Good point and could well be a contributory factor. Except for the central pins,
which have no plastic bush and which also fail.
Duncan McFadyean
On Sunday, January 20, 2002 4:50 AM, Fred Fillinger
[SMTP:fillinger@ameritech.net]
wrote:
> It's curious also that Jim Thursby's post said only one tailplane was
> real sloppy, so that would rule out service loads. I looked back at
> the manual's method re TP12 where you drill the plastic collars. I
> remember doing the plastic all on the bench, and fitting it to TP12
> and pins on same bench, not in the airplane. A rat-tail file don't
> ream nylon quick at all, I remember, but it kept my titanium drills
> that are sharp enough for nasty razor cuts just handling them, out of
> those metal holes to preserve tight fit of the pins. .001" of
> accidental reaming is trailing-edge play already, and maybe the
> increasing slop with flight time is first elongation of the holes in
> the nylon spacer?
>
> Can't challenge your calcs for sure, but maybe pulling G's makes the
> balance weight heavier, making stick force lighter, so more
> aerodynamic load must be built in to counteract - more load on the
> pins. The calc I thus did was crude and potentially questionable, but
> reflected this, and put the 3.8G load on the pins at more then you,
> but seemingly OK.
>
> Cheers,
> Fred F.
>
> McFadyean wrote:
> >
> > The "few" is actually the "many"!
> >
> > 13 ft.lbs times inertial load factor (3.8g)* = 593 ins.lbs
> >
> > Times 1.5* = 889 ins.lbs ultimate
> >
> > Times 1.5* = 1334 ins.lbs ultimate with nominal factor for non-interference
> > fit/potential for light hammering.
> >
> > 2no. 1/4" pins bearing on 0.063" wall of 1.5" dia tube generates a
> > bearing pressure of 27,587 psi.
> >
> > This should be well within the bearing strength of 4130 N.
> > Hence, in my very first contribution to this thread I caveated my response
> > that I could not get the tube to fail by means of calculation.
> >
> > Nevertheless, they do fail and, as Bob Harrison has pointed out, it is not
> > the 4130 tube that fails. Rather, the stainless tube instead. All we need
> > to do now is find out what grade it is.
> >
> > (* Ref. Bruhn "The Analysis of Flight Vehicle Structures", JAR-VLA and
> > others).
> >
> > Duncan mcFadyean
> >
> > On Saturday, January 19, 2002 4:30 AM, Fred Fillinger wrote:
> > > Indeed true, and I see where I made it sound like the mass balance has
> > > no effect, but I measured it. It takes only about 13 foot-pounds to
> > > lift the balance weight. That's about 1/20th of that required to
> > > elongate
> > > two 1/4" holes in 4130 at least, to the point of noticeable play, in
> > > my test. Maybe 13, reflecting inflation, is still a "few?" :-)
> > >
> > > Best,
> > > Fred F.
> > >
> > > > The "few pounds of rotational force" is the weight of the
> > counterbalance
> > > > times the length of the arm it sits on. Same difference in trimmed
> > flight
> > > > where any air-load eccentricity is reacted by the effect of the trim
> > tabs.
|