Principles of Aerodynamics
Flight involves a balance of forces. These forces are THRUST, DRAG, LIFT and WEIGHT.
When Thrust and Drag are equal, the speed of
the aircraft through the air (airspeed) will remain constant in smooth air.
When Lift and Weight are equal, the aircraft
will neither ascend or decend.
Attitude
The Attitude of an aircraft refers to it's relationship to the ground. When in a level attitude, the longitudinal centerline of the aircraft is approximately paralell to the earth's surface. In this attitude, the horizon will appear to be just about on the nose of the aircraft( i.e. the top of the engine cowling is approximately aligned with the horizon).When the nose of the aircraft is above the horizon, this is called a nose high attitude. If the nose is below the horizon, the aircraft is in a nose low attitude.
Center Of Gravity
The weight of the airplane, pilot, passengers, fuel and baggage is distributed throughout the aircraft, as shown by the small downward arrows in the diagram. However, the total weight can be considered as being concentrated at one given point, shown by the larger downward arrow. This point is referred to as the Center of Gravity. If the plane were suspended by a rope attached at the center of gravity ( referred to as the CG) it would be in balance.
The Center of Gravity (CG) is affected by the way an aircraft is loaded. For example, if in a 4 place aircraft, there are 2 rather large individuals in the front seats, and no rear seat passengers or baggage, the CG will be somewhat toward the nose of the aircraft. If however, the 2 front seat passengers are smaller, with 2 large individuals in the rear seats, and a lot of baggage in the rear baggage compartment, the CG will be located more aft.
Every aircraft has a maximum forward and rearward CG position at which the aircraft is designed to operate. Operating an aircraft with the CG outside these limits affects the handling characteristics of the aircraft. Serious "out of CG" conditions can be dangerous.
Aircraft Balance
An aircraft in straight and level flight is similar to a childs "teeter-totter". There is a balance point in the middle (called a fulcrum), with weight on both sides of the fulcrum. For the "teeter-totter" to be in balance, the downward forces on both sides of the fulcrum must be equal.
In the diagram at right, the fulcrum of an aircraft in flight is the center of lift. Generally the CG is forward of the Center of Lift, causing the aircraft to naturally want to "nose down". The elevator located at the aft end of the aircraft provides the counter-balancing force to provide a level attitude in normal flight. Normally, the pilot will "trim" the elevators, by use of the trim tab control in the cockpit, to cause the elevators to provide the correct elevator balance force to relieve the pilot from constant elevator control.
You can readily see that loading of the aircraft, which affects the CG, is a critical consideration in properly balancing the aircraft and it's controllability.
If the pilot pulls back on the control wheel, an "up-elevator" condition results. This forces the tail downward, causing the aircraft to assume a "nose up" attitude. Likewise, a forward movement of the control wheel by the pilot causes a "down elevator" state. This causes the tail to rise, forcing the aircraft into a "nose low" attitude. By use of the elevator trim control (a small wheel or crank in the cockpit), the pilot can cause the aircraft to remain in a nose-up, level, or nose down attitude.As can be seen in the diagram above, when the CG is forward, a greater downward force is required by the elevators to produce a level attitude. Likewise, when the CG is aft, the elevators must produce less downward force to maintain level flight. NOTE: If the CG gets behind the Center of lift (the fulcrum) the aircraft becomes unstable because the CG is aft of the fulcrum. IT MAY BE POSSIBLE TO EXCEED THE TRIM CAPABILITY OF THE ELEVATORS SUCH THAT THE AIRCRAFT ALWAYS WANTS TO NOSE UP, AND BE UNSTABLE. Therefore the pilot must pay attention to proper loading of the aircraft. This will be discussed in greater detain under the subject of Weight and Balance.
Effects Of Attitude Change
When the wing is in a given attitude with respect to the Relative Wind (R W) as shown in the diagram below, the wing produces a Vertical Lift Force (LIFT) which is perpendicular to the Relative Wind..
