Project-1: Powertrain for aircraft in runways

Design of Aircraft Towing Tractor with Electric Powertrain

Aim:

* To study about the aircraft towing process in the airports

* To estimate the load, speed and other operating conditions of the towing tractor in the airport

* To design the towing tractor with electric drivetrain which should be able to fulfll the above requirements

Objective of the Project:

* In Airport the Aircrafts will be there and they will either Take off to attain the flight or they will land in the airport from their flight.

* Usually after when the aircraft is loaded with passengers and luggages, they are ready to take off and while doing that the aircraft has to reach the straight runway inorder to attain required speed for it to fly in the air.

* In this process from the flight boarding area (i.e) the terminal it has to reach the run way and it is called as taxiway.

* For doing this, the flight's propeller power is not used. Instead a aircraft towing tractor is used to push or pull the aircraft to the runway.

* This project is to design the aircraft towing tractor with an electric drivetrain to meet out the needs of towing an aircraft.

* In order to design this towing tractor with an electric drivetrain, first we need to study on certain areas.

Designing Procedures:

* The correct way to design any vehicle is by reverse engineering. This can be done with the below four steps,

1. First the vehicle’s area of usage has to be studied that includes on what conditions we need our vehicle to perform and on what speed, load, and other operating conditions.

2. How much power we are in need to operate our vehicle in the desiered conditions.

3. What are all the necessary things to be calculated and analysed to operate our vehicle in the required conditions

4. Then having those sufficient data we need to go for the appropriate selection of motor and the powertrain.

* The first five questions deals with the first step of vehicle design, giving an idea about the aircrafts and the towing tractors (i.e) about the weight of the aircraft that is to be pushed or pulled using the towing tractors.

* The rest of the project deals on the design of the electric towing tractors to fulfill the above conditions

Questions:

1. Search and list out the total weight of various types of aircrafts.

* In order to design an electric towing tractors we need to know their load operating conditions ,so the weights of various types of aircrafts is shown below.

* The total weight of an aircraft includes the Aircraft’s empty weight + Payload

Aircraft's empty weight - The aircraft’s weight with leftover fuel in the tank + engine oil + other operating fluids

Payload - The fuel weight + passengers weight + luggage weight.

* Here to design an electric powered towing tractor we are considering it to work in the extreme load conditions ,so some of the heaviest aircrafts are listed below

1. The Antonov An-225 Mriya is a cargo aircraft that was designed by the Antonov design beraue in Ukraine . It is the heaviest aircraft ever build with a maximum takeoff load of 640 tonne

2. The Airbus A380 is a wide-body Aircraft manufactured by Airbus. It is the world's largest passenger air liner with a maximum takeoff load of 575 tonne

3. The Boeing 747-8 is a Wide-Body Airliner developed by Boeing commercial Aircrafts, which is the latest and largest variant of the 747, with a maximum takeoff load of 448 tonne

* Also there are lighter aircraft which is used for domestic purposes and some of them are listed below,

4. The Boeing 717 is a domestic passenger aircraft that was designed to accomodate 130 passengers. It's maximum takeoff load is 55 tonne.

5. The Airbus A319 is a passenger Aircraft manufactured by Airbus. It can accomodate upto 150 passengers and it's maximum takeoff load is 64 tonne.

6. The Embraer phenom 300 is a private jet developed by Embraer a brazillian manufacturer, which can take 11 passengers, with a maximum takeoff load of 8 tonne.

* These are some of the types of heavy and light aircraft manufactured. For design purpose we can consider the heavy aircrafts.

2. Is there any difference between ground speed and air speed?

* The aircraft usually operates in land as well as in air and so we need to study their speed differences in ground and in air

* The ground speed and the airspeed usually changes based on the wind component

* If we consider that there is no wind component then,

ground speed = air speed

* If wind is blowing in the tail direction of the aircraft then, (air speed > ground speed)

airspeed = ground speed + wind component

* If the wind is blowing in the head direction of the aircraft then, (air speed < ground speed)

airspeed = ground speed – windcomponent

* Also the gravity too plays a mojor role in increasing the aircrafts speed while it is decending.

