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n13472(at)aol.com
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 Posted: Fri Dec 14, 2018 12:15 pm Post subject: Yak 52TW Fuel Starvation at near zero G |
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They are art of the fittings that connect the lines from each fuel tank
to the sump tank in the belly.
Tom Elliott
CJ-6A NX63727
777 Quartz Ave
PMB 7004
Sandy Valley NV.
89019
Cell 541-297-5497
N13472(at)AOL.COM
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wlannon(at)shaw.ca
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| Posted: Fri Dec 14, 2018 1:57 pm Post subject: Yak 52TW Fuel Starvation at near zero G |
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A few thoughts on fuel flow problems in the CJ;
Dennis, would I be correct in assuming that each flapper valve outlet of the
fuel junction would go to the corresponding side of the Yak 52 header tank?
If so that is functionally the same as the CJ where the flapper valves are
mounted in the header tank walls.
In the early CJ days there were numerous problems with flapper valves stuck
closed due to evaporating Chinese fuel effectively gluing them in place.
Probably could repeat with auto gas and long enough storage time. Have not
heard of any such problem here with avgas.
Also many problems of blocked (or partially blocked) fuel and vent lines due
dead bugs and debris. Many had sat for years after being discarded at or
near their 4000 hr. life limit.
Dented, kinked and otherwise damaged fuel and vent lines also contribute
immensely to unequal fuel flow.
On flapper valves; There are 3 of these in the CJ. One is mounted in the
line from the header tank to the fuel pump for the sole purpose
of preventing back flow of wobble pump fuel which would otherwise just
re-circulate through the wobble pump. It is subject to substantial opening
pressure from the wobble pump. Even Chinese fuel glue could not keep it
closed. An excellent design for the application.
The other two are identical but are used in an application that requires a
slightly different design which moves with fuel flow at minimal differential
pressure. These are designed to require a significantly higher differential
pressure to open (significantly here is a relative term, if one could
measure the pressures involved it would likely be in milligrams/sq. cm. ).
This is evident on inspection:- the area of valve opening surface is about
1/4 that of the surface holding it closed.
In theory, with perfect trim and absolutely no turbulence whatsoever this
valve would require one full and one empty tank to open.
So; I have modified the valves in mine by removing valve seat material to
reduce the surface contact area from 100% to a near line contact. (actually,
since this is a hand filing operation) about a 0.020' to 0.030" contact
band. This allows the normal closing pressure to also react on the opening
side of the flapper resulting in a much closer balance between opening and
closing forces. See attached photos.
I normally never see a fuel imbalance in excess of 2 to 4 lts. except when I
try to do good slow (or point) rolls. In this case the valve flappers are
are doing just that, flapping! After a few I see imbalance up to 15 lts.
or so which returns to normal in a few minutes of level flight.
The valves alone are not the whole problem, I can not stress how important
the condition of the fuel and vent lines are. Both damage, internal
contamination and leakage are critical. I have spent days during CJ
restorations with a hose, hot water, dish washer soap and shop air getting
crap out of the lines. If you are going to do this be sure both ends of all
lines are disconnected. You don't want to blow up the the mains or header
tank in the process.
I had a good example of what a minor discrepancy can do to this fuel system.
A few years back on about 1 hr. flight home checked fuel gauges and shocked
to see a 20 + lt. imbalance. Checking the vent system I found one connector
hose in the RH wing joint area had slipped off! (I had used all new hose
but no clamps thinking they were tight enough). Dumb! But this relatively
small vent pressure change made a huge difference to the entire fuel system.
These valves (In Western terms normally called a vent valve) MUST be mounted
with the flapper hinge at the top! This requirement is a problem for the
Chinese valve as mounted in the header tank since it is threaded into a
welded fitting on the tank wall with a sealing washer.
So; Three things have have to come together at precisely the same time and
place. The valve is at the top, the torque is correct and the sealing washer
is exactly the right thickness. This of course is mission impossible.
Enough for now.
