How does the RPM and Manifold Pressure Gauges work on Aircraft engines?

Question

How does the RPM and Manifold Pressure Gauges work on Aircraft engines?

Answer 1

The RPM or tachometer measures the rotational speed of the engine and the manifold pressure gauge measures the absolute pressure inside of the intake manifold.  Using both of these instruments together a pilot can get a pretty good idea of how much power the engine is producing.  They can also help him avoid operating the engine in a condition which could damage the engine (low RPM and high manifold pressure).

Answer 2

RPM gauges vary on different engines but there few basic types: Mechanical, Electric, and electronic.

Mechanical RPM gauges are basically a cable that is spunn from a gear off the engine.  The other side has a magenet on it that spins inside the RPM gauge with rotates a drag cup wich moves the needle indicating the RPM.  

The electric type is a little more complicated.  Geared into the engine is an AC generator.  Depending on how fast the engine is turning the AC generator will produce a different frequency electic current.  Wires coming from the AC gen. go to the RPM indicator which has a motor in it called a synchonous motor.  The motor turns a magnet like the mechanical RPM indicator which moves the needle.

Now the electronic RPM gauge is hooked up to one of the magnetos and basically counts the number of firing impulses and calculates the RPM from that information and once again displays in on the RPM gauge.

The Manifold gauge is pretty simple.  A pipe is connected to the manifold and run to the instrument.  Inside the instrument is a diaphragm or bellow.  As pressure increases and decreases in the manifold the pressure pushes or pulls on the diaphragm moving the indicator needle.

Answer 3

At low engine power (on ground),you look primarily at just RPM,for reference. When airborne, you use both, but control them separately.

RPM you control with prop pitch or the blue handle.

MP is controlled by the throttle. In order to make an increase in power, adjust prop pitch then throttle. It's just the opposite to make a power reduction. You may also hear power settings like 25/25 for a certain phase of flight. The number on top refers to RPM in 100's, the bottom number refers to MP.

Answer 4

Manifold pressure is necessary when an engine is equipped with a constant speed prop.  It is not needed on a fixed pitch prop because any change in throttle is shown as a change in RPM.