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Heater Control cable "tightening"

TimO

Well Known Member
I'm not sure what the best way to describe this is, but here goes...

I have 2 problems with the heater boxes and cables in the RV-14.

1) The doors don't seal well, so definitely take the time to put a very thin RTV seal bead on the firewall when you are building, to help those vents seal when closed. Mine leak and in the summer it isn't great because when it's 90F outside the last thing you need is more heat coming in.

2) Now that I'm into the season when I need to use cabin heat, I'm glad that it does seem to work well as far as the amount of heat available. But what isn't working for me is the push-pull cable. The way the heater boxes work, if you have the vent door 1/2 open, the airflow will try to shut your vent for you. The push-pull cable detents aren't enough to keep the vent in mid-travel position. Does anyone know a way to "tighten" that up? I don't think there's any adjustment but can something be wrapped around the shaft or maybe shoot some type of glue into the cable detent area that may help add some friction? It wasn't even that cold out the other night but ever 3 minutes I had to pull the heater open again to keep the cabin temp up. If there was some sort of friction adjustment, I think it would have been fine.

I'm going to have to get a fix for that in place within a month or it isn't going to be fun to fly up here in WI.
 
Sealing the doors

Tim,

Not specifc to the cables...

I'm nowhere near flying but I did try an experiment with the heater flapper doors. I attempted to seal the doors with a little RTV on the front side of the firewall. I put tape on the doors, squished them into the thin bead of RTV around their edges, and let it cure. Even though it was a very thin layer it made the doors not want to sit flush against the firewall when they were in the closed position. Im not sure how much pressure the cables apply to keep the doors shut, but they wanted to sit about 1/4" open with the RTV underneath. I assume even the thin layer was interfering with the hinge axis. I felt the results were neither satisfactory or safe compared to the fit without sealant.

A builder with more experience may have better results. Possibly a sliding door would be the ultimate fix but maybe they have their own issues. They work pretty good in my Maule.
 
Check out NAPA auto parts. They do have Beldon type cables that friction twist lock. They fit right in and work great.
 
Circling back to give a small update.

Gasman, thanks for the note. Napa's website kind of stinks, but it did lead me to find a few others on other websites that are potentially good.

James, thanks for the note on the RTV. How thin are you talking? I'm thinking of thin as in barely visible. Maybe if I only put it along the top and top 1" of the firewall door area it wouldn't push the door out too far?
And, of course smear it as thin as humanly possible?
I am just not looking forward to doing anything with it because whether I do it from inside the plane, stuffed into the footwell, or from outside, trying to weave my arms and fingers around the engine mount, it's not going to be fun. I highly recommend that builders fix the sealing issue before they hang the engine. You'll thank yourself later.

Regarding the cable, I did call ACSproducts, who while related isn't the same company as Aircraft Spruce, who makes the heater control cable Van's sells....the CT A-740 cable. That cable is made with detents that are controlled by a little spring clip on the cable. The lady said there's probably nothing that I can do that will fix it, but I should verify that the spring clip is there, and I could at least try to play with the spring clip and bend it and see if I could make it better.

But what they did have to offer is, there is a CT A-700 cable that rather than 3/8" hole mounted, is 7/16" hole mounted but a similar sized knob, that is pushbutton locking. The wire is also slightly thicker at .062 instead of .050, but otherwise the cable would fit into similar spaces and have a positive lock. She said it is good for at least 6 lbs of locking force.

That may be a good way to go. I did find that if I kept pushing the heater knob in the summer and kept force on it, I could keep the leakage down. And so perhaps by giving it a little extra travel I can use the locking effect to keep the vent both locked closed or locked open part way.

So tonight I'll see if I can tweak the spring clips but probably will end up ordering new cables tomorrow. The Van's cables are over $30, so if this works, it may be something that you want to leave out of your kit and just order the A-700 cable from Spruce.
 
We haven't had any problem with the standard cable installations in the two RV-14 prototypes and I have heard any negative feedback from Mitch either.
We do have the cables installed using the standard aircraft practice of adjusting so there is some cushion gap under the knob when the control is in the off position (pushed all the way in)
There is a method of increasing the friction to reduce position creep if anyone decides their installation requires that.
 
