Aerodynamics I

1. Aerodynamics I A study guide on aerodynamics for the Piper Archer

2. Introduction • The purpose of this pilot briefing is to discuss the basic aerodynamics of the Piper Archer. • Please use the following references: - Pilot’s Handbook of Aeronautical Knowledge - Flight Theory for Pilots

3. Basics – The atmosphere • These fundamental basics first must be acknowledged: • Air is a fluid. It can be compressed & expanded • The atmosphere is composed of • 78% nitrogen • 21% oxygen • 1% other gasses

4. Basics – Production of Lift • The production of lift is explained through the theories of Newton and Bernoulli. • Newton is famous for stating three laws of motion and force. • Bernoulli is famous for “explaining how the pressure of a moving fluid (liquid or gas) varies with its speed of motion.” • (PHAK pg. 2-3)

5. Basics – Production of Lift • Newton’s Laws of motion: • Law 1 – A body at rest will remain at rest. A body in motion will remain in motion • For an airplane, inertia keeps it moving. • In contrast, if it is out sitting on the ramp, it will remain on the ramp until an outside force causes it to move. • Law 2 – Force is equal to mass times acceleration (F=MA) • The force that Newton is referencing, is the force that overcomes inertia, which was mentioned in Law 1. This force could be a change in direction or speed. For example, an applied force could cause acceleration.

6. Basics – Production of Lift • Law 3 – For ever action there is an equal and opposite reaction • “In an airplane, the propeller moves and pushes back the air; consequently, the air pushes the propeller (and thus the airplane) in the opposite direction – forward.” In other words, an airplane takes a “bite of air” with the propeller, throwing air back behind over the aircraft. This is the action. The airplane reacts to the propulsion of air, by moving forward. (1)

7. Basics – Production of Lift • Bernoulli’s principle of Pressure: • An increase in the speed or movement or flow will cause a decrease in the fluid’s pressure. • - Example: the Venturi tube Low Pressure (2)

8. Basics – Production of Lift Consider when you blow air over the top of a piece of paper. The paper rises, displaying the effects of lift. The rise of the piece of paper is due to the low pressure that was created between the air stream, and the original paper position. The velocity of the air increased above the paper, thus the pressure decreased, causing lift. (3)

9. Basics – Production of Lift • Bernoulli’s principle: Air going over a wing. Notice the shape of the wing creates a Venturi. The low pressure develops on top of the airfoil.

10. Basics – Air as a fluid • Because air is a fluid, it utilizes the properties of the Coanda effect. This is the tendency for a fluid to follow the object along its flow path. • http://www.youtube.com/watch?v=AvLwqRCbGKY • http://www.youtube.com/watch?v=S-SAQtODAQw • The way the water causes the object to move, is the same concept as the boy blowing over a piece of paper, causing the paper to move upward.

11. Aerodynamics of a wing • Wing construction plays an important role in the aerodynamics of lift that were just discussed. • Some terms to be familiar with: • Camber – This is the curvature of the wing. Notice how the wing is thickest at the middle. It then thins out at the trailing edge. This creates curvature to the wing. The curve of the wing means that the molecules of air traveling on top of the airfoil have a faster velocity than the molecules of air traveling underneath. According to the Bernoulli, this difference in velocity is what contributes to the pressure differential above (LOW) and below (HIGH) the wing. (4)

12. Aerodynamics of a wing • Dihedral – The upward angle that exists between the wings and the fuselage. (5) Chord Line – The exact line from the leading edge to the trailing edge of the wing. (6)

13. Aerodynamics • Now that the basics of lift & wing characteristics are understood, weight and balance must be examined to insure safe flight. The next slides will explore the weight & balance of the Piper Archer (7)