There is also a DRAG component operating parallel to the Relative Wind in opposition to the forward motion of the wing. Drag is created as a natural part of producing lift. These two forces intersect at a point called the CL (center of lift}, or is also called the CP (center of pressure]. The LIFT and DRAG force vectors can be resolved into a single force vector called the RESULTANT force.
Envision if the Angle of Attack is increased. The Vertical Lift decreases in value, and the horizontal force of Drag increases. Therefore, when a pilot wants to slow the aircraft, the nose of the aircraft must be slowly raised into a greater "nose up" attitude, causing drag to increase, thus slowing the aircraft. This increase of angle of attack has limits, however. The wing design of most small aircraft, the wing has a "Critical Angle Of Attack" (somewhere around 18° to 20°) at which point the wing ceases to create sufficient lift to fly, and the wing STALLS. The air flowing over the wing becomes so disturbed that adequate lift to sustain flight ceases, and the aircraft pitches "nose down". This is a STALL.
The primary way to recover from a stall is to push the nose further downward, thus decreasing the Angle Of Attack so that the wing flies again.
Also, envision in the diagram, when the pilot pushes the nose down by use of forward elevator, the Angle of Atack decreases, thus decreasing the drag. Therefore, when power is held constant, the angle of attack (nose high, level, or nose low) provides "Airspeed Control".
Assume for example, an aircraft has been cruising at 120 knots. When the aircraft enters the landing pattern of an airport, the pilot may want to reduce speed to 90 knots. The pilot must reduce power to prevent an altitude increase, and concurrently raise the nose of the aircraft so that the drag is increased sufficiently to slow the aircraft to 90. Later, when on the final approach for landing, the pilot may wish to slow even further, say to 70 knots. Power can be further reduced and the nose raised further, to again increase drag. In addition, the pilot may add 10,20 or 30 degrees of flaps to add an additional drag and lift.
The important point is that ATTITUDE is the primary control of airspeed; not THROTTLE! However, if level flight is to be maintained, appropriate changes in power must be made whenever the pitch attitude is made to prevent gaining or loosing altitude.
The Turn
In order to turn the aircraft, it must be placed into a BANKED state, where one wing is high, the other low. This state is pictured below.
In order to bank the aircraft, the pilot must turn the control wheel (or move the control stick) to the left. The Right Aileron lowers This increases the angle of attack of that part of the right wing, causing the right wing to rise. At the same time, the Left Aileron raises. The angle of attack of that part of the left wing decreases, causing the left wing to lower. This increased lift of the Right and decreased lift of the Left Wing causes the aircraft to roll to the Left.
NOTE: During the time the Right aileron is down, the right wing has MORE DRAG than does the left wing. The effects of this unequal drag is discussed later under Adverse Yaw.
When the aircraft reaches the bank angle the pilot wishes, the ailerons must be neutralized. This causes equal lift by left and right wing, and the aircraft roll stops. Basically, the aircraft will remain in this banked attitude until the pilot rolls the aircraft back to level attitude by operating the control wheel ( or stick) in the opposite direction.
Note in the diagram that some of theTotal Lift ( force T) goes into a Horizontal Force ( H ). This is the force which pulls the aircraft in a circular motion (turn). Note also that the Vertical Lift ( force V) becomes less. If the bank angle becomes large, say 45 degrees, the vertical lift is appreciably less. The pilot may have to hold some up elevator and/or add power to prevent loosing altitude.
Adverse Yaw
During the time that the ailerons are activated, an unwanted effect occurs. In the left turn shown above the pilot turns the control wheel to the left, raising the left aileron, and lowering the right aileron. The intent is to turn left.Unfortunately while the ailerons are activated, the left wing has less drag; the right wing has more drag. This causes the airplane to want to turn to the Right, and not to the left. This tendency to turn in a direction opposite to the intended turn direction is called ADVERSE YAW. So how does the pilot overcome this tendency to initially turn in the wrong direction? He uses the Rudder. By applying just the right amount of rudder in the direction of the turn, the pilot can offset the adverse yaw. When the pilot does this correctly, applying just the right amount of rudder, a Coordinated turn results. If the pilot applies too little or too much rudder, anUn-Coordinated turn results. How the quality of the turn is measured will be covered in the Instruments section.
