Transcript Thrust - Delta State University

Thrust
Lecture 8
Chapter 4
Thrust
• Thrust is the force that must be generated in
order to overcome the natural resistance of
drag.
• Because drag is the force opposite to the
flight path, thrust must be in the direction of
flight.
Thrust
• If thrust is increased greater than drag, the
speed would increase.
• Thrust and drag are equal in straight and
level, unaccelerated flight.
• The thrusting force must equal the retarding
forces in order to maintain a steady speed.
Newton’s three laws
• 1. A body in motion will remain in motion in
constant speed and direction until acted upon an
outside force; A body at rest will remain at rest.
• 2. Force equals mass times acceleration.
• 3. For every action there is an equal and opposite
reaction.
Principles of Propulsion
• Newton’s second law
• A force exerted on an object of so much
mass would accelerate it proportional to the
amount of force.
– F= m x a
– Force = mass times acceleration
– Acceleration is a rate of change of velocity or
change of velocity over time.
Force Velocity
• If a body were initially at rest, a force
would give it some velocity over a period of
time causing acceleration.
• Air has mass, a force applied to,would
accelerate and increase the velocity.
Propeller
• A propeller is wide in diameter & takes in a
relatively large mass of air per second.
• A reasonable amount of thrust can be
created with a small change in velocity
across the propeller plane & a high degree
of efficiency is maintained.
Jet Engines
• Jet engines have a relatively small diameter
& accommodate a smaller amount of air.
• In order to produce significant thrust, they
must accelerate this small mass to a much
larger velocity.
RAMJET
• Ramjet-{simplest form} a nozzle shaped
device in which the air is compressed by the
ram effect of moving through the air.
– Fuel is injected & ignited & the expanded gas
exhausts at high velocity.
– PRO: simple, handle high temps & high speed
– CON: must be in motion at high speed to be
started
PULSEJET
• Pulsejet {variation of ramjet} this engine has a
shutter-like check valve is installed in the air inlet
that works in sync. With the pulse injection of
fuel.
• The fuel is ignited & expansion forces the check
valve shut & a burst of gas out of the exhaust.
• Ram air then force the check valve open as the
internal pressure drops & cycle repeated.
PULSEJET
• Pulsejet required a forward speed in order to
start operation.
• German V-1 “buzz bombs” of WWII
• Usually launched by booster rockets
• Neither the ramjet nor the pulsejet are used
much in conventional aircraft propulsion.
TURBOJETS
• Turbojets were developed in 1930s by Sir
Frank Whittle of England.
• Hans von Ohain of Germany was also
working on a turbojet.
– The Messerschmitt Me-262, the first jetpowered aircraft.
– The Bell P-59, the first American jet aircraft
Commercial Transport
• The de Havilland Comet was the first
commercial jet aircraft, introduced in 1949.
– Then came Boeing 707 and DC 3.
Axial-Flow
• Incoming air is compressed by the compressor &
forced into the burner section
• Fuel is injected & ignited
• The burning gases rapidly expand & force out
through the exhaust
• Then it passes through the turbine, giving rotation
motion.
• Unused exhaust gas is expanded & pressure drops
• Acceleration of this mass of air is a thrusting force
against the engine
Pure Jet
• In a Pure Jet all of the air flows through the
combustion section & gets accelerated to
high velocity.
• Due to inefficiency the turbofan was
designed to increase efficiency but retain
most of the high-thrust ability of the
turbojet.
Turbofan
• In the turbofan the bypass air from the fan
provides the increased efficiency.
• It also derives part of its thrust from the jet
section, accelerates the air to a higher
degree & obtains a siginificant amount of
thrust.
• Turbofan has the larger bypass ratio for
more efficient powerplant.