplane (low mounted), as the wing approaches the stall angle, the turbulent airflow from the wing flows over the tailplane and creates a pre-stall buffet. In a T-tail aircraft, the turbulent airflow from the wings doesn't pass over the tail until after the wings have stalled, therefore there's no pre-stall buffet.
This separation normally occurs as the aeroplane gets slower and the angle of attack of the wing is increased in order to produce the lift necessary to maintain height, but it can also occur if the nose is raised rapidly at high speed (the aircraft initially continues on the same flight path with the nose raised, thus increasing the angle of attack) or during a high 'G' manoeuvre when the wings have to support an increased effective weight.
The angle of incidence of the tailplane is less than that of the wing to ensure it remains un-stalled when the wing stalls.
If a wing drops when close to the stall speed, use opposite rudder to pick it up, not ailerons. If you lower the aileron on the dropping wing in an attempt to raise it, you may put that wing into a stalled condition.
If you use opposite rudder (right rudder if the left wing drops), the yaw that follows will increase the relative airspeed of the dropping wing and the additional lift that's created will level the wings without increasing the angle of attack any more than it already is.
In a 30o angle of bank, the load factor is increased by approximately 15%, and the stall speed is increased by approximately 7% (50 kts X 1.07 = 53 kts - 100 kts x 1.07 = 107 kts).
In a 45o angle of bank, the load factor is increased by approximately 40%, and the stall speed is increased by approximately 20% (50 kts X 1.2 = 60 kts - 100 kts x 1.2 = 120 kts).
In a 60o angle of bank, the load factor is doubled., and the stall speed increases by approximately 40% (50 kts x 1.4 = 70 kts - 100 kts x 1.2 = 120 kts).
The effect of these changes is as follows, and can be summarised by the following example of an aeroplane with a certified stalling speed of 50 kts.
- 50 knots in level flight at maximum gross weight
- 45 knots at 80% of maximum gross weight.
- 54 knots in a 30o angle of bank turn.
- 60 knots in a 45o angle of bank turn.
- 71 knots in a 60o angle of bank turn.
As the aeroplane is approaching the stall speed, the :-
- controls get 'mushy';
- air noise decreases; and
- buffeting occurs.
None of these symptoms occur if the stall is caused by an increase in load factor when the airspeed isn't close to the stall speed.
STALL SPEED DESIGNATIONS
Vso - The stall speed in the landing configuration – i.e. power off, flaps down, wheels down (where applicable).
Vsi - The stall speed in the clean configuration – i.e. power off, flaps retracted, wheels retracted (where applicable).
Although a spiral dive isn't an aerodynamic hazard, it fits into this topic better than anywhere else.
In a spiral dive, the wings aren't stalled, the nose is low, and the airspeed, G forces, and ROD are high.
If the aeroplane is allowed to roll without correction, the sideslip from the inclined lift vector causes the aeroplane to yaw in the direction of the low wing, which then causes the nose to drop.
This yaw increases the relative airspeed of the outside wing and the additional lift that's generated increases the bank angle, which in turn increases the sideslip and pushes the nose even lower. This train of events continues in an ever increasing fashion until the aeroplane is in a spiral dive with rapidly increases forces on the airframe structure.
A prolonged, uncontrolled, spiral dive usually results in structural failure, either before or during impact with the ground, and any attempt to recover by applying rear elevator control will only tighten the turn and increase the load factor on the structure.
The best course of action is to not let it happen in the first place, but if it does, the recover action is simple, just level the wings.
C of G
The elevators provide a force in the pitching plane to balance changes in the C of G, and to control the aeroplane in this plane. The effectiveness of the elevators depends on the magnitude of the force they provide for a given speed (how much the elevators are displaced) and their distance from the C of G (their pivot point).
The CG position, therefore, has a significant effect on stability and stall/spin recovery.
Forward C of G
If the C of G is too far forward, the aeroplane is very stable because of this increase in the moment arm of the elevators, but the nose down couple formed between the C of P and the forward C of G is also increased. This means the download force applied by the stabiliser has to be increased (with the elevators) to balance this additional force (an increase in trim drag).
