Torque rise phenomenon
In the type of helicopter I fly, there is a phenomenon of torque rise in which during take-off the torque rises by itself without any collective lever input. The stages at which it happens distinctly during a normal take-off, is once at roughly about 40-50 kmph and another at about 80 kmph. The amount of torque rise is quite high, about 5-10% (per engine) in each instance and if uncorrected the total torque rise is in the range of 10-20% per engine by the time the nominal take-off speed of 120 kmph is achieved.
The pilot therefore, is forced to glance inside to keep a check on the torque and reduce the collective lever to ensure that the torque remains same as was used to initiate the take-off (hover torque +4% per engine). It also forces the pilot to keep glancing inside during the critical phase of take-off. It especially becomes critical when operating in the regime of Take Off rating, when, if uncorrected, there are chances of over torquing. (There is a voice warning for that though)
Though, I am not convinced for the reason cited for this phenomenon, it has been attributed to flexing of airframe with increase in forward speed, in which the all composite body of the aircraft gets slightly compressed due to aerodynamic forces on it, which in turn leads to an un-intended movement of control rod (without any feedback on the collective lever) leading to increase in pitch on the blades resulting in increase in torque.
Few Questions:
1. Does this sort of torque rise occur in any other helicopter? or atleast anything similar.
2. Though it would be difficult to explain until the airframe is studied in detail, if such a thing of control rods actually moving due to flexing of airframe can occur, can some design modification be done to prevent this un-intended input on control rods?
3. Could this be happening due to any aerodynamic 'actions' on the rotor itself? (which in all probability would be increase in rotor drag, leading to the FADEC compensating by increasing torque by itself to maintain R RPM)