14. Aerodynamics of Weight & Balance Prior to every flight, the weight and balance is calculated for that particular day. It is known that weight x arm = moment. • But, what does that mean? Weight = the actual weight of the object/person in pounds. Arm = “the distance from a datum, to the applied force” • (PHAK 3-9) • The datum in the Piper Archer is right at the tip of the nose of the plane. For example, the arm for calculating fuel is 95 inches. This means that the fuel tanks are located 95 inches aft of the datum. The applied force is the location of the fuel tanks. Moment = the product of the weight multiplied by the arm

15. Aerodynamics of Weight & Balance • To calculate center of gravity, divide the total moment by the total weight. • Center of Gravity (CG) is the center point where all the weight acts through. • The C.G. range for the Piper Archer is 82 inches to 93 inches. • Where is this located? • Answer: Right beneath your feet, as you sit in the pilot’s seat. • “The center of gravity is a point at which an airplane would balance if it were suspended at that point…The center of gravity is not necessarily a fixed point; its location depends on the distribution of weight in the airplane.” (PHAK 8-2).

16. Aerodynamics of Weight & Balance For the Piper Archer, the envelope that the C.G. must remain within is only 11 inches. When C.G. shift is calculated for weight loss during flight (due to burning fuel, decreasing the weight in the fuel tanks), it actually is only shifting a few inches. It is uncommon for the C.G. to reach the forward or aft limits of the envelope. But, it is essential to check it each flight. Well, perhaps you might, but you sure won’t make it off the ground!

17. Aerodynamics of Weight & Balance • Characteristics of a forward CG: • Higher stall speed • Slower cruise speed • More stable • Greater back elevator pressure required • Characteristics of an aft CG: • Lower stall speed • Higher cruise speed • Less stable • (Page 5-23 Commercial Oral Exam Guide)

18. Center of Pressure vs. Center of Gravity • The balance of an airplane depends on the center of gravity and the center of pressure. • But, what is the difference between the two? • The center of gravity is calculated for every flight. • The center of pressure is not calculated. • The center of pressures (CP) is determined by the design of the wing.

19. Center of Pressure vs. Center of Gravity • The center of pressure is dependent upon the shape of the wing and the angle of attack. • So, what is angle of attack? • New term: Angle of Attack (AOA) • Angle of attack is the angular difference between the chord line and the relative wind. (8) (9)

20. Center of pressure vs. Center of Gravity • When the angle of attack changes, the center of pressure changes. For level flight (top picture) the CG is before the center of pressure With a small angle of attack (middle picture) the center of pressure is slightly before the CG With a large angle of attack (bottom picture) the center of pressure is ahead of the CG (10) When the angle of attack increases, the center of pressure moves forward When the angle of attack decreases, the center of pressure moves rearward

21. Center of pressure vs. Center of Gravity • Knowing that the center of pressure moves throughout the flight, what really is it? • “Center of pressure is the point where the resultant force crosses the chord line” (PHAK 2-7) • Now, what is the resultant force? Resultant force is the average between the force of lift and the force of drag. (11)

22. Center of Pressure vs. Center of Gravity Tying it all together, it would make sense that the center of pressure would move during flight. This is because throughout the flight, your lift varies (you climb, descend, level off) and your drag varies (fly with/without flaps). Remember, when you climb and descend, you are increasing, or decreasing your angle of attack. When the forces of lift and drag are constantly changing, the center of pressure is constantly changing. (11)

23. Drag • In the previous slide, drag was introduced into the discussion. • In the Piper Archer, adjusting the position of the flaps, is an example of how the force of drag can be varied. But, it is not the only way that drag can be altered. • There are two main types of drag: • Parasitic Drag • Induced Drag

24. Parasitic Drag • Parasitic drag is the most basic type of drag. It is broken down farther into three subcategories: • Form Drag – Results from the disrupting the airflow going over the surface of the wing • Interference Drag – This occurs at the intersection of air currents. For example, the wing root connected to the fuselage. • Skin Friction – The basic friction that exists from air (a fluid) flowing over an airfoil. • Think of sliding a box on carpet vs. a tile floor • Which one has more friction? • Although a wing is much smoother than carpet in your home, friction still exists.

