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Airplane Flying Handbook
Transition to Jet Powered Airplanes
SLOW ACCELERATION OF THE JET ENGINE
JET ENGINE EFFICIENCY

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Airplane Flying Handbook

Preface

Table of Contents

Chapter 1,Introduction to Flight Training
Chapter 2,Ground Operations
Chapter 3,Basic Flight Maneuvers
Chapter 4, Slow Flight, Stalls, and Spins
Chapter 5, Takeoff and Departure Climbs
Chapter 6, Ground Reference Maneuvers
Chapter 7, Airport Traffic Patterns
Chapter 8, Approaches and Landings
Chapter 9, Performance Maneuvers
Chapter 10, Night Operations
Chapter 11,Transition to Complex Airplanes
Chapter 12, Transition to Multiengine Airplanes
Chapter 13,Transition to Tailwheel Airplanes
Chapter 14, Transition to Turbo-propeller Powered Airplanes
Chapter 15,Transition to Jet Powered Airplanes
Chapter 16,Emergency Procedures

Glossary

Index

Variation of thrust with r.p.m.
Figure 15-6.Variation of thrust with r.p.m.

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]

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.

JET ENGINE EFFICIENCY

Maximum operating altitudes for general aviation
turbojet airplanes now reach 51,000 feet. The
efficiency of the jet engine at high altitudes is the primary
reason for operating in the high altitude environment.
The specific fuel consumption of jet engines
decreases as the outside air temperature decreases for
constant engine r.p.m. and true airspeed (TAS). Thus,
by flying at a high altitude, the pilot is able to operate
at flight levels where fuel economy is best and with the
most advantageous cruise speed. For efficiency, jet airplanes
are typically operated at high altitudes where
cruise is usually very close to r.p.m. or exhaust gas temperature
limits. At high altitudes, little excess thrust
may be available for maneuvering. Therefore, it is
often impossible for the jet airplane to climb and turn
simultaneously, and all maneuvering must be accomplished
within the limits of available thrust and without
sacrificing stability and controllability.

ABSENCE OF PROPELLER EFFECT

The absence of a propeller has a significant effect on
the operation of jet powered airplanes that the transitioning
pilot must become accustomed to. The effect is
due to the absence of lift from the propeller slipstream,
and the absence of propeller drag.

ABSENCE OF PROPELLER SLIPSTREAM

A propeller produces thrust by accelerating a large
mass of air rearwards, and (especially with wing
mounted engines) this air passes over a comparatively
large percentage of the wing area. On a propeller
driven airplane, the lift that the wing develops is the
sum of the lift generated by the wing area not in the
wake of the propeller (as a result of airplane speed) and
the lift generated by the wing area influenced by the
propeller slipstream. By increasing or decreasing the
speed of the slipstream air, therefore, it is possible to
increase or decrease the total lift on the wing without
changing airspeed.

Typical Jet engine acceleration times.
Figure 15-7.Typical Jet engine acceleration times.

 

15-5