Project-1: Powertrain for aircraft in runways
Project 1: Powertrain for Aircraft in runway
1] Search and list out the total weight of various types of aircrafts.
Ans. There are various types and categories of airplane from light to heavy and having different load capacities. Some of the airplanes are listed below,
MTOW = Maximum take-off weight, MLW = Maximum landing weight, TOR = Take-off run, LR = Landing run
These are the name of few aircraft having specification about their different weight in Ton and take-off and landing run in meters.
Different weights associated with aircraft asre as follows,
Standard Empty Weight
The empty weight of an aircraft is the weight of the aircraft without including passengers, baggage, or fuel. Standard empty weight usually includes unusable fuel, full operating fluids, and full engine oil
Basic Empty Weight
The basic empty weight of an airplane is the standard empty weight of the airplane plus optional equipment installed.
Maximum Landing Weight
The maximum landing weight is, as you might imagine, the maximum aircraft weight limit approved for an aircraft to land. Landing above this weight can cause structural damage.
Maximum Ramp Weight
The maximum weight for maneuvering on the ground is called the max ramp weight. Max ramp weight includes the weight of fuel used for start, taxi, and aircraft run-up procedures.
Maximum Take-Off Weight
The maximum takeoff weight (MTOW) or maximum gross takeoff weight (MGTOW) or maximum takeoff mass (MTOM) of an aircraft is the maximum weight at which the pilot is allowed to attempt to take off, due to structural or other limits. The analogous term for rockets is gross lift-off mass, or GLOW. MTOW is usually specified in units of kilograms or pounds.
2] Is there any difference between ground speed and air speed?
Ans: Ground speed:
Ground speed is the horizontal speed of an aircraft relative to the ground. An aircraft heading vertically would have a ground speed of zero. For a reference point picked on the ground, the aircraft moves relative to the reference point at the ground speed. Ground speed is also a vector quantity so a comparison of the ground speed to the wind speed must be done according to rules for vector comparison.
Ground speed can be determined by the vector sum of the aircraft's true airspeed and the current wind speed and direction; a headwind subtracts from the ground speed, while a tailwind adds to it. Winds at other angles to the heading will have components of either headwind or tailwind as well as a crosswind component.
Air speed:
Airspeed is the speed of an aircraft relative to the air. Among the common conventions for qualifying airspeed are indicated airspeed ("IAS"), calibrated airspeed ("CAS"), equivalent airspeed ("EAS"), true airspeed ("TAS"), and density airspeed.
The important quantity in the generation of lift is the relative velocity between the object and the air, which is called the airspeed. Airspeed cannot be directly measured from a ground position, but must be computed from the ground speed and the wind speed. Airspeed is the vector difference between the ground speed and the wind speed.
Airspeed = Ground speed - Wind speed
So the main difference between the Ground speed and Air speed is the relative speed with respect to a reference point, where the Ground speed is the relative speed with reference to a point on ground while the Air speed is the relative speed with reference to the air medium in which it flies.
3] Why is it not recommended to use aircraft engine power to move it on the ground at Airport?
Ans: As we know that the gross weight of the aircraft is quite high, in the order of hundreds of tons and to move them from one place to another will require a huge amount of power which is produced by the aircraft engines. But, to move the aircraft from hangar to the parking space where the passengers may embark on the plane, by using the aircraft engine is not economical. Just to start the aircraft engine and moving to places such as parking bay or for shorter distances on its own power will consume a lot of costly fuel which is not advisable. Also, this will cause a lot of pollution and carbon emission and ni=oise pollution. This is why, we utilize a tow tractor to move aircraft from one place to another for shorter distances.
4] How an aircraft is pushed to runway when its ready to take off?
Ans: The aircraft is pushed onto a runway with the help of a towing vehicle also known as tow tractor when it is ready for take-off. Pushback is an airport procedure during which an aircraft is pushed backwards away from an airport gate by external power. Pushbacks are carried out by special, low-profile vehicles called pushback tractor or tugs.
Although many aircraft are capable of moving themselves backwards on the ground using reverse thrust (a procedure referred to as a powerback), the resulting jet blast or prop wash might cause damage to the terminal building or equipment. Engines close to the ground may also blow sand and debris forward and then suck them into the engine, causing damage to the engine. A pushback is therefore the preferred method to move the aircraft away from the gate.
