its probably an engine related limitation if the engine/prop unit is the principal difference. Some (all?) of the serious aerobatic engines have solid crankshafts to withstand the additional stress from the prop during dynamic manoeuvres like flicking, which produce high gyroscopic loads. The tutor may have a strengthened engine whereas the heron has a more standard engine, but still with inverted systems to allow limited aerobatics.
There is also an issue with the different propeller type. During aerobatics, three bladed props cause less of a fatigue problem for the engine’s crankshaft and hub than an equivalent two bladed unit (for a given engine type), the reason being that the moment of inertia of the two bladed prop varies greatly depending where the blades are in their arc of rotation. The extremes are when the two blades point straight up/down relative to the manoeuvre’s rotational axis and the propeller has a large moment of inertia, and when the propeller is horizontal and the converse is true. This large variation of load (and stress on the propeller mounting, which causes the fatigue issue) is much reduced with a three bladed prop. As well as a greater variation in stress for the two bladed unit, I imagine that the peak stress is also quite a bit higher. Therefore it should be possible, at least in theory, for an engine to be cleared for flicking with a prop with three blades but not with two, although practical considerations may ensure that this is never the case.
That’s my complicated explanation to your straightforward question! The reason for your limit is most likely just because Lycoming or whoever thinks it'll bust the engine.
Anyone know what other mods aerobatic engines have - different bearings, crank case strengthening etc? sickbag?!
