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saber369(at)comcast.net
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 Posted: Mon Dec 07, 2020 12:28 pm Post subject: CJ6 uneven fuel flow |
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Good info Walt... many thanks.
When I first bought my CJ 15 years ago, I was told to wrap the vent on the belly with a fine mesh screen to keep the bugs out. I was also told that the vents going out to each wing tips were installed by Yakity Yaks to help prevent a bug or other debris from causing fuel starvation. Is that true? Can you shed any more light on those vents?
Jon Blake
Saber369(at)comcast.net
| Quote: | On 12/07/2020 12:22 PM Walter Lannon <wlannon(at)shaw.ca> wrote:
From: Walter Lannon (wlannon(at)shaw.ca)
Sent: Sunday, December 06, 2020 9:09 PM
To: yaklist(at)matronics.com (yaklist(at)matronics.com)
Subject: CJ6 uneven fuel flow
Currently there is a lot of interest in the above subject on the Red Star site. Since I have some experience in this area I thought I should respond but since it will be a rather long diatribe I thought the Yak List might be a better venue.
My aircraft is a 1966 model CJ6a, one of four I imported from China in 1993. Did not start my own restoration until after selling my Harvard 4 in 2006 (after 30 years!) It was a 6 year (off and on) 100% restoration including the fuel system where I found some questionable design features. During this period (from 1993) I also licensed a few others and found it was necessary to do a hot soap and water flush of the fuel vent system to remove bug debris.
Many people have noted the fuel feeds from the LH tank first. That is not surprising since the LH vent system is much shorter than the RH and the tube is 8 mm (6 mm ID) for both. Not a huge problem as long as the
tubing is internally clean and totally free of any damage that could restrict air flow. This is critical! There is also some minor effect favoring LH flow via the shorter fuel feed line to the header tank.
The major design problem rests with the header tank flapper valves. There are three of these valves in the aircraft. One is in the fuel line from the header tank to the fuel pump. It’s sole purpose is to direct fuel
from the emerg. (wobble) pump to the engine fuel pump while preventing reverse flow back to the header tank. This is the type of function the valve is designed for and it works perfectly because there is a
distinct difference in pressure on each side.
With reference to the attached photos note the diameter of the flapper as compared to the diameter of the incoming fuel hole. A true “flapper” valve is meant to open (or close) with fuel flow but pressure, though relatively small, is a factor. If we start with equal fuel one flapper will open (probably the LH). In theory as the pressure from the LH tank and the header tank is reduced at some point the RH will open and the cycle will repeat.
But, in fact, the LH plus the header tank fuel is actually applying a force to the valve surface roughly double that of the smaller surface impacted by the RH fuel.
I did some rough math based on tank volume and pressures, etc. and came to the following conclusion: - The major reason this system sort of works is called turbulence! If you could take off, climb and fly a trip with never a bump of any kind you will be out of gas with a full RH tank.
As you probably noted I have modified the valves to allow closing pressure to impact some of the opening surface. My typical fuel imbalance is less than 5 lts. For me this is a hand filing operation (though a CNC genius could probably do it). You MUST retain a seating ring of the original closing surface. If you change that it will never close properly again due to the geometry.
One of the un modified valve photos shows a Chinese character on a wrench flat. In English this reads “HINGE” and must be at the top. Improper positioning could well be the cause of serious imbalance.
Cheers;
Walt
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threein60
Joined: 04 Feb 2016
Posts: 51
Location: Everett Wa
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| Posted: Mon Dec 07, 2020 1:30 pm Post subject: CJ6 uneven fuel flow |
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Great write-up on this. Let me add a little learned knowledge to this discussion. I have no issues with what Walt stated but a few years ago I had fuel starvation while on the ground. I could start the engine on my electrical boost pump but the minute I shut it off, the engine would die. I checked fuel levels, I removed and cleaned the fuel strainer and checked for flow, still nothing. Only when I removed the flapper valve under the floor, the one that feeds the wobble pump, I found the hinge in this flapper was worn and could rotate to a place where it would get stuck closed. This was the first I have ever hear of this happening. I cleaned and lightly refaced both the flapper and flapper seat. I then replaced the worn hinge pivot rod with a new brass one. I have plans to take apart this valve again during this months annual and see if these is any wear taking place on the hinge rod. Lucky I found this on the ground but confident had this happened in flight, my electric driven pump would have supplied enough fuel to get me back safely. Still some thing that should be examined.
