Aerodynamics

In-Flight Forces

There are several forces acting on an airplane while in flight (not stalled). Of these forces, four are always  present in flight and in this manual will be referred to as:

 

• Lift. All upward acting forces.
• Load. All downward acting forces.
• Thrust. All forward acting forces.
• Drag. All rearward acting forces.

Other forces that will be discussed are:

• Aerodynamic force. A force created by the airplane design.
• Centrifugal. A force created when the flight path is curved.

In-flight Forces

Lift

Lift is created primarily by the wings as the airplane moves through the air. However, in certain situations, thrust can contribute to lift as well. The horizontal stabilizer and elevator, vertical stabilizer and rudder, and propeller also create lift. There are many theories of how lift is created. A common thread  through  all  of  these  theories,  whether they are correct or incorrect, is that the air surrounding an airfoil (wing) is displaced as the airfoil  passes through it. The movement of this air is known as relative wind, acts on the airfoil creating the force of lift.

If an airfoil  is shaped, moved, or inclined  in such a way as to produce a net deflection (or  turning)  of  the  relative wind, the local velocity is changed in magnitude and direction; the relative wind is accelerated rearward and downward. Changing the velocity creates a net force (action) on the relative wind which has an equal and opposite force (reaction) that is lift.

Newton’s third law states that for every action there is an equal and opposite reaction.

 

For additional  information  on  how  lift  is  produced  and

some incorrect theories go to:

WWW  /K-12/airplane/short.html 

Factors Affecting Lift

The amount of lifting force needed for flight is dependent upon the load of the airplane. That is to say, that the wings will create enough lift to equal the load.  As  load  is increased by physically increasing the  weight  of  the airplane or by aerodynamic loading, lift must also be increased. An example of this is on takeoff; if the airplane weight is increased, the speed needed to produce the required lift for flight will be higher. Also, in a level turn, back elevator pressure is required to keep the load and lift equal.

Speed and Angle of Attack.

The amount of air moved and the velocity that is imparted to it, directly affects the lifting force. During airspeed changes the balance of lift and load is maintained through the relationship of angle of attack and airspeed (refer to  the  figures  below).  As  airspeed increases, the angle of attack decreases and as airspeed decreases, the angle of attack increases by  maintaining the same lifting force.

The faster the airfoil is moved through  the  air the  greater the velocity of  the  air  that  is  moved,  therefore, less volume of air needs to be moved to maintain  the  same lifting force at higher airspeeds. Higher speed also means a lower angle of attack.

If the speed of the airfoil is slow, the same lifting force can be maintained by moving a greater volume of air. Lower speed means a higher angle of attack. If there is no  air  movement  or  if  the air movement  above the  airfoil  is the  same as below the  airfoil, the  net  lift  is zero.

Wing design (shape and size) is based on desired aircraft performance. A training airplane will have a wing design completely different from a high performance sail plane or high speed fighter. Also, since novice pilots will be  flying the training airplane, slow flight and  stall  characteristics must be predictable. A cambered  rectangular wing with a low aspect ratio has the needed characteristics.

A cambered wing has good slow flying characteristics but its profile and associated high drag, limits the high speed regime of flight. The rectangular wing plan form has the desired stall characteristics and the low aspect ratio yields the performance needed.

Load

Load is the sum of all downward acting forces. In flight, in addition to the physical weight of the loaded airplane there are additional aerodynamic forces that effectively increases the load of the airplane. If a loaded airplane was placed on a scale the number, in pounds, read from the scale is the physical weight of the airplane and any aerodynamic force produced must be added to this number to come up with the total load that lift must oppose. For example, if the scale reads 1,800 pounds and the engineers told us the aerodynamic down force was 200 pounds, the total lift needed to oppose the load is 2,000 pounds.

