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Aim : 1.Search and list out the total weight of various types of aircrafts. 2.Differeniate between ground speed and air speed. 3.Find out why it is not recommended to use aircraft engine power to move it on the ground at Airport 4.Learn about take off power, tyre design, rolling resistance,…
abhijeet dhillon
updated on 21 Jun 2020
Aim :
1.Search and list out the total weight of various types of aircrafts.
2.Differeniate between ground speed and air speed.
3.Find out why it is not recommended to use aircraft engine power to move it on the ground at Airport
4.Learn about take off power, tyre design, rolling resistance, tyre pressure, brake forces when landing.
5.With necessary assumptions, calculate the force and power required to push / pull an aircraft by a towing vehicle.
6.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.
Solution:
1. Aircraft Gross Weight:
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 .
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.
Weight of Various Types of Aircraft Weight :
1.Glider
Glider is a fixed wing plane that is supported in flight by the dynamic reaction of the air against its lifting surfaces, and whose free weight does not depend on an engine. Most gliders do not have an engine, although motor glider have small engines for extending their flight when necessary by sustaining the altitude (normally a sailplane is on a continuously descending slope) with some being powerful enough to take off self launch.
Weight of Glider planes : 110 kg - 200 kg
2.Powered Lift
A powered lift aircraft takes off vertically under engine power but uses a fixed wing for horizontal flight. These aircraft do not need a long runway to take off and land, like helicopters , but they have a speed and performance similar to standard fixed wing aircraft in combat or other situations.
Weight : 160 kgs
3.Fighter Aircraft
A fighter aircraft, often referred to simply as a fighter, is a military fixed wing aircraft designed primarily for air to air combat against other aircraft. The key performance features of a fighter include not only its firepower but also its high speed and manuverability relative to the target aircraft.
Weight : 13500 kgs
4.Transport Planes :
Military transport aircraft or military cargo aircraft are used to airlift troops, weapons and other military equipment to support military operations. Transport aircraft can be used for both strategic and tactical missions, and are often diverted to civil emergency relief missions.
Weight : 250,000 kgs
5.Aerobatic Planes
An aerobatic aircraft is an aerodyne (a heavier-than-air aircraft) used in aerobatics, both for flight exhibitions and aerobatic competitions
Weight : 771 kgs
2.Difference between ground speed and air speed:
Ground Speed | Air speed |
Ground speed is the airplane’s speed relative to the surface of the Earth | Airspeed is the speed relative to the air it is flying in. |
Ground speed is what determines how fast an aircraft will get to its destination | Airspeed is what determines whether there is enough airflow around an aircraft to make it fly |
Ground speed is simply the sum of airspeed and wind speed. | It is independent of ground speed. |
3.Why is it not recommended to use aircraft engine power to move it on the ground at Airport?
Towing is coupling two or more objects together so that they may be pulled by a designated power source or sources. The towing source may be a motorized land vehicle, vessel, animal, or human, and the load being anything that can be pulled
Although many aircraft are capable of moving themselves backwards on the ground using reverse thrust (a procedure referred to as a powerback), the resulting jet prop might cause damage to the terminal building or equipment. Engines close to the ground may also blow sand and debris forward and then suck them into the engine, causing damage to the engine. A pushback is therefore the preferred method to move the aircraft away from the gate.
Although most aircraft are capable of moving themselves backwards on the ground using reverse thrust, a procedure called as a powerback, it is not typically done due to the following reasons:
4.Taxiing 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 (for water-based travel).
The power for taxiing comes from the engines itself. If it is a propeller powered aircraft, the propeller provides thrust and pulls the aircraft, like it would in air. If it is a jet engine, the exhaust air that comes out of the engine creates a forward force due to Newton's third law and moves the airplane.
Airliners have nose wheel steering, controlled by a cockpit control called a tiller. The tiller allows nose wheel to rotate at quite extreme angles making tighter turns possible. It also reduces the fuel consumption as you do not need much engine power during taxiing. If rudders were used, the power has to be increased to make sure sufficient air flows over the rudder surface to make it effective.
Once on the take off roll and the airplane gains sufficient speed i.e. enough air flows over the rudders, pilots go back to using the rudder pedals to maintain the centre line.
Some large airplanes with multi wheel main gears have a tendency to scrub the tyres on the tarmac when nose wheel is steered. To counter this problem, an automatic main gear steering is inbuilt in the aircraft. This makes the main gears turn the opposite direction to that of the nose wheel, greatly decreasing the wear on the tyres.
Small general aviation aircraft uses the rudder for turns during taxi. They don't have a separate control for nose wheel. In such airplanes, to make tighter turns, you could use differential braking. That is, if you want to make a tight turn for example to the left, you would push the upper part of your left rudder pedal which would apply left braking. This allows the airplane to make a sharper turn to the left.
