Basic Aerodynamics To Stability

04/11/23 Author: Harry L. Whitehead 1

AVAF 209 Structures II

III. Basic Aerodynamics A. The AtmosphereB. PhysicsC. The AirfoilD. Lift & DragE. StabilityF. Large Aircraft Flight Controls

Basic Aerodynamics

III. Basic Aerodynamics A. The AtmosphereB. PhysicsC. The AirfoilD. Lift & DragE. StabilityF. Large Aircraft Flight

Controls

Aerodynamics:

The study of objects in motion through the air and the forces that produce or change such motion

Basic Aerodynamics

Controls

The Atmosphere

•In order to fly, we need to create an upward force equal to the weight of the aircraft by using the Atmosphere

•This force comes from the action of the atmosphere on an airfoil

Basic Aerodynamics

Controls

The Atmosphere•Is made of a mixture of gases

•21% Oxygen

•78% Nitrogen

•Rest is mix of inert gases (Argon, Neon, etc.)

Basic Aerodynamics

Controls

The Atmosphere•Mixture remains constant regardless of altitude

•Weight of air changes as altitude changes

•Less weight above as we go up = less ATMOSPHERIC PRESSURE exerted on objects

Basic Aerodynamics

ControlsThe Atmosphere

•STANDARD DAY CONDITIONS

•International Civil Aeronautics Organization (ICAO) has set standards for test data

Basic Aerodynamics

ControlsThe Atmosphere

•STANDARD DAY CONDITIONS

•Allows comparison of test

data from one location or day to any other in world

Basic Aerodynamics

ControlsThe Atmosphere•STANDARD DAY

•Pressure

•Is a force created by the weight of the atmosphere above an object

•Is measured in IN-HG, MM-HG, PSI, or MILLIBARS

Basic Aerodynamics

•Pressure

•In-Hg or mm-Hg

•A tube is filled with Mercury (Hg) and then inverted in a container of Mercury

•Hg will rise and height is measured

Basic Aerodynamics

•Pressure

•In-Hg or mm-Hg

•On a Standard Day at SEA LEVEL (zero altitude), the height will be 29.92 inches 29.92 in-Hg) or 760 millimeters (760 mm-Hg)

Basic Aerodynamics

•Pressure

•In-Hg or mm-Hg

•This is called an ABSOLUTE SCALE measurement as a VACUUM will form in the top of the tube (= ABSOLUTE ZERO PRESSURE)

Basic Aerodynamics

•Pressure

•Atmospheric pressure will decrease by approx. 1 in-Hg for every 1,000 feet increase in altitude

•Known as the LAPSE RATE

Basic Aerodynamics

•Pressure

•An ALTIMETER measures absolute pressure and displays the result in Feet Above Sea Level (ASL)

•Notice KOLLSMAN WINDOW (adjust to varying local conditions)

Basic Aerodynamics

•Pressure

•PSI

•Is a measurement of FORCE / AREA

•The most common units are POUNDS PER SQUARE INCH

Basic Aerodynamics

•Pressure

•PSI

•On a Standard Day at Sea Level, the atmosphere pushes on objects with a force of 14.69 pounds per square inch of area

Basic Aerodynamics

•Pressure

•PSI

•Since ½ of the air in the atmosphere is below 18,000 feet ASL, the pressure there is 7.34 psi

Basic Aerodynamics

•Pressure

•PSI

•Is measured by an Absolute scale and is labeled PSIA

Basic Aerodynamics

•Pressure

•Or a GAUGE scale which uses Atmospheric Pressure as the zero reference (= PSIG)

Basic Aerodynamics

•Pressure

•Millibars

•Are used by Meteorologists (weather forecasters)

•Standard Day at Sea Level is 1013.2 mbs

•1 millibar approximately equals .75 in-Hg

Basic Aerodynamics

•Temperature

•Four scales used:

•Celsius (used to be Centigrade)

Basic Aerodynamics

•Temperature

•Kelvin (Absolute Celsius)

Basic Aerodynamics

•Temperature

•Fahrenheit

Basic Aerodynamics

•Temperature

•Rankine (Absolute Fahrenheit)