Aerodynamic Load

Aerodynamic load that will be discussed in this manual is the force created by the horizontal stabilizer/elevator that is used for lateral stability. And, Centrifugal force is added to the four in-flight forces when an airplane turns. This added force is due to the  curved flight path required to turn the airplane and acts parallel to the earth’s surface towards the outside of the turn. The pilot uses lift to oppose this force. As the bank angle is increased 1 load is also increased. The pilot must then increase the pitch attitude (and angle of attack) to supply the additional lift required to oppose the extra load, thus maintaining the lift/load relationship.

 

Centrifugal Force

Centrifugal force is added to the four in-flight forces when an airplane turns. This added force is due to the curved flight path required  to  turn the  airplane and acts parallel to the earth’s surface towards the outside of the turn. The pilot uses lift to oppose this force. As the bank angle is increased, load is also increased. The pilot must then increase the pitch attitude (and angle of attack) to supply the additional lift required to oppose the extra load, thus maintaining the lift/load relationship.

Thrust

Thrust is the sum of all forward acting forces. An airplane, with an internal combustion engine, produces thrust by converting fossil fuel (chemical energy) into lift by spinning a propeller connected to the engine crankshaft.  A propeller produces lift just as a wing does; however, this lifting force is referred to as thrust.

An airplane in flight has three sources of energy that can be used as thrust. The first is chemical energy, which is the useable fuel in the tanks. This energy can then be converted into the second source of energy, which is airspeed (kinetic energy) and/or the third source, altitude (potential energy). Used individually or combined, these sources of energy can be used as thrust.

Drag

Drag is the sum of all rearward acting forces. Drag acts parallel with and in the same direction as the relative wind. Two types of drag, induced and parasite, are produced by an airplane in flight. Parasite drag is air resistance produced by any part of the airplane that does not generate lift. Landing gear, radio antennas and steps are examples of components that produce parasite drag. Parasite drag increases with an increase in airspeed. Induced drag is produced when lift is produced. Induced drag decreases with an increase in airspeed.

(Refer to the figure below) By combining parasite drag and induced drag, total drag can be plotted as represented by the green line. Both points A and B represent  the minimum total drag and maximum lift to drag ratio (L/D max). Near minimum drag speed is where maximum performance can be obtained for climbs, endurance, and glide distance.

As discussed previously thrust opposes drag so the green curve could also approximate a power curve. Notice that at the extreme left side of the total drag curve the induced drag is high requiring a high power setting but decreases to the right, as airspeed increases, to point A where there is minimum drag and minimum power required. After point A the parasite drag starts to increase requiring an increase in power.

 

Airplane Controls

Flight involves three dimensions, therefore, airplanes are equipped with controls that allow a pilot to direct the airplane’s flight path in three dimensions. The action about each of the three axes can be controlled independently or be combined to have the airplane fly the desired flight path. Each of the axes, longitudinal, lateral, and vertical, has a corresponding control(s) that causes the airplane to move about them. This action or movement is roll, pitch and yaw. At no time should any of these axis be associated with airspeed or altitude.

Ailerons

Ailerons control roll about the longitudinal axis and are used to establish or maintain a desired bank angle. By maintaining a desired bank angle the airplane will turn. By maintaining a 0° bank the airplane will remain on a given heading (assuming coordinated flight). On modern airplanes, the ailerons are connected to the control  wheel  with  cables  and  push/pull  rods.  Right movement of the control wheel lowers the aileron on the left wing and raises the aileron on the right wing creating more lift on the left wing section and less lift on the right wing section. The lift imbalance  results in a roll to the right. With an imbalance of lift comes an imbalance of drag, the raising wing creates more lift and more drag. The result of the drag differential creates a yawing moment to the left. This is opposite the desired direction of turn and is called adverse yaw.

Left and right ailerons are inter connected so that when one moves down the other moves up, however, the down movement is less than the up movement. This is called differential aileron and is designed to reduce adverse yaw by increasing the drag on the lowered wing.

