Project-1: Powertrain for aircraft in runways
PART A
Qn.1) Search and list out the total weight of various types of aircrafts.
Definitions:
Maximum Take off Weight (MTOW)
The maximum takeoff weight (also known as the maximum brake-release weight) is the maximum weight authorised at brake release for takeoff, or at the start of the takeoff roll. The maximum takeoff weight is always less than the maximum taxi/ramp weight to allow for fuel burned during taxi by the engines and the APU.
Maximum Landing Weight (MLW)
The maximum weight authorised for normal landing of an aircraft. The MLW must not exceed the MTOW.
Let us list out the MTOW and MLW of few notable aircrafts.
| Aircraft type | MTOW (kg) | MLW (tonnes) | |
| Airbus A220-300 | 59,000 | 50.80 | |
| Airbus A300-600 | 163,000 | 138 | |
| Airbus A320 | 68,000 | 66 | |
| Airbus A330-300 | 242,000 | 185 | |
| Airbus A340-600 | 367,400 | 256 | |
| Antonov An-225 | 640,000 | 591.7 | |
| ATR 72-600 | 22,800 | 22.35 | |
| Boeing 727-200 | 78,000 | 68.1 | |
| Boeing 737-700 | 70,000 | 58.06 | |
| Boeing 747-400 | 396,900 | 295.742 | |
| Boeing 777-300 | 299,370 |
|
|
| Concorde | 185,000 | 111.1 |
More such weights can be found from the below link:
Qn 2.) Is there any difference between ground speed and air speed?
Ground Speed:
Ground speed is how fast an airplane's shadow would move across the land. If there's a strong wind pushing an aircraft, that's reflected in the ground speed.
Airspeed:
Airspeed, in contrast, is how fast an airplane is really flying strictly under its own power, which is calculated by subtracting the wind speed from the ground speed
Note: On a perfectly still day, the airspeed is equal to the ground speed. But if the wind is blowing in the same direction that the aircraft is moving, the airspeed will be less than the ground speed.
Qn.3) Why is it not recommended to use aircraft engine power to move it on the ground at Airport?
It is not recommended to use FULL engine power to move the aircraft in the airport because of safety reasons and to prevent hazards. Aircraft operated at full thrust could create high volumes of air spreading debris or damaging other aircraft or ground vehicles.
Some of the disadvantages of using aircraft power include:
- Risk of injury to the ground staff personnel who may be in close proximity to the aircraft
- The pilot who is in the cockpit may not have the better view to taxi the aircraft when compared to the view of the pushback truck driver.
- Using aircraft engine power could also consume a considerable amount of the fuel
- Aircrafts when using engine power are prone to much noise.
However, when we are assuming passenger commuter aircraft and large airliners, Aircraft move using their own engine power (Limited Power) all the time on the airport apron and taxiways under their own power to go towards runways and after landing taxi to their assigned gate.
The only time they use external assistance is to pushback from the gate. Large turbofan aircraft need help to reverse via a pushback tractor. The engines are turned on after the pushback is complete and the aircraft can move forward under It’s own power.
To be accurate, some aircraft can reverse using a variable pitch prop or thrust reversing on turbofans but generally reversing is done via pushback tractors. Blasting jet reversers blows debris In the crowded gate area creating foreign object damage hazard for adjacent aircraft, gate equipment or terminal waiting area glass windows.
The above picture shows the pushback tractor which could be used for taxiying.
Qn.4) How an aircraft is pushed to runway when its ready to take off?
Airplanes have engines, and they can use them to move forward along the ground. So they push themselves to the runway. It true that most of the time, a small vehicle is needed to push the airplane backwards away from the terminal building, because even if a plane can reverse it’s thrust, it’s not always safe to hit the buildings with a jet blast. So the little trucks push the jets back away from the buildings, but after that, the plane can apply a small amount of engine power to roll forward.