If the pilot uses too little rudder, the nose of the aircraft wants to stay yawed opposite the turn. The rest of the aircraft wants to "slip" toward the inside of the turn.
If the pilot applies too much rudder, the tail wants to remain outside the radius of the turn, and a "skid" results. Its similar to the rear end of an automobile wanting to skid outside the turning radius of a car.
Therefore, a principle use of the rudder is to control the adverse yaw while rolling into a bank.
Slips
A slip is created by applying rudder in the opposite direction to the turn. This is called Cross Controlling. There are 2 forms of the slip.- Side Slip
- Forward Slip
Side Slip
This manuever is primarially used to compensate for a cross wind while landing. If the wind is from the right of the aircraft, the aircraft will drift to the left side of the runway unless some force is applied in the opposite direction keep the aircraft straight with and on the centerline of the runway. The pilot uses a Right Side Slip to compensate for the leftward drift caused by the wind. The pilot turns the control wheel to the right to initiate a right turn, but simultaneously applies opposite Left rudder just enough to keep the aircraft from turning. Thus the pilot induces just enough right side slip to offset the leftward wind drift. This way, the pilot can keep the aircraft both over the centerline of the runway, and aligned with the runway. This prevents a "side load" on the landing gear on touchdown.
Forward Slip
The forward slip is used primarially on aircraft with no flaps. This configuration is used to loose altitude quickly without increasing airspeed.In this manuever, the pilot simultaneously turns the aircraft left or right, and applies a lot of opposite rudder so the side of the aircraft is presented to the relative wind. It is almost like slipping a sled down a hill somewhat sideways. The pilot maintains this configuration until the desired altitude is lost, whereupon he neutralizes controls to continue straight flight.
Since most modern aircraft have effective flaps to slow the aircraft on landing, and to allow a steeper decent, the forward slip in usually unnecessary. Some aircraft manufacturers state that forward slips should not be made with flaps deployed.
Stalls and Spins
The angle of attack which produces maximum lift is a function of the wing design, and is called the CRITICAL ANGLE OF ATTACK. A stall occurs when the Critical Angle of Attack is exceeded. Smooth air flow across the upper surface of the wing begins to separate and turbulence is created along the wing surface. Lift is lost and the wing quits “flying”. THE STALL IS A FUNCTION OF EXCEEDING THE CRITICAL ANGLE OF ATTACK, AND CAN OCCUR AT ANY AIRSPEED , ANY ATTITUDE, AND ANY POWER SETTING.On most aircraft, the stall starts at the wing root, and progresses outward to the wing-tip. The wings are designed in this manner so that the ailerons are the last wing elements to loose lift. Flap and gear extension affect the stall characteristics. In general, flap extension creates more lift, thus lowering the airspeed at which the aircraft stalls.
Recovery from a stall requires that the angle of attack be DECREASED to again achieve adequate lift. This means that the back pressure on the elevators must be reduced. If one wing has stalled more than the other, the first priority is to recover from the stall, then correct any turning that may have developed.
A CG that is too far rearward can significantly affect the ease of stall recovery. The aft CG may inhibit the natural tendency of the nose to fall during the stall. It may be necessary to force a “nose down” attitude to recover.
Although weight does not have a direct bearing on the stall, an overloaded aircraft will have to be flown at an unusually higher angle of attack to generate sufficient lift for level flight. Therefore the closer proximity to the critical angle of attack can make an inadvertent stall due to pilot inattention more likely.
Snow, ice or frost on the wings can drastically affect lift of the wing. Even a small accumulation can significantly inhibit lift and increase drag. Due to the reduced lift, the aircraft can stall at a higher-than-normal airspeed. Takeoff with ice, snow or frost on the wings should never be attempted.
