Mudge wrote:
OK...let me see if I understand this. The manifold pressure is changed by how much/how little throttle you're applying, right? It's not increased/decreased by a separate control knob/lever of some sort. ie. The more throttle, the higher the MP, (and vice versa), right?
Mudge the enlightened
From the FAA's Airplane Flying Handbook:
Quote:
When performing a climb, the power should be advanced to the climb power recommended by the manufacturer. If the airplane is equipped with a controllable-pitch propeller, it will have not only an engine tachometer, but also a manifold pressure gauge. Normally, the flaps and landing gear (if retractable) should be in the retracted position to reduce drag. As the airplane gains altitude during a climb, the manifold pressure gauge (if equipped) will indicate a loss in manifold pressure (power). This is because the same volume of air going into the engine’s induction system gradually decreases in density as altitude increases. When the volume of air in the manifold decreases, it causes a loss of power. This will occur at the rate of approximately 1-inch of manifold pressure for each 1,000-foot gain in altitude. During prolonged climbs, the throttle must be continually advanced, if constant power is to be maintained.
From the Handbook of Aeronautical Knowledge:
Quote:
On airplanes that are equipped with a constant-speed propeller, power output is controlled by the throttle and indicated by a manifold pressure gauge. The gauge measures the absolute pressure of the fuel/air mixture inside the intake manifold and is more correctly a measure of manifold absolute pressure (MAP). At a constant r.p.m. and altitude, the amount of power produced is directly related to the fuel/air flow being
delivered to the combustion chamber. As you increase the throttle setting, more fuel and air is flowing to the engine; therefore, MAP increases. When the engine is not running, the manifold pressure gauge indicates ambient air pressure (i.e., 29.92 in. Hg). When the engine is started, the manifold pressure indication will decrease to a value less than ambient pressure (i.e., idle at 12 in. Hg). Correspondingly, engine failure or power loss is indicated on the manifold gauge as an increase in manifold pressure to a value corresponding to the ambient air pressure at the altitude where the failure occurred.
The manifold pressure gauge is color-coded to indicate the engine’s operating range. The face of the manifold pressure gauge contains a green arc to show the normal operating range, and a red radial line to indicate the upper limit of manifold pressure.
For any given r.p.m., there is a manifold pressure that should not be exceeded. If manifold pressure is excessive for a given r.p.m., the pressure within the cylinders could be exceeded, thus placing undue stress on the cylinders. If repeated too frequently, this stress could weaken the cylinder components, and eventually cause engine failure.
Manifold Absolute Pressure (MAP)—The absolute pressure of the fuel/air mixture within the intake manifold, usually indicated in inches of mercury.
Hope that helps. BTW, both of those books are a must-read for flight instructors, so if you're looking for sources of good information for yourself as a student pilot, have a look at them. Most of the Jepp, Gleim, or other books are going to be almost the same stuff.
They can be downloaded as searchable PDFs here:
http://www.faa.gov/library/manuals/aviation/
Ryan
_________________
Aerial Photographer with
Red Wing Aerial Photography currently based at KRBD and tailwheel CFI.
Websites:
Texas Tailwheel Flight Training,
DoolittleRaid.com and
Lbirds.com.
The horse is prepared against the day of battle: but safety is of the LORD. - Prov. 21:31
- Train, Practice, Trust.
Login
Register
FAQ
Search
to this in 3 months: 
Help us get her all the way back 