The below link shows a video of how a aircraft undergoes Pushback, Taxi and Take off.
https://www.youtube.com/watch?v=F9OQrY-BBog
Important Note: The modern approach to this pushback is electric. It involves employing motor on the front wheels of the aircraft to move the aircraft either forward, reverse or a 180 degree tilt. This has numerous advantages over pushback trucks such as lesser taxing times, less labor, less maintanence etc. The below link gives more insights about the electric pushback.
The FUTURE of PUSHBACK is ELECTRIC! Without a pushback truck! Explained by CAPTAIN JOE
Qn.5)Learn about take off power, tyre design, rolling resistance, tyre pressure, brake forces when landing.
Take Off Power:
Reduced power or derated takeoffs are normal and frequently used. The performance is carefully calculated prior to each takeoff, ensuring that the proper speeds, power settings and flaps are used for the weight of the takeoff on the runway intended and at the proper temperature. When using this method during takeoff, it is always possible to increase to full power if the situation requires. The captain always makes the final decision whether to use full thrust or derate.
Derates improve engine life and reliability. In addition to lowering operating costs, they decrease the likelihood of an engine failure. All jets use some form of derated or reduced-thrust takeoffs.
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.
It should be taken care that the acceleration overcomes the aerodynamic drag and the rolling resistance friction.
Therefore,
Acceleration, `(dV)/dt= (T-D-F)/M`
where M is the Mass of the aircraft.
The below graph shows the airspeed vs power required generally.
Tyre Design:
Designing aircraft tires is a very intense undertaking. Once an aircraft manufacturer goes to the tire manufacturer with the size and weight requirements, the tire manufacturer begins the process of designing a tire that meets the needs of the aircraft manufacturer as well as all regulatory requirements — including foreign regulations in many cases.
Engineers are challenged with designing tires with cool running, heat-resistant materials while simultaneously exceeding tire service requirements. Tire prototypes are put through rigorous tests that simulate cycles of landings, take-offs, and taxi operations. It is only after the tires pass all mandatory tests and meet the requirements of the aircraft manufacturer and airworthiness authorities that the tires are put into production.
Two types of tires are used in aircraft applications — bias-ply tires and radial tires. Although both types of tires share some similarities in design, there are differences that need to be noted.
Bias-ply tire construction:
A cutaway of a typical bias-ply tire is shown in Figure. Bias-ply tires are popular choices for aircraft tires because of their durability and retreadability. The tire construction consists of the following components:
Tread: The tread is made of rubber mixed with other additives to obtain the desired level of toughness, durability, and resistance to wear. The tread pattern is designed to aircraft operational requirements, with the ribbed tread design used widely due to its good traction under varying runway conditions.
Sidewall: The sidewall is a protective layer of rubber that covers the outer casing ply. It extends from the tread edge to the bead area.
Tread Reinforcing Ply: One or more layers of fabric that strengthens and stabilizes the tread area for high-speed operation. It also serves as a reference for the buffing process when tires are retreaded.
Buff Line Cushion: The buff line cushion is made of rubber compound to enhance the adhesion between the tread reinforcing ply and the breakers or casing plies. It is of sufficient thickness to allow for the removal of the old tread when the tire is retreaded.
Breakers: Breakers are reinforcing plies of rubber-coated fabric placed under the buff line cushion to protect casing plies and strengthen and stabilize the tread area. They are considered an integral part of the casing construction.
Casing Plies: Alternate layers of rubber-coated fabric (running at opposite angles to one another) provide the strength of the tire.
Wire Beads: Hoops of high tensile strength steel wire that anchor the casing plies and provide a firm mounting surface on the wheel. The outer edge of the bead that fits against the wheel flange is called the bead heel. The inner bead edge is called the bead toe.
Apex Strip: A wedge of rubber affixed to the top of the bead bundle.
Flippers: Layers of rubberized fabric that help anchor the bead wires to the casing and improve the durability of the tire.
