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1. 02:39 PM - Re: fuel flow for CH701 (kissell)
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| Subject: | Re: fuel flow for CH701 |
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 https://generalaviationnews.com/2016/04/29/bad-fuel-system-design-contributes-to-accident/, 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 (http://forums.matronics.com/viewtopic.php?t=48650I ; 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..
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=482504#482504
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