Talk:Bypass ratio

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[edit] 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))

[edit] 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)