X-15 Research Results
Aerodynamic Characteristics of
OUR DEPTH OF UNDERSTANDING of how we fly has come
from study of the mechanics of flight and the theory of airflow.
This comprises the science of aerodynamics, which has its roots in
the study of fluid mechanics and concerns all the forces acting on
an airplane as a result of its motion through the air. When an airplane
passes through the atmosphere, the air molecules behave like
a fluid, flowing around the wings and fuselage, tending to stick to
the surface and be dragged along behind, and, under certain conditions,
being compressed. The pressure from this flow exerts the
well-known lift and drag forces, and the less familiar stabilizing
Airflow shows an amazing variety of characteristics, which have
been the subject of intensive theoretical analysis and study in wind
tunnels. At slow speeds, the pressures an airplane generates as it
moves through the air are small relative to the ambient atmospheric
pressure. The balance between these two pressures establishes the
boundaries of the aerodynamic flight corridor. The pressure produced
by motion, called dynamic pressure, increases as the square
of velocity. At Mach 1.2, the dynamic pressure is equal to the
atmospheric pressure; at Mach 6, it is 25 times greater. This increase
in dynamic pressure permits sustained flight at high altitude,
where the atmospheric pressure is extremely low, provided the speed
is high enough.
Pressure forces are also affected by changes in airflow, from its
elastic and viscous characteristics as it flows around an aircraft.
Drastic changes in flow, as previously noted, are encountered in flight
to high speeds. At 4000 mph, the airflow bears little resemblance to
that at 400 mph. It will, in fact, have gone through four regions:
subsonic, transonic, supersonic, and hypersonic.
dramatic photographs of free-flight models of the X-15 being fired
into a wind Tunnel vividly detail the ahock-wave patterns for airflow at
Mach 3.5 (above) and at Mach 6 (below).
The major consequence of flight to high spced is the effect on
airflow, because of the elasticity (compression and expansion) of
air. At the slowest speeds, subsonic, the effects are not pronounced.
As airflow velocities increase, the air becomes compressed, and pressure
begins to pile up ahead of each part of the aircraft, until finally
distinct pressure waves, or shock waves, form. The transonic airflow
region is where shock waves first appear on an aircraft, though
these shocks may be only local in nature. It is a region of mixed and
erratic flow between subsonic and supersonic flow, which causes
abrupt changes in lift and drag forces and airplane stability. As
speed is further increased, local regions of subsonic flow disappear,
and the flow is everywhere supersonic. The air has become further
compressed. The shock waves are now distinct and trail aft in the
form of a wedge, or cone, behind any object that interferes with the
airstream. While a shock wave is normally less than .001-inch
thick, the air undergoes large changes in pressure, density, and
temperature across this minute boundary. These effects are far-reaching,
even extending to the ground in the form of sonic booms. Aerodynamic
theory has been developed that enables the characteristics
of these shock waves to be precisely calculated.
At higher supersonic speeds, the shock waves continue to increase
in strength, bending back to form an acute angle with the aircraft
surfaces. The equations of supersonic flow at this point no longer
apply, and many interactions between shock waves and flow field are
evidenced. One major effect is a loss of lifting effectiveness of the
wings and tail surfaces, because the shock waves attenuate the
aerodynamic forces. Of more significance, the friction of the air
flowing along any surface raises air temperature to many times that of
the surrounding atmosphere. Airflow is now in the hypersonic-flow
region, and the science of thermodynamics is added to aerodynamics.
Though not exactly defined, it is generally accepted as
applying to speeds above about Mach 5. It is an area of multiple
shock waves and interference effects. The difficulty for the
aerodynamicist arises from trying to unterstand the effects of flow that
is discontinuous at each shock wave. Each new geometric shape
calls for reorganization of theory.
By optimizing the shape, size, and relative locations of wing, tail,
and fuselage, an airplane is made highly efficient for one flow
region. But that particular configuration may have many adverse
interference effects when airflow enters a new flight regime. Many
compromises are necessary to achieve one configuration that is
satisfactory from subsonic to hypersonic speeds.