Ply Turnups: The casing plies are anchored by wrapping them around the wire beads, thus forming the ply turnups.
Chafer: A protective layer of rubber and/or fabric located between the casing plies and wheel to minimize chafing.
Liner: In tubeless tires, the liner is a layer of low permeability rubber that acts as a built-in tube and restricts gas from diffusing into the casing plies. In tube-type tires, a thinner liner is used to prevent tube chafing against the inside ply.
Radial-ply tire construction
A typical radial tire is illustrated in the above Figure. With their rigid belt, they provide increased landings and reduced rolling resistance. They have fewer components in their construction and are lighter than similarly sized bias ply tires. Components that differ from bias-ply construction as follows:
Overlay: A layer of reinforcing rubber coated fabric placed on top of the belts to aid in high speed operation.
Belt Plies: A composite structure that stiffens the tread area for increased landings. The belt plies increase the tire strength in the tread area.
Casing Plies: As in bias-ply tires, the casing plies are layers of rubber-coated fabric. However, unlike those in bias plies that run at opposite angles to one another, radial plies run radially from bead to bead.
Chippers: The chippers are layers of rubber-coated fabric applied at diagonal angles that improve the durability of the tire in the bead area.
Rolling Resistance
Rolling resistance is the force resisting the motion when a body(such as ball, tyre or wheel) rolls on a surface. The physical effect of rolling resistance can be seen in the aircraft when aircraft takes off from the runway and while landing at the runway with the help of their wheels.
Rolling resistance can be expressed as
`F_(rr)=mu_(rr)*m*g`
where `F_(rr)`is the rolling resistance force,
`mu_(rr)`is the coefficient of rolling resistance
`m`is the mass (in kg)
and g is the acceleration due to gravity
Tyre Pressure:
Aircraft tires generally operate at high pressures, up to 200 psi (14 bar; 1,400 kPa) for airliners, and even higher for business jets. The main landing gear on the Concorde was typically inflated to 232 psi (16.0 bar), whilst its tail bumper gear tires were as high as 294 psi (20.3 bar). The high pressure and weight load on the Concorde tyres were a significant factor in the loss of Air France Flight 4590.
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 had 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 eliminates the possibility of tire explosion.
The below graph shows the relation between tyre pressure and the rolling resistance:
Brake Forces when Landing:
An aircraft lands at a speed close to 250 Kmph. Hence it becomes necessary to resist its speed within a minimum stipulation of time to bring the aircraft to the rest position. The braking devices which are used to deaccelerate the aircraft are:
1.) Wheel brakes (Including Anti Skid systems and Anti brake systems)
2.) Spoilers
3.) Reverse Thrust
Wheel Brakes: Aircraft brakes stop a moving aircraft by converting its kinetic energy to heat energy by means of friction between rotating and stationary discs located in brake assemblies in the wheels. Brakes provide this critical stopping function during landings to enable airplanes to stop within the length of the runway. They also stop aircraft during rejected takeoff events, in which an attempted takeoff is canceled as the airplane is rolling down the runway before it lifts off from the ground due to engine failure, tire blowout, other system failure or direction from air traffic control.
The most common type of brake used on aircraft is the disc brake. Disc brakes function by exploiting friction between rotating and stationary discs inside the brake. Upon receiving a command signal to brake — from the pilot depressing a foot pedal or from the autobrake system — actuators in the brake move a piston to squeeze the discs together, generating a frictional force that slows the rotation of the wheel. The friction between the discs generates heat as the aircraft’s kinetic energy is converted to heat energy. In this function, the brakes act as a heat sink, absorbing tremendous amounts of heat as the aircraft sheds kinetic energy. During RTO stops, the temperature of carbon disc brakes can exceed 1,800° C.
The above figure shows the Gulfstream G450 main landing gear brake assembly.
