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                      The Stunt Tank  

           Setting up a true Stunt Tank is fairly easy. Defining the stunt tank is what this is about as well. Determining what setup is best can be difficult with all of the various opinions that abound. This article is intended help you with these tasks. The purpose here is to define and understand the functional as well as the plumbing differences with added fuel tank configurations and their plusses and minuses along with the truths and myths. This will also help you decide which plumbing configuration best suits your expected use. I will also show you the actual plumbing installation of two most popular configurations of added tanks. 

For the purpose of this article I will call the configuration I prefer the “Stunt Tank” and the others an Auxiliary Fuel Tank or “Aux. Tank”. I believe that there is only one configuration that is truly a “Stunt Tank”. The difference will be defined and that difference is why I will only use one configuration for aerobatic heli flying. First a few definitions of the names used for these tanks and what the name really means. Then we will trim it down to the two basic configurations used in heli's and what i consider the most appropriate name for each configuration. Some of the names applied to the added tank are Header, Bubbler, Stunt, and Aux Tank, etc. The added tank is generally installed for one main reason, to trap air bubbles in the fuel system, regardless of the name used, but the improper use of the names leads to confusion.

Add on Tank Descriptions:

This next term is often used to describe an add on tank, but the true design associated with that name does not fit our use in heli's or airplanes and when the name is used to describe the add on tank, it is incorrect.. 

  The “Header Tank” design

The basic definition describes its use as a gravity feed tank with a fill at the top and a feed at the bottom and, as its name implies, it uses gravity to provide “Head” pressure to a device. The “Head” pressure comes from the weight of the liquid in the tank, which is located above the device the “Header Tank” feeds. Great fuel system for model “A” Fords and stationary engines, lawn mowers and generators, etc. Does nothing for our heli’s unless you fly in one spot and remain level. Don’t tip the heli in flight with low fuel in the tank and don’t even think about inverted with this tank setup.

Following is an explanation of the two main types as used in helicopters. By the time you finish this article and study the sketches, and it will become clear why the "Stunt Tank" configuration should be the preferred method of set up over the "Aux Tank" set up. Keep in mind that the "Stunt Tank" as defined here will generally serve as any of the others, but the others will not generally serve as the "Stunt Tank". 

   “Auxillary Fuel Tank” design  (Diag. D)
The "Aux Tank" uses a conventional plumbing set up. It has a vent line at or to the top of the tank and a pickup or “clunked” (weighted) flexible line whose fuel pickup end is positioned at the back of the tank. The tank is then installed with the back of the tank, and hence the clunk, to the back of the machine. I have seen a few where the clunk is to the front of the machine.
Although this "Aux Tank" configuration is the most prevalent configuration used, part of my intention is to point out the shortcomings if using for helicopters, in particular where aerobatic flight is concerned. The "Aux Tank" is basically another fuel tank. It is subject to the same problems as the main fuel tank as used in helicopter doing aerobatic flying. The problem with "Aux Tank" plumbing is the high probability of an exposed "clunk" and delivery of air bubbles to the carb. The more bubbles this configuration traps, the more likely this clunk will be exposed to air as well. When the “Aux Tank” configuration is used as a “Bubble Trap” for aerobatic flight in a heli, I call this configuration an “Aux Crash Tank”. If you use this configuration for aerobatic flight, a crash is inevitable somewhere along the way. You just may not realize the true cause.

The “Stunt Tank” design: (Diag. D)

       The only real difference between the “Aux Tank” and the “Stunt Tank” is the pickup line plumbing inside this added second tank.
Where the “Aux Tank” uses a flexible, weighted pickup line, positioned at the rear of the tank, the true “Stunt Tank” uses a  “stand off” pipe, where the inlet inside the tank is located exactly in the center of the tank as viewed from ALL directions.
See Add On Fuel Tank Configurations Diagram ”A" - Stunt Tank".  The advantage of this system in aerobatic flying is that as long as the “Stunt Tank” is a little over half full, the engine will run and there is minimal chance that an air bubble will reach the carb, no matter what the maneuvers, or attitude of the heli. 

 As one example, the most notable difference will show itself under the condition of trapped air from the system, while doing a long tail slide,. See Add On Fuel Tank Configurations Diagram ”B”-Stunt and “C"-Aux - function in tail slide.   One crash from a flame out in a tail slide will convince you to use only the “Stunt Tank” setup. I have had flameouts from long tail slides with no Aux tank in place and from the “clunked” “Aux Tank” set up. I have also had erratic runs in big backward loops that were cured by using the “Stunt Tank” configuration.

What is really going on when you add a Stunt Tank into the system?

