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AOPA Article

A Balancing Tour


Vibration Analysis


















Smooth Moves

Reducing fatigue and wear through better balancing

BY STEVEN W. ELLS (From AOPA Pilot, November 2003.)

Permission to reprint granted 12/04.


Dynamically balancing your prop won't help you if you're flying an airplane with a worn-out engine or beat-up propeller. But if your engine and propeller are in airworthy condition there's a good chance that a dynamic prop balance will transform your airplane into a smoother mover. 

Rockwell Swanson spent $225 for a dynamic propeller balance. "The minute I pushed in the throttle I knew I'd done the right thing," he said. Swanson like almost 90 percent of airplane owners thought the engine in his Cessna Turbo 210 was smooth until he rode in a very well-balanced Beechcraft V35B Bonanza. Swanson's propeller needed balancing in prop-balancing terms the initial reading of imbalance was 0.714 inches per second (IPS); after balancing, the imbalance was reduced to 0.026 IPS. We'll get to the language of propeller balancing later in this article. 

Engine Components Inc. (www.eci2fly.com), manufacturer of Titan cylinders and an experienced engine repair and overhaul facility located in San Antonio, suggests that the crankshaft, connecting rods with bolts and nuts, pistons, piston pins, counterweights, and all counterweight attaching hardware be sent in for balancing. In addition to this general list, Lycoming starter gear supports (also called ring gears or flywheels), Teledyne Continental Motors (TCM) crankshaft alternator face gears and bolts, and rear crankshaft gears for C-series and 200-, 240-, 300-, and 360-series engines must be balanced to achieve the smoothest- running engine. Some of these parts are balanced statically and some are balanced dynamically. When the weight of a nut affects engine balance, it's a strong signal that balancing is important.

 Static balancing

The engines in most of our airplanes have a number of parts that move back and forth (reciprocate) as part of normal operation. Weight variations between like parts (such as pistons) create imbalances and vibrations. Engine manufacturers and engine overhaul facilities strive to match the weights of each piston in a set of pistons, each connecting rod in a set of connecting rods, and each wrist pin in a set of wrist pins. Twenty to 30 years ago, when manufacturing wasn't well controlled, overhaul shops had to stock large quantities of each piston part number, for instance, and weigh each piston in an attempt to match piston weights. This practice is known as tolerance matching or tolerance stacking. Because of advances in manufacturing, and the power of competition, today all manufacturers sell parts sets that are very closely matched in weight.

 Not only are connecting rods matched by weight, they're also matched by the weight distribution between the big end and small end of the rod. All engine manufacturers and all engine rebuilders have written standards outlining their weight tolerances for each component. For instance, TCM Service Information Letter 02-1, titled "Piston Position Identification and Piston Weights," lists piston weights by two-gram divisions. 

Static balancing becomes an issue for owners when a cylinder needs changing. Owners and mechanics must pay attention to the part numbers and weights of both connecting rods and pistons when changing cylinders. Both Teledyne Continental (in Service Information Letter 02-1) and Textron Lycoming (in "Notes on Replacing Connecting Rods and Pistons" in its Key Reprints and Service Instruction No. 1243) detail these issues. Briefly, it's critical that the weights of reciprocating components in opposing cylinders be closely matched. In the past, when cylinders were refurbished by grinding oversize (removing metal to return the cylinder walls to a non-worn profile) the oversized pistons required to maintain the proper piston-to-cylinder bore fit could present imbalance problems, especially if the maintenance technician wasn't aware of the importance of piston weights.

 Again, competition from companies such as Superior Air Parts and Engine Components Inc. (ECI) has lowered new cylinder prices to the point that buying new cylinders is often more cost effective than grinding oversized cylinders. This is especially true if the cylinder time in service is high or unknown. First-run, or other low-time, cylinders can be rejuvenated using ECI's CermiNil coating process these advances have largely eliminated the practice of grinding oversize cylinders and returning them to service.

 Since the weights we're talking about are very small a single piece of typing paper weighs 4.7 grams while TCM pistons range from a low of 706 to a high of 1,640 grams manufacturers allow some piston weight variations. TCM's advanced technician training course book specifies that piston weight differences of up to one-half ounce, or approximately 14 grams, are allowable. Lycoming, in Service Instruction No. 1243, lists certain pistons with an "S" suffix behind the part number. These are known as service pistons and have weights that fall midway between original piston weight limits. As such, they are approved to replace either heavier- or lighter-weight pistons with the same part number.

 Dynamically balanced components

Crankshafts and other rotating parts should be dynamically balanced, or checked for balance, before installation in an engine.

 TCM specifies that its crankshafts are balanced to a maximum imbalance calculated on a two-inch radius at a rotational speed of 600 rpm. This specification means that the maximum imbalance would be akin to a three-quarter ounce weight located at an arm, or distance, from the middle of the crankshaft of one inch. Various other field overhaul facilities advertise lower numbers such as one-half-inch ounces. 

Aircraft Specialties Services, of Tulsa, balances more than 100 crankshafts per week. The company reports that 90 percent of new crankshafts and crankshafts from engines that have been manufactured or rebuilt within the past five to 10 years turn out to be well balanced. Fifty percent of the crankshafts that have been out in the field for more than 10 years need balancing.

 If an owner desires balancing to closer tolerances, or if an imbalance is detected, qualified shops have FAA approval to grind off a small amount of metal from noncritical surfaces of the crankshaft. It doesn't cost much to check the balance of a crankshaft checking ranges from $50 to $80, and a complete dynamic balance usually runs under $200.

