Ring mount- why?

Sam Purpura wrote:

Maybe because a ring mount is better.

Sam, no replies allowed after three bourbons 😉

RW,

".......the engine would vibrate so bad that fuel would spew from the carb vent tubes and shake badly."  

What of the fuel spillage problem with bed rail mounts ? Has the internal  gearbox adjustment  fixed/reduced the problem? OR does it still appear from time to time.?

 "......This was noted as the pilot would be coming down to just below peak torque on throttle down

(912ULS) Peak torque  at about 5100 rpm - please expand the above "...coming down to just below peak torque..." ?

Hi Sean

Long arm engine mounts, mostly bed mount systems, tend to vibrate at a resonance that is determined by the length, diameter and resonant signature of the engine.  In certified engines for example they generally have placards that spell out "bad" RMP ranges and you should not operate in those ranges.  Unfelt harmonic vibrations are why most engine mounts crack. 

The ULS with the overload clutch is good at absorbing many of the harmonics however if the gearbox primary system, the dog and ramp, are lightly loaded then it can still have issues.  In general on a Rotax you will notice this in a few ways.  Normally we simply look for an engine that has no "rattle" while at idle speed on the ground.  If you have to advance the throttle to get rid of the rattle it suggests a under shimmed gearbox dog to main gear.  (you can see the spec in the MML) The other thing that can be an issue is the harmonic that does not get absorbed by the engine mount to airframe, the elastoma may need to be more rigid or perhaps even more soft.  This is to damp out those transient harmonics that tend to be just below the peak torque moment, that 4800 to 5000/5100 range.  It is normally the worst on the deceleration of the engine as the propeller is unloading.  

Cheers

Sorry, just to be clear. "elastomer mounts" are the rubber like things we use to absorb vibration. Whereas peak torque is 5200 the curve is somewhat flat from 5000 to 5200, so coming down from the 5200 there is a buffer.  The reason I suspect Airmaster is pointing you toward 5000 is perhaps to save some fuel.  

You analogy of the tractor is great and exactly what is happening.  With the propeller our engine is under a constant load.  In acceleration the load is being increased all the time.  On deceleration the unloading is what causes abrupt swings in the harmonics and this is primarily seen just from 65 to say 70 percent power (4800 to 5100/5200)  In the Rotax documentation you will see many references to not overload the engine and the importance of being able to operate above 5200.  Getting back to the fuel foaming, or overflow, indeed they may be related to a harmonic. 

Cheers