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| A copy of my original design review along with |
| a few threads of conversation. Please note that |
| my intent is not to slam somebody's product |
| for personal reasons. I try to evaluate what |
| I know for the physics and human factors involved. |
| If anyone should find my facts inaccurate or my |
| reasoning flawed . . . please jump in there and |
| let me know! Bob . . . |
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/Being an Avionics Engineer and having been affiliated
/when several Avionics Shops,being a field engineer
/with ARC/Cessna for several years,and finally
/owner/operator of my own Certified Repair Station
/I am admittedly bias in some of my opinions.
No more bias I'm sure than many of us have who grew up
under the unbrella of certified aviation. I used to repackage
the ARC manuals for publication by Cessna right after Cessna
bought the operation back in the 60's.
/First of all I have read your publication and also several
/of your magazine articles I must say you are a very good
/writer.With a few exceptions I do not disagree with the
/concepts and technics you write about.
. . . but please let me know when you do disagree. It's
not my goal to diseminate the "gospel according to Bob N."
rather to be a gaterhing place for the best information
we can put our hands on. To say that everything I have to
offer is all there is to consider is as bad as the FAA
is now . . . they've damned near regulated the little
airplanes out of business. Not on purpose, I'm sure
but regulation by committee is the best way to kill progress.
Progress in Independence, Kansas just isn't happening
nor do I expect it to.
/Our respective opinions on how or what is the best way to
/accomplish a task is just that,our individual opinions.Even
/if we disagree on the the procedure I respect your right to
/your opinion.
To be sure . . but let us work to differentiate personal
preferences from design rules driven by critical design
review. The piece I've attached speaks directly to feature
issues of the EXP-Bus design . . . if you suspect a shread of
personal preference bias in it, please point it out.
/The one concern I do have is the fact that most kit builders
/see you or I or anyone else state a solution to a particular
/problem and be confused about who is right or who is wrong.
Which is EXACTLY why I publish the 'Connection and spend so
much of my time on the list servers. Good decisions come from
understanding of the issues. The safest builder/pilots are
informed pilots. I cannot put into words the feelings I
get when some guy calls or writes to sort out about a half
dozen "recommendations" from his hangar-mates and arm-chair
designers . . . especially when he's already drilled a LOT
of holes in his panel and mounted expensive but not necessarily
the most useful stuff.
/Please Email me copy of the posting you mentioned and maybe we
/can compare notes.
/Respectfully Larry W. Johnson
Here it is . . . I'd be pleased to know what you think about
it.
FROM: Robert L. Nuckolls, III, 72770.552
Re: Copy of: Re: RV-List: Exp-BUS DC Load Center
>:EXP-Bus ad quoted on a list-server . . .
>ARE YOU WIRING UP A HOMEBUILT AIRCRAFT?
>
>if so, then you should investigate our new product, the EXP-BUS. This
>product can save a lot of time and money wiring up a project. . . .
Compared to what? You still have to cut holes for a PREDETERMINED
number of switches arranged in PREDERTERMINED order. Little chance
for customization. The assembly suggested DOES eliminate the need
for fabricating a breaker panel. So that time and space is saved.
However, fuseblocks and toggle/rocker switches can be used to
fabricate an easily customized power distribution and control
system for about $10 per switched circuit (landing lights,
nav lights, etc) and $0.50 per unswitched circuit (turn coordinator,
nav/com, etc.).
>HOW DO "SOLID STATE FUSES" WORK?
>The purpose of the limiter is identical to that of the breaker, to shut
>off whenever too much current is being drawn from a particular circuit.
>Technically, the devices used are very non-linear thermistors with a
>positive temperature coefficient (PTC). When excess current is drawn,
the
>PTC device heats up and becomes a poor conductor of electricity. This
>shuts down the offending circuit. These components are UL recognized,
and
>manufactured by a major US component manufacturer.
>
Generally true . . .
>AN EXAMPLE
>Suppose that a wire leading to a nav light is chafing against a grounded
>metal part in a wing, such as a rib. If the light is on, and the
>insulation abrades away, the wire will intermittently or permanently
short
>to ground. With a fuse, the overcurrent will blow the element in the
>fuse, before the wire can get hot and start a fire (although the spark
>could ignite fuel vapors before the fuse blows).
True of ANY form of circuit protection whether fuse, breaker OR
PTC resistor.
>With a breaker, the
>bimetallic element in the breaker heats up (more slowly than the fuse)
and
>trips the breaker. With the solid state device, the PTC device gets hot,
>increasing it's resistance, and shutting down current flow to the nav
>lights. Actually, about 1/30th of an amp continues to flow, which is not
>enough current to heat up any wiring. The voltage drop across the PTC
>device keeps the device hot (about 100 degrees C), and the device stays
>"tripped". The load presented to the circuit by the nav lights will
keep
>the device tripped even if the short is intermittant.
>
There are some operational considerations with self-reseting
devices.
An intermittant fault gets you a popped breaker or fuse; an
immediate indication of system difficulties.
>The devices used are made to perform this function, and can do this over
>and over, thousands of times without damage. To reset the device, power
>is removed from the circuit for about 10 seconds (by switching off the
nav
>light) , the device cools and switches back on, and the circuit is
>restored. If the short still exists, the device will immediatly trip
>again.
An important issue here is, "how often does a breaker or fuse
get cycled in an airplane?" The answer is, "almost never." This
being the case, automatic resetability is a non-convenience.
>
>We demonstrated this scenario literally hundreds of times at Oshkosh this
>year. These devices are used in automibiles and also in military
>electronics manufactured by Control Vision. No smoke and no mirrors used
>here. Using these devices, we have produced a PC board with switches
>mounted on it that replaces up to 16 fuses and circuit breakers in a
small
>aircraft. Because these devices are quite inexpensive, we are able to
>offer the entire assembly for $249,. slightly less than the cost of the
>individual circuit breakers it replaces.
