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Fuel Flow, continued.
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Float Flyr

Joined: 19 Jul 2006
Posts: 2704
Location: Campbellton, Newfoundland

Posted: Sun Jan 28, 2007 5:54 am Post subject: Fuel Flow, continued.

Torgeir:



First I understand that you are closing both wing shutoffs as an experiment.

In flight only one valve at a time would ever be closed with the possible

exception of an emergency.



One of the things you seem to have missed is that one tank will have some

pitot pressure on it from the forward facing cap.  The other will have a

partial vacuum from the lift on the wing.  I think you are saying it depends

on which tank has the lost cap as to how the loss of fuel and distribution

of fuel will be accomplished.  In that case I agree.  However that

difference may be minimal.



You suggest a vent to both wings from the header tank.  Why not cut on the

amount of hose being used and just put a cross vent between the two tanks?

And no vent at all from the header.  Pressure difference between the two

tanks (one will always be slightly higher than the other) will cause the

header to purge air through one or the other fuel lines.  Another point is

when filling the wing tanks only one side will be filled at a time allowing

the header tank to fill and purge air thorough the other fuel line.  A cross

vent will allow both wing tanks to feed fuel at any time as well as keep the

air pressure on top of the fuel equal at all times.



The reason I suggest this is there are certified aircraft flying today with

just that set up.



I like manual cameras, manual transmissions and fuel shut offs.  I prefer to

have the choice to control fuel flow during flight.   That requires some

sort of individual shut offs to be installed on the wing tanks.



Others may not consider the that control of any value and prefer to fly

without the shut offs.  They may be right.  They are right for themselves.

I doubt they are for me.



Noel

Soon to install shut offs.

[quote] --

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_________________
Noel Loveys
Kitfox III-A
Aerocet 1100 Floats

pwmac(at)sisna.com
Guest

Posted: Sun Jan 28, 2007 8:07 am Post subject: Fuel Flow, continued.

Kurt,

 I hesitate to insert any further comments on this discussion, BUT--

We were never able to add check valves in our aerospace fluid (fuel) systems without a huge battle from the wise and more experienced engineers. The argument always centered on the fact that none of the various styles of check valves were reliable. It take a significant delta p to gaurentee operation with out leakage. Leakage is a matter of degree but why have one if it does no perform. The last nail in the coffen of adding these devices is that the  added fittings and additional leak paths are just not worth the reduction in system reliability.



Now here we have a Kitfox and fuel flow problems and reliability are paramount to keeping the plane in the air. The stock design of the Kitfox fuel system as 2 wing tanks and a header behind the seat with a shutoff valve & filter downstream of the header tank was well thought out by some very experienced and smart engineers. Tread lightly before assuming that one can change the design without lots of engineering and experience help.



So what is one to do after hearing about these "dark stormy night" stories? First I can give some recommendations:

1) Make sure you're fuel system is really operating as designed. This means fresh tank cap seals that are tested to really seal. Make sure your check list says to verify caps are installed and pointing the correct direction. 

2) Has the fuel hose routing changed since originally installed? No low spots or kinks. 

3) Has the fuel filter been changed recently and how often should you change it? 

4) Have you done recently cleaned your finger strainers? 

All these items affect fuel flow and cross flow.



What did I do about the fuel system design? 1) I modified my fuel tank inlet to use the standard spam can flush filler caps and put the tank vent below the wing. Why? 1) because I wanted an o-ring seal for the tank cap and a flush apearance. I have always found that an o-ring will reliably make a seal whereas a flat gasket is somewhat uncertain. Just look and any modern auto and see the o-ring seal for the gas cap. 2) I made a judgement that the higher pressure below the wing would give more margin for fuel flow than the factory design.

   I have discussed my design with several engineers including the two guys responsible for the factory design and it all agree my design was better and much more costly. In fact at the time of Skystar demise the fuel system redesign would have included these features along with the items which are not mentioned here. I also have a 3/8" tube/hose system for more margin in pressure drop and have individual wing tank shutoff valves for ground maint. The 3/8" lines are good and recommended by the engineers I consulted with, but the valves have pros and cons. The larger valve size is necessary for reduce pressure drop but the extra leak paths is worrysome. 



As far as changing the vent-to tank line I still have the vent header to one tank. Cross feed is a function of the pressure and elevation difference between the two wing tanks. Correct these two things and you will NOT have cross feed. Cross feed problems can be fuel restrictions, cap seal, or vent restriction. Those items must be addressed. Elevation differences most likely are caused by pilot in flight or the parking area on the ground.

 In conclusion: Please do not add check valves. Be sure the fuel system is per the original design before flying. Mods to the factory design are not necessary.



Regards, Paul

=========================================


I have been looking at the check valves myself.  I

think they need .5 psi to open, so they should be

mounted at the header to ensure enough tank pressure

for flow.

Another consideration is a shutoff only on the left

tank.  That prevents cross flow, no chance of having

both tanks closed, and the shutoff could be mounted

where you see it when closed, but not open, just to

remind you.