5. Take off power, tyre design, rolling resistance, tyre pressure, brake forces when landing:
a.Take off Performance :
Take off performance can be predicted using a simple measure of the acceleration of the aircraft along the runway based on force equilibrium.
The forces involved will be,
T – Thrust of propulsion system pushing aircraft along runway.
D – Aerodynamic Drag of vehicle resisting the aircraft motion.
F – Rolling resistance friction due to the contact of wheels or skids on the ground.
During take-off run the imbalance in these forces will produce an acceleration along the runway.
dv/dt = T - D - F /m
The procedure for take-off will be that the vehicle will accelerate until it reaches a safe initial flying speed. The pilot can then rotate the vehicle to an attitude to produce climb lift and it will ascend from the ground. The determination of this safe flying speed or rotation speed, VR, is a critical factor in determining take-off performance.
For safety reasons VR is usually determined as being 1.1 × VSTALL
VSTALL : is the lowest speed that the aircraft can be flown before the airflow starts to separate from wings as the angle of attack becomes too great. The wing is assumed in this case to be in take-off configuration or "clean".
It can be calculated based on knowledge of the aircraft take-off configuration and hence the maximum achievable lift coefficient CL(max). As shown in the previous section , to maintain level flight the lift produced must equal the weight, hence the stall speed can be calculated as,
b.Tyre Design of aircrafts :
When it comes to safety, tyres are one of the most important components of aircraft. They help to absorb the shock of landing and provide cushioning. It also provides the necessary traction for braking and stopping of an aircraft.
An aircraft tyre is designed to withstand extremely heavy loads while landing, take off, taxing and parking. The number of wheels required for aircraft increases with the weight of the aircraft, as the weight of the aeroplane needs to be distributed more evenly.
Carcass plies are used to form the tire. They are sometimes called casing plies. An aircraft tyre is constructed for the purpose it serves.
Unlike an automobile or truck tyre, it does not have to carry a load for a long period of continuous operation. However, an aircraft tyre absorbs the high impact loads of landing and also it’s operating at high speeds for a short time when required.
Retreading – Retreading is methods of restoring a worn tyre by renewing the tread area or by renewing the tread area plus one or both sidewalls. Repairs are included in the tyre retreading process.
Load Rating – Load rating is the maximum permissible load at a specified inflation pressure.
Ply Rating – Ply Rating is used to identify the maximum recommended load rating and inflation pressure for a specified tyre. It is an index of tyre strength.
Speed Rating – The speed rating is the maximum takeoff speed to which the tyre has been tested.
Skid Depth – Skid depth is the distance between the tread surface and the deepest groove as measured in the mould.
Aircraft tyres must have an approved speed and load rating and have sufficient clearance when retracted through landing gear to allow for tyre growth. Tyre growth is the increase in the size of the tyre due to centrifugal forces at high speed.
c.Rolling Resistance :
Rolling resistance, sometimes called rolling friction or rolling drag, is the force resisting the motion when a body (such as a ball, tire, or wheel) rolls on a surface. It is mainly caused by effects; thnon elastic at is, not all the energy needed for deformation (or movement) of the wheel, roadbed, etc., is recovered when the pressure is removed. Two forms of this are hysterisis losses , and permanent plastic deformation of the object or the surface (e.g. soil). Another cause of rolling resistance lies in the slippage between the wheel and the surface, which dissipates energy. Note that only the last of these effects involves friction, therefore the name "rolling friction" is to an extent a misnomer.
The rolling resistance can be expressed as
Fr = c W (1)
where
Fr = rolling resistance or rolling friction (N, lbf)
c = rolling resistance coefficient - dimensionless (coefficient of rolling friction - CRF)
W = m ag
= normal force - or weight - of the body (N, lbf)
m = mass of body (kg, lb)
ag = acceleration of gravity (9.81 m/s2, 32.174 ft/s2)
Rolling Resistance Coefficient | ||
---|---|---|
c | cl (mm) | |
0.001 - 0.002 | 0.5 | railroad steel wheels on steel rails |
0.001 | bicycle tire on wooden track | |
0.002 - 0.005 | low resistance tubeless tires | |
0.002 | bicycle tire on concrete | |
0.004 | bicycle tire on asphalt road | |
0.005 | dirty tram rails | |
0.006 - 0.01 | truck tire on asphalt | |
0.008 | bicycle tire on rough paved road | |
0.01 - 0.015 | ordinary car tires on concrete, new asphalt, cobbles small new | |
0.02 | car tires on tar or asphalt | |
0.02 | car tires on gravel - rolled new | |
0.03 | car tires on cobbles - large worn | |
0.04 - 0.08 | car tire on solid sand, gravel loose worn, soil medium hard | |
0.2 - 0.4 | car tire on loose sand |
d.Tyre Pressure :
Car tyres aren’t just unsafe when they’re old and worn out – you need to ensure you know what tyre pressure you have in your tyres when you drive it because there are critical safety and economic implications.
