Jabiru 912 - Why Not?
From Phil Ainsworth - Joint Managing Director, Jabiru Aircraft
After 4 years development of the Jabiru aircraft a CAA Certificate of Type Approval was awarded on 1 October 1991. One month later, the Italian engine manufacturer (KFM) advised Jabiru that they had decided to cease aircraft engine manufacture. After an exhaustive investigation of all alternatives, including a visit to Italy to examine the possibility of manufacturing the KFM engine under licence, Jabiru decided that the only viable alternative to completely redesigning and recertifying the aircraft was to design, build and certify a new Australian aero engine.
Many people have asked us why we went to the trouble of designing and certifying a new aero engine when all we had to do was to fit the Rotax 912. The following attempts to explain our thought process and our final decision to create the Jabiru 1600 Aero Engine. The first problem to address is the increased weight of the 912. On the hook, ready to install the 912 weighs 81kg (not the advertised weight, but the installed weight complete with exhaust system, oil cooling system, liquid cooling system, carby induction & heat system, oil tanks, fluids, hoses, engine mount frame, support brackets, clamps & hardware).
This compares to the 56 kg of the Jabiru 1600 engine and 63kg for the previous KFM112M. The additional 27kg well in front of the CG position will require additional ballasting. Note that the KFM required 2kg of lead in the Ventral Fin and that the Jabiru engine requires nil ballast. The 912 would therefore require around 6kg of lead to bring the CG within flight range.
The empty weight of the Jabiru aircraft is 235kg plus 32kg (27 additional engine + 6kg ballast) = 267kg which allows only 162kg usable load (430-267), or 2 x 80kg occupants and 3kg of fuel. Note that the Jabiru has an approved MTOW of 430kg which is both a structural and stall speed limitation. Remember that stall speed increases with weight. The Jabiru aircraft has a small (85sqft) wing with slotted and large stall strips and is stretched to its limits to achieve the necessary 40kt flapped and 45kt clean stall speeds. So, lets add more wing, this reduces stall speed but adds weight and doesn't solve anyCG problems, it also increases the need for extra fuselage length, extra tailplane, fin and rudder area to compensate for theadditional wing area. Again more weight!
So let's upgrade the gross weight. This requires a complete revalidation of the airframe structural loads and remember that this is the first composite aircraft to be Type Certificated in Australia, and one of very few in the world. Now we are still overweight but we have, in the process, dramatically changed the design of the aircraft. So, back to flight testing: a full flight test programme which took 7 months of hard work in the first place, will have to be redone. But now we have the additional problem, that the aircraft with 79hp on board now exceeds 100kt straight and level, and therefore we are required to complete a full ground vibration programme at a cost of around $30,000 minimum, and another 3 months work. Also, we must now mass balance the ailerons and rudder - more weight: (say another 5kg). So, we have... ....added extra wing area ....lengthened the fuselage ....increased fin, tailplane & rudder area ....added 5kg mass balance... and we only started with 2 x 80kg occupants + 3kg fuel! As you can see, there is no clear solution to the weight/CG/stall speed/structural problem. But we haven't finished there!
We now have to match the propeller to the geared 912 which means a new propeller design and propeller certification ($10,000), assuming we can transmit 79hp on a 54" diameter propeller at the low final drive RPM output of the 912. We can't inrease the prop. diameter without increasing undercarriage height, because we are at minimum clearance now. So, assuming we have achieved all of the above and probably redesigned the undercarriage for more ground clearance and for increased MTOW (and the extra weight on the nosegear), we now face noise Certification again (another $10,000).
We now have a 912 Jabiru. What have we really got? The market price has increased by at least another £10,000. As an Australian manufacturer we are now totally reliant on an imported engine, as we were with the KFM (and remember that story), with all the vagaries of supply, currency variations, pricing policies of a European supplier. The aircraft now burns 16L/h compared to the Jabiru engine's 12L/h. We just lost another 1 hour endurance and increased our operating costs by $3.60 per hour for fuel, plus the high service costs of the 912. The entire 912 Type Certificated engine has to be sent back to Austria for overhaul at TBO and the 912 UL engine requires a replacement Crankshaft at TBO. Jabiru offers a complete fixed price overhaul for $2500 (labour and parts).
After all of this you might understand why we decided that building an engine, creating another product and a new Australian industry, was more productive than fitting an imported engine. It has not been an easy or trouble free road. Despite detailed testing (some being leading edge technology), we have problems with conrods, crankshafts and oil pressure, but these have all been attended to promptly, at no cost to our customers. All engines are being brought up to the latest Type Certification configuration, which includes the new crankshaft, at Jabiru's expense. When Type Certificated in late February 1994, the Jabiru 1600 engine will be the first aero engine ever designed and certificated in Australia. 1994 will see exports of engines to New Zealand, North America and Europe-exports of Australian technology rather than more imports from Europe.