This additional downward load (just like an increase in weight) requires additional lift (higher angle of attack) to balance it, therefore the stall speed increases with a forward C of G (something to remember during take-offs and landings).
Although an aeroplane with a forward C of G is more stable, more back pressure is required on the elevator controls to raise the nose - a fact to remember on the landing flare.
Aft C of G
If the C of G is too far aft, the aeroplane is less stable in the pitching plane due to the shortened arm reducing the elevator effectiveness. This means more elevator deflection is required to achieve a change in the fore & aft attitude, but less force is required to achieve it. The light back-elevator forces can make it easier to inadvertently enter a stall, and with the reduced elevator effectiveness, the recovery from a stall (and a spin) becomes more difficult as the C of G moves aft.
Because a tail heavy aeroplane requires an upward force on the tailplane to maintain its fore & aft attitude, this additional lift from the tailplane adds to the total lift. This means the wings have to operate at a slightly lower angle of attack to maintain equilibrium, thus reducing the induced drag (a decrease in trim drag), which in turn leads to a slightly better fuel consumption and a lower stall speed.
When flying close to the ground, particularly in a low wing aeroplane, the proximity of the ground :-
- reduces the downwash velocity, which increases the effective angle of attack over the entire wing, and
- inhibits the development of wingtip vortices, thus reducing the induced drag, and increasing lift, which is partially due to the reduction in the spanwise flow that's induced by tip vortices. This increases the lift/drag ratio for that angle of attack.
Ground effect becomes beneficial just after lift-off as the aeroplane accelerates to the 'Take-off Safety Speed' (VTOSS), and generally ceases to be effective when the wings are more than one wingspan from the ground.
#Unlike flaps, spoilers don't create more lift, they spoil lift and create drag.
They extend out of the upper surface of the wing, generally toward the trailing edge and are primarily used on high speed aeroplanes instead of, or in addition to, ailerons. When used in addition to ailerons, the ailerons are used in the lower speed range and are then locked in the neutral position during high speed flight.
When a spoiler is raised it causes a loss of lift on that wing, causing the aeroplane to roll toward that wing without any adverse yaw.
When the control column is moved to the right, the spoilers on the right wing rise, and those on the left wing remain retracted.
When both spoilers are raised at the same time in flight, it allows the aeroplane to descend at a constant power setting and constant speed.
Spoilers are essential in large jet aircraft because :-
- of the considerable time lag when increasing the engine power, the spoiler provides a rapid method of adjusting the sink rate, whilst leaving the power setting constant.
- the size of the ailerons is limited, as most of the trailing edge is taken up with flaps to achieve the required lift at low speeds.
- ailerons lose effectiveness at high Mach numbers.
- when extended symmetrically they act as speed brakes, but can still be varied individually for lateral control.
- spoilers reduce the lift after landing, transferring more weight onto the wheels for more effective braking.
When both spoilers are raised at the same time during the landing roll, lift is destroyed and more of the weight is transferred onto the wheels, thus improving braking effectiveness.
Fixed spoilers are sometimes fitted to the leading edge of a wing root in order to ensure that the root of the wing stalls before the tip does.
The aeroplane designer sets the size and shape of the control surfaces, which have the main influence on control response (control power) , which only leaves airspeed, amount of deflection and slipstream effect for the pilot to consider.
We know that the control surfaces work on aerodynamic principles, and we know that lift (and drag) vary at the square of the speed. It is therefore obvious that the airspeed will play a major part in the effectiveness of the controls.
At a high airspeed, a much smaller deflection (less movement of the pilot's control) will give the same result as a greater deflection at a lower airspeed. If the airspeed is halved, the controls are only 1/4 as effective, leading to the description of sloppy controls at low speed.
You can see from the previous topic, map reading is a skill that needs to be mastered if you're to become a proficient pilot. Like all skills, your proficiency can be improved with practice.
Always orient the map with the direction of flight on the map pointing to the front of the aircraft, so that features on the ground are in the relevant position to those on the map.
