Air bubbles
 

Hi all, I finally got my OS 52 dialed in where it transitions well between idle and full throttle and runs pretty good, got 15 minutes+ on a 6 ounce tank with 1/4 left when Ifinally shut it down, however Idiscovered a problem that will be an issue while flying. Iran the plane up on the ground with the wing off so I could play with the settings. Ipicked it up and pointed the nose up running half and full throttle, and then did simulated moves, up down upside down, and found that when Ipoint the nose down to 45 degrees or so the engine quit, after a couple times doing this Ifound it to be air in the line being sucked up when the nose is down.

How can Iremdedy this? The tank is setup according to the book, and sitting in the correct position. This is in a Kadet LT40.

Or will the fuel be towards the rear of the tank while the plane is moving and be a non issue?

RE: Air bubbles
 

I would ensure the clunk line is of the most flexible fuel line obtainable so when in a downward maneuver, the clunk will fall to the front of the tank. I've found the normal stuff you get on the spool (I use Dubro medium on my engines) isn't super flexible and oftentimes the clunk will not move to the front of the tank. Though most of my aero engines are on airboats and not planes, I dont run into that problem thus the thicker fuel line I use.

RE: Air bubbles
 

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mso-style-noshow:yes; mso-style-priority:99; mso-style-parent:""; mso-padding-alt:0in 5.4pt 0in 5.4pt; mso-para-margin:0in; mso-para-margin-bottom:.0001pt; mso-pagination:widow-orphan; font-size:11.0pt; font-family:"Calibri","sans-serif"; mso-ascii-font-family:Calibri; mso-ascii-theme-font:minor-latin; mso-hansi-font-family:Calibri; mso-hansi-theme-font:minor-latin; mso-bidi-font-family:"Times New Roman"; mso-bidi-theme-font:minor-bidi;}</style><![endif]><p class="MsoPlainText">GS Racing Super-C Silicone Fuel Tubing (Medium,2.4x5.5mm, 50ft) (Pink) (GSC2455TFP) = $29.99from A Main Hobbies

Please ignore the crap at the top of the post. RCUeditor is still in the dark ages ...</p>

RE: Air bubbles
 

There doesnt seem to be enough room in the tank for the line to fall back on itself.

I'm probably chasing a red herring since it didn't stall out when I flew it and its been in a few loops and such.

RE: Air bubbles
 

Gotta admit, you're the first to put a 6 oz. tank on an LT-40 (which is essentially a 60-sized fuse). I have a 14 oz. in my LT - "flop over" is no problem. In fact, I end up cutting the clunk line in half and add a piece of brass tubing to ensure that it doesn't flop over.

Part of the issue is that if you're using a throttle management protocol along with your maneuvers, you are rarely above 1/4 throttle and in vertical down lines, probably at idle. As a result, the flow demand is very low and there is sufficient fuel in the clunk line, tank-to-HS needle line, HS needle to carb line to keep things going.

Brings up an idea, how about adding a loop of fuel line between the tank and your rear mounted carb / HS needle? That will give you a bit of "reserve" fuel for down lines. Also, if you move your tank farther back (yea, I know, CG stuff) that will give you more of a "reserve" fuel line length.

Time to tweak things until it flies the way you want it to fly!

RE: Air bubbles
 

Here are some of the possible places where leaks can occur:

Fuel line ends in fuel tank
Fuel tank stopper
Brass tubing in fuel tank
Fuel line in the tank
Fuel line to remote needle valve
Fuel line from rnv to carb
O ring under carb

The fuel line from rnv to carb seems to always get overlooked. A clunk can turn to the front of the tank and get stuck.

Edit to add leaks from backplates

RE: Air bubbles
 

ive been flying for about 20 years and i was taught that it was not a good idea for the clunk to move forward..... anyone else care to chime in?

RE: Air bubbles
 

After many times ending in troubles with the standard solutions, I ended up using a fuel pump and the life got lot better. Indeed it is not the cheapest way but that is how those pro-guys are flying all the time while all the others are ... fixing tubing most of the time.

Note: some OS Carbs(needle) were 'famous' of generating bubles(small !) inside; there were pretty many posts in the past, especially for OS 52; it is worth checking in RCU search. How big are the air bubbles is indicative - if they are tiny small ones, then the tank is not the problem, the needle is.

Cheers,
Nick

RE: Air bubbles
 

How well is the tank foam rubber padded? That could be a source of air bubbles. A fuel filter before the carb will tend to condense air bubbles and help with that as will a sintered clunk.

Having the clunk go to the front of the tank is inviting a kink in the tubing or getting it stuck in the front where it doesn't usually belong. I have a bad habit of getting the clunk stuck in the front of the tank sometimes after a hard landing and then forgetting to check for that! (ask me how many dead stick landings it takes for this pilot to then realize the clunk is stuck in the front.....:)

Ernie Misner

RE: Air bubbles
 

I understand you don't want the clunk to get stuck, but at 1/2 tank, in a dive, why would you not want the clunk in wet fuel? I mean, air bubbles happen from time to time but one cannot possibly always fly "nose up" to keep the clunk wet.

