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From: William R. Bayne [mailto:[EMAIL PROTECTED]
Sent: Tuesday, November 15, 2005 1:16 AM
To: Ed Burkhead
Subject: WRB Re: [COUPERS-TECH] Stall on Take-off
RLYTECH
On Nov 14, 2005, at 10:46 PM, [EMAIL PROTECTED] wrote:
In a message dated 11/14/2005 10:06:55 PM Eastern Standard Time, [EMAIL PROTECTED] writes:
I opened the wing tank cap it had vacuum pressure on it. I took the gascolator a part. It was clean. Put it back together. The only way I could get the air out was with the primer. I blew through the fuel vent (it is located in the bottom of the fuselage) no problem my checks puffed a little but I could still blow through it.Steve,
A couple of thoughts:
If the wing tank had a vacuum on it as you mentioned, and air rushed into the tank when you removed the wing tank cap, then the wing tanks were definitely not being vented to atmosphere through the header tank.
Makes perfect sense, Wayne.
The fact that the vacuum existed after you shut down and had time to get out and remove a cap implies that the line from the fuselage vent outlet to the header tank, or the line from the wing tank to the header tank was partially plugged.
The fact that the fuel gravity feeds to the carb from the header tank, and not from the wing tanks, and the fact that the engine quit, would lead one to believe that the header tank was not vented to atmosphere either, which would seem to point to the "fuselage vent to header tank line" being plugged.
I would instead conclude that fuel has ceased to flow from the fuselage tank because neither of its vents function. Had Steve opened the fuselage tank first, I believe he would have heard the same "rush of air" he observed into the wing tank.
As I understand the Alon fuel system utilized the original Ercoupe-style fuselage tank cap fitted with a forward-facing vent hole normally pressurized to some degree when the engine is running by both prop wash and air speed. This hole must be clear and unobstructed in order to perform its intended function. Let's call this the #1 fuselage tank atmosphere/pressure vent.
There is a fuselage tank drain line ending in a quick-drain 12" aft of the left-hand exhaust (which has NOTHING to do with tank venting). Fuselage tank overflow lines allow excess fuel (transferred by the fuel pump from the wing tanks) to return to the wing tanks, and also allows wing tank fuel expanding due to heat upward into the fuselage tank. When these are unobstructed and function as intended, they allow pressure to equalize between wing and fuselage tank regardless of circumstance; but they do NOT vent to "atmosphere".
There is a fuselage tank vent line which vents excess fuel overboard should the fuselage tank become completely full (to prevent overflow through the fuselage tank cap vent). Let's call this fuselage tank vent #2. If EITHER of the above-described fuselage tank vents are functioning, it is not possible for Steve's observed "vacuum" to develop.
The fact that you had to "puff" on the fuel vent line to get it to flow MAY be normal if the only other vent to atmosphere is the small hole around the header tank float wire. If the header tank cap is installed, and if you are applying pressure to the only vent in the system, then you have a closed system except for any leaks, such as at the float wire. (Why isn't this vent sealed by the glass tube over the vent wire? Is it missing?)
The glass or plastic tube over the gauge wire was not, to my knowledge, original to any production airframe. I believe it was developed by Larry Blauvelt as an aftermarket item now sold through Skyport. I believe all such caps used on ANY of our fuselage tanks MUST have a drilled forward facing vent for the fuselage tank to operate as intended. The clearance around the wire was likely disregarded, from a design standpoint, as an atmospheric vent of significance.
I would remove the header tank fuel cap and repeat this test. If there is still any resistance to blowing air through with the header tank cap removed, then something must be plugged.
Keep in mind that the vent line may be plugged with "goo" and may not be solidly plugged. This problem might come and go is the goo is moving around in the line at all.
If the fuselage tank cap were replaced with an unvented one, for test purposes, and the tank held pressure, such would confirm blockage in the overboard vent line.
I have not personally seen an Alon with the fuel system vented as yours is, but it seems to me that having all three tanks vented by only one means is a bad idea. I know why they did it that way - to avoid fuel spewing out of the wing tanks on takeoff, but I'd rather have fuel streaming out for a minute or two than have only one vent in the entire system.
Every Ercoupe beginning with the "E" Model has been vented to atmosphere ONLY through the nose tank cap vent. Forney kept this system. I believe Alon added the nose tank overflow line (the second vent), and Mooney kept it. I see nothing intrinsically wrong with this system when it is fully functional as designed.
You asked: <<Question here is could the primer be sucking air at times and get air to the gascolator>>
The primer leaking air would not explain the vacuum on your wing tanks. It is possible that the air in the gascolator came from the primer, but there should be no suction at any time at the gascolator to cause air to flow from the primer to the gascolator. I doubt that the primer is the cause of the problem.
As fuel flows by gravity to the carburetor, the vacuum created should be replaced by more fuel from the fuselage tank. If, however, the o-rings in the primer are not sealing properly, is it possible air flows in easier at this point than fuel from the fuel line? Either will resolve the "vacuum" nature abhors, but the engine can't run on the air.
The Moony M10 Service Manual states, rather interestingly, this:
WARNING: When the fuselage tank has been drained, the fitting at the carburetor must be loostened after refilling the fuselage tank to remove trapped air from line. Drain approximately one pint of fuel."
I would think this would primarily apply to Alons modified per Alon S.B. 16 and/or A-19 (M10-1), but who knows?
If the bird has NOT been modified per S.B. A-19 (M10-1), the introduction thereto states:
"Certain combinations of atmospheric conditions, aircraft attitudes, fuel levels and/or normal flight maneuvers, may result in a condition where fuel starvation can occur."
Of the recommendations which follow, I would consider modification of the carburetor float valve and seat from a .125 to .140 diameter assembly per "Instruction I" first and foremost for implementation given the circumstances. I don't know how much improvement is actually achieved by the remaining modifications recommended but, in my humble opinion, dropping a fuel filter (gascolator) already mounted lower than the carburetor ANOTHER 3.7" is NOT likely to improve the symptoms described.
One other thought that probably is not relevant based on other things that you said, but what kind of fuel are you using? It sounded like every time you had an engine failure, the engine had been "hot soaked" from a previous flight. If you are using auto fuel, you may have experienced vapor lock in the gascolator.
True
(This would not explain the wing tanks being under a vacuum though.)
True.
Consider the above as speculative opinion, as I have never owned an Alon of any variety; and I'm not a certificated mechanic.
Regards,
William R. Bayne
<____|-(o)-|____>
(Copyright 2004)
Regardless of what your problem turns out to be, I would not (and do not) use auto fuel in an Alon. There have been a few instances of engine failures on takeoff after the engine "hot-soaked" for a period of time when auto fuel was used. I can say this without any doubt because the previous owner of my airplane had three engine failures on takeoff with my airplane using auto fuel before he had it modified in accordance with most of SB-19. After that, he never had any more problems. I owned this same Alon before he owned it (I bought it back) and I flew it for about 300 hours with the unmodified fuel system, using only avgas (80 and 100LL) and I never had a problem with vapor lock or any other fuel/power problems. Another reason that I won't use auto fuel is that every time I hear of someone finding algae, or gunk, or sludge in an aircraft fuel system, it is always in a plane that uses auto fuel. I have never heard anyone say that they found a gummed up fuel system in a plane that uses only 100LL. At 5.5 GPH, the risk that auto fuel presents, particularly in an Alon, just isn't worth the savings to me.
All of this is just my opinion - so take it for what it's worth!
Please let us know what you find out. You might save someone else's airplane or rear end!
Best Regards,
Wayne DelRossi
Alon N5618F
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