Engine mounts have it tough. The elastomer doughnut-looking things that live between the engine and airframe to prevent the vibration of the ironmongery up front from beating the rest of the aircraft to flinders not only live in a harsh environment, they are generally forgotten after being installed and are subject to an ongoing identity crisis.
For one, people cant seem to agree what to call them. To most, theyre engine mounts-yet that name also refers to the metal structure that joins the engine to the airframe. Vibration isolators is more technically correct, and we also have heard them referred to as isolators, engine isolators, vibration controllers, and-as Kleenex is used for tissue and Learjet for corporate jets-Lord mounts.
Were going to use engine isolators because thats what the gizmos are intended to do-isolate the rigid airframe from the hammering vibration of the engine and propeller. Well explain how they do what they do, how long they can be expected to last, what they cost and what can go wrong when they wear out.
An engine and propeller combination creates a hideously complex series of vibrations in normal operation. After flying his nearly turbine-smooth Cessna 185, we interviewed professional vibration analyst Steve Gruenberg to get background on the nature of vibration and what it can do to airplanes. He said to think of all vibration as impacts. You just took your hammer and hit the airframe.
To make matters more challenging for aircraft designers, the ideal aircraft structure is lightweight, so it will fly and stiff, to maintain the integrity of the control surfaces. It is the perfect conduit to transmit vibration and shock the length of the vehicle. Thats according to Paul Furhman, senior marketing manager of Lord Corporation, the oldest manufacturer of aircraft vibration isolators (think Lord mounts). He explained to us that its easy to break the transmission path of vibration on a ground vehicle, but difficult to do with an aircraft. The only way that has been found is to use elastomeric engine mounts (isolators).
Vibration from the powerplant(s) does nothing positive for the aircraft or its occupants-it leads to cracking of the structure, loosening of rivets, metal fatigue, damage to instruments and fatigue for the occupants.
The Lord Corporation pioneered the development of rubber vibration isolators (it called them mounts) for aircraft prior to World War II. As general aviation popularity skyrocketed following the war, Lord developed stiff, natural rubber conical mounts for general aviation aircraft that Furhman said were effective in protecting the airframe and instruments from engine vibration. However, they were not very good in damping vibration frequencies that were uncomfortable for the occupants. Furhman said that the vibration in the yoke or stick was often bad enough to cause the pilot significant discomfort over the course of a flight.
In the process of improving its isolators, Lord applied the Dynafocal mount it had developed in the early 1940s to general aviation. The engine mount itself was shaped to point the isolators at the center of gravity of the engine, vastly improving vibration damping. It also mixed other compounds, notably silicon, in with natural rubber, created isolators that were softer-to absorb more vibration at more frequencies-and had improved durability and heat resistance.
Most engine mounting systems use four isolators, although some Cirrus aircraft use six. The Continental engine mounting system supports the engine from below-think of a person holding a beach ball. Lycoming engines are cantilevered out from the firewall, so the top isolators are loaded in tension, the bottom in compression. The isolators must be installed with the correct orientation. Scott Utz, proprietor of Arapahoe Aero at Denvers Centennial Airport, told us that from time to time his shop will get an airplane in with the isolators installed backward.
In general, aircraft and isolator manufacturers say the isolators will last through engine TBO. In operation, we were told by Utz and by Derek DeRuiter, proprietor of Northwoods Aviation in Cadillac, Michigan, that if the airplane is flown frequently, the isolators will last through TBO. However, its more realistic to think of them as good for 10 to 12 years. They should be inspected for condition regularly. They are all made of a compound based on natural rubber, which means that they will eventually solidify and deteriorate.
Heat, oil and avgas are the natural enemies of isolators. Heat shields are usually installed in front of isolators that are near turbochargers or exhaust pipes-any time the cowling is removed, a quick check to see that the heat shield is in place is a good idea. Scott Utz told us that its common for the isolator located nearest the engine oil dipstick to be the first of a set to degrade. This is because pilots routinely allow a few drops of oil to drip when checking the oil quantity and it finds its way to the isolator.
As the isolators wear out, the engine will sag on the mounts and vibration will get worse. As the engine sags it can and will come into contact with the cowling and items within. Common results are the engine ring gear cutting into the lower cowling and chafing of wires and fuel lines. For reference, the cure date is stamped on each isolator.
When the engine begins to sag in the mounts, the aft side of the prop spinner bulkhead will no longer be parallel to the front cowling opening.
Lord publishes guidelines for cleaning and inspection of Lord mounts (isolators) that generally apply to all isolators: wipe oil and dirt off them, do not dip them in cleaning solvents, check bolts and spacers for cracks and damaged threads, and examine bonded rubber isolators for swelling, bond or rubber separation and shear or compression set.
Isolators that show bond separation, oil swelling or flex cracks should be replaced as should those that exceed Lords published maximum allowable tolerances for compression and eccentricity.
Replacing a set of isolators takes anywhere from three to 12 hours. It involves loosening mounting bolts and hoisting the engine a few inches away from the mount, and then loosening or moving cables and lines as needed. Once there is adequate clearance, the bolts can be withdrawn and the replacement isolators installed.
If one isolator goes wears out-usually due to oil damage-the other three probably are not far behind. Because of the time involved with hoisting the engine to replace just one isolator, it usually makes economic sense to replace all of them.
Lord Corporation is the granddaddy of the engine isolator world, makes isolators for most general aviation piston airplanes and is the original equipment supplier for an estimated 80 percent of the new airplanes.
Barry Controls is the OEM for isolators for Beech Aircraft and makes isolators for much of the general aviation fleet. Senior Customer Service representative Greys Castanon told us that the company entered the market in the 1980s and has been progressively expanding its coverage. He echoed the comments we heard from maintenance technicians regarding heat and oil as the enemies of isolator longevity.
Initially known for isolators for the experimental aircraft market, VIP Mounts are also available for a wide variety of production airplanes, particularly aerobatic aircraft. Company representatives Robin MacDonald and Zion Dunn told us that was recently purchased by FTG, Inc., and that the company is also remanufacturing a number of various large engine mounts and doing custom solutions for antiques, classics and warbirds.
Lord, Barry and VIP isolators are available through most aircraft supply houses. Prices vary widely by type of aircraft. We looked at Aircraft Spruces website and found that there were a few airplanes where all three companies made isolators. For a Cessna 177B Cardinal, a VIP isolator was priced at $88.75, the Barry unit was $125.90 and the Lord was $157.85.
Depending on the type of aircraft, we saw isolator prices as high as $800.
After researching this article, we think that price is indicative of value. During our research, we noted that some isolators were advertised as identical to Lord mounts. In our interview with Paul Furhman of Lord, he was adamant that identical applied only to appearance and not isolation efficiency.
Where a maintenance technician would make a recommendation, it was for the Lord product, however, none we spoke with had anything negative to say about Barry or VIP.
Its our opinion that all three manufacturers make satisfactory engine isolators, but if you want the smoothest operation, we give the nod to Lord mounts.