As Unidades de Controle de Combustível Hidromecânico (FCUs) tipicamente fazem usam a pressão do ar de descarga do compressor como uma entrada.
Se bem me lembro, as FCUs com as quais estou familiarizado tiveram pelo menos as seguintes entradas, além dos comandos da alavanca de controle:
- Velocidade de rotação do compressor
- Pressão do Ar de Descarga do Compressor
- Pressão do ar ambiente
Como mecânico e piloto, estudei muitos FCUs e acredito que as entradas listadas acima são padrão em todas as pessoas com as quais estou familiarizado. Isto é certamente verdade dos tipos que eu estudei mais recentemente e dos sistemas que eu atualmente voo. Observe que, embora esses FCUs possam ser designs antigos - em alguns casos, eles são comumente usados em novas turbinas de produção.
A seguir, trechos de alguns dos meus manuais de treinamento sobre um Bendix FCU:
The pneumatic section of the FCU determines the flow rate of fuel to the engine for all operations. The power levers control engine power from idle through takeoff power by operation of the gas generator (N1) governor in the FCU. Increasing N1 rpm results in increased engine power.
For explanation purposes, consider the N1 governor bellows as a diaphragm. P3 air is introduced into the bellows in a manner that sets up a differential pressure on each side of the diaphragm. Therefore, any change in P3 pressure will move the diaphragm. When pressure is increased, the fuel-metering valve attached to the bellows will move in an opening direction to increase fuel flow and increase N1 rpm.
As P3 pressure decreases, fuel flow also decreases which reduces the N1 rpm. The N1 governor increases or decreases P3 pressure in the bellows by varying the opening of relief orifices in the bellows.
The FCU controls engine power by maintaining the requested N1 rpm through the N1 governor. If actual N1 rpm is lower than the desired setting, the N1 governor closes the P3 orifice, allowing pressure to increase. As the pressure increases, the diaphragm moves to open the metering valve, increasing fuel flow, which in turn increases N1 rpm to the speed requested by the governor. When N1 rpm reaches the desired speed, the governor adjusts the P3 orifice to reduce pneumatic pressure to match the fuel pressure required to maintain the desired N1 rpm.