An excellent way to improve your skills at map reading is to spread a map out on a table and drop a spinning pencil on the map. Assume the pencil is your track and you're positioned at the pointy end of the pencil, spend some time studying the features surrounding your imaginary position on the map, and imagine what you'd be seeing if you were in flight at that position.
Another exercise is to pick two towns on the map and estimate the distance and the track between those towns (use the distance between the lines of latitude on the chart as a guide to the distance). Then take a protractor and a ruler and check how accurate your guess was. The more accurate you can make your initial guess, the easier and more accurate in-flight diversions will be.
When map reading in flight, you should always look at your watch first and establish a DR position (as discussed earlier) and then read from your DR position on the map to the ground, locating the features on the ground that you saw on the map. Don't just use one feature, pick several major features on the map, noting their relative size and shape and their distance and direction from each other, and then check your position in relation to the features.
Everything that's on the map will be on the ground but there could be many features on the ground that aren't shown on the map, and if you try read from the ground to the map you're likely to make errors.
There could have been new roads built since the map was made, or a creek that looks prominent to you was just under the dimension required for it to be shown on the chart. However, it you see a prominent feature on the ground that couldn't be mistaken for something else, such as a major road crossing a railway line, a river with a prominent bend, a lake, or an isolated large hill, see if you can locate it on the map and check your distance and direction from the feature. This not working from ground to map, it's a logical cross-checking procedure.
Always, look for major natural features whenever possible, such as the shape of a major river or a range of hills, and where the high point is in relation to the range, rather than man made or less significant features.
If you look for major features on the map during the flight planning stage, and take the time to anticipate what you'll see during your flight, you'll find it much easier to keep track of your position during the flight.
Features such as roads, railway lines, small rivers or creeks, etc should only be used to make the final identification.
This is the easiest one to understand. If the aeroplane is in balanced, straight and level flight, and the thrust is increased, the increased thrust will overcome the existing drag, and the aircraft will accelerate. Because the thrust line is below the drag line, the increase in power will cause the nose to rise.
As the airspeed increases, both the lift and the drag increase (at the square of the speed). If a constant height is required, the nose must be lowered to decrease the angle of attack on the wings by an amount that keeps the lift equal to the weight. This will maintain the height of the aircraft, but the drag still increases as the airspeed increases.
When the drag builds up to a value that's equal to the thrust, the acceleration stops and the speed stabilises.
NOTE :- The engine alignment in the airframe is normally such that the centreline of the engine (the thrust line) is in line with the direction of flight at the normal cruise configuration. If the nose is raised or lowered from this cruise attitude, the thrust line is no longer in line with the direction of flight, and a portion of this thrust is adding to the lift, and is therefore no longer acting to overcome drag.
It's also normal for light aeroplanes to have the thrust line below the drag line. This means that engine thrust is creating a couple that raises the nose and, in the event of an engine failure, without thrust to keep the nose up, it drops automatically, an ideal safety design feature.
A starter motor fitted to light piston-engined aircraft is a series wound electric motor that has a Bendix unit similar to that fitted to the engine in a motor car. When the starter is engaged, the Bendix slides the starter gear along the starter motor shaft and engages it with the engine's ring gear. After the engine starts, the ring gear drives the starter gear and increases its RPM, when this occurs the Bendix throws the starter gear out of engagement and when the starter button is released, the spring on the Bendix unit holds the starter gear clear of the ring gear, ready for the next start.
Some installations have a warning light in the cockpit to inform the pilot that the starter hasn't disengaged. This is usually caused by a faulty starter relay.
If your finger slips off the starter button during the start process, wait for the engine to stop turning before re-engaging the starter. If you re-engage the starter while the engine's still turning, you're likely to damage the teeth on either the Bendix or the flywheel.
Excessive use of a starter motor will cause it to overheat and, when this occurs, the resistance of the electrical windings increases (the electrical resistance of most matter increases with temperature). At the same time, the battery voltage is decreasing and, with less voltage, more current must flow to turn the engine. The increase in current due to the hot starter motor windings and the low battery voltage, rapidly increases the temperature of the starter motor and, if continued, will melt the starter motor windings.
REGULATING & RELIEF VALVES
A pressure regulating valve maintains a constant pressure within the system.
