Fuel draining at uneven rates

[rosslr]

Well, just woke up over here in OZ and see you guys have been busy over night( your day!). As the person who re-ignited this thread, I am appreciative of all your inputs as usual - from the specialist engineers, the experienced kitfox flyers and the the novices (myself). I am not an engineer. But I work with engineers in getting to build collaborative ecosystems with other disciplines. Mixing it up with diverse viewpoints - not only fun but it usually yields outcomes nobody had in mind when we started - as long as people listen, clarify and challenge ... respectfully.

I have already taken a punt on the vent angles but waiting for a nice calm spot of weather to keep the other variables equal. As for the hose lengths - I am thinking of evening them but will have to work out a continual downhill run for the right tank. I get the unbalanced flying contribution but I am still interested in achieving an even draw down, other things being equal. I hear what some of you say about it not being an issue and that the left tank will still fed the engine if the right is dry but try telling that to my hawk-eye flying partner! I will be landing to refuel or flying with a increasingly nervous punter in the right seat!

I was actually wondering if it would be possible to design a vent as a weather cock so that it automatically points the vent into the prevailing airflow? Silly idea?

cheers

ross

[Av8r3400]

Solution:

I put a valve on each side. If one flows more than the other I close the valve. Easy peasy.


Seriously, I don't know a Kitfox or Avid that doesn't flow one over the other. Some are "left tanked" some ore "right tanked". They all do it.

Like was stated earlier, to fix the asymmetrical flow is as easy as stepping on a rudder peddle.

[redbowen]

So I investigated the fuel vent angle and was surprised to see the left hand tank pointing noticeably towards the wing tip. This is the tank that always drains last.
First picture
So I corrected the angle and will give an update on the flight test when it stops raining. Second picture.

[Dick B in KY]

I have flown wing high, wing low, ball out of center both ways and have yet to run out of fuel due to one tank being lower than the other. If it's that much of a concern, stop and refuel.

Dick B

[Danzer1]

Should we start exchanging flaming messages, until the list admin locks the
thread or bans us? That's the usual resolution to thread argument isn't it?

I see no need nor would it be appropriate, it is a healthy topic of discussion - that's all.

There is a pretty easy test to determine the friction drop difference between a 3' and 5' length of 3/8" hose in an open system. If anyone has the spare time to bother - here goes:

Scenario:

Tanks have equal head pressure at start of "flight" and are filled equally.

We are talking about an "open" (vented) system without a return or pump.

Assume engine burn at 6 gph or 3 gph per tank and hose. (You can't flow more than the engine burns) and 6 gph is an easy number to use for this purpose.

Test:

Take an open reservoir larger than 1 gallon and hang it up high with a 3' length of 3/8" hose on the outlet.

Put 2 valves on the hose.

1 to start - stop flow for timing purposes.

1 to meter the flow to achieve filling a measured container with 1 gallon of fluid (water is close enough) in 20 minutes (3 gph flow rate). Might take a few times to get it right at 20 minutes.

Now take the valves and put them on a 5' length and repeat the test - NOT moving the position of the metering valve from the 1st test (3gph per 3 feet).

Now time how long it takes to fill that same 1 gallon container with the 5' hose.

I'd venture to guess, it will take about 10 seconds longer. Or .025 of a gallon per hour = 6.4 tablespoons per hour.

Have fun and thanks for flying Danzer Air!

[jrevens]

I think this discussion from my perspective has gotten a little confused. Engine stoppage because of one tank running out before the other isn't, and never was my concern. I also understand clearly that there are enough variables involved that it is almost a sure thing that most will have uneven flow rates comparing one tank to the other - I've always known that. In my mind we were discussing what does and what does not affect uneven draining of fuel from the wing tanks.

I think I just like to be able to manage the fuel system if I want to, whether to service something downstream of the tanks, or to avoid having to fly in an uncoordinated condition for long periods of time to even things out if I want to. I like to have a nicely balanced airplane - might not be any big deal in a Kitfox, but still desirable to me. If you have roll trim, or a wing leveler, again - no big deal on a long cross-country. My "other" airplane is beautifully balanced, and I can actually make it roll or pitch by just moving my head when trimmed-out in cruise. I like to keep the weight distributed as evenly as possible, so when I'm by myself I guess it might actually be good to have more weight/fuel in the right tank than the left one of a Kitfox. Again, I just like to be able to manage it that way (with valves)... to each his own. I really appreciate all the comments - it's good to learn about these airplanes vicariously through the experiences of others more experienced with them.