Basic Aerodynamics

•Temperature

•Standard Day at Sea Level:

•15o Celsius

•59o Fahrenheit

•2880 Kelvin

•519o Rankine

Basic Aerodynamics

•Temperature

•As we go up in altitude, temperature goes down

•3.54o F or 2o C per 1,000 feet

•ADIABATIC LAPSE RATE

Basic Aerodynamics

•Humidity

•Is amount of moisture in air

•Measured by RELATIVE HUMIDITY

•Is comparison of moisture present to amount air can hold in percent

•Maximum amount is directly proportional to temperature (hotter temp. = more moisture at same Relative Humidity %)

Basic Aerodynamics

•Humidity

•Standard Day is 0% humidity or Dry Air

Basic Aerodynamics

•Density

•Is measure of Mass per unit Volume

•Mass is the amount of matter in an object

•Can think of it as number of molecules

•Weight is the affect of Gravity on a mass

•Since we are dealing with objects near the surface of the Earth, Weight and Mass are used interchangeably in Aerodynamics

Basic Aerodynamics

•Density

•Air density is officially measured in SLUGS PER CUBIC FOOT

•Standard Day at Sea Level = .002378 slugs/ft3

•Formula symbol is the Greek letter Rho ( )•Is a major factor in developing Lift

•Varies directly with Atmospheric Pressure and inversely with Temperature

Basic Aerodynamics

•Aviation uses DENSITY ALTITUDE as important measure of density affects on flying

•Density Altitude

Basic Aerodynamics

•Density Altitude

•Is a measure of an aircraft’s performance (necessary takeoff distance, necessary landing distance, weight-carrying capability, etc.)

Basic Aerodynamics

•Density Altitude

•“The altitude in a Standard Day that has the same density as the Ambient conditions.”

•Is the altitude the aircraft thinks it’s at

Basic Aerodynamics

•Density Altitude

•Computed using a Density Altitude Chart

•Must know PRESSURE ALTITUDE and Ambient Temperature

Basic Aerodynamics

•Density Altitude

•Pressure Altitude is altitude in the Standard Day whose atmospheric pressure matches the local atmospheric pressure

Basic Aerodynamics

•Density Altitude

•Press. Alt. Example:

•Ambient pressure of 28.92 in-Hg

•Since pressure decreases 1 in-Hg/1000 feet, Pressure Altitude = 1,000 feet ASL

Basic Aerodynamics

•Density Altitude

•Dens. Alt. Example:

•Pressure Altitude can also be determined for the location you are by adjusting the Kollsman window to 29.92 and reading the altitude

Basic Aerodynamics

•Density Altitude

•Pressure = 25.92 in-Hg (= ? feet Pressure Altitude)

•= 4,000 feet

•SL (29.92) – actual (25.92) = 4 inches x 1000 ft.

Basic Aerodynamics

•Density Altitude

•Pressure = 25.92 in-Hg (= ? feet Pressure Altitude)

•= 4,000 feet

•Temperature = 80o

Basic Aerodynamics

•Density Altitude

•Density Altitude = 6,500 feet

Basic Aerodynamics

•Density Altitude•Also is affected by the Relative Humidity•Water vapor has about 62% of weight of air = higher humidity = less dense air = higher Density Altitude

•= only affected by about 5%

Basic Aerodynamics

•Density Altitude

•Generally speaking:

BEWARE OF HIGH, HOT AND HUMID CONDITIONS

Basic Aerodynamics

ControlsLaws of Physics which affect Aerodynamics•Bernoulli's Principle

•“If the total energy of flowing air remains constant, any increase in KINETIC energy creates a decrease in POTENTIAL energy”

•Since the LAW OF CONSERVATION OF ENERGY applies, the energies in the flow are only changed

Basic Aerodynamics

ControlsLaws of Physics which affect Aerodynamics•Bernoulli's Principle

•Kinetic energy is measured as Velocity

•Potential energy is measured as Pressure

Basic Aerodynamics

ControlsLaws of Physics which affect Aerodynamics•Bernoulli's Principle•In “throat” of venturi:

•Velocity goes up so all air gets through in same time = pressure down

Basic Aerodynamics

ControlsLaws of Physics which affect Aerodynamics•Newton’s Laws•First Law:

•Law of Inertia

•A body at rest tends to remain at rest and a body in motion tends to remain in motion, until acted upon by an outside force.

