Substituting:
4 × (100) = 6 X P Pz = 1 × 100 Pz = 66.6 p.s.i.g.
A gas which conforms to Boyle's law is termed an ideal gas. When pressure is increased upon such a gas, its volume decreases proportionally and its density is increased. Thus, the density of a gas varies directly with the pressure, if the since gases expand when heated and contract when cooled.
The useful applications of Boyle's law are many and varied. Some applications more common to aviation are: (1) The carbon dioxide (CO2) bottle used to inflate life rafts and life vests;
(2) the compressed oxygen and the acetylene tanks used in welding; (3) the compressed air brakes and shock absorbers; and (4) the use of oxygen tanks for high-altitude flying and emergency use.
Charles' Law The French scientist Jacques Charles provided much of the foundation for the modern kinetic theory of gases. He found that all gases expand and contract in direct proportion to the change in the absolute temperature, provided the pressure is held constant. In equation form, this part of the law may be expressed.
ViTz = V211 or Vz = I= This equation means that with a constant volume, the absolute pressure of a gas varies directly with the absolute temperature, Examples of Charles' law: A cylinder of gas under a pressure of 1,800 p.s.i.g. at 70° F. is left out in the sun in the tropics and heats up to a temperature of 130° F. What is the new pressure within the cylinder? The pressure and temperature must be converted to absolute pressure and temperature.
Formula or equation:
P1 = T= Pz Using the Rankine system:
70° F. = 530° absolute 130° F. = 590° absolute Substituting:
1,800 + 14.7 530 Pz - 590 Then:
P2 = (590) (1,814.7) 530 Pz = 2,020 p.s.i.a.
Converting absolute pressure to gage pressure:
2,020.0
- 14.7 2,005.3 p.s.i.g.
Free balloon flights into the stratosphere, the expanding gases of jet-propelled aircraft, and the effects of clouds and weather on instrument recordings may be explained by the use of Charles' law. Here are practical applications of a law of physics that aid the pilot, air controller, and aerographer in their work. Flying is made safer when humans are able to apply this law in handling weather data so vital to aviation.
General Gas Law The facts concerning gases discussed in the preceding sections are summed up and illustrated in figure 7-8. Boyle's law is expressed in (A) of the figure, and the effects of temperature changes on pressure and volume (Charles' law) are illustrated in (B) and (C), respectively.
By combining Boyle's and Charles' laws, a single expression can be derived which states all the information contained in both. This expression is called the general gas law, a very useful form of which is given in the following equation.
(NOTE: The capital P and T signify absolute pressure and temperature, respectively.)
PAL - PE 229
Then:
Volume 179.4 540 = = 760 Temperature constant Pressure constant Pz = (179.4) (760) constant = ₽z 540 PęV½ Pr = 252.5 p.s.i.a.
Converting absolute pressure to gage pressure:
252.5
- 14.7
237.8 p.s.i.g.
(А) (B) (C) 2. Four cubic feet of a gas at 75 p.s.i.g. and 80° F. are compressed to 237.8 p.s.i.g. and heated FIGURE 7-8. The general gas law. to a temperature of 300° F. What is the volume of the gas resulting from these changes?
Formula or equation:
three equations are special cases of the general An examination of figure 7-8 reveals that the PIV: = I= Pavz equation. Thus, if the temperature remains constant, T1 equals T2, and both can be eliminated from the general formula, which then reduces to Using the Rankine system:
the form shown in (A). When the volume remains 80° F. = 540° absolute constant, V1 equals V½, reducing the general equa- 300° F. = 760° absolute tion to the form given in (B). Similarly P, is equated to P2 for constant pressure, and the Substituting:
equation then takes the form given in (C).
(75 + 14.7) (4) (237.8 + 14.7) V½ The general gas law applies with exactness only to "ideal" gases in which the molecules are as- 540 760 sumed to be perfectly elastic. However, it de- Then:
scribes the behavior of actual gases with sufficient accuracy for most practical purposes. (89.7) (4) = (252.5) V½ Two examples of the general equation follow: 540 760
1. Two cu. ft. of a gas at 75 p.s.i.g. and 80° F.
are compressed to a volume of 1 cu. ft. and then heated to a temperature of 300° F. What is the V2 = 540 × 252.5 358.8 × 760 new gage pressure? V½ = 2 cu. ft.
