Editing Turbofan (section)

===Efficiency===
[[File:Gas turbine efficiency.png|thumb|Propulsive efficiency comparison for various gas turbine engine configurations]]

[[Propeller (aeronautics)|Propeller]] engines are most efficient for low speeds, [[turbojet]] engines for high speeds, and turbofan engines between the two. Turbofans are the most efficient engines in the range of speeds from about {{convert|500|to|1000|km/h|kn mph|abbr= on}}, the speed at which most commercial aircraft operate.<ref name= grc_nasa>{{cite web|url= http://www.grc.nasa.gov/WWW/K-12/airplane/aturbf.html |title= Turbofan Engine |publisher= GRC NASA |access-date= 2010-11-24}}</ref><ref name="Neumann_2004_1984_pp228-230">{{cite book | last = Neumann | first = Gerhard | author-link = Gerhard Neumann | year = 2004 | orig-year = first published by Morrow 1984<!--removed excess additional detail--> | title = Herman the German: Just Lucky I Guess |publisher = Authorhouse | location = Bloomington, Indiana, US | isbn = 1-4184-7925-X | pages = 228–30}}</ref>

In a turbojet (zero-bypass) engine, the high temperature and high pressure exhaust gas is accelerated when it undergoes expansion through a [[propelling nozzle]] and produces all the thrust. The compressor absorbs the mechanical power produced by the turbine. In a bypass design, extra turbines drive a [[ducted fan]] that accelerates air rearward from the front of the engine. In a high-bypass design, the ducted fan and nozzle produce most of the thrust. Turbofans are closely related to [[turboprop]]s in principle because both transfer some of the gas turbine's gas power, using extra machinery, to a bypass stream leaving less for the hot nozzle to convert to kinetic energy. Turbofans represent an intermediate stage between [[turbojet]]s, which derive all their thrust from exhaust gases, and turbo-props which derive minimal thrust from exhaust gases (typically 10% or less).<ref name= srm>[http://www.srmuniv.ac.in/downloads/turbofan-2012.pdf "The turbofan engine"] {{Webarchive|url=https://web.archive.org/web/20150418181832/http://www.srmuniv.ac.in/downloads/turbofan-2012.pdf |date= 2015-04-18}}, p. 7. [[SRM Institute of Science and Technology]], Department of Aerospace Engineering.</ref> Extracting shaft power and transferring it to a bypass stream introduces extra losses which are more than made up by the improved propulsive efficiency. The turboprop at its best flight speed gives significant fuel savings over a turbojet even though an extra turbine, a gearbox and a propeller are added to the turbojet's low-loss propelling nozzle.<ref>{{cite book | title = Gas Turbine Theory | edition = 2nd | last1 = Cohen | last2 = Rogers | last3 = Saravanamuttoo | publisher = Longmans | year = 1972 |isbn = 0-582-44927-8 |page = 85}}</ref> The turbofan has additional losses from its greater number of compressor stages/blades, fan and bypass duct.{{clarify|date=May 2021}}

Froude, or propulsive, efficiency can be defined as:

<math display="block">\eta_f = \frac {2}{1 + \frac {V_j}{V_a}}</math>

where:
* {{math|{{var|V}}{{sub|{{var|j}}}}}} = thrust equivalent jet velocity
* {{math|{{var|V}}{{sub|{{var|a}}}}}} = aircraft velocity