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  #61  
Old 06-09-2004, 12:28 AM
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mplafleur mplafleur is offline
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Quote:
Originally Posted by Newbie Wimp
John..
Good spotting with the ignition fuses..
I also understand the reason to go with fuses however..
Have you considered using W31 style switch/breakers on the ignition??
The advantage being that if one blows, the switch (Toggle) turns itself off.. therefore you can spot the problem..
It seems to me to be one of those occasions where its nice to know what's going on..

Great reading.. Fly more !!
You could always put a 1K Ohm resistor in series with an LED across the fuse. If any fuse should blow, the LED will light up.

I myself would likely use polyfuses in combination with the LEDs. The polyfuse is more reliable than a wire fuse and they will reset themselves. Look at www.raychem.com for more info. I have used them at work before.
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  #62  
Old 06-09-2004, 01:39 AM
Newbie Wimp Newbie Wimp is offline
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Default W31's

Yup.. I agree and my first thought was the LED's idea but then since there are already toggle switches for the IGN it is very simple to swap for W31's and that is it..

I personally aren't a fan of polyfuses but that is the great thing.. everyone is different..

Cheers
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  #63  
Old 06-09-2004, 01:51 AM
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Why would you want them to reset?
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  #64  
Old 06-09-2004, 04:06 AM
Newbie Wimp Newbie Wimp is offline
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Cool Poly's

What he means is.. they reset themselve's when power is removed..
They work in a mode that when they are in their "Blown" state, they are actually in a high resistance state that requires a small amount of current to remain flowing in order to remain "Blown".. when the power is removed (The switch turned off) they reset back to the normal low resistance state..
They give no indication of their state and so normally have an LED/Resitor accross them to indicate the "Blown" state..
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  #65  
Old 06-09-2004, 07:56 AM
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Good Stuff John,
So it was electrics eh?
Best of luck with the flight testing.
Adam
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  #66  
Old 06-09-2004, 09:02 AM
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Here's a better link to a pdf. Raychem calls them Polyswitches.

http://www.circuitprotection.com/lit...ormcatalog.pdf

The one drawback I see is that I can't find a pluggable form factor.
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  #67  
Old 06-09-2004, 12:39 PM
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The fuse protects the wire right?
So if a fuse blows, then the wire is being damaged, or is getting ready to.
If thats true then why would you want to reset the fuse? Wont that likly damage the wire, or just blow the fuse again?
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  #68  
Old 06-09-2004, 04:37 PM
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Quote:
Originally Posted by LargePrime
The fuse protects the wire right?
So if a fuse blows, then the wire is being damaged, or is getting ready to.
If thats true then why would you want to reset the fuse? Wont that likly damage the wire, or just blow the fuse again?
PPTC devices (Polymeric Positive Temperature Coefficient) devices are actually non-linear thermisters. They limit the flow of high current during fault conditions. The devices reset after the fault is cleared.

Typically, a fuse in the plane is used to protect against an overload of the system wiring. Usually we fuse a system and some individual components.

Here is an example of current carrying capabilities of coper wire. Note, this is in free air and these values must be derated when bundling.

http://www.alphawire.com/pages/383.cfm?partner=0&part=0

I've put the above table into one of the attached speadsheets.

Now I'm going to take another path and and talk about how to determine wire size.

Wire size:

The National Electric Code (NEC) specifies the following formular to determine the wires size in circular mils (CM) for a constant load of I amps, wire length L in feet, and voltage drop V.

CM = (25 x I x L) / V

The CM size can be converted to AWG using the tables below or the link above. This standard is a practical limit determined by NEC. NEC has established that a 2% maximum voltage drop is acceptable. If you want less voltage drop, go ahead and use it.

For example, if I wanted a 6 foot cable to supply my 40A panel. I would size the wire thussly:

CM = (25 x 40A x 6')/0.24v = 25,000 circular mils

This would require a 6 gauge wire. How did I get that? I used the following attached tables:
Attached Files
File Type: xls Current Capacity.xls (16.0 KB, 31 views)
File Type: xls Wire Gauge Resistance and Drop.xls (14.5 KB, 31 views)
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  #69  
Old 06-09-2004, 04:52 PM
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Excellent post mike.

Thats one for the archives.

Now what part of that explained why you would want to reset a blown fuse?
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  #70  
Old 06-09-2004, 05:18 PM
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Quote:
Originally Posted by LargePrime
Excellent post mike.

Thats one for the archives.

Now what part of that explained why you would want to reset a blown fuse?
As stated before, it will reset when the fault goes away. If the fault is still there, the polyswitch will stay open, thus no current.

