Re: bypass ratios and engine efficiency

Date:         04 Jan 97 03:55:48 
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In article <airliners.1996.3047@ohare.Chicago.COM>, "P. Wezeman"
<> writes:

>   It is true that high bypass engines are more efficient at low speeds
>and low bypass engines are more efficient at high speeds. The new F-22
>fighter will have low bypass P&W F119 turbofans optimized for cruising at
>supersonic speed (mach 1.5?), and the Concorde's straight turbojets give
>very good propulsive efficiency at mach 2.2. However, the bypass ratio of
>any of the present high bypass engines is still too low to give optimum
>efficiency at the cruise speeds of subsonic airliners. To do so would
>require even larger ratios with bigger, heavier fans, less ground clearance,
>more chance of hoovering up foreign objects, perhaps require a reduction
>gear to drive the fan, and other complications that people would rather
>not deal with as long as fuel is cheap.

Do we want to continue this debate on the Concorde thread or the 757/767
thread?? (Over to you Karl!)

[Moderator's note: I vote for door #3 -- a new thread.  KLS]

When we talk about engine efficiency we need to be fairly precise as to what
we mean? Cycle efficiency, propulsive efficiency or overall efficiency,
 Cycle efficiency is related to the internal cycle of the engine and is
improved by high pressure ratios and high component efficiencies. Propulsive
efficiency is related to the speed of the aircraft and the speed of the
exhaust and is what most people focus on. Propulsive efficiency is given by
the ratio of work done in propelling the aircraft to the energy injected into
the airstream and can be reduced to the formula (2xV0)/(VJ+VO).

In order to get maximum efficiency out of an engine you therefore need to
make VJ=VO, which of course is quite useless as this gives you an engine with
zero thrust. Conclusion : you aren't looking for the most efficient engine.

What you are in fact looking for is the most efficient transportation system.
Simplistically this may considered to be the smallest aircraft to transport
the payload over the prescribed range. The shorter the range and the more
demanding the field performance (e.g. regional jets) the less significant
fuel efficiency becomes relative to engine weight and smaller bypass ratios
will tend to be optimum.

With respect to Concorde (other thread!) they would have needed a turbojet to
get adequate thrust at Mach 2 out of an engine with acceptable weight and
size. I guess however with todays technology that running to hotter
temperatures and with modern lighter materials that the optimum cycle would
in fact be a low bypass ratio turbofan.

Gerald Wilson