content_image/599581.jpg content_image/599580.jpg
<img src='content_image/599581.jpg'>

Fan speed (N$_{1}$) is the primary indication of thrust on most turbofan engines. Fuel flow provides a secondary thrust indication, and cross-checking for proper fuel flow can help in spotting a faulty N$_{1}$ gauge. Turbofans also have a gas generator turbine tachometer (N$_{2}$). They are used mainly for engine starting and some system functions. 

In setting power, it is usually the primary power reference (EPR or N$_{1}$) that is most critical and is the gauge that first limits the forward movement of the thrust levers. However, there are occasions where the limits of either rpm or temperature can be exceeded. The rule is: movement of the thrust levers must be stopped and power set at whichever the limits of EPR, rpm, or temperature is reached first. 

## Thrust To Thrust Lever Relationship 

In a piston-engine, propeller-driven airplane, thrust is proportional to rpm, manifold pressure, and propeller blade angle, with manifold pressure being the most dominant factor. At a constant rpm, thrust is proportional to throttle lever position. In a jet engine, however, thrust is quite disproportional to thrust lever position. This is an important difference that the pilot transitioning into jet-powered airplanes must become accustomed to. 

On a jet engine, thrust is proportional to rpm (mass flow) and temperature (fuel/air ratio). These are matched and a further variation of thrust results from the compressor efficiency at varying rpm. The jet engine is most efficient at high rpm, where the engine is designed to be operated most of the time. 

As rpm increases, mass flow, temperature, and efficiency also increase. Therefore, much more thrust is produced per increment of throttle movement near the top of the range than near the bottom. 

One thing that seems different to the piston pilot transitioning into jet-powered airplanes is the rather large amount of thrust lever movement between the flight idle position and full power as compared to the small amount of movement of the throttle in the piston engine. For instance, an inch of throttle movement on a piston may be worth 400 horsepower wherever the throttle may be. On a jet, an inch of thrust lever movement at a low rpm may be worth only 200 pounds of thrust, but at a high rpm that same inch of movement might amount to closer to 2,000 pounds of thrust. Because of this, in a situation where significantly more thrust is needed and the jet engine is at low rpm, it does not do much good to merely “inch the thrust lever forward.” Substantial thrust lever movement is in order. This is not to say that rough or abrupt thrust lever action is standard operating procedure. If the power setting is already high, it may take only a small amount of movement. However, there are two characteristics of the jet engine that work against the normal habits of the piston- engine pilot. One is the variation of thrust with rpm, and the other is the relatively slow acceleration of the jet engine. 

## Variation of Thrust with RPM 

Whereas piston engines normally operate in the range of 40 percent to 70 percent of available rpm, jets operate most efficiently in the 85 percent to 100 percent range, with a flight idle rpm of 50 percent to 60 percent. The range from 90 percent to 100 percent in jets may produce as much thrust as the total available at 70 percent. [Figure 15-6] 

<img src='content_image/599580.jpg'>

