Header Tanks
 

Just wondering if somebody could explain how header fuel tanks work and when they would be used.

Its just that I noticed somebody using one on an aircraft I was thinking of getting - they had a Saito 125 FS on the machine. I've never come across them on a model before.

Are header tanks specifically for four stroke engines?

Cheers?

RE: Header Tanks
 

I've seen header tanks primarily used on helicopters, but I've also used them on airplanes myself.

The idea is that the small (~2oz) header tank remains completely full of fuel (no air), so vibration doesn't cause the fuel to get air bubbles in it or foam up. The main tank will get air bubbles and foam up, but very little air gets into the header tank from there. It works pretty well. I used one on a small SPAD that I built. I couldn't seem to isolate the engine vibration from the fuel tank very well on that plane (which is what you usually do to keep the fuel from foaming), so the addition of a header tank helped quite a bit.

You just connect the header tank between the main tank and your needle valve. I use my needle valve line to fill the tank, so I just fuel/defuel the same way I would with a single tank setup. The header tank fills first, then overflow exits it's vent line and fills the main tank, then when that's full, fuel comes out of the main tank's vent line (which goes to the muffler's pressure tap).

RE: Header Tanks
 

Header tanks work well in a variety of situations like pkh stated above. . I used one on both a heli and a Goldberg Extra that I was having problems with with a Super Tiger 90 installed. It kinda fools the engine/carb into thinking the fuel tank is always full and
thus almost eliminating the "going lean " condition that occurs when the fuel level drops in the main tank. The header tank stays full
as long as the main tank has fuel to supply. It can be a good alternative to using a pump but can be used on 2 strokes as well as
4 strokes.

RE: Header Tanks
 

Thanks pkh,

I'm assuming then that if the header tank is always full, then there's no need for a clunk (in the header tank)? Is that right?

Cheers

RE: Header Tanks
 

You do want a clunk in the header tank for a few reasons. First, you want to be able to completely empty both tanks for storage, so you want a clunk at the bottom of both tanks to empty them. Second, if you can't see the header tank (e.g. if it's enclosed inside an airplane fuselage), you'll want to be able to use all the fuel in the header tank if you happen to fly too long and empty the main tank. Third, a little bit of air will end up getting into the header tank, and you want to be sure you've got a clunk in there to pick the fuel up from the bottom, and not get the air that will bubble up to the top.

RE: Header Tanks
 

Thanks for the brilliant feedback - I have one other question?

Is the position of the header tank (in relation to the main tank and/or main needle to the carb) critical?

RE: Header Tanks
 

Hi!
Totally unnecessary if you have a good tank set-up!
Best tank is to use a Tettra "Bubbleless" tank!
second ...use a ordinary tank set up with "Uni-flow set-up" (two clunks)!

RE: Header Tanks
 

Good question. I've always tried to keep the center of both tanks about level with the carb. I believe you want the approximate center of mass of the fuel in both tanks about level with the carb (in level flight) to help keep it from siphoning. You really want the muffler/carb pressure/vacuum to determine the fuel flow, not a siphoning effect.

RE: Header Tanks
 

Thanks guys,

Jaka,

Can you explain the "Uni-flow set up" please - sounds interesting!

Cheers

RE: Header Tanks
 

Jaka,

I totaly agree. All my aircraft have Tettra " Bubbleless" tanks. the only way to go.

Ed S

RE: Header Tanks
 

The header tank does not eliminate the "going lean" condition. It does provide a collection point for any bubbles that might form which will then be vented back to the main tank. Location of the header tank is inconsequential, other than adding additional head loss due to the length of fuel lines, I say inconsequential because the pressure in the header tank will be determined by the pressure from the muffler on the main tank and the difference in fuel height between the main tank and spray bar. That is until the main tank goes empty. Then the header tank becomes the main tank. If the header tank is located above the main tank, you will see a sudden increase in head pressure at the spray bar, causing the engine to go rich. If its located below the main tank, when the main tank empties, the pressure at the spray bar will drop suddenly causing the engine to go lean. Most people locate the header tank either above or on line with the main fuel tank.

Brad

RE: Header Tanks
 

In cases of problamatical fuel installations I totally agree with Jaka and Ed Smith above. A bubbleless type tank will provide superior fuel flow characteristics to a conventional clunk type muffler pressure system (bubbless tanks have no clunk) and is superior to a header tank setup as well as a much simpler installation. Bubbless tanks are typically hard mounted with no need for foam supension.

John

RE: Header Tanks
 

So how do you fill the bladder-type tanks, and how do you know when they are full?

Do you reverse the pump and suck the air out of the bladder before you fill it? Does the pump just kick the filler fitting off the line when its full?

