SLOW ACCELERATION OF THE JET ENGINE

In a propeller driven airplane, the constant speed propeller keeps the engine turning at a constant r.p.m. within the governing range, and power is changed by varying the manifold pressure. Acceleration of the piston from idle to full power is relatively rapid, somewhere on the order of 3 to 4 seconds. The acceleration on the different jet engines can vary considerably, but it is usually much slower.

Efficiency in a jet engine is highest at high r.p.m. where the compressor is working closest to its optimum conditions. At low r.p.m. the operating cycle is generally inefficient. If the engine is operating at normal approach r.p.m. and there is a sudden requirement for increased thrust, the jet engine will respond immediately and full thrust can be achieved in about 2 seconds. However, at a low r.p.m., sudden full power application will tend to overfuel the engine resulting in possible compressor surge, excessive turbine temperatures, compressor stall and/or flameout. To prevent this, various limiters such as compressor bleed valves are contained in the system and serve to restrict the engine until it is at an r.p.m. at which it can respond to a rapid acceleration demand without distress. This critical r.p.m. is most noticeable when the engine is at idle r.p.m. and the thrust lever is rapidly advanced to a high power position. Engine acceleration is initially very slow, but changes to very fast after about 78 percent r.p.m. is reached. [Figure 15-7]

Figure 15-7.Typical Jet engine acceleration times.

Even though engine acceleration is nearly instantaneous after about 78 percent r.p.m., total time to accelerate from idle r.p.m. to full power may take as much as 8 seconds. For this reason, most jets are operated at a relatively high r.p.m. during the final approach to landing or at any other time that immediate power may be needed.