A pressure relief valve limits the maximum pressure within a system in case the regulating valve fails.
The terms 'regulating valve' and 'relief valve' are often interchanged and used to describe either valve. The construction and operation of both valves is virtually the same in that they're both spring loaded valves that open at a set pressure. The difference is that a regulating valve is usually partially open at all times to maintain a constant pressure whereas a relief valve is closed at all times unless the pressure builds up to a point where the relief valve opens to prevent damage to the system.
If the relief valve is capable of by-passing the entire output of the pump, it's called a full flow relief valve (FFRV).
Simple systems using a constant displacement pump have a pressure regulating valve downstream of the pump that opens to direct fluid back to the reservoir or the inlet side of the pump when the pressure reaches a pre-set value.
All systems, regardless of the type of pump fitted, will have a pressure relief valve fitted in the pressure side of the system to limit the maximum pressure to a value that won't damage the hydraulic hoses or components in the event of a regulating valve failure.
Some larger systems using a constant displacement pump utilise a system pressure regulator (sometimes called an automatic cut-out valve [ACOV] or an unloading valve) to maintain system pressure and reduce the load on the pump when the system isn't in use by opening a by-pass back to the inlet side of the pump (or the reservoir) whenever the demand on the system is low. When a large hydraulic unit is actuated, the pressure in the system drops and a pressure operated check valve in the system opens to allow the output of the constant delivery pump to be directed into the system to actuate the unit. When the actuation is finished, the pressure builds up again and the check valve seals the system off from the pump, allowing the pressure to be maintained by the accumulator until another demand for fluid flow is required. During this period of rest, the output from the pump is by-passed back into the reservoir, thus reducing the power demand on the pump, but the system pressure is retained.
This type of pressure control system shouldn't be confused with the more common system that uses a pressure regulating valve,
The major difference between the system fitted an ACOV and one without an ACOV is as follows.
- The system without an ACOV doesn't require an accumulator for the system to work, but when one is fitted, the pump supplies the fluid for all hydraulic loads, leaving the accumulator as an emergency source of pressure if the pump fails.
- The system with an ACOV does require an accumulator to make the system work. After the pump has filled the oil side of the accumulator, the ACOV isolates the pump from the system. The accumulator then supplies the fluid for all hydraulic loads until the system pressure drops. When this occurs the ACOV reconnects the pump to the system to restore the hydraulic pressure in the system.
Some systems also have thermal relief valves (TRVs) fitted in sections of the system that are between check valves that would contain the fluid in that section after the system was shut down. They only come into operation after the system is shut down, and are designed to relieve any pressure build-up due to thermal expansion in the 'locked' sections after the system is shut down. These valves are similar to a pressure relief valve, but are set to a higher pressure.
If a high flying aircraft lands at a tropical aerodrome, after the system is shut down, the temperature of the extremely cold fluid increases and creates a dangerous rise in pressure within the system. The TRV then cracks open slightly to relieve the pressure in this region of the system when check valves etc, prevent the normal pressure relief valve from relieving the excess pressure in that region.#
A jet pump works on the principle of a venturi, by pumping a small quantity of fuel under pressure into the throat of a venturi, a larger quantity of fuel is drawn into the venturi to be transferred to another tank, or to keep a collector tank full under low fuel conditions when the aircraft's attitude changes.
GREAT CIRCLES, SMALL CIRCLES & RHUMB LINES
Climbing at the maximum rate of climb speed ensures the aircraft reaches its most economical cruising height in the minimum time, and as jet aircraft burn far more fuel per distance covered at low level than they do at altitude, a rapid climb to the cruising altitude decreases the overall fuel burn for the sector (or increases the aircraft's range).
Although the maximum rate of climb speed achieves the maximum gain in height in the minimum time, there's generally a reasonable range of airspeeds where the amount of excess power is only slightly less than at the BROC speed (Vy), and the use of a slightly faster speed may achieve almost the same ROC, but cover a greater distance in doing so.
As you gain altitude, you must increase the TAS to achieve the same IAS, and at 40,000', the IAS is approximately half the TAS.