RE: Header Tanks
 

Some of the information I have read above I feel is misleading and difficult to understand. Here is my experience, you decide. I use a 2 OZ header tank on two of my planes. On my SPAD Flying Lawnmower the main fuel tank is located inside the wing, in front of the 2" high spar, and mounted sideways. I mount the header tank on the engine mount, right behind the engine. The center of the main tank is probably about 12" behind the engine. I have never had a fuel problem with this arrangement. The mower did loops and rolls, flew inverted, but could not go vertical long since it only used an OSFX engine driving an APC 12X4 prop. In four years of frequent flying I never had a dead stick. My second plane with a 2 OZ header tank is a pusher SPAD Canard. The main tank is mounted in front of the wing, backward, about 12" (maybe a little more) away from the carburetor. The header tank is mounted just in front of the engine. When I go vertical with the Canard I sometimes hear a slight engine problem but the engine has NEVER stopped. I plan to move the main tank about 3" closer to the engine to improve the fuel flow. The Flying Lawnmower never once in four years of flying had a fuel problem. On both planes, muffler pressure goes to the main tank pressure connection; the main tank outlet connects to the header tank inlet; the header tank outlet goes to the carburetor. I have never notice bubbles in the lines from the main tanks to the header tanks. The line is very long and very visible on both planes.

RE: Header Tanks
 

The tank is filled just like any other tank, through the feed line. You know when it is full just like any other tank, fuel comes out of the overflow.

Ed S

RE: Header Tanks
 

Do you think that a header tank will cure a problem i have with my exisisting aeroplane? :

The engine behaves during all phases of flight and during 'normal' aerobatics, however the moment I flick the aeroplane with full elevator, the engine quits - every time! - I'm convinced its because of the weight of fuel during the high 'G ' manoeuvre exceeding the muffler pressure and draw from the engine - allowing the fuel to flow back into the space in the main tank.

If the header tank is always full, then under high G conditions the fuel will have no where to go.

What are your views?

RE: Header Tanks
 

The high G-Maneuver is not likely to cause fuel starvation for the following reason. Fuel line pressure is at the spray bar is a function of the difference in fuel height between the spray bar and the fuel level in the tank plus the pressure from the exhaust system. In the high G move, the fuel pressure at the bottom of the tank will increase due to the centripetal force, since that force is acting toward the belly of the airframe. Pressure outside the spray bar has not changed, so the increased differential pressure between the atmosphere and the bottom of the fuel tank will result in a rich condition, not a lean condition.

Secondly, have you ever tried pinching the fuel line and seeing how long it takes for the engine to quit? Its not instantaneous, since there is some fuel remaining in the line, and in the crank case. The high-G maneuvers normally happen very quickly, and not slow enough for the engine to starve.

I'd look at engine tuning, making sure the low end is properly adjusted, and that you're not running the high end too rich. Also change you're glow plug. Just because its new, doesn't mean its good.

Brad

RE: Header Tanks
 

Thanks Brad,

I've not thought about it in that way - essentially you think I might be getting a 'rich cut' or 'flood' as opposed to fuel starvation. I'll definitely look into this and fine tune my adjustments prior to my next flight - I'll also change the plug for what its worth. Cheers.

RE: Header Tanks
 

A bladder tank solves all of those problems. No matter what the manouvre or the attitude of the airplane. there is always fuel at the pickup.

Ed S

RE: Header Tanks
 

HGi;Maybe a silly question but where can I get the tetra,etc tanks?Live in area with no hobby shops all mail order but don't think have seen them at tower?Thanks

RE: Header Tanks
 

Go Here.

http://www.centralhobbies.com/Fuel/fueltnk2.html

Ed S

RE: Header Tanks
 

Hi Ed Smith
If the problem is that fuel pressure to the carburetor is very low (but adequate for level flight) because the fuel tank is far away and/or very low relative to the height of the carburetor, I do not understand how the bladder tank will solve this. I have always been interested in the bladder tank. Please read above where I have described my use of a header tank. I believe that when my carburetor sees very low pressure, or perhaps even zero pressure because my plane is going vertically up, the vacuum at the carburetor throat is still able to draw fuel from the header tank. I want to understand this better.

RE: Header Tanks
 

www.darrolcady.com
You can get them here too.
The idea of the bladder or even Jett's bubbless tanks are that the fuel is inside a bag (bladder is a mis-representation since there is no pressure applied from this bag) that has been sucked dry of air and filled with fuel as mentioned. The exhaust pressure is on the amibient side of the bag within the tank so no air or pressure hits your fuel. The bag is full of fuel till it runs out without any air replacing the volume.

RE: Header Tanks
 

Hi Villa,

I have never used a header tank so I realy cannot answer your question. For many years before the bladder tanks came along I used the "Uniflow" style of tank. I try and position the tank on the balance point of the airplane. There is logic to this.

The picture shows a "U-Can-Do" powered by an OS 90 FS. The bladder tank can be seen on the balance point a long way from the engine. There is no pump involved. What can I say? it works.

Ed S

RE: Header Tanks
 

So how do you know when the bag is full? Is there a one-way valve on it for overflow?