Hi Gang, (especially Tiger and Super Cheetah owners),
>
>
>
> Following the threads on this subject, I want to clarify some things so
> everyone is on the same page.  First, the reason we have the problem is
> twofold. The original spinners on the Tigers and Cheetahs had the backplate
> sandwiched between the propeller hub and the spacer.  These (especially
> Tigers) were cracking like they were made of glass, in the late 70s, so the
> Grumman Engineers went to work to develop a fix in the form of (first) a
> similar unit to the old AA-1 series, which had a fixed aluminum forward
> bulkhead riveted into the dome.  It still cracked.  The next-gen was what we
> have now (SK143-1 & -2) which took some time to get certified.  (There was a
> service letter available from Grumman that allowed the planes to fly with
> the entire spinner and aft bulkhead removed until the new design spinner
> system was available.
>
>
>
> There was an unexpected problem with installing the new spinners in that the
> McCauley prop bolts were barely long enough to be legal with the original
> spinners.  The new aft bulkhead was twice the thickness of the original plus
> the forward bulkhead and its required steel doubler were added for a total
> additional 0.123 (to 0.128 with a TCB composite front bulkhead) of
> bulkhead material what subtracts from the available thread penetration of
> the prop. bolts.  Thats an eighth of an inch!  For this reason the SK-143
> kit instructions said to omit the washers under the bolt heads which caused
> serious damage to the metal of that steel doubler.  Thats all for the first
> part of the twofold problem.  The 2nd part was that the propeller drive
> bushings (the things pressed into the crankshaft flange) were specified at
> Lycoming (by some dummy) to be of the length normally found in engines that
> would get a constant speed prop installed on it.  Two of them are shorter
> than the other four.  When the starter ring gear support assy. is installed
> there is none of the shorter two protruding through the front of the support
> and only a tiny bit of the other four.  When the backplate is placed over
> these they barely peek through the holes enough to get the propeller
> spacers counter bores to have anything to slide over.  Now someone has to
> start the prop bolts into the drive bushing threads and run them in until
> they contact the forward bulkhead doubler and draw it snug to the propeller.
> What happens to cause the punch out of the bulkhead is it tends to slip off
> of the drive bushings and rest on the prop bolts.  When it is torqued down,
> the leading edge of the bushings and the counter bores of the prop spacer
> acct as a die and receiver punch that clips out a crescent shaped piece of
> the bulkhead at each of the 4 protruding bushings.  (This will cause the
> holes in the spinner not to align with all the ones in the aft and forward
> bulkheads and should stop any mechanic who is awake from continuing and
> start looking for a problem.  We see some spinners with elongated holes that
> were filed out to force it to allow all the screws to be installed.  Of
> course the back of the spinner will wobble.)
>
>
>
> NOW, try to picture all this going on with one person doing the job.  The
> maintenance manual revision, which covered the new spinner system (that was
> standard on about the last 320 or so Tigers in 1979) suggested taping the
> aft bulkhead to the nose cowl to hold it in place while the prop was
> installed.  This does kind of work but still two people doing the
> installation is much better though still not foolproof.
>
>
>
> When I did the first installation of the Sensenich 76EM8S10-0-(pitch)
> propellers, on the 180 HP Cheetah conversion development, Sensenich did not
> have the right length bolts since they had never built the dash 10 spacer
> version of the 76EM8 prop.  We had to use McCauley bolts until them got them
> made and certified.  They designed the new bolts < inch longer than the
> McCauley bolts which allowed using the washers and still getting adequate
> penetration through the drive bushing threads.  This was in early 1981 and
> the problem of punched out aft bulkheads had not yet become a wide spread
> problem.  Soon after the STC was issued for converting Cheetahs to 180 HP,
> Ameromod Corp. was awarded STC SA1195NW approving the installation of the
> same series of Sensenich propellers on AA-5B aircraft.  When the punched
> bulkhead thing did become noticed as a more common a problem, nothing was
> done about it in form of even a service letter, let alone any attempt to
> correct the root source.
>
>
>
> In late 1985, after the split of Ameromod Corporation and we formed Air Mods
> N.W., I certified the 2nd STC for installing the Sensenich propellers on
> Tigers.  Part of this new STC covered a fix for the bulkhead syndrome by
> authorizing the replacement of the two shortest drive bushings with much
> longer ones (from the O-360-A4M model engine) OR replacing all 6 of them.
> The Cheetahs and Travelers never had the problem due to longer drive
> bushings which held the aft bulkhead in place during installation.  This
> offered the same effect on the Tiger when the bushings were replaced.
>
>
>
> Another fix STC SA3326NM offered is a much more detailed installation
> instruction for putting the propeller on without damaging the bulkhead.  It
> also covers what can be considered for the airworthiness of a damaged
> bulkhead.  If there are still two undamaged holes in it, it can be used with
> the longer drive bushings or possibly indexed differently than recommended
> in the maintenance manual.  (This can help offset the cost of the two drive
> bushings if you choose to replace them. they cost around 500 bucks for the
> pair.)
>
>
>
> The bottom line of all this verbiage is that there is a way to eliminate the
> problem of installation damage.  As for the statement made by Dave Fletcher
> regarding the requirement of checking the torque of the bolts at annual.  He
> is absolutely correct.  However, removing the prop does create the risk of
> damaging the bulkhead on reinstallation.  If you know it was not damaged
> before, and the prop was correctly installed, I would not recommend doing
> more than to break the torque, back out the bolts far enough to inspect the
> front bulkhead for cracks, and re-tighten the bolts.  If you do not know,
> and cannot verify, the correctness of the installation it is a good idea to
> pull the prop and inspect the aft bulkhead.  If the bolts are installed
> without the washers, you should add them.  I use an MS27183-18 washer
> instead of the AN960-8 which has a greater outside diameter and spreads the
> pressure of the highly torqued.   bolt causing less damage to the doubler
> and forward bulkhead lessening the tendency for the bulkhead to crack around
> the outside of the print of the washer.
>
>
>
> OK, that is your lesson in Grummanology from The GURU for today.  If
> anyone wants to discuss it with me, please call me or e-mail direct.
>
>
>
> Ken Blackman
>
> Air Mods N.W.