3. Why is it not recommended to use aircraft engine power to move it on the ground at Airport?

* The aircraft usually does not use its engine powered to reach the runway from the airport terminal.

* This happens because, when the aircraft engine power is used the propeller blades starts rotating and it eventually pulls the dust and other particles on the ground into the propeller and it can damage the propeller blades.

* Also there are other reasons like propeller will cause lot of wind disturbances around the aircraft which will be a problem for the nearby buildings and other aircrafts setteled nearby.

* So to avoid all these things towing tractors are introduced for getting the job done.

4. How an aircraft is pushed to runway when its ready to take off?

* When aircraft is ready with all the passengers and lugages the aviation, pushback is an Airport procedure during which an Aircraft is pushed backwards away from an airport gate by external power.

Aircraft Towing Tractors or Tugs

* Pushbacks are carried out by special, low-profile vehicles called pushback tractors or tugs.

* These are heavy weighing low profile vehicles with enourmous torque to do the job of pushing/pulling the large aircraft from the terminals to the runway.

* The Tractors are designed with extra weight purposely to offer them enough Traction to push/pull heavy aircraft.

* Some of the categories of airraft tugs is shown below as per, IATA Airport Handling Ground Support Equipment Specification

* There shall be four categories of tractors defined in accordance with the maximum aircraft weight,

Category 1 – aircraft less than 50,000 kg. (110,000 lb.)

Category 2 – aircraft less than 150,000 kg. (330,690 lb.)

Category 3 – aircraft less than 260,000 kg. (573,196 lb.)

Category 4 – aircraft more than 260,000 kg. (573,195 lb.)

* The minimum tractor weight for each class,

Category 1 – 4,000 kg. (8,800 lb.)

Category 2 – 12,000 kg. (26,455 lb.)

Category 3 – 18,000 kg. (39,683 lb.)

Category 4 – 40,000 kg. (88,184 lb.)

* This is about the aircraft Towing tractors and its weights.

5. Learn about take off power, tyre design, rolling resistance, tyre pressure, brake forces when landing.

(i) Take off power

* Takeoff is the phase of flight in which an aerospace vehicle leaves the ground and becomes airborne.

* For aircraft that take off horizontally, this usually involves starting with a transition from moving along the ground on a runway

* The takeoff speed required varies with air density, aircraft gross weight, and aircraft configuration

* The power from the engine of the aircraft used to achieve the flight is called take off power

* For light aircraft, usually full power is used during takeoff.

* Large transport category (airliner) aircraft may use a reduced power for takeoff, where less than full power is applied in order to prolong engine life, reduce maintenance costs and reduce noise emissions

(ii) Tyre Design:

* The aircraft tyre is usually specially designed as the tyre has to withstand more load and also higher speed at the same time.

* The number of tyres required for any aircraft increases with aircraft's weight, as the weight of the aircraft needs to be distributed more evenly.

* Aircraft's tread pattern are designed to facilitate stability in high crosswind conditions, to channel water away to prevent hydroplaning and for braking effect. some of the tyre design is listed below.

Bias Ply Tyre:

* The bias ply tyre consists of casing plies running diagonally at approximate right angles to one another.

* The number of plies and the angles at which they are laid dictate strength and load capacity.

* High-performance bias aircraft tyres feature inter-tread reinforcing fabric (ITF). This provides additional high-speed stability, reduces tread distortion under load, protects the casing plies from damage and can act as wear indicators on retreadable tyres.

Radial Tyre:

* Radial tires are the most modern design of aircraft tyres. It is a design that is similar to that of car tyres.

* Typically, the radial tyre offers more landings per tread and lighter weight than a bias tyre, though sometimes at the expense of retread lives.