The design of the aileron surface itself has also been improved by the “Frise type” aileron. With this type of aileron, when pressure on the control stick or wheel is applied to one side, raising one of the ailerons, the leading edge of that aileron (which has an offset hinge) projects below the wing into the airflow and increases drag. This helps equalize the drag created by the lowered aileron on the opposite wing and thus reduces adverse yaw.

The Frise type aileron also forms a slot so that the air flows smoothly over the lowered aileron. This helps to make the aileron more effective at slow speeds. However, despite these improvements, some rudder action is still needed whenever ailerons are applied.

 

                                                                 

Elevator

Elevator controls pitch about the lateral axis and is used to establish or maintain a desired pitch attitude. The elevator is attached to the horizontal stabilizer and connected to the control wheel with cables. The horizontal stabilizer is attached to the airplane with a negative angle of incidence and in flight the horizontal stabilizer/elevator produces lift in the opposite direction to wing lift. This force is not only used to control the pitch attitude but also used for lateral (pitch) stability.

Some airplanes have a movable horizontal surface called a “stabilator, which serves the same purpose as the horizontal stabilizer and elevator combined. When the cockpit control is moved, the complete stabilator moves not just the elevator portion as with the conventional stabilizer/elevator combination.

Rudder

Rudder controls yaw about the vertical axis and is used to maintain coordinated flight. The rudder is attached to the vertical stabilizer and connected to the rudder pedals with cables. Being a symmetrical airfoil, the rudder can produce an equal amount of lift in either direction.

Flaps

Flaps are considered high lift devices and change the lift coefficient on the wing section they are installed decreasing the stall speed. However, on low speed airplanes, the decrease in stall speed is small and flaps are used primarily to increase drag. By increasing drag on an approach to landing, the approach angle can be steeper while maintaining a slow airspeed allowing obstruction clearance and landing nearer the beginning of the runway.

Extending flaps increases camber, on that section of the wing, increasing the lift coefficient and drag coefficient. After the flaps have been extended and the new configuration stabilized, lift will be equal to load and at the same value as before flaps were extended but at a slower airspeed. The flaps are not being used to increase the total lift, but to have the same lift at a slower speed or the same speed with a lower pitch attitude.

As flaps are extended, there will be a pitching moment due to  streamlining; the wing will seek minimum profile. On low wing airplanes this requires nose up elevator pressure and on most high wing airplanes nose down elevator pressure will be required.

Trim Devices

Trim Tabs. When an airplane’s flight conditions (attitude, airspeed, loading, etc.) and configuration are changed, the control pressures required to maintain the new flight conditions are affected by the resulting changes in aerodynamic  forces.  The  placement  of  fuel, passengers baggage, or cargo sometimes results in the airplane being wing heavy, tail heavy, or nose heavy. To counteract such unbalanced  conditions the  pilot will find  it necessary to continually exert pressure on the  control stick or wheel, or on the rudder pedals. Over a period of time this becomes annoying and fatiguing. To relieve the pilot of this tiring effort, most airplanes are equipped with trim tabs  with which to trim the airplane for balanced flight.

A trim tab is a small, adjustable surface, located on the trailing edge of the aileron, rudder, or elevator control surface. It is used to maintain balance in straight and level flight and during other prolonged flight conditions without the  pilot having to  hold pressure on the controls. This is accomplished by deflecting the tab in the direction opposite to that in which the  primary control surface must be held. The force of the  airflow  on the  tab  causes the main control surface to be deflected to a position that will correct the unbalanced condition of the airplane.

In many airplanes the trim tabs may be  adjusted by controls in the cockpit, while in some of the older types they may  be adjustable only on the  ground. Those which can be controlled from the cockpit provide a trim control wheel or electric switch. To apply a trim force, the trim wheel or switch must be moved in the desired direction. The position in which the trim tab is set can usually be determined by reference to a trim indicator.

