Hip Bone/Leg Bone - Pt. II (I think- I've lost count)

[Ryan Smith]

As much as I have enjoyed becoming intimately familiar with the old lady, the further I dig, the less consensual it becomes. Pretty soon, I may have to call the police.

The wiring in the airplane over the years is; well, let's just call it interesting. I can see why so many folks rewire their airplanes. If it doesn't end up happening by the time the airplane gets back up in the air, it will probably occur shortly thereafter. Lots of scary stuff behind the panel that's being addressed.

I am in the middle of trying to figure out the situation with the rheostat. After some calculating and digging, the Ohmite Model H 25W 100Ω that I purchased will not work well in the system. I am not confident that the rheostat that was pulled out of the airplane was the original one, nor am I confident that the rheostat that I checked on the 49 A model that's been living on the field for the last several months is either. After being told that I need to change out the original equipment 1816 bulbs, as well as do a hold host of other complicated things to the airplane to make the lights work, I called a time out and did some digging. The bulbs in the A model are all 1816s, but the rheostat was a 5W 30Ω versus some unknown wattage and presumably 50Ω one that came from our airplane. Doing some calculations, the 0-15Ω Ohmite rheostat that Aircraft Spruce sells will do the trick for the purposes of getting all of the original-style lights (plus compass light, plus radio lights) working safely and properly. First lesson learned: don't reinvent the wheel.

Next, while trying to figure out what the second wire soldered to the center terminal of the rheostat is going to, I noticed a visibly cut, original wire that had been taped up. Luckily it had the number printed on it, "C28".

Great! I'll just consult the IPC to determine the home for this wire!

WRONG.

C28 does not exist in the wiring diagram. Pulling the wire loose, I trace it through a grommet going through the main panel behind the front kick panel upholstery and running up the front of the doorpost out into never-never land. After pulling it loose from the bundle to the top of the door post, I notice it doesn't have any resistance. I flick of the wrist tugs the wire free from the bundle, and the plot thickens.

Originally, I thought that maybe I was reading the wire wrong somehow and it was "C82", which goes to the stall warning horn. The stall warning horn works, and has the original wire running to it. The airplane has never (to my knowledge) had a heated pitot tube, Kohlsman or otherwise. All of the lights have worked at annual, so not a nav light or a landing/taxi light. The wiring diagrams in the IPC appear to be very complete, often referencing optional wiring, but there are some visible jumps in wire numbers. Was this to accommodate optional equipment? Did Cessna provide labeled wire for any service kit installations? Would this be documented in any of the service kits? I have checked the A model IPC and couldn't find anything referencing that wire number...figuring that since this is a relatively early B model (S/N 20408) that perhaps it was a holdover from older wiring.

No dice. Not super critical, but I am curious.

Lastly (two-part question), there is a second (newer Tefzel) wire that is/was soldered to the center terminal of the rheostat. The only wire going to center terminal should be coming from the fuse. After pulling my hair out, my IA just walked in and mentioned that the only two wires routing from under the panel, and up the front of the doorpost on the starboard side should be to the dome light or the nav light. He told me to pull the fuse for the dome light and see if it still worked, but then back pedaled and said that since I have already unsoldered the rheostat again that it it would be difficult. I replied that by process of elimination, I would find out what that wire went to, because when I turned the master switch on, either the nav light wouldn't work, or the dome light. Dome light was the winner, but that opens up another can of worms - the dome light SHOULD be on the same fuse as the landing light and cigarette lighter. I'm not in any risk of overloading the current fuse that it's on (on the rheostat, puts it on the panel lights and (optional, not installed) Stewart-Warner heater), but should I put it back to the proper/placarded fuse? Also, since the load on the fuse with the panel lights is so small with no heater, is it safer to put a 5A fuse rather than the 15A one that's in there? If there is a problem with the panel lights, the airplane will burn up before the fuse burns. Seems like a poor decision.

Eventually, circuit breakers will be retrofitted, but that will probably not happen until a rewire occurs. We're working with what we have now, just making safer and functional.

I appreciate any leads.

[Bruce Fenstermacher]

Ryan, I'm thinking about all you said here with regards to the different sizes of rheostats and the rheostats in general with regard to the legs of the rheostat.

One thing that stands out is several times you mention the center terminal. A rheostat can have 2 or 3 legs. Left, center and right. If it is a two leg rheostat, it is the same as a 3 leg with either the left or right leg removed. In a 3 leg rheostat the center leg is the wiper, the left and right legs are each end of the resistance wire.

