Talk:Bypass ratio

Regarding introduction

My hat off to the author.

A very good exposition except at the point where you might put the uninitiated off which was where the fundamental principle had to be conveyed.

You know your stuff. But as my historiography professor said, 'do not assume too much technical knowledge on the part of your reader."Mark Lincoln 02:59, 21 July 2007 (UTC)

Maybe someone can improve the phrasing, but it seems pretty good considering this is an article about a technical detail of the engines. Simpler material is in the turbofan article. (SEWilco 03:32, 27 October 2007 (UTC))

Description???

Isn't this bit completely wrong? The problem isn't avoiding adding energy to the engine and burning out the turbine, the problem is that air is coming too fast out the back of a turbojet for subsonic aircraft, and you want to slow it down- if anything you want to *lean* out the engine, because the air is taking away energy; there's no point in having a jet going out the back at mach 2 if you're only flying at mach 0.85; the air just ends up going backwards wayyy too fast. But it's difficult to lean out the engine too much, because you hit minimum combustion limits.WolfKeeper 03:28, 16 November 2007 (UTC)

I also am somewhat confused by the description in the arcticle. My turbine propulsion prof gave me the impression that bypass ratios are chosen to maximize fuel consumption at the desired cruising speed. Higher ratios give better specific fuel consumption (SFC), but at the cost of exhaust velocity. Here's the break-down: thrust is equal to exhaust velocity times mass frow rate; since turbofans consume something like 20 times the amount of air as it does fuel, it can be assumed that the mass flow is mostly air-- therefor, air breathing jet engine thrust is equal to mass flow rate times the difference in velocity of the aircraft and exhaust. Or: F=M*(E-V), where F is thrust, M is mass flow rate, E is exhaust velocity, and V is free stream velocity (forgive my use of improper symbols, I don't know how to use subscripts here). So, in the most basic sense for a given amount of fuel consumed, an engine designer can trade exhaust velocity for mass flow rate; the higher the bypass ratio the higher the M, but the lower the E. If your plane needs to go rather fast, then E needs to be particularly high (always greater then V). If your plane doesn't need to go fast, then you can trade off E for more M by increasing the bypass ratio, as this has the the benefit of better SFC. "Lean out" the engine is perhaps not the best of terms, but basically Wolfkeeper is on the right track: higher then necessary exhaust velocity is wasted fuel. You want to push the pypass ratio as high as possible to maximize SFC. (In contrast to commercial engines are powerplants of fighters designed for "super-cruise". These engines have very low (or non-existent) bypass ratios to keep the exhaust velocity as high as possible, thus maximizing thrust at high speed. Though this penalizes SFC, it's still better fuel economy then an afterburner (i.e. reheat). An F-15, for instance, has relatively high bypass ratio engines compared to an F-22. This results in the F-15 probably having better SFC at subsonic speeds, but at supersonic speeds the F-22 has better SFC since the F-15 must use it's burners.) Nwilde (talk) 16:55, 10 May 2008 (UTC)

OK, having just read the Turbofan article, I'll stick by what I remember from school and endorse Wolfkeeper's orginal comment: this article is "completely wrong".Nwilde (talk) 17:01, 10 May 2008 (UTC)

Fix

This is the most poorly-written article I have ever read on Wikipedia. I'm an aerospace engineer, and I can't even understand it. In every sentence, the author arranges his points in the worst possible order and then loads each sentence with unnecessary transitions like "hence" and "would imply". These mask what is otherwise a fifth-grade writing ability. It needs entirely rewritten.

Szuhay (talk) 14:42, 7 January 2010 (UTC)

Exhaust Velocity

Does anyone agree with me that this statement "Lower bypass ratios are appropriate at high speeds because the exhaust velocity must exceed the airspeed to give forward net thrust" is flatly wrong? Thrust is derived from accelerating air and regardless of how much you accelerate it it provides some thrust. There is no "magic number" like the velocity of the aircraft whereby you start getting net positive thrust. I'm a mechanical engineer and I do think this article has problems. It does get the fundamental point of why bypass is important, i.e. It decouples fuel combustion stoichiometry from mass-flow rate, and that momemtum is linear while kinetic energy is a square relationship. But the description here is not what I would expect to see in an engineering text. Allowing that this is an attempt to explain the concept to laymen, it should also give a greater degree of technical discussion for those with engineering or scientific training to better grasp the concepts.Pmarshal (talk) 02:39, 14 December 2011 (UTC)

The standard reference frame used to analyse jet engines is the fairly obvious one where the engine is stationary. In this frame of reference the air approaches the engine at the airspeed, but leaves the engine at the effective exhaust velocity. If the exhaust velocity is lower than the airspeed, then the air has slowed down through the engine and the engine exerts a net drag.GliderMaven (talk) 13:56, 28 December 2011 (UTC)

I understand what you are saying. Depending on whether you have a stationary or moving frame of reference, the exhaust velocity has two different values. I am referring to a either a stationary frame where the exhaust is moving "backwards" in relation to motion at some speed greater than zero, or in a moving frame where the scalar speed is greater than the airspeed opposite to the direction of travel. I can't actually think of any examples in current engines where exhaust velocities are ever "negative" using these criteria. This is more of a concern with ramjets where inlet drag can easily exceed thrust (therefore providing a net drag not a net thrust). I'm more concerned about the confusion this wording caused (since I have already removed it) rather than it's precise technical accuracy, because of the ambiguity of stationary/moving reference frame implications. If you are more up on supersonic engine performance than I am (which is likely) you might want to add a discussion of inlet drag and the conversion of supersonic external flows to subsonic internal flow.Pmarshal (talk) 20:07, 9 January 2012 (UTC)

Is this article redundant?