Walt
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hkgibby(at)yahoo.com
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| Posted: Fri Dec 14, 2018 3:06 pm Post subject: Yak 52TW Fuel Starvation at near zero G |
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Walt -
I remember you posting this and your flapper modification years ago...thanks for the refresher for those unfamiliar. I know at least one CJ that has the flappers in the header tank removed and has had no issues. Many of us have bladders with a modified vent system which allows for more control of fuel balancing. I blow my out my vent lines (fuel tank caps off;) via the ram air belly vent every so often to ensure I get no fuel accumulation into the vent lines which can happen and cause loss/reduction of pressure. Don’t really have a way of verifying if any fuel is in the vent lines but it gives me peace of mind. Is removal and inspection of the header tank recommended beyond a full overhaul or say every xx years?
Hoot
Sent from my iPhone
[quote] On Dec 14, 2018, at 1:54 PM, Walter Lannon <wlannon(at)shaw.ca> wrote:
A few thoughts on fuel flow problems in the CJ;
Dennis, would I be correct in assuming that each flapper valve outlet of the
fuel junction would go to the corresponding side of the Yak 52 header tank?
If so that is functionally the same as the CJ where the flapper valves are
mounted in the header tank walls.
In the early CJ days there were numerous problems with flapper valves stuck
closed due to evaporating Chinese fuel effectively gluing them in place.
Probably could repeat with auto gas and long enough storage time. Have not
heard of any such problem here with avgas.
Also many problems of blocked (or partially blocked) fuel and vent lines due
dead bugs and debris. Many had sat for years after being discarded at or
near their 4000 hr. life limit.
Dented, kinked and otherwise damaged fuel and vent lines also contribute
immensely to unequal fuel flow.
On flapper valves; There are 3 of these in the CJ. One is mounted in the
line from the header tank to the fuel pump for the sole purpose
of preventing back flow of wobble pump fuel which would otherwise just
re-circulate through the wobble pump. It is subject to substantial opening
pressure from the wobble pump. Even Chinese fuel glue could not keep it
closed. An excellent design for the application.
The other two are identical but are used in an application that requires a
slightly different design which moves with fuel flow at minimal differential
pressure. These are designed to require a significantly higher differential pressure to open (significantly here is a relative term, if one could measure the pressures involved it would likely be in milligrams/sq. cm. ). This is evident on inspection:- the area of valve opening surface is about 1/4 that of the surface holding it closed.
In theory, with perfect trim and absolutely no turbulence whatsoever this valve would require one full and one empty tank to open.
So; I have modified the valves in mine by removing valve seat material to reduce the surface contact area from 100% to a near line contact. (actually, since this is a hand filing operation) about a 0.020' to 0.030" contact band. This allows the normal closing pressure to also react on the opening side of the flapper resulting in a much closer balance between opening and closing forces. See attached photos.
I normally never see a fuel imbalance in excess of 2 to 4 lts. except when I try to do good slow (or point) rolls. In this case the valve flappers are are doing just that, flapping! After a few I see imbalance up to 15 lts. or so which returns to normal in a few minutes of level flight.
The valves alone are not the whole problem, I can not stress how important the condition of the fuel and vent lines are. Both damage, internal contamination and leakage are critical. I have spent days during CJ restorations with a hose, hot water, dish washer soap and shop air getting crap out of the lines. If you are going to do this be sure both ends of all lines are disconnected. You don't want to blow up the the mains or header tank in the process.
I had a good example of what a minor discrepancy can do to this fuel system. A few years back on about 1 hr. flight home checked fuel gauges and shocked to see a 20 + lt. imbalance. Checking the vent system I found one connector hose in the RH wing joint area had slipped off! (I had used all new hose but no clamps thinking they were tight enough). Dumb! But this relatively small vent pressure change made a huge difference to the entire fuel system.
These valves (In Western terms normally called a vent valve) MUST be mounted with the flapper hinge at the top! This requirement is a problem for the Chinese valve as mounted in the header tank since it is threaded into a welded fitting on the tank wall with a sealing washer.
