MIG-5 DIS200 Pressure Fuel System
| Twin engine Combat planes must have very reliable engine operation. The first twins I built used clunk tanks assisted by muffler pressure. A tank in each nacelle and a throttle servo on each engine was required to run the engines. For the MIG 5 I wanted to simplify the fuel system and make it more reliable. A pressure bladder is simply a length of 1/4" latex tubing fill to pressurize it with fuel. In SSC airplanes bladder tanks have had great acceptance. The challenge with a twin is to use one bladder to fuel both engines. A single bladder allows a single throttle to be used to shut off the fuel flow and kill the engines. As the MIG5 design was developed the initial thinking was to put the bladder in the fuselage above the wing. The thought was to use a one way valve at the rear of the bladder to fuel the bladder and a Tee fitting in the front of the bladder to split the fuel stream to the engines. Another requirement was to be able to easily remove and replace a bladder. After much thought the idea of putting the tank in the fuse was abandoned due to the complications of getting the bladder in and out. This led to the adoption of a wing mounted fuel bladder. To keep the balance of the twin equal on both sides, a tank design was developed that would use two small bladders connected by a Tee fitting. Please look at the drawing below to see this design. The design was started by first making a "bladder compartment". This is simply a clear plastic sleeve purchased at Home Depot or Lowes. Its real purpose is to slide over fluorescent lights to contain the glass splinters if the lamp is broken. To determine how long the tube compartment would have to be a length of bladder was made up, slipped into the 1-5/8" diameter tube and filled. Testing showed that one inch of 1/4" latex fuel bladder would hold two ounces of fuel and it would expand slightly over two inches in length. The second photo below shows the mocked up duel chamber bladder system and the fittings used to assemble it. The tubing, and all fittings are available on line from www.mcmaster-carr.com and can be found in the on line catalog on pages 97 and 112. The third photo shows the fatal flaw of the dual chamber system - only one side would inflate. As soon as one side started inflating the pressure to fill that side was much less than required for the other side to "break" and fill. After what seemed like hours of testing the idea was abandoned and the 10" long compartment with a single single bladder was decided upon. |