Powertrain for aircraft in runways

Aim:

• Search and list out the total weight of various types of aircrafts.
• Is there any difference between ground speed and air speed?
• Why is it not recommended to use aircraft engine power to move it on the ground at Airport?
• How an aircraft is pushed to runway when its ready to take off?
• Learn about take off power, tyre design, rolling resistance, tyre pressure, brake forces when landing.
• With necessary assumptions, calculate the force and power required to push / pull an aircraft by a towing vehicle
• 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.

Important aspects of Designing:

• 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.
• How much power we are in need to operate our vehicle in the desired conditions.
• What are all the necessary things to be calculated and analysed to operate our vehicle in the required conditions
• Then having those sufficient data we need to go for the appropriate selection of motor and the powertrain.

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

The aircraft gross weight (also known as the all-up weight and abbreviated AUW) is the total aircraft weight at any moment during the flight or ground operation. An aircraft's gross weight will decrease during a flight due to fuel and oil consumption. An aircraft's gross weight may also vary during a flight due to payload dropping or in-flight fueling. At the moment of releasing its brakes, the gross weight of an aircraft is equal to its takeoff weight. During flight, an aircraft's gross weight is referred to as the en-route weight or in-flight weight. An aircraft's structural weight capability is typically a function of when the aircraft was manufactured, and in some cases, old aircraft can have their structural weight capability increased by structural modifications.

1.Maximum design taxi weight (MDTW)

The maximum design taxi weight (also known as the maximum design ramp weight (MDRW)) is the maximum weight certificated for aircraft manoeuvring on the ground (taxiing or towing) as limited by aircraft strength and airworthiness requirements.

2.Maximum design takeoff weight (MDTOW)

Is the maximum certificated design weight when the brakes are released for takeoff and is the greatest weight for which compliance with the relevant structural and engineering requirements has been demonstrated by the manufacturer.

3.Maximum design landing weight (MDLW)

The maximum certificated design weight at which the aircraft meets the appropriate landing certification requirements. It generally depends on the landing gear strength or the landing impact loads on certain parts of the wing structure.The MDLW must not exceed the MDTOW.The maximum landing weight is typically designed for 10 feet per second (600 feet per minute) sink rate at touch down with no structural damage.

4.Maximum design zero-fuel weight (MDZFW)

The maximum certificated design weight of the aircraft less all usable fuel and other specified usable agents (engine injection fluid, and other consumable propulsion agents). It is the maximum weight permitted before usable fuel and other specified usable fluids are loaded in specified sections of the airplane. The MDZFW is limited by strength and airworthiness requirements. At this weight, the subsequent addition of fuel will not result in the aircraft design strength being exceeded. The weight difference between the MDTOW and the MDZFW may be utilised only for the addition of fuel.

5.Minimum flight weight (MFW)

Minimum certificated weight for flight as limited by aircraft strength and airworthiness requirements.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

• The Antonov An-225 Mriyais 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.
• The Airbus A380is a wide-body Aircraft manufactured by Airbus. It is the world's largest passenger air liner with a maximum takeoff load of 575 tonne.
• The Boeing 747-8is 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.
• The Boeing 717is a domestic passenger aircraft that was designed to accomodate 130 passengers. It's maximum takeoff load is 55 tonne.
• The Airbus A319is a passenger Aircraft manufactured by Airbus. It can accomodate upto 150 passengers and it's maximum takeoff load is 64 tonne.
• The Embraer phenom 300is a private jet developed by Embraer a brazillian manufacturer, which can take 11 passengers, with a maximum takeoff load of 8 tonne.

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

Airspeed vs. Ground Speed

As mentioned above, true airspeed is simply the speed at which an aircraft is moving relative to the air it is flying in. As such, it’s also the speed at which the air is flowing around the aircraft’s wings.`Ground speed, on the other hand, is the aircraft’s speed relative to the ground. One thing that should be noted here is that it’s its horizontal rather than vertical speed – an aircraft climbing completely vertically would have a ground speed of zero. In other words, while airspeed is what determines whether there is enough airflow around an aircraft to make it fly, ground speed is what determines how fast an aircraft will get to its destination.

• 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) so,airspeed = ground speed + wind component
• If the wind is blowing in the head direction of the aircraft then, (air speed < ground speed)so, airspeed = ground speed – wind component
• Also the gravity plays a major 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?

Taxing is the movement of an aircraft on the ground, under its own power, in contrast to towing or push-back where the aircraft is moved by a tug. The aircraft usually moves on wheels, but the term also includes aircraft with skis or floats.

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.

An airplane uses taxiways to taxi from one place on an airport to another; for example, when moving from a hangar to the runway. The term "taxiing" is not used for the accelerating run along a runway prior to takeoff, or the decelerating run immediately after landing, which are called the takeoff roll and landing rollout, respectively.It is not recommended for an aircraft to use its engine power as most aircrafts have jet propulsion engines using gas turbines. An average A320 aircraft can produce upto 105KN of thrust. This is enough to burn a small populated village if the combustion goes unburnt to the turbine and hence creating flames outside the engine. Another reason is that the jet engines used in aircrafts blast a lot of flames once combustion starts. When taxiing, aircraft travel slowly. This ensures that they can be stopped quickly and do not risk wheel damage on larger aircraft if they accidentally turn off the paved surface. Taxi speeds are typically 30 to 35 km/h (16 to 19 kn).The use of engine thrust near terminals is restricted due to the possibility of jet blast damage.

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

Taxiing refers to the movement of an aircraft on the ground, under its own power. The aircraft moves on wheels. An airplane uses taxiways to taxi from one place on an airport to another; for example, when moving from a terminal to the runway.