So; Three things have have to come together at precisely the same time and place. The valve is at the top, the torque is correct and the sealing washer is exactly the right thickness. This of course is mission impossible.
Enough for now.
Walt
--
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Flibob
Joined: 25 Jun 2014
Posts: 28
Location: Indiana
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| Posted: Fri Dec 14, 2018 4:08 pm Post subject: Yak 52TW Fuel Starvation at near zero G |
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Thanks Walt, great treatise on flapper valves. Now to find time to check mine.
GiddyUp,
Bob
On Dec 14, 2018, at 4:54 PM, Walter Lannon <wlannon(at)shaw.ca> wrote:
A few thoughts on fuel flow problems in the CJ;
Dennis, would I be correct in assuming that each flapper valve outlet of the
fuel junction would go to the corresponding side of the Yak 52 header tank?
If so that is functionally the same as the CJ where the flapper valves are
mounted in the header tank walls.
In the early CJ days there were numerous problems with flapper valves stuck
closed due to evaporating Chinese fuel effectively gluing them in place.
Probably could repeat with auto gas and long enough storage time. Have not
heard of any such problem here with avgas.
Also many problems of blocked (or partially blocked) fuel and vent lines due
dead bugs and debris. Many had sat for years after being discarded at or
near their 4000 hr. life limit.
Dented, kinked and otherwise damaged fuel and vent lines also contribute
immensely to unequal fuel flow.
On flapper valves; There are 3 of these in the CJ. One is mounted in the
line from the header tank to the fuel pump for the sole purpose
of preventing back flow of wobble pump fuel which would otherwise just
re-circulate through the wobble pump. It is subject to substantial opening
pressure from the wobble pump. Even Chinese fuel glue could not keep it
closed. An excellent design for the application.
The other two are identical but are used in an application that requires a
slightly different design which moves with fuel flow at minimal differential
pressure. These are designed to require a significantly higher differential pressure to open (significantly here is a relative term, if one could measure the pressures involved it would likely be in milligrams/sq. cm. ). This is evident on inspection:- the area of valve opening surface is about 1/4 that of the surface holding it closed.
In theory, with perfect trim and absolutely no turbulence whatsoever this valve would require one full and one empty tank to open.
So; I have modified the valves in mine by removing valve seat material to reduce the surface contact area from 100% to a near line contact. (actually, since this is a hand filing operation) about a 0.020' to 0.030" contact band. This allows the normal closing pressure to also react on the opening side of the flapper resulting in a much closer balance between opening and closing forces. See attached photos.
I normally never see a fuel imbalance in excess of 2 to 4 lts. except when I try to do good slow (or point) rolls. In this case the valve flappers are are doing just that, flapping! After a few I see imbalance up to 15 lts. or so which returns to normal in a few minutes of level flight.
The valves alone are not the whole problem, I can not stress how important the condition of the fuel and vent lines are. Both damage, internal contamination and leakage are critical. I have spent days during CJ restorations with a hose, hot water, dish washer soap and shop air getting crap out of the lines. If you are going to do this be sure both ends of all lines are disconnected. You don't want to blow up the the mains or header tank in the process.
I had a good example of what a minor discrepancy can do to this fuel system. A few years back on about 1 hr. flight home checked fuel gauges and shocked to see a 20 + lt. imbalance. Checking the vent system I found one connector hose in the RH wing joint area had slipped off! (I had used all new hose but no clamps thinking they were tight enough). Dumb! But this relatively small vent pressure change made a huge difference to the entire fuel system.
These valves (In Western terms normally called a vent valve) MUST be mounted with the flapper hinge at the top! This requirement is a problem for the Chinese valve as mounted in the header tank since it is threaded into a welded fitting on the tank wall with a sealing washer.
So; Three things have have to come together at precisely the same time and place. The valve is at the top, the torque is correct and the sealing washer is exactly the right thickness. This of course is mission impossible.
Enough for now.
