EXTERNAL FUEL TANKS

ON A

RANS S7S

2011/11/17

 

 

There are times when a little more fuel than the stock 18 gallons would be nice to have. I wanted the additional fuel but did not want it as far aft as a baggage tank would put it because we were planning a trip which would include some survival equipment and a rear seat passenger. I also wanted tanks that could be easily removed for the bulk of the time when I was flying locally.

 

This is the terrain that got me thinking about having lots of fuel:

 

 

But additional endurance isnít the only reason for the tanks. Most of my flying is on floats but now that I have the 100hp engine, marina gas is not high enough octane so carrying extra fuel could be helpful. There is another advantage to having two separate fuel sources (I can select left or right external). High octane gas is only needed for full power operation. It is perfectly acceptable to cruise using lower octane gas. So nowI could run with high octane in the wing tanks which would be used for takeoff but I could fill up the external tanks with lower octane marina gas which could then be used during cruise power settings (or visa versa).

 

A third reason for these tanks is that they allow you to get full use of the fuel in the mains knowing that you have more time in the fuel in the external tanks.

 

These tanks are available from dealers who stock parts for the Chinook and Beaver. I got my pair from LEAF but they are available other places including the Chinook manufacturer, ASAP, in Vernon BC. They cost about $100 per tank plus fittings and hardware.

The tanks hold about 5 gallons each. The installed weight of the tanks total is 15 lbs. I estimate the cg to be at 42Ē which is well ahead of the wing tank arm of 52Ē

 

I made use of the existing spade/eye bolts at the jury struts to bolt on the SS brackets (.090 304 SS). Strictly speaking these bolts should be another dash number longerto get a few threads showing but they cost over $20 a piece. I plan to weld up some new ones from stainless. The Chinook uses a similar system but only clamps the brackets to their struts with 2 stainless clamps. I used the bolt and one clamp at the front and just the bolt at the rear. The aluminum channel is made from 040 6061 and takes a bit of fitting to clear the caps on the tank. The tanks come with two 1/4" aluminum lined holes for the mounting bolts.The horizontal bolt at the rear strut has a tube on it so that the aluminum, channel wonít be deformed when you tighten.

 

You canít make a simple U bracket at the front because the tank needs to be moved to the rear about 3/4" so the filler cap can clear the strut. That is why it is an odd shape.

 

This mounting feels pretty solid.

 

The filler cap and fuel outlet cap are stock plumbing parts. The fuel line cap from the dealer has a long barb going straight thru which I didn't like because it would result in a big loop of fuel line above the tank. I bought new ones (3/4" MIP plastic plugs) and made up barbs into a 90 degree path. You can buy the screen/check valve pick up from the dealers. Most people get rid of the check valve because they are unreliable. I just expanded mine to let the balls drop out. That is a quick disconnect in the line. The lines are held on to the rear strut leading edge with aluminum tape.

 

Below the flap handle I mounted a left/right/off valve. After the existing Rans fuel shut off, I put in a T to connect to this new valve output. The new valve moves pretty easily and once when I was poking around there I inadvertently moved it to on. Without check valves and with the main valve open fuel will drop down from the main tanks into the selected outboard. To prevent this Iíll mount a plate with a slot in it to hold the valve handle in the off position. The plate will flex up to release the handle.With this plumbing you can select either the mains of one of the external tanks.

 

 

Here is the routing near the fuselage:

There could be a better method than punching a hole in the fabric like maybe a quick disconnect at the fuselage. Let me know if you have one.

 

The engine mechanical pump easily pulls in the fuel from these tanks. For safety on subsequent installations where there was no backup pump, Iíve added and electric pump after that T with a cigarette lighter plug for power.

 

This is a link to Chinook††† ASAP who sell the tanks:http://www.ultralight.ca/Framesets/chinook/chfuelfr.htm

Probably the guys with show quality planes might not like the cosmetics of this especially when these lines would stay on even when the tanks are removed. Functionality suits me just fine however.

 

Peter

 

Peterc at pipcom dot com