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fuel flow for CH701
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Joined: 18 May 2008
Posts: 17
Location: Manchester, NH

PostPosted: Mon Sep 22, 2008 7:28 pm    Post subject: Re: fuel flow for CH701 Reply with quote

Bob Kissell, the originator of this thread, accidentally sent this to just me instead of the whole list, so I'll cut-and-post it here for everyone to read. Kudos to Bob for taking the experiment to the next step to answer the question we probably all have regarding a completely full tank and a completely dry tank.

From Bob:

Thanks for the reviews since I posted my "study". Since then I have flown a cross-country with snorkel vents installed. The flight took 1.25 hours and I started with nearly full tanks. I cruised at 80 mph IAS and when I arrived at my destination the fuel gauges were at nearly identical positions. A major improvement from the original Zenith caps. On the return trip I filled one tank to the brim and the other to a lesser amount but nearly full. When I arrived after a 1.5 hour cruise (head wind this time) I measured the fuel depth of both tanks and had about a 1/3 inch delta. SInce I started with an imbalanced fuel the higher tank must have siphoned (flowed) to the lower tank. I am very happy with these results.

For you guys wishing to have snorkel vents it is easy to modify the Zenith caps. I purchased some steel brake line stock with fittings and flares on both ends. I drilled the center of each cap with a hole to match the diameter of the line. The brake line was cut and the threaded fitting removed and then bent with a tubing bender to create the right angle, two were made. The tube was then inserted into the hole and aligned so that the flared end faced forward after the cap is locked into position on the tank. Important..the cap position from tank to tank varies so you must trial fit these vents. Once the vent position is determined I brazed the tube to the cap with a MAP gas torch and brass brazing rods. The gasket was not damaged by the heat, but don't push it. The caps were then cleaned and painted. Total cost was about 5$ for the brake line material, if you have the rest of the tools. Also don't forget to fill the original vents along the cap edge, I used epoxy.

I plan to do an empty tank test soon and will report the results. I will drain one tank and clamp the fuel line so no fuel can flow into the empty one from the tank with fuel and no air can possibly enter the system if drawn by the engine. I will take off on one tank with half fuel and once over a very large empty field at high altitude remove the clamp. This will duplicate the case of one tank running dry with plenty of fuel in the other. Then orbit around for 30 minutes or so and see what happens. If the engine falters then I will replace the clamp, turn on boost and restart or land in the big field and then restart.

I am fairly certain though that the engine will continue to run and I will land will fuel in the empty tank. Wish me luck though...

Bob Kissell N701UB Dayton, Ohio

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Joined: 05 Dec 2006
Posts: 18
Location: Beavercreek, Ohio

PostPosted: Fri Aug 17, 2018 1:38 pm    Post subject: Re: fuel flow for CH701 revisted Reply with quote

I recently received a phone call from someone at the NTSB regarding the Zenith fuel system. I have not yet followed through with the query but it did cause me to re-examine the Zenith fuel problem which I wrote about more than ten years ago.

In an article I found regarding a crash from fuel failure in a CH-750, I analyzed what was in the article and present here my analysis of how fuel starvation can occur under very special conditions. In the original article ( ; I mentioned how a difference in pressure, developed by the slip stream above the vented fuel caps can impact fuel flow. I did discuss the case when the fuel tanks are full and the possibly of fuel venting in flight but did not discuss what happens when fuel runs low in one tank.

The crash article explains that there was about 6 gallon of fuel left in one tank and the the other was empty. The puzzle was how can the engine starve for fuel when there is six gallons of fuel in one tank.

Using the dimensions of the Zenith 701 tank (10 gallon) each, six gallons would require about 3.25 inch of depth in the tank. The tank has a floor area of about 427.4 square inch and there is 231 cubic inch in a gallon of fuel so 6 gallon has 1386 cubic inch of volume. Divided by the tank floor area results in 1386/427.4= 3.2428 inch of required depth in the tank for the 6 gallons.

So now the question becomes how much pressure difference between the two tanks does it take to support 3.25 inch of fuel height. Each cubic inch of fuel weighs 0.0272728 pound, ( see previous article using 6.3 pounds per gallon), so 3.25 times 0.0272728 = 0.0884 pound. Since this was all done relative to square inches the pressure difference is thus about 0.09 psi.

This indicates that if the fuel caps and air flow combination created a pressure difference of 0.09 psi between the two tanks, the fuel in the tank with the lower pressure would settle at abut 3.25 inch higher than the fuel level in the other tank and would maintain this level difference throughout the flight, providing the conditions did not change. Eventually the tank with the higher pressure (lower level) would run out of fuel before the other tank and the tank with fuel (lower pressure) would still have about 6 gallons of fuel that would not be usable because the pressure difference would be supporting (lifting) the fuel as before. The tank fuel lines tied together would likely fill with air being pulled from the empty tank towards the one with fuel.

The aircraft fuel pump would not be able to draw fuel since the line has only air in it. It can be seen that very little pressure difference is required to result in a substantial volume of fuel trapped and unusable. This "head" or difference in fuel level is proportional to the difference in tank pressure. As this shows it is very important to that the pressure on the top side of each tank are kept the same if all of the fuel is to be usable. In the article I wrote ten years ago I solved this problem on my aircraft by adding snorkel tubes to my fuel caps which created about +0.1 psi of tank pressure (measure value) during flight. The exact pressure is not that important, but both of them being the same is. Another recommendation was to add a vent tube to connect the top of each tank together to equalize the pressure. This is not easy on the Zenith designs and in my case I had already built the aircraft. The snorkel vents, described in the earlier article have been in use on my aircraft for 10 years and have solved my fuel level so far. I hope this helps you builders make good fuel systems in particular on Zenith aircraft. I have found my aircraft,N701UB to be a joy to fly and very reliable. The crash article was perhaps a little harsh towards the design, I think it's pretty good and thank Zenith for it. But there are always room for improvement..

regards Bob Kissell, Dayton, Ohio N701UB 400+ CH701 flight hours and climbing..

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