From: Craig Wall <corax@concentric.net>
Newsgroups: rec.aviation.homebuilt
Subject: Re: How wings produce lift
Date: 10 Mar 1999 22:28:45 PST

David Pincus wrote:

> Craig Wall wrote:
> >
> > Terry Schell wrote:
> >
> > > highflyer wrote:
> > > <snip
> > > >
> > > > I do know that, during the war, they decided the P-38 needed to
> > > > have counterrotating props.  They installed them on the P-38-J
> > > > which was a quite common P-38 model.  In the P-38-J both props
> > > > rotated outboard at the top.  Think about that for a while.
> > >
> > > Are you sure they counter-rotated the "wrong" way?  That seems strange.
> > > Any ideas on the rationale? Could it be related to the fact that the
> > > rudders on this bird were directly in the propwash, unlike most twins?
> >
> >  Yep, they do, and I once got to ask Kelly Johnson that very same
> > question.  And he answered me.
> >
> >   Craig Wall
>
> The counterrotation props were used on ALL USAAF models of the
> Lightning, including the original pre-production YP-38s. Only the
> British export Model 322 Lightnings had the same rotation on both props.
>
> As Craig has related Mr. Johnson's comments, the ORIGINAL direction of
> rotation of the prop wash on the rudder and horizontal stab was part of
> the problem that caused a number of test a/c to loose their tail
> asssemblies during high speed dives, near-mach tuckunder not
> withstanding. Many fixes were tried, including switching the left and
> right engines. This help reduce the stresses on the tail assembly,
> though it took the underwing dive brakes to finally tame the near-mach
> dive problems. The dive flaps were instituted during the late "K" series
> (-25 series I think) but the countrarotating propellor switch happened
> earlier on in the production run.
>
> David Pincus

  I didn't relate any comments at all, yet!

  I met Kelly in about '82 or '83 at a seminar in Norman, Oklahoma.  I asked
him about the P-38 props turning outboard at the top, giving two critical
engines.

  I though I knew the answer: because the spiral flow off the props was
opposite the tip vortices, the ship should be cleaner. And it is.  But that
wasn't the reason he gave....

  He just said "it made a better gun platform".

  That was it.  The entire purpose of the airplane was to shoot, and anything
that made it do that job better was the deciding factor in all decisions like
the prop rotation, etc...

  He said they actually tried it in every possible combination of prop
directions, and that's the one that worked best and gave the highest gunnery
scores.

  That day I learned to *focus*.

  Craig Wall