reply to post by Yankee451
I guess the easiest thing to do, is to repeat what I posted in another thread....yes, I just copied it, rather than re-typing the whole thing
again:
~~~~~~~~~~~~~~~
The MAIN difference, from the 1970s and now? The engines.
Please research into the:
- Turbojet
- Low-bypass Turbofan
- High-bypass Turbofan
That is the order of technology development. The earliest "straight" turbojets, common mostly on fighters and only a few early commercial jets,
produced all of their thrust from the exhaust gases. The pattern of the airflows, and the efficiency of the fuel burning process was very
different.
And, they were gas-guzzlers.
Then, the Turbofan was developed....most iconic is the Pratt & Whitney JT-9D. Used on the Boeing 707, 727, and early 737. And the early DC-9. All
(low bypass) designs. Their contrails were more common than from the straight turbojet engines, but still wouldn't always form cohesively. Only
under a more narrow range of conditions would they persist for any time.
Larger jets, widebodies, as they were developed meant more powerful engine technology...and thus, the HIGH-bypass turbofan designs. The
majority...anywhere form 80 to about 90% and more of the thrust comes from the huge fan, out in front...acts just like a multi-bladed propeller. In a
shroud, which increases its efficiency.
These engines are very fuel efficient, and have different exhaust characteristics and final temperatures. (Cooler).
And, THAT is what makes the contrails formed by these engines much, much more prevalent, and longer lasting. In fact, in a way, it harkens back to
the days of WWII, and the era before jets, the 1950s....with the BIG piston engines, and big propellers. The props produced a column of air that
surrounded and enveloped the exhaust gases, much as the big turbofan fans do today. And, the exhaust temps of piston engines are lower than most
early jet engines...but, the newer jets are closer, nowadays. ALL of these factors combine...to include the retirement of the older types of engines,
AND the large increase in overall jet traffic as well.
Have you seen the examples from WWII? Was quite a problem for the Allies, late in the War, as they began daytime bombing raids into Axis territory.
At night, contrails can't be seen...but daytime, they were terrible as they drew unwanted attention:
See above, about this:
.
I know they don’t appear in any of the photos of my childhood and youth, not that they would always be there, but I do catch them in the
photos and videos I take these days; you can’t seem take a photo or video which includes a patch of sky without catching one.
There must be movies that include them.
I'm afraid it's that simple. Changing engine design, combined with MORE airplanes. And, selective memories in some cases, from people's personal
recollections of their pasts.
edit on 6 February 2011 by weedwhacker because: (no reason given)