There can be circuits where a wire is attached to both the left and right leg but I doubt you would ever see this in a circuit such as the one for our panel light dimmer circuit. The positive wire can be attached to either the left or the right leg, depending on which way you want to turn the knob to add resistance (and dim the lights) and the other wire to the center. The positive wire could also go to the center leg with the other wire attached to either the left or the right leg depending on which way you want to turn the knob to add resistance (and dim the lights).

Here is a picture of a rheostat:

imgres.jpg

The fact you don't seem to know where the wire coming from the center leg goes is a problem.

One of the problems with this circuit is the rheostat resistance and watts needs to be sized to balance the rest of the total resistance of the bulbs. Changing just one bulb from one type to another could through the balance off and have negative effects if the bulb does not have the same resistance as the one it's replacing. Also adding or subtracting a bulb or adding radio lights changes things.

[Bruce Fenstermacher]

As for which fuse the wire goes to remember the fuse protects the wire. All the wire past the fuse must be sized to carry loads exceeding the fuse rating. If the wire you want to move is not rated to carey 15 amps, you can't move it to that circuit.

[Ryan Smith]

I found the source of the second wire coming to the center terminal in the rheostat. It goes to the dome light; I may not have made that clear in my post.

That said, the dome light should be going to a different fuse, instead it's on the same fuse, effectively, as the instrument lights/heater. Given that the airplane never had a Stewart-Warner heater installed, this isn't an issue of carrying load, but it is an issue of the wire NOT being on the placarded/designed fuse.

Do I leave it as-is, or do I move it to where it should go?

[Bruce Fenstermacher]

The dome light is not dimmed. It should not be going to the rheostat at all. Dome light wire 68 goes to the landing light/cigarette lighter fuse. Assuming the wire is still the original 18 gauge or larger, you should put it back on that fuse.

On the rheostat the left or right leg depending on which way you want to turn the knob turn on/up your lights, is where the wire from dome light/heater fuse should go. The wire to the lights bulb socket centers should go to the center leg of the rheostat.

All this according to the schematic fig 71 of the IPC for a '52 B model serial 20267-25372.

Your wiring is not correct. Maybe could explain the smoke.

[lowNslow]

Interesting discussion since I am in the middle of my own rewire efforts. You both bring up an interesting point about the dome light. Cessna has a #18 wire on a 25A fuse, #18 wire should have no more than a 10A fuse to prevent overheat/fire, how is this right? They addressed the cigar lighter with AD79-08-03 but not this.

I have also been running into some inconsistency in regard to wire numbering. For the most part mine agree with the numbers for the serial number 25373 and on chart (mine is SN25400) while in a few instances it agrees with the SN 20267-25372 chart. My guess is because mine is close to the earlier model they used up some of there old stock of wiring in production.

[Bruce Fenstermacher]

Good catch Karl. I wasn't looking at the fuse size. I'll bet this is a IPC mistake and the wire might actually go to the 10 amp instrument fuse. Can anyone easily verify this on their '52 or prior? The dome light does go to a 10 amp fuse '53 and after in the schematic fig 72 serial 25373- 26995.

[Bruce Fenstermacher]

Here is a schematic of the stock '52 instrument light circuit.

52 Inst Light Circuit.png

For simplicity we will say the bus or battery is supplying 13v.

We are somewhat uncertain of the correct wattage and ohms the rheostat should be. Researching this site, George reports the original rheostat to be 10 watt, 60 ohms. However that coincidentally is the size of the fixed resister found only in the compass light circuit. I could not find the rating for the rheostat anywhere in Cessna documents I have. In another post George suggests he knows a 25 watt, 100 ohm rheostat works well in his system (a '53 powering unknown bulbs and not like a 52)

The GE 1816 bulbs which are the stock bulbs are rated at 4 watts, 13V. Using Ohms law that means these bulbs pull .33 amps each and have 39 ohms of resistance. We don't know anything about the compass bulb but the resistance is probably minuscule. In series with the compass light is a 60 ohm resister. So we have a total of 7 -39 ohm bulbs and a 60 ohm resister, all in parallel to ground. This totals only 5 ohms of resistance. I cheated and used an online calculator to get this.