* The Rolling resistance force is mainly Experienced by the Tyre when the vehicle moves over any surface.

* The rolling resistance force is given by the expression,

Frr = urr * (m.g) ,where (m.g) is the weight in Newton and (urr) is the coefficient of rolling resistance.

* The coefficient of rolling resistance (urr) depends on many factors like the ,Tyre pressure,Tyre Temperature, Material of the Tyre, Thread pattern of the Tyre and also the Road Roughness and Liquids on the Road.

* These factors determine the rolling resistance force.

(iv) Tyre Pressure:

* The pressure generally is very high in an aircraft tire to reduce the rolling resistance and to withstand the weight. Usually it will be around 200 to 300 psi of pressure and they change based on the type of the aircraft.

* Tests of airliner aircraft tyres have shown that they are able to sustain pressures of maximum 800 psi.

* Aircraft tyres are usually inflated with nitrogen to minimize expansion and contraction from extreme changes in ambient temperature and pressure experienced during flight.

(v) Brake force while Landing:

* The brake force while landing is done by three different process and they are,

Aircraft disc brakes:

* Aircraft has disc rotors in the landing gear, used to brake the wheels while touching the ground. These brakes are operated hydraulically or pneumatically.

Trust reversers:

* Thrust reversers allows thrust from the engines to be used to slow the aircraft. This is also very effective in bringing down the speed of the aircraft. By using a negative angle plane, we are turning the wind that is coming into the propeller to flow in the opposite direction

Air Brake:

* Air brake dedicated flight control surface that work by increasing the drag force, air brakes are designed to increase drag while making little change to lift, resulting in a higher stall speed.

* These are some of the types of braking methods used to bring down the aircraft's speed while landing the flight.

6. With necessary assumptions, calculate the force and power required to push / pull an aircraft by a towing vehicle.

* The power required to push/pull an Aircraft can be estimated by the fillowing three steps,

The forces acting on the aircraft should be analysed

↓

The operating speed should be analysed

↓

The required power can be estimated

1. The forces acting on the aircraft should be analysed:

* The forces that will be involved in moving the aircraft with the towing tractor is studied and there are mainly two forces acting in this condition.

(i) - Rolling Resistance Force

(ii) - Air drag force

* The greadability and acceleration forces will not be there that much because the towing tractor operates in such a working environment, so we neglect those two forces.

(i) Rolling resistance force

* The Rolling resistance force is mainly Experienced by the Tyre when the vehicle moves over any surface.

* The rolling resistance force is given by the expression,

Frr = urr * (m.g) ,where (m.g) is the weight in Newton and (urr) is the coefficient of rolling resistance.

* The coefficient of rolling resistance (urr) depends on many factors like the ,Tyre pressure,Tyre Temperature, Material of the Tyre, Thread pattern of the Tyre and also the Road Roughness and Liquids on the Road.

* Here we are taking the heaviest flight as a reference to design our electric powered towing tractor because if we design for heavy application all the light aircrafts can be push/pulled too.

Calculation:

Weight of the Aircraft (Antonov AN-225) = 640000 Kg

Weight of the towing tractor = 50000 kg (To provide Enough Traction)

Rolling resistance coefficient of towing vehicle (Urr) = 0.001 (Assume)

Rolling resistance coefficient of an aircraft tire (Urr) = 0.006 (Assume)

Rolling resistance force of tow tractor = Urr * (m.g) = 0.001 * (50000*9.81)

Rolling resistance force of tow tractor = 0.49 KN

Rolling resistance force of Aircraft = Urr * (m.g) = 0.006 * (640000*9.81)

Rolling resistance force of Aircraft = 37.67 KN

The total Rolling Resistance Force = Rolling Resistance force of aircraft + Towing tractor = 0.49 + 37.67

The total Rolling Resistance Force = 38.16 KN

(ii) Air Drag force

* The air drag force is the force that occurs due to the Air striking effect on the vehicle when it moves.

