The reason I use balloon tanks is that they can be crammed into a very small space. This is a necessity on some of my models.

   This compactness also enables a favourable tank location relative to the needle valve.

   Since no tank access hatch is required, the engine bulkhead can be backed-up by a strong closed-box structure.

   When fuelled, the tank contains no air. So only neat fuel can be delivered to the engine.

   The plumbing is very simple; only one pipe from tank to engine. This pipe is disconnected for filling.

  The photo shows two sizes. The small one takes about 2oz, uses a 9" balloon, and has a cap dia of 15mm. The larger one takes about 8oz, uses a jumbo balloon, and has a cap dia of 22mm.

   I have used them with 5% nitro castor-based fuel. They remain usable for about 3 months. I remove them from the aircraft after a flying session, and store in a light-proof bag.

   You need to devise a refuelling procedure which ensures zero air in the balloon. For a small tank I employ a 60ml syringe and a spring clip. For a large tank, I use a reversible pump and a measuring bottle.

   Do not overfill. The balloon must not pressurise the fuel.

   Prior to first-use, a new balloon should be air-inflated to increase its slack capacity.