>
There are breakers and then there are breakers. You can buy
thermal breakers for $3 to $30 apiece depending on size
and relative "quality." $3 breakers will protect 16 circuit
for about $50. Of the 16 circuits, perhaps 5 will have switches
for another $35 bringing the total hardware costs to $85.
Switch panels have to be fabricated no matter what. Breaker
panels can be replaced with fuseblocks which pushes the labor
close to zero. If one assumes that $20 Klixon breakers are
the standard of comparison, then yes $249 IS less than $320.
>This is not a scam, we are offering this product with a 90 day money back
>guarantee, and a 1 year warranty. A builder can save time,
A little . . .
> money,
Depends . . .
>panel space,
Compared with clasic breaker panel/
switch panel installations, yes. . . .
>troubleshooting,
Vague . . . what troubles? Popped breakers and fuses on a
finished airplane are extremely rare and to my way of thinking,
the positive indication afforded by these devices is preferable
to the intermittant condition which is masked by the self
re-setting current limiters. . . .
>and weight.
Again, compared to what? Fuse blocks and switches are the
lightest,
lowest cost, and fastest installation you're going to find.
Further,
it's easiest to maintain in terms of individual switch replacment
and if necessary one can replace the entire bus bar and protection
system in 10 minutes with a screwdriver and needle nose pliers
There are several products of this genre on the market. They
all feature higher cost, longer installation times, greater
weight and higher parts counts than the fuseblock and switch
configuration of power distribution and control. Both types I
saw at OSH were not well thought out with respect to vibration
resistance. Both dropped wire segments directly into soldered
pads of etched circuit boards with no mechanical support. Parts
counts on both products was high. One version soldered
one edge of an etched circuit board to a row of toggle switch
connections . . . a real nightmare for replacement of a single
switch . . . Further, it used an electrical connection to a switch
and their entrepreneurship, I cannot recommend these products as
cost
or performance effective in amateur-built airplanes.
Regards,
Bob . . .
AeroElectric Connection
////
(o o)
o00o(_)o00o
| |
| Go ahead, make my day . . . . |
| Show me where I'm wrong. |
72770.552ompuserve.com
http:\\www.aeroelectric.com
Also, here's a bit of a conversation I had with a builder in
New Zeland . . .
FROM: Robert L. Nuckolls, III, 72770.552
DATE: 5/5/97 9:43 PM
Re: "solid state fuses"
So, fresh out of the box, you've invested time and effort on the system
that did not go directly to the production of your airplane.
>Isn't that what "Experimenting" is all about :). Unlike many builders I
>spend a lot of time exploring the build process and design issues.
>One of the great joys I have discovered with homebuilding is the time I
>spend working out how to do something and exploring different methods of
>achieving the task.
Oh most certainly!!! I too have many creative features in the machines
I fly and encourage any and all to exercise their rights and desires
to do the same. But we were discussing shortcuts of various flavors
in getting a system installed. My suggestion was that the EXP Bus did
not deliver on it's advertising hype. I thought you were taking a
position
to the contrary.
---------------
Okay, explain that to me. Even for your own case, you've already spent
more time evaluating and modifying the EXP Bus than I say should be
required to install and wire it's equivalent collection of parts.
>Quite probably, but then I knew that before I purchased the unit. My
>requirements were for a unit that could hook into my experimental
>microprocessor based flight management system. Having had a good chat
>with Jay Humbard who designed the unit we came to the conclusion that
>their unit combined with their own monitoring board would actually save
>me around six months of development time.
Okay, we're not talking apples and apples here and we need to make sure
that folk listening in know it. The EXP Bus is attractive to you for
personal reasons that go beyond the considerations I outlined in my
original design review.
-------
Based on your own experience, how would you recommend that anyone
purchase
and apply the EXP Bus to their own project, and compared to what other
technique is anything saved?
>The recommendation is simple but probably won't please you too much as it
is
>more oriented to personal psychology than to hard technical . . .
(a piece of your message dropped out here but I get the gist of it)
On the contrary, it pleases me very much that you have the time, talent
and willingness to explore new techiques and to share your experiences
with
others.
> . . . .Aero Electric Connection, wire up their own fuse panel and do all
>the cabling themselves then this is by far the more preferable but lets
not
>forget those that need a more gentle introduction to the fine art to
electrical cabling.
Then I don't think we disagree on much . . . you're willing to pay the
asking
price for the EXP Bus product to accomplish a certain task. You believe
that
it saves some cabling time and I say it needs to be demonstrated. I
sugested
that after you add the wires nessary to remotely mount the switches,
you'll have
as many or more wires that with any of the techniques I recommend. I
don't think
there's any disagreement between the costs of do-it-all-yourself and
using products
like the EXP Bus. You didn't speak to any of the other arguments I made.
What I want people to understand is the differnce between USING a product
and
RECOMMENDING that anyone else use the product. Your application for the
EXP Bus is unique and doesn't demonstrate much for the guy who is
spending
hundreds of dollars in anticipation of reaping the "savings" offered. I
too
dabble in technologies and techniques that I am proud of and willing
to talk about at length. However, when asked to recommend hardware and
techniques
for an amateur builder, I have a totally different responsability.
Regards,
Bob . . .
AeroElectric Connection
////
(o o)
oOOo(_)oOOo
| |
| Go ahead, make my day . . . |
| Show me where I'm wrong. |
72770.552ompuserve.com
http://www.aeroelectric.com
|