Basic Aerodynamics

ControlsLaws of Physics which affect Aerodynamics•Newton’s Laws•Second Law:

•Law of Acceleration

•Acceleration of a body is directly proportional to the force applied and inversely proportional to the mass of the body or a = F / m

•Or more useful to us: F = ma

Basic Aerodynamics

ControlsLaws of Physics which affect Aerodynamics•Newton’s Laws•Third Law

•Law of Reaction

•For every Action there is an Equal and Opposite Reaction

Basic Aerodynamics

ControlsThe Airfoil•Flight Forces•As we looked at before, there are four forces being applied to an airplane in flight:

•Lift (up)

•Weight (down)

•Thrust (forward)

•Drag (aft)

Basic Aerodynamics

ControlsThe Airfoil•Flight Forces•In order to understand these forces, we need to look at VECTORS:

Basic Aerodynamics

ControlsThe Airfoil•Flight Forces•A Vector is an arrow whose length shows a value and it points in the direction the value is being applied

Basic Aerodynamics

ControlsThe Airfoil•Flight Forces•To combine vectors, we place them with their starting points joined (as on the left below)

Basic Aerodynamics

ControlsThe Airfoil•Flight Forces•And by COMPLETING THE SQUARE we can get the RESULTANT vector (the combination of the other two)

Basic Aerodynamics

ControlsThe Airfoil•Flight Forces•If two forces are exactly opposing each other (such as Lift and Weight) and have the same value, the resultant is zero

Author: Harry L. Whitehead 54

Basic Aerodynamics

ControlsThe Airfoil•Flight Forces•In STRAIGHT AND LEVEL, UNACCELERATED FLIGHT, Thrust and Drag are equal, Lift and Weight are equal, and the aircraft continues in a straight line with no change in altitude

•The forces are said to be in EQUILIBRIUM

Basic Aerodynamics

ControlsThe Airfoil•Flight Forces•In order to climb, we must increase the Lift Vector so there is no longer an equilibrium between Lift and Weight

•The Resultant of the two is an upward force

Basic Aerodynamics

ControlsThe Airfoil•Flight Forces•In order to go faster (Accelerate), we must increase the Thrust vector to get a Resultant forward

•Etc.

Basic Aerodynamics

ControlsThe Airfoil•Flight Forces•Thrust is created by the POWERPLANT and PROPELLER•Weight is the effect of Gravity on the aircraft•Drag is created by movement of the aircraft•Lift is created by the Airfoils used as Wings

Basic Aerodynamics

ControlsThe Airfoil•An Airfoil is a specially designed surface which produces a reaction to air flowing across it•Two theories:

•Bernoulli’s Principle•Newton’s Laws

Basic Aerodynamics

ControlsThe Airfoil•Subsonic airfoils can be Asymmetrical or Symmetrical•Most airplanes use Asymmetrical wings

•Blunt, rounded LEADING EDGE•Max. thickness about 1/3 of distance from L.E. to TRAILING EDGE

Basic Aerodynamics

ControlsThe Airfoil•There are many basic airfoil shapes

Basic Aerodynamics

•Early were very thin with definite camber

•The Clark-Y was the standard through the 1930s

•NACA developed the “modern” asymmetrical shape in the 30s and it was used for decades = smoother airflow and greater lift with less drag

Basic Aerodynamics

•As aircraft started to get near Mach 1, the subsonic shapes caused shock waves to form and destroy lift and increase drag tremendously

•Supersonic airfoils were designed with sharp Leading and Trailing edges and the max thickness about ½ of the chord distance

Basic Aerodynamics

•Next came the Supercritical design

•Reduces the velocity of the air over the upper surface and delays the drag rise occurring with the approach of Mach 1

