Stability and control studies are concerned with motion of the center
of gravity (cg) relative to the ground and motion of the airplane about
the cg. Hence, stability and control studies involve the use of the six
degree of freedom equations of motion. These studies are divided into
two major categories: (a) static stability and control and (b) dynamic
stability and control. Because of the nature of the solution process, each
of the categories is subdivided into longitudinal motion (pitching motion)
and lateral-directional motion (combined rolling and yawing motion).
While trajectory analyses are performed in terms of force coefficients
with control surface deflections either neglected (untrimmed drag polar)
or eliminated (trimmed drag polar), stability and control analyses are in
terms of the orientation angles (angle of attack and sideslip angle) and
the control surface deflections.
In hindsight, the evolution of aerovehicle and aeropropulsion systems looks
like the result of a master plan. The evolution began with the piston engine
and propeller, which constituted the best propulsion system for the initially
low flight speeds, and had an outstanding growth potential up to about
450 mph. In the early 1940s, when flight technology reached the ability to
enter into the transonic flight speed regime, the jet engine had just demonstrated
its suitability for this speed regime. A vigorous jet engine development program
was launched. Soon the jet engine proved to be not only an excellent transonic
but also a supersonic propulsion system. This resulted in the truly exploding
growth in flight speed, as shown in Fig. 3.
It is interesting to note that military development preceded commercial
applications by 15-20 years for both the propeller engine and the gas turbine
engine. The reason was that costly, high-risk, long-term developments conducted
by the military sector were necessary before a useful commercial application
could be envisioned. After about 75 years of powered flight, the aircraft has outranked
all other modes of passenger transportation and has become a very important
export article of the United States.
The evolutions of both aerovehicle and aeropropulsion systems have in no
way reached a technological level that is close to the ultimate potential! The
evolution will go on for many decades toward capabilities far beyond current
feasibility and, perhaps, imagination.
How Jet Propulsion Came into Existence
The idea of airbreathing jet propulsion originated at the beginning of the 20th
century. Several patents regarding airbreathing jet engines had been applied for by
various inventors of different nationalities who worked independently of each other.
From a technical standpoint, airbreathing jet propulsion can be defined as a
special type of internal combustion engine that produces its net output power
as the rate of change in the kinetic energy of the engine's working fluid.
The working fluid enters as environmental air that is ducted through an inlet
diffuser into the engine; the engine exhaust gas consists partly of combustion
gas and partly of air. The exhaust gas is expanded through a thrust nozzle or
nozzles to ambient pressure. A few examples of early airbreathing jet propulsion
patents are as follows:
1) In 1908, Lorin patented a jet engine that was based on piston machinery
2) In 1913, Lorin patented a jet engine based on ram compression in supersonic
flight (Fig. 4b), the ramjet.
3) In 1921, M. Guillaume patented a jet engine based on turbomachinery; the
intake air was compressed by an axial-flow compressor followed by a combustor
and an axial-flow turbine driving the compressor (Fig. 4c).
These patents clearly described the airbreathing jet principle but were not
executed in practice. The reason lies mainly in the previously mentioned
strong interdependency between aerovehicle and aeropropulsion systems. The
jet engine has, in comparison with the propeller engine, a high exhaust speed
(for example, 600 mph and more). In the early 1920s, the aerovehicle had a
flight speed capability that could not exceed about 200 mph. Hence, at that
time, the so-called propulsive efficiency of the jet engine was very low (about
30-40%) in comparison to the propeller, which could reach more than 80%.
Thus, in the early 1920s, the jet engine was not compatible with the too-slow
aerovehicle. Also, in the early 1920s, an excellent theoretical study about the possibilities
of enjoying jet propulsion had been conducted by Buckingham of the
Bureau of Standards under contract with NACA. The result of this study was
clear--the jet engine could not be efficiently employed if the aerovehicle could
not greatly exceed the flight speed of 200 mph; a flight speed beyond 400 mph
seemed to be necessary. The consequences of the results of this study were
that the aircraft engine industry and the scientific and engineering community
had no interest in the various jet engine inventions.
In the anshu aerospace-about flight-Flight is a relatively simple and widely studied phenomenon. As
surprising as it may sound, though, it is more often than not
misunderstood. For example, most descriptions of the physics of lift
fixate on the shape of the wing (i.e., airfoil) as the key factor in
understanding lift. The wings in these descriptions have a bulge on the
top so that the air must travel farther over the top than under the wing.