Formula or equation:
Avogadro's Law An Italian physicist, Avogadro, conceived the theory that "at the same temperature and pres- Using the Rankine system: sure, equal volumes of different gases contain equal numbers of molecules." This theory was 80° F. = 540° absolute proven by experiment and found to agree with 300° F. = 760° absolute as Avogadro's law. the kinetic theory, so it has come to be known Substituting:
Dalton's Law (75 + 14.7) (2) = Pг(1) If a mixture of two or more gases which do not 540 760 combine chemically is placed in a container, each 230
equally and undiminished in every direction throughout the column-forward, backward, and sideways-so that the containing vessel is literally filled with pressure.
If a gas is used instead of a liquid, the force is transmitted in the same manner. The one difference is that gas, being compressible, provides a much less rigid force than the liquid, which is incompressible. (This is the main difference in the action of liquids and gases in fluid power systems.)
(A) (B) FIGURE 7-9. Transmission of force:
(A) solid; (B) fluid.
gas expands throughout the total space and the absolute pressure of each gas is reduced to a lower value, called its partial pressure. This reduction is in accordance with Boyle's law. The pressure of the mixed gases is equal to the sum of the partial pressures. This fact was discovered by Dalton, an English physicist, and is set forth in Dalton's law: "A mixture of several gases which do not react chemically exerts a pressure equal to the sum of the pressures which the several gases would exert separately if each were allowed to occupy the entire space alone at the given temperature."
Pascal's Law The foundations of modern hydraulics and pneumatics were established in 1653 when Pascal discovered that pressure set up in a fluid acts equally in all directions. This pressure acts at right angles to containing surfaces. Thus, in figure 7-10, if the liquid standing on a square inch (A) at the bottom of the tank weighs 8 pounds, a pressure of 8 p.s.i. is exerted in every direction at (A). The liquid resting on (A) pushes equally downward and outward. The liquid on every square inch of the bottom surface is pushing downward and outward in the same way, so that the pressures on different areas are in balance.
At the edge of the tank bottom, the pressures act against the wall of the tank, which must be strong enough to resist them with a force exactly equal to the push. Every square inch of the bottom of the tank must also be strong enough to resist the downward pressure of the liquid resting on it. The same balance of pressures exists at every other level in the tank, though of lesser pressures as one approaches the surface. There- Transmission of Forces Through Fluids When the end of a bar is struck, the main force of the blow is carried straight through the bar to the other end (see A of figure 7-9). This happens because the bar is rigid. The direction of the blow almost entirely determines the direction of the transmitted force. The more rigid the bar, the less force is lost inside the bar or transmitted outward at right angles to the direction of the blow.
8 p.s.i.
When a force is applied to the end of a column of confined liquid (B of figure 7-9), it is transmitted straight through to the other end and also FIGURE 7-10. Pressure acting on a container.
Surface Head FICURE 7-11. Pressure relationship with shape.
fore, the liquid remains at rest; it does not leak out and the tank does not collapse.
One of the consequences of Pascal's law is that the shape of the container in no way alters pressure relations. Thus, in figure 7-11, if the pressure due to the weight of the liquid at one point on the horizontal line (H) is 8 p.s.i., the pressure is 8 p.s.ì. everywhere at level (H) in the system.
Pressure due to the weight of a fluid depends, at any level, on the height of the fluid from the level to the surface of the fluid. The vertical distance between two horizontal levels in a fluid is known as the head of the fluid. In figure 7-11, the liquid head of all points on level (H) with respect to the surface is indicated.
Pressure due to fluid head also depends on the density of the fluid. Water, for example, weighs
62.4 lb../eu. ft. or 0.036 lb./cu. in., but a certain oil might weigh 55 lbs./cu. ft., or 0.032 lb./cu. in.
To produce a pressure of 8 p.s.i., it would take 222 inches of head using water, and 252 inches of head using the oil (see figure 7-12).
on a unit area of the surface acted upon. In hydraulics and pneumatics, pressure is expressed in pounds per square inch. Thus, pressure is the amount of force acting upon 1 square inch of area.
