Wilksch diesel conversion.

[MikeGodsell]

Very interesting report in latest LAA mag by Peter Fines on his Wilksch powered Jodel. But I would like to know a bit more about the aerodynamics of new engine installations. For instance common sense suggests that the new engine should be fitted with its thrust line identical to the old one, and at such a distance fore or aft on the longitudinal axis as to keep the aircraft CG in its original place. But Peter's Wilksch engine has a thrust line 7cm above the original 0-200, which surely must change the thrust-drag couple of the aircraft and need a change of tailplane incidence to compensate. This in its turn causing more drag and a change in handling. Is there an easy way to calculate these things before cutting metal, or is it done by trial & error? With the undoubted advantages of diesel over avgas burners more of us will be contemplating diesel conversions, and would like to know how to make the calculations.

[Trevor Lyons]

ARV Super2s are fitted with a variety of engines. The original aircraft had the Hewland, an inverted 3-cyl 2T engine whose reduction gear brought the prop-line below the line of the crankshaft. This allowed the cowling to slope down, enhancing the already good visibility. But some ARVs have instead either a MidWest rotary or the Rotax flat-four. (All three Rotax types, 912, 912S and 914 have been used; and perhaps the 912S is optimum). It is worth noting that both the MidWest and the Rotax engines have reduction gearboxes that, unlike the Hewland, RAISE the thrust line above the crankshaft (or eccentric shaft, the rotary equivalent).

The MidWest ARV has a thrust line about 6 cm above that of the Hewland. This allows for a slightly bigger prop and/or better prop clearance, which is good! But the higher thrust-line must have altered the thrust drag couple, to give more of a nose-down force; and given that ARVs must be careful not to be nose-heavy, this is a shame! Some in-flight photos of MidWest ARVs tend to suggest that the elevator has to give more downforce than is ideal; so there may be more drag in the cruise than with the Hewland ARV.

As you may have read under the BB thread "ARV Super2 News", the Opus Super2 has just received FAA certification in the USA. It will go into production at first with a Rotax 912, and perhaps with a 912S in due course. It will be interesting to see what Opus have done to accommodate any thrust-line issues.