25. Induced Drag • The second main type of drag is induced drag. • Induced drag is defined as drag that is simply a byproduct of lift. • There is no way to minimize it. • Rather, it is simply a result of the wing developing lift. • No system can be 100% efficient. Whenever a wing is developing lift, induced drag is consequently being generated.

26. Induced Drag – Wingtip Vortices • Induced drag is a result of wingtip vorticies. • What are wingtip vortices? • This is the wake that is generated from the wingtips. They are counter-rotating vortices that are caused from air spilling over the end of the wing. • “This pressure differential triggers the rollup of the airflow aft of the wing resulting in swirling air masses trailing downstream of the wingtips” (PHAK 12-13). • The pressure difference the PHAK is referencing is the Low pressure above the wing, countered with a High pressure below the wing. • http://www.youtube.com/watch?v=E1ESmvyAmOs

27. Wingtip Vortices Always land beyond an aircraft generating significant wingtip vortices. Rotate prior to their rotation point. ALWAYS give yourself plenty of time to avoid them. Remember to sidestep upwind. Problem: Have you ever seen a Piper Archer out climb a 727? Probably Not. So, what good will it do to rotate prior to their rotation point if you can’t remain high above their climb out path? You will eventually fly through them. Time will solve this problem so that the vorticies can dissipate. (12)

28. Wingtip Vortices

29. Wingtip Vortices • Imagine an infinite wing…would it have wingtip vortices? • Answer: No. This is because an infinite wing would not have wingtips, therefore it would not develop wingtip vortices. • Wingtips generate induced drag. Therefore if an infinite wing does not have wingtips, it would not generate induced drag.

30. Introduction to Stability • An important design characteristic to be aware of with the Piper Archer is stability. • What is stability? • Stability is the tendency for the aircraft to correct back to the original state. • There are two types of stability: • Static • Dynamic

31. Introduction to Stability • Static Stability is the aircraft’s initial response following a disturbance. Positive static stability means that initially, the aircraft will return to its original position. After being disturbed, it wants to go back. Neutral static stability means that initially, the aircraft will remain in a new position after being disturbed. Negative static stability means that initially, the aircraft will continue away from its original state, after being disturbed. (14)

32. Introduction to Stability (15) Dynamic stability is the aircraft’s response over a period of time. • Is an example of positive dynamic stability. Over time, the aircraft desires to correct back to the original state. • (B) Is an example of neutral dynamic stability. Over time, the aircraft will continually find a new position. • (C) is an example of negative dynamic stability. Over time, the aircraft will continue away from the original state.

33. Introduction to Stability • So, what types of stability does the Piper Archer have? • Knowing that the Archer is used as a training aircraft, it has positive static stability, and positive dynamic stability. • This means that initially, and over time, the aircraft wants to return to the original state.

34. Introduction to Stability • When discussing stability, the words “maneuverability” and “controllability” might also be included in the conversation. • It is important to understand the difference between them: • Maneuverability - The ability to change attitude and withstand stresses • Controllability - The aircraft’s response to pilot imputs Maneuverability, controllability, and stability are each unique design characteristics, don’t mistake them for the same thing!

35. Concepts to understand • It is important to understand some base aerodynamic concepts about the Piper Archer. The first is the idea of ground effect. So, what is it exactly? • Ground effect- “Fly an airplane just clear of the ground (or water) at a slightly slower airspeed than that required to sustain level flight at higher altitudes” (PHAK 3-7). • Ground effect alters: • Upwash • Downwash • Wingtip vorticies

36. Concepts to understand • According to the definition on the previous slide, being in ground effect allows you to maintain level flight at slower airspeeds than you normally would, if you were up at altitude. • Why? • When the aircraft is that close to the ground, there is a reduction of induced drag. Because the effects of aircraft upwash, downwash, and wingtip vortices are altered, the inherent drag due to lift, is decreased. • When the aircraft generates less drag, consequently the airspeed, when operating in ground effect, can be reduced.

