Re: Delta Ratio

From: (Robert Dorsett)
Organization: Netcom Online Communications Services (408-241-9760 login: guest)
Date:         18 Sep 96 13:52:14 
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In article <airliners.1996.1889@ohare.Chicago.COM> (MikeM727) writes:
>In article <airliners.1996.1875@ohare.Chicago.COM>,
>>Delta is sometimes used to mean the ratio of either ambient or engine
>>total pressure to sea level static standard pressure, 1013.25mb or
>>14.69psia. At constant temperature the thrust of a jet engine is directly
>>proportional to this value , which therefore has to be taken into account
>>in field performance calculations.
>This sounds to me more like a description of EPR, Engine Pressure Ratio.
>I've never heard EPR referred to as "Delta".  Of course the term "delta"
>can be used to mean  any differential.

Delta is a correction constant.  It is the ratio of an ambient value to
a normalized value, usually sea level in one standardized atmosphere
definition or another.  There are other such constants, such as theta
(temperature) and sigma (density).

These constants are used extensively in a variety of aerodynamic calculations.
The idea is to be able to perform tests in an arbitrary configuration, then
to "normalize" the results and thereby be able to potentially extrapolate
the results to *other* arbitrary configurations.

As a trivial example, consider the airspeed indicator, which yields
airspeed performance compared to sea level.  It will only show true airspeed
AT sea level, in the conditions the airspeed indicator was calibrated for.
To come up with true airspeed, you would use the equivalent airspeed
(indicated airspeed corrected for compression and measurement effects)
and divide it by the square root of sigma, the density ratio.

Engine Pressure Ratio is completely different.  It's the ratio of the
air pressure at engine exhaust to engine input.  It shows engine performance
in the here and now, normalized in a format such that pilots can easily
understand.  Look at it this way: the correction constants help define
the real-world *limits* for EPR settings.

Flight crews rarely have to deal with any type of correction constants.

Robert Dorsett                         Moderator, sci.aeronautics.simulation