Spoilers
Spoilers serve several very important functions. In flight, they effectively increase the aircraft’s rate-of-descent without causing an accompanying increase in airspeed. Once the main landing gear is firmly on the runway, the pilot deploys the spoilers to their fully extended position, creating the maximum amount of drag to slow the aircraft from its touchdown speed of approximately 120-to-140 mph to a more proper braking speed of 60-80 mph. Once the aircraft has decelerated to taxi speed, spoilers are stowed.
Above figure shows the spoiler being deployed.
Reverse Thrust
Reverse thrust is a control system found in many airplanes with jet engines that changes the direction in which air is exhausted from the engine. Upon reading this, we may assume that reverse thrust is the equivalent to a car or truck’s reverse control, but this isn’t necessarily true. Airplanes can’t reverse direction in midair. Rather, reverse thrust is used primarily to assist pilots in decelerating their airplane prior to landing. When engaged, it changes the direction in which air comes out of the airplane’s engines, allowing the airplane to slow down in preparation of landing.
Part B:
Qn.6 A)With necessary assumptions, calculate the force and power required to push / pull an aircraft by a towing vehicle.
Let us the consider the scenario of push the Airbus A320-200 (World's most predominantly used Passenger Aircraft).
Weight of Airbus A380= 79 tonnes= 79,000 Kgs
Let us consider the towing vehicle to be Mototok Spacer 8600
Weight of the Mototok Space 8600= 5400 Kgs. (from the data sheet)
Let us analyse various forces to be overcome to push the aircraft.
1. Rolling resistance force
`F_(rr)= mu_(rr). M.g`
`=> F_(rr)= mu_(rr). weight`
let `mu_(rr)`=0.002
That implies Frr= 0.002*79000= 158 N
Frr of Mototok towing vehicle= 0.002*5400=10.8 N
2. Aerodynamic resistance force:
Let us assume that speed of pushback or pushforward is 5.4 km/hr. (i.e 1.5 m/s) [From the datasheet]
Let,
Drag coefficient (Cd)= 0.025
Air Density `rho` = 1.125 kg/m^3.
Frontal Area (A): 150 m^2
Velocity (V)= 5.4 kmph or 1.5 m/s
`F_(ar)= 1/2* C_d* rho*A* v^2`
`=> F_(ar)=1/2* 0.025*1.125*150*1.5^2`
`=> F_(ar)= 4.74 N`
Therefore, airdrag force is 4.74 N
Note: Generally Towing vehicles are designed such that their frontal area is very less. Hence the air drag force due to them is very small and can be neglected.
3. Hill climb resistance
The taxiways on the airport are constructed flat. Hence gradient or hill climb force is neglected.
4.) Inertia force of the aircraft
Mass of the aircraft= 79000/9.8= 8061.22 kg
Inertial force of the aircraft= `m *(v/t)` where t is the towing time.
Let the towing time be 30 seconds.
That implies inertial force of the aircraft is 8061.22*(1.5/30)= 403 N
5.) Inertial force of the Towing Vehicle
Mass of the towing vehicle= 5400/9.8= 551 kg
Inertial force = 551* (1.5/30)= 27.55 N
Therefore, Total force requirement is 158+10.8+4.74+403+27.55= 604.09 N= 610 N (approx)
Power required
`P= F* v`
`=> P= 610 *1.5= 915 W`
Therefore the power required to push / pull an Airbus A320 at 5.4 km/hr by the Mototok Spacer 8600 towing vehicle is 915 W
Qn 6B) Develop the model for the calculated force and power using Simulink.
The simulink model is developed is shown below.
Hence, basic blocks are used to implement the expressions of Rolling resistance force , Aerodynamic drag force ,Gradient force and inertial forces. The forces are then added to find the net force required. The force is then multiplied with the velocity to obtain the total power required.
Qn 7A) Design an electric powertrain with type of motor, it’s power rating, and energy requirement to fulfill aircraft towing application in Simulink. 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.
