Jet engine performance: Difference between revisions

Revision as of 02:58, 19 November 2023

Revision as of 09:47, 19 November 2023

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==Performance deterioration==

[[File:Pratt & Whitney JT8D-17A 1.JPG|The [[Pratt & Whitney JT8D]] has a full length fan duct which is a rigid case construction which resists inlet air loads during aircraft rotation. Compared to the later JT9D it has relatively loose clearances between rotating and stationary parts so blade tip rubs as a source of performance deterioration were not an issue.<ref>https://ntrs.nasa.gov/citations/19810022654,"Aircraft Engine Diagnostics", JT-8D Engine Performance Retention, p. 66</ref>]]

[[File:P&W JT9D cutaway.jpg|right|The Pratt & Whitney JT9D with a big increase in thrust over the JT8D raised awareness how to transfer engine thrust to the aircraft without bending the engine too much and causing rubs and performance deterioration.<ref>Flight International, 13 November 1969, p. 749</ref>.]]

Gas path deterioration and increasing EGT coexist. As the gas path deteriorates the EGT limit ultimately prevents the take-off thrust from being achieved and the engine has to be repaired.<ref>Aircraft Engine Diagnostics, NASA CP 2190, 1981, JT8D Engine Performance Retention, p. 64</ref>

The engine performance deteriorates with use as parts wear, meaning the engine has to use more fuel to get the required thrust. A new engine starts with a reserve of performance which is gradually eroded. The reserve is known as its temperature margin and is seen by a pilot as the EGT margin. For a new [[CFM International CFM56]]-3 the margin is 53&nbsp;°C.<ref>https:smart cockpit.com, CFM Flight Operations Support, page 37</ref><ref>Performance of the Jet Transport Airplane, Young 2018, {{ISBN|9781118534779}}, Fig 8.19</ref> Kraus<ref>https://reposit.haw-hamburg.de/handle/20.500.12738/5576,"Further investigation of engine performance loss, in particular exhaust gas temperature margin, in the CF6-80C2 jet engine and recommendations for test cell modifications to record additional criteria, Tables 2.1–2.4</ref> gives the effect on increased fuel consumption of typical component degredationdegradation during service.

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[[File:Pratt & Whitney JT8D-17A 1.JPG|The [[Pratt & Whitney JT8D]] has a full length fan duct which is a rigid case construction which resists inlet air loads during aircraft rotation. Compared to the later JT9D it has relatively loose clearances between rotating and stationary parts so blade tip rubs as a source of performance deterioration were not an issue.<ref>https://ntrs.nasa.gov/citations/19810022654,"Aircraft Engine Diagnostics", JT-8D Engine Performance Retention, p. 66</ref>]]

[[File:P&W JT9D cutaway.jpg|right|The Pratt & Whitney JT9D with a big increase in thrust over the JT8D raised awareness how to transfer engine thrust to the aircraft without bending the engine too much and causing rubs and performance deterioration.<ref>Flight International, 13 November 1969, p. 749</ref>.]]

File:Silnik Lis-2 - przekrój - Muzeum Nauki i Techniki Warszawa.jpg|[[Klimov VK-1]] centrifugal impeller showing that the blades have rubbed on the shroud causing increased clearance and leakage losses.

File:Marbore IV.jpg|[[Turbomeca Marboré]] IV engine showing ___location of leakage between impeller blades and stationary shroud, shown sectioned and painted blue. This is the leak path for a centrifugal impeller equivalent to an axial blade tip to casing clearance.<ref>AGARD CP 237 'Gas Path Sealing in Turbine Engines', Figure 6(a)</ref> The clearance between the impeller vanes and their shroud is visible and has to be as small as possible without causing rubbing contact. This keeps leakage to a minimum and contributes to the efficiency of the engine.