Clunks

[John Gallard]

 I know I'll be accused of worrying about a problem that does not exist but this comes under the heading of 'So how does that work then?' The following video shows that the clunk spends a lot of time 'out of the fuel'. During the time it is out of the fuel I imagine air, under pressure finds its way into the fuel line, granted the engine will run until the reserve of fuel in the line is used up, but what happens when the air reaches the carb? I have seen the effect on the ground of a row of bubble reaching the carb....the engine cuts! This has been bugging me since a club colleague (albeit a heli guy) saw me assembling my tank at the field last week and told me I had done it incorrectly, the clunk should be free to move in every direction including folding back on itself (without a kink)

http://richter-lackierung.de/aktualisierung_2006/5MBVersionflycamefuntana.wmv

[BB]

You have to also remember that quite a few of those manouvers require little throttle during the aero'.

 

BB

 

[flytilbroke]

The video is pretty good and shows clearly how fuel moves around

 

I have been unable to get any tubing which would allow the clunk to get to all areas of a tank when used in tank sizes from 8oz to 14oz. I have only used Glow engines so far.

 

The model in the video, to me looks like it is using a Petrol fueled engine with Tygon tube and a felt covered clunk. The felt retains some fuel, feeding the demand until it gets into the main fuel again.

 

Many guys do not like Felt around the clunk due to small fibres clogging either the inline filter or the Carbureter valve.

 

 

[John Muir]

I don't know how you could allow the clunk to go to the front of the tank without getting stuck there. How does your heli flying pal do it? I thought heli tanks were much the same as fixed wing ones.

 

In the video you'll notice the fuel only runs to the front when the plane is in a steep, or vertical dive. The guy does this repeatedly to demonstrate his point. For the fuel to run forward, the plane has to be decelerating, so the engine must be idling and the prop acting as a brake. When this is happening the engine needs very little fuel and will keep going on whats sloshing about in the cylinder and fuel lines. When the throttle's opened up again, there might be a small hesitation while it draws fuel, but it won't be as bad as on the ground because the prop is being turned by the airflow which will help keep the engine going. If the engine was providing thrust, accelerating the plane, the fuel would be at the back of the tank anyway, even in a dive.

 

Obviously, if you kept the plane in a steep dive long enough, the engine would stop through lack of fuel, but by then the engine's pick up would be the least of your worries.

 

JM

[Stephen Grigg]

Enjoyed the music

[Terry Whiting]

The only time I know of  a clunk folding forward and getting stuck is through a abrupt stop,   and that just can't happen when flying.

 

I have always made sure my clunk is 1/4" from the end of the tank.

To see bubbles when on the ground you must have only half a tank of fuel and a hole in the clunk tube where it is connected to the feed pipe.

I personally only ever used Slec tanks (square)  

[JohnP25]

This seems to be quite an issue with very different schools of thought. How much clunk should the clunk clunk?

Those many years long gone, fuel tanks were supplied with a very soft silicon tube which would allow the clunk to flop anywhere in the tank.

I was taught by the experts of that time to adjust the bend in the breather / pressure pipe to follow the shape of the tank front. It had to be close enough not to allow the clunk to pass behind it. Further, the ordinary clunk would be changed to either sintered bronze or felt covered. This was done to eliminate bubbles in the case of the fuel in the tank foaming due to vibration and act as an in-line filter. The instruction supplied with the tank on how to set the pipe length were always followed. The tank was also pack in foam to avoid the effects of vibration.

 The modern fuel tank is not supplied with the soft pipe, but ordinary fuel tube. I am not sure what this means. Does it mean that a stiffer tube is good enough? I managed to source some of the soft tube in Nottingham a few years ago and bought enough to last a while.

 It may not matter through my style of flying. I do not have a huge fuel tank but one that will give me about 15 to 20 minutes with my TX timer set for 13, the rest being for going round once or twice in case the strip is occupied. Normally there is very little fuel left in the tank on landing.

 I do not think that the modern pipe will flop to the front of the tank as it is normal pipe. Adding stiffeners in the middle then seem rather pointless unless of course the tank is that big.

 The issue seems to be that the breather pipe is not set properly at the front of the tank. Maybe that should be discussed further.

[JOHN WATERS]

I have found the best clunks are sintered bronze

with that you don't have to cut your line to the

engine which leads to other problems like

sucking air