Replies to This Discussion

Permalink Reply by Kevin McCune on November 28, 2009 at 7:39pm

Here is what I have come up with, well me a n a lot of input from others. The sight tube may be a level switch instead?

Kevin

Attachments:

• N701DZ fuel system.jpg, 39 KB

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Permalink Reply by Ralph Sanson on September 9, 2009 at 5:02am

I've had my engine lost power 6 times due to this. My tanks are larger than standard, 42.5 litres each from CZAW which probably doesnt help. All occurrences were running a third or less fuel, and drawing off a single tank. 5 times (3 in the one flight!!) I was doing a steep spiral descent and feeding off the down wing side. The 6th time I was in level flight but had been doing some uncoordinated flight just prior.

Normal recovery occured in all cases by levelling out and a couple of times switching to both tanks selected.

I am more careful to select both when fuel level is low for approach and landing.

Have considered adding header tanks behind the door pillars area one for each side a length of extruded pipe 5" dia sitting upright on the baggage shelf with inlet at top and outlet below?

Ralph

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Permalink Reply by Victor Menkal on October 21, 2009 at 2:00pm

Im currently trying to decide on the best fuel system for my 750 prior to starting the wings and have also wondered about this issue. As well, also trying to find a simple way to decrease "dead" storage in the tanks.

Came across flop tubes which are used in aerobatic planes.

http://www.aircraftspruce.com/catalog/appages/flopetube.php

Does anyone have any experience with this system and pros and cons.

The one draw back I could see would be that the intake would be slightly lower than the outlet fitting which could lead to air lock problems under the right conditions.

Cheers Vic

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Permalink Reply by Chumphol Sirinavin on October 30, 2009 at 11:09pm

My wing tanks have two outlets each. One at the low forward corner, another at the low rearward. Fuel from the two outlets join at a T at wing root. Then, fuel from both wing join at the gascolator. With tanks connected as such, filling one tank is actually filling all. There is no fuel selector valve.
If anyone knows of any problem that could happen with this (too) simple fuel system, please let me know.

Thanks in advance

Champ

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Permalink Reply by Victor Menkal on November 1, 2009 at 7:11pm

Hey Champ, Im definitely not an expert on this but I had considered the option of two intakes for steep descents at low fuel levels but realized that if the fuel was low enough to need the forward intake, the the aft intake would be drawing air and would still cause fuel starvation. It could be Im missing something with the dual intake configuration and if so please advise.

The use of a header tank seems to be a tried and true solution but I am a bit squeamish about having a fuel tank in the cockpit. Havnt found anyone with experience with flop tubes yet but will keep researching and advise. I noticed my weed wacker has one and it works well when I use it for mosquito control in a vertical orientation.

On the advice of my EAA advisor, Im also going with two separate lines, gascolators and fuel filters and a selectable LR BOTH fuel selector. The reasons for this is that if there is water, icing, vapour lock or filter blockage in one system you can switch to the other. Another bonus is you can use the fuel selector to help balance the plane or equalize tanks as required.

Please keep in mind my playground is a very remote area and fuel problems are common due for a number of reasons.

That being said, there is a lot to be said for the ol KISS principle, which, IMHO, is one of the outstanding qualities of Mr. Heinz's design.

Please advise what you finally come up with.

Cheers Vic

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Permalink Reply by Victor Menkal on November 1, 2009 at 7:29pm

Sorry, I should have noted that the Tee needs to be below the level of the tanks for the dual intake system to prevent fuel starvation. Id be a bit worried if was located at the wing root.

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Permalink Reply by Chumphol Sirinavin on November 2, 2009 at 9:35pm

Thanks you Victor for the comment.

When only one fuel tank outlet is feeding the engine through one arm of the T, as long as there is a constant fuel flow, air won't enter the fuel line. Fuel is heavier than air. Also, when wings level, the wing dihedral puts the fuel outlets higher than the T held flush with the lower wing skin at wing root.

The only drawback of my fuel setting may be that I would not be able to use weight of fuel to keep wings balanced (more or less) in flying a long cross wind leg.

