Put prop balancing in your smooth-engine arsenal
Landing-light filaments flicker and fail. Instrument needles bounce against faces. Rivets fret; aluminum flexes and cracks. Vibration — it's nobody's friend, but a natural condition of the internal- combustion engine mated to a propeller. In excessive amounts, vibration will shorten the life of the airplane and its accessories, and make a three-hour trip seem like a month of listening to Uncle Joe's Trailways anecdotes. In the great collective consciousness of pilots, some engine-and-airframe combinations come to be known wistfully as "real smooth" or, less positively, as "just a plain old shaker."
Vibration in a piston-engine airplane comes almost entirely, as you surely surmise, from the powerplant itself. There are some airframe-buffeting effects, but they are minor compared to the powerplant's output. After all, the whole concept of taking linear motion (the piston's rising and falling in the cylinder) and converting it into rotational energy and then finally to the propulsion of pockets of air by the propeller seems to lend itself to a bad case of the shakes. Indeed, as the industry comes up with increasingly sophisticated means of measuring vibration, many new variables associated with powerplant vibration have surfaced, as well. Owners moving up from piston twins to turboprops usually comment early on about the kerosene-burner's impeccable smoothness.
Why is the piston engine such a trembler, then? Put simply, vibration comes from an imbalance of forces. Few engineers try to design an engine with no vibration, in part because, while it's theoretically possible to completely counteract the significant forces of each piston's coming to an abrupt stop at each end of its stroke, it's not entirely practical. Theoretically, an opposed engine could have its major reciprocating masses balanced and thereby counteract this part of its vibration signature. However, because of the forces involved, even a small imbalance results in perceptible vibration. Plus there are too many in-service variables to upset this fine plan, such as the migration of oil and carbon build-up on reciprocating and rotating components.
Moreover, by their very design, typical opposed-piston powerplants will have some residual vibration, no matter how well balanced internally. Why? Rather than having the cylinders perfectly opposed, they are slightly offset front to back; to have them perfectly aligned would require a knife-and-fork connecting rod style, a la Harley-Davidson (and only Harley-Davidson) motorcycles. This slightly side-by-side positioning of the opposed cylinders creates what's called a rocking couple; no matter how finely balanced, every opposed aircraft engine will have some residual rocking-couple vibration.
Another cause of vibration — imbalances in the engine's rotating parts, such as the crankshaft and camshaft. Can something as small as a cam create noticeable vibration? Surely it can. Remember that it's common now in automobiles to use counterbalancers — a weight spinning off-center on a shaft, essentially — to quell vibration. And they don't have to be very large to be effective, either; a few ounces will do the trick. Crank length also affects engine vibration, because as sturdy as they seem on the bench, cranks continually flex in use — loading and unloading — subtly throwing off the best efforts of engineers to create balance. Four- cylinder engines benefit from shorter cranks but give away in the vibration game because of their fewer, more widely-spaced power pulses. Eights, which you would expect to be exceptionally smooth, tend to suffer from tremendous higher-order vibration because of the flexing of the long, limber crank.
Differences in combustion events — one cylinder producing more power than another — also produce vibration. Cylinders near the back of the engine also have a greater influence because they are farther from the greatest area of crankshaft-bearing structure and that stabilizing gyroscope out front called the propeller. If your Lycoming's number five cylinder is weaker than its number six, you'll get increased vibration.
As an aircraft owner, there are relatively few things you can do to affect the vibration characteristics of your engine. By far the simplest and most popular of these is propeller balancing. Now, every propeller comes from the factory statically balanced; that is, the blades are matched in weight so as to preclude any gross imbalance. This is not performed dynamically, however, and not while attached to the engine that will become its mate on the airframe.
Prop balancing, a steadily growing industry for the last half decade or so, came to us from the helicopter world. With long blades and plenty of mass, dynamic balancing is critical to rotorcraft to help eliminate vibration and, more important, to stave off the deleterious effects of blade oscillations on the hardware itself. A wildly out-of-balance rotor system will quickly tear itself apart.
