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dsavarese0812(at)bellsout
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 Posted: Sat Dec 15, 2018 5:31 am Post subject: Fuel Imbalance (Re: Yak 52TW Fuel Starvation at near zero G |
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FWIW, the single vent system Yak 52's (pre '89) models have a fuel imbalance issue as well, especially with full tanks and with the airplane sitting on the ramp in the sun. The net of it is the vent system gets blocked with fuel. Particularly the vent line on the right tank because of the design of the vent system. As everyone knows, vent lines must be clear of fuel in order for fuel to feed. With a blocked vent line, no fuel can feed from that tank. That's why we roll the airplane up on one wing and fly straight to force the fuel out of the vent line.
BUT there is a solution...a second vent to the tank. This can also apply to the CJ. Install this vent check valve on the top of the tank and run the vent tube overboard. Most likely location is near the fill port so you'll have access to the inside of the tank to tighten the nut on the bottom of the vent check valve. The vent check valve is used in race cars and should the race car were to turn over, the vent check valve would prevent the fuel from leaking out of the tank. With the vent check valve installed, during flight the tank will always have a clear vent line.
Here is a link to where it can be purchased and also a photo of the vent check valve installed on the top of a fuel filler plate on a Yak 52 fuel bladder. One is required for each fuel tank.
https://fuelsafe.com/thv45/
From: Greg Wrobel <clouddog22(at)gmail.com>
To: yak-list(at)matronics.com
Sent: Saturday, December 15, 2018 7:32 AM
Subject: Re: Re: Yak 52TW Fuel Starvation at near zero G
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:
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
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Ttail
Joined: 24 Jun 2013
Posts: 120
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| Posted: Sun Dec 16, 2018 11:32 am Post subject: Re: Fuel Imbalance (Re: Yak 52TW Fuel Starvation at near ze |
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No image Dennis be interested to see the image of this vent installed.
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