Miles,
Can you summarise specifically what the Sport Aviation findings were?
(rather than sending us all scurrying to a competitor magazine of yours!).
The NACA reports documenting the original research had three principal
findings related to exhaust improvements that resulted in overall drag
reductions (but none of them related to thrust recovery):
1. The cowl opening around the exhaust tailpipe should be as tight as
possible to minimise air leakage.
2. The amount of tailpipe protruding into the airstream should be as
short as possible or nothing (i.e. cut flush with surface of the cowl, with
suitable protection to the fuselage).
3. the pipe should be angled back, but for the purpose of reducing the
length of plume poking into and disrupting the airstream, not as a means of
thrust recovery. And this was determined on aircraft with 1000hp and more!.
Mention was made of using the exhaust energy to provide a venturi effect
around the cowl air exit, for the purpose of encouraging airflow through the
cowls and improving cooling.
That said, the Spitfire was well documented as gaining some extra speed with
exhaust stubs turned backwards; these days one might call that wartime
"spin".
Any turbo'd engine would already have had its share of exhaust energy
recovery. Don't be greedy David!
I would have thought that the optimum angle to present the tailpipe end to
the airstream in order to reduce backpressure was perpendicular (i.e.
exhaust pipe pointing straight down; must try experimenting by blowing
across one arm of a manometer.)
The automotive world has had its share of "exhaust thrust recovery" gizmos
over the decades; none taken up by OEMs or the racing fraternity. QED!
Duncan McF.
----- Original Message -----
From: "Miles McCallum" <milesm@avnet.co.uk>
Subject: Re: Europa-List: Exhaust Departure Angle
>
> Re exhaust thrust equations -- Look at the CAFE article in Sport Aviation
> from
> about Jan or Feb of 1997. However, they contain some pretty severe
> mistakes.
> Also look at sport aviation from a couple of months later for a page
> (colored
> yellow) that contains a response from Jim Cunningham providing corrections
> for their equations.
>
> M
> >
> >
> > Greetings All,
> >
> > I recently enjoyed reading Ken Paser's "Economy with Speed".
> > It is essentially a study of the effects of applying 30-70 year old
known
> > aeronautical design best practices (from NACA reports) to the
improvement
> > of a modern homebuilt. The results is a much awarded and honored
> > Busby Mustang 20 or 30 mph faster than others of its design.
> >
> > One technique involves canting the muffler discharge into the axis of
> > flight to both mitigate the drag of the exhaust plume and simultaneously
> > gain a small amount of thrust.
> >
> > Has anyone experimented with the Europa's exhaust? Best angle, etc?
> > I know you donj't want to go straight back to avoid melting or staining
> the
> > fuselage.
>
>
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