* The vehicle moves through a Air medium of particular density value with certain speed ,this causes the air to strike against the vehicles body and it causes a drag force on the vehicle

* The Aerodyanamic drag is given by the expresssion,

Fad = 1/2*rho*A*v^2*c_d

* Here (Rho) gives the density of air medium,(A )is the frontal area of the vehicle which strikes the air, (v) is the veocity with which the vehicle moves and (c_d )is the coefficient of drag which varies for different shapes of the frontal area.

* The above expression gives the Drag force experienced by the vehicle at particular speed.

Calculation:

The velocity of the Aircraft while Towing = 25 KMPH = 6.95 m/s (Assume)

Density of Air medium (rho) = 1.225 (Assume)

The Frontal area of the Aircraft = 20 m^2 (Assume)

The coefficient of Drag of the Aircraft = 0.026 (Assume)

Air drag force = 0.5 * rho * A * v^2 * cd = 0.5 * 1.225 * 20 * 6.95^2 * 0.026 = 0.015 KN

The Air Drag Force Acting = 0.015 KN (Which is very minimum in Magnitude)

* Therefore the total force that the motor needs to overcome = Rolling resistance of the aircraft + Drag force acting on the Aircraft.

* Total forces acting = 38.16 + 0.015

The Total Force that the motor needs to overcome inorder to Push/pull the Aircraft = 38.175 KN

2. The Power Requiered:

* The power requireded by the motor is given by the velocity with which the towing operation takes place.

* The average speed of Push/Pull = 20 kmph = 5.5 m/s

* Power required by the motor = Total forces * velocity = 38.175 KN * 5.5

Power required by the motor = 210 Kw

* The torque output of the towing tractor can also be calculated by the below steps,

Fte = T . G / r

where, Fte – The Total Tractive force (Rolling resistance force + Air drag force) ; T – Torque output ; r – Radius of tire ; G – Gear ratio

For a Heavy Duty Towing tractor we assume,

G – 11 (For delivering high torque) ; r – 25 inches = 0.635 m

T = Fte * r / G = 38200 * 0.635 / 11

The Torque requiered to Push/pull for the above stated condition = 2205 Nm

* Now for pushing or pulling the aircraft our electric towing tractor requires the following power and torque figures,

1. Power = 210 Kw

2. Torque = 2.2 KNm

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.

Design of Electric Powertrain for Aircraft Towing Tractor:

Energy required to be delivered

↓

Motor selection based on Power requirements

↓

Power Converters To upscale the armature voltage as required

↓

Switching Techniques With Duty Cycle ratio For speed Control Of the Motor

* Energy is the capacity to do work. Energy is power integrated over time.

* Assuming the operational time of the Towing truck is around 10 minutes for one cycle

10 minutes = 0.16 Hour

Energy (KWh) = Power (KW) * Time (h) = 210 * 0.16

Energy (Kwh) = 33.6 KWh

* Therefore the battery must be able to store the energy of 33.6 KWh and should deliver the energy throughout the run time in order for the motor to produce the 2.2KNm of torque instantaneously.

* Thus the storage of energy in batteries with high performance ultracapacitors are selected to fulfill the required power delivery.

Motor Selection:

* Here we have selected a BrushlessDC Motor type that has a power rating of 210 KW and with the gear ratio and the transmission it should be able to deliver 2.2KN.m of torque instantaneously.

The Battery armature voltage (V) = 400V (Assume)

The Battery armature current (A) = 400A (Assume)

* For getting the maximum power output from the motor (210 KW), the voltage applied to the motor terminals must be 800V (Assume) at the motor's positive and negative terminals.

Powerconverters:

* Here we use a DC-DC Boost converter to upscale the voltage level from the battery to the required level so that when supplied to the motor we can get the desiered power output from the motor.

* Bidirectional DC-DC converters find applications in places where battery charging, regenerative braking, and backup power are required.

* The power flow in a bidirectional converter is usually from a low voltage end such as a battery or a supercapacitor to a high voltage side and is referred to as boost operation.

