POWERTRAIN FOR AIRCRAFT IN RUNWAYS

 

AIM :- To analyze the aircraft design parameters.

 

OBJECTIVE :- a) To list out the total weight of various types of aircraft.

                      b) To differentiate between ground speed and air speed.

                      c) To understand why aircraft engine power is not used move it on the ground at airport.

                      d) To learn about take-off power, tyre design, rolling resistance, tyre pressure and brake forces while landing.

                      e) To calculate the force and power required to push/pull an aircraft by towing a vehicle.

                      f) To design an electric powertrain with the type of motor, power rating and energy requirement to fulfill aircraft towing application.

                     g) To draw a block diagram of Powertrain.

 

THEORY :-

The design of an aircraft draws on a number of basic areas of aerospace engineering. These include aerodynamics, propulsion, light-weight structures and control. Each of these areas involves parameters that govern the size, shape, weight and performance of an aircraft.

The weight of the aircraft is needed to be calculated by the pilot for every flight to ensure the aircraft remains within weight limitations set by the manufacturer. These limitations are to ensure the aircraft is able to perform and recover from all flight maneuvers and withstand structural loading.

                                             

                                                                                                                 Fig: 1.1 Antanov AN-225

The table shows some of the most common aircraft flying around our skies today:

  

Aircraft	Empty weight            (kg)	Max Fuel Weight             (kg)	Max Cargo weight            (kg)	Max Gross Take Off Weight                   (kg)
Antanov        AN-225	285,000	300,000	190,000	640,000
Airbus      A380-800	277,000	254,000	84,000	575,000
Boeing     777-300ER	168,000	145,500	67,100	352,000
Boeing        787-10	135,500	101,500	57,300	254,000
Boeing       737-900	44,700	24,000	20,200	85,000
Airbus      A320-100	42,600	22,100	20,000	68,000
Embraer          190	28,000	13,000	13,100	48,000
Gulf stream         G650	24,500	21,190	3,000	45,200
Bombardier           CRJ900	21,850	8,900	2,760	36,500
Bombardier          0400	17,800	5,300	8,500	28,000
Learjet 75	6,300	2,750	1,300	9,700
Cessna Citation            CJ4	4,700	2,650	1,000	7,800
Beechcraft King            Air         B100	3,200	1,450	1,900	5,400
Diamond        DA50RG	1,450	155	560	2,000
Beechcraft           Bonanza           G36	1,150	200	385	1,660
Cessna 206H	990	235	520	1,630
Sirrus SR22	1,000	220	430	1,630

Aircraft Empty Weight (AEW) :- It is referred to as Operating Empty Weight and it is the weight of the aircraft with all its equipment on board but no fuel, no passengers or no cargo.

Aircraft Maximum Gross Takeoff Weight (MGTW) :- This is the maximum permissible weight the aircraft can weigh as it lifts off the ground at takeoff. This includes the aircraft’s empty weight, the fuel, the crew, the passengers and all the cargo.

Aircraft Useful Load (AUL) :- This is the weight that a pilot can place in the aircraft i.e, fuel, crew, passengers and cargo.

Ground speed v/s Air speed

Aerodynamic forces are generated by an object moving through a fluid (liquid or gas). A fixed object in a static fluid does not generate aerodynamic forces. Hot air balloons lift because of buoyancy forces and some aircraft like Harrier use thrust to lift the vehicle, but these are not examples of aerodynamic lift. To generate lift an object must move through the air or air must move past the object. Aerodynamic lift depends on the square of the velocity between the object and the air.

                                                

                                                                                                                           Fig : 1.2

The airplanes have two different types of speed :

• Ground speed :- It is the aircraft’s speed relative to the ground.
• Air speed :- It is the speed at which an aircraft is moving relative to the air it is flying in. As such it is also the speed at which the air is flowing around the aircraft’s wings.

In other words, while airspeed determines whether there is enough airflow around an aircraft to make it fly, ground speed determines how fast an aircraft will get.

Wind’s Effect on Ground Speed

The relationship between airspeed and ground speed is fairly simple. Ground speed is the sum of airspeed and wind speed.

If the aircraft is flying in the same direction as the wind is blowing, the aircraft experiences a tail wind, and its ground speed is higher than its airspeed. On the other hand, if the wind is blowing against the direction the aircraft is  travelling in, the aircraft experiences headwind, and its ground speed is lower than its airspeed.

For pilots, both airspeed and ground speed are important. While the first helps them to make sure they are flying fast enough to take off, not to stall and so on, the second one helps them to figure out how long will it take them to get from one place to another.