One other thing - I wouldn't have probably even thought of valves initially if it weren't for my good friend Stan Specht, who recommended them highly to me. Stan has flown his 80 hp Mod. IV Speedster well over 2000 hours to every State in the union but Hawaii. He's also flown to almost every Province in Canada, and we'll most likely get to those couple he missed when we make our Alaska flight in a year or so. He's flown into every public & many private airports in Colorado. He knows all about flying a Kitfox cross country. I really respect that guy, and he's the reason I decided to build a Kitfox (well, that and the fact that my dad had an attraction to them before he passed away in 2000). I'm gonna' love this little airplane so much!

[HighWing]

I have already taken a punt on the vent angles but waiting for a nice calm spot of weather to keep the other variables equal. As for the hose lengths - I am thinking of evening them but will have to work out a continual downhill run for the right tank. I get the unbalanced flying contribution but I am still interested in achieving an even draw down, other things being equal. I hear what some of you say about it not being an issue and that the left tank will still fed the engine if the right is dry but try telling that to my hawk-eye flying partner! I will be landing to refuel or flying with a increasingly nervous punter in the right seat!
cheers

ross

A couple of things - maybe three on the quote above. I do understand about Hawk-Eye. I have one of those too, but now she just watches from the ground. But, the low tank will never fully empty before the fuller tank begins to flow. It is a matter of head pressure as Jeff mentioned. Regarding the fuel routing. Frankly, I never liked the down the cross tube behind pilot and passenger heads routing. I run mine immediately aft above the quarter window, then a gentle bend brings it forward again to the header tank. I like that routing for two reasons. 1. the fuel line is now where it is close to impossible to be stressed or damaged from loading the baggage sack and 2. I place my glass fuel filters above the quarter windows so I can monitor actual fuel flow from time to time. On my inflight low fuel indicator test, I saw on both sides the big bubble form in the filters. I am wondering if that routing would create opportunities for lengthening the right feed line while maintaining the desired continuous downward flow.

One more thought not related to the above quote. Valves in the lines would most certainly cure the flow imbalance if monitored regularly. I chose to go with what nature provides primarily, because, I don't want a sputtering engine to remind me I closed the low tank valve and forgot to open the valve to what was previously the low tank. Most accidents are caused by pilot error, but we all know that.

[redbowen]

For those with Fuel valves on the feed lines to the header tank, there is a service bulletin #29 from Skystar that suggests not having those valves. I'm sure you all have found them safe, but here is the bulletin.




December 15, 1992

SERVICE BULLETIN #29

SUBJECT: HEADER TANK AND FUEL LINE ROUTING FOLLOW-UP

(Follow-up to Service letter #22) MANDATORY

TO: ALL Kitfox™ BUILDERS WITH HEADER TANKS

FROM: SKYSTAR AIRCRAFT

Service Letter #22 was an advisory to all Kitfox™ owners who utilize a header tank in conjunction with one or more wing tanks. It dealt with the possibility of fuel not flowing to the header tank from the wing tank if the wing tank fuel pick up is un-ported, causing air to enter the line. Then fuel flow stoppage to the header tank, can cause the header tank to drain; resulting in fuel starvation to the engine.

This Service Bulletin will deal with specific recommendations and instructions on how to deal with this potential problem. The overall object, is to have the fuel flow from the wing tank to a rear mount header tank and then forward, to the firewall with the greatest amount of droppage as possible (gravity feed with no high point) . It has also been determined that deleting the individual fuel valves from each wing tank, helps prevent, air entrapment in the fuel lines.

RECOMMENDATIONS

(1) : All front mount header tank systems must be removed and a rear mount header tank must be installed. (SkyStar Aircraft is offering a new, rotationally molded header tank for easier installation of a retrofit rear mounting position, at a special reduced price for this Bulletin.)

(2) : Each wing tank fuel valve must be removed from the headrack.