As the altitude's increased, the power required curve moves UP and to the RIGHT, and the power available curve moves down. This reduces the difference between the power available and the power required (the excess power), which reduces the maximum rate of climb.
#Water on a runway reduces the friction between the tyres and the runway and therefore reduces braking effectiveness.
Aquaplaning (hydroplaning) occurs if the tyre's tread can't clear the surface water from underneath the wheel. If this occurs, a wedge of water is pushed in front of the tyre, which lifts the tyre off the ground and totally eliminates any braking or steering actions.
The higher the tyre pressure, the greater the likelihood of aquaplaning occurring.
To minimise the chance of aquaplaning on a wet runway, there should be positive contact followed by heavy braking.
Some runways have grooves (saw cuts) across the touch-down area of the runway to assist with draining the runway and reducing the chance of aquaplaning occurring.
Aquaplaning is likely to occur if the aircraft's ground speed is 9 times the square root of the tyre pressure in pounds per square inch (PSI).
Questions requiring the use of this formula are only found in the JAR exams, but a fixed wing pilot flying relatively fast aircraft needs to have an understanding of when aquaplaning is likely to occur.
For practical purposes, there's no need to get the calculator out, if you take the approximate Square root of the tyre pressure and multiply it by 9, you'd be aware of the possibility of aquaplaning before it occurred.
Most light aircraft run a tyre pressure of between 60 psi and 150 psi, and as the square root of 60 is around 8 (8 x 8 = 64), its estimated aquaplaning speed using the above formula is 69.7 kts, using the approximation quoted above, it would be 72 kts.
Moving to the high end of tyre pressures, the estimated aquaplaning speed using the above formula is 110.2 kts, using the approximation quoted above (12 x 12 = 144), it would be 108 kts.
All the lift is generated by the wings, but most of the aircraft's weight is in the fuselage, and this generates large bending stresses on the wings and wing roots.
The wings have to provide enough lift to overcome the gross weight, and the greater the percentage of gross weight there is in the fuselage, the higher the bending loads at the wing root.
In a non-stressed skin construction, the majority of the bending loads are taken by the spar, but the skin takes some of the loads. As the wing bends up, the upper skin is being compressed, and the lower skin is being stretched (in tension).
To remove as much weight from the fuselage in order to alleviate this bending load, most aircraft carry the fuel in the wings. Large transport aircraft often have fuselage mounted tanks as well (and in some large aeroplanes, tanks are mounted in the tail as well to assist in balancing the aircraft longitudinally).
Mounting the engines in or under the wing also relieves some of this bending load, thus allowing lighter wing structures to be used.
RADIO MAGNETIC INDICATOR
A Radio Magnetic Indicator (RMI) in its basic form, is a slaved compass with an ADF needle.
Most of the later instruments have two indicator needles, one thicker than the other to minimise confusion. The thin needle (with a single line on it) is the #1 needle and the thicker needle (with 2 lines on it) is the #2 needle.
Two selector switches are located in the lower corners of the instrument, the left switch (with a single line on it) connects the #1 needle to either the #1 VOR or the #1 ADF. The right switch (with two lines on it) connects the #2 needle to either the #2 VOR or the #2 ADF.
Unlike a normal VOR indicator, when a RMI needle is selected to a VOR station, the needle points to that station, making it easy to make a positive radio fix, using the two selected aids (VOR's and/or NDB's). If you select one VOR that's co-located with a DME station, you get a position fix with the intercepting lines crossing at 90o, and you can't get any more accurate than that, unless you use more than 2 aids.
Should one of the aircraft's receivers fail, or not be tuned to a valid frequency, the relevant needle will move to the 270o position, and remain steady, regardless of any changes in the aircraft's heading.
If the gyro fails, a DG flag will show, indicating that the RMI is only operating as a Directional Gyro, and needs to be re-set every 15 minutes.
A critical point needs to be calculated for long over-water flights, and other flights where there are no landing areas en-route. This means that, in the event of an in-flight emergency, an immediate decision can be made as to whether it's quicker to continue or turn back.
If there was no headwind or tailwind component, the critical point would be the halfway point along the track.
