When a non-turbocharged engine is running, the manifold pressure will always be less than the ambient air pressure, which is approximately 30" Hg at sea-level. This is due to the piston in the cylinder acting like an air pump. Any change in engine RPM will cause a change in the manifold pressure due to the amount of cylinder volume swept per minute by the piston. This becomes significant when changing power settings. For example, assume a take-off power setting of 2700 RPM and a manifold pressure of 26" Hg is being used and when airborne a power setting of 24" Hg is required. If the manifold pressure is reduced to 24" Hg followed by an RPM reduction to 2400 RPM, the manifold pressure will rise back up about 1" Hg to 25" Hg. This will require a further throttle adjustment to return the manifold pressure back to the required 24" Hg. The initial rise in manifold pressure is due to the decrease in the cylinder volume swept per minute causing less suction through the manifold. To avoid having to adjust the throttle/ manifold pressure twice, simply throttle back to a manifold pressure of 1" Hg lower than that required. For example here, throttle back to 23" Hg followed by RPM reduction. The manifold pressure will then rise back up to the required 24" Hg.
A check of the aircraft’s flight manual (POH) will show a table for RPM/MP combinations. From the table it can be seen a manifold pressure of 2-3" Hg above over-square is permissible. This can be used to advantage during the initial letdown from the cruise altitude. Reducing power by 200300 RPM first, instead of reducing MP, will not only reduce noise but it will also increase the MP as already mentioned earlier. Increased MP will produce an increase in cylinder head temperature, counteracting the drop in temperature normally associated with reduced power settings used during the descent. When the RPM has been reduced to its limit in accordance with the POH table the manifold pressure must be reduced to avoid exceeding the engine’s operating limits. Also, check the cylinder head temperatures is not exceeded. Finally, after reaching circuit height, reduce the manifold pressure from the descent/cruise setting before selecting full fine/flat pitch for landing; otherwise, the high RPM associated with full fine/flat pitch will cause excessive noise and can be objectionable to people on the ground.
The prop thrust is proportional to the manifold pressure, and RPM/blade angle. A change of MP has the greatest effect on thrust variation with RPM being constant. Conversely, changing the RPM/blade angle has less effect on thrust developed. During an approach to land, RPM is maintained at a fairly high level by using fine/flat pitch, power is varied by throttle use. In the event of a ‘go-around’, maximum thrust is quickly supplied by increasing the manifold pressure while the CSU turns the blades to a coarser pitch setting to absorb the increased power.
The CSU is a very reliable device but faults can, and do occur. A blocked oil feed line to the CSU will cause the blades to lock – on to the pitch setting in use at the time the blockage occurs. It maybe possible to remedy the fault by cycling the prop-pitch control to clear the blockage. However, if the blockage persists, the flight should be terminated as soon as convenient. Keeping in mind a fine/flat pitch setting can over-speed the prop and a coarse pitch can be a problem if a go-around is necessary after a baulked landing.