•NASA developed the GAW series for General Aviation aircraft and give higher lift with lesser drag than the “modern”

Basic Aerodynamics

ControlsLift and Drag•In order to generate Lift, an Airfoil must have an ANGLE OF ATTACK ()

•This is defined as the angle between the CHORD and the RELATIVE WIND (= opposite the FLIGHT PATH)

Basic Aerodynamics

•Don’t confuse this with the ANGLE OF INCIDENCE

•The angle formed between the Chord and the Longitudinal Axis of the airplane

Basic Aerodynamics

•If the is positive = the Leading Edge is higher than the Trailing Edge = generate Lift in the Upward direction •Negative = downward Lift

Basic Aerodynamics

•As the increases, the amount of Lift also increases

Airfoil simulation

Basic Aerodynamics

•This can be shown graphically using the COEFFICIENT OF LIFT or CL

Basic Aerodynamics

•Notice the CL is positive even to a small negative

Basic Aerodynamics

•And the CL peaks at some positive

Basic Aerodynamics

ControlsThe Airfoil•In order to generate Lift, an Airfoil must have an ANGLE OF ATTACK ()

•Also, the CL starts to drop off if the gets higher

•This is called a STALL and starts at CLmax or CRITICAL

Basic Aerodynamics

•Stall is a SEPARATION OF AIRFLOW from the upper wing surface = rapid decrease in lift

Basic Aerodynamics

•This occurs at the same regardless of speed, aircraft weight, or flight attitude

Basic Aerodynamics

•To eliminate this condition = reduce the below critical

Basic Aerodynamics

ControlsLift and Drag•Other Factors Affecting Lift:

•Airspeed•Faster = increased Lift•Lift is increased as the square of the speed•For example:

•At 200 mph a wing has 4 times the lift of the same airfoil at 100 mph•At 50 mph the lift is ¼ as much as at 100 mph

Airfoil simulation

Basic Aerodynamics

•Wing Planform•View of the wing from above or below

Basic Aerodynamics

•Wing Planform•Rectangular: excellent slow flight and stall occurs first at root of wing (= good aileron control)

Basic Aerodynamics

•Wing Planform•Elliptical: most efficient = least drag for given size but difficult to manufacture and stalls all along Trail. Edge

Basic Aerodynamics

•Wing Planform•Modified (or Moderate) Tapered: more efficient than Rectangular and easier to build than Elliptical but still stalls along Trailing Edge

Basic Aerodynamics

•Wing Planform•SweptBack (and Delta): Good efficiency at high speed but not very good at low

Basic Aerodynamics

•Camber•Curve of the wing•Increased Camber = increased airflow velocity over the top surface and more downwash angle = more lift•It also tends to lower the Critical •Trailing Edge Flaps use this to allow more lift at a slower airspeed for landing and takeoff

Airfoil simulation

Basic Aerodynamics

•Aspect Ratio•Is the Ratio of the Wing’s SPAN to the average Chord•Higher Aspect Ratio (“long and skinny”) = increased lift and lower stalling speed•Used on Gliders and TR-1 spy plane

Basic Aerodynamics

•Wing Area•Is the total surface area of the wings•Must be sufficient to lift max weight of the aircraft

•If wing produces 10.5 pounds of lift per square foot at normal cruise speed = needs Wing Area of 200 square feet to lift 2,100 pounds of weight

Airfoil simulation

Basic Aerodynamics

ControlsLift and Drag•Drag

•Is the force opposing Thrust•Is the force trying to hold the aircraft back as it flies and generally limits the maximum airspeed•Is created by any aircraft surface that deflects or interferes with the smooth air flow around the aircraft•Drag is classified as two types:

•Induced •Parasite

Basic Aerodynamics

ControlsLift and Drag•Induced Drag

•The Airfoil shape (type of airfoil and amount of Camber) and Wing Area create a force which comes from the same forces as those which create Lift•It is Directly Proportional to the Angle of Attack ()

Basic Aerodynamics

•As increases, the high pressure on the bottom of the wing flows around the wing tips and “fills in” some of the low pressure on top•This creates a WINGTIP VORTEX and destroys some of the wing’s lift or increases its drag