I have modified the general description for this article, but there is a considerable amount of overlap between the articles for turbofan and axial compressor. All three essentially are discussing the same thing. Should these articles be merged?Pmarshal (talk) 23:24, 14 December 2011 (UTC)

Re-written most of article, added reference

I have re-written most of the article in a style more appropriate for an engineering topic. I have left the last portion as-is since I have no source material for this and I do not know the origin of the tabular data.Pmarshal (talk) 08:32, 27 December 2011 (UTC)

This paper describes why turbofans are better than turbojets. The formula on page 507 is particularly important for this article. Someone with formula skills should put it in. TGCP (talk) 16:47, 21 July 2012 (UTC)

Sources

This article seems pretty skimpy on sources. There are plenty of good textbooks out there that have good explanations of what bypass ratio is and qualitative / mathematical explanations of its effects on performance. Books by Mattingly (Elements of Propulsion or Aircraft Engine Design) come to mind. SkycraftAero (talk) 12:40, 17 July 2013 (UTC)

reasons for deletions

If P&W and GE developed most of the very large bypass engines in the US who did the rest and what were they? How did the US not best the UK in engine design until the advent of the CF6/JT9D? eg S. Hooker seems to come across as bested in 1944 when on a visit to GE To my concern I found they were running a 4000lb engine and to add to my dismay also a 4000lb axial turbojet designed by Alan Howard(Not much of an Engineer). I don't see any particular significance in "besting" or firsts but just point this out because these kind of claims need a fair bit of research. How does no bpr reduce intake ramp aerodynamic drag? No explanation. No cite. RR started development of the high BPR turbofan Does "the" really mean "the"? in which case Gunston says Recognising the impact the TF39 engine would have RR began detailed investigation of high bpr engines so RR started "a" not "the" Who cares who started? but a good reason to stay away from claims like this without reliable sources.Pieter1963 (talk) 12:37, 26 June 2015 (UTC)

concise definition replaced with homemade version

The definition for bpr was recently copied from article turbofan. It is concise/unambiguous and able to be cited from numerous sources. It has been replaced with a DIY version which has a couple of poorly-chosen words, disk and un-combusted. Disk is usually used to mean the solid hub the blades are attached to, so air flows through the fan, or fan blades, not the fan disk.

uncombusted. The air flowing through the fan is air. Keep it simple. The added word is not only redundant, it is also possibly ambiguous, ie excludes all afterburning engines (where fan air is 'combusted') from the definition.Pieter1963 (talk) 19:53, 14 July 2015 (UTC)

My negative feedback on the Principles section

I present the chunks of the 1st paragraph of the Principles section in the left hand column below, with my comments in the right hand column:

Caption text

Orginal Text	My comments
If all the gas power from a gas turbine is converted to kinetic energy in a propelling nozzle, the aircraft is best suited to high supersonic speeds	Abstract and nuanced analysis for somebody who knows how it already works. Meaningless for someone learning
If it is all transferred to a separate big mass of air with low kinetic energy, the aircraft is best suited to zero speed (hovering)	This could be interpreted to mean a larger slow jet engine core is better. Not what he meant. Also, it's probably lower kinetic energy per mass of air, but more air, so not exactly "low kinetic energy" overall. But we haven't defined what exactly it is that has lower kinetic energy.
For speeds in between, the gas power is shared between a separate airstream and the gas turbine's own nozzle flow in a proportion which gives the aircraft performance required	He's talking about the separate stream without 1st defining where or what it is here.
The first jet aircraft were subsonic and the poor suitability of the propelling nozzle for these speeds due to high fuel consumption was understood, and bypass proposed, as early as 1936 (U.K. Patent 471,368).	Why are we giving a history in the principles section?
The underlying principle behind bypass is trading exhaust velocity for extra mass flow which still gives the required thrust but uses less fuel	Again, this could be interpreted to mean a larger slow jet engine core is better. Not what he meant. Also, we're talking about the benefits of the strategy without defining what the strategy is.
Frank Whittle called it "gearing down the flow".[7] Power is transferred from the gas generator to an extra mass of air, i.e. a bigger diameter propelling jet, moving more slowly	There is no "gas generator" in the diagrams
The bypass spreads the available mechanical power across more air to reduce the velocity of the jet.[8] The trade-off between mass flow and velocity is also seen with propellers and helicopter rotors by comparing disc loading and power loading.[9] For example, the same helicopter weight can be supported by a high power engine and small diameter rotor or, for less fuel, a lower power engine and bigger rotor with lower velocity through the rotor."	This would all be fine once the bypass concept is defined.

I would add an introductory sentence to the Principles section (or the Intro section?) as follows:

″Looking at the diagram, a bypass jet engine takes energy from the low pressure turbine (being pushed by the burning/expanding gases from the combustion chamber) and transmits that energy via the low pressure drive shaft to the front "fan". The front fan pushes a large volume of air through bypass channel around the engine core . Pushing this large mass of air helps provides an additional propelling force, in addition the thrust at the rear nozzle from the hot expanding gases" — Preceding unsigned comment added by BenjaminGSlade (talk • contribs) 03:29, 14 October 2021 (UTC)