Engine Mount Cross-Bearers - Alternate for DynaFocal?

Brian Jackson

Platinum Member
Joined
Jul 17, 2004
Messages
3,524
Location
Hamburg, New Jersey USA
Aircraft
GyroBee Variant - Under Construction
Greetings.

Ralph's original GyroBee documentation shows the basic configuration for the DynaFocal mount by Leaf which is no longer in production. I have read here that it is OK to use flat stock for this item and leave un-bent for a 503 Rotax. My question to the experts here is in regards to material thickness. The DynaFocal uses 3/8" thick bar stock that is 2" wide. Could I substitute with 1/4" thick that is 3" wide, providing the same cross-sectional area?

This seems reasonable on the surface, but it's the little things I perhaps haven't considered that prompts this question. I don't have a crystal clear understanding yet of Barry mount thicknesses, etc. Any guidance here would be greatly appreciated. Thank you.

Respectfully,
Brian Jackson
 
Brian: We need not mourn the "Dynafocal" mount. It wasn't really configured to work properly in the usual Gyrobee engine setup. The "focal" line was located assuming gearbox mounted with the prop shaft up (on the same same side as the sparkplugs) while a 'Bee installation should be box-down to minimize HTL.

I would not go to 1/4" stock, of any width. Cross-sectional area is only one consideration. Rigidity in up-down bending is another. You have to see one of these things start on one cylinder to truly appreciate what ragged torque pulses are possible. The engine rocks like a wet dog. These torque pulses, of course, end up as up-down bending forces on the mount bars, trying to bend or crack them through the central 1/4" mounting holes.
 
Thank you, Doug.

Understood completely and heeding your advice. I have not test-run the engine yet so I don't have a good mental picture of the vibrational loads. However I would not have proceeded with using thinner material without first consulting the Forum.

From your memory, has there ever been a failure of this item with a 503? Not specifically the DynaFocal but the 3/8" flat bar stock in general in this application.
 
Brian:

The four threaded bosses on the bottom of the crankcase will break off first. I speak from hard experience.

Via a chain of stupid errors, I managed to get the heavy steel bridle cable that connected the ballistic chute to my Kolb Firestar (FW UL plane) into the prop at full throttle on takeoff. This shredded the (wood) prop and snapped the bosses off the case. The engine, with about 12" of splintered prop left, ended up hanging off the aircraft by the carb and choke cables. Had it departed, I'm afraid the plane would have made like a lawn dart. As things turned out, there was runway enough for an engine-out landing.

Destructive testing, indeed. I had the crank alignment checked, then put the innards of this engine into a new case and it became my Gyrobee engine. It's a 447, but obviously the concept that the bosses break first will extend to any inline Rotax case.

As an extra precaution, I had the AEROTEC model of the 3/8" bars made of 2024-T3 instead of the cheaper 6061-T6, but I'm not sure that was anything more than unnecessarily "gilding the lily."
 
Finished fabrication of engine mounts yesterday. The rubber Barry mounts (#22001 YLW/WHT medium) inserted nicely into the 3/4" holes of the upper bar, albeit very snugly. The inserted portion is .79" diameter so it must compress .02" on all sides in order to insert fully. This makes for a tight fit. I didn't see where reaming the holes to the exact part diameter was required, only that an .040 chamfer was necessary per specs.

Will share pics when time permits. Turned out beautifully.
 

Hi Flyboy. I've been posting progress photos in this thread. The mounts are near bottom of page 1 linked above.
 
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