Computing Force, Pressure, and Area A formula, similar to those used with the gas laws, is used in computing force, pressure, and area in fluid power systems. Although there appears to be three formulas, it is only one formula written in three variations, where P refers to pressure, F indicates force, and A represents area.
Force equals pressure times area. Thus, the Force and Pressure In order to understand how Pascal's law is applied to fluid power, a distinction must be made between the terms "force" and "pressure." Force may be defined as a push or pull. It is the push 8 Ibs. 8 Ibs.
or pull exerted against the total area of a par- Water Oil ticular surface and is expressed in pounds. As previously stated, pressure is the amount of force FIGURE 7-12. Pressure and density relationship 232
circular. Either the diameter or the radius (one half the diameter) may be given. The radius in inches must be known to find the area. Then, the formula for finding the area of a circle is used.
This is written A = Tr?, where A is the area, 7 is 3.1416 (3.14 or 37 for most calculations), and r' indicates radius squared.
FIGURE 7-13. Device for determining the arrangement of the force, pressure, and area formulas.
formula is written:
F = PXA.
Pressure equals force divided by the area. By rearranging the formula, this statement is condensed into:
P= Since area equals force divided by pressure, the formula is written:
A = P Figure 7-13 illustrates a device for recalling these formulas. Any letter in the triangle may be expressed as the product or quotient of the other two, depending upon its position within the triangle.
For example, to find area, consider the letter "A" as being set off by itself, followed by an equal sign. Now look at the other two letters.
The letter "F" is above the letter "P"; therefore, A = F/P.
In order to find pressure, consider the letter "p" as being set off by itself, and look at the other two letters. The letter "F" is above the letter "A"; therefore, P = F/4.
In a similar manner, to find force, consider the letter "F" as being set off by itself. The letters "p" and "A" are side by side; therefore, F= PXA.
Sometimes the area may not be expressed in square inches. If it is a rectangular surface, the area may be found by multiplying the length (in inches) by the width (in inches). The majority of areas to be considered in these calculations are Pressure and Force in Fluid Power Systems In accordance with Pascal's law, any force applied to a confined fluid is transmitted equally in all directions throughout the fluid regardless of the shape of the container. Consider the effect of this in the system shown in figure 7-14, which is a modification of B in figure 7-9. The column of fluid is curved upward to its original level, with a second piston at this point. It is clear that when the input piston (1) is pushed downward, a pressure is created through the fluid, which acts equally at right angles to surfaces in all parts of the container.
Referring to figure 7-14, if the force (1) is 100 Ibs. and the area of the piston is 10 sq. in., then the pressure in the fluid is 10 p.s.i. (110). This pressure acts on piston (2), so that for each square inch of its area it is pushed upward with a force of 10 pounds. In this case, a fluid column of uniform cross section is considered so that the area of the output piston (2) is the same as the input piston (1), or 10 square inches. Therefore, the upward force on the output piston (2) is 100 pounds, the same as applied to the input piston
(1). All that has been accomplished in this system was to transmit the 100-pound force around a Force 1 = 100 Ibs. Force 2 = 100 Ibs.
Input Output
• piston (1) piston (2).
10 sq. in. 10 sq. in.
Pressure 10 Ibs. per sq. inch FIGURE 7-14. Force transmitted through fluid.
Force 1 = 100 Ibs. Force 2 = 100 Ibs. Force 1 = Force 2 = 20 1bs. Output 200 1bs.
piston (2) Piston 1 Piston 2 20 sq. in.
10 sq. in. 10 sq. in Input piston (1) 2 sq. in.
Pressure 10 1bs.
- per sq. inch Pressure 10 Ibs. per sq. inch FIGURE 7-15. Transmitting force through small tube.
bend. However, this principle underlies practically all mechanical applications of fluid power.
At this point it should be noted that since Pascal's law is independent of the shape of the container, it is not necessary that the tube connecting the two pistons be the full area of the pistons. A connection of any size, shape, or length will do if an unobstructed passage is provided Therefore, the system shown in figure 7-15, in which a relatively small, bent tube connects two cylinders, will act exactly the same as that shown in figure 7-14.
