Melmoth 2 inherited its engine mount brackets and isolators-the rubber pads the engine is actually bolted to-from Melmoth 1, which had in turn acquired them, in 1971, from a Cessna Skymaster that was already far from new. It’s surprising that any rubber compound can retain a whiff of elasticity after 35 years or more in the heat, gusty winds and general turmoil of an engine compartment, but those dampers still felt more like rubber than wood to the touch. Nevertheless, the engine vibration struck me as excessive. Now, the quality of the vibrations coming from an engine and propeller is to some degree a matter of personal perception. One pilot found the airplane quite smooth. But even after trying to convince myself that it was all in my mind, I still thought it felt harsh; and so I decided to replace the isolators.
Looking at a listing of Lord mount types, I was surprised to discover that there is a huge variety of them, and a bewildering number exists specifically for my Continental TSIO-360 engine. Naturally, Melmoth was not one of the airframe types mentioned in the list. I called Paul Snyder at Lord for advice about which mount type to select, and he recommended a J9613-31.
The old dampers resembled two thick discs stacked one on the other. The new ones were more like hollow cups. A complete assembly-there are four in all-consists of two of these cups clamped, face to face, to a thin steel plate that is part of the engine mount. A cylindrical spacer holds the two cups in a fixed relation, somewhat compressed, so that the engine is supported on the north and south poles of a more or less spherical rubber shell whose equator is secured to the airframe.
Curious about why so many different types are available for the same engine, I called Lord again, and this time spoke with Product Support Specialist Tom Law.
Law explained that isolators are tailored to different engine mounts. Mounts vary both in the detail geometry of the brackets that support the isolators, and also in their own susceptibilities to flexure and vibration. The function of an isolator is to stop engine vibration on its way to the airframe by being particularly flexible in the frequency range where the predominant vibrations occur. Its design is constrained, however, by the requirement that the isolators also support the weight and thrust of the engine during maneuvers and limit its freedom to move within the cowling, particularly, in the case of recips, during the shaky moments of starting and shutting down. The engine mount must be sufficiently rigid to support and restrain the engine, but sufficiently compliant to damp the characteristic frequencies that would occur, for example, at cruise rpm.
All modern engines have “focalized” mounting points; in Lycomings they are typically at the back of the engine, on Continentals typically on the bottom. Focalization, which came into use with big radial engines during World War Two, means tilting the axes of the isolators so that they aim at a single point. By tinkering with the shape and orientation of the isolators and the so-called “L value,” which is the ratio of the isolator’s stiffnesses in tension/compression and shear, this point, called the “elastic center,” can be made to coincide with the center of gravity of the engine-propeller combination. The effect of focalization is to simplify the engine’s response to its own internal impulses, removing rotational components and making it easier to isolate and damp vibrations.
Most general aviation isolators are designed for smoothness in cruise and are relatively compliant, because G loadings are normally low. Acrobatic airplanes put greater demands on isolators; for instance, Lord supplies Patty Wagstaff with specially stiff mounts to handle the huge G and gyroscopic forces to which she subjects her Extra 300S.
Lord recommends installing new isolators at overhaul. Although most isolators are made of natural rubber, they are doped with preservative compounds and normally remain intact and compliant for more than the typical eight to ten year overhaul interval. The main reason for changing them is sag; slowly the rubber creeps under the weight of the engine, deforming the isolators and moving the engine out of position. Some operators rotate isolators to counteract sag, but unless the mounting bracket provides a mechanical means of holding the isolators in place, there is some chance that once sag has set in, an isolator will rotate back into its original position.
Installing isolators by myself had always seemed to me to be a major hassle, but with the help of a couple of friends who knew how to do it, the job was accomplished easily in an hour. The difference was dramatic; particularly around 2500 rpm, Melmoth 2‘s engine now runs with an almost automotive smoothness. Other than installing proper door seals and soundproofing, and maybe making better seats, I doubt that anything else I do will have such a noticeable effect on my and my passengers’ comfort.
Flying Objects This morning, while I was as usual driving 17 miles out of my way to get my favorite morning latte and avoid starting the day’s work, I heard a report on NPR that Serbian revelers firing their guns into the air to celebrate a wedding had brought down a small sport plane. The reporter went on to note that neither of the two fliers aboard, who survived their crash landing, had a pilot’s license.