Facing Major Gaps in Knowledge
At the time the X-15 was designed, theory and empirical data
(much of it from previous research airplanes) provided a good
understanding of the mechanics of airflow for speeds to about Mach
3. But there were major gaps in aerodynamic knowledge above
this speed. Some of these gaps were bridged by wind-tunnel tests
of scale models of the X-15. However, although models of the X-15
were tested in many supersonic and hypersonic wind tunnels,
they were of very small scale - 1/15 or 1/50 - and no verification had
been made of the results from small-scale models for flight at
hypersonic speeds. Moreover, wind tunnels approximate flow conditions
rather than exactly duplicating them. Hence, a valuable part of the
X-15 program would be to verify or modify the picture of hypersonic
flow derived from these experimental techniques and from theoretical
Over the years, various analytical techniques have been developed
by which basic aerodynamic characteristics can be extracted from
flight measurements of airplane response. In general, it was found
that these techniques could be extended to the X-15's ranges of speed
and angle of attack. However, since most X-15 maneuvers are of
a transient nature, the evaluation of dynamic motions was aided
considerably by using the flight simulator to "match" the actual flight
maneuvers. New techniques were required for the analysis of aerodynamic
heating, however. Since the thermocouples provide only a
measure of the response of the structure, techniques were developed
on a digital computer to determine heat flow from the air into the
Details of Hypersonic Flow Emerge
From these analyses, the details of hypersonic flow began to unfold.
The results confirmed many of its nonlinear characteristics. The
data also confirmed another peculiar trend of hypersonic flight: the
reduced importance of the wings for lift. At Mach 6 and 25° angle
of attack, the large fuselage and side fairings on the X-15 contribute
70 percent of the total lift, enough to permit reentry from an altitude
of 250 000 feet with fuselage lift alone.
As the shock waves trail aft from the fuselage nose, canopy, side
fairings, wing leading edge, and other protubtrances, they interfere
with the flow and cause further changes in flow angle and pressure
forces. The wing and fuselage also induce a swirling motion in the
airflow as it sweeps aft. Another significant change in flow occurs
whenever the airplane pitches to a different angle of attack, for this
alters the position of the shock waves sweeping aft.
The consequences of these interactions become apparent when
flow impinges on the tail surfaces, which provide the means of control
as well as the major part of the stability. They may have a favorable
effect on the balance between stability and control. In the case of
pitch control, the X-15 can be maneuvered to higher angle of attack
at Mach 6 than at Mach 3.
At high angle of attack, the changes in flow angle influence the
forces on the lower vertical tail, which becomes more effective. The
upper vertical tail, on the other hand, comes into a region of lower
pressure, and loses much of its effectiveness. The lower vertical tail
is able to offset this, though, and provides adequate directional
stability to the highest angle of attack attainable - a lack of which
proved so disastrous to the X-1A and X-2.
In solving the directional-stability problem, a new difficulty
manifested itself. The force on the lower vertical tail that
stabilizes the airplane also tends to roll the plane whenever
the counterbalancing force on the upper vertical tail is lacking.
This type of motion has always plagued pilots,
and aircraft designers try to obtain a balance
between the rolling and yawing motions that the pilot must counteract.
On conventional aircraft, which have virtually all the vertical
tail above the fuselage, the roll is in a direction that eases the pilot's
control problem. In the X-15 configuration, however, yawing produces
an adverse rolling moment, which severely complicates the pilot's
This adverse roll was of great concern during reentry flight at
high angle of.attack, and will be dealt with in more detail in a later
section. It is sufficient to point out here that it was a major problem
during the flight program. Fortunately, the lower half of the
lower vertical tail is jettisoned prior to landing. Thus, a logical
first approach to the stability problem was to remove this surface,
thus reducing the magnitude of the adverse moment. This also reduced
directional stability to marginal levels at certain other flight
conditions, but a positive increment of stability was obtained by the
use of the speed brakes. Various combinations of lower vertical tail
and speed-brake position have enabled the X-15 to explore a wide
range of aerodynamic characteristics; in effect, to simulate several
different aircraft configurations.
From this, designers have gained a clearer understanding of the
delicate balance between stability and control for reentry at high
angle of attack. The X-15 program has provided insight into theoretical
methods used to calculate flow conditions and forces for
hypersonic flight. Because of the complexities of hypersonic flow,
the calculations are normally made for an isolated fuselage, wing,
and tail, to which are added the incremental effects of mutual
interference from shock waves and flow. Another assumption of the
theories is that the airplane is treated as composed of straight
surfaces, a cone-cylinder, for the fuselage and flat plates for the wings
and tail. The theoretical methods also derive from assumptions of
flow conditions at low angles of attack. Yet, these methods were
successfully used to include the high-angle-of-attack flight of the
X-15. In some cases, pressures on the wing and fuselage could be
computed from simplified theories that ignore interference effects.
