From: geoffm@purplehaze.Corp.Sun.COM (Geoff Miller)
Newsgroups: sci.aeronautics
Subject: Re: Augmenting cooling by exaust pipe placement
Date: 16 Apr 92 18:12:04 GMT
Followup-To: sci.aeronautics
Organization: Sun Microsystems, Menlo Park, Ca.
NNTP-Posting-Host: purplehaze.corp.sun.com



In article <1992Apr16.143028.26700@dartvax.dartmouth.edu>
Charles.K.Scott@dartmouth.edu (Charles K. Scott) writes:

>What the guy was saying was that if you placed the exaust pipes so that
>they terminated slightly inside a carefully designed cowl, the exiting
>gas pulses would carry the air surrounding the pipes with it and drag
>it outside the cowl.  This created low pressure at the tail end of the
>engine compartment which would help draw the air over the cylinders at
>low speeds or when the aircraft was on the ground.

This idea was used on the Convair 240/340/440 series of airliners back
in the Fifties.  The exhaust pipes terminated inside the cowling, just
in front of a pair of trumpet-shaped devices called augmentors; there
were two per engine.  This setup obviated the need for cowl flaps.  If
you've ever seen a Convair-Liner that hasn't been converted to turboprop
engines (540/580/600), you might've noticed how smooth the cowlings are
compared to those on contemporary machines like the DC-6 (which uses the
same engine, incidentally, the P&W R-2800).

------

>This seems pretty logical to me, but I'm not an engineer.  What do you
>guys think?  Also, I'm not intending to use an air cooled engine but
>I'm thinking that the same technique should work for a radiator as
>well.  The added thrust the guy was talking about was from the exaust
>being directed back towards the tail instead of straight down as is the
>case with lots of lightplanes.

Can't see any reason why it shouldn't work.  As far as extra thrust is
concerned, the Convairs were able to achieve this with the above design.
Not that it was like strapping on a JATO rocket or anything, of course,
but it makes sense to take advantage of any extra energy when you can 
instead of simply dumping it overboard.

------

>I'm also thinking that I should probably have some sort of cowl flap
>that could be lowered (deflected) into the airstream to increase the
>negative pressure inside the radiator tunnel.  In my mind, I'm placing
>the cowl flap to the rear of the radiator and just before the
>terminated exaust pipes which I'm picturing to be at the firewall.

Seems to me that if the whole thing is designed right, you wouldn't 
have to pay the drag, weight and complication penalties of the cowl
flap mechanism.  But if you insist on the flap, it would do more good
*aft* of the augmentors than in front of or beside them.  Your objective
would be to reduce pressure downstream of the terminated exhause pipes
so as to encourage airflow.  If the flap was set up as you describe, it
would work against the tendency of the exhaust and ambient air to flow
into the augmentors, because it would lower the pressure in front of
them.

------

>So I actually have two questions I guess.  Is the idea sound?  And is
>it possible it would help with a liquid cooled engine (the cooling help
>is mostly for taxing and hot air climbout).

The basic concept is demonstrably sound.  But you never specified whether
you were thinking in terms of a sigle- or multiengined aircraft.  If
you're talking about a single, then the extra room required for this 
gear behind the engine will move the engine forward, which would change
its moment arm.


Geoff


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Geoff Miller                    + + + + + + + +        Sun Microsystems
geoffm@purplehaze.Corp.Sun.COM  + + + + + + + +     Menlo Park, California
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