Stall recognition can come several ways. Modern aircraft are equipped with stall warning devices (usually an audible signal) to warn of proximity to the critical angle of attack. The aircraft may vibrate, control pressures are probably "mushy", the "seat of the pants" sensation that the aircraft is on the verge of loosing lift, and other sensations can tip off the pilot of an impending stall. Practice of slow flight and stalls at altitude is invaluable training in stall recognition.
A spin is a stall that has continued, with one wing more stalled than the other. The aircraft will begin rotation around the more stalled wing. The spin may become progressively faster and tighter until the stalled condition is "broken" (stopped).
Usually spin recovery procedures are covered in the Pilot Operating Handbook (POH) for the given type of aircraft. If one is not available, the following is the suggested spin recovery technique.
a. Close the throttle. Power usually aggravate the spin. b. Stop the rotation by applying opposite rudder. c. Break the stall with positive forward elevator pressure. d. Neutralize the rudder when rotation has stopped. e. Return to level flight.
Secondary Controls
Trim
There are some secondary "pilot-controlled" functions which can "trim" the elevators, rudder and ailerons for improved straight and level flight, thus freeing the pilot from constant attention and control of the major control surfaces. Most small aircraft have only Elevator Trim. As the sophistication of an aircraft design increases, Rudder Trim and Aileron Trim are added. In most cases, the elevator trim is an additional moveable surface on one of the elevator assemblies which causes a “nose up”, “level”, or “nose Down” trimmed attitude.In some aircraft, rudder trim and aileron trim can also be accomplished by pilot from the cockpit.. The purpose of the trim action is to compensate for variations in aircraft loading or other minor factors which may tend to cause deviation from straight and level flight.
Flaps
Flaps are moveable surfaces on the trailing edge of the wing similar in shape to the ailerons. they are usually larger in surface area. They are located on inboard end if the wing next to the fuselage. Both sides are activated together so they do not produce a rolling action like the ailerons.Flaps are usually deployed in "degree" increments. In small aircraft deployment is usually in 10 degree increments from zero degrees (non-deployed) to 40 degrees maximum. Larger or more sophisticated aircraft may have a different range of settings. Normally, the flaps operate electrically through a 4 or 5 position switch located on the instrument panel. In earlier aircraft the flaps were operated using a manual flap handle.
Deployment of flaps increases both the lift and drag of the wing. Flap activation increases the angle of attack across the wing / flap section. At 10 degrees, more lift than drag is produced. As the flap angle is increased more drag and less lift is produced for each increment of deployment.
The primary use of flaps is in landing. They permit a steeper decent without increase in airspeed. Flaps may be used in certain take-off situations(usually 10°) on short or soft fields.
Dynamics of Flight
Drag
Just as wind friction causes drag in an automobile, aerodynamic friction and displacement of air during flight creates aerodynamic DRAG. Drag occurs any time that air is displaced from its normal stable condition. Air has density and weight, and although compressible, it still requires energy to displace.- INDUCED DRAG occurs as a by-product of lift.
- PARASITE DRAG results from friction with surfaces and appendages, and impact with structures such as struts, landing gear, antennas, etc..
Induced Drag
Induced drag results from the creation of lift. The amount of drag depends on the airfoil design of the wing, its camber and angle of attack. Also due to the way the air flows across the wing during flight, vortices are generated at the wing tips which add to the induced drag component. .Parasite Drag
Parasite drag is an unwanted resistance of the air to an object traveling through e air. The 3 types of parasite drag are Form, Skin and Interference as demonstrated below.
