Step 5. Decide on the type of propulsion system

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

 D_P = { 4P_max / (π n_P P_bl ) } ½ 

And reworked to yield

P_bl = 4P_max / π n_P D_P²

Given D_P =    Propeller diameter

            P_max =  Max power per engine (hp)

            n_P =     Number of blades

            P_bl =     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 P_bl 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 P_bl    Even the smallest lightly loaded in the FAR 23 twins range of P_bl 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.  

Step 4. Prepare a preliminary (scaled) drawing of the cockpit and fuselage.

The above picture represents an increase of the fuselage length just aft of the wing trailing edge of one full bulkhead. Plus nose is longer than the old position of the spinner. All up taking the length from 5m to 6m, at this stage the tail fin ruder and the all moving horizontal stabiliser have not been increased in size.  Further examination is still required.

The main wheels have also been moved aft an inch or two to allow for clearance to the longer (in span) central box spar. Increased from 2m to 3m.

Step 3. Select the type of configuration to be designed

Given it is nearly always desirable to place the fuel, payload and empty weight C of G at the same longitudinal location. Doing this limits the C of G travel, resulting in a configuration with less wetted area, due to less need for trim control.


Using the definitions of outlined in Chapter 3.3 “Configuration Possibilities” on page 95 of part II, the following description applies given the mission statement. This aeroplane is land based, conventional, twin tractor engines mounted in wing nacelles, a cantilever low wing, zero sweep, full span flaps with droop ailerons and a tricycle retractable undercarriage.

Given this is a development of a known design; many of the choices are already made in terms of configuration layout. The fewer changes from the original design configuration, the better!

Step 2. Perform a comparative study of simular aeroplanes

I have found other examples accross the internet of light multi engine aircraft.  Unfortunatly no have been anywhere near my megar budget.


1. Company: Zenith Aircraft. (USA)

Model: Gemini CH620

Type: Kit development

Website: www.zenithair.com/gemini/gem-what.htm

Note: 2 Jabiru engines, fixed pitch and fixed gear, development is on hold.

2. Company: Tecnam. (Italy)

Model: P2006T

Type: Certified aircraft

Website: www.tecnamaircraft.com/Tecnam_P2006T.htm

Note: 2 Rotax engines, 4 seater, retractable gear. As at 2008 cost EUR $280,000 for a basic model

3. Company: Aeroprakt (Russian)

Model: A28 and A36

Type: Complete aircraft.

Website: www.aeroprakt.kiev.ua/eng_html/main.html

Note: A36 built on request and A28 still under development.  I did find the A36 listed on a US disributors web site for $175,000 USD

If anybody finds any more, please leave me a comment.

Step 1. Review Mission Statement

This step does seem straight forward.  No changes yet and work done thus far is inline with the original concept. 

Preliminary Design Sequence I

According to Part II, there are 16 steps in this sequence of design. They are referred to as class one methods. For a summary of each of these steps can be found on page 11 of chapter 2. Note these sizing methods only have an accuracy of ± 10%.   So step by step I will try and follow them and post the results here. Some of these steps look complicated, oh well...

Sizing to cruise speed requirements

From Chapter 1, part 3, page 162 thru 165 it can be seen that formula for cruise speed is;

V = 77.3{h_p(W/S)/σC_D(W/P)}^1/3                     note: V is in mph

or

V = (550SHPh_p / 0.5ρSC_D) ^1/3

	Sea Level		5000 ft
	mph	kts	mph	kts
Vcr @ 50% power =	123	107	130	113
Vcr @ 75% power =	141	123	148	129
Vcr @ 100% power =	155	135	163	142

Note: with 5 hours of usable fuel on board, at 75% horse power then range is 600 Nm at sea level.