That's why I suggested super flexible line. It makes far more sense that way than than to not allow the clunk to move much. Then again, a 6oz tank is really small.

Then again, what do I know. I use stuff for which is was never intended, so my thought process is different.

RE: Air bubbles
 

1. Tell me how long it will run with the nose pointed down.
2. How long can you fly in that attitude?

If the answer to the first question is longer than the second, it doesn't matter.


You don't want t the clunk forward in the tank, if it gets stuck there you will flame out climbing or inverted.
You won't be at full throttle with the nose pointed down, and the line contains enough fuel for the downline.

RE: Air bubbles
 

Whoa, hold on, didn't mean to start a war on clunks. All I was asking is how to avoid a stall out when the nose is down. Keep in mind I was holding the plane while the engine was running, so no G-forces involved to push the fuel back. <div>
</div><div>I totally forgot to put the perspective of the fuel in play while the plane is in motion and my static test is probably not a good test. </div><div>
</div><div>
</div><div>Anyhow, </div><div>
</div><div></div><div>
</div><div>I did notice this when idling, small air bubble rise up from the carb, and when I throttle up, causes it to misfire a bit.  Pretty sure this is the reason it stalls out when I have to throttle up for a go around when landing.   I will do a search on this to see what the fix is, and just fly it for now, It's most likely going to fly just fine. </div>

RE: Air bubbles
 

G'day

I have built more planes than I can remember and have never had a problem with the engine dying in a dive. I usually use the same tube in the tank as outside it.

I have had some Chinese tanks which came with very light walled tube fail. What usually happens is that the thin tube splits and falls off the tube at the front of the tank. The result is that the plane flies OK for a few minutes then the engine suddenly stops when the fuel level falls below the tube. Since this happened, I have replaced the tube in all these tanks and no problems since.

I have also had some Chinese tanks split around the bung. Perhaps I was too enthusiastic and tightened the plug too much? I suspect so.

These days I use DuBro tanks for the simple reason that they are all I can get. They seem OK to me and I have not had any failures with them and the tube they supply.

Cheers

Mike in Oz

RE: Air bubbles
 

+1 to cheap fuel filter:)

RE: Air bubbles
 

Get a Tettra bubbleless tank and be done with the problem forever.

Denis

RE: Air bubbles
 

Purchase a "Tettra" bubbles fuel tank from Central Hobbies and your problem will be solved.

RE: Air bubbles
 

Hi!
Hold on!
There is no problem with your tank as long as there is not air bubbles when you run the engine as normal!
The engine will eventually cut out in a long dive! But that is no problem because you cannot dive for that long ...after just a few seconds the ground will grab you!;)

You should not! I repeat NOT istall the lines in your tank so that the fuel line can be bent forward!!!!
The standard procedure is to have the clunk line 0,5-1cm from the back of the tank ...and not bother with the fuel line going forward in a dive...because the fuel line should not!!!!

RE: Air bubbles
 

I used to think that the fuel line should be able to move towards the front of the fuel tank, when the plane is in a nose down attitude. But now I am not sure it needs to. The Gee forces applied to the plane, and fuel tank tend to force the fuel to stay at the rear so the clunk remains in the fuel. I think even in a stall-spin manuever, the fuel tends to stay at the back of the fuel tank as the plane is falling nose down. So I am not sure if the clunk needs to go forward anymore. Maybe it is a myth that has propagated around all these years.  I think if one really needs the clunk to go forward and they are using the larger fuel tanks, they only need to use a heavier clunk to overcome the more stiff fuel line.

RE: Air bubbles
 

The fuel will stay at the rear of the tank in a dive. If it didn't, we would all deadstick during dives. The clunk is never supposed to end up at the front of the tank.

RE: Air bubbles
 

Thats what I thought!  A static nose down run doesnt have the fuel moving backwards because the plane inst moving. Call me Homer, DOH!

RE: Air bubbles
 

Hi!
The fuel will fall forward in a "no acceleration dive"!!!! But that isn't a problem because the fuel remaining in the line is enough to have the engine run for several seconds...that is more time than you need before the earth grabs you!:D

RE: Air bubbles
 

How many seconds until you get the air bubbles? Figure maybe 45 mph with a LT-40 is 66 ft/sec. If you do a 10 second dive that's 660 feet drop.

Have you ever held a LT-40 to a 660 ft dive at full throttle? They climb at full throttle.

I don't believe any of my clunks move much forward. I place them 1/2" in from the rear of the tank.</p>

RE: Air bubbles
 

If your tank center isn't approx. lined up with the carb that can cause troubles in various attitudes such as stright up or down. How? If that tank center is off the your tuning will always be a compromise and either nose up or nose down will compound the tuning problem in the worst way or direction. A too low tank will necessitate an overly rich LS needle or idle setting and then pointing the nose straight down pushes it over the edge rich, wet plug, and deadstick. A hot plug can help with this compromise even the OS F.

Thanks,

Ernie Misner

RE: Air bubbles
 

Don't ever fly 45 degrees nose down. :D

RE: Air bubbles
 

I totally understand, and stand corrected. I haven't gotten in the air yet, so I'm not up to speed on everything. I can run an engine fine, but flying..... not so much... (yet.)