Aircraft Systems

density (higher density altitude) decreases the power output will increase or decrease as appropriate, with changes in of the engine. As altitude changes, the position of the throttle airspeed and propeller load. For example, once a specific must be changed to maintain the same rpm. As altitude is rpm has been selected, if aircraft speed decreases enough to increased, the throttle must be opened further to indicate the rotate the propeller blades until they contact the low pitch same rpm as at a lower altitude. stop, any further decrease in airspeed will cause engine rpm to decrease the same way as if a fixed-pitch propeller were Adjustable-Pitch Propeller installed. The same holds true when an aircraft equipped with The adjustable-pitch propeller was the forerunner of the a constant-speed propeller accelerates to a faster airspeed. As constant-speed propeller. It is a propeller with blades whose the aircraft accelerates, the propeller blade angle increases to pitch can be adjusted on the ground with the engine not maintain the selected rpm until the high pitch stop is reached. running, but which cannot be adjusted in flight. It is also Once this occurs, the blade angle cannot increase any further referred to as a ground adjustable propeller. By the 1930s, and engine rpm increases. pioneer aviation inventors were laying the ground work for automatic pitch-change mechanisms, which is why the term On aircraft equipped with a constant-speed propeller, power sometimes refers to modern constant-speed propellers that output is controlled by the throttle and indicated by a manifold are adjustable in flight. pressure gauge. The gauge measures the absolute pressure of the fuel-air mixture inside the intake manifold and is more The first adjustable-pitch propeller systems provided only two correctly a measure of manifold absolute pressure (MAP). At pitch settings: low and high. Today, most adjustable-pitch a constant rpm and altitude, the amount of power produced propeller systems are capable of a range of pitch settings. is directly related to the fuel-air mixture being delivered to the combustion chamber. As the throttle setting is increased, A constant-speed propeller is a controllable-pitch propeller more fuel and air flows to the engine and MAP increases. whose pitch is automatically varied in flight by a governor When the engine is not running, the manifold pressure gauge maintaining constant rpm despite varying air loads. It is the indicates ambient air pressure (i.e., 29.92 inches mercury most common type of adjustable-pitch propeller. The main (29.92 "Hg)). When the engine is started, the manifold advantage of a constant-speed propeller is that it converts pressure indication decreases to a value less than ambient a high percentage of brake horsepower (BHP) into thrust pressure (i.e., idle at 12 "Hg). Engine failure or power loss horsepower (THP) over a wide range of rpm and airspeed is indicated on the manifold gauge as an increase in manifold combinations. A constant-speed propeller is more efficient pressure to a value corresponding to the ambient air pressure than other propellers because it allows selection of the most at the altitude where the failure occurred. [Figure 7-9] efficient engine rpm for the given conditions. The manifold pressure gauge is color coded to indicate the An aircraft with a constant-speed propeller has two controls: engine’s operating range. The face of the manifold pressure the throttle and the propeller control. The throttle controls gauge contains a green arc to show the normal operating power output, and the propeller control regulates engine range and a red radial line to indicate the upper limit of rpm. This regulates propeller rpm, which is registered on manifold pressure. the tachometer. Once a specific rpm is selected, a governor automatically adjusts the propeller blade angle as necessary to maintain 3300 the selected rpm. For example, after setting the desired rpm 2255 3355 during cruising flight, an increase in airspeed or decrease in MANIFOLD PRESS propeller load causes the propeller blade angle to increase 2200 4400 as necessary to maintain the selected rpm. A reduction in airspeed or increase in propeller load causes the propeller IN Hg 1155 ALg. 4455 blade angle to decrease. 1100 5500 The propeller’s constant-speed range, defined by the high and low pitch stops, is the range of possible blade angles for a constant-speed propeller. As long as the propeller blade angle is within the constant-speed range and not against Figure 7-9. Engine power output is indicated on the manifold either pitch stop, a constant engine rpm is maintained. If pressure gauge. the propeller blades contact a pitch stop, the engine rpm 7-6