The main purpose of an added tank is to trap bubbles and basically, all added “inline” tanks will do this. The second most important need is for the tank to continue to deliver fuel, under your intended flying conditions, even after it has trapped air bubbles. These are the bubbles in the fuel line headed for the carb, caused by the end of the pickup line in the main tank being exposed to air. If these bubbles make it to the carburetor, consequences will occur. The first is that the engine will run leaner, but not necessarily quit.  Most pilots overcome this problem by running a richer main needle setting than would be needed if these occasional series of  bubbles were not in the system. The aerobatic pilot does not often realize this is what is really going on. Some will state that they never have a problem with bubbles and the heli flies/runs fine without the added tank, so therefore its not needed. The truth is, either of two main plumbing configurations to be discussed here will “fix” that problem, if it exists in the machine, and will usually allow the needle mixture to be set from a few clicks, to several clicks leaner, without jeopardizing the engine from lean momentary episodes. This can be verified with a lock in temp monitor, one that records and holds the highest temp reached during the run. Those who have added an inline tank and discover they CAN set the needle leaner or have more power evidence this fact. It's just that the reasoning is not always quite on target. 

That result then, leads to erroneous logic, and that is: “Because the fuel supply is “closer” to the carb, fuel is delivered easier”, so I can now run leaner needle settings. Another misconception is that if the added tank is on line with the center of the carb, this is what makes it “work” or that the added tanks acts as a “header tank” and this helps fuel delivery. It really doesn't add pressure feed to the carb from gravity assist. 

I will try to clear up some of the misconceptions:

• 1)      When adding a second inline tank, it will actually INCREASE the resistance of the fuel to flow to the carb. This is because there is a distance increase from the end of the pickup line inlet “clunk”, in the main tank, to the carb inlet. This distance has been increased by added fuel line required to plumb the system. The longer this distance, the greater the resistance to flow and the leaner the engine will run. If you add a secondary tank and can set the needle considerably leaner, it is because you have been pulling a lot of air somewhere in your flight. It could be from certain aerobatics or from fuel foaming due to vibration or both.

• 2)       As long as the added tank is in the “shadow” of the Main tank, it will not add any gravity feed (head pressure). The main tank location, high or low, as it relates to the centerline of the carb, will determine the ”head pressure” effect. This is mainly due to main tank volume versus "add on" tank volume. This add on tank is usually around 1-2 oz. capacity and will have little effect over a 10 or 12 oz capacity main tank if both are in the same "zone".

• If the main tank is low to carb centerline, a good needle setting at full tank will go lean at low fuel level. Conversely, with the low mounted main tank, the engine will run rich when the heli is inverted with a full tank. Placement of the added inline tank will have no effect from an added “head pressure” standpoint, if it is located in the shadow of the main tank, close to the centerline of the main tank and not the centerline of the carb. It (the add on tank) could add some effect if it were located higher or lower than the outer parameters of the main tank.
  The position close to or far from the carb will have no head pressure affect, either when in upright or inverted flight. There can be some effect though, in exaggerated nose down or nose up flight from a forward or aft location of the added tank. The best location is on centerline of the main tank, closer to the carb as opposed to closer to the main tank to negate head pressure effects in extreme  nose down attitude or nose up attitude. The difference being that the stunt tank won't combine with the head pressure weight of the main tank to add to - rich/nose down, lean/nose up as the tank will be outside the "shadow" of the main tank in these attitudes. The best way to cure this problem is to position the main tank on centerline with the carb. (click here to see Rap 60 tank mod)

•  Most machines will demonstrate a richer running condition when the main tank is emptied and the system goes to the added tank only. This again goes to the distance from fuel pick up to carb. Because you are pulling fuel from the added tank only, a shorter distance from carb to end of pickup where there is fuel, the distance and flow resistance is reduced and the engine goes rich. The amount that the engine richens can vary. Some machines will demonstrate a significant increased richness, on some it almost unnoticeable. I suspect that it is a difference in muffler pressure that makes the difference.

• 4)      It should be noted here that fuel pumps and pressurized systems added to the system will not fix any fuel draw problems related to height/location of the main tank. If that system does not have regulated flow to the carb, as determined by “demand” at the carb and not supply to the carb, it will still demonstrate high tank/low tank differences. This means that a pumped system, even with “loop back” feed, will not change the problem of a tank that is mounted low to the carb center line. It will only move the needle settings to a different range (needle can be in further).

• A pumped and/or pressurized system will help with a tank that is located farther from the carb, say, quite far aft of the engine. This again is due to increased flow resistance, related to the distance from the carb to the pickup clunk, that the pump overcomes, but it does not overcome the differences of head pressure from fuel levels in the main tank or Main tank height as it relates to carb center line.

  Misc. Notes:
 The “Aux Tank” can be used very successfully in a scale heli or for basic forward flight where the machine is not subjected to aerobatic flight motions.

 There is another configuration for the plumbing of 2 tanks, called “Dual or Twin Tanks” that are plumbed similar to the “Aux tank” set up, and is used in twin engine set ups or for extra capacity only. click here for info. Twin Tanks Setup
:)

         11/29/02
         Pete Schmidt
         AMA 6314
Pete’s Hobbies/Precision RC Products