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What the fuel does in a control line tank

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What the fuel does in a control line tank Empty What the fuel does in a control line tank

Post  batjac Sat Apr 09, 2022 1:34 am

I was just directed to this video from a Stunthanger post.  I know it's pretty obvious, but I've never seen a video taken of the fuel sloshing about in flight.  I thought it was worth a look.





The Sloshed Mark
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Post  KariFS Sat Apr 09, 2022 4:05 am

That was interesting! Made me a bit dizzy though... Watched only about the first 3 minutes Smile

If you look closely, you can see the clunk moving around too.
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Post  balogh Sat Apr 09, 2022 4:21 am

Thanks for sharing this, Mark. Especially when the tank is near empty, the uninterrupted supply of fuel to the engine is not guaranteed because of the free " sloshing" of the fill, no matter the centrifugal force. This is why I have always used party balloons in my RC planes, where the tank never contains air, it simply collapses when the fuel is gradually consumed...no need for a heavy clunk as payload either...
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Post  davidll1984 Sat Apr 09, 2022 6:51 am

From now on, fuels have anti-foaming agents Looking at it, we understand why RC Plane Small Cox Logo Babe Bee .049
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Post  Ken Cook Sat Apr 09, 2022 8:03 am

For one thing, your looking at a profile. Profile planes running on the ground are subjected to side to side vibration. Even a incorrect prop choice can cause this as it's allowing the engine to run in a zone where it consistently resonates.  When the plane leaves the ground and flies about 1.5 laps in the air, most of that settles down. A quality inline filter will resolve 90% of  the foaming issues on fuel delivery. Other factors come into play and one is the engine mounting. ARF's for instance are complete crap, the wood blows and the engine crushes into the beams which while it may work one day, two weeks later the gremlins make a visit. Just because you see bubbles and foaming in the tank doesn't mean the engine isn't going to work properly because it can. I have had two identical planes with similar engines and tanks require two totally different approaches on achieving a good engine run.

         Looking at the beginning of the video I see no muffler pressure line so I can only assume that this tank is setup for uniflow but I can't confirm that. The uniflow tank allows for 3 styles of engine run. One needs to try all 3 to confirm what is going to work properly. While most flying stunt don't generally run a standard vent configuration, sometimes it may be required due to other factors at work. Uniflow works great when it works, unfortunately this doesn't happen in control line.  This is also why most r/c flyers will say that learning control line is more difficult to obtain a decent engine run.

         A clunk isn't needed in control line. Shape, location and the configuration of the tank plumbing is the importance. Most clunks need to be reworked to work properly. In other words you want the tank to feed to the last drop. Most clunk tanks leave quite a bit of fuel at the end. This can be problematic if the clunk grabs a little fuel and the engine continues to run, then it leans, then it almost quits, then it picks up fuel again. This can go on for several laps until the fuel is depleted . Not a very gracious approach if your flying a official flight.  The radius in the rear corner of the tank and the round clunk is responsible for this and the clunk should be filed on a hard angle in order for it to go into the corner more. Unfortunately, this reduces the weight of the clunk. This is why one should shim the rear of the tank out and also adjust your leadouts to yaw the plane inboard slightly. You want that pickup in the fuel load at all times. While in the video on the ground, you can see the fuel bouncing all over but in the air the fuel load is solid and clunk was  very close to becoming uncovered in a few of those shots.

