Re: Those swinging engine pods!

From:         drinkard@bcstec.ca.boeing.com (Terrell D. Drinkard)
Organization: Boeing Commercial Airplane Group
Date:         30 Mar 94 00:06:43 PST
References:   1 2 3
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In article <airliners.1994.1061@orchard.chicago.com>,
Dickey Bradley F <bdickey@cc.brynmawr.edu> wrote:
>I remember the Business Week article over the summer about this little
>problem (It was mentioned on this group, so I daresay the archives 
>should list the date, etc - I know it was during June/July 93 sometime..)
>The article mentioned that this was bad, particularly if the inboard
>engines went missing, since they tend to  take the outboard engines with 
>them.

Only half true.  If the #3 engine tears loose, the gyroscopic force
(assuming the engine is still turning) can cause it to impact the #4
engine.  This is not true of the #2 engine.

>The question is, if an engine falls off a plane during flight, is that
>an automatic downer for the aircraft, or are there certain admissible
>scenarios?  If an outboard engine falls off a 747 in flight, can the
>aircraft recover?

I, for one, can think of no plausible scenario wherein it would be
permissible to have an engine depart the aircraft without immediately
landing.  The 747 can be safely flown with as many as two engines gone.
The real question is what caused the engines to depart the airframe?  An
engine failure, such as a siezed engine, can be recovered without undue
problems (I dare not say it is easy - the pilot community would skin me
alive!).

>Shortly after reading the Business Week article, I made personal note
>of the 'swaying engine pods' phenomenon on a PIA flight to Lahore.  It 
>really is rather disconcerting if you don't know what is going on.  Does
>anyone know what degree of freedom they have - is it just in a single
>dimension perpendicular to the fuselage longtitudinal axis, or can they
>move back and forth as well?  Are there any clean functions that describe
>the movement, or is it erratic?

If memory serves, the engine/nacelle has a couple of hundred degrees of
freedom, nearly all of which are highly damped and quite small to begin
with.  The primary engine/nacelle mode is a 'hump' mode, which looks like
it is bouncing up and down on the strut.  This is relatively lightly damped
and is perfectly regular.  I don't do the code to predict flutter, but I
believe it to be rather clean; therefore, I would expect the movement to be
easily described.

If by 'back and forth' you mean can the engines move longitudinally, the
answer would be no.  That is the thrust line, and you could expect
something in the vicinity of 8,000lb thrust per engine even at cruise.

Terry

-- 
Terry
drinkard@bcstec.ca.boeing.com
"Anyone who thinks they can hold the company responsible for what I say has
more lawyers than sense."