Clunk!

[Adrian Smith 1]

As a long time user of RotoFlow tanks, Gary I can tell you there is not a problem with sustained diving flight if that is the issue you have. There are great tanks and are a fit & forget item (as much as they can be) . The only issue is cost, but I found to get reliability sometimes you have to pay ........... Anyway hope you manage to sort it.

Edited By Adrian Smith 1 on 26/03/2018 13:31:39

[John Stainforth]

At risk of stating the obvious and making a fool of myself, when flying inverted with the nose down, couldn't the fuel line be in the air whilst the vent line is submerged? The angular momentum of the spin may also be tending to fling the fuel to the front of the tank if the tank is in front of the CG.

[Denis Watkins]

Posted by John Stainforth on 26/03/2018 13:42:06:

At risk of stating the obvious and making a fool of myself, when flying inverted with the nose down, couldn't the fuel line be in the air whilst the vent line is submerged? The angular momentum of the spin may also be tending to fling the fuel to the front of the tank if the tank is in front of the CG.

You are right John, Brian Winch investigated this with a camera inside the model and the fuel in view during manoeuvres, and did follow such patterns that defied gravity

[Gary Manuel]

Posted by john stones 1 on 26/03/2018 11:49:17:

Tis one of lifes great mysteries, when you've drawn a conclusion, can you tell me why your other petrols behave, given you fly same style with those.

I don't think I've sustained a steady nose down spin for so long with any other plane.

This Weston Capiche is really steady in right way up and inverted SLOW spins with about 1/4 throttle. It is more nose down when inverted though, which is probably why I'm seeing the problem. I'm probably talking about getting on for 10 seconds.

The other factor is that I use normal felt clunks in my other planes, which have a bit of an internal reservoir underneath the felt. . I would have used the same in this one but the 2 clunks I had that I thought were felt, were not.

Posted by Adrian Smith 1 on 26/03/2018 13:30:57:

As a long time user of RotoFlow tanks, Gary I can tell you there is not a problem with sustained diving flight if that is the issue you have. There are great tanks and are a fit & forget item (as much as they can be) . The only issue is cost, but I found to get reliability sometimes you have to pay ........... Anyway hope you manage to sort it.

Edited By Adrian Smith 1 on 26/03/2018 13:31:39

I don't think a Rotoflow tank would behave any different to a tank with a normal clunk. If the nose is pointing downwards, the Rotoflow clunk would still be out of the fuel with a half empty tank.

It's not sustained diving that's the problem. In a sustained down line, the throttle would be chopped so that the engine is using very little fuel. I'm having the problem in a spin, with a fair amount of throttle, to keep the "flattish" spin going. Fuel is being used, but the clunk is above the fuel line.

I dare say that once this problem has gone away, and I gain more confidence in the engine not cutting, I'll reduce the throttle and allow the model to flatten out a bit more.

Posted by John Stainforth on 26/03/2018 13:42:06:

At risk of stating the obvious and making a fool of myself, when flying inverted with the nose down, couldn't the fuel line be in the air whilst the vent line is submerged? The angular momentum of the spin may also be tending to fling the fuel to the front of the tank if the tank is in front of the CG.

Don't be afraid of making a fool of yourself - I do it all the time.

If you look at the video in the second post on page 2 of this thread, you will see that this is exactly what is happening - and my spins are much slower and sustained than the ones in the video. Here's the video embedded, but obviously the associated discussion isn't here. See the above link for the discussion.

 

Posted by Denis Watkins on 26/03/2018 13:56:04:

Posted by John Stainforth on 26/03/2018 13:42:06:

At risk of stating the obvious and making a fool of myself, when flying inverted with the nose down, couldn't the fuel line be in the air whilst the vent line is submerged? The angular momentum of the spin may also be tending to fling the fuel to the front of the tank if the tank is in front of the CG.

You are right John, Brian Winch investigated this with a camera inside the model and the fuel in view during manoeuvres, and did follow such patterns that defied gravity

Do you have a link to where Brian Winch investigated this Denis?

Thanks again chaps - I'm sure that I can resolve this by putting a better clunk on and maybe changing my flying style for this model. I have the bit between my teeth now though, and I want to get to the bottom of what I see as a problem with current tank designs.

It is of interest (to me at least) that RC boaters use medical IV bags (drip / plasma bags) to avoid getting air bubbles or water(!) into their carbs. They evacuate the bag first, then pump fuel into it, then away they go. As fuel is sucked out of the bag, the bag collapses - hey presto, pure fuel is delivered. This would work in a plane, but the risk of puncture in use or rupture after a crash makes it unsafe to use. If however, an IV was inserted into a solid tank, which was vented to the outside, the outer tank would protect the bag, and air would be expelled during filling of the bag and sucked in as the bag emptied. I'm not seriously considering this, but it would be one way of solving the problem completely.

Edited By Gary Manuel on 26/03/2018 16:16:19

[John Stainforth]

I should have read your early post more carefully - I missed the excellent video, which shows very clearly what I had in mind.

