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"On October 14, 1947, the
Bell X-1 became the first airplane to fly faster than the speed of sound. Piloted
by U.S. Air Force Capt. Charles E. "Chuck" Yeager, the X-1 reached a speed of 700 miles per hour, Mach
1.06, at an altitude of 43,000 feet." This article appeared in Air Trails a year later in order
to help introduce and explain supersonic flight to the modeling public. One of the most unanticipated
aspect of supersonic flight was the reversal of aileron control in the transition region. Aerodynamists
quickly figured out what was happening and made design alterations to remedy the problem. BTW, 'Muroc'
mentioned here isMuroc Air Force Base, which was later re-named to Edwards AFB.
Beyond the Sound "Barrier"
If you have fired a gun, you have dabbled in supersonics. But how much do you know? Is there
a sonic barrier? What is compressibility? Here is a clear account of what happens to air and plane at
high speed.
By David A. Anderton
Next to the atomic bomb, there. has been more misinformation thrown around on the general subject
of high-speed flight than on anything else this season. To qualify as an expert, all you need know are
a few catch phrases - "sonic barrier," "compressibility," "guided missiles." These, coupled with almost
anything said hurriedly in between, will make you an authority. If your wife has recently had one of
the new "supersonic" permanents, that should help too.
The whole country has broken out in a rash of high speed lately. It started at the time the Skystreak
broke the world's speed record and then repeated the performance by bettering its own record. More recently,
there has been an asinine exchange of "we done it first" releases between the British and our NACA on
the subject of achieving supersonic speeds. While we are considering these claims, without much regard
for how such speeds were achieved, let' s not overlook the kid next door with a basement rifle range.
He has been getting supersonic velocities every time he pulled the trigger of his .22, and with very
little effort and no special equipment. As a matter of fact, probably the first supersonic velocities
in this country were reached by rifle bullets shortly after the Civil War. To ballistics experts, the
word "supersonic" is no stranger.
But there are many questions about high-speed flight that have to be answered in a simple form if
the average person is to acquire some sort of knowledge about the subject. For instance, what is this
business about the sonic barrier, anyway? Why do we have to pierce it? Of what value are the missile
experiments and the speed runs at Muroc? How did the speed of sound get tied up with the whole thing
?What is Mach number and what good is it? Taking first things first, maybe we can work some sense out
of these questions in a logical order.
Perhaps the best way to start is with a simplified approach to an understanding of compressibility.
This involves mentioning how the speed of sound got tied up in the whole thing, and why we speak of
critical Mach numbers. So, if you are willing to accept the concept of air molecules as little elastic
particles that bounce around madly when agitated, we may go on from there.
Fig. 1 - Mach cone at sonic.
One of the unique things about a fluid such as air is that if a sudden change in pressure occurs
somewhere, the disturbance moves along a spherical wave-front. (You can see a two-dimensional picture
of this when next you heave a rock into a still pond.) Sound, it has been found from basic studies,
travels as a wave-form sort of energy, expanding in space in spherical waves from the source. It doesn't
take much to see that sound will very likely travel with the same velocity as the pressure disturbance
we mentioned, because sound is a series of controlled pressure disturbances in the air - alternate compressions
and rarefactions, if you will. Well, that is what happens - the sound wave-front travels with the same
speed as the pressure disturbance. Furthermore, the wave-front, or little compression wave, weakens
in intensity as it gets farther away from the source. Remember these two things.
Now we are ready to talk about airflow. We want to consider three speed ranges: subsonic, trans-sonic
and supersonic. You'll note that all of these contain the syllables "sonic"; that's because they are
all compared to the speed of sound: Why? Actually, for convenience in speech; more correctly; they should
be compared to the propagation speed of a pressure disturbance in air. Those last nine words are merely
saying, as we learned above, the velocity (propagation speed) of sound (a pressure disturbance in air).
