THE FOUR FORCES OF FLIGHT
During flight, the four forces acting on the airplane are lift, weight, thrust, and drag. Lift is the upward force created by the effect of airflow as it passes over and under the wings. It supports the airplane in flight. Weight opposes lift. It is caused by the downward pull of gravity. Thrust is the forward force which propels the airplane through the air. It varies with the amount of engine power used. Opposing thrust is drag, which is a backward, or retarding force that limits the speed of the airplane.
The arrows which show the forces acting on an airplane are often called vectors. The magnitude of a vector is indicated. Y the arrow’s length while the direction is shown by the arrow’s orientation. When two or more forces act on an object at the same time, they combine to create a resultant.
Lift is the key aerodynamic force. It is the force that opposes weight. In straight-and-level, unaccelerated flight, when weight and lift are equal, N airplane is in a state of equilibrium. If the other aerodynamic factors remain constant, the airplane neither gains or looses altitude.
When an airplane is stationary on the ramp, it is also in equilibrium, but the aerodynamic forces are not a factor. In calm conditions, the atmosphere experts equal pressure on the upper and lower surfaces of the wing. Movement of air about the airplane, particularly the wing, is necessary before the aerodynamic force of lift becomes effective.
During flight, however, pressures on the upper and lower surfaces of the wing are not the same. Although several factors contribute to this difference, the shape of the wing is the principal one. The wing is designed to divide the airflow into areas of high pressure below the wing and areas of comparatively lower pressures above the wing. The pressure differential, which is created by the movement of air about the wing, is the primary source of lift.
The basic principle of pressure differential of subsonic airflow was discovered by Daniel Bernoulli, a Swiss physicist. Bernoulli’s Principle simply stated, says, “as the velocity of a fluid (air) increases, its internal pressure decrease.”
One way you can visualize this principle is to imagine air flowing through a tube that is narrower in the middle than T the ends. This type of device is usually called a venturi.
It is not necessary for air to pass through an enclosed tube for Bernoulli’s Principle to apply. Any surface that alters airflow causes a venturi effect. Figure 1-14
The wing of the airplane is shaped to take advantage of this principle. The greater curvature on the upper portion causes air to accelerate as it passes over the wing. The resulting pressure differential between the upper and lower surfaces of the wing creates an upward force. This difference in pressure is the main source of lift. (Figure 1-15).
When air flows over the curved upper surface of a wing, it increases in velocity. This increase reduces the pressure above the wing and produces the upward force of lift.
NEWTON’S THIRD LAW OF MOTION
The remaining lift is provided by the wing’s lower surface as air striking the underside is deflected downwards. According to Newton’s Third Law of Motion, “for every action there is an equal and opposite reaction.” The air that is deflected downwards also produces an upward (lifting) reaction.
Since air is much like water, the explanation for this source of lift may be compared to the planing effect of skis on water. The lift which supports the water skis (and the skier) is the force caused by the impact pressure and the deflection of water from the lower surfaces of the skis.
Under most flying conditions, the impact pressure and the deflection of air from the lower surface of the wing provide a comparatively small percentage of the total lift. The majority of lift is the result of the decreased pressure above the wing rather than the increased pressure below it.
An airfoil is any surface such as a wing, which provides aerodynamic force when it interacts with a moving stream of air. Remember, an airplane’s wing generates a lifting force only when air is in motion about it.
LEADING EDGE – This part of the airfoil meets the airflow first.
TRAILING EDGE – This is the portion of the airfoil where the airflow over the upper surface rejoins the lower surface airflow.
CHORD LINE – The chord line is an imaginary straight line drawn through an airfoil from the leading edge to the trailing edge.
CAMBER – The camber of an airfoil is the characteristic curve o& its upper and lower surfaces. The upper camber is more pronounced, while the lower camber is comparatively flat. This causes the velocity of the airflow immediately above the wing to be much higher than that below the wing.
RELATIVE WIND – This is the direction of the airflow with respect to the wing. If a wing moves forward horizontally, the relative wind moves backwards horizontally. Relative wind is parallel to and opposite the flight path of the airplane.
You shouldn’t confuse the actual flight path with the flight attitude of the airplane. For example, the airplane’s fuselage may be parallel to the horizon while the aircraft is descending.
ANGLE OF ATTACK – This is the angle between the chord line of the airfoil and the direction of the relative wind. It is important in the production of lift.
AIRFOIL DESIGN FACTORS
Wing design is based on the anticipated use of the airplane, cost , and other factors. The main design considerations are wing platform, camber, aspect ratio and total wing area.
PLANFORM – refers to the shape of the airplane’s wing when viewed from above or below. Each platform design has advantages and disadvantages.
CAMBER as noted earlier, affects the difference in the velocity of the airflow between the upper and lower surfaces of the wing. If the upper camber increases and the lower camber remains the same, the velocity differential increases.
There is of course a limit to the amount of camber that can be used. After a certain point, air will no longer flow smoothly over the airfoil. Once this happens, the lifting capacity diminishes. The ideal camber varies with the airplane’s performance specifications, especially the speed range and the load-carrying requirements.