All my models develop tank and fuel supply problems from time to time, and then it is difficult to avoid dead-sticks if one throws the planes around the sky!

[Adrian Smith 1]

I think you can see why I made the move to 60cc electrics, as all these fuel issues go away, Gary! Seriously though I hope the suggestions here help to to resolve it sooner rather than later.

[Gary Manuel]

Posted by John Stainforth on 26/03/2018 16:34:57:

I should have read your early post more carefully - I missed the excellent video, which shows very clearly what I had in mind.

All my models develop tank and fuel supply problems from time to time, and then it is difficult to avoid dead-sticks if one throws the planes around the sky!

The irony John, is that if I had been throwing it around the sky, the occasional splash of fuel into the clunk might have kept the engine running.

On this occasion, I was flying too smoothly - a rare event as other will testify.

Edit - regular felt clunks ordered. I might also install an in-line filter to act as a reservoir although I recognise that the usual mesh filter used for nitro is not fine enough for petrol carbs.

Edit 2 -Why not one of THESE?

Edited By Gary Manuel on 26/03/2018 17:21:12

[John Stainforth]

Gary,

You are probably right. I steady spin is probably more detrimental for fuel supply than more erratic manoeuvres.

What kind of clunk systems do the top aerobatic model pilots use?

[iqon]

Also dont forget - petrol will make it more floppier, dont rush into altering stuff you dont have too, as posted earlier fly a tank through it without doing the acrobatic stuff, its no wonder it`s getting confused. Do you normally empty tank when done, next time dont.

[Gary Manuel]

John,

Not sure but I suspect they use the standard clunk arrangement. Put it this way, I've bought supposedly ARTF TOC specification models which came with standard arrangement tanks.

Out of interest, I've found out that full sized aerobatic planes have a small header tank between the main tank and the carb. The header tank is fitted with a clunk, much the same as ours. See second question HERE.

[Gary Manuel]

Posted by iqon on 26/03/2018 17:45:07:

Also dont forget - petrol will make it more floppier, dont rush into altering stuff you dont have too, as posted earlier fly a tank through it without doing the acrobatic stuff, its no wonder it`s getting confused. Do you normally empty tank when done, next time dont.

Funny you should say that - I've found that petrol makes the pipes go stiffer, rather than floppier.

It's an aerobatic model that was designed to do aerobatic stuff - and it needs testing. No point flying around in circles till my timer tells me it's time to land. But I may need to do this just to confirm that another fault isn't leading me to a wrong diagnosis of the problem.

Yes I do normally empty the tank after a days flying.

Edited By Gary Manuel on 26/03/2018 18:00:09

[Gary Manuel]

Update.

The collapsible air bag system inside a solid outer bottle I spoke about earlier is already available to purchase and used by Pylon Racers.

They are available pre assembled in various sizes up to 12 oz (which is too small for me) and need a pump to ensure that all the air is removed from the bag. See HERE for details of tanks and pumps.

Here's a couple of videos that show the tank ......

.... and the pump.

[Ron Gray]

@Gary - try one of those larger, designed for petrol (mowers, chainsaws and the like) filters, that will give you the reservoir that I was on about which will allow you to fly through the dry clunk period. Fuel bladders and the like is overthinking it!

[Gary Manuel]

Ron - I've already said I'm just going to put a proper felt clunk and maybe an inline filter.

I'm sure that my problem will go away once I do this.

The rest of the discussion was out of interest (mine if nobody else's).

[Martin McIntosh]

The two I built for others used header tanks. No problems reported. Plenty of room for one in a Cap especially with a smaller main tank than a glow would use.

[Denis Watkins]

Posted by Gary Manuel on 26/03/2018 18:51:24:

Ron - I've already said I'm just going to put a proper felt clunk and maybe an inline filter.

I'm sure that my problem will go away once I do this.

The rest of the discussion was out of interest (mine if nobody else's).

Always interesting, and relevant Gary

[Ron Gray]

Posted by Gary Manuel on 26/03/2018 18:51:24:

Ron - I've already said I'm just going to put a proper felt clunk and maybe an inline filter.

Fair enough, I'll leave you too it then!

[Gary Manuel]

Posted by Martin McIntosh on 26/03/2018 19:20:04:

The two I built for others used header tanks. No problems reported. Plenty of room for one in a Cap especially with a smaller main tank than a glow would use.

Hi Martin.

Thanks for joining the discussion and bringing a (slightly) different solution into it.

After doing a bit of reading up about header tanks, I think I can see how this works - and I like it.

Excuse me rambling here - I'm just collecting my thoughts:

As I understand it, a header tank sits between the main tank and the carb. The outlet from the main tank clunk line goes to a fixed pipe as close as possible to the top of the header tank. The outlet from the header tank is taken from a fixed pipe close to the physical centre of the tank (or a clunk?) to the carb via a "T" connector going to a fuel dot. The "T" connector / dot is important because the process of filling the tank from the fuel dot pushes any air from the last flight out of the header tank via the pipe at the top, into the main tank and out of the vent pipe at the top of the main tank.