Aircraft Control on Ground

Planes move by pulling or pushing themselves through the air, rather than by applying engine power to spin their wheels, and thus have no forward or reverse gears. Like ground vehicles engines, the aircraft engines can’t run backwards. The airplanes generally do not reverse and usually pushed back by the pushback tugs. Under the norms, after the aircraft is pushed back into the bay by a tow truck, chocks have to be put on by the ground staff to stop the wheels from moving even if the aircraft engines get started inadvertently.

It is not recommended for an aircraft to use engine power to move it on the ground as most aircraft have jet propulsion engines using gas turbines. An average A320 aircraft can produce up to 150 KN 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. Also the jet engines used in the aircraft’s blast a lot of flames once combustion starts. When taxing, aircraft travel slowly, this ensures that they can be stopped quickly and do not risk wheel damage on larger aircraft if they accidently turn-off the paved surface. Taxi speeds are typically 30-35 km/h. The use of thrust near terminals is restricted due to the possibility of jet blast damage. 

The risk which may arise due to engine ground running relates to the potential for loss of control of the aircraft by the pilot. Damage can occur to the aircraft itself, other aircraft nearby or to airside structures. In addition there is a risk of injury to ground support personnel who may be in relatively close proximity to the aircraft.

The best from of mitigation is to ensure that all persons authorized to supervise or directly participate in engine ground running from the flight deck, who are not pilots or flight engineers currently rated on the specific aircraft type, are in receipt of suitable initial training and that there is a proper system for both initial and recurrent qualification for engine ground running duties.

It is also important to understand about take-off power, tyre design, rolling resistance, tyre pressure, brake forces while landing of the aircraft :

Take-off Power :  Take-off is the phase of flight in which an aerospace vehicle leaves the ground and becomes airborne. For aircraft travelling vertically, this is known as lift-off. For aircraft that takes off horizontally, this usually involves starting with a transition from moving along the ground on a runway.

For light aircraft, usually full power is used during take-off. Large transport category 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 emission. In some emergency cases, the power used can be be increased to increase the aircraft’s performance.

Tyre Design : The 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 aircraft needs to be distributed more evenly. They help to absorb the shock of landing and provide cushioning. It also provides the necessary traction for braking and stopping of an aircraft. Carcass piles are used to form the tire.

                                             

                                                                                                                           Fig : 1.3

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 take-off 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

Nowadays, almost all the airliners are using tubeless tyres as it is more advantageous over tube type. Also, they use radial tyre over bias tyre. The radial tyres are more expensive than bias-ply tyres. Radial tyres are in demand because of their lower life cycle cost and long term value.

Each tyre manufacturer produces their own designs of the tyre with different treads patterns, wear patterns, wear limits, etc and in some cases different names for the same tyre. But all tyres must meet the approved standards before being accepted by the FAA or CAA.

All commercial aircrafts tyres approved under FAA requirement Technical standard order (TSO) C62. A TSO is a minimum performance standard for specified materials, parts and appliances used on civil aircraft.

Rolling Resistance :  Rolling is the combination of forces that works against the forward motion of the vehicle.

F_rr = μ_rrmg

The coefficient, is a function of the tire material, tire structure, tire temperature, tire inflation pressure, tread geometry, road roughness, road material and the presence or absence of liquids on the road. The value varies with speed. Tire pressure increases, its value decreases.

The tests are made to determine the rolling friction of airplane wheels and tires under various conditions of wheel loading, tire inflation pressure and ground surface. For comparable conditions, either on a concrete runway or on firm turf, the standard type wheels and tires had the lowest values of rolling-friction coefficient; the values for the low pressure tires is only slightly higher. The highest coefficient is obtained with the extra low-pressure wheels and tires. In general, the variation in rolling-friction coefficient with either wheel load or tire inflation pressure was fairly small. The effect on take-off, with the exception of ground-surface condition, was generally quite small.

Tyre Pressure : Aircraft tires generally operate at high pressures, up to 200 psi for airliners, and even higher for business jets. The main landing gear on the Concorde was typically inflated to 232 psi, whilst its tail bumper gear tires were as high as 294 psi. Higher the pressure, the firmer the tyre and more strength it has to support the plane. Airplanes tires are usually inflated with nitrogen (Nitrogen is an inert gas) 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 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.

Brake Forces while Landing : 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. Aircraft brakes work in conjunction with other brake mechanisms such as thrust reversers, air brakes and spoilers. Thrust reversers are surfaces that are deployed into the path of the jet blast from the engines to redirect propulsive thrust in a direction that opposes the motion of the aircraft. Air brakes and spoilers are flight control surfaces that create additional aerodynamic drag when deployed into the path of air flowing around the aircraft.

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 generate 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.