What is left in the total circuit in series with this 5 ohms is the rheostat. To figure out what size resistance that might be lets assume half of the full voltage of 13v or 6.5v would make the bulbs dim enough. In this case a rheostat of 5 ohms which when turned to full resistance would drop the voltage to the lamps in that half of the circuit to 6.5v. If you used a 10 ohm rheostat it would drop the voltage to 4.3v or a third of 13v. And if you used a 15 ohm rheostat adjusted, to full on, the voltage to the lamps would be 3.25v or a quarter of the 13v. Of course light given off by a incandescent bulb is not linear and so as I said likely half to a third of the power will pretty much kill the light. **See my note below about this.

Now how much wattage do we need for the rheostat. Looking at our examples in the last paragraph. The total circuit has a max of 10 ohms resistance which will pull 1.3 amps. Amps remain constant in the circuit but in the first case the voltage across the 5 ohm rheostat is 6.5v. 6.5v x 1.3a according to ohms law is 8.45 watts. The second example of 10 ohms has 8.6v across the rheostat and 8.6 x 1.3a is 11.18 watts. The third example of a 15 ohm rheostat has 9.75v across it and 9.75v x 1.3a is 12.65 watts. Like everything having to do with an airplane we like to have some wiggle factor. I'd double the wattage and use at least at a 25 watt rheostat. Hmm guess what Aircraft Spruce sells. 25 watt 8 or 15 ohm Ohmite rheostats. http://www.aircraftspruce.com/catalog/e ... eostat.php

Now just for kicks lets calculate the wattage required should you use a 100 ohm rheostat in the above circuit. 100 ohms gives us 12.3v across the rheostat and 12.3v x 1.3a is 15.6 watts.

So how hot could a rheostat get at 15 watts. Remember the old fashion large outdoor Christmas lights size C9. They are 7 watts. Could you hold one in your hand let alone two. Not old enough to remember the C9 bulbs, you surely remember the smaller C7s which are 5 watts. Try holding on to three of them sometime let alone one.

Don't skimp on wattage when it comes to rheostats. Larger would be better at handling the heat but watts is watts and eventually a larger mass of the larger rheostat will get hot unless it's size effectively cools it better than the smaller versions.

And don't just go willy-nilly adding or changing bulbs in this circuit. doubling the amount of these bulbs doubles the wattage requirement and the heat it will make.

** If your rheostat does not have a dead zone at the low end of it's travel which acts as an off switch, your lights might be off but your rheostat is continually creating heat. Make sure you have a dead spot at the end. If you don't you might add one with a small smooth dab of high temp epoxy or paint.

Before I close I should say I'm not an electrical engineer. However I've had some basic electrical education and I've been dabbling with it since I was young. I don't practice this everyday and so I review references which I did to write this. I could have made a mistake in my application of theory but I don't believe I have. Perhaps someone more practiced could collaborate my calculations.

[Ryan Smith]

Here is a schematic of the stock '52 instrument light circuit.

52 Inst Light Circuit.png

For simplicity we will say the bus or battery is supplying 13v.

We are somewhat uncertain of the correct wattage and ohms the rheostat should be. Researching this site, George reports the original rheostat to be 10 watt, 60 ohms. However that coincidentally is the size of the fixed resister found only in the compass light circuit. I could not find the rating for the rheostat anywhere in Cessna documents I have. In another post George suggests he knows a 25 watt, 100 ohm rheostat works well in his system (a '53 powering unknown bulbs and not like a 52)

The GE 1816 bulbs which are the stock bulbs are rated at 4 watts, 13V. Using Ohms law that means these bulbs pull .33 amps each and have 39 ohms of resistance. We don't know anything about the compass bulb but the resistance is probably minuscule. In series with the compass light is a 60 ohm resister. So we have a total of 7 -39 ohm bulbs and a 60 ohm resister, all in parallel to ground. This totals only 5 ohms of resistance. I cheated and used an online calculator to get this.

What is left in the total circuit in series with this 5 ohms is the rheostat. To figure out what size resistance that might be lets assume half of the full voltage of 13v or 6.5v would make the bulbs dim enough. In this case a rheostat of 5 ohms which when turned to full resistance would drop the voltage to the lamps in that half of the circuit to 6.5v. If you used a 10 ohm rheostat it would drop the voltage to 4.3v or a third of 13v. And if you used a 15 ohm rheostat adjusted, to full on, the voltage to the lamps would be 3.25v or a quarter of the 13v. Of course light given off by a incandescent bulb is not linear and so as I said likely half to a third of the power will pretty much kill the light. **See my note below about this.