During each flight, any air sucked into the clunk finishes up in the header tank. It will not be able to get out of the header tank and into the carb because the pickup is in the centre of the tank, which should always be in fuel unless the air bubble at the top of the header tank reaches the pickup pipe.

The header tank is effectively acting like the in-line filter that I'm thinking of adding, in that it acts as a fuel reservoir. The main difference is that the in-line filter might let some air reach the carb depending on the models attitude, but the header tank will only release air once the air bubble fills the tank as far as the fuel pickup pipe in the centre.

Apologies for rambling, but doing so helps me get me head round it. The question is, have I understood it correctly?

[Gary Manuel]

Posted by Ron Gray on 26/03/2018 22:14:31:

Posted by Gary Manuel on 26/03/2018 18:51:24:

Ron - I've already said I'm just going to put a proper felt clunk and maybe an inline filter.

Fair enough, I'll leave you too it then!

Apologies if I came across as being curt Ron. It was not intentional and I very much appreciate your input - in fact my decision to go this way was mainly down to your earlier suggestion.

[Petrol Pete]

Hi Gary, I have been having exactly the same problem with a dle20, I've changed the clunk, replaced the fuel one, striped and cleaned and overhauled the carb and still it runs for about 2 mins in the air and cuts as you back off on the throttle, I then found it would run flat out and cut on backing off the throttle to a lower setting. I then ran it in the garden flat out for about 2 mins( poor Neighbours) and backed it off and it cut, the only way to stop it was to riches the idle up about 1/2 a turn which is quite a lot on a petrol that stopped it and it kept going whatever. I haven't flown it yet as I'm on holiday . The other point of note more for me was I was running it on 50-1 mix which I see is debatable on small engines. I can remember a zenoa 62 I had years ago did similar with a carb bend on it and people said it was fuel condensing on the bend in cold weather ? Pete

[Gary Manuel]

Hello Pete - sorry you've been having trouble keeping your DLE20 running. It does sound more like a mixture problem than a plumbing issue. Persevere and I'm sure you'll sort it.

Oil mixture is very much open to debate, but I'm sure that manufactures err on the side of caution. I'm using a 30:1 mix of good quality fully synthetic (Mobil 1 Racing 2t) in all my petrol engines now, mainly to avoid multiple fuel cans. Everything from DLE55's, DLE111, Zenoah Z62 and NGH 38GF run well on it. Excess oil doesn't appear to be causing a problem.

[Petrol Pete]

Hi Gary , I think it's a carb mixture problem, maybe caused by carb icing whatever but as these carbs use the idle jet to be part of the mixture in the upper rev range the idle mixture is relevant to the total mixture and anything that reduces the flow will end up with a weak mixture Pete

[Ron Gray]

Posted by Gary Manuel on 26/03/2018 22:46:01:

Posted by Martin McIntosh on 26/03/2018 19:20:04:

The two I built for others used header tanks. No problems reported. Plenty of room for one in a Cap especially with a smaller main tank than a glow would use.

Hi Martin.

Thanks for joining the discussion and bringing a (slightly) different solution into it.

After doing a bit of reading up about header tanks, I think I can see how this works - and I like it.

Excuse me rambling here - I'm just collecting my thoughts:

As I understand it, a header tank sits between the main tank and the carb. The outlet from the main tank clunk line goes to a fixed pipe as close as possible to the top of the header tank. The outlet from the header tank is taken from a fixed pipe close to the physical centre of the tank (or a clunk?) to the carb via a "T" connector going to a fuel dot. The "T" connector / dot is important because the process of filling the tank from the fuel dot pushes any air from the last flight out of the header tank via the pipe at the top, into the main tank and out of the vent pipe at the top of the main tank.

During each flight, any air sucked into the clunk finishes up in the header tank. It will not be able to get out of the header tank and into the carb because the pickup is in the centre of the tank, which should always be in fuel unless the air bubble at the top of the header tank reaches the pickup pipe.

The header tank is effectively acting like the in-line filter that I'm thinking of adding, in that it acts as a fuel reservoir. The main difference is that the in-line filter might let some air reach the carb depending on the models attitude, but the header tank will only release air once the air bubble fills the tank as far as the fuel pickup pipe in the centre.

Apologies for rambling, but doing so helps me get me head round it. The question is, have I understood it correctly?

Take this scenario, header tank half full of air due to previous aero antics, then go nose down. Picture the carb feed pipe in the header tank, wouldn’t it be in the air bubble?

[Denis Watkins]

The idea with a header is that it remains full, as per heli boys

[Ron Gray]

I’m probably being extremely dense here but how does the header remain full if the supply line clunk in the main tank waggles around in the air (clunk not submerged)? I can understand it with pressurised systems (glow) but Gary’s is a gasser.