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School of Aerospace, Mechanical and Manufacturing Engineering

Part C Learning Guide INTRODUCTION TO AIRCRAFT

Lecture 5 A

Introduction to Aircraft Propulsion II

March 2013

Introduction to Aircraft

1

School of Aerospace, Mechanical and Manufacturing Engineering

Lecture 5A: Introduction to Aircraft Propulsion II
Introduction
History of Gas Turbines Since we are talking about turbines, there is an intimate relationship with turbo-machinery, hence the first important aspect to the history of gas turbines in the invention of the modern centrifugal pump by Appold in 1851. Then in 1902 Renault patented the centrifugal supercharger. This marks the application of turbo-machinery to engines. In 1908 Lorin proposed the forerunner of the motorjet (we will see this shortly), where air flow from behind the propeller was directed to a combustion chamber to produce jet thrust at the rear of an aircraft. Next in 1917 Morize (in France) proposes and Harris (in England) patents the motorjet. In a motorjet a conventional engine is used to drive a compressor which pumps compressed air into a combustion chamber, where fuel is added and then burnt to produce jet thrust at the rear of the aircraft. Finally, as we saw in Week 2 (History of Aviation), in 1930 Sir Frank Whittle patents the centrifugal flow turbojet. The design of Whittle is a direct evolution of the previous turbomachinery developments, and their application to aircraft. In it, he uses a Centrifugal compressor to achieve the compression ratio that Otto knew we needed inside an engine to make it more efficient. Lecture 5 A Introduction to Aircraft Propulsion II March 2013

Introduction to Aircraft

2

School of Aerospace, Mechanical and Manufacturing Engineering

Jet Propulsion Concept To understand the basic physics behind jet propulsion consider an enclosed cylinder. The “fluid” inside the cylinder is at a higher pressure than the “fluid” outside the cylinder. That is, P1 > P0. The pressure difference is balanced by the stress within the pressure vessel. No think what would happen if I instantly cut this cylinder in half. The pressure on the inside would cause the “fluid” within the pressure vessel to push out against the outside pressure, over the area of the cylinder. However, these two pressures are not equal, and the resultant is that the fluid will flow out with a velocity v, and from Newton’s Third Law, we know this excess pressure over the area, will gives us a force, which will be equal and opposite to the thrust of the cylinder, which will accelerate it. That is,

F A F AP = A A F = AP = A( P − P0 ) 1 P=
Now if we had some way to pump fluid at the original pressure into the pressure vessel we could sustain this thrust. This principle can be applied to a “jet ski” or a “jet aircraft”. The simplest example is that of a balloon. Definition Why do we use the term “Turbojet” when talking about the quintessential “jet engine” used on aircraft? Why the prefix “turbo”? This relates to turbo machinery. In turbo-machinery two things are possible, rotational energy can be converted into fluid energy, or conversely fluid energy can be converted into rotational energy. Why “jet”? Simple, we are utilizing the jet propulsion concept, where a fluid is forcefully propelled from a nozzle as we saw before. Hence we have a fluid “jet”. Lecture 5 A Introduction to Aircraft Propulsion II March 2013

Introduction to Aircraft

3

School of Aerospace, Mechanical and Manufacturing Engineering

Types of Gas Turbines Turbojets: There are two general subcategories of turbojets. These are: 1. Centrifugal flow turbojets, where the gas flows radially outwards relative to the line of the thrust, and 2. Axial flow turbojets, where the gas flows straight through the engine along the line of the thrust. There are advantages and disadvantages of both. The first practical design was centrifugal, while all large modern designs are axial. For axial flow turbo get...
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