Now how much wattage do we need for the rheostat. Looking at our examples in the last paragraph. The total circuit has a max of 10 ohms resistance which will pull 1.3 amps. Amps remain constant in the circuit but in the first case the voltage across the 5 ohm rheostat is 6.5v. 6.5v x 1.3a according to ohms law is 8.45 watts. The second example of 10 ohms has 8.6v across the rheostat and 8.6 x 1.3a is 11.18 watts. The third example of a 15 ohm rheostat has 9.75v across it and 9.75v x 1.3a is 12.65 watts. Like everything having to do with an airplane we like to have some wiggle factor. I'd double the wattage and use at least at a 25 watt rheostat. Hmm guess what Aircraft Spruce sells. 25 watt 8 or 15 ohm Ohmite rheostats. http://www.aircraftspruce.com/catalog/e ... eostat.php

Now just for kicks lets calculate the wattage required should you use a 100 ohm rheostat in the above circuit. 100 ohms gives us 12.3v across the rheostat and 12.3v x 1.3a is 15.6 watts.

So how hot could a rheostat get at 15 watts. Remember the old fashion large outdoor Christmas lights size C9. They are 7 watts. Could you hold one in your hand let alone two. Not old enough to remember the C9 bulbs, you surely remember the smaller C7s which are 5 watts. Try holding on to three of them sometime let alone one.

Don't skimp on wattage when it comes to rheostats. Larger would be better at handling the heat but watts is watts and eventually a larger mass of the larger rheostat will get hot unless it's size effectively cools it better than the smaller versions.

And don't just go willy-nilly adding or changing bulbs in this circuit. doubling the amount of these bulbs doubles the wattage requirement and the heat it will make.

** If your rheostat does not have a dead zone at the low end of it's travel which acts as an off switch, your lights might be off but your rheostat is continually creating heat. Make sure you have a dead spot at the end. If you don't you might add one with a small smooth dab of high temp epoxy or paint.

Before I close I should say I'm not an electrical engineer. However I've had some basic electrical education and I've been dabbling with it since I was young. I don't practice this everyday and so I review references which I did to write this. I could have made a mistake in my application of theory but I don't believe I have. Perhaps someone more practiced could collaborate my calculations.

Thanks, Bruce. This is pretty much what we've come up with. The Aircraft Spruce 0-15Ω was ordered last night after we figured out what would work. I would be curious to see the differences between George's system and mine (stock 1952 with (7) 1816 bulbs and the addition of lights for two radios and a compass light with wheat grain bulbs), because the Ohmite Model H 25W 100Ω rheostat gives me no adjustability...it's either on or off in 1/10 of a turn. After some calculations on switching out to 1864 (28V bulbs) or going to 756 bulbs to use the 100Ω rheostat that I purchased, I put a stop to it and had a friend that's smarter than I run some calculations.

Also, to clarify, the dome light that's connected to the rheostat is connected to the center terminal, only to receive power and fuse protection. Dimming the rheostat does nothing to the dome light as it's not connected to either of the ends of the rheostat. Not correct, I know, but I'm more concerned with it not being on the placarded fuse. The fuse that it's on is plenty big enough.

Has anyone replaced the stock 15A fuse for the panel lights with a smaller (5A or similar) fuse for safety? This airplane doesn't have the optional gas-powered heater, therefore the load on that fuse is much lower.

Lastly...you said that you had built up the end of the rheostat with some CA at one point in an older post that I dug up. Did this work well to insulate the wiper, or do I need to get some specialty epoxy or something from Radio Shack?

[Bruce Fenstermacher]

Ok now I see the picture of the dome light wire. Since studying the wiring schematic today for the better part of the day I realize that if your plane is wired like the schematic the power goes in to the rheostat at the center leg. So in this case the dome light wire is only using that lug as a convenient place to tie into that fuse circuit.

In fact your dome light wire installation may be the standard place it was attached as karl pointed out it would be incorrect to have an 18 gauge wire protected by a 25 amp fuse as the IPC depicts. probably another mistake in the IPC.

As for the 5 amp instead of the 15 amp. So long as the wiring will support 15 amps, AND the load on the circuit does not exceed 5 amps or get close, I see no reason you couldn't switch it out to a 5 amp fuse.