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neal.george(at)gmail.com
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 Posted: Mon Oct 18, 2021 6:38 pm Post subject: Fwd: Aero Electric Post |
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Bob et al- One of my neighbors has experienced two sheared alternator couplings recently. Prior to the first event, his electrical system ran trouble-free for approximately 500 hours. More detail in his narrative below.
Neal
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Within the last ~250 hours I've had two alternator shear couplings fail on my RV-6. The first failed after ~248 hours of flying, the second failed at less than two. The alternator is a Plane Power FS1-14B (30A, internally regulated, bolted to the accessory pad) and the airplane is wired using Bob Nuckoll's Z-11 diagram in the Aero Electric connection. This is the only alternator on the plane.The airplane is a full-up Dynon system (10" Skyview, ADS-B, transponder, wx rcvr), a Garmin GTR200 radio, landing light, strobes, nav light. None of the lights were turned on between repairing the first failure and the second one.The going failure theory is an intermittent connection in the field circuit that induces a heavy alternator load by turning it on and off but there isn't anything in the 1Hz data traces from the Dynon that indicates this is the case. Nor is there evidence of any loose connections or nicked wires in the field circuit.The image shows the volt and amp trace for the last 1.7 hour flight. The failure occurred around count #6184. The noise on the front end is startup and taxi. The blip around 2291 was plugging in an iPad.Any ideas on the failure mode?
Keystone Aerospace LLC
Gregg Costabile, President/Owner
(850) 585-3149
gcostabile(at)cox.net (gcostabile(at)cox.net)
[img]cid:93E6C79F-AC9D-40B8-828A-1B2D31EEFF76[/img]
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Ceengland
Joined: 11 Oct 2020
Posts: 380
Location: MS
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| Posted: Tue Oct 19, 2021 6:06 am Post subject: Fwd: Aero Electric Post |
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On Mon, Oct 18, 2021 at 9:42 PM Neal George <neal.george(at)gmail.com (neal.george(at)gmail.com)> wrote:
| Quote: | Bob et al- One of my neighbors has experienced two sheared alternator couplings recently. Prior to the first event, his electrical system ran trouble-free for approximately 500 hours. More detail in his narrative below.
Neal
======
Within the last ~250 hours I've had two alternator shear couplings fail on my RV-6. The first failed after ~248 hours of flying, the second failed at less than two. The alternator is a Plane Power FS1-14B (30A, internally regulated, bolted to the accessory pad) and the airplane is wired using Bob Nuckoll's Z-11 diagram in the Aero Electric connection. This is the only alternator on the plane.
The airplane is a full-up Dynon system (10" Skyview, ADS-B, transponder, wx rcvr), a Garmin GTR200 radio, landing light, strobes, nav light. None of the lights were turned on between repairing the first failure and the second one.
The going failure theory is an intermittent connection in the field circuit that induces a heavy alternator load by turning it on and off but there isn't anything in the 1Hz data traces from the Dynon that indicates this is the case. Nor is there evidence of any loose connections or nicked wires in the field circuit.
The image shows the volt and amp trace for the last 1.7 hour flight. The failure occurred around count #6184. The noise on the front end is startup and taxi. The blip around 2291 was plugging in an iPad.
Any ideas on the failure mode?
Keystone Aerospace LLC
Gregg Costabile, President/Owner
(850) 585-3149
gcostabile(at)cox.net (gcostabile(at)cox.net)
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Likely unrelated to failure, but,
What caused the current to drop from ~25A to zero, then jump back to ~25A at the 230 point?
Again likely unrelated to failure, but,
5A continuous seems pretty low for a full Dynon system plus transponder plus ADSB (if that means 'Out'), plus weather receiver, plus the idle current for a comm radio. Have you verified that number using a separate amp meter in-line with the bus?
To the shear coupling failures:
Is there any 'soft' element in their coupling to handle any torsional resonance issues? Failing a shear coupling that quickly almost sounds like either misalignment issues or torsional resonance related fatigue.
Charlie
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nuckolls.bob(at)aeroelect
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| Posted: Tue Oct 19, 2021 2:09 pm Post subject: Fwd: Aero Electric Post |
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| Quote: | The going failure theory is an intermittent connection in the field circuit that induces a heavy alternator load by turning it on and off but there isn't anything in the 1Hz data traces from the Dynon that indicates this is the case. Nor is there evidence of any loose connections or nicked wires in the field circuit.
The image shows the volt and amp trace for the last 1.7 hour flight. The failure occurred around count #6184. The noise on the front end is startup and taxi. The blip around 2291 was plugging in an iPad.
Keystone Aerospace LLC
Gregg Costabile, President/Owner
(850) 585-3149
gcostabile(at)cox.net (gcostabile(at)cox.net)
|
There is no way to externally 'induce a heavy load' in the
alternator drive shaft by manipulating the field voltage.
Recall that alternator driving torque is proportional
to field flux which is proportional to field excitation
current. The field winding is an inductor . . . the reactive
opposite of a capacitor. Rapid changes in applied voltage to
a capacitor will indeed produce higher currents. Not so
with an inductor. A magnetic field building in response
to applied voltage changes in an inductor are OPPOSED
by that building field. This is called 'counter emf'.
This physical constraint tells us that an alternator's
shaft torque cannot be caused to 'spike' in response
to an application of normal voltage. Voltage available
to excite the field can only come from the bus which
in this case is battery driven. Further, the speed with
which shaft torque changes is moderated by the field's
natural tendency to RESIST variations in current due
to changes in applied voltage.
Example in point. Modern switching regulators in
TC aircraft operate at switching frequencies on
the order of 200 to 1000 cycles per second. An
oscilloscope on the field terminal will show a
variable pulse width, square wave peak to peak
amplitude equal to bus voltage.
Looking at the b-lead terminal, one cannot see
any artifacts that might correspond to those
large, rapid fluctuations in applied field
voltage.
So the idea that a plastic shear coupling can
be 'overstressed' by some anomaly in field
excitation simply has no foundation in physics.
It is far more likely that material or a process
used to fabricate the coupling is deficient.
Bob . . .
Un impeachable logic: George Carlin asked, "If black boxes
survive crashes, why don't they make the whole airplane
out of that stuff?"
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neal.george(at)gmail.com
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| Posted: Wed Oct 20, 2021 2:18 pm Post subject: Fwd: Aero Electric Post |
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Gentlemen - A new Tempest-branded alternator coupler was installed and tested yesterday.
New graph and comments from Gregg follow…
Neal
The graph below was from the test flight yesterday, 19 Oct. The gray line is normalized rpm (actual rpm/150) from the left mag, blue line is volts, amber is amps. Takeoff power was applied around 22:13:13. The alternator field was turned on at around 13:31. The dips at 11:25, 12:55. 14:25, 16:49, etc are curious.
This graph is from “black box data” which is recorded at 16 Hz. The one Hz data shows similar results.
The flight to Gainesville today was uneventful on all counts. I’m now thinking material failure due to heat and possibly bad stock, as Bob mentioned in his email to you yesterday (thanks for passing that along).
[img]cid:image002.png(at)01D7C5AE.51121580[/img]
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