Walt
--
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dsavarese0812(at)bellsout
Guest
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| Posted: Fri Dec 14, 2018 5:08 pm Post subject: Yak 52TW Fuel Starvation at near zero G |
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Hi Walt,
Yes you are correct on the flapper valves in the 52. Based on what you said about the CJ, Jon Blake may have a flapper valve hanging up.
Dennis
Sent from my iPad
[quote] On Dec 14, 2018, at 4:54 PM, Walter Lannon <wlannon(at)shaw.ca> wrote:
A few thoughts on fuel flow problems in the CJ;
Dennis, would I be correct in assuming that each flapper valve outlet of the
fuel junction would go to the corresponding side of the Yak 52 header tank?
If so that is functionally the same as the CJ where the flapper valves are
mounted in the header tank walls.
In the early CJ days there were numerous problems with flapper valves stuck
closed due to evaporating Chinese fuel effectively gluing them in place.
Probably could repeat with auto gas and long enough storage time. Have not
heard of any such problem here with avgas.
Also many problems of blocked (or partially blocked) fuel and vent lines due
dead bugs and debris. Many had sat for years after being discarded at or
near their 4000 hr. life limit.
Dented, kinked and otherwise damaged fuel and vent lines also contribute
immensely to unequal fuel flow.
On flapper valves; There are 3 of these in the CJ. One is mounted in the
line from the header tank to the fuel pump for the sole purpose
of preventing back flow of wobble pump fuel which would otherwise just
re-circulate through the wobble pump. It is subject to substantial opening
pressure from the wobble pump. Even Chinese fuel glue could not keep it
closed. An excellent design for the application.
The other two are identical but are used in an application that requires a
slightly different design which moves with fuel flow at minimal differential
pressure. These are designed to require a significantly higher differential pressure to open (significantly here is a relative term, if one could measure the pressures involved it would likely be in milligrams/sq. cm. ). This is evident on inspection:- the area of valve opening surface is about 1/4 that of the surface holding it closed.
In theory, with perfect trim and absolutely no turbulence whatsoever this valve would require one full and one empty tank to open.
So; I have modified the valves in mine by removing valve seat material to reduce the surface contact area from 100% to a near line contact. (actually, since this is a hand filing operation) about a 0.020' to 0.030" contact band. This allows the normal closing pressure to also react on the opening side of the flapper resulting in a much closer balance between opening and closing forces. See attached photos.
I normally never see a fuel imbalance in excess of 2 to 4 lts. except when I try to do good slow (or point) rolls. In this case the valve flappers are are doing just that, flapping! After a few I see imbalance up to 15 lts. or so which returns to normal in a few minutes of level flight.
The valves alone are not the whole problem, I can not stress how important the condition of the fuel and vent lines are. Both damage, internal contamination and leakage are critical. I have spent days during CJ restorations with a hose, hot water, dish washer soap and shop air getting crap out of the lines. If you are going to do this be sure both ends of all lines are disconnected. You don't want to blow up the the mains or header tank in the process.
I had a good example of what a minor discrepancy can do to this fuel system. A few years back on about 1 hr. flight home checked fuel gauges and shocked to see a 20 + lt. imbalance. Checking the vent system I found one connector hose in the RH wing joint area had slipped off! (I had used all new hose but no clamps thinking they were tight enough). Dumb! But this relatively small vent pressure change made a huge difference to the entire fuel system.
These valves (In Western terms normally called a vent valve) MUST be mounted with the flapper hinge at the top! This requirement is a problem for the Chinese valve as mounted in the header tank since it is threaded into a welded fitting on the tank wall with a sealing washer.
So; Three things have have to come together at precisely the same time and place. The valve is at the top, the torque is correct and the sealing washer is exactly the right thickness. This of course is mission impossible.
Enough for now.
