How Jet thrust is created
for flying
1
• The aircraft is to be moved forward, forcing it
to run through still air at a high speed. Only
then necessary lift is created for it to fly. This is
a continuous requirement.
• This forward thrust for the aircraft comes from
one of the two sources: i) a rotating propeller
powered by an engine or, ii) a pure Jet engine.
2
Thrust requirement for aircraft
Lift
Drag
For level
Weight
flight
Lift = Weight
For an aircraft to fly Drag = Thrust
3
Differential form of thrust generation,
Thrust based on acceleration of mass
Thrust based on velocity change
in a time period
Acceleration, Velocity, Momentum and Force are vector
quantities. They all have specific magnitude & direction
4
Thrust needs to be created for all flight
regimes of the aircraft:
• Take-off – normally maximum thrust
• Climb – reducing from maximum
thrust
• Cruise – normally minimum thrust
• Manoevres – variable thrust
• Acceleration & Deceleration - variable
• Descend – Low thrust
• Landing – Just Less than maximum
thrust 5
• Thrust is a mechanical force which is generated
through the reaction of accelerating a mass of
gas, as explained by Newton's III Law of motion.
• A gas or air, used as a working fluid is
accelerated to the rear and the engine attached
to the aircraft are accelerated in the forward
direction.
• To accelerate the gas, we need some kind of
propulsion system. A propulsion system is an
energy consuming machine which accelerates a
gas/ air.
6
• But if we are dealing with a fluid (liquid or
gas) and particularly if we are dealing with
a moving fluid, keeping track of the mass
gets tricky. For a moving fluid, the most
important parameter is the mass flow rate.
• Since the mass flow rate already contains
the time dependence (mass/time), we can
express the rate of change of momentum
across the propulsion device as mass flow
rate times the change in the flow velocity.
7
A Jet Engine Schematic
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Jet Engine fundamentally is a Heat Engine
Jet
Thrust
Air
Waste
Heat
Combustion is the energy input in to the engine and is
key to the operation of a jet engine
9
Fundamental varieties of jet engine
Pure
Turbojet
Prop-jet
Unducted or
Ducted fan
10
The thrust equation is given by:
Fn = [( ṁ . V)e – (ṁ. V)a ]+ [(pe - pa).Ae]
Momentum Thrust Pressure Thrust
Normally, the magnitude of the pressure thrust is
small relative to the ṁxV term.
This is the uninstalled thrust of the engine
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A Bypass Jet engine
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Prof. Bhaskar Roy, Prof. A M Pradeep, Department of Aerospace, IIT Bombay
Lect - 2
A mixed flow bypass jet engine
13
Prof. Bhaskar Roy, Prof. A M Pradeep, Department of Aerospace, IIT Bombay
Lect - 2
A pure turbojet engine with afterburner (mixed flow)
14
Lect - 2
Mechanical Creation of Thrust by all components
15
Prof. Bhaskar Roy, Prof. A M Pradeep, Department of Aerospace, IIT Bombay
Lect - 2
Mechanical Forces created by various
components of a Jet engine
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Prof. Bhaskar Roy, Prof. A M Pradeep, Department of Aerospace, IIT Bombay
Lect - 2
Total and Static Pressures acting on various parts of a jet engine
17
Lect - 2
• All Mechanical loads (e.g. gas loads) are to
be borne by various components of the
engine. These components also transfer the
load to the structure of the engine. The
engine, rigidly attached to the aircraft, and
provides pull (thrust) for motion.
• The loads are variable and continuous
during all the operations of the engine.
• The load bearing components, e.g.
bearings, struts etc. need to be designed and
installed to withstand these continuous and
variable forces. Otherwise the engine is liable
to fail under mechanical loads.
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A high Bypass Turbofan Engine
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A modern very-low bypass engine
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