The large amount of heat generated during aircraft braking is dissipated by means of passive or active cooling. Passive cooling occurs due to natural conduction of heat through the disc material and to surrounding components, radiation of heat off the brake and wheel assembly, and convection as air flows past and through the brake assembly. Active cooling is achieved by using fans that force air through the brakes. Fans are either built into the wheel or part of the external ground support equipment that is positioned against the wheel and removed once sufficient temperature drop has occurred.

Modern commercial aircraft are equipped with an autobrake system that controls the deceleration of the aircraft by automatically optimizing the application of brake force independent of pilot operation of the brake pedals to minimize stopping distance, reduce pilot workload and smooth braking action.

Another common feature of aircraft brake systems is a skid control function. Similar to anti-lock brakes on an automobile, skid control systems prevent wheel lockup. They minimize stopping distance and prevent the negative effects of skidding on tires such as unnecessary tire wear and blowout. Skid control is accomplished by applying an optimal brake force that maximizes the coefficient of friction and remains just below the amount of force that would cause skidding. In one implementation, the proper force is arrived at by continuously sensing wheel speed and comparing it to a calculated aircraft velocity. The difference between the two values represents wheel slip and brake force is reduced if it rises above a specified value.   

Force and Power Required to Push/Pull an Aircraft by a Towing Vehicle

The forces involved in moving the aircraft with the towing tractor is studied and there are mainly two forces acting in this condition :

• Rolling Resistance Force
• Aerodynamic drag force

We will take the reference of the heaviest aircraft i.e Antanov AN-225 to design our electric powered towing tractor so that it is applicable for all the light aircrafts too.

Rolling Resistance force on Aircraft

Weight of Antanov AV-225 = m₁ = 640,000 kg

Rolling Resistance coefficient =  μ_rr1 = 0.004

Rolling Resistance Force,

F₁ = μ_rr1*m₁*g

F₁ = 0.004 * 640000 * 9.81

F₁ = 25.1136 KN

Rolling Resistance Force on Aircraft Tug

The weight is an important factor when it comes to aircraft tugs. In order to move an airplane, tugs have to be very heavy, although they can cheat themselves into heaviness. From an economic point of view, light tugs are much more cost-efficient, but they need to be well equipped.

Weight of Towing vehicle = m₂ = 25,000 kg

Rolling Resistance coefficient = μ_rr2 = 0.001

Rolling Resistance Force,

F₂ = μ_rr2*m₂*g

F₂ = 0.001 * 25,000 kg * 9.8

F₂ = 0.24525 KN

Total Rolling Resistance Force = F₁ + F₂

                                       F_A = 25.1136 + 0.24525

                                        F_A = 25.35885 KN

Aerodynamic drag force acting on the Aircraft

Air density = ρ = 1.225

Frontal Area of the Aircraft = 200 m²

Velocity of the Aircraft while towing = 36 kmph = 10 m/s

Coefficient of Drag of the Aircraft = 0.025

Air drag force = 0.5 * ρ * A * v² * cd

                          = 0.5 * 1.225 * 200 * 10² * 0.025

                          = 0.30625KN

Total Force = Total rolling resistance force + Air drag Force

                     = 25.35885 + 0.30625

                     = 25.6651 KN

The total force that the motor needs to overcome in order to push/pull the Aircraft is 25.6651 KN.

Power required to push/pull the aircraft

The average speed of push/pull = 24 kmph = 6.67 m/s

Power is evaluated from the total force at given speed,

P_te = F_te*v

P_te = 25.6651*6.67

P_te = 171.186217 KW

The power required to push/pull an aircraft by towing a vehicle is 171.186217 KW.

SIMULINK MODEL :https://drive.google.com/file/d/1oSsIdwF07CKg8ZBBCYtbZytJTFozMBE2/view?usp=sharing

                         

                                                                                                                           Fig : 1.4

Procedure :

1. Use constant blocks to define parameters of rolling resistance and aerodynamic drag force.
2. Use the product blocks to get the required forces.
3. Add these forces in order to calculate the total Tractive force.
4. Multiply the total tractive force with the velocity of the vehicle and calculate the power.
5. The power required by the vehicle is displayed with the help of display block.

 

Electric Powertrain Design with type of Motor

An EV powertrain has 60% fewer components than the powertrain of an ICE vehicle. The components are described below:

• Battery Pack – The battery pack is made up of multiple Lithium-ion cells and stores the energy needed to run the vehicle. Battery pack provide direct current output.
• DC-AC Converter – The DC supplied by the battery pack is converted to AC and supplied to the electric motor. This power transfer is managed by a motor control mechanism (also referred to as the Powertrain Electronic Control Unit) that controls the frequency and magnitude of voltage supplied to the electric motor in order to manage the speed and acceleration as per driver’s instruction communicated via acceleration/brakes.
• Electric Motor – Converts Electrical energy to Mechanical energy that is delivered to the wheels via single ratio transmission. Many EVs use motor generators that can perform regeneration as well.
• On-board Charger – Converts AC received through charger port to DC and controls the amount of current flowing into the battery pack.