Pushbacks at busy aerodromes are usually subject to ground control clearance to facilitate ground movement on taxiways. Once clearance is obtained, the pilot will communicate with the tractor driver (or a ground handler walking alongside the aircraft in some cases) to start the pushback. To communicate, a headset may be connected near the nose gear.
Since the pilots cannot see what is behind the aircraft, steering is done by the pushback tractor driver and not by the pilots. Depending on the aircraft type and airline procedure, a bypass pin may be temporarily installed into the nose gear to disconnect it from the aircraft's normal steering mechanism.
Once the pushback is completed, the towbar is disconnected, and any bypass pin removed. The ground handler will show the bypass pin to the pilots to make it clear that it has been removed. The pushback is then complete, and the aircraft can taxi forward under its own power.
5] Learn about take off power, tyre design, rolling resistance, tyre pressure, brake forces when landing.
Ans:
A] Take Off:
Take-off is the phase of flight in which an aerospace vehicle leaves the ground and becomes airborne. For light aircraft, usually full power is used during take-off. Large transport category (airliner) aircraft may use a reduced power for take-off, where less than full power is applied in order to prolong engine life, reduce maintenance costs and reduce noise emissions. In some emergency cases, the power used can then be increased to increase the aircraft's performance. The aircraft is permitted to accelerate to rotation speed (often referred to as Vr). The term rotation is used because the aircraft pivots around the axis of its mainlanding gear while still on the ground, usually because of gentle manipulation of the flight controls to make or facilitate this change in aircraft altitude.
Fixed-wing aircraft designed for high-speed operation (such as commercial jet aircraft) have difficulty generating enough lift at the low speeds encountered during takeoff. These are therefore fitted with high lift devices, often including slats and usually flaps, which increase the camber and often area of the wing, making it more effective at low speed, thus creating more lift. These are deployed from the wing before takeoff, and retracted during the climb. They can also be deployed at other times, such as before landing.
The speeds needed for take-off are relative to the motion of the air (indicated airspeed). A headwind will reduce the ground speed needed for take-off, as there is a greater flow of air over the wings. Typical take-off air speeds for jetliners are in the range of 240–285 km/h (130–154 kn; 149–177 mph). For a given aircraft, the take-off speed is usually dependent on the aircraft weight; the heavier the weight, the greater the speed needed.
Take off performance can be predicted using a simple measure of the acceleration of the aircraft along the runway based on force equilibrium. The forces involved will be,
T – Thrust of propulsion system pushing aircraft along runway.
D – Aerodynamic Drag of vehicle resisting the aircraft motion.
F – Rolling resistance friction due to the contact of wheels or skids on the ground.
During take-off run the imbalance in these forces will produce an acceleration along the runway.
`(dV) / dt = (T - D - F) / m`
where dV/dt is the acceleration along the runway and m is the mass of the vehicle.
B] Tyre Design:
Tyres are one of the most important components of aircraft. They help to absorb the shock of landing and provide cushioning. It also provides the necessary traction for braking and stopping of an aircraft.
An aircraft tyre is designed to withstand extremely heavy loads while landing, take off, taxing and parking. The number of wheels required for aircraft increases with the weight of the aircraft, as the weight of the aeroplane needs to be distributed more evenly.
A Boeing 737NG & 737MAX uses 6 wheels, Boeing 787 uses 10 wheels, Boeing 777 uses 14 wheels and Airbus A380 uses 22 wheels. Aircraft tyres work under extreme conditions, carrying up to 340 tons and accelerating at over 250 km/hour at takeoff, in addition to enduring varied environmental stress when in flight and taxiing.
Design and Construction of aircraft tyre
Carcass plies are used to form the tire. They are sometimes called casing plies. An aircraft tyre is constructed for the purpose it serves.
- They support the weight of an aircraft while it is on the ground.
- It also provides the necessary traction for braking and stopping of an aeroplane.
- Tyre also helps to absorb the shock of landing and provide cushioning the roughness of takeoff, roll-out, and taxi operations.
Let’s go through some technical aspects of the tyre.
Retreading – Retreading is methods of restoring a worn tyre by renewing the tread area or by renewing the tread area plus one or both sidewalls. Repairs are included in the tyre retreading process.