An accelerometer, along with a timing device that is used to determine both prop speed and indexing, makes up the heart of the modern prop-balancing system. For most piston engines, a solid- state accelerometer is mounted on the crankcase, close to the propeller flange. It does what its name implies, sensing the rapid movements of the crankcase that eventually feel like vibration to us.
Information from the accelerometer is fed to a portable computer, and an acceleration graph will be produced. This graph will display the amount of vibration measured in velocity, usually inches per second, at various frequencies. The relationship to the base frequency — the speed of the engine — determines any vibration's order. A first-order vibration, then, occurs at crankshaft speed. A second-order vibration is one centered at twice crankshaft speed, and so on.
Prop balancing attempts to reduce vibration principally at the first order — an unbalanced two-blade propeller will send out strong vibration on the first order because its center of mass is not aligned with the center of rotation.
We had an Aerospatiale Trinidad TC balanced by A&P Norman Heath, using the Microvib system. The Microvib is one of the newest systems intended for field dynamic prop balancing; some of the Microvib principals had been with Chadwick Helmuth, widely considered the pioneer in the field. While the Chadwick system uses a strobe to determine prop phase, the Microvib system employs a small optical sensor aimed at the back of the prop; a swatch of reflective tape on the back of one blade helps the sensor to find the zero-index point. Then, with the accelerometer bolted to the crankcase parting line, it's time to run the engine. For the Trinidad, we used 2,200 rpm, a reasonable percentage of the engine's 2,545- rpm maximum; however, the static runup did not have the turbo system creating any appreciable boost.
Several runs may be made to fine-tune the placement and amount of weight bolted to the prop flange to blank out the first- order vibration. When having the prop balanced, make sure the technician installs the counterweights on something quite hefty; simply screwing them through holes on the spinner backplate doesn't cut it. Even a few grams of weight will create a tremendous amount of force at takeoff speeds. One of the best aspects of the Microvib setup is that the computer will produce before-and-after graphs showing accelerations at various frequencies. In the Trinidad, the after page shows a remarkable decline in first-order vibration, from 0.257 inches per second (IPS) to 0.037 IPS. That means the two small 3/16-inch bolts used as weights in the prop flange, when properly placed, indeed make a big difference.
Still, the big six-cylinder Lycoming continued its shaking ways, with strong oscillations at 0.5, 2, 2.5, and 3 times crank speed. The half-order vibrations are generally imbalances in combustion events, whereas the second order is typically a reciprocating-mass imbalance and a bit of that rocking couple.
But what's important to understand is this: Prop balancing, while unquestionably a good thing, will not cure all of the engine's shakes. In flight, the Trinidad felt marginally smoother, but the general roughness noted by the pilots remained largely intact. And it's hard to know exactly why. We asked several engine experts; the general consensus is that the big Lycomings are a bit rougher than the Continentals, perhaps because they are not as well balanced, or maybe because of the vastly different mounting schemes. The Lycoming is typically hoisted by the back of the engine, whereas many of the large Continentals use a bed mount, cradling the engine from beneath. In any case, prop balancing didn't convert this TIO-540 from a longshoreman to an English butler. It's better now, but nobody will mistake it for a turbine.
Some other things to consider. A new and unusual vibration in your engine should be tracked to its source immediately. We heard one report of a fellow who had his airplane's prop balanced, the camshaft timing and lift checked, the ignition system overhauled, and so on — all in search of a new, strange vibration. Finally, metal began showing up in the filter and on oil analysis; a teardown revealed that the center main bearing had spun and the vibration was the crank slamming against the cases.
Many familiar with prop balancing say flat out that it should be used as a fine-tuning device, that a serious vibration problem may well be something other than an off-balance prop — engine mounts collapsed or something on the engine touching an airframe component under the cowling, for example. For the price — about $150 for a single — dynamic prop balancing is something every airplane owner ought to consider; it's inexpensive and certainly will not hurt anything. Chances are your landing lights and instruments will thank you for it.
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