If there was any sort of headwind or tailwind component, the critical point will always move into the wind, more so with a low airspeed.
NOTE:- When considering a CP for a multi engine aircraft that suffers an engine failure or any aircraft that suffers a problem that reduces its TAS, the reduced TAS due to the problem must be used when calculating the return GS.
Distance 240 nm.
TAS 120 knots.
Headwind component 20 knots.
On the outbound leg, the ground speed is 100 knots and on the return leg it would be 140 knots.
The formula for working this out is: -
i.e. from the CP, at a GS of 100 knots, it would take 1 hour to travel the 100 nm on to the destination, and at a GS of 140 knots, it would take 1 hour to travel the 140 nm back to the departure point.
If you were travelling in the opposite direction (B to A), the CP would be in the same place - i.e. 240 X 100 divided by 100 + 140 = 100 nm from B, which is 140 nm from A.
NOTE :- In the above formula, G/S ON + G/S HOME is equal to 2 X TAS.
To convert the above formula into a single set of entries for the electronic calculator, enter the details as follows :-
Distance X G/S home / 2 / TAS (or ETAS).
In the above example, the G/S ON is 100 kts, and the G/S HOME is 140 kt, therefore the TAS (or ETAS) must be 120 kt.
By entering the data in the format shown, the result is also 140 nm (240 X 140 / 2 / 120 = 140).
When entering the ground speeds into an OEI CP formula, you only use the OEI ground speeds ON and HOME.
#If the engine fails, centripetal forces would move the propeller into fine pitch and the aeroplane's airspeed causes the propeller to windmill, which creates a considerable amount of drag and can cause further damage to the engine. In a multi engine aeroplane, this asymmetric drag would make the aeroplane very difficult to control. An engine failure during take-off is the most difficult to control due to the high power setting on the good engine(s) at the time when the rudder is least effective due to the low airspeed.
For this reason, many multi-engine aeroplanes have a means of setting the blade angle so that the chord line of the blade is in line with the direction of flight. This stops the propeller from rotating and ensures the propeller is producing the least amount of drag. This is referred to as feathering the propeller.
If an engine failure occurs during take-off, the amount of drag on the dead engine, combined with the high power setting on the remaining engine, makes the pilot's actions during ensuing few seconds critical to the safety of the flight, and leads many of the larger aeroplane manufacturers to fit propellers that automatically feather if the engine fails, which leaves the pilot free to fly the aeroplane without having to manually feather the propeller during this critical phase of flight.
Manual feathering is achieved by moving the pitch control lever rearward and through the gate into the feather position (or pushing the feather button on some larger aeroplanes).
In a single acting system, feathering is accomplished by removing oil pressure and allowing the counterweights and the feathering spring to move the blades into the feathered position.
In either system, as the blades reach this feathered position, a latch is automatically engaged to hold them at that angle.
A single acting system usually has an accumulator to supply the oil pressure necessary to unlock the feather latch and move the propeller back toward fine pitch, where it will windmill due to the aeroplanes airspeed and allow the engine to be re-started if necessary.
In some larger aeroplanes using a double acting system, a separate electrically driven feathering pump is supplied to provide the higher pressure needed to move these larger propellers into feather, and to supply the pressure to un-feather them when required.
The separate pump has the added advantage of avoiding the risk of accidental feathering due to a malfunction within the CSU.#
ASYMMETRIC BLADE EFFECT
If the propeller shaft is not pointing in the direction the aircraft is travelling in, the distance of the blades through the air per revolution is not the same.
It's most noticeable in tail wheel aircraft when the tail is still on the ground during the take-off roll. At this time, the blade that's moving upward is travelling the least distance, and the down-going blade is travelling the furtherest distance.
In a right hand propeller (one that rotates clockwise as viewed from the rear), the down-going blade (the one with the longer path) is on the right hand side of the propeller disc, and as the blade travelling the furthest distance per revolution has a greater effective pitch than the one travelling the shorter distance, its angle of attack increased, which causes it to produce more thrust.
In a single engine aeroplane, this causes the aeroplane to yaw to the left.
