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Jabiru Aircraft
Pty Ltd
Installation Manual
Jabiru 3300 Aircraft Engine
REVISION
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Dated : 1
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Dec 2016
Page: 53 of 56
i.
Wherever possible the cowl outlets should be vertical openings with lips that come close to the
propeller
– as close a possible without the blades hitting the cowls.
ii.
The propeller blade must have significant pitch and chord in the section which passes over the
outlets.
iii.
The cowl openings should each be reasonably small. As each blade passes the opening it will
create a suction in the cowl behind it, but if the cowl opening is large this effect will be dissipated.
Alternatively, larger openings can be divided up by fitting louvers or vanes.
Augmentor type exhausts (Figure 62) can also be used to suck air out of the cowlings.
In pusher installations the inlets into the cowl are harder to get right than in a tractor installation. Intake
ducts should be as straight as possible with no sharp corners or other restrictions to the flow.
The position of the cowl air inlets is critical
– inlets on the upper surface of the aircraft are generally in low
pressure zones while those on the underside are normally in high pressure zones. Depending where the
inlet is located, the area ratio between inlet and outlets may need to be modified.
12.8
Amphibian or Seaplane Installations
Water taxiing requires relatively high power settings for long periods and this is often the most critical
condition for cooling systems in these aircraft.
Increased duct size (scooping more air through the engine) may be necessary.
For amphibian or seaplane aircraft using a pusher engine installation the methods outlined above can use
the propeller to suck air out of the cowls, but ultimately the effect is limited and can conflict with cooling
requirements in other modes of flight. For these installations some form of active venting for the cowls
–
such as flaps, fans or an augmentor-type exhaust system (See Figure 62) may be required.
Figure 62. Augmentor Exhaust System
12.9
Slow Speed Installations
Installations where the cruise speed is below around 70
– 80 knots are considered slow speed installations.
Jabiru ram air ducts are available for slow speed installations. These are larger than the ducts used for
faster aircraft.
Increased duct size (scooping more air through the engine) may be necessary for slow speed installations.
Increased outlet size and more aggressive outlet lips may be required.
In some of these installations where the airframe has a lot of drag it is preferable to do away with cowls
altogether and run an open installation. Aircraft such as the Thruster (Vision), Drifter, X-Air and some RANS
models are examples of this. In these cases large ram air ducts are used, and the rest of the engine is
exposed to the propeller wash for cooling.
Air is sucked out of the cowl and entrained
with the exhaust inside the augmentor tube.