                                         

                                                                                                                           Fig : 1.5

TABLE FOR ASSUMED PARAMETERS

1.	Mass of aircraft	640,000 kg
2.	Mass of the towing vehicle	25,000 kg
3.	Coefficient of Rolling resistance for aircraft	0.004
4.	Coefficient of Rolling resistance for towing vehicle	0.001
5.	Drag coefficient	0.025
6.	Air density	1.225 kg/m3
7.	Frontal Area	200 m2
8.	Velocity	10 m/sec
9.	Battery Voltage	550 V
10.	Motor Voltage	400 V
11.	Duty Cycle	80%

MS Excel file : https://docs.google.com/spreadsheets/d/1iy4OO5Tnrco_ELHdaf-tA10NcSTMOEoB/edit?usp=sharing&ouid=105698778706441167018&rtpof=true&sd=true

Duty Cycle Estimation : It is the ratio of the full-power pulse’s duration to the entire PWM interval period, usually expressed as a percentage.

When a controller is sending the full voltage of the power source to the motor, the motor sees a PWM signal with a duty cycle 100%. PWM equivalent voltage is equal to the power supply voltage times the duty cycle. Therefore, we will take the parameters based on the calculation done in towing of an aircraft:

Motor considered for the drivetrain is DC7 block

Power required = 171.186 kW

Battery Voltage = V_in = 550 V

Motor required Voltage = V_out = 400 V

 

Duty Cycle = V_out/V_in

                    = 400/550

                    = 0.7272

As there will be 10% losses, the voltage demanded by the motor is

D = 0.7272 * (1 + 0.1) = 0.8

Thus, the voltage required from the battery is 440 V.

An electric powertrain with DC7 motor is designed with its power rating, and energy requirement to fulfill aircraft towing application. The electric powertrain is a simpler system, comprising of far fewer components than a vehicle powered by an internal combustion engine. The main components include Engine, Transmission and driveshaft.

An FTP75 drive cycle is used as a reference input. The DC7 drive features closed loop speed control with four quadrant operation. The speed control loop outputs the reference armature current of the machine. Using a PI current controller, the drive cycle corresponds to the battery current is derived.

The main advantage of this drive compared to other DC drives, is that it can operate in all four quadrants. In addition, due to the use of high switching frequency DC-DC converters, a lower armature current ripple is obtained.

                              

                                                                                                                          Fig : 1.6

FTP-75 Drive Cycle :

                     

                                                                                                                           Fig : 1.7

This drive cycle is defined by a series of tests defined by the US Environment Protection Agency (EPA) to measure tailpipe emissions and fuel economy of passenger cars.

The characteristics of the cycle are :

• Distance travelled: 11.04 miles (17.77 km)
• Duration: 1874 seconds
• Average speed: 21.2 mph (34.1 km/h)

The FTP-75 is also used to estimate the range in distance travelled by an electric vehicle.

BATTERY OUTPUT :

                      

                                                                                                                           Fig : 1.8

The battery SOC has decreased from 100% to 99.37% in 7 seconds. The RMS value of current is 29.25 A and that of voltage is 8.032 V.

MOTOR OUTPUT :

                      

                                                                                                                             Fig : 1.9

The above graph shows the duty cycle of the IGBT switches. The positive and negative duty cycle for IGBT switch 1 & 4 is 57.423% and 43.010% respectively. Similarly, for IGBT switch 2 & 3 the positive and negative duty cycle is 43.235% and 56.746% respectively.

The armature voltage is developed at the terminals of the armature winding of a DC machine during generation of power. Here, the RMS value for the armature voltage is 4.468 V. The armature current 2 is higher than the armature current 1 which is due to the number of turns in the windings.

A pulse with a long duration imparts more energy to the motor, increasing the speed of the motor. A short duration pulse reduces the available energy and the motor spins more slowly. The motor sees changes in pulse energy just like when batteries are added or removed.

 

CONCLUSION :  The total weight of various aircrafts were listed in the table and understood the difference between ground speed and air speed. Also, we understood the use of towing vehicle in moving the aircraft. Then we learned about take-off power, tyre design, rolling resistance, tyre pressure and brake forces while landing. The force and power required to push/pull an aircraft was calculated with necessary assumptions and a Simulink model was created for the same. Further, an electric powertrain was designed with a DC7 type motor and duty cycle was estimated for the same. Then, an excel sheet was created with the help of assumed parameters. All the models were simulated successfully and result were obtained.

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