Car tyre pressure is a measurement of how much air is in your pneumatic tyre, and ensures the tyres wear evenly and maintain the correct level of grip on the road surface. This is commonly expressed as PSI, or pounds-per-square-inch.
5.Force and Power required to push / pull an aircraft by a towing vehicle.
The force to move an air plane on an absolute flat runway with no wind at constant low speed is equal to the rolling resistance between the plane and the tarmac.
The rolling resistance for the air plane can be calculated as :
Fr = c mr ag (1)
where
Fr = rolling resistance (N, lbf)
c = rolling resistance coefficient
mr = rolling mass (kg, lbm)
ag = acceleration of gravity (9.81 m/s2)
The friction force for the puller can be calculated to
Ff = μ W
= μ mf ag (2)
where
Ff = frictional force (N, lb)
μ = friction coefficient
W = weight (N, lbf)
mf = friction mass (kg)
The drag force acting on the tow will be :
The drag force D exerted on a body traveling though a fluid is given by
C is the drag coefficient, which can vary along with the speed of the body. But typical values range from 0.4 to 1.0 for different fluids (such as air and water)
ρ is the density of the fluid through which the body is moving
v is the speed of the body relative to the fluid
Now the tractive force or the pulling force required by the tow will be
Pulling force = Drag force + friction force + Rolling resistance force
Now let us consider aircrafts with the following specifications :
Now as we have seen that various kinds of planes have different weights ,the amount of pulling force also will be different .Therefore we will consider a range of weight :
Range of weight = 200 kgs - 250000 kgs '
The towing speed considered is 5 m/s
Area range : 50 - 100 m^2
Coefficient of rolling resistance : 0.02
Coefficient of friction : 0.01
Now we use a script from matlab which will help us calculate the pulling force required :
clc
clear all
%Inputs
c=0.002%Rolling resistance coefficient
m=linspace(200,25000,2000) % Weight
g=9.81 %acceleration of gravity
u=0.6%friction coefficient
A=linspace(50,100,2000)%area of planes
v=2 % velocity
Cd=0.8 %Drag Coefficient
%Calculations
F1=c*m*g %Rolling resistance force
F2=u*g*m%Friction force
F3=0.5*A*1.2*Cd*(v)^2
Ft=F1+F2+F3
plot(m,Ft)
title('Tractive Force vs Weight of planes')
xlabel('Weight of planes(kg)')
ylabel('Tractive Force')
axis([0 2000 0 20000])
Results:
7. Electric powertrain with type of motor, it’s power rating, and energy requirement to fulfill aircraft towing application:
Consider the powertrain with the following specifications :
Vehicle Drive: 2.0 HP, 36 VDC Traction Motor mated with High-Efficiency Helical Gear Dana Axle Assembly
Batteries: Six 6 VDC, 190 AMP-HR Batteries (6 hr rate), wired in series.
Vehicle Speed, Loaded: 3.6 mph / 5.8 kph
Lift Cradle Capacity: 1,500 lbs / 680 kg
Now this can be represented in simulink in the following manner :
The following components were used :
1.PWM Module :
Pulse width modulation speed control works by driving the motor with a series of “ON-OFF” pulses and varying the duty cycle, the fraction of time that the output voltage is “ON” compared to when it is “OFF”, of the pulses while keeping the frequency constant.The power applied to the motor can be controlled by varying the width of these applied pulses and thereby varying the average DC voltage applied to the motors terminals. By changing or modulating the timing of these pulses the speed of the motor can be controlled, ie, the longer the pulse is “ON”, the faster the motor will rotate and likewise, the shorter the pulse is “ON” the slower the motor will rotate.In other words, the wider the pulse width, the more average voltage applied to the motor terminals, the stronger the magnetic flux inside the armature windings and the faster the motor will rotate and this is shown below.
2.Motor :
The motor is a dc motor having two terminals ,the rotor R and the case c which is fixed to a stationary frame .The motor produces the amount of torque required and gives it to the gearbox.
3.Gearbox,wheel axle and Mass
The gearbox input shaft is connected to the output of the motor and is given gear ratio of 10 to produce the required torque .The gearbox gives the required torque to the wheel axle which has the mass attached to it .
Results :
Torque produced at wheel axle for a mass of = 1000 kgs :
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