Load Rating – Load rating is the maximum permissible load at a specified inflation pressure.
Ply Rating – Ply Rating is used to identify the maximum recommended load rating and inflation pressure for a specified tyre. It is an index of tyre strength.
Speed Rating – The speed rating is the maximum takeoff speed to which the tyre has been tested.
Skid Depth – Skid depth is the distance between the tread surface and the deepest groove as measured in the mould.
Aircraft tyres must have an approved speed and load rating and have sufficient clearance when retracted through landing gear to allow for tyre growth. Tyre growth is the increase in the size of the tyre due to centrifugal forces at high speed.
Aircraft tyres may be Tube-type or Tubeless
Tubeless tyres are more advantageous over tube-type. There is no longer the use of tube-type tyre in recent aviation. Nowadays all airliners are using tubeless tyres. Tubeless that are meant to be used without a tube has the word TUBELESS on the sidewall of the tyre.
Aircraft tyres may be Radial or Bias
Almost all airliners are using Radial tyre. Bias is an older design, and it’s mainly used for road vehicles. Radial tyres have the word RADIAL on the sidewall. Radial tires are more expensive than bias-ply tyres. Radial tires are in demand because of their lower life cycle cost and long term value.
The basic difference between a radial tyre and bias (cross-ply) tyre is the lay of the plies within the tyre carcass. The remainder of the tyre is very similar in construction in both.
Tyres are critical components of aircraft. The choice of tyre totally depends on the aircraft manufacturer. Radial tyres having a feature of rigid belt and flexible carcass which providing more number of landings and less rolling resistance.
Another means of classifying an aircraft tyre is by the direction of the plies used in the construction of the tyre, either bias or radial. Traditional aircraft tyres are bias-ply tyres.
Construction of Aircraft Radial & Bias Tyre
A bias-ply tyre has the fabric bias oriented with and across the direction of rotation and the sidewall. Since fabric can stretch on the bias, the tyre is flexible and can absorb loads. Tyre strength is obtained by adding plies.
1. Apex strip: A triangular piece of stiff rubber which is fitted onto the bead core to taper the rigid bead into the flexible body plies in the sidewall.
2. Bead heel: Outer edge of the bead.
3. Bead toe: Inner edge of the bead.
4. Breaker: Breakers are fabric plies placed directly on the upper-most ply beneath the tread rubber. They disperse stress throughout the tire. This breaker is utilized in a limited number of Bridgestone aircraft tires.
5. Chafer strip: Reinforces the beads and protects the cord body-plies from damage from rim friction or from damage when replacing the rim.
6. Carcass plies: Each nylon ply is held by wire beads across the tire to keep the tensile strength at the proper level, and is folded to the side of the sidewall. Each ply is provided with cushion rubber to reduce shearing force produced between the cords of the tire when it is deformed. Multiple coated-nylon cords are crossed alternately on the bias to make the carcass.
7. Decoration line: A projecting, embossed line molded between the shoulder and sidewall of a tire. Its purpose is for decoration only.
8. Flipper strip: A strip of material which is wrapped around the bead bundle and filler to add support and help taper the assembly into the sidewall.
9. Innerliner: A sheet of special rubber laminated to the inside of the first casing ply of the tubeless tire so as to ensure retention of pressure when the tire is inflated.
10. Ply turn-ups: The turned-up ends of plies around the bead. Other plies lock the ends into place.
11. Tread reinforcing fabric: Nylon cord plies located in the tread between the tread groove and the outer ply of the carcass which reduce tread squirm and increase stability for high speed operations.
12. Rim line: This line is provided near the beads to allow inspection of how the tire fits with the rim.
13. Shoulder: The area on the side of the tire between the sidewall and the tread.
14. Sidewall: The side area of the tire located between the buttress and the beads. Composed of rubber, it protects the carcass from damage.
15. Tread: A layer of rubber on the outer circumference of a tire, its major functions are to protect the casing from cuts, withstand high speed during take-off and landing, provide traction, resist wear, provide durability under a wide range of temperatures and to ensure stability (in particular, the effective discharge of water on wet runways). The tread compound is designed to suit a wide range of operations.
