|
Flap Positions
On UL and Calypso models, you have to operate with the flaps set slightly down to ensure you get the full flap deflection to achieve the stall speed required for a Microlight. If you wish to retain the flaps running 'clean' when retracted and keep the stall speed setting it is best to purchase from ST a different flap plate which allows you to achieve this. Fuel All current Jabiru engines are able to run on Avgas 100LL or Mogas above 95 octane. However the PFA have now produced a formal procedure for approval of Mogas in your Jabiru and this needs to be followed and signed off by your Inspector prior to running with Mogas. The PFA's move has been motivated by eco considerations and the thought that Avgas will be phased out at some point in the future. I personally like to run with a bit of a mix in my tank because Avgas promotes cooler running, and makes the mix less likely to be prone to detonation. Also the boiling temp for Mogas is not much above ambient. Poor tank venting can cause fuel draw problems. Mogas is more prone to vaporisation. Incorrect float levels can also provide a major influence on smooth running. Earlier engines required shims under the cylinder bases to lower the compression for Mogas use. Kinked fuel lines from the tank to the engine and fuel pump valves can also adversely contribute to restricted fuel draw. Wing tanks and gravity feed are a good idea. Reminds one of the model T Ford situation. Fuel tank under the seat, with very little fuel going up hill fuel drain could become uncovered. Many a motorist of yesteryear overcame this by simply reversing up the hill. When we think about mechanical pump, boost pump, and gravity feed, we’ve got a lot on our side. Wing tanks now have a sight gauge, which is an indicator only of fuel content, accurate flight planning with fuel burn figures is the prudent way to fly. Fuel Pump and Fuel Pressure A few owners have taken it upon themselves to install a pipe to allow cool air to flow to the fuel pump. This is a good idea especially if using Mogas, which has a higher likelihood of fuel vaporisation. While we have never done this ourselves or found a need to do it, isn’t a bad idea though. While on the topic of fuel pumps, recently we came across a problem. We had things running really well, 3lb spring driving around 2lb of fuel pressure to the Carby and pump pushrod of correct length, when out of the blue the diaphragm in the pump was changed to a heavier type (black) by the manufacturer. This set up with the normal spring led to fuel starvation. After much testing we found we only needed to change the spring to a “gold coloured” one of 4lb rating. This the allowed the pump to deliver around 2 lb of fuel pressure and up to 3lb with the boost pump on. Engines exhibited a lower fuel pressure on the dyno and various investigations followed. The confusing issue was that it was not happening all the time. Following a successful solution to problem, necessary advice was sent to owners and dealers of all engines involved and follow up mailing of new spring and directions for the job. Changing the spring is not a difficult job. The fuel pump change occurred from s/n 1292 (2200) and s/n 347 (3300). Fuel Burn Jabiru models are being produced with fuel tanks in the wings. This has been a welcome upgrade. Again simple sight gauges provide an indication of fuel quantity. Prudent flight planning would consider fuel required (fuel burn) from the trip. A new engine will burn more in the early hours, here we can estimate. Best advice is to begin the trip with full fuel; a few hours later refill tanks and calculate exact fuel burn. You now have a known endurance. Expect 5-10 hours of engine life before significant drops in temps and freeing up of the engine. Fuel burn will also be affected. In general for a Jabiru aircraft the 4 cylinder at 75% power will use between 13-16 litres per hour while the 6 cylinder will use between 22-26 litres per hour. There are variables here also that will affect this. Fuel Type As stated in the manual, Jabiru engines are able to run on Avgas 100LL or Mogas above 95 octane. Operation on Mogas has always some risk of vaporisation and detonation and slightly higher operating temps. Fuel may not always be uniform depending on company additives, time of year etc so the use of Avgas is the preferred option. Earlier engines required shims under the cylinder bases to lower the compression for Mogas use.
|