Basic Aerodynamics

•The strength of the Vortex is proportional to aircraft speed, weight, and configuration

•These can be dangerous for small aircraft flying behind a large aircraft

Basic Aerodynamics

•This effect can be reduced by installing WINGLETS on the tips of the wings

•Reduce the Vortex = increased lift and reduced drag

Basic Aerodynamics

•This effect can also be reduced by installing TIP TANKS on the tips of the wings

Basic Aerodynamics

•And/or by installing DROOPED TIPS

•Used on STOL (Short Take Off/ Landing) aircraft or those designed for heavy and slow flight

Basic Aerodynamics

•This can also be shown by looking at the COEFFICIENT OF DRAG (CD) of the airfoil

•CD is proportional to Angle of Attack () and increases as increases

Basic Aerodynamics

ControlsLift and Drag•Angle of Attack and Drag

•By combining the CL and CD curves we get a “Family” of curves for any given airfoil•Includes a combination known as Lift-to-Drag Ratio (L/D)

Basic Aerodynamics

•Peak L/D (L/Dmax) occurs at a given which is the most efficient for the airfoil to operate at

Basic Aerodynamics

•Unfortunately, this may be at too low an to generate enough lift to fly (may not be able to fly fast enough)

Basic Aerodynamics

ControlsLift and Drag•Parasite Drag

•Is the drag produced by the aircraft itself and is proportional to Airspeed•Is disruption of the airflow around the aircraft•4 types:

•Form Drag•Skin Friction Drag•Interference Drag•Profile Drag

Basic Aerodynamics

•Form Drag•Created by any structure which extends into the airstream•Is directly proportional to the size and shape of the structure•Includes struts, antennas, landing gear, etc.•Streamlining reduces Form Drag

Basic Aerodynamics

•Skin Friction Drag•Caused by the roughness of the aircraft’s skin

•Includes paint, rivets, skin seams, etc.•Causes small swirls (eddies) of air = drag

•Improved by flush riveting and cleaning and waxing the skin

Basic Aerodynamics

•Interference Drag•Occurs when various air currents around the aircraft structure intersect and interact with each other

•Example: mixing of air where fuselage and wings meet

•Improved by installing FAIRINGS

Basic Aerodynamics

•Profile Drag•Drag formed by the Frontal Area of the aircraft•Can’t be changed or affected by anything except Retractable Landing Gear

Basic Aerodynamics

•Combined Parasite Drag Airspeed Effect

•Parasite Drag increases exponentially as airspeed increases•IS LOWEST AT LOW AIRSPEEDS and increases rapidly

Basic Aerodynamics

•Can best be reduced by Retractable Landing Gear & streamlining•Weight and complication is more than compensated by decrease in Parasite Drag at higher airspeeds

Basic Aerodynamics

ControlsLift and Drag•Total Drag

•Induced Drag is also somewhat dependent on Airspeed (indirectly)•Since it is Inversely Proportional to and since the is highest at low airspeeds = Induced Drag is highest at low airspeeds and drops off rapidly

Basic Aerodynamics

•By combining the two Drag curves, we get Total Drag•At low airspeeds, Induced Drag predominates so curve goes down•At higher airspeeds, Parasite Drag predominates so curve goes up

Basic Aerodynamics

•At some airspeed it will be at its lowest value = most efficient airspeed to fly at = best Lift/Drag Ratio or L/Dmax

•However, like L/Dmax when looking at the curve, it may not be possible to operate at this airspeed

Basic Aerodynamics

ControlsLift and Drag•Other Design Considerations

•Other factors affect the structure and design of an aircraft while in flight besides just Lift and Drag•These are:

•Load Factor•Propeller Factors

•Engine Torque•Gyroscopic Precession•Asymmetrical Thrust•Spiraling Slipstream

Basic Aerodynamics

ControlsLift and Drag•Load Factor

•Load Factor is a function of Banking an aircraft•You can also think of it as creating a curved flight path = CENTRIFUGAL FORCE puts more downward force (LOAD) on the structure

