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F
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.
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