Re: Vee engines

["Rich Wagner" <drsavage@domain.elided>]

Godfrey wrote:

>Radial aircraft engines are typically not high rpm engines: the 
>articulated connecting rods weigh more than the individual rods and 
>longer crank throws which limits rpm, and they are more complex 
>mechanically. They are designed for long running at relatively constant 
>rpm and make power through supercharging and displacement, not through 
>rpm.

Hold the phone!!!  Radial engine articulating rods (proper term) are
exceptionally light.  Have you ever hefted one?  To use the R3350 as an
example, the engine develops about 2,700 HP at 3,000 RPM.  Each cylinder
sees a mean effective pressure of about 216 psi.  I don't have the bore or
stroke of the engine available to me here, so I can't tell you what the
maximum compressive load on each articulating rod is, but it's MANY times
the load seen by the connecting rods in your Alfa.  (If the engine were
"square", it would have a bore and stroke of 6.2 inches.  That would give a
mean effective load of 6,521 lbs for each articulating rod!!  That comma
isn't misplaced.  Each rod has to be designed to handle a peak load of well
over 10,000 lbs.  The real numbers will be slightly lower, because the 3350
is undersquare.  By comparison, your 2 liter Alfa engine sees a mean
effective pressure of about 147 psi, and a mean effective load of 1,263 lbs
on its connecting rods, a fraction of that seen by the radial.)

  Each cylinder in the 3350 generates 150 HP.  That's as much power going
through each connecting rod as is generated by an entire 2.5 liter Alfa 6!!
 Indeed, a piston of 6 inches diameter and its associated articulating rod
are a substantial translating mass (the rods are much lighter than the
pistons, by the way), but power for power, I'll bet they're actually
lighter than the assembly in your 2 liter Alfa.  Another important point: 
your Alfa engine sees loads like that for a TINY fraction of its life.  The
3350 will see loads like that for over 90% of its life.

The real reason radials turn so slow, is that they are (because of their
configuration) DYNAMICALLY UNBALANCED.  Running them at higher RPM requires
a great deal more structure be built into the airplane to handle the
vibratory loadings.  Also, at higher RPM, those vibratory loadings work
against all of the fasteners and material within the engine, causing it to
shake itself apart--like my old VW Golf diesel.  It's not that difficult to
translate a balanced 6 inch piston and rod back and forth over 3,000 times
per minute.  But with an unbalanced system, you're not likely to do it
without shaking the entire township to pieces.  That's the situation we're
talking about here.  It's the reason radials were turned as slow as
possible, and it's also the reason radials eventually lost out to less
efficient, less dependable (at the time), more expensive gas turbines. 
It's next to impossible to build a human-comfortable airplane using radial
engines.

Last, radial engines are far LESS complex than an equivalent inline engine
(lighter too).  The articulating rod to master rod bearings are very simple
bushings, and see VERY little rotation.  That connection is directly
analogous to the connection at the wrist pin.  It gets precious little
wear.  The bushings merely have to be sized correctly for the load--and
they were.  In a twin row engine, you end up with 18 cylinders, only three
main bearings, and two direct drive cam sets.  Compared to an inline
engine, there is precious little in there to fail, making a radial
dependable and light.  The problem is, they all shake like hell.


Rich Wagner
Montrose, CO
'82 GTV6 Balocco

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End of alfa-digest V7 #927
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