With the choice of two Jabiru 2200 engines, mounted on the wing in nacelles, only the propellers need to be considered. According to step 5.6 on page 128, chapter 5, part II, the propeller diameter and number of propeller blades can be determined by the following formulas
DP = { 4Pmax / (π nP Pbl ) } ½
And reworked to yield
Pbl = 4Pmax / π nP DP2
Given DP = Propeller diameter
Pmax = Max power per engine (hp)
nP = Number of blades
Pbl = Propeller blade power loading (hp/ft2)
Given the Jabiru engine manufacture recommends and produces a 60 inch diameter, two blade propeller for their motors. First impression is to go with this; however there is a problem with the geometry and requirement to be “road able”. The distance between the fuselage and engine nacelle is limited, and maximum propeller diameter can be no bigger than 48 inches or 1200mm (a proximity only 100mm clearance to fuselage).
Utilising the Pbl formula above and comparing the results for smaller diameter propellers we find
| | Pmax | DP | nP | Pbl |
| Jabiru Propeller | 85 | 60 inch or 5’ | 2 | 2.2 |
| Smaller Diameter | 85 | 48 inch or 4’ | 2 | 3.4 |
| Smaller Diameter plus more Blades | 85 | 48 inch or 4’ | 3 | 2.3 |
Please note only the two and three blade propellers are considered as the geometry of any more blades will exceed the max allowable width to transport the aircraft “on the road” with the propellers still attached.
Values for FAR 23 certified twins range 2.8 to 4.8 Pbl Even the smallest lightly loaded in the FAR 23 twins range of Pbl have higher values, suggesting a smaller diameter prop would be appropriate at 43 inches. However it can be seen from the above table, that a three bladed propeller at max diameter, yields a very simular blade power loading when compared to the original Jabiru installation and should not have an adverse effect on engine life or performance.
The above figure shows the deposition of engines on the wing stubs; note the physical limitations, such that with the wings detached the centre section span must be less than 3m wide to be legally road able in NSW metropolitan areas.
