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Jabiru Aircraft
Pty Ltd
Installation Manual
Jabiru 3300 Aircraft Engine
REVISION
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Dec 2016
Page: 50 of 56
Figure 57 shows an aircraft at varied angles of attack to the surrounding air. The cowl inlets and outlets
must both be designed to work effectively at all angles which the aircraft will normally experience.
Figure 58 shows two different cowl outlets
– one is basically an opening in the flat bottom of the cowl, while
for the other the opening is oriented at 90° to the airflow direction. Vertical orientations (Deep Outlet) give
better pressure differentials and are less affected by aircraft angle of attack than horizontal (Long Outlet).
Figure 58 also shows the lower firewall section of a Jabiru Aircraft. The lower part of the fuselage has two
large ramps moulded in which increase the depth and area of the cowl outlet (and also provides mounting
points for the rudder pedals). This type of feature is not mandatory for good engine cooling but it does help.
An alternative is to make the bottom corner of the firewall as smooth and rounded as possible to help airflow
and minimise the outlet restriction.
Some aircraft types have a flange running around the firewall. Particularly on metal types, this flange is a
useful way of mounting the cowls. However, if the flange runs across the edge of the firewall where the cowl
outlet is located then it causes a significant flow restriction. Figure 59 shows a drawing of the lower section
of a firewall with a flange of this type. Wherever possible flanges across the cowl outlet should be avoided.
Alternatively a fairing can be built inside the cowl to smooth airflow over the lip & reduce flow restriction.
Figure 56: Lip to aid cooling as installed on a Jabiru.
Figure 57. Affect of Angle of Attack on Cowl Outlets
Line shows direction
of ambient airflow.