What causes repetitive cracking of spinners and spinner bulkheads? What is the proper method of installing spinners?
(Spinner cracks, spinner cracking, bulkhead cracks, bulkhead cracking, backplate cracks, backplate cracking, propeller backplate cracking, install my spinner, spinner installs, spinner installation, spinner screws, smoking rivets, smoking screws)
Technical Editor:
I have not read an authoritative analysis of all of the causes of spinner and backplate cracking. On the assumption that the parts are correct for the application and are properly made, I suspect three primary factors, in this order:
(A) Badly unbalanced prop (includes an unbalanced spinner)
(B) Improperly fitted/installed prop and spinner combo
(C) Internal engine imbalance
My belief is that Beech requires replacement of spinners and bulkheads as a balanced assembly; a practice that is apparently widely ignored. There is normally a set of index marks (or index labels) inside the spinner and on the bulkheads, showing how to orient them during assembly to retain balance. I don’t know whether a spinner shop like ASEI (link on BAC; more info at bottom of this note) performs re-balancing following repairs, but I suspect that they do. You can check with them before buying new parts. They may also be able to aid diagnosis.
If the prop has been field-dressed repeatedly for nick repair, and has never been re-balanced, it is almost certainly out of balance. It doesn’t need frequent re-balancing; but metal removal during dressing, despite efforts to stay consistent on both blades, eventually leads to the need. While the spinner assembly should first be balanced “standalone”, the final prop balancing will include the backplate. The final check may or may not include the spinner itself, depending on the equipment being used.
The spinner is (normally) supposed to be a snug fit over the bulkheads. It should require some hand-slapping to get the holes to line up. This is also true of spinners on Hartzell constant-speed props, where the prop piston dome serves as the front bulkhead pilot. Heavy-duty Mylar packing tape (like the premium 3M brand) can be used for shimming out clearances. Don’t bother trying to get by with the usually thin junk Mylar tape that tears easily; it will rapidly chafe away. If you can easily slip your spinner on and off, it is probably too loose on the bulkheads. In addition to this factor, many of the spinners are supposed to have fiber or Nylon washers under the heads of the spinner screws. This allows stresses to equalize, rather than concentrating them at a tight screw. They also keep the screw from fretting and corroding the spinner, which creates stress risers leading to cracks. A ‘smoking screw’ should be immediately investigated, before more damage is done. Very seldom do I find the proper washers on the spinner screws. I don’t see a reference to Nylon washers for the 1963 spinners, in the poor images in the BAC 1963 IPC download. Nonetheless, virtually all of the spinner and prop manufacturers now specify a fiber or Nylon washer; regardless of what the original airframe parts list calls for.
SPINNER INSTALLATION INSTRUCTIONS!
The spinner needs to be properly shimmed on the front bulkhead. Many owners and A&Ps think it’s a good thing when they can slip the spinner on and all the screw holes line right up. But that means that there is insufficient support (clearance) at the front bulkhead. That results in a very large loading on the aft screws, because the forward bulkhead can’t do its job. The front bulkhead has to be shimmed to a proper fit. My personal preference is the relatively inexpensive premium, heavy-duty 3M clear Mylar packaging tape. While it costs around $6 per roll in most places, roughly 50% more than the cheaper tapes, it is far thicker and tougher. This tape is NOT applied to the ID of the spinner. It is nearly impossible to get an installation there that is free of wrinkles and uneven overlaps.
You clean the bulkhead (or constant-speed prop dome) well, and start the tape (doesn’t matter where). You hold the cut end firmly, with the tape equally overlapping the front and back side of the bulkhead, and pull the tape really tight while you make the wrap. The tape should stretch to a tight fit on the bulkhead, with no wrinkles in the contact area. The two overlapped areas will pull slightly toward the center due to the stretch, which keeps them out of the way of the spinner. You complete two full wraps, and cut the tape exactly where the original end began (no overlap, no gap).
Then try the spinner fit. I use clean cotton gloves when handling a nice spinner, as it will be getting a lot of hand work. It should go on readily until the screw holes are showing no more than 1/3 to 1/2 in each screw hole, with the spinner ‘square’. If they don’t show any overlap with the trial fit, remove the spinner and peel off a single layer of the tape. Make sure it is trimmed exactly where the first lap began. With the spinner back in place, and the holes partially showing, ‘hand slap’ the spinner back onto the bulkhead, by using your palm to slap it rearward as you rotate the prop (if you must). Be sure not to take both hands away from the spinner at once. Even though it may seem tightly secure, it can easily pop off and hit the deck, leaving you with a nice dent. And unless the dent is pretty centered on the nose, a side dent can cause vibration. Once you have the first couple of screw holes close enough to get screws started (with fiber or Nylon washers), get those in; then slap the spinner toward the less aligned holes. The first screws should be installed in the center area of the spinner flange, midway between the prop cut-outs. Then work the spinner and screws into place, moving equally out to the final screws next to the prop blades.
On some older and smaller spinners on other makes of planes, this is less critical than it is on most of our fleet. It is also slightly less critical on the shorter and rounder 1963 Musketeer spinner, if it has been replaced with the later, thicker replacement dome (and nearly all have). But there is no sense in cutting corners on spinner installation, whether it is thought to really be needed or not. You can pay an A&P for a lot of extra fifteen minute time blocks before you approach the cost of repairing a spinner. Not to mention the hassle factor.
You might want to verify that the internal bulkheads have the proper high-strength AN525 washer-head screws (if they have screws). Also that the spinner screws are correct; if they have been replaced with stainless-steel screws, they should be the proper heat-treated aircraft-grade SS, not the cheaper commercial-grade often sold in kits for access panel SS screws. Contrary to popular belief, not all SS hardware is created equally. For that reason, not all of it is equally legal for aircraft applications. Probably half or more of the SS hardware you see on planes is technically illegal for the application, as it is not a MilSpec hardware grade. Also note that anywhere a SS fastener is used against aluminum or painted aluminum, a Nylon washer is required to prevent corrosion. On an original Cadmium-plated steel screw, the Cadmium is sacrificial (much like the zinc on galvanized steel). When a SS screw is used, the aluminum is sacrificial; not an attractive situation. Smoking screws on access panels often tell this tale as well.
And finally, if the engine has dynamic counterweights and they become de-tuned (too long a story to explain here), they can produce vibrations that lead to cracks, thrown alternator belt/broken alternator brackets, etc. That doesn’t apply to a 1963. You can look up your engine suffix letters and decode them, to see whether your engine has this form of internal balancing.
ASEI is located just 1/4 mile south of Hartsfield Atlanta International Airport. Phone: 404-559-1946, Fax: 404-559-1975 Hours 9:00am – 6:00pm EST Monday through Friday