flymeaway

My Feedback: (6)

My fuel tank is set up with a fill line and a pick up line. Is it necessary to T a separate clunk in the supply line closer to the stopper so as air bubbles won't enter the system? My thought is that when in a downline with say less than a half tank of fuel, won't the clunk by above the fuel level and if so, will air enter the line and possibly cause the motor to stumble or quit?

Thanks for any insight.
Bob

Mastertech

My Feedback: (31)

No need. Single pickup line is all thats needed. Model airplanes have been flying like this for 50 years.

krayzc-RCU

My Feedback: (31)

gas or glow?

flymeaway

My Feedback: (6)

Yes, and I've been flying them for five years like this so maybe it's a stupid question, but I'm a little gun shy.



Gas or glow?

Gas.


The reason I'm questioning this is I just repaired my 27% gas Extra (ripped the gear off and broke a stab landing in a bean field) that I lost the motor on a downline - not enough altitude for a go around and too much airspeed and missed the runway. The reason for the engine quitting was my fault - I didn't put a barb on the brass tube for the supply line and the wire tie I used wasn't enough to keep the tube on. The system picked up enough air on the upline for the motor to quit on the downline.

So, as long as I'm in there doing things right, I was wondering if double pick up lines are necessary and if anyone is doing their tanks this way.

krayzc-RCU

My Feedback: (31)

i asked gas or glow as a gas engine never does ingest an air bubble because of the way the carb is set up.

JoeAirPort

My Feedback: (41)

Only one clunk is required on the pickup line. This question about where gas goes in the tank during a downline has come up before.

Mastertech

My Feedback: (31)

Not a stupid question. Trying new things is how we make advances. Asking why we've always done something one way rather than another is a good thing. I doubt you lost the fuel on the way up but most likely on the way down. Either way, you could use a header tank but to be honest I don't think you need one.

aaxiss

Well you seem to know the cause , Put barbs on the brass tubes both inside and out, safety wire the fuel line ( I use some thin stainless leader wire from the local fishing shop. like .029 ).

To safety wire the fuel line, loop wire around the fuel line just in back of the barbs 2 times, grab ends of wire with some small vice-grip type locking pliers ( or you can use safety wire pliers if ya have em, they lock too and have nifty knob on the end to spin wire) Trim twisted wires up about 5-8 twists up , and cover cut end and fold out of way best you can , those little wires are like needles..

I like to crimp an extra rigging crimp like those used on pull-pull rudder cables over the ends of the sharp wires so I don't end up donating blood if I have to fumble around my tank.

hauckf

I am not doubting krayzc-RCU's statement, but can anyone explain why a gas engine does not ingest an air bubble, or point me to a thread, etc.? I've read a ton of threads on RCU and info on other sites on Walbro carbs and have never read an explanation, or for that matter, any mention of that fact. Thanks.

Fly_Fly

I have had 2 dead stick landings with my Comp ARF Yak 55-SP It was when I was down to approx 1/3 of a tank.
a mate of mine picked up my Yak none down and pointed out that the clunk was sitting in thin air..!!
Another club member explained that he always uses a Walbro felt clunk and black flexible neoprene fuel line.
This allows the clunk to act as a clunk. I have since put one of the said Walbro clunks in all of my fuel tanks and have not experienced the dead stick problem since. You can buy the Walbro clunks from any good mower shop. Also OS Engines make one identical to the Walbro one.
I hope this is of some help.

Cheers Ivan

Composite ARF Yak 55-Sp 2.1
Carbon Fibre 3 bladed 21 X 11.5 Mejlick
DL-55 with Mtw Canaster & Header
710 ml Dubro Tank
Hitec Servos
Futaba Fasst

Tired Old Man

My Feedback: (1)

Been saying good things about the Walbro felt clunks for a long, long time. Then someone pops up and talks about some fibers found on a carb screen they found in their engine. In thousands of flight hours I haven't seen felt fibers in carb screens so I'm wondering where they come from in other people's carbs.

The Walbro clunks break up air bubbles and pick up the last drop of gas in a tank, making them an excellent choice for the equipment list.

It would have to be a mighty long down line for a carb to run out it's residual fuel from the upline. To get high enough for the carb to run dry going down the plane would have to have become invisible on the upline.

Fly_Fly

Well T_O_M I do have a bad habit of flying up around the clouds and I also have my Yak set up for a high idle on a switch to yet again avoid the good ol dead stick situation.
so with the long down line and a higher than usual idle I have run dry a couple of times, But in using the felt clunk and soft black in tank line, I appear to have fixed the problem.
So,, in a long down line, even if the clunk decides to stay at the top of the tank, (unlikely) but if it does there is a bit more fuel in the felt to help feed the engine, so you have a full fuel line and a felt backup if that makes sense?

Works for me

NB: It was suggested to change the clunk after a year or so just in case of the felt deterioration, ? That a heck of a lot of flights so I'm happy to do that !

Cheers Ivan..

JoeAirPort

My Feedback: (41)

I think the stiff tygon is the culprit in most of these downline issues. That and clunks/clunk lines that have fallen off.

Rcpilot

My Feedback: (78)

The design of the Walbro basically won't let an air bubble work it's way into the venturi. I can explain it in detail, but it'll take a 5000 word essay to explain it.

There's a pump and a "bowl" that the fuel must pass through before entering the venturi. Passing an air bubble and causing a flame out just isn't going to happen.......... not if you have fuel in the tank and the motor is running. Of course you can suck a tank dry, but then you expect it to flame out, right?

Don't worry about a flame out from an air bubble in a gas engine. You'd have to fly so stinkin' high and then dive it straight down for probably 5 minutes before it would run the carb dry.


Just for fun:
If you want to know how long your engine will run before the carb is sucked dry, simply start it and get it running good. Nice and warmed up. Now go to idle and pull the clunk line off the carb. Set your stop watch and grab a soda. Have a smoke. I'll bet you it runs at idle for at least 2 or 3 minutes. How many of us will experience a 2 minute or 3 minute down line at idle?

Fine print:
Do that at your own risk. I don't need to tell anyone of the risk involved with pulling a fuel line off a running gas engine.