When the flight crew tried to take off with the freshly overhauled airplane, the bending stresses in the blade roots from the improperly damped vibrations were bad enough to cause a low-cycle fatigue failure of one of the blade roots just at liftoff on that first takeoff.

Shouldn't this have been detected on the run up.? :-\ Every major repair I ever made required a run up of some sort, before I signed the log book.

From : Don Stackhouse

There was a test run, and a run-up prior to flight. Neither of those are likely to detect an incipient fatigue crack. Also, just because there isn't much vibration apparent in the airframe does not mean that there aren't any of significance in the engine and propeller, and vice-versa.

Props with four or more blades are especially tricky in this regard, because of "reactionless modes"; i.e.: vibration modes where the blades can be flapping around like birds' wings, but the loads in the individual blades exactly cancel each other, so that no net reaction forces are transmitted back to the engine and airframe. The blades are busy beating themselves to death, but nobody except the prop itself is aware of it. This is one of the issues I have to deal with in the 26" diameter giant scale variable pitch constant speed four-blade props I'm presently developing for Warbird Prop Drives.

For this reason, certification of a new combination of prop/engine/airframe has to be verified by a vibrations survey, either by "similarity" to another sufficiently equivalent installation that had a successful survey, or more often by conducting a full ground and flight survey on the production prototype airplane itself. The prop is shake tested in the lab to determine all the nodal points and natural frequencies, and then strain gauges are mounted in all the key locations on the blades identified by the shake test. A large slip ring with a couple dozen silver tracks (for better conductivity and less noise in the signal) is mounted to the hub, and the data from the strain gauges goes out of the prop through the slip ring to a multi-track recorder. The slip ring may also carry out data from strain gauges on the crankshaft. The plane is then operated throughout its operating envelope on the ground and in the air (this may take a number of flights to get all the data from all the gauges), and the data analyzed for any indications of resonances that could cause dangerous stresses in the prop or engine. Airframe parts such as engine mounts and supporting structure may be studied as well. Even things like accessory mounting brackets can sometimes be an issue. Pilots are not likely to enjoy flying a plane that has things like alternators falling off without warning!.

Without a vibrations survey, there could be high vibrational stresses in the blades that cannot be easily detected in the rest of the airframe. When you repeatedly apply stress and then release it, the damage within the material accumulates, until a fatigue crack is formed. With continued stressing, that crack grows until finally there is not enough uncracked material left to carry the normal loads on the part. At that point the remaining material fractures and the part comes apart, typically ruining someone's entire day in the process.

The vibrations survey measures the stresses in the blades. If any high stresses are found, then the blades are redesigned to change their resonant frequencies and mode shapes in an attempt to get them outside of the prop's normal operating range. The vibrations survey is then repeated, and the process goes on until an acceptable design is developed. With today's computer prediction techniques, they can usually get a safe design on one or two tries, but there can still be surprises, especially on unconventional layouts. Pushers are one class that tends to be problematical, it's tough to predict just what sort of disturbed aerodynamic garbage the airframe is going to feed into the prop.

The stresses must be low enough that a fatigue failure is statistically unlikely to occur in the life of the aircraft, plus a very healthy safety margin. Folks take blade failures on full-scale aircraft pretty seriously; about the only things that rival a blade failure for the ability to create mayhem and catastrophe are things like major in-flight fires (which can be a consequence of a blade failure), and failures of things like wing spars. If there is any question about the fatigue life of the part, and there is no way to design the problem area out, the prop can be life-limited, setting a maximum number of hours or events the prop is allowed to see before it has to be taken out of service and then mutilated in some way so that there is no possibility of the critical parts being inadvertently used again. Lomcevaks and snap rolls in aerobatic aircraft often fall into this category, with a limit on how many of those per flight hour the plane is allowed to do without requiring the prop to be "retired".

Steel parts are fairly easy from a fatigue standpoint because ferrous alloys exhibit a clear "endurance limit", a stress level below which the material has a fatigue life of an infinite number of cycles. For typical steels it's about 40% of the material's ultimate strength. Aluminum is not so lucky; any level of stress, no matter how small, eventually will cause a fatigue failure. However, if the stresses are kept sufficiently low, that fatigue life can be so much longer than the maximum number of cycles that part could ever see in service that the life is for all practical purposes equivalent to unlimited.

At first glance this would seem to favor steel as a material for propeller blades. Unfortunately, about the only way to make steel blades light enough is to make them hollow, which then creates problems such as condensation and drainage, and the major issue of how to inspect the insides of the blades for corrosion and cracks. Steel blades have been tried in the past, but historically their safety record has been pretty spotty.

Wood is somewhat unique in that its endurance limit is generally considered to be in excess of its ultimate strength. There can be exceptions to that, but in general, if a wooden part does not fail due to one cycle of the highest stress it sees in service, it will not fail from fatigue. Rot, termites, mold, carpenter ants, UV damage from sunlight, temperature and humidity changes, etc., but not from fatigue. Therefore, the FAA does not require a vibrations survey for wood props, but does require a successful test run at full power and RPM plus an additional safety margin.

In the case of the Beech 18 blade failure, the specified prop/engine/airframe combination had been successfully vibration surveyed. However, when the mechanic installed the wrong dampers in the engine, he altered the entire prop/engine/airplane combination to one that had NOT been successfully tested, in fact one where a dangerous resonant frequency occurred right at the takeoff RPM. The huge stresses in the blades at that RPM and power setting caused a low-cycle fatigue crack to form and grow to catastrophic size in only a few minutes running time.

The key point here is that props accumulate fatigue cycles incredibly fast. For example, a four-and-a-half order fatigue stress on an engine turning 2300 RPM at takeoff power (such as the P&W R-985 in the Beech 18) adds up to 100,000 fatigue cycles in less than 10 minutes!

As far as the discussion about the torsional dampers installed in the flywheels of car engines, this is a different approach to the same problem of mitigating the effects of torsional vibration. The flywheel dampers don't eliminate the vibration, but by coupling the engine to the drivetrain through what amounts to a large spring, they attempt to keep the vibrations inside the engine and impede them from getting into the drivetrain. Unfortunately, in the case of a propeller on an airplane, this approach opens up a whole Pandora's box of other issues. Conversely, the pendulum dampers mounted on the crankshaft attempt to cancel out the vibration at its source. Both approaches can be valid, depending on the details of a given application.

Don Stackhouse
DJ Aerotech