In twin engine aeroplanes with engines that rotate in the same direction, if the left engine fails on take-off, the thrust from the right hand engine, which is offset to the right of the disc when the nose is raised, will cause a greater asymmetric yaw than if the right engine failed at this time. For this reason, some twin engine aeroplanes have engines that rotate in opposite directions in order to keep the down-going blade inboard on each engine, thus minimising the asymmetric yaw that occurs during an engine failure, particularly on take-off with high power and a nose-up attitude.
This effect is also noticeable when flying slowly due to the high nose attitude required to generate the required lift at low speed.
TWO SPEED SUPERCHARGERS
This type of gear driven supercharger was developed for large radial engines fitted to high-flying aircraft in the Second World War, and have little relevance to current day light aircraft, particularly helicopters, but as the Australian CASA have started questioning this topic in both the fixed wing and helicopter CPL exams, I have included it in this course.
The only one of these engines that I know of that progressed into civil operations was the Pratt & Whitney R-4360 (4,360 cubic inches), four of which were fitted to the Super Constellation. They are a 28 cylinder engine (4 rows of 7) producing 3,500 HP and weighing over 1.5 tonne, so you are hardly likely to find one in any of your Cessna's or Piper's, let alone a helicopter.
All of the engines fitted with a 2-speed supercharger are altitude engines, meaning they cannot be operated at full throttle at low altitudes without detonation occurring.
They have a cockpit selector for LOW BLOW (low supercharger RPM) and HIGH BLOW (high supercharger RPM). LOW BLOW is used at low altitudes, and HIGH BLOW is selected at a specified altitude. The gear selection is done via a hydraulic clutch.
As the aircraft climbs above sea level, the throttle is continuously opened to maintain a constant MP up to the Low Blow critical altitude, with a constant MP, the power output increases slightly as it climbs due to the reduced back pressure in the exhaust system and the reduced crankcase pressure, which increases the pressure differential across the piston (as it does in a normally aspirated engine). The normal change-over procedure is to reduce power, select HIGH BLOW, and then open the throttle until the required MP is achieved, the change-over altitude is usually somewhere above 10,000'.
After selecting HIGH BLOW, the power output increases until reaching the HIGH BLOW critical altitude. Above this altitude, the power output commences to drop below the rated power.
Viscosity is a measure of the oils resistance to flow (its internal resistance) at a given temperature. The greater its resistance to flow, the higher the viscosity (SAE 20 is thinner than SAE 100 oil), in other words a high viscosity oil flows slowly, and a low viscosity oil flows freely. The oil's resistance to flow decreases (becomes less viscous) as the oil temperature increases, and the viscosity index is a measure of how much the viscosity changes with a change in temperature.
BAKC-3.2.1(d)(ii) - BAKC-3.3.1
The performance rating is the current terminology for what used to be called the octane rating (anti-knock rating) of AVGAS fuel. It is a measure of a fuel's resistance to detonation.
By blending certain additives with fuels it's possible to increase the octane rating of fuel and increase its resistance to detonate. Now days, the anti-knock rating of a fuel is expressed as a performance number.
AVGAS is often "dual rated" , and when two performance numbers are given e.g. 100/130, the numbers mean the fuel has an octane rating of 100 when used as a lean mixture and 130 in a rich mixture. The performance number is smaller for the lean mixture because the engine runs hotter when lean and higher temperatures mean a greater risk of detonation.
To avoid confusion and to minimise errors in handling aviation gasoline, it's common practice to designate the grade by just the lean mixture performance, i.e. Avgas 100/130 becomes Avgas 100.
Each aircraft engine has a minimum grade of fuel approved for use - i.e. "Min grade Avgas 100LL".
High compression engines require fuel with a higher octane rating to prevent detonation.
Use of automotive petrol (Mogas), or Avgas of a lower grade than recommended, is very dangerous because of the higher risk of engine damage due to detonation.
Avgas burns slower than Mogas and the ignition timing is set to ensure that peak pressure in the cylinder isn't obtained before TDC. If you use a faster burning fuel (Mogas), peak pressure may be obtained before TDC and detonation (self-ignition) may occur before the spark plug fires. Mogas also has a lower Octane Rating (resistance to detonation) than Avgas, which further increases the chance of detonation.