Walt
--
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Ttail
Joined: 24 Jun 2013
Posts: 122
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| Posted: Fri Dec 14, 2018 7:54 pm Post subject: Re: Yak 52TW Fuel Starvation at near zero G |
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Awesome info thanks Walt
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Clouddog
Joined: 02 Jun 2016
Posts: 122
Location: Lebanon, TN
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| Posted: Sat Dec 15, 2018 4:29 am Post subject: Yak 52TW Fuel Starvation at near zero G |
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Regarding the CJ6. I was one of the fortunate to go to China with members
of the Southeast region a few months ago. The highlight of our 2 week
journey was visiting the factory that is currently building the latest CJ6A
aircraft (by the way, no significant changes were noted). I will not go
into the details of the visit or our journey but during our visit to the
factory, we were able to sit down with the factory manager , CJ6A
engineers, and production staff. When you ask a question , you do not want
to ask in a way that they preceive that you are accusing them of a design
flaw so our wording was carefully chosen. One of our questions related to
fuel imbalance so we carefully asked if they were aware of the fuel
imbalance that occures during flight or while sitting on a sloped ramp. To
our amazement, they were familiar. To our further amazement, nothing has
been done to correct the issue! They know it exist but there has been no
changes in design to correct it.
Please realize, our discussions were through interpreters as the only
Chinese we knew were Píjiǔ (beer) and Báijiǔ (clear types of alchohol)
pronounced pee-joe and bye-joe respectively. Ok then, you have now been
enlightened to some important Chinese words of the day. In any case, we
are not sure if they truly understood our question or we truly understood
their answer but there you have it.
Now what does this have to do with engine hesitation at the top of a loop
in a YAK52.....absolutely nothing! Just seems we were beating flapper
valves to death in a CJ and went astray. We have installed elaborate
venting systems in our CJ's to fix fuel imbalance and have all these
questions and theories. Maybe the Chinese are telling the truth on this
one. The CJ has a fuel imbalance issue created by flappers/ check valves
or gremlins and apparently it is not going away. From my personal
expierence, I keep the fuel 3/4" - 1" below the rear lip of the fill port
as mentioned in the Chinese Flight Manual. This equates to 1 gallon less in
the tanks versus topping it off. Also attempting to fly wings level and in
trim (thats a full time, max concentration effort for me by the way) and
most of the time, my fuel burns symmetrical but not always. Apparently I'm
not concentrating hard enough. Darn flapper valves or are they flipper
valves? Who knows!
On Fri, Dec 14, 2018, 16:07 Walter Lannon <wlannon(at)shaw.ca wrote:
[quote] A few thoughts on fuel flow problems in the CJ;
Dennis, would I be correct in assuming that each flapper valve outlet of
the
fuel junction would go to the corresponding side of the Yak 52 header tank?
If so that is functionally the same as the CJ where the flapper valves are
mounted in the header tank walls.
In the early CJ days there were numerous problems with flapper valves stuck
closed due to evaporating Chinese fuel effectively gluing them in place.
Probably could repeat with auto gas and long enough storage time. Have not
heard of any such problem here with avgas.
Also many problems of blocked (or partially blocked) fuel and vent lines
due
dead bugs and debris. Many had sat for years after being discarded at or
near their 4000 hr. life limit.
Dented, kinked and otherwise damaged fuel and vent lines also contribute
immensely to unequal fuel flow.
On flapper valves; There are 3 of these in the CJ. One is mounted in the
line from the header tank to the fuel pump for the sole purpose
of preventing back flow of wobble pump fuel which would otherwise just
re-circulate through the wobble pump. It is subject to substantial opening
pressure from the wobble pump. Even Chinese fuel glue could not keep it
closed. An excellent design for the application.
The other two are identical but are used in an application that requires a
slightly different design which moves with fuel flow at minimal
differential
pressure. These are designed to require a significantly higher
differential
pressure to open (significantly here is a relative term, if one could
measure the pressures involved it would likely be in milligrams/sq. cm. )
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ggtyler
Joined: 05 Mar 2016
Posts: 29
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| Posted: Mon Dec 17, 2018 2:06 pm Post subject: Re: Yak 52TW Fuel Starvation at near zero G |
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Thank you for all your input. I'll look into all the possible solutions posted here and flog the airplane near the airport on the next few aerobatic sessions.
If I come up with anything definitive, I'll update this thread.
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