I was lookin around today and ran across this article. I am sure these guys have no clue what they are talking about ... but what if they did hmmm makes you wonder huh...
sprag clutch article:
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).
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.
We have already shown through some fundamentally simple calculations (Vibration Basics) that the destructive 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).
IF a one-way clutch which actually HAD 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. So 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 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 approach, 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 will fracture the clutch outer race in a relatively short time.
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, 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.
continued ...
(1) Perhaps the most important point to be made by this presentation is that PSRU’s, propellers, and engine mounts are complex devices. PSRU’s for conversions are the incarnation of EXPERIMENTAL. There is no significant body of accumulated experience behind most of them. Treat them as such.
(2) TEST STAND OPERATION IS NOT THE SAME AS FLIGHT.
(3) Do not be misled by the false belief that "if you cannot FEEL vibration, then none exists". Destructive vibration can occur at frequencies far beyond the range humans can detect.
(4) The life expectancy of your PSRU and propeller (and by extension, you, your passengers and your aircraft) depends on a complex mixture of factors including the dimensions, stiffnesses, materials, heat-treatments, and quality control of critical components, the metallurgical goodness of the gear material (AGMA GRADE), the accuracy (AGMA QUALITY) of the gears, the reduction ratio, the Jm of the propeller, the talent of the designer(s), and other factors.
(5) Be SUSPICIOUS of (a) any PSRU that allows the use of a light flywheel, (b) of any PSRU that has a torsionally-rigid coupling between the engine crankshaft and the PRSU drive gear, or (c) of any PSRU that which can be bought without a drive coupling system engineered for THAT PSRU and YOUR engine.
(6) RUN (don’t walk) away from anything with a one-way clutch in the propeller drive system, or with magical (unexplainable) properties. Be careful and be cynical.
(7) Before buying a PSRU, have the seller satisfy you (with data, not allegations) that the SYSTEM will operate correctly with your selection of engine, propeller, and gear ratio. Don’t be satisfied with marketing drivel.
(8) Remember that, unlike the experimental world, TBO numbers in the real world are established on the basis of substantial experience and documented evidence, both practical and scientific. Far too many TBO's in experimental aviation are no more than the product of some peddler's wishful thinking.
(9) Ask to talk to the designer. Don’t be afraid to ask him the relevant questions (see FAQ's). Maybe the designer knows about gears but didn’t do the vibration analysis. Maybe the designer has no engineering knowledge at all, but has money enough to buy CAD software and CNC machining equipment, and can talk faster than you. Maybe the "...thousand hours this PSRU has in service" came from 10 hours each on 100 airboats. Ask about previous litigations involving their products. Speak to actual customers.
(10) Don’t accept one of the countless variations on the theme:
THIS PRODUCT WAS DESIGNED BY THE (best, most experienced, most aviation-knowledgeable) EXPERT IN (the world, the USA, Germany, Uganda) BUT HE (is too busy, is undercover for the CIA, has been abducted by aliens, only speaks Swahili) AND CAN’T TALK TO YOU.
(11) Be cautious of most race-car experts. There are a FEW brilliant race-engine guys out there, but their perspective is often quite different than yours. Remember, long life for a typical ski-boat engine is 100 hours. Long life for a Nextel (Winston) Cup or Busch engine is 3 hours. (Some race-car guys think long life is 15 seconds, measured in 5-second chunks.)
REMEMBER: Many Lycomings and Continentals live thousands of hours at 75% power without a failure. In fact, much of the grief with "LycoNentals" comes from disuse.
(12) While it is well recognized that the Federal Aviation Regulations defining design and testing standards (PARTS 23, 33 & 35) do not apply to experimental aircraft, nevertheless those regulations provide a rich source of MINIMUM DESIGN STANDARDS which a prudent aircraft designer would be well advised to study, especially with regard to structural and reliability issues.
So I wonder ... I always thought I wanted one of those things but maybe not now. cause for a pause??
sprag clutch article:
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).
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.
We have already shown through some fundamentally simple calculations (Vibration Basics) that the destructive 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).
IF a one-way clutch which actually HAD 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. So 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 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 approach, 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 will fracture the clutch outer race in a relatively short time.
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, 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.
continued ...
(1) Perhaps the most important point to be made by this presentation is that PSRU’s, propellers, and engine mounts are complex devices. PSRU’s for conversions are the incarnation of EXPERIMENTAL. There is no significant body of accumulated experience behind most of them. Treat them as such.
(2) TEST STAND OPERATION IS NOT THE SAME AS FLIGHT.
(3) Do not be misled by the false belief that "if you cannot FEEL vibration, then none exists". Destructive vibration can occur at frequencies far beyond the range humans can detect.
(4) The life expectancy of your PSRU and propeller (and by extension, you, your passengers and your aircraft) depends on a complex mixture of factors including the dimensions, stiffnesses, materials, heat-treatments, and quality control of critical components, the metallurgical goodness of the gear material (AGMA GRADE), the accuracy (AGMA QUALITY) of the gears, the reduction ratio, the Jm of the propeller, the talent of the designer(s), and other factors.
(5) Be SUSPICIOUS of (a) any PSRU that allows the use of a light flywheel, (b) of any PSRU that has a torsionally-rigid coupling between the engine crankshaft and the PRSU drive gear, or (c) of any PSRU that which can be bought without a drive coupling system engineered for THAT PSRU and YOUR engine.
(6) RUN (don’t walk) away from anything with a one-way clutch in the propeller drive system, or with magical (unexplainable) properties. Be careful and be cynical.
(7) Before buying a PSRU, have the seller satisfy you (with data, not allegations) that the SYSTEM will operate correctly with your selection of engine, propeller, and gear ratio. Don’t be satisfied with marketing drivel.
(8) Remember that, unlike the experimental world, TBO numbers in the real world are established on the basis of substantial experience and documented evidence, both practical and scientific. Far too many TBO's in experimental aviation are no more than the product of some peddler's wishful thinking.
(9) Ask to talk to the designer. Don’t be afraid to ask him the relevant questions (see FAQ's). Maybe the designer knows about gears but didn’t do the vibration analysis. Maybe the designer has no engineering knowledge at all, but has money enough to buy CAD software and CNC machining equipment, and can talk faster than you. Maybe the "...thousand hours this PSRU has in service" came from 10 hours each on 100 airboats. Ask about previous litigations involving their products. Speak to actual customers.
(10) Don’t accept one of the countless variations on the theme:
THIS PRODUCT WAS DESIGNED BY THE (best, most experienced, most aviation-knowledgeable) EXPERT IN (the world, the USA, Germany, Uganda) BUT HE (is too busy, is undercover for the CIA, has been abducted by aliens, only speaks Swahili) AND CAN’T TALK TO YOU.
(11) Be cautious of most race-car experts. There are a FEW brilliant race-engine guys out there, but their perspective is often quite different than yours. Remember, long life for a typical ski-boat engine is 100 hours. Long life for a Nextel (Winston) Cup or Busch engine is 3 hours. (Some race-car guys think long life is 15 seconds, measured in 5-second chunks.)
REMEMBER: Many Lycomings and Continentals live thousands of hours at 75% power without a failure. In fact, much of the grief with "LycoNentals" comes from disuse.
(12) While it is well recognized that the Federal Aviation Regulations defining design and testing standards (PARTS 23, 33 & 35) do not apply to experimental aircraft, nevertheless those regulations provide a rich source of MINIMUM DESIGN STANDARDS which a prudent aircraft designer would be well advised to study, especially with regard to structural and reliability issues.
So I wonder ... I always thought I wanted one of those things but maybe not now. cause for a pause??