ASPECT RATIO – is the relationship between the length and width of a wing. It is one of the primary factors in determining lift/drag characteristics. At a given angle of attack, a higher aspect ratio produces less drag for the same amount of lift.
WING AREA – is the total surface area of the wings. Most wings don’t produce a great amount of lift per square foot so wing area must be sufficient to support the weight of the airplane. For example, in a training aircraft at Norma” operating speed, the wings produce only about 10.5 pounds of lift for each square foot of wing area. This means a wing area of 200 square feet is required to support an airplane weight of 2100 pounds during straight-and-level flight.
You can see that design has a lot to d9 with a wing’s lift capability. Platform, camber, aspect ratio, and wing area are some of the major design factors.
Once the design of the wing is determined, the wing must be mounted on the airplane. Usually it is attached to the fuselage with the chord line inclined upward at a slight angle, which is called the angle of incidence.
PILOT CONTROL OF LIFT
You can change the angle of attack and airspeed. You can also change the shape of the wing by lowering the flaps. Keep in mind that anytime you do something to change lift, drag is affected. If you increase lift, drag increases. Drag is always a by-product of lift.
Changing the angle of attack or airspeed are the two primary ways to control lift.
CHANGING ANGLE OF ATTACK
You have direct control over angle of attack. During flight at normal operating speeds if you increase the angle of attack, you increase lift. Anytime you move the control column fire or aft during flight, you change the angle of attack. At the same time, you are changing the coefficient of lift. The coefficient of lift (CL) is a way to measure lift as it relates to angle of a attack. CL is determined by wind tunnel tests and is based on airfoil design and angle of attack. Every airplane has an angle of attack where maximum lift occurs.
A STALL is caused by the separation of airflow from the wing’s upper surface. This results in a rapid decrease in lift. For a given airplane, a stall always occurs at the same angle, regardless of airspeed, flight attitude or weight. Since training airplanes normally do not have an angle of attack indicator, the important point to remember is that you can stall an airplane at any airspeed, in any flight attitude, or any weight.
Stall characteristic s vary with different airplanes. In most training airplanes, the onset of a stall from a level flight attitude is gradual. The first indication may be provided by a stall warning device or a slight buffering of the airplane.
All you have to do to recover from a stall is to restore the smooth airflow. The only way to do this is to decrease the angle of attack to a point below the critical angle.
To recover from a stall, you must decrease the angle of attack.
An airplane always stalls when the critical angle of attack is exceeded, regardless of weight or airspeed. The indicated airspeed of a stall, however, varies with weight and configuration. Manoeuvring can also affect stall speed.
Although the stalling or critical angle of attack does not vary with weight, the stalling speed does. It increases slightly as the weight of the airplane increases and decreases as weight decreases. This means you need slightly more airspeed to stay above the stalling speed in a heavily loaded airplane. If you want to fly an airplane at a given weight, there are many combinations of airspeed and angle of attack which will produce the required amount of lift.
The faster the wing moves through the air, the greater the lift. Actually lift is proportional to the square of the airplane’s speed. For example, at 200 knots, an airplane has four times the lift of the same airplane travelling at 100 knots if the angle of attack and other factors are constant. On the other hand, if the speed is reduced by one-half, lift is decreased to one quarter of the previous value.
Lift is proportional to the square of the airplane’s velocity. If the airplane’s speed is doubled, lift increases fourfold.
Although airspeed is an important factor in the production of lift, it is only one of several factors. The airspeed required to sustain an aircraft in straight-and- level flight depends upon the flap position, the angle of attack, and the weight.
ANGLE OF ATTACK AND AIRSPEED
The relationship between angle of attack and airspeed in the production of lift is not as complex as it may seem. Angle of attack establishes the coefficient of lift for the airfoil. At the same time, lift is proportional to the square of the airplane’s speed. Since you can control both angle of attack and airspeed, you can control lift.
Total lift depends on the combined effects of airspeed and angle of attack. When speed decreases, you must increase the angle of attack to maintain the same amount of lift and conversely, if you want to maintain the same amount of lift at higher speed, you must decrease the angle of attack.
When properly used, flaps increase the lifting efficiency of the wing and decrease stall speed. This allows you to fly at a reduced speed while maintaining sufficient control and lift for sustained flight. Remember, though, that when you retract the flaps, the stall speed increases.
Flaps allow you to steepen the angle of descent on an approach without increasing airspeed.
The ability to fly slowly is particularly important during the approach and landing phases. For example, an approach with full flaps permits you to fly slowly and at a fairly steep descent angle without gaining airspeed. This allows you to touchdown at a slower speed. In addition, you can land near the approach end of the runway, even when there are obstacles along the approach path.
In training airplanes, configuration normally refers to the position of the landing gear and flaps. When the gear and flaps are up, an airplane is in the clean configuration. If the gear is fixed rather than retractable, the airplane is considered to be in clean configuration when the flaps are in the up position. During flight you can change configuration by raising or lowering the gear, or by moving the flaps. Flap position affects the chord line and angle of attack for that section of the wing where the flaps are attached. This causes an increase in camber for that section of the wing and greater production of lift and drag. In contrast, landing gear position normally has little or no effect on chord line or angle of attack.