Multiplication of Forces In figures 7-14 and 7-15 the systems contain pistons of equal area in which the output force is equal to the input force. Consider the situation in figure 7-16, where the input piston is much smaller than the output piston. Assume that the area of the input piston (1) is 2 sq. in. Pushing down on the input piston (1) with a force of 20 pounds creates 10 p.s.i. (22) in the fluid. Although this force is much smaller than the applied force in figures 7-14 and 7-15, the pressure is the same.
This is because the force is concentrated on a relatively small area.
This pressure of 10 p.s.i. acts on all parts of the fluid container, including the bottom of the output piston (2). The upward force on the output piston
(2) is therefore 10 pounds for each of its 20 square inches of area, or 200 pounds (10 X 20). In this case, the original force has been multiplied tenfold while using the same pressure in the fluid as before. Obviously, the system would work just the same for any other forces and pressures, so 1/10" FIGURE 7-16. Multiplication of forces.
that the ratio of output force to input force is always the same.
The system works the same in reverse. Consider piston (2) in figure 7-16 as the input and piston
(1) as the output. Then the output force will always be one-tenth the input force. Sometimes such results are desired.
Therefore, if two pistons are used in a fluid power system, the force acting on each is directly proportional to its area, and the magnitude of each force is the product of the pressure and its area.
Differential Areas Consider the special situation shown in figure
7-17. Here, a single piston in a cylinder has a piston rod attached to one side of the piston which extends out of the cylinder at one end.
Fluid under pressure is admitted to both ends of the cylinder equally through tubes. The opposed faces of the piston behave like two pistons acting against each other. The area of one face is the full area of the cylinder, for example, 6 sq. in.
The area of the other face is the area of the cylinder minus the area of the piston rod, which is 2 sq. in., leaving an effective area of 4 sq. in.
on the right face of the piston. The pressure on both faces is the same, in this case, 20 p.s.i.
Applying the rule just stated, the force pushing the piston to the right is its area times the pressure, or 120 pounds (20 X 6). Similarly, the force pushing it to the left is its area times the pres.
sure, or 80 pounds. Therefore, there is a net 234
20 1bs. sq. inch Tube.
Tube
14.7 1bs. 14.7 1bs.
Tube per sq. inch per sq. inch.
Piston Cylinder Piston rod 40 Ibs. •
14.7 Ibs. per sq. inch Area Area 14.7 14.7 6 sq. in. 2, sq. inches
14.7 14.7 FIGURE 7-17. Differential areas on a piston.
14.7 14.7 unbalanced force of 40 pounds acting to the right, and the piston will move in that direction.
The net effect is the same as if the piston and cylinder were just the size of the piston rod, since all other forces are in balance. 14.7 Volume and Distance Factors In the systems illustrated in figures 7-14 and 7-15, the pistons have areas of 10 sq. in. each.
Therefore, if one of these pistons is pushed down 1 inch, 10 cu. in. of fluid is displaced. Since liquid is practically incompressible, this volume must go somewhere. In the case of a gas, it will compress momentarily, but will eventually expand to its original volume. Thus, this volume moves the other piston. Since the area of this piston is also 10 sq. in., it will move 1 inch in order to accommodate the 10 cu. in. of fluid. The pistons are of equal areas, and will therefore move equal.
distances, though in opposite directions.
Applying this reasoning to the system in figure 7-16, it is obvious that if the input piston (1) is pushed down 1 inch, only 2 cu. in. of fluid is displaced. In order to accommodate these 2 cu. in.
of fluid, the output piston (2) will have to move only one-tenth of an inch, because its area is 10 times that of the input piston (1). This leads to the second basic rule for two pistons in the same fluid power system, which is, that the distances moved are inversely proportional to their areas.
- 14.7 1bs. per sq. inch' FIGURE 7-18. Effects of atmospheric pressure.
Effects of Atmospheric Pressure Atmospheric pressure, described previously, obeys Pascal's law the same as pressure set up in fluids. As illustrated in figure 7-14, pressures due to a liquid head are distributed equally in all directions. This is also true of atmospheric pres- 235
Higher Pressure Higher pressure Hower pressure here ligh velocity B c FIGURE 7-19. Pressures and velocities in sure tud velocities in a venturi tube.
sures. The situation is the same if these pressures act on opposite sides of any surface, or through fluids. In A of figure 7-18 the suspended sheet of paper is not torn by atmospheric pressure, as it would be by an unbalanced force, because atmospheric pressure acts equally on both sides of the paper. In B of figure 7-18, atmospheric pressure acting on the surface of the liquid is transmitted equally throughout the liquid to the walls of the container, but is balanced by the same pressure acting directly on the outer walls of the container. In C of figure 7-18, atmospheric pressure acting on the surface of one piston is balanced by the same pressure acting on the surface of the other. The different areas of the two surfaces make no difference, since for a unit of area, the pressures are in balance.