Detonation can burn holes in the top of pistons, bend connecting rods, and even cause cylinders to be blown off the crankcase. THIS IS NOT A NICE THOUGHT FOR A PILOT.
Using a fuel of a higher grade than recommended is less serious; however the additives used to increase the performance number can cause spark plug fouling and exhaust valve corrosion.
NOTE :- The only red aviation or automotive fuel that has ever been available has a much lower performance number (octane rating) than Avgas 100 or 100LL.
Therefore if you get a question asking the likely consequences of having ordered Avgas 100 or 100LL, and on conducting a fuel drain, finding the fuel is red, the correct answer is that the engine is prone to detonation under high power, regardless of whether the examiner meant Avgas 80/87 or Mogas.
CLIMBING & DESCENDING TURNS
In a turn, the outer wing is travelling faster than the inner wing and is therefore producing more lift.
In a level turn, the increased lift on the outer wing creates a tendency to over-bank (roll the aeroplane further into the turn).
In a climbing turn, this tendency to over-bank is accentuated by the fact that the outer wing also has a higher angle of attack. This means the pilot must use less aileron control than would be required in a level turn at the same angle of bank.
In a descending turn, although the outer wing is travelling faster and creating more lift, the inner wing has a higher angle of attack, which tends to even-out the lift distribution.
Many of the calculations that are necessary during a flight to keep track of your ETA, fuel usage, etc, can be done mentally to an acceptable level of accuracy if you practice mental arithmetic rather than picking up a calculator every time you're making a simple calculation.
For navigation, some of the simple calculations that you can do are listed below.
DISTANCE FLOWN OR DISTANCE REMAINING
Every 6 minutes you're covering 1/10 of your groundspeed, so if you're doing 135 kts and have 27 nm to run, it'll take you 12 minutes (two 6 minute groups of 13.5 nm). If the distance to run was 37 nm, after 12 mins you'd have 10 nm to run, which is near enough to 2/3 of a 6 minute block, so you should arrive at your destination after a further 16 minutes.
If you divide your route into quarters, an estimation of the time remaining can be made as you reach the first 1/4 mark on your chart, and re-assessing it at the 1/2 way and 3/4 way-points.
If it took you 28 minutes to reach the 1/4 mark, you'd have 84 minutes to run (call the first one 30 minutes, multiply it by 3 and deduct 6 minutes [3 lots of 2 minutes]). This is easier, and less prone to error, than saying to yourself, three eights are 24, remember the 4 and carry the 2, and three times two is 6, plus the 2 makes it 8, so the answer is 84.
If it took you a further 27 minutes to reach the 1/2 way point, your ETI to the destination is 54 minutes (2 x 30, minus 2 times 3).
If it then took you another 26 minutes to reach the 3/4 point, it means your groundspeed is increasing, and it should take you another 25 minutes to reach your destination.
This method will keep you within the required 2 minute accuracy of advising your ETA, and regardless of whether you're using a computer to calculate your ground speed and then using that ground speed to calculate your ETA, the above method is an excellent cross-check, and it's also an excellent means of keeping track of whether you have sufficient fuel left to complete the flight.
If you reach the 1/2 way point and have used 1/2 your fuel, you had better be looking for an alternate that's closer than your planned destination, but if you're at the 1/2 way point and a similar amount of fuel will get you to your destination with your reserves intact, unless something changes drastically, you should make it, but check it again at the 3/4 point.
POWER REQUIRED CURVE VARIATIONS
As either the gross weight or the density altitude increase, the power required curve moves up and to the right.
If the gross weight increases, the angle of attack must be increased in order to maintain the lift equal to the increased weight at a given airspeed. This increases the induced drag, which increases the power required.
If the density altitude increases, the drag remains constant for a given CAS, but the TAS must be increased to maintain the same CAS, and this requires an increase in power. You will also note that in the curve showing an increased D/A, the power available curve has moved down, reducing the margin between the power available and the power required even further.