Thrust Lecture 8 Chapter 4 Thrust Thrust is

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 1930 s 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.