The speed ranges are generally defined by giving limiting Mach numbers. (Is it still necessary to
say that Mach number is the ratio of the speed of the airplane or missile to the speed of sound?) Subsonic,
for instance, goes from a Mach number of 0 to one of 0.85; trans-sonic, from 0.85 to 1.2; supersonic,
from 1.2 on up. These boundaries, by the way, are drawn with a wide brush, mostly for convenience:
At subsonic speeds, flow conditions are simple. Suppose we think about any airfoil shape, and see
what happens. The air, blowing at the ,wing, divides and goes above and below the surface. The reason
it divides is because each air molecule was told what was coming. And the reason it was told is that
a little area of high pressure, creating a disturbance, builds up on the nose of the airfoil. The air
particles flow from areas of high pressure to low pressure; they can be repelled, in a sense, from high
pressure areas. So the oncoming airstream divides and flows around the wing. And just so long as the
speed of that oncoming air is low compared with the propagation speed of a pressure disturbance in air
(velocity of sound), the warning reaches out upstream, and the air molecules are told in time.
Suppose now we increase the speed of the plane, or of the air over the wing. We know 'that the air
passing over the upper surface of a wing is traveling faster than an equivalent particle passing outside
the wing along a chord line. (A straight line is the shortest distance between two points.) Slowly the
airplane approaches sonic speed, and if we could see it, here's what would happen.
The air on the upper surface of the wing (or for that matter, a cowling, canopy, fuselage nose or
duct inlet) gets very close to, and finally reaches sonic speed while the rest of the airplane is traveling
somewhat slower. And at sonic speed, a shock wave forms - and those eight words are going to take a
lot of explaining. Chapters have been written on the mechanics of shock waves, but most of them are
too technical. Too many make the mistake of assuming that a shock exists, and then proving, thermodynamically,
that it can. If we think about the propagation of compression waves some more, we can probably get a
good picture of what happens in sonic flow. In fact, we can draw some pictures that will show exactly.
For this, we need a compass and a scale. Let some convenient unit represent the speed of sound, and
other speeds be expressed by multiples or fractions of that unit. Suppose the air is traveling at sonic
speed; we can draw a number of points, all one unit apart, which represent positions of a wing leading
edge, for example, on successive seconds. In Figure 1, ten points were used. At time = 10 seconds, the
sound wave has just begun to propagate. At the distance corresponding to nine seconds, it is one second
old. It can be represented on paper by a circle of one-unit radius. This goes right on down the line
to the zero point, at which point the pressure sphere is ten seconds old.
Fig. 2 - Mach cone at M = 2.0.
See what's happening? All these spheres of disturbance are tangent, and they show an envelope of
common boundaries. If you do this for enough points, a straight line envelope results, perpendicular
to the air stream. This envelope represents the sum of many little pressure disturbances, adding and
reinforcing each other until they are very strong. This envelope is a shock wave. Simple?
If you draw the same sort of a diagram for a supersonic speed, say twice that of sound, you get the
conical shape shown in Figure 2. These shock wave envelopes are sometimes called Mach cones, or if observed
or drawn on a plane surface, called Mach waves.
Here, then, are physical pictures of what happens to the air over a wide range of speeds. There are,
however, lots of other things to consider. The way these shock waves were so easily drawn can't begin
to hint at the trouble they may cause. Some of these troubles have been named "compressibility effects,"
and have been ascribed to the so-called "sonic barrier." We've seen how and why these shock waves form
at near-sonic speeds, but what effect do they have?" Let's talk about this for high subsonic Mach numbers.
One of the seeming paradoxes of supersonic flight apparently violates the laws of continuity at first
glance.
A normal shock wave, and by normal we mean perpendicular to the airflow, has supersonic velocity
upstream of it, and subsonic downstream! Picture an aircraft flying at just under sonic speed, and with
a normal shock standing out from the upper surface of the wing. Ahead of the shock, the air flows faster
than the plane is traveling; behind the shock, the air flows slower than the airplane's velocity. Confusing,
isn't it?
Actually, however, everything is all right. There are compensating pressure and density changes so
that the equations of continuity are satisfied. But worrying about this, or trying to calculate the
process through the sonic speed range, has annoyed aerodynamicists.
