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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.
100
90
80
70
60
50
40
30
20
10
0
0 10 20 30 40 50 60 70 80 90 100
Percent Maximum Thrust
Percent Maximum R.P.M.
VARIATION OF THRUST WITH R.P.M.
(Constant Altitude & Velocity)
Figure 15-6.Variation of thrust with r.p.m.
8
6
4
2
60% 100%
Time to Achieve Full Thrust (sec.)
R.P.M.
78%
Figure 15-7.Typical Jet engine acceleration times.
Ch 15.qxd 5/7/04 10:22 AM Page 15-5
15-6
For example, a propeller driven airplane that is allowed
to become too low and too slow on an approach is very
responsive to a quick blast of power to salvage the
situation. In addition to increasing lift at a constant
airspeed, stalling speed is reduced with power on. Ajet
engine, on the other hand, also produces thrust by
accelerating a mass of air rearward, but this air does
not pass over the wings. There is therefore no lift bonus
at increased power at constant airspeed, and no
significant lowering of power-on stall speed.
In not having propellers, the jet powered airplane is
minus two assets.
• It is not possible to produce increased lift
instantly by simply increasing power.
• It is not possible to lower stall speed by simply
increasing power. The 10-knot margin (roughly
the difference between power-off and power-on
stall speed on a propeller driven airplane for a
given configuration) is lost.
Add the poor acceleration response of the jet engine
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