FW and Gyros
- Oct 17, 2014
- Cessna 152, 172, 172RG, 177, 206 -- Piper 180 -- RV-7A -- Calidus -- RAF2000 -- Sport Copter II and
- Total Flight Time
As any long-suffering RAF owner has learned, there are many design/material issues to the gyro.
The engine/PSRU mount plate has been known to crack, and it happened to me last year.
When my TIG welder friend repaired the OEM mount, he cautioned me against relying too heavily on rewelded aluminum (which loses its temper),
and forecast that it would probably crack again in about 25 hours. Well, his forecast unfortunately proved correct, as it cracked after 22.9 hours.
I found it on a post-flight inspection, having felt/noticed nothing in its previous half-hour flight. The mount could not be further repaired.
I've vowed to never order anything from RAFSA, as our USA owner experience has often shown that a local part can be made
better, stronger, faster, and often cheaper. (RAFSA parts are still made in Canada, but shipped to South Africa to prop up the
dwindling fiction that RAFSA does their own mfg. Thus, USA customers pay for the bloated shipping costs involved.)
So, I cast about for a local CNC machinist, and a pilot buddy knew of a reputable fellow who had a home-based shop who wasn't nervous
about making aeronautic parts. I sent along the mount, horseshoe, and jackshaft.
While 7075-T6 aluminum would have certainly sufficed (for a long while, at least), I and others (including Jim Vanek) thought that the vitality
of the mount deserved chrome moly steel. Such would probably never fail, and if it did, it could be repaired. There would be a moderate weight penalty,
but with its location near engine cg (while also reducing RAF propthrust vertical cg), the effect would be insignificant.
I wished that the mount thickness (designed for aluminum, but not needed in 4140 steel) could be reduced, but standoff dimensions had to be
maintained for the jackshaft and crankshaft bearing plate bracketry.
The machinist and I agreed that the horseshoe should also be made of 4140, to maintain like heat expansion with the mount.
I asked that all bolt holes and surface edges be radiused, to reduce odds of stress risers. (I believe that the OEM mount's sharp edges all around
contribute to its frequent cracking. Also, IMO, the pinch bolt force places too much stress on the lower horseshoe bolt holes, and the mount usually cracks there.)
It's rather a complex part to design in SolidWorks, but within the capability of most CNC guys. There is a LOT of trig to input!
There were 22 small holes to be drilled, a few of them tapped, plus the larger holes for the jackshaft, starter, and flywheel area.
MODS: He widened the top area from 4.875" to 5.25", requiring a longer pinch bolt (AN8-60A).
Cross-section was increased for the two lateral horseshoe bolts.
His weight-reducing measure was a very nice looking "bow-tie" 0.5" deep inset, handsomely rounded out inside with a ball mill.
My machinist estimated its strength as "about quadruple" the OEM aluminum mount. I think that'll do, heh.
His plate bolted right up; evidence of high-quality work. Workmanship was first-rate.
I just landed today after 0.7 hours, and all's well. Feels great to be rid of that crack-prone OEM mount!
Anybody who has a failed RAF mount and wishes to replace it with something far superior (either in 7075 or 4140) may PM me for a price quote.
P.S. I'd briefly considered going to a geared redrive at this point, but it would have taken longer, been more expensive, and required a
reverse pitch prop. Also, had I been thinking of going to the EJ2.5 motor and flipping the redrive, this would have been the perfect time
to have a mount plate made for that. But, since I'm getting a Sport Copter M2 this year and selling the RAF, I hadn't time or will to convert
from carbed 2.2. That can be the next owner's project if necessary. (I think that my RAF would make a great first gyro for a newbie, as nearly
all the bugs have been worked out, or a good training ship for a CFI who wants to add the RAF to his stable. I'm about to do its 500 hour
checklist. All components are strong and reliable, and its engine is 80/80/80/80 making no metal. Asking price will be mid-$30k.)