I have bladder tanks so my vents are extended to their respective wing tips with a separate vent for the header tank. I find that I tend to fly more pressure on my left rudder because my knee board is on my right and lifting my right knee a little makes writing easier. This tends to cause a noticeable fuel difference over a 2 hour period. I know, bad pilot bad. Very early on before I had my bladder tanks installed, I had to do an emergency divert because my one tank was almost dry and the opposite tank wasn't moving the needle yet. Only after landing I noticed the needle move. Constant fuel management has become the norm. I optioned to not install the vent shut off values after and incident I had with another RPAer. This pilot closed one of his vents to overcome the fuel flow difference and forgot to open the vent again. On roll out with the engine pulled to ideal, the engine died on the runway. It would start but than die again. When we got it to parking, we noticed one bladder tank was sucked flat and dry..... The same side with the closed vent. Human factors are human factors! Unless you design a system to incorporate some warning when the pressure builds or the vent is closed indication, this only complicates memory items that needs to be attended and eventually can lead to failures.
Larry Pine
On Monday, December 7, 2020, 10:48:19 AM PST, Walter Lannon <wlannon(at)shaw.ca> wrote:
From: Walter Lannon (wlannon(at)shaw.ca)
Sent: Sunday, December 06, 2020 9:09 PM
To: yaklist(at)matronics.com (yaklist(at)matronics.com)
Subject: CJ6 uneven fuel flow
Currently there is a lot of interest in the above subject on the Red Star site. Since I have some experience in this area I thought I should respond but since it will be a rather long diatribe I thought the Yak List might be a better venue.
My aircraft is a 1966 model CJ6a, one of four I imported from China in 1993. Did not start my own restoration until after selling my Harvard 4 in 2006 (after 30 years!) It was a 6 year (off and on) 100% restoration including the fuel system where I found some questionable design features. During this period (from 1993) I also licensed a few others and found it was necessary to do a hot soap and water flush of the fuel vent system to remove bug debris.
Many people have noted the fuel feeds from the LH tank first. That is not surprising since the LH vent system is much shorter than the RH and the tube is 8 mm (6 mm ID) for both. Not a huge problem as long as the
tubing is internally clean and totally free of any damage that could restrict air flow. This is critical! There is also some minor effect favoring LH flow via the shorter fuel feed line to the header tank.
The major design problem rests with the header tank flapper valves. There are three of these valves in the aircraft. One is in the fuel line from the header tank to the fuel pump. It’s sole purpose is to direct fuel
from the emerg. (wobble) pump to the engine fuel pump while preventing reverse flow back to the header tank. This is the type of function the valve is designed for and it works perfectly because there is a
distinct difference in pressure on each side.
With reference to the attached photos note the diameter of the flapper as compared to the diameter of the incoming fuel hole. A true “flapper” valve is meant to open (or close) with fuel flow but pressure, though relatively small, is a factor. If we start with equal fuel one flapper will open (probably the LH). In theory as the pressure from the LH tank and the header tank is reduced at some point the RH will open and the cycle will repeat.
But, in fact, the LH plus the header tank fuel is actually applying a force to the valve surface roughly double that of the smaller surface impacted by the RH fuel.
I did some rough math based on tank volume and pressures, etc. and came to the following conclusion: - The major reason this system sort of works is called turbulence! If you could take off, climb and fly a trip with never a bump of any kind you will be out of gas with a full RH tank.
As you probably noted I have modified the valves to allow closing pressure to impact some of the opening surface. My typical fuel imbalance is less than 5 lts. For me this is a hand filing operation (though a CNC genius could probably do it). You MUST retain a seating ring of the original closing surface. If you change that it will never close properly again due to the geometry.
One of the un modified valve photos shows a Chinese character on a wrench flat. In English this reads “HINGE” and must be at the top. Improper positioning could well be the cause of serious imbalance.
Cheers;
Walt
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AlaskaChang801
Joined: 26 Jan 2018
Posts: 36
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| Posted: Wed Dec 16, 2020 4:45 pm Post subject: Re: CJ6 uneven fuel flow |
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Good to see this article generating some interest with us fairly new CJ owners. I know for one, I'm running strictly on aging information from the longtime owners and while it's great that the knowledge is out there, I'm worried as the seemingly older guys are dumping their CJ's and we as a group are losing knowledge base. So with that, it seems the original post had photos, but they've long gone. So is there a way to post them again??
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