         Many in our club offer their gallon jugs a spritz of Armor All. I have done this myself at times and NEVER have seen ill effects in doing so. I have read the bs that every one suggests will happen. I have been flying with a guy for 40 years and I will say he's been doctoring his fuel cans with Armor All since the 80's. I haven't seen the plugs foul that so many suggest or a engine shutting off. I only use it when I can't achieve a decent run. As long as your pickup or clunk is submerged, bubbles generally are a non issue, the problem can really occur when you introduce a uniflow pipe within 3/8'" near the pickup. This can cause turbulence which can impact engine run and the uniflow setup becomes compromised. This may require the uniflow pipe to be backed off of the pickup. This is also a time when running muffler pressure to the uniflow pipe may or may not calm the issue down. Again, it takes patience and multiple tests. Therefore, just because you see the fuel bubbling in the tank as in the video assume that this is bad.
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Post  Surfer_kris Sat Apr 09, 2022 9:33 am

There is a nice video below thats shows what the fuel does in RC-planes. The clunk does it job very well in every manoeuvre except for extended dives, there is no way the clunk will be able to correct for that.

Sintered clunks are better than the regular ones (with a single hole in them) as they will keep a little bit of fuel in them, and they also tend to wick up the last bits of fuel when the tank is nearing empty.

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Post  Oldenginerod Sat Apr 09, 2022 5:09 pm

Surfer_kris wrote:There is a nice video below thats shows what the fuel does in RC-planes. The clunk does it job very well in every manoeuvre except for extended dives, there is no way the clunk will be able to correct for that.

Sintered clunks are better than the regular ones (with a single hole in them) as they will keep a little bit of fuel in them, and they also tend to wick up the last bits of fuel when the tank is nearing empty.


That pick-up sure spent a lot of time out of the fuel. I'm not sure how it kept running. Obviously it's only using a small amout of fuel at idle in those moments, but I'm surprised it didn't die when they throttled back up with a huge air bubble on the line.
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Post  Surfer_kris Sun Apr 10, 2022 12:18 am

The engine is actually running of another tank, inside the fuselage.
The "external" one seen in the video is only there in order to se what happens inside a less than half-filled tank, and the fuel lines are simply blocked off.
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Post  Ken Cook Sun Apr 10, 2022 6:39 am

One thing I didn't mention in my above post is something I also do when mounting my tanks. I choose to mount the same method as shown in the video. I wrap the tank with 3-4 layers of bubble wrap. One thing I've discovered over the years is that I think sometimes the noses are too stiff. For instance, if you build a stock Ringmaster, the doublers end 1" past the leading edge, if you hit the ground the nose breaks off right at this point. I take 15 min epoxy, glue it directly back on. After about the 2nd or 3rd time this happens the engine runs are terrific. This serves as a double standard if your using the Fox .35. This is essential for a Fox .35 break in. I don't know why they don't tell you this in the instructions. I think it's a combination of slamming the crank and the dirt it ingests. It acts like a lapping compound and assists in breaking in your Fox. It's worked for me.
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Post  Surfer_kris Sun Apr 10, 2022 8:06 am

Fox used to sell a lapping compound that one would feed into the engine while running, "Fox lustrox" I think it was called. It always sounded funny to me and I'm pretty sure it didn't help for their reputation around here... Wink
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Post  fredvon4 Sun Apr 10, 2022 9:22 am

I still have a packet or two of the majik Duke Fox powder that came in New Fox 36 MK IV and MK VI I bought
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Post  getback Mon Apr 11, 2022 3:26 pm

Ken Cook wrote:             One thing I didn't mention in my above post is something I also do when mounting my tanks. I choose to mount the same method as shown in the video. I wrap the tank with 3-4 layers of bubble wrap. One thing I've discovered over the years is that I think sometimes the noses are too stiff. For instance, if you build a stock Ringmaster, the doublers end 1" past the leading edge, if you hit the ground the nose breaks off right at this point. I take 15 min epoxy, glue it directly back on. After about the 2nd or 3rd time this happens the engine runs are terrific. This serves as a double standard if your using the Fox .35. This is essential for a Fox .35 break in. I don't know why they don't tell you this in the instructions. I think it's a combination of slamming the crank and the dirt it ingests. It acts like a lapping compound and assists in breaking in your Fox. It's worked for me.
Is this stiff nose thing the reason your using a type of tile , can' remember the name but in another post /build you had stated this material ? that is crazy about the dirt instating.
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