But the key to closer agreement between theory and fact was through
approximating as many of the interaction effects and nonlinearities
as possible. One flaw in the theories was uncovered, however. In
the region of the horizontal tail, the flow is too complex for available
theories to predict the amount of control for maneuvering to high
angles of attack.
The X-15's aerodynamic measurements have verified the aerodynamic
results of various wind-tunnel tests. Supersonic and hypersonic
tunnels have rather small test sections, some only nine inches in
diameter. This requires the use of very small models, a fact that
increases uncertainty when the results are extrapolated to a
full-scale airplane. However, measurements in six supersonic and
hypersonic wind tunnels at NASA's Langley Research Center and Ames
Research Center, and at the Massachusetts Institute of Technology
and Jet Propulsion Laboratory, have shown remarkable agreement
with flight results. Significantly, this was the first correlation with
full-scale flight data.
One area of discrepancy was found - in drag measurements. The
tunnels provided accurate measurements of all the various components
of drag except that produced by the blunt aft end of the
airplane. This component was found to be 15 percent higher on
the actual airplane - another area for further research.
From this emerging profile of aerodynanic flow has come a clearer
understanding of the peculiarities of the forces from subsonic to
hypersonic speeds and to 25° angle of attack. In addition, it has
helped pin down some flaws in aerodynamic theory and wind-tunnel
testing. As valuable as this research has been, it is of a rather
complementary nature. But in the field of aerodynamic heating,
fundamental contributions to hypersonic aerodynamics have been made.
This is, perhaps, a normal consequence, since it was an area with
significant unknowns, not only during the feasibility studies and the
design but until recent flights. Whereas consideration of aerodynamic
forces was basically an extension of previous experience,
aerodynamic heating of an airplane by the airflow was a completely
new factor. Not the least of the difficulties has been to develop
flight-test procedures and techniques to analyze structural heating
from a high-temperature airflow.
One part of the problem that was well understood from the beginning
pertained to the heating of air particles as aircraft speeds
increased. As the particles are pushed out of the path of the airplane,
some are accelerated to the speed of the plane and undergo
a huge change in kinetic energy. This energy is imparted to the
molecules in the form of heat, which raises the air temperature an
amount proportional to the square of the velocity. At Mach 6, this
heat energy raises air temperature to 2500° F, although only within
a thin layer of air near the leading edges of the aircraft's wing and
tail surfaces, cockpit canopy, etc.
The heat flow from the high-temperature air into the external
skin of an airplane presents a complex problem, less well understood.
Some early theoretical analysis dates from the 1900's, but,
paradoxically, scientists at that time were concerned with the
transmission of heat energy from the airplane to the atmosphere; they
were trying to solve the problem of cooling aircraft engines. But
the basic mechanism is identical for the X-15 - the transfer of heat
energy between a fluid and the surface over which it passes.
When the X-15 entered the picture, in the early 1950's, several
theories of a semi-empirical type had been developed. The methods
were based on assumed flow conditions with approximate solutions,
and, although showing some agreement, they showed significant
differences. Experimental results were meager, and one thorough
series of tests, conducted to determine which theory was more
accurate, showed trends that contradicted theoretical analysis. The
basic problem is insufficient understanding of the flow properties
in the layer of air near the skin.
Analysis shows that the heat-energy flow into the skin from
high-temperature air increases in approximate ratio to the cube of the
velocity. Thus, at Mach 6, the X-15 absorbs eight times more heat
than it encounters at Mach 3. (This assumes that loss of heat energy
from the aircraft by radiation from the structure back to the
atmosphere is small, which is the case for the X-15. At higher structural
temperatures, radiation is a predominant feature, which aids
in cooling the structure.)
Heat flow is also a function of air pressure, and the regions of
highest heating are found on frontal and lower surfaces that
encounter the full impact force of airflow. An alleviating effect comes
from flights to high-altitude, low-air-density conditions. In this
region, even high air temperatures transfer little heat into the structure.
Conversely, the highest structural temperatures encountered
with the X-15 have been at Mach 5 and relatively low altitude.