When the system blows a fuse (I'll use polyswitch and fuse to mean the same), it may only be one component of the system that is bad. Turn all components off and turn on one at a time. When the fuse blows again, turn the last unit back off and go on to the next one...
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  #71  
Old 06-09-2004, 05:29 PM
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Dam am i glad you are close buy
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  #72  
Old 06-10-2004, 06:11 AM
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Default IAS-CAS-TAS

Ok, can't find the post where we talk about doing a TAS(TrueAirSpeed) calculation and you didn't mention it on your site write up. But remember that to be most accurate, you would not base your TAS calculation on IAS(IndicatedAirSpeed). You base it on CAS(CalibratedAirSpeed).

CAS is IAS corrected for instrument and position error, something you don't know with your homebuilt.

So normally you take your IAS, do a CAS conversion, then calculate TAS. If you just use IAS to calculate TAS you might still have a difference from GPS but you should kinda be close anyway.

In the past they've never had the benefit of GPS to validate TAS calculations so I'm sure all handbooks aren't 100% accurate there.

Tom
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  #73  
Old 06-10-2004, 01:35 PM
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Clutch Cargo Clutch Cargo is offline
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Thumbs down I agree to disagree...

About fuses..
I use em, but I hate em. A disposable circuit protection device.

Re-setable circuit protection devices (circuit-breakers, polyfuses) have somewhat of an advantage in an aircraft application. IMHO
There are several causes for a short circuit or a circuit with too high of a load for the circuit to handle. Not all of these become permanent dead shorts. Sometimes components fail under differing loads, wires rub against metal and terminals fail to terminate. The conditions may change after the initial overload condition. Most circuit breakers are thermally tripped. A given load heats up a bi-metal strip or some other temperature sensitive device and it mechanically breaks the circuit. They are approved for aircraft use because they are reliable. If they are used frequently in a bad circuit they can go bad, (personal experience from machine tool industry) but they can give years of service without failure. I have had bad fuses right out of the box, some of fuse media is so small that it is hard to see with the naked eye, you have to meter it to check it. Some larger fuses become capacitors and resistors because they partially blow so they show good on a meter but won't pass amperage.
Back to the aircraft application. I personally have had occasion to reset breakers mid-flight. At night, alone in a pattern during a high pilot work load, is a bad time to find a flashlight and start checking fuses. It's a bad time to push circuit breakers too, but you don't need your eyes to feel for popped CBs. You need your eyes to fly the airplane. Trust me, when the electric system malfunctions, especially at night, your hands and eyes are VERY busy.
So, not all overload events are permanent. Sometimes you may need the overloaded system to land safely. With CBs you can quickly determine a permanent or temporary situation (without looking). Protecting the wire from turning the airplane into a toaster is just one aspect of overload protection. It also protects other circuits from low voltage and unexpected current flow.

If you have some sort of indicator LEDs planned for your fuse panel, make sure you isolate the power supply to them from the main buss.

I know there have been many good and logical reasons for using fuses in aircraft. Even the Sladecraft uses them. So it can't all bad.

I won't use them in mine. I been there, done that. Got the T shirt and some permanent wedgie marks

un con fused
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  #74  
Old 06-10-2004, 06:22 PM
Newbie Wimp Newbie Wimp is offline
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Question KISS etc

I can't help but think some sometimes the KISS principle is a little misunderstood when it comes to electrical ciruits..
The basic principle of having enough backup to not require re-setting CB's (often not a good practice anyhow) is a good one but I'm sure the principle is not there to keep people in the dark (No pun intended) about what is actually going on.
Take John Slade's ignition example.
Great.. he had enough backup to get back without having to replace the fuse.. but the issue I have with the design is that at no stage in the flight did he actually know what was going on !! To my way of thinking.. this ain't good design. In fact he had to pull his plane apart before he discovered it since there were other possibilities..
Now it is also true that you can put LED's and such in the circuit which would tell him whet is happening.. this is fine in some situations but then is this the KISS principle?? It seems to be getting more complex to me.
If his ignition switches were breaker style ignition switches then the problem is solved in that he would know what's happening.. no extra parts are needed to be wired in and he can relax in the knowledge that he has sufficient backup and everything else is OK.
Kiss is a great principle but as a pilot you need to know what the hell is going on.
Food for thought??
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  #75  
Old 06-10-2004, 07:22 PM
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Allow me a counter point.

What did it matter what happened?

Number one rule is fly the plane. Should he be flipping a breaker or flying the plane?
The real issue is "what is the point of a fuse or breaker?"
The real point of a fuse OR breaker is to protect the WIRE, and thats it.
If the wire is in danger a fuse or breaker protects it.
Fuses protect the wire better than a breaker at less cost and weight with higher reliability.
In an aircraft, if a wire is in danger, something is seriously WRONG and needs to be FIXED on the ground, and only on the ground.

That is just what john's system did. He was not fooled into thinking he needed to know what was wrong. He was not tricked into perhaps fixing it. He flew the plane, because he had no other foolish options to entertain.

P.S. and off topic
I understand and respect your decision to post as the newby.
I have the question of why? You need in no way to respond, I am just curious.
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