Form Drag
The form of the object and the effective frontal surface it presents to the air has a significant effect on the amount of drag generated. As shown in (A), a surface such as shown would present much more "frontal" drag than it would is it were rotated 90 degrees as in (B). Drag can be reduced in the aircraft design by streamlining objects such as wing struts to minimize the frontal appearance to the air flow.Skin Friction
Even though the form in (A) above is re-oriented to shape (B), there is still some form drag. In addition, there is friction between the skin of the surface and the air flow. It is obvious that if the surface is dirty, has frost, ice or other obstructions, that drag will increase. Effects of skin drag can be reduced by smooth surfaces and flush riveting in the design, and by keeping the surface clean and waxed by the owner.Interference Drag
Surfaces at angles to each other as in (C) create turbulence in the region of the joint. This occurs most frequently at the intersection of the fuselage and wingPitch, Power and Performance
The amount of lift that a wing generates is a function of it's design (camber, area, etc.), speed through the air, air density, and angle of attack. The effects of air density will be treated in more detail in a later chapter.
The three aircraft shown above can all be in constant altitude flight, but at different airspeeds. Maintaining a fixed altitude at a given airspeed requires the pilot to control two factors; (1) Angle of Attack and (2) Power. The angle of attack is controlled by the up, neutral, or downward trim position of the elevators. The power, is controlled by the "power setting" of the engine and propeller. For a "fixed pitch" propeller, this means adjusting the engine RPM. For a variable pitch propeller, this means adjusting both the throttle and the propeller pitch control.
The left aircraft could be at a 10 degree nose-up attitude with an indicated airspeed of say 70 nautical miles per hour (knots). The center aircraft could be at cruise with a 0 degree attitude and 110 knots. The right aircraft could be in a slightly high speed decent at minus 3 degrees of pitch and an indicated airspeed of 140 knots (abbreviated kts).
The pilot can control the Pitch, Power and Performance of the aircraft and can fly at a considerable range of attitudes, speeds and power settings.
Lift versus Drag
An aircraft with a given total gross weight can be operated in level flight over a range of power settings and airspeeds. Since Lift and Weight must be equal in order to maintain level flight, it is obvious that there is a relationship between Lift (L), Airspeed (V), and Angle of Attack (AT). This relationship can be "generalized" with the following expression. (Note: the expression is not an exact equation).Lift = Angle of Attack x Velocity
Since angle of attack and speed also have a relationship to Induced Drag and Parasite Drag, the relationship of Lift/Drag is shown by the graph below.
Ground Effect
An aircraft can be flown near the ground or water at a slightly slower airspeed than at altitude. This is known as Ground Effect.
The airflow around the left aircraft at altitude can flow around the surface of the aircraft in a normal manner. The airflow around thr right aircraft is disturbed by the proximity to the ground. The normal downwash of air produced by the wing and tail surfaces cannot occur, and the air becomes compressed under these surfaces. A "cushioning" effect occurs which allows the airplane to fly at slightly slower airspeed than at altitude.
The maximum ground effect occurs at approximately 1/2 the wingspan above the ground. It is this effect which causes the plane to seem to float when near the ground on landing. It also allows the aircraft to be "pulled" off the ground before adequate climb speed is achieved.
Load Factor
The load factor is the total load supported by the wings divided by the total weight of the airplane. In straight and level flight, the load factor is 1; i.e. the weight supported by the wings is equal to the weight of the loaded aircraft. The load factor is described as 1G Force. With a load factor of 1, the G force is 1. In other terms, the load supported by the wings equals the total weight of the loaded aircraft.In a turn, the weight of the aircraft increases due to the addition of centrifugal force. The rate of turn determines the total weight increase. A faster turn (steeper bank) generates greater centrifugal force. The centrifrugal force is straight out from the center of the turn. When the downward weight of the aircraft is mathematically resolved with the horizontal centrifrugal force, the load on the wings is the Resultant Load.
The effects of the bank angle is shown in the graph on the right. The G Force is shown on the Left Side, and the Bank Angle is shown on the bottom of the graph.
The maneuver of most importance to the private pilot is the forces in a turn. The most critical time is in turns in the traffic patern, when airspeeds are low, and the attention to bank angle and airspeed may be distracted by other duties.
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