There is some conjecture as to whether the manufacturer's balance tolerances are close enough. Lycoming says that its engines are carefully balanced to the degree that is necessary and concedes that its engines are not balanced to the point of perfection Lycoming company officials say it's not necessary because of the slow crankshaft rotational speeds of Lycoming engines. They further state, in the engine balance section of the Key Reprints portion of the company Web site (www.lycoming.textron.com), "Additional internal balancing contributes little to engine smoothness." 

On the other side of the issue are engine rebuild shops that insist that close-tolerance static and dynamic balancing is critical, and warn owners that without combining fine-tuned balancing with their practice of balancing all cylinder combustion chamber volumes (a practice that's designed to match the power output of each cylinder on an engine) an engine will never be electric-motor smooth. There are strong advocates for both approaches, but everyone agrees that crankshaft balancing is money well spent. 

If I were overhauling my aircraft's engine, I'd pay the small amount of extra cash to get the crankshaft balanced to the smallest imbalance the shop could deliver. A smooth-running engine lessens both metal and pilot fatigue, reduces avionics and instrument maintenance, and impresses passengers. Having said that, the smoothest-running general aviation engine I have ever flown behind was installed in Dick Bicknell's Cessna 182. The 182's carbureted six-cylinder TCM O-470 engine is not renowned for its smoothness; in fact, uneven fuel distribution because of its log-type induction manifold causes large splits in cylinder exhaust gas temperatures (EGTs). Split EGTs indicate that there's a variance in the strength of the cylinder power impulses, which have an effect on an engine's net smoothness. Bicknell's engine overhaul was done by a competent shop, but no extraordinary balancing was specified. He said the smoothness was the result of a dynamic propeller balance. 

Dynamic propeller balancing

Dynamic propeller balancing is accomplished by ground running an engine-propeller installation and then attaching small weights, most often to the backing plate of the propeller spinner, to reduce the imbalance of the rotating mass. 

ynamic prop balancing is only effective at decreasing vibrations that occur at propeller rpm. These vibrations are called first-order vibrations because they occur once for each crankshaft-propeller (assuming the propeller is not geared) rotation. Excessive half-order (half-order vibrations occur when the spark plugs fire and the power stroke starts in the cylinders) and first-order vibrations are destructive rivets loosen, wear in avionics and instruments is accelerated, and pilot fatigue increases since engine-mount vibration isolators aren't very effective at dampening these frequencies.

 Dynamic balancing is quick and easy. A minimum of one vibration sensor is attached to the engine just aft of the propeller. This sensor converts motion into electrical signals that are fed into a processor box. This box plots the magnitude of the mass imbalance, and the location (azimuth) of the imbalance with reference to the propeller disk. A solution is derived and simple weights (almost always in the form of aircraft-quality screws, nuts, and washers) are attached to the aluminum propeller spinner backing plate. The engine is then run up on the ground; usually the tester selects an arbitrary rpm representative of cruise power such as 2,200 or 2,300 rpm, although Jim Beech of Dynamic Solutions Systems says that 2,000 is high enough since this rpm is above the excitement rpm of the engine mounts. The processor is then able to plot the effect of the initial weight solution and create a final solution.

 Kent Felkins of Felkins Aircraft Service in Tulsa has been doing dynamic propeller balancing for 15 years. He is a believer. Using the 0.2-IPS standard as a baseline, Felkins has kept records showing that the propellers on 88 percent of 580 airplanes he tested were out of balance.

 Manufacturers of balance equipment recommend rechecking the balance every 400 to 600 hours. Felkins, an experienced balancer, cautions that anytime the prop is removed to change an alternator belt on a Lycoming engine, for instance the propeller-engine combination should be rebalanced.

 The goal of balancing is to bring the center of the rotating weight (mass) into alignment with the rotational center of the crankshaft. Because there are manufacturing tolerances built into propellers that allow them to be mounted onto the crankshaft flange, removal and reinstallation require rebalance.

If the center of the rotating weight mass is one one-thousand of an inch out of alignment with the centerline of the crankshaft, Felkins' rule of thumb says that this creates a 0.3 IPS imbalance. All the manufacturers of balancing equipment agree that 0.3 IPS is too high.

 Imbalance is measured in inches per second, which is an expression of velocity. Imbalance also can be expressed in units of gravity (Gs) or in displacement (in milliliters or 0.001) caused by the imbalance. 

 Is your airplane out of balance?

Based on Felkins' figures, most pilots have never flown a smooth airplane. That, coupled with the fact that personal definitions of smoothness are subjective, often makes skeptics out of airplane owners. Yet Cirrus, Lancair, Mooney, and other high-end airplane manufacturers balance their propellers before delivery. There are some telltale signals that often indicate an out-of-balance condition.

 If you feel a shiver go through your airplane as you reduce prop rpm prior to landing, it's probable that the prop is out of balance. The vibration is caused by a harmonic vibration between the frequency of the out-of-balance prop and the dampening frequency of the engine mounts. Typically this happens between 1,800 and 1,200 rpm.

 If instrument needles vibrate, or the compass never settles down, this indicates a condition that propeller balancing may improve. Do you have to stop every two hours during a cross-country to get some circulation back into your body? Does your arm, or one or both feet, go to sleep during cruise flight when rested against the cockpit or floorboards?

 All of these scenarios are out-of-balance indicators. More than one pilot has reported that the effects of getting his propeller balanced has made flying fun again simply because of the reduced fatigue associated with a smooth-running airplane.