- The "Linear Coupling Device" -
A Dangerous Hoax
One of the most dangerous designs we have seen is a PSRU that contains a sprag-clutch with a magic-sounding name ("LCD", or "Linear Coupling Device"). Surreal properties are claimed for this magic device, specifically, that "...it isolates all the engine shock and torsional excitation from the gearbox".
That’s a curious claim in view of the fact that the clutch is downstream of the input gear in the examples we have seen (located in the output shaft in one case, and in the idler shaft in another). But, on the other hand, nothing claimed by that particular peddler is surprising.
It’s even more curious that the purveyor of this PSRU alleges that "...the magic device can couple and decouple as fast as 75 times per second, which breaks the resonance feedback loop". However, the manufacturer of this "device", which is no more than a commercial sprag clutch, states in their engineering data that the maximum rate ever achieved with this clutch is 20 Hz. However, that PSRU peddler has demonstrated himself to be completely untroubled by contradicting published engineering data or widely-accepted scientific facts.
We have already shown through some fundamentally simple calculations (VIBRATION BASICS) that the destructive PSRU vibration loads are applied at frequencies ranging from 240 Hz to 320 Hz. (three to four times faster than the max CLAIMED rate of the wondrous "device", and more than 10 times its ACTUAL rate).
SO IF a one-way clutch which actually HAS a maximum rate of 75 Hz were driven by an 8-cylinder engine, it would become a solid coupling above 1125 RPM, barely above idle. Therefore it is clear that, even if this "device" COULD couple and uncouple at 75 HZ, it would be completely inert at the frequencies which excite the destructive vibrations.
The design of this particular PSRU caused the first mode resonant frequency of the propulsion system I examined (a V8 engine, this vendor's PSRU, and a composite propeller) to be about 3700 RPM (right at the bottom of the cruise range). At max takeoff RPM (4700) the system was not even close to crossover RPM (5230). Imagine the FORCES AT WORK to destroy that system.
There are other severe problems with this sprag-clutch implementation, however. The PSRU vendor claims that the clutch decouples the propeller from feeding back vibratory energy into the system. It purportedly does this by disengaging the engine from the PSRU during the part of the torque cycle in which the instantaneous engine torque is low (a torque "valley"). So far, so good.
But when the next cylinder of the engine fires, the PSRU input shaft accelerates freely (it is not driving any load because the magic-clutch has decoupled). When the shaft speeds match, the output shaft is still at a relatively constant speed while the input shaft is accelerating rapidly. Suddenly, BANG, the clutch re-engages, imposing an extremely high shock load on the gearbox and propeller because the re-engagement of the clutch picks up the entire engine output over a period of a few microseconds. These high shock loads (hoop stress) fractured the vendor's early version of the clutch outer race in a relatively short time. The only way it could be made to survive was to make the walls VERY thick ( heavy ) And that simply moved the impact loading problem to another component.
Further, even if the clutch did reduce the gain of the resonance feedback loop, it does nothing to isolate the gears or the propeller from the torsional excitation (engine peaks times transmissibility) the engine produces.
Worse yet is the fact that with a one-way clutch in the PSRU, when the pilot reduces the engine power in flight (for a quick descent from altitude, for example) the propeller becomes freewheeling. Thus, if the airspeed remains high during the descent, the propeller can easily overspeed and shed a blade. Compounding the problem, the pilot is unlikely to detect the overspeed until a disaster happens, because both the sound of the engine and the reading of the tachometer confirm to him that RPM is decreasing.
The vendor of the PSRU-with-sprag-clutch adamantly denied that the propeller will freewheel when power is reduced. Then later, to validate his design, he produced an engineering analysis of his PSRU by a university professor. That study states unequivocally that the propeller will freewheel when power is reduced. When questioned about that discrepancy, the vendor responded: "The professor does not know what he’s talking about". Amazing?
A client of ours proved in flight tests that the propeller on this PSRU does freewheel when power is reduced, exactly as stated here. He also found, to his chagrin, that when it freewheels in the landing flare, it removes the expected propeller drag and greatly extends the rollout.
Note that helicopter gearboxes use a sprag (one-way) clutch between the engine and the rotor drive gearbox. The purpose of that clutch is to automatically disconnect the rotor system from the engine in case of an engine failure. That allows the rotor system to be propelled by the sinking of the helicopter through the air, thereby allowing the pilot to maintain control of the aircraft during the descent and to use the energy stored in the rotor system to decelerate the aircraft to a successful landing. It’s called "autorotation".
We have used a pair of sprag clutches in the input coupling to our double-engine gearbox (the MARK-14) for the purpose of automatically disconnecting a failed engine from the prop and allowing the remaining good engine to continue to power the prop. Because of those sprag clutches, the Mark-14 allows the prop to freewheel when power is reduced (described above), but it is an acceptable compromise since the gearbox was designed primarily for a race application.