I agree with Tim O. Both my heater vents leak and I live in the "Sun Shine State" of Australia (similar to Florida). I don't need or want the heat inside the cabin. I thought my vents looked pretty good when I built them and installed them and I'm very surprised to find they leak.

I wish I knew what Tim O and I now know when we were building. Its a minor issue but most annoying in warmer weather

I am considering removing the scat hoses for summer to alleviate the problem. Does anyone see any problems with removing the hoses?

Tim thanks for the research into the cables, I'm very tempted to buy a new set of cables.
 
RTV seal

Tim,

The layer between the door and the firewall was pretty thin. I tried for enough thickness for the door to seat against but when I peeled it back off it was paper thin. Since I don't know how much tension the cable will keep on the door in the closed position I'm back at the stock installation.

Again, a smart person may have better luck.
 
Tighten cable

The way to tighten a cable that is used for the heat flapper is to make a small (not to sharpe) bend in the cable. This will increase the friction on the wire inside the spiral wound core and stop wire inside the cable from sliding due to vibration.
 
After seeing Tim's problems with the heater doors not fully closing, and observing a tendency of one of my doors to "gap", I opted to get the A-700 cables hoping they would exert more pressure in the closed position. Unfortunately, I'm not flying yet (in fact not yet installed) so can't offer a PIREP.
 
Keith, do you have a pic of the cables you ordered? How do you like the looks of them? Any reason that you think I shouldn't go for it?

BTW: I did add a slight cushion to the cable so that it doesn't bottom out but you can push it and get the heater door closed and still have a tiny bit of travel left. That wasn't enough to get it to stay closed and sealed though.

I may try to kink mine and see how it goes, but I have a feeling the A-700's are in my future. We use the heat from now thru May and on many higher altitude flights, so that's a long time to screw around without things being just right.
If you don't live in the midwest (or north of here), you really don't know what true cold is. It's not worth having the heater valves close on me for any length of time. The leakage issue can wait until spring now.
 
Tim, I flew in temps with mine that I needed the cold air vents wide open (not as hot as you I'm sure) but did not notice any heat at my feet. I did not go looking for leaking hot air but did not notice any either. Obviously you noticed this before going looking for the source?
 
Ron, I did notice the leaking air right away, although the doors appear flush and tight at least when I'm not flying. Can't see them when I am. The tell-all was when I pushed the heater knob forward to put more pressure on it (since I left about 1/8" cushion), the leakage goes much away. The catch is, the detents aren't strong enough to hold the knob in that hard. And, they aren't strong enough to hold the vent from closing when in mid travel now that it's cold. So I know right where the heat is coming from. Just have to fix it.

To reply to AussieFlyer...I see I missed replying. I think you could remove the SCAT for summer if you felt like it. You'd probably want to cap off the inlet
too. The thing is, I wouldn't be too keen about leaving the heat muff around the exhaust then. I don't know enough about the ramifications to be qualified to say anything one way or another, but, that would create an area in the heat muff that could get hot around the pipe. So you probably should then remove the heat muff too. That would make it much less convenient though. The idea is, as long as there is air flowing thru the muff it won't get too hot. So I'd either remove the whole heater system or just seal off the doors on the firewall.

Personally if you want a temporary fix, I'd try some aluminum tape over the heater vent door seams. It's cheap enough and you can just peel it off if you want heat. So if you find yourself visiting a cold destination, you can quickly undo your workaround.

Again, I'm not an expert and maybe the muff wouldn't overheat, but I can just see that as a potential area that the exhaust pipe would then be hotter.
 
Ron, I did notice the leaking air right away, although the doors appear flush and tight at least when I'm not flying. Can't see them when I am. The tell-all was when I pushed the heater knob forward to put more pressure on it (since I left about 1/8" cushion), the leakage goes much away. The catch is, the detents aren't strong enough to hold the knob in that hard. And, they aren't strong enough to hold the vent from closing when in mid travel now that it's cold. So I know right where the heat is coming from. Just have to fix it.

