The explanation about the physical reasons of Mach pitch-up given in the article, "Eurofighter FCS software to counteract Mach pitch-up" (Flight International, 7-13 May), does not reflect reality - caused by a mix-up of reasoning.
At first, when the aircraft passes through Mach 1 from subsonic to supersonic speeds, the aerodynamic centre (centre of pressure) moves rearwards. Thus, with the assumption of constant position of centre of gravity, an additional nose-down pitching moment is generated. Counteraction of this moment requires additional nose-up moment normally generated by the rearward elevator, caused by an additional downward directed negative lift force, causing additional trim drag. To counteract these performance penalties, fuel is transferred rearwards to shift the centre of gravity rearwards, and to reduce the negative lift and additional trim drag. For a fighter aircraft with unstable characteristics like the Eurofighter, the pass through Mach 1 means more stability or less agility in the supersonic regime. It is counteracted through the canard, which destabilises the aircraft. Pumping fuel aft would re-destabilise the aircraft and increase instability and thus manoeuvrability.
At second, transonic pitch-up is caused through heavily non-linear aerodynamic behaviour, which mainly "happens" at the leading-edge of the wing. Since this is normally in front of the centre of gravity, additional nose-up pitching moments are being generated. Due to the generally non-linear aerodynamic behaviour in this regime, these pitch-ups can occur rapidly and heavily in magnitude (attitude and load factor changes) reducing handling qualities of the aircraft considerably. Counteraction of these occurrences can be achieved through measures within an aircraft's stability controller, thus improving handling qualities through software measures and reducing pilot workload.
Christoph Oelker
Pichl, Austria
Source: Flight International