16. Undertread: A rubber layer provided for use in retreading the tire and which provides adhesion of tread to the cord body. Bridgestone makes this rubber layer thick enough to ensure a high number of retreads.
17. Wire beads: Constructed of steel wire encased in rubber, they help to maintain the shape of the tire and keep it securely fixed to the rim. The wire beads are wrapped with fabric.
18. Venting hole: The hole through which gas is vented from inside the carcass plies.
A radial tyre has the fibre strands of the ply fabric oriented with and at 90° to the direction of rotation and the tyre sidewall. This restricts flexibility bidirectionally and the flexibility of the sidewall while it strengthens the tyre to carry heavy loads.
1. Apex strip: A triangular piece of stiff rubber which is fitted onto the bead core to taper the rigid bead into the flexible body plies in the sidewall.
2. Aramid cord protector: The tire is constructed with an aramid ply insert between the tread and the top belt to prevent damage to the tire belts.
3. Bead heel: Outer edge of the bead.
4. Bead toe: Inner edge of the bead.
5. Belt plies: Angled belt plies which serve to add strength to the tire especially in the tread area.
6. Chafer strip: Protective layer of rubber and fabric which prevents chafing between cord body plies and the wheel.
7. Carcass plies: Layers of rubber-coated fabric plies which have the cords running radially from bead to bead. These provide the tire with strength in the sidewall area.
8. Innerliner: A sheet of low permeable rubber laminated to the inside of the first casing ply of a tubeless tire to insure retention of air when tire is inflated.
9. Ply turn-ups: The turned-up ends of plies around the bead. Other plies lock the ends into place.
10. Rim line: A line provided near the beads to allow inspection of how the tire fits with the rim.
11. Sidewall: The side area of the tire located between the shoulder and the beads. Composed of rubber, it protects the carcass from damage.
12. Tread: The functions of the radial tread are the same as those defined for the bias tire.
13. Undertread: A rubber layer provided for use in retreading the tire and which provides adhesion of tread to the tire carcass. Bridgestone makes this rubber layer thick enough to ensure a high number of retreads.
14. Wire beads: Two wire bundles (one for each bead) which help to maintain the shape of the tire and anchor it securely to the rim.
15. Venting hole: The hole through which gas is vented from inside the carcass plies.
Punture and Cut Resistance of Tyre
Most radial tyres are resistant to penetrations and cuts, especially in the tread area due to their unique construction process. In fact, only a small portion of the radial tyre will be affected in case of any damage.
Bias-ply tires are cut resistant in the sidewall area. However, if there is any cut in the tread area, it can affect only a small portion of tyre.
Traction Properties for Aircraft Tyre
Due to stiffer tread in radial tyres, they distribute weight evenly. These results provide consistent contact patch and greater traction, which means the tyres wear less and longer tread life.
While bias-ply tyres do not offer as much smoothness in terms of riding quality, their strong and stable sidewalls more than make up for it, especially this use in heavy-duty carriage trucks and buses.
What is the purpose of Chines in aviation tyre
Chines are also called deflectors. Chine tyre used on the nose wheel of aircraft, specially fuselage-mounted jet engines. It diverts runway water away from the engine inlets.
Chines are circumferential protrusions that are moulded into the sidewall of nose tyres that deflect water sideways to help reduce excess water ingestion into the engines. Tyres may have chines on one or both sides, depending on the number of nose tyres on the aircraft.
C] Rolling Resistance:
The force that resists the motion of a body rolling on a surface is called the rolling resistance or the rolling friction. The friction between aircraft and runway will be proportional to the normal force exerted by the aircraft on the runway.
F=μ(W−L)
The normal force will be the difference between Weight of aircraft and Lift, the friction coefficient will be typically of a magnitude of 0.02 for a standard tarmac runway.
D] Tyre Pressure:
An aircraft tyre is designed to withstand extremely heavy loads for short durations. The number of tires required for aircraft increases with the weight of the aircraft, as the weight of the airplane needs to be distributed more evenly. Aircraft tire tread patterns are designed to facilitate stability in high crosswind conditions, to channel water away to prevent hydroplaning, and for braking effect.
