Today's Message Index:
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1. 10:24 AM - Fw: CJ6 uneven fuel flow (Walter Lannon)
2. 12:28 PM - Re: Fw: CJ6 uneven fuel flow (JON BLAKE)
3. 01:30 PM - Re: Fw: CJ6 uneven fuel flow (Larry Pine)
4. 09:06 PM - Re: Fw: CJ6 uneven fuel flow (Walter Lannon)
Message 1
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| Subject: | Fw: CJ6 uneven fuel flow |
From: Walter Lannon
Sent: Sunday, December 06, 2020 9:09 PM
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=99s 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
=9Cflapper=9D 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 =9CHINGE=9D and must be
at the top. Improper positioning could well be the cause of serious
imbalance.
Cheers;
Walt
Message 2
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| Subject: | Re: Fw: CJ6 uneven fuel flow |
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 t
he 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@comcast.net
> On 12/07/2020 12:22 PM Walter Lannon <wlannon@shaw.ca> wrote:
>
>
>
>
> From: Walter Lannon mailto:wlannon@shaw.ca
> Sent: Sunday, December 06, 2020 9:09 PM
> To: yaklist@matronics.com mailto:yaklist@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 re
spond but since it will be a rather long diatribe I thought the Yak List mi
ght be a better venue.
>
> My aircraft is a 1966 model CJ6a, one of four I imported from China i
n 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 in
cluding the fuel system where I found some questionable design features. D
uring 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 remo
ve bug debris.
>
> Many people have noted the fuel feeds from the LH tank first. That i
s 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 favor
ing 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 f
rom the header tank to the fuel pump. It=99s sole purpose is to dire
ct fuel
> from the emerg. (wobble) pump to the engine fuel pump while preventin
g reverse flow back to the header tank. This is the type of function the v
alve 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 flappe
r as compared to the diameter of the incoming fuel hole. A true =9Cf
lapper=9D valve is meant to open (or close) with fuel flow but pressu
re, 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 L
H tank and the header tank is reduced at some point the RH will open and th
e 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 impa
cted by the RH fuel.
>
> I did some rough math based on tank volume and pressures, etc. and ca
me to the following conclusion: - The major reason this system sort of work
s is called turbulence! If you could take off, climb and fly a trip with n
ever 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 pre
ssure 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 geni
us 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 wr
ench flat. In English this reads =9CHINGE=9D and must be at th
e top. Improper positioning could well be the cause of serious imbalance.
>
>
> Cheers;
> Walt
>
>
>
>
>
>
Message 3
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| Subject: | Re: Fw: CJ6 uneven fuel flow |
Great write-up on this. Let me add a little learned knowledge to this discu
ssion. I have no issues with what Walt stated but a few years ago I had fue
l 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 f
uel levels, I removed and cleaned the fuel strainer and checked for flow, s
till nothing. Only when I removed the flapper valve under the floor, the on
e that feeds the wobble pump, I found the hinge in this flapper was worn an
d could rotate to a place where it would get stuck closed. This was the fir
st I have ever hear of this happening. I cleaned and lightly refaced both t
he 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 mo
nths annual and see if these is any wear taking place on the hinge rod. Luc
ky 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 p
ressure 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 noticeab
le 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 ma
nagement has become the norm. I optioned to not install the vent shut off v
alues 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 n
oticed 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 t
o incorporate some warning when the pressure builds or the vent is closed i
ndication, 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@shaw.ca> wrote:
=C2-=C2-From: Walter Lannon Sent: Sunday, December 06, 2020 9:09 PMTo:
yaklist@matronics.com Subject: CJ6 uneven fuel flow=C2-Currently there i
s a lot of interest in the above subject on the Red Star site.=C2- 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 ve
nue.=C2-My aircraft is a 1966 model CJ6a, one of four I imported from Chi
na in 1993.=C2- Did not start my own restoration until after selling my H
arvard 4 in 2006 (after 30 years!)=C2- It was a 6 year (off and on) 100%
restoration including the fuel system where I found some questionable desig
n features.=C2- During this period (from 1993) I also licensed a few othe
rs and found it was necessary to do a hot soap and water flush of the fuel
vent system to remove bug debris.=C2-Many people have noted the fuel feed
s from the LH tank first.=C2- That is not surprising since the LH=C2- v
ent system is much shorter than the RH and the tube is 8 mm (6 mm ID) for b
oth.=C2- 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!
=C2- There is also some minor effect favoring LH flow via the shorter fue
l feed line to the header tank.=C2-=C2-=C2- =C2-The major design pr
oblem rests with the header tank flapper valves.=C2- There are three of t
hese valves in the aircraft.=C2- One is in the fuel line from the header
tank to the fuel pump.=C2- It=99s sole purpose is to direct fuelfro
m the emerg. (wobble) pump to the engine fuel pump while preventing reverse
flow back to the header tank.=C2- This is the type of function the valve
is designed for and it works perfectly because there is a distinct differe
nce in pressure on each side.=C2- =C2-With reference to the attached ph
otos note the diameter of the flapper as compared to the diameter of the in
coming fuel hole.=C2- A true =9Cflapper=9D valve is meant to
open (or close) with fuel flow but pressure, though relatively small, is a
factor.=C2-=C2- If we start with equal fuel one flapper will open (prob
ably the LH).=C2- In theory as the pressure from the LH tank and the head
er 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 forc
e to the valve surface roughly double that of the smaller surface impacted
by the RH fuel.=C2-I did some rough math based on tank volume and pressur
es, etc. and came to the following conclusion: - The major reason this syst
em sort of works is called turbulence!=C2- If you could take off, climb a
nd fly a trip with never a bump of any kind you will be out of gas with a f
ull RH tank. =C2-As you probably noted I have modified the valves to allo
w closing pressure to impact some of the opening surface.=C2- My typical
fuel imbalance is less than 5 lts.=C2- For me this is a hand filing opera
tion (though a CNC genius could probably do it).=C2-=C2- You MUST retai
n a seating ring of the original closing surface.=C2- If you change that
it will never close properly again due to the geometry. =C2-One of the un
modified valve photos shows a Chinese character on a wrench flat.=C2- In
English this reads =9CHINGE=9D and must be at the top.=C2- I
mproper positioning could well be the cause of serious imbalance.=C2-=C2
-Cheers;Walt=C2-=C2-=C2-=C2-=C2-
Message 4
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| Subject: | Re: Fw: CJ6 uneven fuel flow |
Hi Jon;
Thank you!
I have never seen the wing tip vent installation but assume that was
it=99s purpose. Should work as long as it terminates in a
positive (or neutral) pressure area.
I seem to remember from my misspent youth that Cessna produced a lightly
spring loaded vent valve that was installed in the fuel cap. I have
been intending to
track that down for years!!!
We really should have an emergency vent of some type in the CJ. ONE is
not enough! In the T6/Harvard (and most aircraft) each tank is
separately vented.
Over the years I have seen three T6 fuel tanks where you could touch the
tank bottom from the filler hole due to a plugged vent.
Cheers;
Walt
From: JON BLAKE
Sent: Monday, December 07, 2020 12:26 PM
Subject: Re: Yak-List: Fw: CJ6 uneven fuel flow
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@comcast.net
On 12/07/2020 12:22 PM Walter Lannon <wlannon@shaw.ca> wrote:
From: Walter Lannon
Sent: Sunday, December 06, 2020 9:09 PM
To: yaklist@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=99s 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
=9Cflapper=9D 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 =9CHINGE=9D and must be
at the top. Improper positioning could well be the cause of serious
imbalance.
Cheers;
Walt
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