Inlet cones - Definition and Overview

Introduction

Inlet cones (sometimes called shock cones) are a component of some supersonic aircraft. They are primarily used on ramjets, such as the turboramjets of the SR-71 or the pure ramjets of the D-21 Tagboard and Lockheed X-7. More examples of inlet cones can be found on the Su-7 Fitter and the MiG-21 Fishbed, both of which use conventional jet engines.

Purpose

The main purpose of an inlet cone is to slow down the flow of air to subsonic speeds before it enters the engine. Most engines require subsonic airflow to operate properly, and require a diffuser to prevent supersonic airflow inside the engine. At supersonic speeds, a shock wave forms on the leading edges an aircraft, causing significant pressure and drag. The shock wave has the added effect of slowing down the air that passes through it. The inlet cone is shaped so that the shock wave that forms on its point is directed to the lip of the intake; this allows the engine to operate properly in supersonic flight. As speed increases, the shock wave becomes increasingly more narrow. As a result, some inlet cones have the ability to move front-to-back to allow operation in a wider range of speeds.

Inlet cones on turbojets

Shock waves may also cause the compressor blades of a conventional jet engine to melt or shatter. Inlet cones prevent shock waves from forming inside the engine. Inlet cones are not used with turbojets anymore; they have been replaced by rectangular inlets, which serve the same purpose as inlet cones, but with a simpler construction.

Inlet cones on ramjets

When used on ramjets, the inlet cone has the additional purpose of compressing air by forcing the air through an increasingly small space. This is called ram air compression. It does the same thing as the compressor blades of a turbojet.

References

• Benson, T. (2004). High Speed Aerodynamics Index (http://www.grc.nasa.gov/WWW/K-12/airplane/shortc.html). Retrieved Nov. 19, 2004.

• Eden, P. & Moeng, S. (2002). Modern Military Aircraft Anatomy. Aerospace Publishing Ltd. ISBN 1-58663-684-7.

• Scott, J. (May 9, 2004) Turboramjet (http://www.aerospaceweb.org/question/propulsion/q0175.shtml). Retrieved Nov. 19, 2004.