It has been determined, from wind tunnel tests, that the normal shock wave on a wing moves aft as
the airplane speed approaches sonic. Just a fraction of a mile per hour below sonic, the shock wave
is at the trailing edge of the wing, and a bow shock is beginning to form at the leading edge. At sonic
speed, the shock attaches at the trailing edge, and the bow shock builds up strongly. What this does
to the airflow over a wing is the real danger in the "sonic barrier." Under conditions of such a shock
system, the flow over the wing is first subsonic, then part supersonic and part subsonic, then almost
all supersonic, then all supersonic. Since airflow speed alters the pressure distribution over a wing,
the air loads on the wing change terrifically during this procedure. These changes may, in fact, become
severe enough to rip the wings off completely. And the control surfaces in the wake of all this flow
shift are affected, too. The changes in flow-angle off the wing trailing edge (usually called "downwash")
cause large variations in the tail loads, resulting in violent pitching motions and loss of control.
Fig. 3 - Sweepback kids the air.
Fortunately, there are means of keeping an aircraft relatively free of compressibility troubles.
For instance, wind tunnel tests have shown that a thin airfoil section is capable of delaying compressibility
effects to a higher subsonic Mach number than a thick section. An example of this can be quoted. For
the NACA 16-500 series, the critical Mach number of a 20% thick section is about 0.68. By thinning the
same section to 5%, the critical Mach number is raised to 0.76. At sea level, this means an increase
in speed of 61 miles per hour, from 519 mph to 580 mph. Not bad, not bad!
Following this lead, aircraft designers began to consider thinner wing sections for fighters and
high-speed bombers. But right away, they ran into another problem: a thin wing has less physical dimension
for structure than a thicker one. Structure is what we needs most as aircraft speeds go higher and higher,
because the loads get greater and greater. However, it was possible to build a thin, strong wing, once
the designers could be weaned away from conventional practice. (The Bell XS-1 has a very thin wing,
on the order of 8%, and uses an ingenious and different type of construction.)
A great contribution was made by the Germans during the last months of World War II. Our technical
missions to Germany reported that all of their new aircraft designs featured wings with large amounts
of sweepback in order to delay compressibility effects. (A good disussion of sweepback was presented
in the February 1948 Air Trails.) However, the war ended before the Germans were able to get many practical
flight tests on such wings. Their capabilities were indicated by the fact that most of the calculated
performance data showed top speeds around 0.9 Mach number, much faster than any fighters we were playing
with at the time, or would have had in time. As a matter of fact, we still don't have such fighters.
There are two things to remember about sweepback. First, all it does is to kid the air into thinking
that it is flowing over a thinner section than the wing actually has. Figure 3 shows what is meant.
If the wing is a 10% thick section, then the thickness is 10% of line AB, following usual nomenclature.
But the air flows along AC, which is longer. Consequently, section AC is an airfoil of less than 10%
thickness, and as was stated above, we gain a fraction of a Mach number that way. If the angle of sweepback
is 45°, that section at AC is only about 7.1 % thick.
The second thing to remember about sweepback is that the reasons for sweeping a wing are completely
different for subsonic speeds than they are for supersonic speeds. Subsonic reasons have been covered
above. At supersonic speeds, the wing leading edge is swept to keep it completely inside the Mach cone
so that the flow over the wing will be uniform (Figure 4a). It is difficult to calculate what happens
if the Mach wave crosses the wing surface some place (Figure 4b).
The only way of avoiding compressibility effects on control is to locate the controls out of the
wake of the wing. That is why the xS-1 and Skystreak have elevator and stabilizer perched high on the
vertical tail. Going a step further, there's a good reason for tailless aircraft. If there's no tail,
there are no compressibility troubles with the tail. (It's not as easy as that, but that's another story.)
So, by doing these things - thinning the wing, sweeping it back, getting the tail out of the way
- an airplane can be kept relatively trouble-free up to a pretty high subsonic Mach number. And remember,
the present world's speed record (Skystreak, 650.6 mph) represents only about 0.82 Mach number. We have
a long way to go to reach sonic.
