Re: Are Two Engine 757 & 767 Jets Dangerous?

Date:         27 Dec 96 13:32:18 
From:         "P. Wezeman" <pwezeman@blue.weeg.uiowa.edu>
Organization: The University of Iowa
References:   1 2 3 4 5 6
Followups:    1
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On 12 Dec 1996, Jan Polcher wrote:

> I was told once that low by-pass engines are more efficient at cruising
> speeds while the high by-pass engines have an advantage in the low speed
> sections of the flight. Is there hard evidence for this ?
>
   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.

                        Peter Wezeman, anti-social Darwinist

                             "Carpe Cyprinidae"