Aircraft tires generally operate at high pressures, up to 200 psi (14 bar; 1,400 kPa) for airliners, and even higher for business jets. Tests of airliner aircraft tires have shown that they are able to sustain pressures of maximum 800 psi (55 bar; 5,500 kPa) before bursting. During the tests the tires have to be filled with water, to prevent the test room being blown apart by the energy that would be released by a gas when the tire bursts.
Aircraft tires are usually inflated with nitrogen to minimize expansion and contraction from extreme changes in ambient temperature and pressure experienced during flight. Dry nitrogen expands at the same rate as other dry atmospheric gases (normal air is about 80% nitrogen), but common compressed air sources may contain moisture, which increases the expansion rate with temperature.
The requirement that an inert gas, such as nitrogen, be used instead of air for inflation of tires on certain transport category airplanes was prompted by at least three cases in which the oxygen in air-filled tires combined with volatile gases given off by a severely overheated tire and exploded upon reaching autoignition temperature. The use of an inert gas for tire inflation will eliminate the possibility of a tire explosion.
E] Brake forces when landing
Bringing an aircraft down to the runway from 37,000 feet is only part of the challenge. It’s not over until the aircraft is safely parked at its gate.
A large amount of brake force is required to stop the aircraft after landing on the runway since the speed of the aircraft is quite high in the range of 250-300 kmph. In order to calculate the brake force required to stop the aircraft, we have to first estimate the kinetic energy carried by it and this can be determined by,
`K.E = 1/2*m*v^2`
Once the energy has been calculated, and if we assume the minimum distance in which the vehicle has to be stopped we can calculate the brake force required and the same has to be applied on the wheels of the aircraft.
Stopping a 200-tonne aircraft landing at 180 mph (290 kmph) requires a lot of braking force. The 787 has one brake unit on each of the eight wheels on the main gear assembly. On other aircraft types, the brake units are powered by the hydraulics system. An electrical signal is sent from the flight deck to hydraulic actuators near the main landing gear. Here, hydraulic fluid at 3,000 pound per square inch (20.68 KN/m2) is used to force the brake unit against the wheel, thus slowing it down. This system works fine, but the pipes and actuators that form this part of the hydraulic system come at a considerable weight cost. Extra weight means more fuel burn, which in turn increases costs and carbon emissions.
Electrical brakes
The brake system on the 787 Dreamliner is controlled by the pilots pressing the tops of the rudder pedals under their feet, demanding the rate of braking which they require. This sends an electronic signal to the Left and Right Brake System Control Units (BSCUs).
These then send signals to the four Electronic Brake Actuator Controllers (EBAC) which control the rate of braking on the wheels. Each wheel has four Electric Brake Actuators (EBA), a kind of piston which presses against the carbon brake discs.
The brake disks themselves are made up of two parts. Firstly, there are the rotors. These are connected to the wheel by drive tabs (in the video below they are the black rectangles on the rotor disks). As these drive tabs are in contact with the inside of the wheel, they spin at the same speed. Depending on the brake manufacturer, there are either four or five of these rotors on each brake assembly.
The second part of the disks are the stators. These sit around each rotor and are fixed in place and thus don’t move. As the wheel turns, the rotors spin round inside the stators.
When the brakes are applied, the four EBAs apply pressure to the first stator. This in turn squeezes the stationary stators up against the spinning rotors and it’s this friction which slows the wheel down.
Anti-Skid Protection
When landing on slippery runways, there’s a chance that the wheels may start to skid as the brakes are applied. To stop this from happening and to maintain maximum effective braking, each wheel has anti-skid protection.
Using a variety of sources to determine the aircraft speed, the brake units know how fast the wheels should be spinning. If that speed drops significantly, it’s because the current brake pressure on that wheel is too great and the wheel is just skidding over the surface.
In this situation, the anti-skid system automatically reduces the braking on that wheel to a point where the skid stops before reapplying the pressure. All this is done in a fraction of a second.
Autobrake
Pressing on the brakes is pretty straight forward when taxiing in a straight line at low speed. However, when we’re landing in strong winds, it can be a little tricky. We need to use our feet on the rudder pedals to line the nose of the aircraft up with the runway centerline at the last moment. Then, whilst holding that position, slide our feet up to press the toe brakes. Really not very easy when moving at 160 mph.
