is regulated by air pressure supplied to cylinder through pipe 20 this pressure is maintained at a low level during draw machine shut down by a preselected setting of transducer 66 governed by the amount of voltage tapped off from potentiometer 54 and fed through amplifier 56 to the transducer when the draw machine is started by actuation of switch 26 the amplifier 56 receives an additional input signal developed across potentiometer 52 which is proportional to the voltage output of tach generator 30 and also to draw machine speed these first signals across potentiometer 52 are combined with a second signal of constant magnitude developed across potentiometer 54 in amplifier 56 to produce third signals at the output of amplifier 56 that increase with increasing speed of the draw machine 12 these increasing signals are lagged upon actuation of the start switch 26 so that the clapper pressure reaches steady state about 2 seconds after the draw machine speed reaches steady state while the capacitor of resistor capacitor circuit 58 60 connected to the input of amplifier 56 is charged each time the draw machine is stopped by opening switch 26 relay 64 operates to connect switch 62 to the capacitor 58 thus discharging it through ground conversely when switch 26 is closed and the draw machine is running relay 64 operates to move switch 62 to the open position the increasing output from amplifier 56 increases the output air pressure of transducer 66 which is increased by a factor of 3 in passing through amplifier 68 before being fed to cylinder 19 to increase the pressure on flap 17 in accordance.