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kerrjohna(at)comcast.net
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 Posted: Mon Jun 25, 2012 7:21 am Post subject: Engine out..deadstick landing required..question on header |
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fluid physics would suggest that the head pressure is determined by the fuel level, not the header tank position. Likely no the cause of the engine out.
John Kerr
From: kitfoxjunky(at)decisionlabs.com
To: "kitfox-list matronics.com" <kitfox-list(at)matronics.com>
Sent: Monday, June 25, 2012 7:27:41 AM
Subject: Engine out..deadstick landing required..question on header tank location
Had someone have me take a look at his KF3. He recently had a dead stick landing. Only damage was a broken prop but now he only flies high and over his home field as he does not trust the plane.
He is putting this down to bad fuel, as it happened right after he switched tanks. Problem is with a new prop the airplane started and ran fine on the ground after the incident.
I was looking at the fuel system, and I noted that the header tank is mounted on the tube structure near the back of the seat. It is a plastic tank, and looks to me like the bottom is almost at the floor level. In my KFIV I do not run a 582..I have a 912s, but the header tank is mounted high up behind the baggage area.
Is a header tank not supposed to provide a head of fuel to the engine? How could it do that if it was mounted below the level of the carbs?
Wanted to ask those out there more in the know on the two stroke setups.
Gary Walsh
KF IV Anphib 912S
C-GOOT
[url=about:blank]www.decisionlabs.com/kitfox[/url]
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mikeperkins
Joined: 22 May 2007
Posts: 123
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| Posted: Tue Jun 26, 2012 2:47 pm Post subject: Re: Engine out..deadstick landing required..question on hea |
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Sorry to hear about the outcome of your glider experience. Not fun, and expensive.
A power loss associated with switching tanks would only occur if the header tank contained mostly air. That could happen if the header vent line were obstructed.
True that the head pressure comes from the fuel tanks, not the header tank. But wing tank head pressure is not always reliable. A lot of high-wing aircraft do not have a header tank.
In a Kitfox, the purpose of the header tank is to provide fuel when both wing tanks are unported (during slips and negative pitch with low fuel, etc.). Also, without a header tank with two ports, a T would be necessary. However, T's are notorious for creating vapor lock (a bubble lock) when located where head pressure is very low, and so the header tank eliminates the need for a T. In any case, the intent of the header tank behind the seat is not in itself to supply head pressure.
The folding-wing design contributes to the tendency for the hoses to have a rise near the wing hinges which are prone to vapor locking. And thus the header tank also provides insurance against temporary vapor locking in those humps and gets rid of the need for a T.
Being a Model I builder, before Service Bulletin #29 was written I became aware that some builders were mounting their fuel valves conveniently to the fuselage structure adjacent to the wing tanks, nearly level with the fuel tank outlet, and in some cases even above the outlets. With practically zero head pressure here, we eventually concluded there was the possibility for air to be trapped in the valve innards as well as in a hose hump.
Since fuel is notorious for not liking to be pulled by a pump and for not overcoming vapor locks, aka ever-expanding fuel bubbles (because of its characteristically-high vapor pressure), the pump engine pump was not able to draw fuel. So if both valves or both fuel hose humps were vapor-locked, no fuel would get to the engine. (This is one of several reasons the EAA and FAA had to prove back in the ‘80’s that auto fuel would work in certificated aircraft and why there are not mogas STCs for all aircraft. But all gasoline, including 100LL, has some vapor pressure - it's just that mogas comes equipped with more of it.)
The answer for the Kitfox fuel flow problems was to put the valves down much lower and to prevent any rises in the hoses and fuel lines prior to the header tank. It was discovered after SB #29 was written that this was ok to do, but the SB remained in effect, probably for liability reasons.
My wing tank valves for 400 hours have been about 3” above the seat-backs, no fuel problems. It is not convenient for in-flight operation; the intention was mostly for fueling and de-fueling purposes anyway. This location reduces the possibility for vapor lock because there is a 20” drop from the wing tank’s fuel outlet before reaching the valve. And care was taken to avoid fuel hose humps in the hinge area. For flight-emergency purposes, I installed a master fuel valve on the floorboard under the left door near the carry-through truss.
Running a gravity fuel supply test is always good idea, but it does not account for vapor lock caused by combinations of flight conditions, differences in the gasoline used, and high temperatures, etc.
It is best to put fuel pumps near the tanks so that fuel is pushed, rather than pulled, using perhaps a pair of Facet pumps. Low wing aircraft would do best this way, obviously not a Kitfox issue, but something worth mentioning.
If the selector valves are up high, they should be relocated lower, but it is ok to have them as long as there is a significant fuel-system drop before them. And the vent line should be checked for blockage. Obviously filters and fuel lines in general should be checked. A mud dauber when the lines are apart for maintenance can cause havoc. Also, a filler cap vent blow-test should be a checklist item. A pair of Facet pumps hear the tanks would also overcome a variety of fuel-system sins.
Hope this helps.
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Mike Perkins
Havana, Illinois
Model I, 532, B gearbox, GSC prop |
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