37. Concepts to understand When you enter ground effect, the following phenomena occur: “On entering ground effect: 1. Induced drag is decreased 2. Nose-down pitching moments occur 3.The airspeed indicator reads low Upon leaving ground effect: 1. Induced drag is increased 2. Nose-up pitching moments occur 3The airspeed will read higher (correctly)” Page 72 Flight Theory for Pilots

38. Concepts to understand According to the diagram, in ground effect, less thrust is required to maintain any given velocity, compared with the thrust required out of ground effect. Because less thrust is required, also displays the correlation as to why you can maintain level flight at a slower airspeed. “Therefore, the wing will require a lower angle of attack in ground effect to produce the same lift coefficient or, if a constant angle of attack is maintained, an increase in lift coefficient will result” (PHAK 3-7). (16)

39. Concepts to understand • Where exactly are the limits of the ground effect region? • The maximum altitude that an aircraft can experience ground effect depends on the wing span of that specific airplane. • Generally, the distance of about ½ the wing span, determines the region of ground effect. • The wingspan of the Piper Archer is 35.5 feet. • To estimate the altitude you will enter/exit ground effect, take 35.5 divided by 2 = appx. 18 • About 18 feet, above field elevation, is the upper limit that the aircraft will experience the phenomena associated with ground effect.

40. Concepts to understand • Another concept to be familiar with is the idea of adverse yaw. Adverse Yaw – You change the camber (shape) of the wing with the ailerons when executing a turn. The upward wing has more lift than the lower wing. In adverse yaw, the aircraft tends to slip towards the upward wing due to the differential of lift. In a turn, an increase in lift results in an increase in drag. The more drag on the upward wing causes a shift/twist around the vertical axis resulting in an uncontrolled turn. Adverse Yaw is one explanation for why a rudder is essential. (17)

41. Concepts to understand • Stalls are an important maneuver in the fundamentals of flight training. But, understanding the aerodynamics behind a stall is equally important. • When does an airplane stall? • When it exceeds the critical angle of attack. • Remember from previous slides that angle of attack is the angle between the chord line and the relative wind.

42. Concepts to understand • Lift is developed when air is flowing over the surface of a wing. When a stall occurs, airflow over the top of the wing is disturbed. The airstream is no longer smooth, and the production of lift is reduced. • A stall occurs first at the wing root, then works out toward the tip. This design characteristic is so that you still maintain aileron control as long as possible. • http://www.youtube.com/watch?v=9eoboZNL9R8

43. Concepts to understand • Referencing the definition of when a stall will occur, note that there is not an airspeed associated with it. • It does NOT matter what speed your aircraft is flying at. A stall will ALWAYS occur when you exceed the critical angle of attack. • The speed is irrelevant for determining when a stall will happen. But, the manufacture does provide us with a respective airspeed that a stall will usually occur. It is incorrect to believe that flying at this speed will produce a stall. Rather, the critical angle of attack will normally be exceeded at those approximate speeds.

44. Concepts to understand • In the Piper Archer there are two airspeeds associated with stalls: • Vs and Vso • Vs is the stall speed without flaps (Clean configuration) • Vs = 50 knots • Vso is the stall speed with flaps (Dirty Configuration) • Vso = 45 knots Stall speeds are based upon 1G, wings level, unaccelerated flight.

45. Concepts to understand • Why would there be a difference in stall speed with or without flaps? • First recognize the purpose of flaps. Flaps are designed to maintain lift at slow airspeeds. • When flaps are added, the camber of the wing is changed. Because the chord line has increased, it is now easier for lift to develop. • This is why a stall the respective stall speed is slower with an airplane configured with flaps.

