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Dual Tank safer?

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Old 06-24-2003, 12:25 PM
  #1  
Dazzler_BE
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Default Dual Tank safer?

OK, here's the deal.

What happens when you're running low on fuel - and your flying in a downward way, either to lower your height, or coming out of a loop, or other aerobatic maneouver. My guess would be: the fuel inside the tank comes on a lower level than the clunk, making it impossible to suck fuel. Engine dies...
OK, some people have posted in previous posts that the time between "no more fuel" and "having back fuel" will not be long enough for the engine to die... but what if is does?

Here an image I made up to illustrate:


1) level flight with full Fuel tank --> no problems
2) inverted flight with full Fuel tank --> no problems
3) flying up with full Fuel tank --> no problems
4) flying down with full Fuel tank --> no problems

5) level flight with almost empty Fuel tank --> no problems
6) inverted flight with almost empty Fuel tank --> no problems
7) flying up with almost empty Fuel tank --> no problems
8) flying down with almost empty Fuel tank --> PROBLEMS !!!


I've seen posts talking about dual fuel tank setups, whereas the fuel tank running to the carb will always be full as long as the second one is not completely empty.

So, assuming this, If I put up a fuel tank in a plane, let's say 16oz, and add a small one of say, 2oz between the big one and the carb, the small one actually works as a buffer, which is always completely filled with fuel, so no matter what maneouver I'm pulling, the clunk will always be in the fuel ?
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Old 06-24-2003, 03:42 PM
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Default Dual Tank safer?

Centrifical force keeps the clunk in the fuel unless you have the tank messed up or you are doing some really strange maneuver.

Helis run header tanks which are always full by design until the main tank is empty. It is slick but in most planes, there is hardly room for one tank, let a lone two. that is the problem with it.

When you start sucking air, from dooing too many aerobatics when the fuel gets low, the engine doesn't just quit, it usually surges, telling you it is time to get more. You still have pleanty of time to get down.

Research header tanks in helis and see how they work. It is a nice way if you have room, but it would have the same problem when it starts to empty if you fly too long.
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Old 06-24-2003, 05:24 PM
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Default Dual Tank safer?

Flyboy, "gravity" keeps the clunk in the fuel!

Dazzler_BE, you've pretty much got it right:

The header tank acts as a buffer by replacing consumed fuel with more fuel from the main tank. The main tank of course can only replace consumed fuel with air via the vent.

If your main tank has a low enuf fuel level to start sucking air when in a nose down attitude (figure 8 of your graphic), then your small 2nd tank (header tank) will begin to draw air from the main tank.

However, because your header tank was full up to this point in time, its clunk will still be fully submerged in fuel, as in figure 4 of your graphic.

Example: I use 16 oz tank in the fuselage that feeds a pair of 2 oz header tanks behind each engine on a twin. This way, even if my engines have different consumption rates, an engine out situation due to an empty tank won't be an issue untill at least the last 2 oz of fuel/engine.

Hope that helps.
-E
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Old 06-24-2003, 06:31 PM
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Default Dual Tank safer?

Ok, wrong word, when you are upside down pulling negative Gs, gravity. Bonehead mistake. What can I say. Was doing 3 things at once.
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Old 06-24-2003, 07:51 PM
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Default Re: Dual Tank safer?

Originally posted by Dazzler_BE

So, assuming this, If I put up a fuel tank in a plane, let's say 16oz, and add a small one of say, 2oz between the big one and the carb, the small one actually works as a buffer, which is always completely filled with fuel, so no matter what maneouver I'm pulling, the clunk will always be in the fuel ?
Yes this is correct as the others have mentioned however if you are really concerned the use a Jett or Tetra bubbless tank avaliable up to 16 oz. These have no clunk an will never deliver air just liquid untill empty additionally they do not have to be mounted in foam. These tanks have a far superior fuel delivery performance to a conventional system and it make no differance what attitude the tank is Mounted.

John
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Old 06-25-2003, 12:17 AM
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Default Dual Tank safer?

I know in my tank installations the clunk is at the rear of the tank when I am low in fuel and on my vertical downline following a stall turn. What I don't know is if the fuel is at the front or rear of the tank. My clunk stays at the rear of the tank and is only free to move to the end area of the tank. With the average 40 cc gas engine will consume 1 oz.minute, what I don't know is how much fuel will the fuel line hold and for me, a 5-10 second downline at idle is about as long as it gets.
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