Basic Aerodynamics

•So in order to maintain altitude = need to pull back on the yoke or stick and increase the engine’s power to increase the overall Lift component

Basic Aerodynamics

•Load Factor is the Ratio of the load supported by the wings to the actual weight of the aircraft•Below about 20o Bank Angle it is equal to 1G in force

•= the weight is not being increased

Basic Aerodynamics

•As the Bank Angle increases above that the “G-force” also goes up exponentially

•For example: at about 60o of Bank, the Load Factor is 2

•The wings feel the aircraft weighs twice as much as normal

Basic Aerodynamics

•The FAA establishes LIMIT LOAD FACTORS for airplanes to be designed to•= the maximum Load Factor the aircraft can withstand without permanent deformation or structural damage

Basic Aerodynamics

•For a NORMAL CATEGORY airplane = 3.8 positive Gs and 1.52 negative Gs

•For a UTILITY CATEGORY = 4.4 positive Gs and 1.76 negative Gs

Basic Aerodynamics

•For an ACROBATIC CATEGORY airplane = 6 positive Gs and 3 negative Gs

Basic Aerodynamics

ControlsLift and Drag•Propeller Factors: Torque

•Torque is a force applied to the airplane from the Reaction to the spinning Propeller (Newton’s 3rd Law)•It causes a roll to the left = opposite of the normal rotation of U.S. designed engines

Basic Aerodynamics

ControlsLift and Drag•Propeller Factors: Torque

•On single-engine airplanes, it’s common to use aileron trim tabs to compensate• On multi-engine airplanes, it’s common for the engines to rotate in opposite directions which cancels out the Torque Effect

Basic Aerodynamics

ControlsLift and Drag•Propeller Factors: Gyroscopic Precession

•A rotating Propeller also acts like a GYROSCOPE and exhibits two gyroscopic characteristics:

•RIGIDITY IN SPACE•PRECESSION

Basic Aerodynamics

•Precession is the phenomenon which says that any force applied to a Gyroscope is felt 90o later in direction of rotation

Basic Aerodynamics

•Any rapid change in aircraft pitch = a precessive force applied to the prop.

•Most commonly felt by Conventional Gear airplanes just prior to Takeoff when the tail wheel is raised

Basic Aerodynamics

•This causes a downward force (action) applied to the prop•Which causes a reaction 90o later = yaw to the left

Basic Aerodynamics

ControlsLift and Drag•Propeller Factors: Asymmetrical Thrust

•At high aircraft angles of attack and during rapid climbs, the prop blades see differing angles of attack during their rotation

Basic Aerodynamics

•The side of the prop “disk” on which the prop blade is descending has a higher than the ascending blade = more lift•NOTE: rotation is clockwise as viewed from the pilot’s seat

Basic Aerodynamics

•This change in comes from the vertical movement and a corresponding change in Relative Wind of the airfoil

Basic Aerodynamics

•Since the airfoil (prop) is rotating in addition to flying, the Relative Wind is now made of two factors:

Basic Aerodynamics

•The Flight Path vector and a vertical (rotation) vector•Descending blade (right side) = vertical vector is down

Basic Aerodynamics

•Which gives us a new Relative Wind and a higher

Basic Aerodynamics

•Since the descending (right) side of the prop has a higher it is also producing more Thrust•The opposite occurs on the ascending side and it produces less Thrust• = tendency to yaw to

left in rapid climb

Basic Aerodynamics

ControlsLift and Drag•Propeller Factors: Spiraling Slipstream

•On a single-engine airplane, the SLIPSTREAM from the propeller “wraps” itself around the fuselage in a Spiraling manner•It will generally then strike the left side of the Vertical Stabilizer and cause a yaw to the left

Basic Aerodynamics

•Since this is a function of how much air the prop is pushing which is directly proportional to the Thrust being produced = more yaw at higher power settings

Basic Aerodynamics

•It’s not uncommon to find the Vertical Stabilizer installed with a slight offset to the left to cause a constant compensating force•This is usually set up to balance the Slipstream affect during Cruise flight

Basic Aerodynamics

Controls

Basic Aerodynamics To Stability

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