10 Jan Helicopter Manifold Pressure and RPM Control
The manifold pressure of a piston powered helicopter is the way we tell how much power is developed by the engine. During normal helicopter flight the rotor rpm is kept constant therefore by varying the collective pitch and thereby varying both the thrust and drag produced by the blades the function of the engine is to keep the rotor rpm constant or in the normal operating range.
When flying a piston powered helicopter equipped with a governor the function of the governor is to automatically control the throttle in order to keep the rotor rpm constant so in essence the pilot only has to monitor the manifold pressure or set a specific manifold pressure to achieve a specific result. For the purpose of this explanation the result that gets referred to will be either to climb, descend or fly straight and level.
The problem that most students have is controlling the rotor rpm and manifold pressure combination in a non governor or correlator operated helicopter. The crux of the problem lies in the fact that the throttle by itself controls the rotor rpm as well as the manifold pressure and the collective by itself does the same. For example let’s say we increase the engine rpm from 3000 to 3100 rpm by opening the throttle the MAP will also increase and let’s say it does so from 18″ to 20″ manifold pressure. Now lets say we leave the throttle alone (constant) and we pull the collective up the manifold pressure will increase because there is a linkage (correlator) between the collective and the fuel injection unit or carburettor, however in most cases the rotor rpm will drop due to increased drag on the blades. If we push the collective down whilst keeping the throttle constant the MAP will decrease but the rotor rpm will increase due to less drag on the blades.
Now most people only focus or rather fixate on the engine and rotor rpm and chase the rpm up and down between the lower and upper red line with the throttle totally forgetting what the implications are on the manifold pressure. For example lets assume we have a manifold pressure of 20″ but we sit with low rotor rpm, if the throttle only is opened to increase the rpm the manifold pressure will also increase lets say to 22″ thereby causing the helicopter to behave differently with this “new” power setting. In the previous example if we wished to maintain 20″ MAP whilst increasing the rotor rpm the correct action would have been to open the throttle and at the same time lower the collective pitch lever slightly.
One way of figuring out which control to use being it just the throttle, just the collective or a combination of both is to always have a target MAP in mind to achieve a desired phase of flight. In other words if one wants to climb, descend or fly straight and level give yourself a manifold pressure value to achieve this phase of flight and then think what the effect will be on this value before one simply opens or closes the throttle. After implementing this system for a while it will become a habit and by seeing certain combinations of rotor rpm and MAP values the right corrective action can be taken without thinking about at all.
© Harry Helicopter Training Syllabus 2008