Block Diagram of power train of towing vehicle:
The above block diagram shows the basic topology of an electric power train. We need to design the each and every component of the power train based on our need. Let us do these calculations below.
Design of Motor:
From the previous question, we could say that the total power required for the towing application is 915W
Let us analyse the torque requirement of the motor:
`T= (F*r)/(G*eta_g)`
where `G`is the gear ratio of the transmission system and `eta_g`is the gear efficiency
For Mototok's towing vehicles, G=27 and radius of the wheels (r) = 0.15 m
Assuming efficiency of gear transmission as 95%,
Torque T= (610* 0.15)/(27* 0.95)= 3.567 Nm
`omega= P_(out)/ T_(out)`
`=> omega= 915/3.567=256.5`rad/s= 2450 rpm (approx)
Considering all the above requirements, let us choose the ABB "M2BAX 80MB 2" three phase squirrel cage induction motor.
Motor Specifications:`V_(bat)= P_i/I_i`
Output Power: 1.1 kW
Rated torque: 3.67 Nm
Rated speed: 2840 rpm.
In our case, the motor needs to generate an output power of 915W. It is given in the data sheet that the efficiency of the motor ABB "M2BAX 80MB 2" at full load is 79.6%= 80% approx
Therefore, Input Power= Output power/ efficiency= 915/0.8= 1143.75 W.
This implies that 1143.75 W of power has to be given from the battery. [ by assuming power converter efficiency as 100%]
Battery Design:
As the rated torque of the motor is almost equal to the required torque, let us assume that the full load current is supplied by the battery to the motor.
`I_i= P_i/ (sqrt 3* V_l*cos phi)`
That implies Ii= 1143.75/(1.732*415*0.83)= 1.917 A [From the data sheet, Line voltage Vl=415V and Power factor at full load is 0.83)
Therefore current supplied by the battery is 1.917 A.
That implies Battery voltage `V_(bat)= P_i/ I_i`
Vbat= 1143.75/1.917= 596.6 Volts
Therefore battery voltage must be 596.6 Volts
In a DC chopper, Vo= D. Vin where D is the duty cycle. Here Max Vo allowed is the voltage rating of the motor.
Vo cannot exceed 415V as it is the rated voltage of the motor. Vin= 596.6 V
=> D_max= 415/596.6=0.695
Therefore the duty cycle can range from 0 to 0.695 to control the speed of the towing vehicle from zero to highest.
Energy requirement of the battery
`E= P_i*t`where t is the total time time of discharge of battery.
Assuming total time of discharge as 120 seconds, E= 1143.75*120= 137250 J
That implies E= 137250/(3600)= 38.125 Wh
Therefore the minimal rating of the battery must be 38.125 Wh
Assumed Parameters
Note: Some of the parameters are not assumed but taken from the datasheets.
| Parameter | Assumed values |
| Weight of the Aircraft (Airbus A320) | 79000 kg |
| Weight of the Towing vehicle (Mototok Spacer 8600) | 5400 kg |
| Co. efficient of rolling resistance (`mu_(rr)`) | 0.002 |
| Drag co.efficient (Cd) | 0.025 |
| Air Density `rho` | 1.125 `(kg)/ m^3` |
| Frontal area of the aircraft | 150 m^2 |
| Velocity or Speed of the towing vehicle | 1.5 m/s |
| time of towing | 30 seconds |
| Gear ratio of transmission system of Mototok Spacer | 27 |
| Gear transmission efficiency | 0.95 |
| Radius of the Towing vehicle wheel | 0.15 m |
| Gradient of the surface (in degrees) | 0 |
| Efficiency of the motor | 0.8 |
| Power factor of the motor | 0.83 |
| Rated voltage of the motor | 415 V |
Qn.7b)Design the parameters in excel sheet.
The excel sheet is attached below.
Links for data sheets of ABB Induction motor and Mototok Spacer 8600:
https://new.abb.com/products/3GBA081320-ASCCNBA/m2bax-80mb-2