Maximum Cruise Rating

This rating is approximately 80 to 85% of the maximum power matched to the maximum cruise-speed capability. Unless there is a need for higher speed, typical cruise is performed at a 70 to 75% power rating, called the cruise rating. This gives the best fuel economy for the LRC. In a holding pattern in an airport vicinity, engines run at still lower power, barely maintaining altitude while waiting for clearance to proceed. The rating depends on the weight of an aircraft; at the end of cruise (lightweight), an approximate 65% rating is sufficient.

Idle Rating

This rating is at approximately 40 to 50% of maximum power and is intended for an engine to run without flameout but also produces practically no thrust. This situation arises at descent, at approach, or on the ground. During a descent, it has been found that better economy can be achieved by descending at partial throttle, at about a 60% power rating. This results in a shallower glide slope to cover more distance and consume less fuel.

Representative engine performances of various types at takeoff, maximum climb, and maximum cruise ratings are given in this section for an ISA day. Engine manufacturers also supply performance data for non-ISA days, which is more critical for hot and high-altitude conditions when engines produce considerably less power. To protect engines from heat stress, a fuel-control system is tuned to cut off power generation to a flat-rated value (at an ISA-day engine rating) up to a hot day that can be 20◦C above the ISA day. This book does not address non-ISA–day performances; in the industry, they are supplied.

10.11.1 Piston Engine

There are several ways to present piston engine performances. Figure 10.41 shows a Lycoming IO-360 series 180-HP piston engine. Readers may obtain the appropriate engine chart from manufacturers of other engines, or this graph may be scaled for coursework.

Readers should note that the power ratings are given in rpm. A Lycoming IO360 series takeoff is conducted at a maximum 2,700 rpm, whereas a climb is conducted at 2,500 to 2,600 rpm and cruise at 2,100 to 2,400 rpm. A partial throttle descent can be accomplished at 1,800 to 2,000 rpm. Idle is below 1,800 rpm (i.e., around 1,200 to 1,400 rpm; not shown). A fuel-flow graph is shown separately in Figure 10.42.

Piston engine power depends on the amount of airmass inhaled, which is indicated by the rpm and manifold pressure, pmanifold, at a particular ambient condition. A throttle valve controls airmass aspiration; when it is closed, there is no power (i.e., pmanifold = 0). When it is fully open and the engine is running at full aspiration, suction is created and the pmanifold reads the highest suction values. If there is less propeller load at the same rpm, less power is generated and the valve could be partially closed to inhale less airmass in order to run at equilibrium. At a low rpm, the aspiration level is low and there is a limiting pmanifold line. Therefore, the variables affecting engine power are rpm, pmanifold, altitude, and atmospheric temperature (nonstandard days). If an engine is supercharged, then the graphs indicate the

Figure 10.41. Lycoming IO-360 series 180-HP piston engine. (Courtesy of Lycoming Engine – retraced maintaining high fidelity)

effect. Figure 10.41 shows the parameters in graphical form; how to use the graphs is explained herein.

Figure 10.41 shows two graphs that must be used together. The left-hand graph provides the starting point for reading conditions at sea level, and the ISA day

Figure 10.42. Lycoming IO-360 seriesfuel flow graph