Balance Tabs. Balance tabs look like trim tabs and are hinged in approximately the same  places  as  trim  tabs would be. The essential difference between the two is that the balance tab  is coupled to  the  control surface by a rod, so that when the primary control surface is moved in any direction the tab automatically is moved in the opposite direction. In this manner the airflow over the tab counter balances some of the air pressure against the primary control surface and enables the pilot to more readily move and hold it in position. These tabs also act as trim tabs as described above.

Servo Tabs. Servo tabs are very similar in operation and appearance to the trim tabs  previously  discussed. Servo tabs, sometimes referred to as flight tabs,  are used primarily on large airplanes. They aid the  pilot in moving the control surface and in holding it in the desired position. Only the servo tab moves in response  to movement of the pilot’s flight control, and the force of the airflow on the servo tab then moves the primary control surface.

 

Use of Flight Controls

The following will always be true, regardless of the airplane’s attitude in relation to the earth:

• When back pressure is applied to the elevator controls the airplane’s nose rises in relation to the pilot.
• When forward pressure is applied to the elevator control, the airplane’s nose lowers in relation to the pilot.
• When right pressure is applied to the aileron control, the airplane’s right wing lowers in relation to the pilot.
• When left pressure is applied to the aileron control, the airplane’s left wing lowers in relation to the pilot.
• When pressure is applied to the right rudder pedal, the airplane’s nose moves to the right in relation to the pilot.
• When pressure is applied to the left rudder pedal, the airplane’s nose moves to the left in relation to the pilot.
  

The preceding explanations should prevent the beginning pilot from thinking in terms of up or down” in respect to the  earth, which  is only a  relative state to the pilot. It will also make understanding of the functions  of  the controls much easier, particularly when performing steep banked turns and the more advanced maneuvers.

After learning how the airplane will react when the flight controls are used, the pilot must learn how to use them properly. Rough and erratic usage of all or any one of the controls  will  cause  the   airplane  to   react  accordingly; therefore,  the   pilot  must  form  the   habit  of  applying pressures smoothly and evenly. The amount of force the airflow exerts on a control surface is governed by the  airspeed and the degree that the surface is moved out  of its neutral or streamlined position. Since the airspeed will not be the same in all maneuvers, the actual amount the control surfaces are moved is of little importance;  but it is important that the pilot maneuver the airplane by applying sufficient control pressures to obtain a desired  result, regardless of how far the control surfaces are actually moved.

The pilot’s feet should rest comfortably against the rudder pedals.  Both  heels should  support the  weight of the  feet on the cockpit floor with the ball of each foot touching the individual  rudder pedals The legs and feet should  not be tense;  they  must  be  relaxed  just  as  when  driving  an automobile.

When using the rudder pedals, pressure should be applied smoothly and evenly by pressing with the ball of one foot just as when using the brakes of an automobile. Since the rudder  pedals  are  interconnected  and  act  in  opposite directions, when pressure is applied to one  pedal, pressure on the other must be relaxed proportionally. When  the rudder pedal must be moved significantly, heavy pressure changes should be made by applying the pressure with the ball of the foot while the  heels slide along the  cockpit floor. Remember, the ball of each foot must rest comfortably on the rudder pedals so that even slight pressure changes can be felt.

Lift is used to create the action about each of the axes; increasing or decreasing the download on the horizontal stabilizer/elevator will alter the pitch up or down; lowering or raising ailerons will change the lift at the ends of the wings and the airplane will roll right or left;  rudder pressure will create lift on the vertical stabilizer/rudder to yaw the nose right or left. As the control surfaces move, the force of lift is changed on that section of the airfoil. Air flows smoothly around an airfoil (unless stalled) and does not “hit” the control surface.

During flight, it is the pressure the pilot exerts on the control wheel and rudder pedals that causes the airplane to move about its axes. When a control surface is moved out of its streamlined position (even slightly), the air flowing past it will exert a force against it and will try to return it to its streamlined position. It is this force that the pilot feels as pressure on the  control  stick or whee! and the rudder pedals.