Anders Troberg wrote:
That's true, but in terms of thrust forces, it doesn't matter. Once that air is inside the intake tube, it is ALL pulled inward in a single direction - just as the air is ejected from a tailpipe.
Yep, but where is the acceleration of the incoming air taking place? Inside the intake or outside it? Since the intake usually is more or less the same cross section area all the way, I would guess that most of the acceleration is done outside, while the air still is sucked from all directions.
Well, in thinking this over, I have to mostly agree with you. What I missed was that obviously, the largest part of the acceleration of the intake mass is where we get it moving into
the intake, and that is essentially omnidirectional. Eric has noted on his "Advanced FWE" that the flare size and shape are fairly critical, which obviously has to do with optimizing this action. While the Bernoulli
effect on the air speed takes place as soon as we get inside just past the flare, there is still some acceleration in the straight pipe during the early part of the inflowing gust, though, due to the gradually decreasing chamber pressure, and that's what I was thinking of.
The combined effect of this is a jetting action into the chamber, which is a negative thrust effect (if the intake pipe is aimed rearward a la
Chinese / Thermojet / FWE), and of course, the same thing happens later on at the tailpipe. What we have is a "momentum rectifier" rather than a "flow rectifier" intake. Nothing wrong with that!
It is really amazing that Eric was able to show how this could be powerfully optimized to form a "negative thrust" engine. As I said before, the thing to do is to optimize the negative thrust effect at the front end of a linear form factor engine. Practically speaking, it beats adding a recuperator hands down. The desirable qualities of this would probably disappear at high forward speed, however, unless we provided some variable-geometry damping or some other unsavory complication (but maybe just a streamlined cap would be enough).