* During regenerative braking, the power flows back to the low voltage bus to recharge the battery (buck mode). As a backup power system, the bidirectional DC-DC converter facilitates the safe operation of the vehicle when ICEs or electric drives fail to drive the motor.

* The boost converter converts the 400V of the armature voltage to a high voltage level of 1000 V.

Speed control of the vehicle:

* The electric vehicle uses power from the battery source to drive the motor and then further to drive the wheels in order to move the vehicle.

* The speed and torque generated from the motor is very important to drive the vehicle as per the Driver's desire.

* If we connect the battery directly to the motor the motor will spin with a speed and torque characteristics which will be based on the full power of the battery voltage.

* We does not require full power in all the driving conditions so we need to adopt a power controller between the battery and the motor so that the motor speed may be controlled by the driver to drive the vehicle as desired.

* For controlling the vehicle power we use the soft switching operation technique here.

* The amount of power can be controlled by changing the duty cycle value.

* Considering the losses we take 80 % of the duty cycle to deliver the maximum power of the motor

Motor terminal voltage = Armature voltage * Duty cycle = 1000 * 0.8

* With 80% Duty Cycle the Voltage @ Motor Terminals = 800 V , Which gives the full power with all the gearings, instantaneously.

* Thus by varying the Duty Cycle value the speed of the vehicle is controlled.

Result:

* By answering the first five questions the need for Aircraft towing tractor is known and the rest of the project deals with the design of the towing tractors.

* The reference Aircraft Taken here is the Heaviest Aircraft ever build and so our designed towing tractor is a heavy duty operational vehicle.

* The power requirements are calculated for the particular load to be pushed/pulled and to achieve this power at the motor output, the motor type and its voltage at the terminals are discussed.

* Then the power converters are used to upscale the battery armature voltage to the desiered high voltage value so that the motor may spin at full power and requiered torque can be achieved using a Bidirectional DC - DC Converter.

* Finally the speed control of the vehicle is done with Switching techniques where the duty cycle adjustment can change the motor speed from minimum to maximum.

* The parameters assumed for designing this vehicle is taken from onine references and are listed below

S.No	Assumed Parameters	Assumed Values
1.	Weight of the Aircraft	640000 kg
2.	Weight of the Towing Tractor	50000 kg
3.	Rolling resistance coefficient of aircraft tyre Urr (@200 psi in New Asphalt )	0.006
4.	Rolling resistance coefficient of towing tractor tyre Urr (in New Asphalt )	0.001
5.	The velocity of the Aircraft while Towing	6.95 m/s
6.	Density of Air medium (rho)	1.225 Kg/m^3
7.	The Frontal area of the Aircraft	20 m^2
8.	The coefficient of Drag of the Aircraft	0.026
9.	Towing tractor Gear ratio "G" (for high Torque output)	11
10.	Towing Tractor tyre radius "r"	0.635 m
11.	Operational time of the Towing for one cycle	0.16 Hour
12.	The Battery armature voltage	400 V
13.	The Battery armature current	400 A
14.	The motor terminal voltage for full power output (Taken with reference from Tesla cars)	800 V @ motor terminals
15.	Power converters used	Bidirectional DC - DC Boost converter
16.	The maximum duty ratio for achieving full power	80%

* The Electric powered Heavy Duty Towing Tractor for Aircraft is designed for opeating in extreme load conditions.

Online references:

1. https://www.sats.com.sg/Tenders/Documents/CT1809G033/4%20Appendix%20C%20Annex%20A%20Technical%20Specification%20Compliance.pdf

2. https://en.wikipedia.org/wiki/List_of_airliners_by_maximum_takeoff_weight

3. https://www.aerospecialties.com/aviation-ground-support-equipment-gse-products/tld-ground-support-equipment/tld-tmx-550

4. https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19780005100.pdf

5. https://en.wikipedia.org/wiki/Drag_coefficient