There are several common types of flaps. The plain flap is attached to the wing by a hinge. When deflected downwards, it increases the effective camber and changes the wing’s chord line. Both of these factors increase the lifting capacity of the wing. The split flap is hinged only to the lower portion of the wing. This type of flap also increased lift, but it produces greater drag than the plain flap because of the turbulence it causes. The slotted flap is similar to the plain flap. In addition to changing g the wing’s camber and chord line, it also allows a portion of the higher pressure air beneath the wing to travel through a slot. This increases the velocity of the airflow over the flap and provides additional lift. Another type of flap is the Fowler flap. It is attached to the wing by a track and roller system. When extended. It moves rearward as well as down. This rearward motion increases the total wing area, as well as the camber and chord line. The Fowler flap is the most efficient of these systems. As you might expect, it also is the most expensive.
Although the amount of lift and drag created by a specific flap system varies, we can make a few general observations. As the flaps are extended, at first they will produce a relatively large amount of lift for a small increase in drag. However, once the flap extension reaches approximately the midpoint, this relationship reverses. Now, a significant increase in drag will occur for a relatively small increase in lift. Because of the large increase in drag beyond the half-flap position, most manufacturers limit the takeoff setting to half flaps or less.
The weight of the airplane is not a constant. It varies with the equipment installed, passengers, cargo, and fuel load. During the course of a flight the total weight of an airplane decreases as fuel is consumed. Additional weight may also occur during some specified flight activities such as crop dusting, fire fighting or sky diving flights.
In contrast the direction in which the force of weight acts is constant. It always acts straight down towards the centre of earth.
Thrust is the forward-acting force which opposes drag and propels the airplane. In most airplanes, this force is provided when the engine turns the propeller. Each propeller blade is cambered like the airfoil shape of a wing. This shape, plus the angle of attack of the blades, produces reduced pressure in front of the propeller and increased pressure behind it. By accelerating a relatively large mass of air through a relatively small velocity change, the propeller produces thrust, the force which moves the airplane forward.
During straight and level, unaccelerated flight, the forces of thrust and drag are equal.
You increase thrust by using the throttle to increase power. When you increase power, thrust exceeds drag, causing the airplane to accelerate. This acceleration, however, is accompanied by a corresponding increase in drag. The airplane continues to accelerate only while the force of thrust exceeds the force of drag. When drag again equals thrust, the airplane ceases to accelerate and maintains a constant speed. However, the new airspeed is higher than the previous one.
When you reduce thrust, the force of drag causes the airplane to decelerate. But as the airplane slows, drag diminishes. When drag has decreased enough to equal thrust, the airplane no longer decelerates. Once again it maintains a constant airspeed. Now, however, it is slower than the one previously flown.
As you have seen drag is associated with lift. It is caused by any aircraft surface that deflects or interferes with the smooth airflow around the airplane. A highly cambered, large surface area wing creates more drag (and lift) than a small moderately cambered wing. If you increase airspeed, or angle of attack, you increase drag (and lift). Drag acts in opposition to the direction of flight, opposes the forward-acting force of thrust, and limits the forward speed of the airplane. Drag is broadly classified as either parasite or induced.
Parasite drag includes all drag created by the airplane, except that drag directly associated with the production of lift. It is created by the disruption of the flow of air around the airplane’s surfaces. Parasite drag normally is divided into three types: form drag, skin friction drag, and interference drag.
Parasite drag increases with an increase in airspeed. It consists of all drag not associated with the production of lift.
Form drag is created by any structure which protrudes into the relative wind. The amount of drag created is related both to the size and shape of the structure. For example, a square strut will create substantially more drag than a smooth or rounded strut. Streamlining reduces form drag.
Skin friction drag is caused by the roughness of the airplane’s surfaces. Even though these surfaces may appear to be smooth, under a microscope they may be quite rough. A thin layer of air clings to these rough surfaces and creates small eddies which contribute to drag.
Interference drag occurs when varied currents of air over an airplane meet and interact. This interaction creates additional drag. One example of this type of drag is the mixing of air where the wing and the airframe join.
Each type of parasite drag varies with the speed of the airplane. The combined effect generally is proportional to the square of the airplane’s speed.(Figure 1-26)
Induced drag is the main by-product of the production of lift. It is directly related to the angle of attack of the wing. The greater the angle, the greater the induced drag. Since the wing usually is at a low angle of attack at high speed and a high angle of attack at low speed, the relationship of induced drag to speed also can be plotted.
Induced drag is associated with the production of lift. It is directly related to angle of attack and increases as angle of attack increases. Normally, induced drag decreases as airspeed increases.
Total drag for an airplane is the sum of parasite and induced drag. The total drag curve represents these combined forces and is plotted against airspeed.
Courtesy of: Private Private Manual, Jeppesen Sanderson CO, USA, 1991