Bernoulli's Principle Bernoulli's principle was originally stated to explain the action of a liquid flowing through the varying cross-sectional areas of tubes.
In figure 7-19 a tube is shown in which the cross-sectional area gradually decreases to a minimum diameter in its center section. A tube constructed in this manner is called a "venturi," or "venturi tube."
As a liquid (fluid) flows through the venturi tube, the three vertical tubes act as pressure gages, filling with liquid until the pressure of the liquid in each tube equals the pressure of the moving liquid in the venturi.
The venturi in figure 7-19 can be used to illustrate Bernoulli's principle, which states that:
The pressure of a fluid (liquid or gas) decreases at points where the velocity of the fluid increases.
In the wide section of the venturi (points A and C of figure 7-19), the liquid moves at low velocity, producing a high pressure, as indicated by the height of the liquid in the vertical tubes at these two points. As the tube narrows in the center, it must contain the same volume of fluid as the two end areas. In this narrow section, the liquid moves at a higher velocity, producing a lower pressure than that at points A and C, as indicated by the height of the column of liquid in the vertical tube above point B of figure 7-19.
The venturi principle, in any of a number of shapes and sizes, is used in aircraft systems. They may be referred to as restrictions or orifices. For example, an orifice is generally installed in a hydraulic line to limit the rate of fluid flow. A hydraulically operated aircraft landing gear, when being extended, will tend to drop with great speed because of the weight of the mechanism.
If a restrictor is installed in the hydraulic return line the extension of the gear will be slowed, thus preventing possible structural damage.
ATMOSPHERE General Aviation is so dependent upon that category of fluids called gases and the effect of forces and pressures acting upon gases, that a discussion of the subject of the atmosphere is important to the persons maintaining and repairing aircraft.
Data available about the atmosphere may determine whether a flight will succeed, or whether it will even become airborne. The various components of the air around the earth, the changes in temperatures and pressures at different levels above the earth, the properties of weather encountered by aircraft in flight, and many other detailed data are considered in the preparation of Pascan and Torricelli have been credited with developing the barometer, and instrument for measuring atmospheric pressure. The results of their experiments are still used today with very little improvement in design or knowledge. They determined that air has weight which changes as altitude is changed with respect to sea level.
Today scientists are also interested in how the atmosphere affects the performance of the aircraft and its equipment.
Composition of the Atmosphere The atmosphere is a complex and ever changing mixture. Its ingredients vary from place to place and from day to day. In addition to a number of gases, it contains quantities of foreign matter such 236
as pollen, dust, bacteria, soot, volcanic ash, spores, and dust from outer space.
The composition of the air remains almost constant from sea level up to its highest level, but its density diminishes rapidly with altitude. Six miles up, for example, it is too thin to support respiration, and 12 miles up there is not enough oxygen to support combustion. At a point several hundred miles above the earth, some gas particles spray out into space; some dragged by gravity fall back into the ocean of air below; others never return, but travel like satellites around the earth;
and still others like hydrogen and helium escape forever from the earth's gravitational field. Physicists disagree as to the boundaries of the outer fringes of the atmosphere. Some think it begins 240 miles above the earth and extends to 400 miles; others place its lower edge at 600 miles and its upper boundary at 6,000 miles.
There are also certain nonconformities at various levels. Between 12 and 30 miles, high solar ultraviolet radiation reacts with oxygen molecules to produce a thin curtain of ozone, a very poisonous gas without which life on earth could not exist. This ozone filters out a portion of the sun's lethal ultraviolet rays, allowing only enough to come through to give man sunburn, kill bacteria, and prevent rickets. At 50 or 65 miles up, most of the oxygen molecules begin to break down under solar radiation into free atoms, and to form the incomplete molecule, hydroxyl (OH) from water vapor. Also in this region all the atoms become ionized.
