ENGINE MOUNTS (VIBRATION ISOLATORS)
I hope by now you've had a chance to flight test your along with the other installations Barry sent you parts for. I'd like to review your findings someday.
As we discussed, there are some facts and "general" rule of thumb guidelines to be aware of and follow regarding the use of engine vibration isolators. Normally, each engine, propeller and truss combination will have a unique vibration signature and natural frequency. A change in any one of the three will change the vibration characteristics of that installation.
Under perfect conditions, the elastomeric "mounts" or vibration isolators are designed to suit these individual combinations on an installation-by-installation basis. Choice of elastomeric compounds and spring rates (stiffnesses) are also determined by the overall flexibility matrix of the individual installation. Just because an isolator physically fits an installation envelope and related mount hardware doesn't necessarily make it the proper part.
So much for perfect conditions!
The real world (especially homebuilts) requires a combination
of best effort and budgetary restrictions when choosing a suitable isolator. The best
starting point for most installations is to use the same or cross-referenced equivalent
isolator as used in the aircraft the engine came out of (especially if it's a factory
airframe). Another choice is to find a factory aircraft using that engine and use
isolators normally installed on the chosen aircraft.
New isolators should have current cure dates less than one (1) year old to insure best performance. Ambient air alone will cause elastomers in installed isolators to lose their resiliency and deteriorate. Once the isolators are installed, they need to be inspected for wear and fatigue at the same time as other external engine parts.
Isolators should be kept free of dirt, oil and other petroleum based fluids at all times. Oils will soften the elastoiner making them ineffective. Spilled fluids can be cleaned off using isopropyl alcohol or electronic contact cleaner.
Prolonged temperatures over 200' will dry out and harden most elastomers over time. Heat will initially cause the elastomer to soften and "drift", but will eventually harden it, making the isolator ineffective. Extreme ambient temperatures during tie down and storage are just as destructive to the elastomer, and contribute to short service life. Isolators should be changed every time the engine is removed from the airplane. They will take a permanent set" within 48 hours of installation and are virtually impossible to replace in an identical fashion.
Wear for installed insolators varies with flight hours, and is not consistent from aircraft to aircraft. Each inspection should include looking for debonds and tears in the elastomer.Debonds are a physical separation of the metal outer or inner plate from the elastomer. A good rule of thumb is 30% circumferential or 30% depth separation.
Tears or splits will show in the bulged or center portion of the isolator and follow the same rule of thumb. They should not exceed 30% circumferential, 30% elastomer depth or 30% elastomer width separation. Any one of these conditions is cause for replacement, at which time all isolators should be changed. Equal spring rates and loads are essential to proper isolator performance.
Since no two installations are identical, and aircraft hours per year are so different, service life is reflective' of those conditions. At very best though, five year old isolators or installations should be changed out. Other items for inspection include excessive deflection, mounting bolt integrity, security and tightness.
Mounting trusses should be made as stiff as possible. This normally allows a "softer" isolator to be used, resulting in a "smoother" ride. Full ringed trusses are best. They also keep engine deflections to a minimum. The "horseshoe" type trusses are very flexible, and usually require stiffer isolators. This in turn allows more engine deflections, that require larger cowlings.Actual truss members need to be checked during installation for non-concentric bolt attachment and engine mounting flange points. When a truss "leg" or member is pre-loaded during installation to center the bolt or isolator, undue stresses are put into the isolator that can cause it to prematurely fail. Be sure to look for this, especially on rear mounting engines without a completed truss ring at the isolator mounting plane.We have an 0-200 isolator configuration in place on Bruce Evans' EZ, and have an installation in the works for an 0-290. I'm gathering data and will keep you informed. So far, Bruce seems very happy. Sketches showing the modifications are available if you would like to publish them in the Canard Pusher.
If you have any question, or if any of your builders have questions on engine isolators, don't hesitate to contact me.
A Unit of Barry Wright
Engine Vibration Isolators"