Why do we have to reach sonic speeds, or beyond? First, let's not fool ourselves by assuming that
transports or light-planes are going to fly at supersonic or even near-sonic speeds in the conceivable
future. The faster you go, the more it costs - above certain limits. A rocket-propelled transport is
very unlikely for domestic service. The answer to this question lies in its military implications. We
want military aircraft, or missiles, that can go fast and be safe for a pilot, or free from the danger
of breaking up in flight if we carry no pilot.
But we don't know what happens through sonic speed. There has not been enough experience with trans-sonic
flight to draw any general conclusions. Only test work can provide the experience and data so urgently
needed. We do know, from hundreds and thousands of wind tunnel tests and flight tests, what generally
happens below Mach numbers of 0.85. Occasionally even here, something eludes us and causes trouble,
in spite of 45 years of aeronautical development.
At supersonic speeds, one can judge fairly well what happens to wingless missiles or bullets, because
of the long background of ballistic studies here and abroad. The never-never land of the trans-sonic
range still has to be explored and mapped.
That's why we make missile experiments. That's why the NACA drops special models from aircraft to
reach sonic speeds, and why the Skysteak and Skyrocket were developed. We know nothing about the trans-sonic
range except what we guess, or extrapolate.
Wind tunnels? Not at trans-sonic speeds - the tunnel will "choke" with a shock wave, probably at
the model, because of the reduction in test section area. Only recently did Lockheed announce its ingenious
"hump" technique for obtaining trans-sonic speeds in a high-speed tunnel. If you've seen their releases,
you know that in the tunnel test section, the air is locally accelerated by a hump on the floor. A drawback
here is that a small model must be used, to avoid blocking off too much area. Further, it must be a
half-model, complete only to the centerline of the aircraft, in order to avoid unsymmetrical effects
due to the presence of the floor.
That leaves only flight tests of many sorts. Piloted aircraft are approaching sonic speeds from the
low side, accelerating towards a Mach number of one. Dropping specially prepared models has been done
too, because they reach super-sonic speeds on the way down. Fastening a small model half-wing to a fighter's
wing gives an open-air tunnel using the "hump" technique, a la Lockheed. Or rockets can be fired; and
data taken as they decelerate through sonic speed.
All this is for education. We must probe, not pierce, the sonic range; map it, not ride through it;
respect it, not fear it. We must know more, we must do research work, we must be capable of intelligent
design of extremely fast aircraft. The value of the XS-1, the Skystreak, the Skyrocket and other aircraft
to follow, lies in their ability to fly, under control, closer and closer to the speed of sound.
Fig. 4 - Supersonic sweepback.
As with other developments, new and technical, a great deal of misinformation has been circulated
on the subject of high-speed flight. Some of these have been collected for refutation here.
Fallacy 1: If a rocket or ramjet engine cuts at supersonic speeds, the
resultant deceleration will kill the pilot.
Acceleration or deceleration, which are the same except for choice of direction, are caused by a
difference between thrust and drag. A little simple algebra and one of Sir Isaac Newton's handy laws
show this neatly.
We know that force (F) equals mass (m) times acceleration (a):
F = ma
Further, in power-on, level flight, force is thrust minus drag, mass is weight (W) divided by gravity
(g) and acceleration (a) is just that. So we rewrite the equation:
T - D = (Wa)/g
Now, the ratio of a, the acceleration produced, to g, the acceleration of gravity, is spoken of as
the number of g's of acceleration. If a = 64 feet/second/second, we call it a 2g acceleration. So we
can again rewrite the equation to get the ratio of a to g on one side:
a/g = (T-D)/W
What this last equation says is that the number of g experienced by the pilot will be the difference
between thrust and drag, divided by the weight. Incidentally, for the special case of vertical rocket
flight where drag is small, the right hand numerator is thrust minus weight, not drag.