46. The topics addressed in this briefing are critical to develop a solid understanding about the basic aerodynamics regarding the Piper Archer. • If you have any questions about topics covered in this presentation feel free to contact a RMC flight instructor, or professor. • FLY SAFE!

47. Resources Used • Pilot’s Handbook of Aeronautical Knowledge FAA-H-8083-25, 2003, U.S. department of Transportation Federal Aviation Administration. • Flight Theory for Pilot’s, Charles Dole, Fourth Edition, Jeppesen-Sanderson Training Products • Commercial Oral Exam Guide, Michael D Hayes, Sixth Edition, ASA

48. Resources Used Continued… • (1) http://www.sciencetoymaker.org/balloon/images/newton.gif • (2) http://www.neam.co.uk/MathsFolder/venturi.gif • (3) http://www.coolscienceexperimentsforkids.com/wordpress/wp-content/uploads/2009/02/paper_lifted_by_air.jpg • (4) http://www.mansfieldct.org/schools/MMS/staff/hand/flightpressureonawing_files/image007.jpg • (5) http://knoxmodelairplane.com/_borders/clip_image004.jpg • (6)http://www.researchsupporttechnologies.com/boomerang_site/Boomerang%20aerodynamics3_files/angle%20of%20attack.jpg • (7) http://www.cartoonstock.com/lowres/wpa1258l.jpg • (8)http://images.google.com/imgres?imgurl=http://content.answers.com/main/content/img/oxford/Oxford_Sports/0199210896.angle-of-attack.1.jpg&imgrefurl=http://www.answers.com/topic/angle-of-attack&usg=__WyvMspHN9_L4SDhMIfsK5gf9VnI=&h=544&w=518&sz=55&hl=en&start=7&itbs=1&tbnid=uHQI8iAeKLy5JM:&tbnh=133&tbnw=127&prev=/images%3Fq%3Dangle%2Bof%2Battack%26hl%3Den%26gbv%3D2%26tbs%3Disch:1 • (9) http://images.google.com/imgres?imgurl=http://resources.yesican-science.ca/100_years/images/attack1.png&imgrefurl=http://resources.yesican-science.ca/100_years/lift.html&usg=__UblbFSnzuShPfuFAcedC8Uf-2w0=&h=416&w=300&sz=22&hl=en&start=15&itbs=1&tbnid=3kW88gsx18m_TM:&tbnh=125&tbnw=90&prev=/images%3Fq%3Dangle%2Bof%2Battack%26hl%3Den%26gbv%3D2%26tbs%3Disch:1 • (10)http://images.google.com/imgres?imgurl=http://www.free-online-private-pilot-ground-school.com/images/CP_angle_of_attack.gif&imgrefurl=http://www.free-online-private-pilot-ground-school.com/aerodynamics.html&usg=__Q3CvhSjlrmiFNsP_HDI5Hn3jnTY=&h=658&w=589&sz=12&hl=en&start=1&itbs=1&tbnid=0Wc1faGAVIEQGM:&tbnh=138&tbnw=124&prev=/images%3Fq%3Dangle%2Bof%2Battack%26hl%3Den%26gbv%3D2%26tbs%3Disch:1 • (11)http://2.bp.blogspot.com/_fX9doSZqagk/SNcYVKrM9OI/AAAAAAAAAvk/BP8gYVm3gXU/s200/Figure+2-9+Force+vectors+on+an+airfoil.jpg • (12) http://www.paragonair.com/public/docs/FAA-Handbooks/8083-25_AC61-23C_PHAK/_61-23C_Fig_06-18.jpg • (13) http://www.atlasaviation.com/AviationLibrary/wake%20turbulence/accov.jpg • (14) http://www.flightlearnings.com/backup/wp-content/uploads/2009/08/4-18.jpg • (15) http://www.free-online-private-pilot-ground-school.com/images/damped_undamped.gif • (16) http://www.free-online-private-pilot-ground-school.com/images/ground_effect_drag_lift.gif • (17) aviatorthings.com