Studies of the atmosphere have revealed that the temperature does not decrease uniformly with increasing altitude; instead it gets steadily colder up to a height of about 7 miles, where the rate of temperature change slows down abruptly and remains almost constant at - 55° C. (218° K.) up to about 20 miles. Then the temperature begins to rise to a peak value of 77° C. (350° K.) at the 55-mile level. Thereafter it climbs steadily, reaching 2,270° C. (2,543° K.) at a height of 250 to 400 miles. From the 50-mile level upward, a man or any other living creature, without the protective cover of the atmosphere, would be broiled on the sunny side and frozen on the other.
The atmosphere is divided into concentric layers or levels. Transition through these layers is gradual and without sharply defined boundaries. However, one boundary, the tropopause, exists between the first and second layer. The tropopause is defined as the point in the atmosphere at which the decrease in temperature (with increasing altitude) abruptly ceases, between the troposphere and the stratosphere. The four atmosphere layers are the troposphere, stratosphere, ionosphere, and the exosphere. The upper portion of the stratosphere is often called the chemosphere or ozonosphere, and the exosphere is also known as the mesophere.
The troposphere extends from the earth's surface to about 35,000 feet at middle latitudes; but varies from 28,000 feet at the poles to about 54,000 feet at the equator. The troposphere is characterized by large changes in temperature and humidity and by generally turbulent conditions.
Nearly all cloud formations are within the troposphere. Approximately three-fourths of the total weight of the atmosphere is within the troposphere. The temperature and absolute pressure in the troposphere steadily decrease with increasing altitude to a point where the temperature is approximately - 55° C. (or 218° K.), and the pressure is about 6.9 Hg on a standard day.
The stratosphere extends from the upper limits of the troposphere (and the tropopause) to an average altitude of 60 miles. In the stratosphere the temperature decline virtually stops; however, at 18 or 20 miles up, it often descends to - 63° C.
(210° K.).
The ionosphere ranges from the 50-mile level to a level of 300 to 600 miles. Little is known about the characteristics of the ionosphere, but it is thought that many electrical phenomena occur there. Basically, this layer is characterized by the presence of ions and free electrons, and the ionization seems to increase with altitude and in successive layers. The temperature increases from about 200° K. at the lower limit to about 2,500° K.
at the upper limit. These extremely high temperatures in the upper altitudes do not have the same significance as would corresponding temperatures at sea level. A thermometer reading in this region would be determined more by solar radiation than by the temperature, because of the energy of the particles.
The exosphere (or mesosphere) is the outer layer of the atmosphere. It begins at an altitude of 600 miles and extends to the limits of the atmosphere. In this layer the temperature is fairly constant at 2,500° K., and propagation of sound is thought to be impossible due to lack of molecular substance.
Atmospheric Pressure The human body is under pressure, since it exists at the bottom of a sea of air. This pressure is due to the weight of the atmosphere. The pressure which the atmosphere applies to a square inch of area is equal to the weight of a column of air one square inch in cross section which extends from that area to the "top" of the atmos- Since atmospheric pressure at any altitude is due to the weight of air above it, pressure decreases with increased altitude, Obviously, the total weight of air above an area at 15,000 feet would be less than the total weight of the air above an area at 10,000 feet.
Atmospheric pressure is often measured by a mercury barometer. A glass tube somewhat over 30 inches in length is sealed at one end and filled with mercury (Hg). It is then inverted and the open end placed in a dish of mercury. Immediately, the mercury level in the inverted tube will drop a short distance, leaving a small volume of mercury vapor at nearly zero absolute pressure in the tube just above the top of the liquid mercury column. The pressure acting upward at the lower end of the tube above the level of mercury in the dish is atmospheric pressure. The pressure acting downward at the same point is the weight of the column of mercury. Thus, the height of the column of mercury indicates the pressure exerted by the atmosphere.
This means of measuring atmospheric pressure gives rise to the practice of expressing atmospheric pressure in inches of mercury (in. Hg) rather than in pounds per square inch (p.s.i.). It may be seen, however, that a simple relationship exists between pressure measurements in p.s.i. and in inches Hg.