Yet we all know that wings fly quite well upside down, where the
Thunderbirds in Figure , with wings of almost no thickness at all.
To cover for this paradox, we sometimes see a description for inverted
flight that is different than for normal flight. In reality, the shape of the
wing has little to do with how lift is generated, and any description
that relies on the shape of the wing is misleading at best. This assertion
will be discussed in detail in Chapter 1. It should be noted that the
shape of the wing does has everything to do with the efficiency of the
wing at cruise speeds and with stall characteristics.
Let us look at three examples of successful
wings that clearly violate the descriptions that
rely on the shape of the wing as the basis of lift.
The first example is a very old design. Figure
shows a photograph of a Curtis 1911 Model
D type IV pusher. Clearly, the air travels the
same distance over the top and under the bottom
of the wing. Yet this airplane flew and was
the second airplane purchased by the U.S.
Army in 1911.
Fig. 3 Aircraft speed trends.
improve their aircraft system and, in 1906, conducted longer flights with safe
takeoff, landing, and curved flight maneuvers. While the flight speed was only
about 35 mph, the consequences of these first flights were enormous:
1) Worldwide interest in powered flight was stimulated.
2) The science of aerodynamics received a strong motivation.
3) The U.S. government became interested in power flight for potential
defense applications, specifically reconnaissance missions.
In 1909, the Wright brothers built the first military aircraft under government
contract. During World War I, aircraft technology progressed rapidly. The flight
speed reached about 150 mph, and the engine power attained 400 hp. After World
War I, military interest in aircraft systems dropped, but aircraft technology had
reached such a degree of maturity that two nonmilitary application fields could
1) Commercial aviation, mail and passenger transport (first all-metal
monoplane for passenger and mail transport, the Junkers F13, in 1919, sold
2) Stunt flying leading to general aviation (sport and private transportation).
In the period from 1920 to 1940, the speed increased from about 150 to
350 mph through evolutionary improvements in vehicle aerodynamics and
engine technology, as discussed previously. At the end of World War II, the
flight speed of propeller aircraft reached about 400-450 mph, and the power
output of the largest reciprocating engines was about 5000 hp. This constituted
almost the performance limit of the propeller/reciprocating engine propulsion
system. Today, the propeller/reciprocating engine survives only in smaller,
lower-speed aircraft used in general aviation.
In the late 1930s, jet propulsion emerged that promised far greater flight
speeds than attainable with the propeller or piston engine. The first jet-propelled
experimental aircraft flew in the summer of 1939 (the He-178), and in early 1941,
the first prototype jet tighter began flight tests (He-280). In 1944, mass-produced
jet fighters reached a speed of about 550 mph (Me-262).
In the early 1950s, jet aircraft transgressed the sonic speed. In the mid-1950s,
the first supersonic jet bomber (B-58 Hustler) appeared, and later the XB-70
reached about Mach 3. Also during the 1950s, after more than 15 years of military
development, gas turbine technology had reached such a maturity that the
following commercial applications became attractive: 1) commercial aircraft,
e.g., Comet, Caravelle, and Boeing 707; 2) surface transportation (land, sea);
and 3) stationary gas turbines.
In the 1960s, the high-bypass-ratio engine appeared, which revolutionized
military transportation (the C5A transport aircraft). At the end of the 1960s,
based on the military experience with high-bypass-ratio engines, the second generation
of commercial jet aircraft came into existence, the widebody aircraft.
An example is the Boeing 747 with a large passenger capacity of nearly 400.
Somewhat later came the Lockheed L-1011 and Douglas DC10. By that time,
the entire commercial airline fleet used turbine engines exclusively. Advantages
for the airlines were as follows:
1) Very high overall efficiency and, consequently, a long flight range with
2) Overhaul at about 5 million miles.
3) Short turnaround time.
4) Passenger enjoyment of the very quiet and vibration-free flight, short travel
time, and comfort of smooth stratospheric flight.
5) Community enjoyment of quiet, pollution-free aircraft.
By the end of the 1960s, the entire business of passenger transportation was
essentially diverted from ships and railroads to aircraft. In the 1970s, the
supersonic Concorde with a flight speed of 1500 mph (the third generation of
commercial transport) appeared with an equivalent output of about 100,000 hp.