Re: Delta Ratio

From:         GWilson404@aol.com
Date:         01 Oct 96 23:56:38 
References:   1
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In article <airliners.1996.1916@ohare.Chicago.COM>, ehahn@mallard.mitre.org
(Ed Hahn) writes:

>I think GWilson is trying to say that the Delta ratio is the ratio of
>ambient pressure at a particular altitude to sea level pressure,
>independent of vehicle speed.  Thus, Delta can be tabulated on
>standard atmosphere tables.  Jet engine performance is a function of
>Delta.

My answer to the original question seems to have generated a lot of further
debate. As Robert Dorsett correctly pointed out delta and EPR are quite
different things and in fact delta like theta and sigma is merely a
normalising coefficient which allows you to correct back to sea level
conditions.

If anyone is still following the argument ,however, it may just be worth
pointing out that theta for jet engines nearly always relates to total
temperature at engine face i.e. always includes the kinetic temperature rise.
Sigma on the other hand always relates to static density conditions as total
density is a meaningless concept. Delta, however, can be used for either
static (ambient) pressure or total pressure (i.e. including the kinetic
pressure rise). thus at Mach 1 20000ft static delta is around 0.5 and total
delta is around 0.95. If it isn't clear from the context, you therefore need
to ask which type of delta is meant : however the total type of delta is
often denoted by a subscript T.....but you still need to be careful about the
intake pressure recovery assumed (or not)!

Bottom line is that Ed Hahn's comment is correct in respect of a delta used
to correct field performance data (as per the original question), but in
general delta does not necessarily ignore the speed term.

All clear now???? Well I guess those who actually need to use it knew anyway.

Now it really does get interesting when you have to consider real gas effects
in hypersonic flows (air breathing spaceplanes)..........

Gerald Wilson