Re: Thrust/Horsepower of modern engines

Date:         27 Aug 99 03:08:03 
From:         "P. Wezeman" <pwezeman@blue.weeg.uiowa.edu>
Organization: The University of Iowa
References:   1
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On 7 Aug 1999, Don Stauffer wrote:

> I am working on a book on internal combustion engines ( I am including
> turbines as ICs).  I have a chapter of comparisons of modern engines for
> various applications.  Horsepower figures into many of the comparisons.
> I wish to use thrust horsepower (thrust times velocity) for the
> turbojets/turbofans.
>
> Can I use max thrust and cruise velocity together?  That is, can modern
> turbofans develop max thrust at normal cruise speeds?

	No, they cannot: the maximum thrust is maximum take-off thrust,
and is produced only at low speeds. For a typical modern turbofan with
a bypass ratio of about 5 to 1 the maximum cruise thrust is about
two thirds take-off thrust. The power delivered to the jet efflux is
the same at both speeds, but since kinetic energy increases with the
square of speed, for a given power the change in velocity for the air
flowing through the engine, and hence the thrust, is greater at
low speed. Some of the newer engines such as those used on the Boeing
777 have bypass ratios of 8 or 10 to 1, and the reduction in thrust
at cruise speed would be greater. Conversely, for a straight turbojet,
static thrust and cruise thrust are about the same.
	There is another consideration if you want to give the actual
thrust horsepower produced in airline use. Subsonic jetliners, with
flaps and slats retracted, are designed to fly at a maximum indicated
airspeed of about 250 knots. They reach their cruise true airspeeds
of mach .75 to .85 at their cruising altitude of 35,000-45,000 feet,
where air density is one fourth or one fifth of what it is at sea level,
and engine thrust is reduced proportionately. Actual cruise thrust is
about one fifteenth or one eighteenth of the aircrafts weight, the ratio
being the same as the aircraft's lift to drag ratio. At some point in
the flight the thrust horsepower will be at a maximum, I suspect at
a fairly low altitude.

                        Peter Wezeman, anti-social Darwinist

                             "Carpe Cyprinidae"