To reply to AussieFlyer...I see I missed replying. I think you could remove the SCAT for summer if you felt like it. You'd probably want to cap off the inlet
too. The thing is, I wouldn't be too keen about leaving the heat muff around the exhaust then. I don't know enough about the ramifications to be qualified to say anything one way or another, but, that would create an area in the heat muff that could get hot around the pipe. So you probably should then remove the heat muff too. That would make it much less convenient though. The idea is, as long as there is air flowing thru the muff it won't get too hot. So I'd either remove the whole heater system or just seal off the doors on the firewall.

Personally if you want a temporary fix, I'd try some aluminum tape over the heater vent door seams. It's cheap enough and you can just peel it off if you want heat. So if you find yourself visiting a cold destination, you can quickly undo your workaround.

Again, I'm not an expert and maybe the muff wouldn't overheat, but I can just see that as a potential area that the exhaust pipe would then be hotter.

If you reconfigure the heat system, you MUST do it in a way that keeps air flowing through the heat muffs if you leave them installed.
 
Tim:

Here are photos of the A-700 locking cables:


Vans "detented" cable on the left, "locking" on the right.
Shafts at mount measure 0.365" on Vans, 0.425" diameter on locking cable.
 

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Thanks Keith, great pics! Last night I kinked my cables and went flying. The kink was enough to keep the vent from creeping when I turned the heat on. But, the kink didn't help when I tried to keep the valve completely shut. Or it at least didn't help much.

I was on the fence on ordering cables until spring, because for the next 6 months I won't care if it leaks, but after seeing your photos they look nice enough that I'm going to just order them today. That way it's on my to-do list and I can get it done during the winter. That looks like a much better way to go.

Scott, thanks for commenting on the heat muff thing. That was what I was thinking, but you verified it. I think you could cause issues by not having airflow thru the muff.
 
That was what I was thinking, but you verified it. I think you could cause issues by not having airflow thru the muff.

Yes, it can cause an extreme over heating of the pipe under the muff, and the muff itself. Depending on localized air flow, it might not be a problem, but it definitely can be.
 
Tim, THANK YOU, for continuing to do these "shakedown reports" on the RV-14a.
With a new model like this, there will be a myriad of little things, like this heater cable, that needs to be understood and finessed. Your willingness to articulate the issues and put them out there for discussion, is highly valuable for all of us, a few months behind you. :D

Greg Novotny
RV-14a N14ZP
fiberglass....everywhere
Gig Harbor, WA
Kailua-Kona, HI
 
The way to tighten a cable that is used for the heat flapper is to make a small (not to sharpe) bend in the cable. This will increase the friction on the wire inside the spiral wound core and stop wire inside the cable from sliding due to vibration.

Right! and that is the way McFarlane induces friction in their cables. Mine came that way with a tag NOT to straighten it. It has 4-5 sine wave wiggles in the casing. Maybe .040 displacement?
 
Keith, thanks again for the pics. I ordered those cables back when I made my last post, but hadn't yet put them in. A few more flights and depending on the heater vent position the valve was still creeping either closed or open, even with the kinked cable.

Today I decided to dedicate the day to swapping in the nicer A-700 cables. It was a very easy swap because other than the hole enlargement to the threaded end there really was no major difference. When I got done I finally had a flight where when I pushed the vents closed, I had a minimized heat leakage, and when you put them in a specific position they will stay there.
Very happy I upgraded the cables. Thanks for taking those pics for me and posting them.
 
Tim - glad to hear the cables work as hoped. You don't mind the additional expense when it produces the desired results.

As an RV-14 "still under construction," I really appreciate the time you take to keep the rest of us up to date on tweaks you are making.
 
Just replying as an update now that it's summer again.

The new control cables are a big help. I do still have a slight leak on the pilots side yet, so I may have to resort to adding something thin for a film on the flapper door to completely get it to seal the hot air out. But without those locking cables with the pushbutton, it would be a losing battle keeping the heat out.

120+ hours on it now and it's been going great.
 
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