Enjoy your day,

Champ

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Permalink Reply by Phill Barnes on November 2, 2009 at 4:17pm

Hi I am not an expert in the 701 but I read all of the discussions daily and have found this one very interesting. My question is, would it be possible to have two individual small header tanks in the fuse so that the dual fuel lines from each tank terminate in their own header tank and then a single fuel line from each header tank go directly to the fuel selector. This way you would have two separate fuel systems with no air entering the fuel valve. Would this solve the fuel flow problem?

Regards
Phill Barnes

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Permalink Reply by Victor Menkal on November 3, 2009 at 11:27am

Hi Champ and Phil. Great thread .... lot better to think this stuff out now rather than at 2000'.

Champ, please note that my background is in water and dirt engineering and I am absolute newbie with plane stuff so please take this with a grain of salt (OK, an entire salt shaker). But here goes.

I modeled the plane at an extreme down angle of 45 degrees (OK makes the math easier as well). I call this the angle not exceed (ANE). As an amatuer builder I can make up my own acronyms. I measured my tank where the forward intake would be at 450mm from the aft intake.

At 45 degrees down angle the aft intake would thus be 318mm above the forward intake.

I believe the dihedral angle of the wing provides a 2.15% slope and the distance from the tank to the wing root is 318mm. If my math is correct that would give a drop of 6.8mm from the tank to the wing root due to dihedral.

So now looking at the fuel flow. If I just had one isolated line to the forward intake, the fuel would have to flow uphill 318mm before it drops down to the gascolator (as per Phils nice drawing). I believe this would work even with just gravity flow due to siphoning (the column of fuel pulling down is higher than the column of fuel being raised). The fuel pump would help with its suction.

Now if I put a hole in the fuel line at the top of the "hill" (the aft intake), the suction is now broken and the downward pulling column of fuel cannot lift the fuel from the forward intake.

I believe the simple solution is to use the system as pictured in Phil's great drawing where the tees are located at the header tank location with the tees located more than say 400mm below the level of the tanks.

Phew! OK I have to admit that I set up a little model with a tuperware container and some poly fuel lines and made airplane noises as I "flew" my gas tank around the house before I attempted the math :). Try this out Champ, its a lot of fun regardless of the rude comments from on lookers.

Very elegent simple solution, with the only draw back being the need for two fuel line penetrations at the wing roots.

For myself, the concern is welding the second intake flange. Not sure if I can find anyone with the skills for this type of welding. Thus still looking at the ol flop tube but havnt been able to find anyone with any experience with these puppies. I think Ill give EAA a shout and see if I can contact an aerobatic type aircraft engineer for advice and will keep you posted.

That being said I have ordered in a couple of welding flanges in case I use the dual intake/ dual fuel line option.

Hope this helps and please check the math!

Cheers Vic

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Permalink Reply by Chumphol Sirinavin on November 4, 2009 at 6:40am

Victor,

Big thanks for your great concern.

I had been doing some mild aerobatic, such as Steep Turn and Wing Over. 45 degrees of bank and nose down had been exceeded momentarily. May be, the other wing tanks kept the fuel running. There were 4 of 5 gallon tanks, 2 per wing, that were inter-connected. I will be extra careful next time, especially when 45 degree nose down with wings level, as you have experimented. And there better be no hole in any of my fuel lines. :^)

Shown in the crude sketch below is a 10 gallon tank with little fuel in a sustained 45 degree dive. The T is shown lower than wing to simplify things a bit. The lower fuel outlet (front one in this case) is still some hight above the carburetor, and keep the siphoning going even when the T connector is above the fuel level.

My big tanks have 2 outlets 61 cm. apart, while the smaller ones 20 cm. To show all 4 tanks of two wings in step banking and diving is too much at this beer time. It is still months before I finish this rebuild. Meanwhile, I am all open for suggestions.

Anyway, nylon T is far cheaper and lighter than any header tank for the same job.

Enjoy your beer..

Champ

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Permalink Reply by Victor Menkal on November 4, 2009 at 12:51pm

Hey nice pic Champ! That is a lot more clear than the math. As shown, this is the option I will be using. OK if I put your pic in my build log?

My apologies if I misunderstood the configuration. I had the tee located directly below the aft pickup in my calcs.

Four tanks huh? You should have enough range to come up and visit when you are done!

Good luck with the build and keep us posted on the results.

Cheers Vic

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Permalink Reply by Chumphol Sirinavin on November 4, 2009 at 7:39pm

OK with me, Vic.

Happy homebuilding.

Champ

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