When aircraft is ready with all the passengers and luggages the aviation, pushback is an Airport procedure during which an Aircraft is pushed backwards away from an airport gate by external power.The aircrafts always moves on the ground following the yellow lines, to avoid any collision with the surrounding buildings, vehicles or other aircrafts. The taxiing motion has a speed limit. Before making a turn, the pilot reduces the speed further to prevent tire skids. Just like cars, there is a certain list of priorities during taxiing. The aircrafts that are landing or taking off have higher priority. The other aircrafts have to wait for these aircrafts before they start or continue taxiing.The thrust to propel the aircraft forward comes from its propellers or jet engines. Steering is achieved by turning a nose wheel or tail wheel/rudder; the pilot controlling the direction travelled with their feet. The use of engine thrust near terminals is restricted due to the possibility of jet blast damage. This is why the aircrafts are pushed back from the buildings by a vehicle before they can start their own engines for taxiing.

• 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.

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.

Tire Terminology

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. Inner liner: 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. Under tread: 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.

Balance marker

A balance marker, consisting of a red dot, is affixed on the sidewall of the tire immediately above the bead to indicate the lightweight point of the tire.

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. Radial aircraft tyres differ to bias aircraft tyres in that the plies all run radially from bead to bead at approximately 90° to the centre-line of the tyre. Angled belt plies are laid between the tread and the top casing ply, resulting in a flatter tread and adding strength to the tyre.

Tire Terminology

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. Inner liner: 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. Under tread: 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.

Balance marker

A balance marker, consisting of a red dot, is affixed on the sidewall of the tire immediately above the bead to indicate the lightweight point of the tire.

(iii) Rolling Resistance:

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 = weight in Newton

urr = 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:

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.

Large drogue parachutes: It is used by several former and current military and civilian aircraft (examples include the American B-52 and the soviet Tu-134 and Tu-144) and in the Space Shuttle.

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

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

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 = weight in Newton

urr = 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:

Let us assume

Weight of the Aircraft (Airbus A-380) = 575000 Kg

Rolling resistance coefficient of an aircraft tire (Urr) = 0.004

Weight of the towing tractor = 50000 kg

Rolling resistance coefficient of towing vehicle (Urr) = 0.001

Then,

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) = 0.004 * (575000*9.81)

Rolling resistance force of Aircraft = 22.56 KN

The total Rolling Resistance Force = Rolling Resistance force of aircraft + Towing tractor = 0.49 + 22.56

The total Rolling Resistance Force = 23.05KN

(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 Aerodynamic drag is given by the expresssion,

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

Where,

Rho = density of air medium

A = frontal area of the vehicle which strikes the air

v = veocity with which the vehicle moves

c_d = coefficient of drag which varies for different shapes of the frontal area.

Calculation:

Let us assume

The velocity of the Aircraft while Towing = 30 KMPH = 8.33 m/s

Density of Air medium (rho) = 1.225

The Frontal area of the Aircraft = 20 m^2

The coefficient of Drag of the Aircraft = 0.026

Air drag force = 0.5 * rho * A * v^2 * cd = 0.5 * 1.225 * 20 * 8.33^2 * 0.026 = 0.022 KN

The Air Drag Force Acting = 0.022KN

Therefore the total force that the motor needs to overcome = Rolling resistance of the aircraft + Drag force acting on the Aircraft hence the total forces acting = 23.05 + 0.022

The Total Force that the motor needs to overcome in order to Push/pull the Aircraft = 23.07KN

2.The Power Required:

The power required 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 = 23.07 KN * 5.5

Power required by the motor = 126.8Kw

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 = 23000 * 0.635 / 11

The Torque required to Push/pull for the above stated condition = 1.327 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.

The parameters assumed for designing this vehicle is taken from online references

S.No	Assumed Parameters	Assumed Values
1.	Weight of the Aircraft	575000 kg
2.	Weight of the Towing Tractor	50000 kg
3.	Rolling resistance coefficient of aircraft	0.004
4.	Rolling resistance coefficient of towing tractor	0.001
5.	The velocity of the Aircraft while Towing	8.33 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	0.33 Hour
12.	The Battery armature voltage	350 V
13.	The Battery armature current	350 A
14.	The motor terminal voltage for full power output	630 V @ motor terminals
15.	Power converters used	Bidirectional DC - DC Boost converter
16.	The maximum duty ratio for achieving full power	70%

Assuming the operational time of the Towing truck is around 20 minutes for one cycle

20 minutes = 0.33 Hour

Energy (KWh) = Power (KW) * Time (h) = 126.8 * 0.33

Energy (Kwh) = 41.8 KWh

Therefore the battery must be able to store the energy of 41.8 KWh and should deliver the energy throughout the run time in order for the motor to produce the 1.32KNm 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 126.8 KW and with the gear ratio and the transmission it should be able to deliver 1.32KN.m of torque instantaneously.

The Battery armature voltage (V) = 350V

The Battery armature current (A) = 350A

For getting the maximum power output from the motor (126.8 KW), the voltage applied to the motor terminals must be 700V (Assume) at the motor's positive and negative terminals.

Power Converters:

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 desired power output from the motor.The 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. 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 350V of the armature voltage to a high voltage level of 900 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 70 % of the duty cycle to deliver the maximum power of the motor

Motor terminal voltage = Armature voltage * Duty cycle = 900 * 0.7

With 70% Duty Cycle the Voltage at Motor Terminals = 630 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.

Block diagram of powertrain:

Result:

• The reference Aircraft Taken here is the Heavy Aircraft 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.