Hi all,

Saturday went out flying with my Kadet MkII, it's still very windy here in NL (force 4/5) but overall flying went well. I did my own takeoffs and flight, but my instructor did the landings as it was too windy for me (not feeling like doing repairs if I can avoid them ).

Anyway on my last flight of the day I made some loops and 'wild manouvres (ok, trying to recover from the loop and turbulence)' and started to line up the plane to hand over to my instructor for landing when I noticed something wasn't right. The engine stopped and that with the plane at a good distance away with the wind blowing the plane further from the field.

My instructor took over and managed to land the plane as softly as humanly possible. Then I had to crawl my way through wet mud & heavy clay, but found the plane and no damage at all!!!

So that was all good in the end, leaves the question why the engine stopped. It's an OS .40LA, never stopped before and tank was still half-full when it died. When diagnosing it turned out that when you hold the plane nose down for a few seconds with a ~half-full tank (with engine running) airbubbles were sucked in and the engine would sputter badly but mostly keep running. Except during the last flight when either the bubble was too big or something else caused it to stop.

Anyway to cut a long story short, I was advised to not use the standard clunk that came with the tank but rather a 'Filt-Clunk'.
I borrowed one from someone who had a spare and flew with it today and the problem seems to be solved. This clunk sort of keeps a reserve of fuel even when the clunk is not dipped in the fuel for a brief time.

Sofar my experience with it is good, it seems to have solved the problem. If it keeps up I wil never build a plane without it anymore!

Thought I'd share this. I never gave my clunk a second thought but they can make the difference between a normal landing or a deadstick....Anyone have similar experiences?

Johan

Johan,
Do you have a link to a vendor that sells one of these. It does sound interesting.

Neil

Hi Neil,

I borrowed mine from a fellow flyer but I know he bought them in a HS here in the Netherlands. BTW the guy who runs the shop has been the dutch champion model aerobatics for a couple a years in a row. He's the one who recommends them apparently.

In any case I did a google search and found [link=http://www.jperkinsdistribution.co.uk/list.php?subcat=63&cat=Fuelling%20accessories&Navm ain=Field%20equipment]this[/link] site. It's the 8th one on the list (part number 5508085).

The filt sucks full of fuel and there is actually a small reservoir under the filt so it has a small buffer. Seems to work sofar.....

Good luck....

Johan

Correction. I see the correct English is actually FELT clunk (not filt). Also on the website they seem to be aimed at petrol flyers but don't see why they shouldn't work with Glow....

I have one of these in my gasser, didn't know anything about the reservoir though.

Reservoir is too big a word in this context.

If you pull the felt away you will see a hollow space that will hold a few milliliters of fuel. All I can see is that before this clunk I had airbubbles (nose down with a half empty or so tank) and afterwards they were gone....

Johan,

The clunk is an interesting piece. I'm not doubting your experience with them but just commenting since you did ask. Personally, I've never needed such a device, not even in my aerobats which I do throw around mercilessly. I've had similar problems such as yours when there was a hole in my lines, the stopper was not on tight and one time when the stopper had expanded and leaking through the holes.

Thanks for the info.

Hi sfsjkid,

Totally agree with what you're saying, however I inspected the tubing very carefully and could not find any punctures/leaks. I still use the same fuel-lines with the new clunk and it seems to have solved my problem, but time will tell. I can believe that this new clunk has a better safety margin though....

What I am wondering about (more in general) is indeed when you look at the 'physics' of the tank and clunk... When you take a half-full tank and hold it 'nose-down' the clunk will come out of the fuel (as it doesn't fall forward far enough to stay under) which in my mind can only have the result of sucking air rather than fuel....Aren't bubbles unavoidable in such a case or am i missing something?

Anyway...More things to keep us occupied

Cheers,

Johan

Johan,

Under the conditions you state, yes, bubbles and deadstick are unavoidable. And you are also correct about this being food for thought.

Upon reflection, I normally do most of the "radical" stuff at the begining of the tank and leave plenty of reserve before landing, just in case something happens to tie up our runway. Typically I'll land with 2 to 3 minutes of fuel left. On those flights where I've forgotten about how long I've been up, I'll get a warning from the sputtering my engine does and return to level flight. Usually before the engine dies.

I've not considered this before but basically, the clunk line seems to be acting as the buffer. Unless you happen to hit exactly the right conditions, even on a down line, the clunk should fall forward slightly, and all the way forward depending on the angle. Addtionally, I believe the exhaust pressure insures that there is at least a little fuel.

The filter you mentioned probably does give you more time in that the felt is soaked with fuel and is more likely to be in the fuel during your nose down test, even with a very low fuel level. It does seem to be a good idea, I'll consider it should I start having problems.

Thanks!

Keep in mind, in a downline, fuel moves to the back of the tank, along with the clunk. If the clunk gets stuck forward then you have a problem but if you are in a dive, inertial forces move the fuel to the BACK of the tank, it will not pool up at the front of the tank and dry out the line. What sometimes happens though is that in a downline, high speed snap, you can snap fuel out of the line due to the intertial forces to the side - and forward since the plane slows down so fast.

This does not make sense. The only way for inertial forces to move fuel to the back of the tank on the down line is if the plane is accelerating more than the force of Gravity. I think most of us probably cut back on the throttle on the down line, letting gravity do most of the work. Since there is drag against the plane, the fuel will pool in the front.

This would be mitigated by the amount of fuel already in the clunk line. By cutting the throttle, you significantly reduce the fuel draw, so you get more time before you're sucking air. I normally use a longer fuel supply line, so I can put a long loop between the tank and the engine. This has not affected engine performance significantly, and reduces the number of deadsticks in a diving maneuver. Another way to address the problem is to take a lesson from the Heli guys and use a 2 oz header tank between the main tank and the engine. This effectively increases the volume of the supply line. When the plane returns to straight and level flight, any air bubbles that may have moved into the header tank vent back to the main tank, and the 2 oz of fuel would support a two to three minute dive. The height of the header tank relative to the carb is not significant, so it can be placed in any convenient location within the fuselage.

Brad

This has been discussed before.

http://www.rcuniverse.com/forum/m_22...tm.htm#2251490

and also check out MinnFlyer's post here which again, is the same thing I stated.

http://www.rcuniverse.com/forum/m_16.../tm.htm#169335

I'll throw this in the mix. What if the flying conditions had more to do with the clunk out of the fuel? Gusty conditions, the plane getting bounced around, all that. There are times when the clunk is out of the fuel when doing aerobatics, but it never last very long. You take Johan's situation, the plane is low on fuel and being tossed around by the wind. Those forces aren't the normal aerobatic forces. I've been flying when wind are gusting and have seen my plane literally stop in mid air. Nothing you do in aerobatics duplicates this.

Johan, your instructor acted admirably. Landed your plane safely, then let you slog your way through the mud to retrieve it.

Hi Kwigen,



I'm not complaining. The guy is very good!

And yes it was very gusty (quite spectacular actually). I would surely have needed a barf-bag had I been in that plane.