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From: mat@hou4b.UUCP
Newsgroups: net.aviation
Subject: Re:  New Engine Unveiled
Message-ID: <1177@hou4b.UUCP>
Date: Wed, 3-Oct-84 17:24:37 EDT
Article-I.D.: hou4b.1177
Posted: Wed Oct  3 17:24:37 1984
Date-Received: Thu, 4-Oct-84 03:47:56 EDT
References: <12388@sri-arpa.UUCP>
Organization: AT&T Information Systems Laboratories, Holmdel, NJ
Lines: 33

With regards to the counter-rotating unducted fan ...

	Yes, the two fans interact.  No, they don't interfere.  The
fans are not supposed to pull the air around and around ... they are
supposed to push it BACK (cuz thats what makes the plane go forward ...)
The counter-rotating fans reduce torque effects, which apparently are
more severe with the unducted fan than with conventional ducted fans.
The whole point of the ducts is to avoid all the interesting effects that
occur around the tips of prop blades, while still getting as much momentum
into the airstream as possible.

[Back to Freshman Physics]

	Note momentum, not energy.  Energy goes as the square of velocity, and
momentum as the first power.  Conservation of momentum holds -- so that going
moving twice as much air at half the speed will take half as much power.  The
speed in question is the difference between the speed (relative to the
aircraft) that the gasses come in at and the speed they leave at; when talking
about the energy, it is the differences in the squares of the speeds. (NB
Power is energy/unit time...)

	If instead of pushing exhaust gasses and propwash, you are pushing
against something that is (almost) infinitely massive (like the earth) all
of your energy goes into the thing that is moving.  It follows that as the
mass of what you are pushing against (or ejecting) goes down, the amount of
energy you lose in it goes up.  This is one advantage that fans have over
straight jets ... they move a greater mass, and so have to speed it up less
to get a given momentum out of it.
-- 

	from Mole End			Mark Terribile
		(scrape .. dig )	hou4b!mat
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