In general, the thrust of any supersonic airplane is not going to be much more than three or four
times its weight, if that much, The V-2, for example, has a thrust roughly twice its weight. The XS-1
has somewhat less thrust than weight at launching, somewhat more at empty weight. For the equilibrium
conditions of level flight, thrust and drag are equal; otherwise the airplane speeds up or slows down.
Therefore, if the engine is cut at maximum speed, this equilibrium is upset, and the only force acting
is drag. But we saw that drag, equaled thrust at top speed, and that thrust was maybe only three times
the weight. Put those numbers in the equation, and what kind of a terrific deceleration do you get?
3g! My maiden aunt could take that with her corset off, and not come apart.
It would be possible, it seems, to get a little more than 3g. For instance, if the plane were still
accelerating to its maximum speed, and then the engine cut, the change from acceleration to deceleration
could be a little more than 3g, but certainly nowhere near the 100g body-ripper of the misinformed.
Well, somebody says, how about the . way the drag builds up through the sonic range? If the plane
is slightly super-sonic and begins to slow down, the drag will build up faster, which will slow down
the aircraft even more, and so on. That's correct. But the airplane, in order to get through the sonic
speed range, had to have more thrust than drag at sonic speed. If we consider what happens coming back
from the supersonic speed, we can see that the deceleration increases from the reduction in speed, and
that the maximum deceleration will occur where the drag, in pounds, is highest. We know that happens
at, or very close to, sonic speed, and we also know that that drag will not be more than two or three
multiples of. the aircraft weight. It's still only about 3g or so.
The British De Havilland 108 used for high speed research. Sweptback all-wing configuration
greatly retards compressibility.
Fallacy 2: Mach waves are opaque, and the pilot of any supersonic plane
will therefore be flying blind.
No! That fallacy has appeared twice in magazines of national circulation. It is not true.
If a shock wave were opaque, you could see an opaque cone in the air marking the location of a bullet
you just fired. Have you ever seen such a phenomenon, even on a well-lit indoor target range?
There are two sources from which this fallacy may have sprung. First, there are condensation shocks.
These are analogous to a shock wave, except that they can occur at very low speeds. For instance; these
can be observed during a flight through clouds or fog. Little wisps of denser fog come off the prop
and the wing. Also, in landing in a moist atmosphere, a plane may suddenly shed a long sheet of condensed
water vapor, which looks opaque. High altitude vapor trails are another example of condensation shocks.
But all these only occur under special conditions of atmospheric pressure, temperature and humidity.
Generally, supersonic planes will be flying at very high altitudes where the water content of the atmosphere
is very low indeed. We would not expect to get condensation shocks very often under such conditions.
Second, there are pictures of bullets, for example, taken in flight, showing the Mach waves as heavy,
dark lines. Don't be fooled - these are taken by special optical means. There are three of these means:
schlieren, shadowgraph, and interferometer. Without going into the details, suffice it to say that all
three of these methods make use of the density changes through a shock wave. These density changes bend
the light rays much as a lens does, and vary the illumination intensity on a photographic plate. What
the plate "sees" then, are gradations in density. And the more rapid the gradation, the sharper the
line or area on the plate. Consequently, a Mach wave, which has a large density change across it, shows
as a dark line of strong intensity. Remember, a shock wave .is no more opaque to the unaided eye than
any other hunk of compressed air.
Fallacy 3: There is a sonic barrier, a stone wall of compressibility.
The only sonic barrier is the headwind built into an airplane by the designer, or by military or
other requirements. There is no reason that any airplane cannot exceed the speed of sound, or any reasonable
multiple thereof, if the power is available, and the design is clean.
But to make the design clean, aye, there's the rub. Cockpits protrude, wings and inlet ducts have
rounded leading edges, access doors and landing gear fairings make for slight unevenness in contour.
So, only if the designer sets out to build a purely research aircraft, can he use the external lines
he must to avoid the built-in headwinds. It is axiomatic that the cleaner the airplane, the lesser the
drag, the faster it will go.
Posted February 7, 2015
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