Only a small fraction of the total heat energy of the air is
conducted into the aircraft structure. The predominant factors are
the heat-conduction and -insulation characteristics of the hot boundary
layer of air enveloping the aircraft. Where this layer of air
flows in even streamlines along a surface, the heat transfer is small
and predictable. But here the viscosity of air is the chief difficulty.
One of air's most intransigent characteristics is that boundary-layer
flow that starts out in smooth streamlines suddenly changes to a
turbulent, eddying type of flow. This turbulent flow is not unusual.
It is the normal condition of the flow over much of the X-15. But
it introduces problems of major proportions. In spite of never-ending
efforts to understand the mechanics of it, it remains a largely
unpredictable phenomenon, even for subsonic flow.
With the X-15 and succeeding airplanes, boundary-layer flow
assumes major significance because of its effect on aerodynamic
heating. Turbulent flow breaks up the insulating properties of airflow
near the surface, and can increase the heat flow by a factor of
six over non-turbulent, or laminar, flow. The irregular nature of
the flow, moreover, makes calculation of the heat transfer across the
boundary layer a highly speculative proposition.
well-dotted sketches above indicate locations of hundreds of research and
systems sensors aboard the X-15. The sensors measure pressures, temperatures,
strains, accelerations, velocities, control positions, angles, and physiological
The outline drawing below shows maximum temperatures that the X-15 has
experienced to date, and where they were recorded.
Consequently, the research contribution of the X-15 data to
aerodynamic heating has been through clearer understanding of
heat transfer and local flow conditions across a turbulent boundary
layer. This pioneering work showed initially that heat flow into the
X-15 was 30-40 percent lower than predicted by available theories.
This large discrepancy, while favorable to keeping structural
temperatures low during flight to high speed, stimulated further
analysis of the flow conditions.
It appeared at first that the answer might lie in the difference
between the type of shock wave assumed for the theories and the kind
encountered in flight. Theory was based upon flow around pointed
surfaces, with the shock wave attached to the surface and trailing
aft in a straight line. In actuality, the blunt leading-edge surfaces
of the X-15 produce curved shock waves which remain positioned
ahead of the leading points. These differences were disproven as a
factor, however, through a series of research flights with a specially
fabricated vertical tail with a sharp leading edge, which duplicated
the theoretical model. No measurable difference from heat transfer
with a blunt leading edge was detected.
An exact understanding of the differences between theory and
fact is still to be found. Accurate knowledge of heat flow into the
X-15 structure has been obtained, however. From these data,
empirical factors have been developed that enable designers to
predict structural temperatures for proposed flight trajectories with
good accuracy. They are confident that these techniques can be
used to predict temperatures to Mach 10 or 12 and smooth the
path for future hypersonic aircraft.
The second part of the boundary-layer-flow problem, which concerns
the point at which the flow becomes turbulent, remains as
obscure as it was in 1954. Boundary-layer flow typically becomes
turbulent whenever the viscosity forces binding the streamlines
together are overcome by the pressure forces of the airflow along
the surface. On the X-15 wing, this normally occurs anywhere
from 4 to 12 inches aft of the leading edge. lt has not been possible
to correlate the viscosity and pressure forces so as to provide a means
for accurately predicting this phenomenon. Lacking this knowledge,
designers are forced to make conservative assumptions for the higher
heating of turbulent flow, as in the case of the X-15.
One of the many tools of the X-15's research is this multiple-probe pressure rake,
mounted on the forward fuselage to measure boundary-layer airflow at hypersonic
speeds. Below the rake is one of the 140 holes cut in the aircraft's skin to measure
surface pressures. Above and behind the rake is a pressure probe, used only
during landing, for the pilot's airspeed indicator.
Thus, hypersonic flow has yet to reveal all its secrets. Enough is
known, though, to provide a basic understanding of the pressures
and heat input along the wing and the fuselage. In localized areas
with large discontinuities interference effects, the flow is too complex
to yield to a generalized analysis. For example, the wing-fuselage
juncture, tail-fuselage juncture, and canopy obstruction
create chaotic combinations of multiple shock waves and cross-flow
conditions, especially at high angle of attack. Since these effects
are synonymous with uneven pressure and heating, the loads and
thermal stresses are equally obscure. Sometimes the magnitude of
the unknowns was uncovered only when localized structural failures
occurred, unexpectedly and dramatically upsettung the tempo of the