With the right tank full, it can still feed thru the

vent line, if the main line is shut off.  After a bit

this no longer applies, but the left tank is

completely controllable thru the only line.

Right now I use a hose clamp on one line or the other

only, and in cruise only.  I am listening to all of

you for better ideas.

Kurt S.  S-5

--- Marco Menezes <msm_9949(at)yahoo.com> wrote:

 	  Quote: 	
		   At the risk of adding fuel to the embers of this

 string (sorry), how about Andair check valves

 instead of shut-offs at each wing tank? Wouldn't

 that solve the problem of cross-fed, total fuel

 depletion from one missing or loose cap, where

 single tank isolation is not wanted or needed?

 
	


 

_________________________________

SISNA...more service, less money.

http://www.sisna.com/exclusive/

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torgemor(at)online.no
Guest

Posted: Sun Jan 28, 2007 1:26 pm Post subject: Fuel Flow, continued.

Hi Noel,



I'll put the answers in between lines below.

On Sun, 28 Jan 2007 14:53:39 +0100, Noel Loveys <noelloveys(at)yahoo.ca>  

wrote:



 	  Quote: 	
		   



 Torgeir:



 First I understand that you are closing both wing shutoffs as an  

 experiment.


	


Yes, the meaning was to isolate the system and show that the header tank  

is indeed filled by gravity only. This is true as the header is vented to  

the same pressure as the right main tank,  since there is no pressure  

difference between them, the header will be filled by the gravity only  

-when you open the right wing tank valve.

Sure this is an experiment that's only to be carried out on ground. But  

you know we can simulate exactly what’s going on with the aircraft on  

ground, yes no risk here.

Yes, you can add the pressure difference, run the engine and see for  

yourself -this is no magic pure physics.



 	  Quote: 	
		   In flight only one valve at a time would ever be closed with the possible

 exception of an emergency.


	


Of course you're right here, this is how my system is today -only  

difference I have a large header tank, almost a "panel tank". I'll always  

open the closed one, then I shut the other.



With a true "cross vented" system those valves is really not needed, but  

for maintenance -and fuel management, they are a must.

 	  Quote: 	
		  

 One of the things you seem to have missed is that one tank will have some

 pitot pressure on it from the forward facing cap.  The other will have a

 partial vacuum from the lift on the wing.  I think you are saying it  

 depends

 on which tank has the lost cap as to how the loss of fuel and  

 distribution

 of fuel will be accomplished.  In that case I agree.  However that

 difference may be minimal.


	


No I haven’t missed the head pressure, but you're right this pressure is  

very low(in absolute value), compared to the low pressure over the wing.  

One other thing, I've avoided to use the word vacuum, thou everyone of us  

understand this. In the physics vacuum is zero pressure, -as out in the  

space. The pressure we're living in is approx. 14.7 lbs. or ~1 BAR. So  

absolute pressure is the one with reference to zero pressure (the space).



Even an aircraft without the head (pitot) pressure, would experience the  

same thing.



The flow either way is about the same in numbers if the right or the left  

tank cap has the same amount of leakage, but the BIG difference is that  

the header is not filled by the gravity if we have a "certain" leakage at  

the LEFT fuel cap. Here you will have a full left fuel tank, but the fuel  

cant flow to the header due to the air flow upward in the fuel line all  

the time.



If you look at the opposite leakage, leakage at the right cap, this  

airflow will go through the vent line and the fuel will flow in the fuel  

line as proposed to be.



Yes, it is here those engineers have missed!



So crossed vented tanks tied down to the header is the cure, or, -the fix.



 	  Quote: 	
		  

 You suggest a vent to both wings from the header tank.  Why not cut on  

 the

 amount of hose being used and just put a cross vent between the two  

 tanks?

 And no vent at all from the header.  Pressure difference between the two

 tanks (one will always be slightly higher than the other) will cause the

 header to purge air through one or the other fuel lines.  Another point  

 is

 when filling the wing tanks only one side will be filled at a time  

 allowing

 the header tank to fill and purge air thorough the other fuel line.  A  

 cross

 vent will allow both wing tanks to feed fuel at any time as well as keep  

 the

 air pressure on top of the fuel equal at all times.


	




Sure Noel, you're right here, and I'll like to add; this exactly according  

to a drawing I'll made for this list many years ago, showing both wing  

tanks crossvented with a T connection down to the header tank.  I don't  

know if the sportflight's photo share is working, but sure this drawing is  

there.

 	  Quote: 	
		  

 The reason I suggest this is there are certified aircraft flying today  

 with

 just that set up.




	






 	  Quote: 	
		   I like manual cameras, manual transmissions and fuel shut offs.  I  

 prefer to

 have the choice to control fuel flow during flight.   That requires some

 sort of individual shut offs to be installed on the wing tanks.



 Others may not consider the that control of any value and prefer to fly

 without the shut offs.  They may be right.  They are right for  

 themselves.

 I doubt they are for me.


	


I could have been write the same thing myself..



 	  Quote: 	
		  

 Noel

 Soon to install shut offs.




	


Hoping this may clear up things.



Cheers



Torgeir.

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