One cu. in. of mercury weighs 0.491 pound. Therefore, a pressure of 30 inches of mercury would be the equivalent of:
0.491 × 30 = 14.73 p.s.i.
A second means of measuring atmospheric pressure is with an aneroid barometer. This mechanical instrument lends itself to use on airplanes much more adequately than does the mercury barometer. Aneroid barometers (altimeters) are used to indicate altitude in flight. The calibrations are made in thousands of feet rather than in p.s.i.
For example, the standard pressure at level is 29.92 in. Hg, or 14.69 p.s.i. At 10,000 feet above sea level, standard pressure is 20.58 in.
Hg, or 10.10 p.s.i. Altimeters are calibrated so that if the pressure exerted by the atmosphere were 20.58 in. Hg, the altimeter would point to 10,000 feet. In other words, the altimeter is calibrated so that it indicates the altitude at which the prevailing atmospheric pressure would be considered standard pressure. Thus, the altitude read from the altimeter, being dependent upon atmospheric pressure, is called pressure altitude (H,).
Actually, an altimeter will read pressure altitude only when the altimeter adjustment is set at
29.92 inches Hg.
A third expression is occasionally used to indicate atmospheric pressure. Atmospheric pressure may be expressed in atmospheres. For example, a test may be conducted in a pressurized chamber under a pressure of six atmospheres. This merely means that the pressure is six times as great as standard sea level pressure.
Atmospheric Density Since both temperature and pressure decrease with altitude, it might appear that the density of the atmosphere would remain fairly constant with increased altitude. This is not true, however, for pressure drops more rapidly with increased altitude than does the temperature. The result is that density decreases with increased altitude.
By use of the general gas law, studied earher, it can be shown that for a particular gas, pressure and temperature determine the density. Since standard pressures and temperatures have been associated with each altitude, the density of the air at these standard temperatures and pressures must also be considered standard. Thus, a particular atmospheric density is associated with each altitude. This gives rise to the expression "density altitude," symbolized Ha. A density altitude of 15,000 feet is the altitude at which the density is the same as that considered standard for 15,000 feet. Remember, however, that density altitude is not necessarily true altitude. For example, on a day when the atmospheric pressure is higher than standard and the temperature is lower than standard, the density which is standard at 10,000 feet might occur at 12,000 feet. In this case, at an actual altitude of 12,000 feet, we have air which has the same density as standard air at 10,000 feet. Density altitude is a calculated altitude obtained by correcting pressure altitude for temperature.
The water content of the air has a slight effect on the density of the air. It should be remembered 238
that humid air at a given temperature and pressure is lighter than dry air at the same temperature and pressure.
Water Content of the Atmosphere In the troposphere the air is seldom completely dry. It contains water vapor in either of two forms: (1) Fog or (2) water vapor. Fog consists of minute droplets of water held in suspension by the air. Clouds are composed of fog. The height to which some clouds extend is a good indication of the presence of water in the atmosphere almost up to the stratosphere.
As a result of evaporation, the atmosphere always contains some moisture in the form of water vapor. The moisture in the air is called the humidity of the air. Moisture does not consist of tiny particles of liquid held in suspension in the air as in the case of fog, but is an invisible vapor truly as gaseous as the air with which it mixes.
Fog and humidity both affect the performance of an aircraft. In flight, at cruising power, the in use. Some merely show the expected reduction in power due to humidity; others show the boost in manifold pressure necessary to restore full takeoff power.
The effect of fog on the performance of an engine is very noticeable, particularly on engines with high compression ratios. Normally, some detonation will occur during acceleration, due to the high BMEP (Brake Mean Effective Pressures) developed. However, on a foggy day it is difficult to cause detonation to occur. The explanation of this lies in the fact that fog consists of unvaporized particles of water. When these particles enter the cylinders, they absorb a tremendous amount of heat energy in the process of vaporizing. The temperature is thus lowered, and the decrease is Fog will generally cause a decrease in horsepower output. However with a supercharged engine, it will be possible to use higher manifold pressures without danger of detonation.
effects are small and receive no consideration.
During takeoff, however, humidity has important effects. Two things are done to compensate for the effects of humidity on takeoff performance.
