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Download Aerodynamic Principles I - University of Notre Dame
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Aerodynamics Getting to the Point Orville Wright Wilbur Wright Written for the Notre Dame Pilot Initiative By the Pilots of the University of Notre Dame “Teaching the Science, Inspiring the Art, Producing Aviation Candidates!” Four Forces of Flight Lift opposes Weight Thrust opposes Drag In straight, unaccelerated flight, L = W & T = D Lift created by pressure differential around wing. High pressure on lower surface and low pressure on the upper surface – low pressure caused by increased airflow velocity over top of airfoil. Weight – downward force of gravity Drag – rearward retarding force Thrust – forward force propelling airplane through air Airfoils What is NACA? National Advisory Committee for Aeronautics Chartered in 1915, operational from 1917-1958 The National Aeronautics and Space Act of 1958 created NASA from NACA Aerodynamic Surfaces Aerodynamic Surfaces Prop Jet B727 Spoilers Airfoils - Nomenclature Low p High p Chord line - straight line connecting the leading and trailing edges of an airfoil Camber line – locus of all points equidistant from top and bottom of airfoil Camber – distance between chord line and camber line Thickness – maximum distance between top and bottom surfaces of wing Leading Edge Trailing Edge Wingspan (b) Aspect Ratio (AR = b2/S) Frost If wing is below dewpoint which is below freezing, frost will form Sublimation of air to solid ice crystals Disrupts smooth airflow over the wing Why is this bad? Decreases lift Increases drag Frost removed before take-off Rime Ice Clear Ice Angle of Attack Angle between wing chord line and relative wind The angle of attack at which airplane stalls does not change Published NACA Data – NACA 2415 Airfoils - Nomenclature Flaps Plain Flap Flaps increase lift and decrease stall speed Flaps allow steep rate of descent for approaches without increasing airspeed Split Flap Fowler Flap -Fowler Flap effectively increases the wing area by rolling backwards on a roller system. Slotted Flap -Slotted Flap allows high pressure air underneath wing to join airflow above wing. This effectively increases velocity of top airflow and thus increases lift. Laminar v. Turbulent Laminar flow about a sphere Laminar v. Turbulent Turbulent flow about a sphere Bernoulli’s Principle - Lift Low Pressure High Pressure A1V1=A2V2 “As the velocity of a fluid increases, its internal pressure decreases.” From Newton’s 2nd (F=ma) Shown by Venturi tube Bernoulli’s Principle Again Courtesy of FAA: Pilot’s Handbook of Aeronautical Knowledge, AC 61-23B Bernoulli’s Principle Again Courtesy of FAA: Pilot’s Handbook of Aeronautical Knowledge, AC 61-23B Bernoulli’s Principle Again Courtesy of FAA: Pilot’s Handbook of Aeronautical Knowledge, AC 61-23B Lift Vector Courtesy of FAA: Pilot’s Handbook of Aeronautical Knowledge, AC 61-23B Drag Types Induced drag is the unavoidable by-product of lift and increases as the angle of attack increases Parasite drag is caused by any aircraft surface that deflects or interferes with smooth airflow around airplane Skin-friction drag - between the outer surfaces of the aircraft and the air through which it moves. Reduced by using glossy, flat finishes on surfaces Form drag - resistance of air to the shape of the aircraft. Form drag can be reduced by streamlining the aircraft shape. Drag – Body Comparison sphere cylinder airfoil Wingtip Vortices – “Twin Tornadoes” A few words on wingtip vortices: ‘High pressure on the lower surface creates a natural airflow that makes its way to the wingtip and curls upward around it to the area of low pressure. When flow around the wingtips streams out behind the airplane, a vortex is formed. These twisters represent an energy loss and are strong enough to flip airplanes that blunder into them.’ Wingtip Vortices Why Winglets? Equivalent to span extension w/o increased wingspan Reduces wingtip vortices Reduces drag NASA B-727 Wingtip Vortex Test Flight Learn more about winglets: http://www.airspacemag.com/ASM/Mag/Index/2001/AS/htww.html Drag – Ground Effect TIP: On a soft-field runway, you can takeoff at a lower speed and then accelerate while in “Ground Effect.” Drag vs Angle of Attack Relationship between drag and angle of attack Torque / P-factor (Left-Turning Tendencies) Newton’s 3rd law: “For every action there is an equal and opposite reaction.” Propeller rotates CW when viewed from pilot’s seat. Torque reaction rotates the airplane CCW about longitudinal axis P-factor (asymmetrical thrust) caused by descending blade taking a greater “bite” of air than ascending blade at high angle of attack Stability & Control Inherently stable airplane returns to its original condition after being disturbed. Requires less effort to control Center of Gravity concerns: Unable to compensate with elevator in pitch axis Weight and Balance becomes critical – taught in a coming lecture Stability & Control pitch The 3 axes of motion: roll, pitch, yaw roll yaw Tail Placements Looks like the A-10 Also called “H-Tail” Canards Stabilizer located in front of the main wings Used on the Wright Flyer More aerodynamically efficient than an elevator b/c canards provide positive lift Accident Report – Loss of Elevator AIRCRAFT FINAL REPORT THE AIRCRAFT HAD JUST BEEN REPAIRED AFTER RECEIVING TORNADO DAMAGE. THIS REPAIR INCLUDED REMOVAL AND REPLACEMENT OF THE ELEVATOR CONTROL TUBE. THE PILOT TAXIED TO THE RUNWAY FOR THE PURPOSE OF A TEST FLIGHT. ALL FLIGHT CONTROL CHECKS APPEARED NORMAL. AFTER LIFT-OFF, THE PILOT INTENDED TO LEVEL OFF AT 5 TO 10 FEET, THEN TOUCH DOWN AGAIN. HOWEVER, AFTER THE AIRPLANE BECAME AIRBORNE, HE LOST ELEVATOR CONTROL, AND THE AIRCRAFT CLIMBED STEEPLY TO 50 TO 75 FEET. THE PILOT THEN REDUCED POWER, THE AIRCRAFT'S NOSE DROPPED, AND THE AIRCRAFT DESCENDED. WITH NO ELEVATOR CONTROL, THE PILOT WAS UNABLE TO ARREST THE DESCENT, AND THE AIRCRAFT IMPACTED THE GROUND. A POST-CRASH EXAMINATION REVEALED THAT A BOLT AND NUT WERE MISSING FROM THE ELEVATOR CONTROL LINKAGE, WHICH ALLOWED THE LINKAGE TO BECOME DISCONNECTED. AIRCRAFT 1 CAUSE REPORT FAILURE OF MAINTENANCE PERSONNEL TO PROPERLY REINSTALL A BOLT AND NUT IN THE ELEVATOR CONTROL LINKAGE, WHICH RESULTED IN A DISCONNECT OF THE LINKAGE AND LOSS OF ELEVATOR CONTROL.