(3) : The fuel line routing, should flow directly from the wing tank to a rear mounted header tank. From the header tank, fuel line routing should travel down to the bottom of the door frame, forward (under the door frame) to the firewall, across the firewall to the shut-off valve.

NOTE: For those operating wing tanks in conjunction with the large, front main tank, the wing tank fuel lines should route:

forward from the wing tank, above the door frame work, down the diagonal bracing in the windshield area, and plumbed directly into the filler neck of the main tank. The wing tank fuel valve can be installed in the headrack above the door. Because of the capacity and fuel visibility in the main tank, the question of fuel flow stoppage from the wing tanks is not as critical. With the fuel routed into the filler neck, you will have a constant visual on all fuel.

Enclosed, you will find in-depth instructions for the mounting and retro-fit mounting of the rear mounted header tank. You will note that some of these instructions are in the form of revised manual pages. Those who have the new format construction manual should exchange these pages for those in their manual. The new pages are marked “Revision” 1.

If you have questions regarding this Bulletin or procedures, please feel free to call our Technical Support Department. The Header Tank Replacement Kit, front mount to rear mounted header tank, is part number #69141 and will normally retail for $119.99. Those ordering in response to this Bulletin will be able to purchase this kit (#69141) for the price of $76.00. Those aircraft which presently utilize a rear mounted header tank, may use there existing header tank but need to modify the routing, extra #47000 Fuel Line for this modification and the Replacement Kit can be ordered from the Customer Service Department. Phone: (208) 466-1711 Fax: (208) 466-7194

Rear Mount Header Tank (#44071) Installation

Preface

Read each step carefully to determine the specific application to your installation. Some steps may not, in there entirety, deal with your application; but, may contain points that you should be aware of and act upon.

1. Drain all fuel from the wing tanks and header tank. Try to eliminate fumes and ““ground”” the aircraft.

2. Remove baggage sack and seat back cover.

3. Remove the existing fuel line and #29001 Wing Tank Fuel Shut Of f Valve from each wing tank to the front rear mounted header tank. (For existing Rear Mount Modification, skip to Step 9.)

4. Remove the existing aluminum #65032 Front Header Tank from the firewall bulkhead.

5. Attach the #44071 Header Tank (new rotationally molded) to fuselage diagonal brace tubing, above and behind the control system. as per Fig CFO-4b.

6. Install #94002 Elbow Fittings in the #44071 Header Tank inlets and outlet. For single wing tank aircraft us a #94005 Plastic Plug in place of the #94002 Elbow. Use a good grade thread sealant on all threads. Be sure to use a wrench on the hex portion of the brass adapter bushing, to prevent its turning when tightening the Elbow. Orient the barbed, inlet Elbows outboard and outlet Elbow to the left. Also install a #94000 Vent Elbow using above procedure and orient the barbed end to the right. See Fig. CFO-4b.

7. Install the #44000 Rubber Fuel Line Hose between the Wing Tank Outlet and the Header tank. Route the fuel lines to insure a continuous drop, from the Wing Tank to the Header Tank. Secure these lines to the fuselage structure with tie wraps. Make sure the Hose is long enough to allow folding of the wings. Route the Hose under the headrack carry through brace as shown in Fig WO-3-l. (For those with an existing, Aluminum Rear Mount Header Tank, slip the hose over the inlet nipple and secure with a hose clamp.)

8. If not previously accomplished, install a return vent

[kmach]

Allot has changed since 1992 !

I for one , will not be removing my valves between my wingtanks and header. They are too useful , for inflight fuel management , uneven ground parked situations to prevent fuel transferring and dumping out of the low tank and for maintenance.

[happiestflying]

I too am having this problem.

My longest cross-country was bringing the plane home to Washington from Colorado, during which flight I had uneven flow but lots of fuel left in reserve, even after the longest leg which was 5 hours and 10 minutes. That was two years ago.

It seems that the problem is getting worse, so even though I muck around with full tanks, when I get home the left wing carries substantially more fuel than the right, and I don't have the courage to run them down to the point where the right wing shows nothing, just in case the opinions here are in error. Would rather just find and fix the issue, so will try the suggestions here and report back. Have tried the ball-off-center suggestions, but haven't yet re-aligned the gas cap pitot.