# Aerodynamics Getting to the Point

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Aerodynamics Getting to the Point
-K 2, 9 / G 1, 12 / HW1 -K 5, 6 ? 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 What is the name of the “condition” that the air has to speed up over top of airfoil? Kutta Condition?

Airfoils What is NACA? National Advisory Committee for Aeronautics
Chartered in 1915, operational from The National Aeronautics and Space Act of 1958 created NASA from NACA

Aerodynamic Surfaces

Aerodynamic Surfaces Prop Jet B727 Spoilers
-Spoilers pop up hydraulically on passenger jets to disrupt flow over wing during landing…they slow down airplane -Top Gun Speed Brakes (Chap 26, 138:30, Chap 10, 31:27) How do speed brakes work? 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) -Chord is line from leading edge to trailing edge -Camber is line equidistant from top and bottom of airfoil at every given point along chord -symmetrical airfoil at 0 angle of attack yields 0 lift S = planform wing area

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 -Rime ice forms a brittle and frost-like surface. It develops when small drops, like those in stratiform clouds or in light drizzle, freeze on impact without spreading. It is rough and opaque. An example is like the frost in a home freezer. -Clear ice forms a hard and glossy surface. It develops when water droplets which touch the airplane flow across the surface before freezing. This ice can accumulate as a smooth sheet. Clear ice is likely to form in areas of large water droplets, such as in rain or cumuliform clouds.

Angle of Attack Angle between wing chord line and relative wind
The angle of attack at which airplane stalls does not change Stall speed does not change with altitude, loading, or airplane weight Larger angle of attack, more lift

Published NACA Data – NACA 2415
-Lift goes up with increasing angle of attack -Drag goes up with lift -Show stall angle

Airfoils - Nomenclature
Right most picture: Cambered airfoil:Zero lift angle is negative, i.e. at zero degrees angle of attack, airfoil is still producing lift For symmetric airfoil, zero lift at zero degree angle of attack

Flaps Flaps increase lift and decrease stall speed
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 allows high pressure air underneath wing to join airflow above wing. This effectively increases velocity of top airflow and thus increases lift. -Fowler Flap effectively increases the wing area by rolling backwards on a roller system. Graph on left basically shows that for the same angle of attack, a wing with flap extended produces more lift 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
A1V1 = A2V2 Continuity the mass flow into the control volume must equal the mass flow exiting the control volume “As the velocity of a fluid increases, its internal pressure decreases.” From Newton’s 2nd (F=ma) Shown by Venturi tube Low Pressure High Pressure A1V1=A2V2

Bernoulli’s Principle Again
Next 3 slides link together to help student understand why airfoils generate lift 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

Courtesy of FAA: Pilot’s Handbook of Aeronautical Knowledge, AC 61-23B
Lift Vector -This resultant vector plus the forward thrust of the propeller causes the airplane to go. 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. More lift, more induced drag Ground effect Parasite Drag hinges, etc. Skin-friction drag ice or frost on wings increases skin-friction drag

Drag – Body Comparison sphere cylinder airfoil
Form Drag, airfoil is streamlined and thus has less drag, frontal surface area of airfoil is smaller than both sphere and cylinder airfoil

-Draw a picture of birds on the chalkboard? 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

Drag – Ground Effect TIP: On a soft-field runway, you can takeoff at a lower speed and then accelerate while in “Ground Effect.” -Ground Effect is the result of the interference of the ground surface with the airflow patterns about the airplane -Vertical component of airflow around wing is restricted, which alters the wing’s upwash, downwash & wingtip vortices -Could explain complicated aerodynamics, induced flow, induced angle of attack, etc. -Wing requires a lower angle of attack in ground effect to produce same lift So, if same angle of attack is maintained, more lift is produced than usual -Within one wingspan above ground, best effect when within ½ wingspan above ground Soft Field Take-off

Drag vs Angle of Attack Why is minimum at 4 degrees?
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 -Torque effect is greatest at low airspeed, high angle of attack, high power -P-factor causes yawing to left. -Spiraling slipstream

Stability & Control Inherently stable airplane returns to its original condition after being disturbed. Requires less effort to control Airplanes normally pitch down when power is reduced, b/c downwash on elevators is decreased resulting in less effective control Bad Center of Gravity difficulty recovering from stall, less stable at all airspeeds 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

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 -Middle pictures is the Beech Starship Canards are unstable

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.