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alexpeterson(at)earthlink
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 Posted: Mon Apr 10, 2006 4:24 pm Post subject: Why use starter contactor? |
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| Quote: |
--> <emjones(at)charter.net>
A dump of what I think I know and some vaguely suspicions--
Stuck Starters--The starter motor pinion is 1.2 inch in
diameter, the ring gear is about 12 inches in diameter--a
1:10 ratio. The starter (non-geared type) turns at several
thousand RPM to turn the engine at several hundred RPM. When
the engine is running at maybe 4000 RPM, an engaged pinion
would turn the starter at 40,000 RPM.
1) Is this even possible?
2) Since a PM starter would be a generator, what would be the effect?
3) Would not a pilot have any way to know this?
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I don't know about all aircraft starters, but the starter pinion assembly on
a Lamar starter will not allow the starter to be spun up by the engine.
There is a centrifugal release on the pinion (small gear on starter) which,
once released above some rpm, causes the pinion to retract. The retraction
is caused by the (driven) pinion threading its way along a coarse thread. I
only know the Lamar starter, but I would hope all starters act similarly. It
would be a crappy design if it weren't tolerant of people holding the start
button on too long.
Alex Peterson
RV6-A N66AP 750 hours
Maple Grove, MN
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nuckollsr(at)cox.net
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| Posted: Mon Apr 10, 2006 5:34 pm Post subject: Why use starter contactor? |
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At 07:21 PM 4/10/2006 -0500, you wrote:
| Quote: |
<alexpeterson(at)earthlink.net>
>
> --> <emjones(at)charter.net>
>
> A dump of what I think I know and some vaguely suspicions--
>
> Stuck Starters--The starter motor pinion is 1.2 inch in
> diameter, the ring gear is about 12 inches in diameter--a
> 1:10 ratio. The starter (non-geared type) turns at several
> thousand RPM to turn the engine at several hundred RPM. When
> the engine is running at maybe 4000 RPM, an engaged pinion
> would turn the starter at 40,000 RPM.
>
> 1) Is this even possible?
> 2) Since a PM starter would be a generator, what would be the effect?
> 3) Would not a pilot have any way to know this?
I don't know about all aircraft starters, but the starter pinion assembly on
a Lamar starter will not allow the starter to be spun up by the engine.
There is a centrifugal release on the pinion (small gear on starter) which,
once released above some rpm, causes the pinion to retract. The retraction
is caused by the (driven) pinion threading its way along a coarse thread. I
only know the Lamar starter, but I would hope all starters act similarly. It
would be a crappy design if it weren't tolerant of people holding the start
button on too long.
|
When you think about it, some kind of over-run mechanism is
essential to the survival of a starter motor. Cranking RPM is
always a small fraction of running rpm. A "hard" geared
connection between the starter ring gear and the starter's
armature would be stressed very hard when the engine fires
of and attempts to accelerate the starter motor's mechanisms
as well.
In modern starters with high gear ratios between armature and
pinion shaft, the potential for stress is even higher. Should
the gear train stand the stress of accelerating the armature,
the next potentially destructive boundary to cross is keeping
the wires in the armature and bars on the commutators intact.
Centrifugal forces are high on armature operating at its
design speed. Spin it up by 5 to 10x its design speed and
the windings are thrown out or commutator bars extrude out
of their moorings.
Inertially engaged drives (Bendix) have a helix that pushes
the pinion gear out when the starter motor is accelerating
the system. If the engine starts and tries to push the
starter, then the same helix will retract the pinion. Starters
with positive engagement solenoids have some form of overrunning
clutch to disconnect the slower starter from the rapidly accelerating
engine.
Some feature to prevent back-driving a geared starter while
the pilot senses that the engine has started and releases
the start command is not an option.
Bob . . .
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gmcjetpilot(at)yahoo.com
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| Posted: Tue Apr 11, 2006 9:23 am Post subject: Why use starter contactor? |
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Bob wrote:
>Of course, continuous duty contactors are used only for
>battery contactors and are ALREADY closed in flight so g-loading
>is not a matter for consideration
Bob, I know of "master relays" can open in flight if mounted so positive
G's act to open the device. It has happened to a few RV's. It is not as
critical as the starter, but a master opeining mid aerobatic flight could
cause a little havoc.
Mickey wrote:
>My battery
| Quote: | contactors are laying on their side, so the only G force that
will cause them to open will be acceleration, and my engine
won't have that much power.
|
Also, Call Van about sideway mount relays, they fail early due to galling.
Van's had a service bulletin / notice about 10 years ago on this.
B&C's starter contactor is designed to mounted Horz.
For a typical vertical relay, mount the starter relay so Pos G's
keep it open and the master relay is held closed by pos G's.