To help us get the braking underway as soon as we touch down, we have the autobrake system.This provides automatic braking at a preselected rate as soon as the aircraft senses that it is on the ground. It also provides full braking pressure in the case of a rejected takeoff if the speed is above 85 knots (98 mph).
Brake Temperature indications
With friction comes heat. As a result, each brake unit displays its temperature on the wheel synoptic page in the flight deck. Here, numerical values relating to brake temperature are shown next to each wheel. A value of 0-4.9 is in the normal range. When a temperature becomes 5.0 or above, an advisory message is displayed to the pilots.
Should the brakes become too hot, there’s a chance that the heat transferred to the wheels could cause the tires to explode. To stop this from happening, when a certain temperature is reached, fuse plugs in the tires melt. This allows the air to be released safely and slowly deflate the tires.
Parking Brake
Every so often we need to engage the brakes and keep them on. This is particularly useful on long taxis to the runway and obviously when parked at the gate. The park brake is set by fully pressing down both toe brakes and pulling the parking brake lever up. With this set, we can then release the pressure from the pedals. To release, we just press the brake pedals again.
Spoilers
Have you noticed the large panels on the top of the wings raise up on touch down? These are the called the spoilers as they literally spoil the airflow over the wing. This dumps any remaining lift the wings are generating, allowing the wheels to take all the weight and achieve maximum efficiency from the brakes.
Reverse Thrust
The final part of the braking process comes from reverse thrust. Just after we have touched down, we pull two levers on top of the thrust levers to engage the reverse thrust, a sort of “reverse gear” for jet engines.
This causes blockers inside the engine to deploy and a door in the side of the engine to slide backward. The air which normally leaves the engine out the back is deflected forward by the blockers and out through the door.
6] With necessary assumptions, calculate the force and power required to push / pull an aircraft by a towing vehicle.
Ans. In order to calculate the tractive force and power required to push/pull aircraft by the towing vehicle, we need to understand the forces that acts on the towing vehicle which can be listed as follows,
a) Force acting due to rolling resistance,
b) Force acting due to drag.
A] Rolling resistance Force (`F_(rr)`)
The force due to rolling resistance experienced by towing vehicle carrying aircraft can be be given by the formula as shown,
`F_(rr) = mu_(rr) * m * g`
where,
m = total mass of the towing vehicle + gross weight of the airplane (kg)
g = gravitational acceleration (m/s^2)
`mu_(rr)` = coefficient of rolling resistance
The rolling coefficients for air filled tires on dry roads can be estimated by,
`mu_(rr)` = 0.005 + (1 / p) (0.01 + 0.0095 (v / 100)2)
where
`mu_(rr)` = rolling coefficient
p = tire pressure (bar)
v = velocity (km/h)
Coefficient of rolling resistance is affected by various factors such as tire pressure, tire diameter, tire width, tire construction, tire tread, surface roughness etc
Let us assume the aircraft that is being towed in this case be Airbus A330,
Mass of A330, m1 = 240000 kg
Mass of towing vehicle, m2 = 15000 kg
Total mass, m = m1 + m2
m = 240000 + 15000 = 255000 kg
g = 9.81 m/s2
Assuming a tyre pressure (p) of 15 bar and vehicle velocity (v) as 10 kmph
`mu_(rr) = = 0.005 + (1 / 15)* (0.01 + 0.0095* (10 / 100)^2)`
`mu_(rr) = 0.0056`
Therefore, `F_(rr) = 255000 xx 9.81 xx0.0056`
`F_(rr) = 14010 N or 14.01 KN`
B] Force due to Air drag
The force due to air drag on towing vehicle carrying aircraft can be be given by the formula as shown,
`F_(ad) = 1/2 * rho * A * C_d * v^2`
Where,
ρ = density of air
A = frontal area
Cd = coefficient of air drag
v = velocity of towing vehicle with aircraft
Lets assume some values for the force calculation,
ρ = 1.225 kg/m3
A = 20 m2
Cd = 0.0265
v = 10 kmph or 2.78 m/s
Therefore, `F_(ad) = 1/2 xx 1.225 xx 20 xx 0.0265 xx (2.78)^2`
`F_(ad) = 2.5 N or 0.0025 KN`
Total tractive force required to tow the aircraft on the runway at a speed of 10 kmph will be,
`F_(te) = F_(rr) + F_(ad)`
`F_(te) = 14.01 + 0.0025`
`F_(te) = 14.012 KN`
The power require to pull/push the aircrft using towing vehicle will be,
`P = F_(te) * v`
`P = 14.012 xx 2.78`
P = 39 KW
7] Design an electric powertrain with type of motor, it’s power rating, and energy requirement to fulfill aircraft towing application. Estimate the duty cycle range to control the aircraft speed from zero to highest. Make all required assumptions. Prepare a table of assumed parameters. Draw a block diagram of powertrain.