It’s odd how the media pick up on the bureaucratic aspects of things; perhaps they do it under the influence of the official spokespeople whom they interview, and who, being bureaucrats, see the presence or absence of certain properly completed documents as the essential aspect of any event. I’m not sure how it is in Serbia, but here a pilot’s license does not protect you from stray bullets. What I want to know is, were the shooters’ guns properly registered?
At any rate, it appears that Serbian weddings may require TFRs.
A propos, Scot Paltrow, a Wall Street Journal reporter and, incidentally, cousin to Gwyneth, who is the ex of Ben Affleck, who in turn was once made airsick and happy at the same time by an acrobatic flight in actor-producer Tony Bill’s Siai-Marchetti 260-the links to aviation go on and on-anyway, Scot sent me an story from the AP wire regarding yet another bizarre human activity that also seems to deserve a TFR: firing pumpkins into the air from specially made compressed-air cannons.
According to the story, a competition is held annually in Delaware to see who can shoot a pumpkin highest into the air and have it come down intact, at least until the moment it hits the ground. The story omitted some details that I would have found interesting, like the size of the pumpkins used, how the judges measure the height attained by a flying pumpkin, how they know that the falling pumpkin is still intact before it lands, and what the muzzle velocity of a typical pumpkin cannon is. It’s all about muzzle velocity, of course, but a joking reference to 5,000 mph in the article is obviously not in the ballpark. At this writing, the height record is 3,199 feet. Although I could readily calculate the terminal velocity of a falling pumpkin given its radius and weight, my high school algebra is not up to the problem of calculating the muzzle velocity required to loft a given pumpkin to, say, 4,000 feet, through a resisting medium like air. This will be a good exercise for the calculus buffs among our readers; in the meantime, watch for pumpkins in Delaware airways.
A Lonely Impulse of Delight I subscribe to The New York Review of Books and await its strangely irregular comings with impatience. I enjoy its style and assume that its writers’ judgments on most matters are authoritative.
Nevertheless, it was with some dread that I found, in the issue of November 6 of last year, a review of several books about the Wright brothers, early aviation history, and flying in general. All too often, I have been disappointed to read accounts, in what I believed to be reliable publications, of subjects well known to me, only to find them amateurish and full of silly errors of fact, interpretation or emphasis. Now I was worried that my beloved New York Review would also turn out to have feet of clay.
I am glad to say that the author of this review, Roger Shattuck, who flew transports in the Pacific during the Second World War, passed the test. He discussed with precision, perspective and insight the nature and dimensions of the Wrights’ contribution, and pierced the miasma of hyperbole that surrounds it. He also evaluated several books of the Why I Love to Fly variety without sliding into sappiness.
The review ended with an unexpectedly introspective discussion of various commonly expressed explanations of the lure (for pilots, at least; many people loathe it) of flying. Comparing the pleasures of flight, somewhat arbitrarily, with those of literature, Shattuck concluded: “For many of us flying occupies a privileged status. But it does not trump literature.”
“Us,” I suspect, means readers of The New York Review. Many of us Flying readers may take a different view, or at least feel that in the great world there is ample time and space for both flying and literature, and no need to put them in opposition to one another.
All this is by way of introduction to a bit of lit with which Shattuck began his closing reflections, and which he says inspired him to enlist in the Army Air Corps. It is a short poem by the Irishman and non-pilot William Butler Yeats. I suppose that the great popularity of that other aviation poem, “High Flight,” which is infinitely inferior to this one, is due to its linking of flight with religion, whereas Yeats, writing just after the First World War, links it, perhaps less palatably to fliers, with death. At any rate, to those in whom a love of literature has not been entirely eclipsed by their love of flying, I want to offer this moving poem. It is called “An Irish Airman Forsees His Death,” and, set to suitable music, could be (in an alternate universe) the anthem of the Air Force Academy.
I know that I shall meet my fate Somewhere among the clouds above; Those that I fight I do not hate, Those that I guard I do not love; My country is Kiltartan Cross, My countrymen Kiltartan’s poor, No likely end could bring them loss Or leave them happier than before. Nor law, nor duty bade me fight, Nor public men, nor cheering crowds, A lonely impulse of delight Drove to this tumult in the clouds; I balanced all, brought all to mind, The years to come seemed waste of breath, A waste of breath the years behind In balance with this life, this death.
