Thrust

1
Thrust

• Lecture 8
• Chapter 4

2
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.

3
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.

4
Newtons 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.

5
Principles of Propulsion

• Newtons 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.

6
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.

7
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.

8
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.

9
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

10
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.

11
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.

12
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 jet-powered
  aircraft.
• The Bell P-59, the first American jet aircraft

13
Commercial Transport

• The de Havilland Comet was the first commercial
  jet aircraft, introduced in 1949.
• Then came Boeing 707 and DC 3.

14
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

15
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.

16
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.