Ans: In order to design an electric power-train for a towing vehicle for purpose of push/pull of an aircraft from one place to another, we will have to take into consideration various parameters that electric motor may require to overcome or satisfy so as to perform the desired operation.
The aircraft taken into consideration for this example is Airbus A-330-300 having maximum take-off weight 240000 kg. Following are the assumptions that are made for designing the powertrain for towing vehicle,
a) Maximum take-off weight of the the aeroplane, m1 = 240000 kg
b) Weight if the towing vehicle, m2 = 54000 kg
c) Air density, ρ = 1.225 kg/m3
d) Speed of vehicle, v = 10 kmph or 2.78 m/s
e) Coefficient of rolling resistance, μrr = 0.0056
f) Coefficient of drag, Cd = 0.02
g) Tyre diameter, d = 0.853 m and radius, r = 0.426 m
h) Frontal area, A = 20 m2
i) Gear ratio = 11
j) Vehicle operation time for single to and fro run = 30 min or 0.5 hrs
k) Battery Voltage = 280 V
l) Battery Current = 200 A
m) Power converter = Boost DC-DC converter
From the above calculations, we have seen that the power require to move the aircarft at a speed of 10 kmph is 39 KWh. The motor torque required to pull the aircraft if a gear ratio of 11 is used can be calculted as,
`T = (F_(te) * r) / G`
`T = (14.012 xx 0.426) / 11`
`T = 540 N.m`
Now, consider the movement of aircraft from hangar and its return trip back to hangar takes place in 30 min, then the energy required to power the trip can be calculated as shown,
Energy (E1) = 39 x 0.5 KWh
E1 = 19.5 KWh
If we consider the vehicle to complete at least 3 trips on a complete charge then the Battery pack energy should be,
E = E1 x 3
E = 19.5 x 3
E = 58.5 KWh
The battery pack should be able to store 58.5 KWh of energy so as to complete 3 return trips of the towing vehicle from hangar to runway and back to hangar and provide a maximum torque of 540 Nm when running at 10 kmph speed.
Boost DC DC converter
Boost converter is employed to control the speed of BLDC motor. The AC supply is applied to the bridge rectifier, the bridge rectifier converts ac supply into dc supply. That dc supply is applied to boost converter to boost the input voltage (i.e.) if input 15v dc means boost converter output voltage is greater than 15v dc voltage. That dc voltage is given to the three phase inverter; it converts the dc voltage into three phase ac voltage. Three phase ac voltage is connected to the BLDC motor. The bldc motor have hall sensor. The hall sensor output is feedback to the controller. The three phase inverter Pulse depends on the hall sensor of bldc motor.
Speed control of motor
The speed of the motor is needed to be controlled at every instance of time depending on the torque requirement. As we know that, when we connect the battery supply directly to the motor without any control, then the motor will deliver the full rated speed and torque characteristics which is not the desired result at start of operation. We need to gradually increase the speed and torque values, and for that we need to control the power that is being supplied to the motor by means of an motor controller. The motor controller will vary the applied battery voltage to motor depending on the torque and speed requirement. Also, we do not wish to deliver the maximum motor power at total battery voltage but at a value lower than that and to calculate the duty cycle that must be applied we must calculate the power input,
P(in) = Battery voltage * Battery current
P(in) = 280 * 200 = 56 KW
Duty cycle, D = P / P(in)
D = 39 / 56 = 0.7
Therefore, a duty cycle of 70% can be applied to deliver the power required to propel or push the vehicle carrying aircraft at a speed of 10 kmph. We can vary the duty cycle from 0 to 70% depending on the vehicle speed and torque requirement.
The block diagram for the electric powertrain is as follows,