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

Commercial Transport The de Havilland Comet was the first commercial jet aircraft, introduced in –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.