Development of the Jabiru J170-D
Earlier this year our J170 model came in for some attention to improve its handling and usability. The J170-C had received a handling upgrade in January 2011 which had been well received by customers and we wanted to take the aircraft a step further. The basic idea was to move the engine forward which has the effect of moving the empty CG of the whole aircraft forward. In a J170 the entire payload (crew, fuel and baggage) all are positioned aft of the CG, so by moving the empty CG forward the aircraft is, in theory, able to carry more payload without running into CG issues.
Easily said, but as it turned out this simple idea caused a raft of technical issues. With the longer engine mount, the engine now had too much leverage on the airframe so that at low speed and high power the aircraft wanted to follow the engine around to the left - no matter what the aileron and rudder said. The directional stability also suffered a bit as Dutch Roll made an unwelcome return. Finally, moving the engine had increased the inertia of the airframe enough that in some of the more unusual and extreme spin conditions the aircraft didn't recover as it should have. A few months of work later, the final list of changes is a long one: To cater for the engine move we needed a new engine mount, cowlings, exhaust, ram-air ducts and throttle cable. We fitted new-shape ailerons which worked better at low speed and to improve the directional stability we fitted a new-shape fin and rudder. These actually work so much better that we were able to change back to the old (small) ventral fin under the tail - though the aft CG limit of the aircraft did move forwards slightly. we also changed the angle of attack of the horizontal tail to improve the elevator performance.
All in all the testing on the J170 in the last 12 months or so has totalled up to over 40 flight hours and something like 250 individual spins - all carried out by independent test pilot Keith Engelsman. Structural testing also required the application of about 16 different load cases on the fin and horizontal tail. Because of all the changes compared to the earlier J170-C we called the finished aircraft the J170-D. The finished aircraft flies similarly to a J170-C - directional stability is quite good and does not need too much pilot input. Banking turns without using the rudder result in the balance ball coming out of centre by about half a ball - which is about the same as the larger J230. Dutch Roll - which is the tendency of the aircraft to draw a “∞“ shape on the horizon with its nose - is also much improved over the original prototype. Again it's now similar to the J230 which is a pretty "solid" handling aircraft.
The new engine mount uses more engine thrust offset than the previous version which has made the aircraft quite neutral with different power settings - on take off just a tiny amount of right rudder is required to keep the aircraft straight while on landing (low power) a small amount of left rudder is needed. This is a real plus in a go-around situation where the pilot needs to go from very low to full power quickly and at low speed - previously when the power was applied the torque reaction tried to drag the aircraft off to the left but now it stays straight with much smaller inputs. Instructors and students particularly will appreciate this feature.
Similarly the new angle of the horizontal tail makes the aircraft easier to trim - on a recent cross-country flight in a J170-D we found that the amount needed to move the trim lever between different modes of flight climb, cruise, descent - was very small. The aircraft was sensitive to trim and just needed a touch here and there to cope with major changes in power or angle. So in the end the bright idea turned out to be a bigger job then we expected - but the end result is worth it in an aircraft which is significantly improved in many areas and should make an excellent trainer for years to come.