Most sport planes, even RV's pull much less negative G's than Pos.
As for hard core acro planes I wounder what they do also?
Cheers G
---------------------------------
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nuckollsr(at)cox.net
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| Posted: Tue Apr 11, 2006 10:50 am Post subject: Why use starter contactor? |
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At 10:18 AM 4/11/2006 -0700, you wrote:
| Quote: |
Bob wrote:
>Of course, continuous duty contactors are used only for
>battery contactors and are ALREADY closed in flight so g-loading
>is not a matter for consideration
Bob, I know of "master relays" can open in flight if mounted so positive
G's act to open the device. It has happened to a few RV's. It is not as
critical as the starter, but a master opeining mid aerobatic flight could
cause a little havoc.
|
Where's your data George? You are no doubt aware of the exponential
increase in holding force as a magnetically driven armature seats
in the closed position. Let's noodle this through a bit.
If a 12v contactor will close at 7 volts, then current that
flows at 7 volts represents the same FORCE as would be applied
to close a contactor due to g-loading. The same contactor drops
out at 1.5 volts; about 1/5th the current required to
close it. Now if we bias the coil at 14 volts, then force keeping
it closed is now about 10X more than it takes to close it. If my
measured 4-5 g's was the closure acceleration, then opening
accelerations must be on the order of 50g's or more. So much
for back-of-the-napkin reasoning . ..
I just went to the bench and energized an S701-1 contactor
(el cheeso S-W-R continuous duty) and pulled it out of
fully engaged positing with a spring tester. It unseats at 6.5 to
7 KgF. The plunger and contact ring weigh in at 60 gm so
that gives us more than 100:1 ratio between mass of
the assembly and force to dislodge it. This substantiates
the back-of-the napkin estimate above.
I'll suggest that nobody is going to open ANY energized
battery contactor with ANY manner of aerobatic maneuver.
| Quote: |
Mickey wrote:
>My battery
>contactors are laying on their side, so the only G force that
>will cause them to open will be acceleration, and my engine
>won't have that much power.
Also, Call Van about sideway mount relays, they fail early due to galling.
Van's had a service bulletin / notice about 10 years ago on this.
B&C's starter contactor is designed to mounted Horz.
For a typical vertical relay, mount the starter relay so Pos G's
keep it open and the master relay is held closed by pos G's.
Most sport planes, even RV's pull much less negative G's than Pos.
As for hard core acro planes I wounder what they do also?
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????? These contactors . . .
http://www.aeroelectric.com/articles/contactor/strtrctr.jpg
. . . have acceleration-sensitive axis normal and parallel
to the mounting base. This contactor . . .
http://www.aeroelectric.com/Pictures/Contactors/s701-1.jpg
. . . has an acceleration-sensitive axis parallel to the mounting
base.
Which style is "designed to be mounted horz". I don't recall
seeing terms like "vertical" or "horizontal" used to
describe intended orientation for any of these products.
We've seen all these form factors mounted in a variety of
orientations. Help us understand the concerns.
Bob . . .
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gmcjetpilot(at)yahoo.com
Guest
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| Posted: Wed Apr 12, 2006 1:39 pm Post subject: Why use starter contactor? |
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Bob wrote:
>Where's your data George? You are no doubt aware of the
| Quote: | exponential increase in holding force as a magnetically driven
armature seats in the closed position. Let's noodle this through
a bit.
|
Bob, I don't know what the holding force is, and I believe you're
100% correct the force is huge and probably unlikely to come
unstuck or open under normal positive G's regardless of mounting
position.
To answer you question about my data, I think I stated it but will
repeat. A RV builder / pilot reported his panel went dark with
Pos G's maneuvers about 6 or more years ago. From this report
posted in Van's Aircraft News letter (The RVator), but don't have
a date or page number reference. So lets put it in the rumor /
hearsay column.
Since there is 4000 RV's and Van's sells the contactors they
have some good service history info and that is what I base
my comments on.
I am 100% sure Van's Aircraft does indeed recommend the
master contactor/relay be mounted so Pos G's works to keep
the plunger (contact) close. Why not do that? Also as I said
they also say keep vertical contactor vertical since galling
occurs with side ways mounting. The excepting is the B&C
starter contactor that is horz mount.
With a typical Cole-Hersee mater relay that is the pointy end
up. So given the choice, which you should have, I would mount
it per Van's recommendation, since it makes sense, even if its
not absolutly an issue as you say due to the big holding force.
I don't want to beat a dead horse, but thanks for your insight.
I have a call into Cole-Hersee to see what they say is the
G's limits are. If I find any DATA I'll write.
Cheers George
---------------------------------
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