Since humid air is less dense than dry air, the allowable takeoff gross weight of an aircraft is generally reduced for operation in areas that are consistently humid. Secondly, because the power output of reciprocating engines is decreased by humidity, the manifold pressure may have to be increased above that recommended for takeoff in dry air in order to obtain the same power output.
Engine power output is calculated on dry air.
Since water vapor is incombustible, its pressure in the atmosphere is a total loss as far as contributing to power output. The mixture of water vapor and air is drawn through the carburetor and fuel is metered into it as though it were all air. This mixture of water vapor, air, and fuel enters the combustion chamber where it is ignited Since the water vapor will not burn, the effective fuel/air ratio is enriched and the engine operates as though it were on an excessively rich mixture.
The resulting horsepower loss under humid conditions can therefore be attributed to the loss in volumetric efficiency due to displaced air, and the incomplete combustion due to excessively rich fuel/air mixture.
The reduction in power that can be expected from humidity is usually given in charts in the flight manual. There are several types of charts Absolute Humidity Absolute humidity is the actual amount of the water vapor in a mixture of air and water. It is sometimes expressed in g./cu.m. (grams per cubic meter), sometimes in lbs./cu. ft. The amount of water vapor that can be present in the air is dependent upon the temperature and pressure.
The higher the temperatures, the more water vapor the air is capable of holding, assuming constant pressure. When air has all the water vapor it can hold at the prevailing temperature and pressure, it is said to be saturated.
Relative Humidity Relative humidity is the ratio of the amount of water vapor actually present in the atmosphere to the amount that would be present if the air were saturated at the prevailing temperature and pressure. This ratio is usually multiplied by 100 and expressed as a percentage. Suppose, for example, that a weather report includes the information that the temperature is 75° F. and the relative humidity is 56 percent. This indicates that the air holds 56 percent of the water vapor required to saturate it at 75° F. If the temperature drops and the absolute humidity remains constant, the relative humidity will increase. This is because less water vapor is required to saturate the air at the lower temperature.
Sling type Continuous reading type FIGURE 7-20. Wet-bulb hygrometers.
Dew Point The dew point is the temperature to which humid air must be cooled at constant pressure to become saturated. If the temperature drops below the dew point, condensation occurs. People who wear glasses have had the experience of going from cold outside air into a warm room and having moisture collect quickly on their glasses.
This happened because the glasses were below the dew point temperature of the air in the room.
The air immediately in contact with the glasses was cooled below its dew point temperature, and some of the water vapor was condensed out. This principle is applied in determining the dew point.
A vessel is cooled until water vapor begins to condense on its surface. The temperature at which this occurs is the dew point.
Vapor Pressure Vapor pressure is the portion of atmospheric pressure that is exerted by the moisture in the air (expressed in tenths of an inch Hg). The dew point for a given condition depends on the amount of water pressure present; thus a direct relationship exists between the vapor pressure and the dew point.
Wet- and Dry-Bulb Temperatures Vapor pressure and humidity may be determined from charts based on the wet- and dry-bulb temperatures (figure 7-20). The dry-bulb temperature is obtained from an ordinary thermometer.
The wet-bulb temperature is obtained from a thermometer which has its bulb covered with a thin piece of wet cloth.
Because of moisture evaporation from the wet cloth, the wet bulb will read lower than the dry bulb. The more rapid the evaporation, the greater will be the difference in readings. The rate of evaporation is dependent upon the degree of saturation of the air. In using the wet-bulb thermometer, it must be moved through the air at a rate of about 1,200 f.p.m. to give a worthwhile reading. This is accomplished by mounting both the wet- and dry-bulb thermometers on a frame which can be hand rotated around a pivot and the desirable rate of 1,200 f.p.m.
attained.
If the air is saturated, no evaporation will take place and the wet- and dry-bulb temperatures will be the same. Thus, these two temperatures coincide at the dew point.
Physical Laws Related to the Atmosphere Although air is composed of various gases and must be treated as a mixture for certain purposes, it is treated as a uniform gas in aerodynamic calculations. Air is a fluid since it has the fluid property to flow, and it is also a gas, since its density is readily varied.
As is usual in engineering work, certain simplifying assumptions are made. One standard assumption is that in dry air there is no water vapor present. Ordinary flight-takeoff charts may be corrected for vapor pressure, but subsonic 240