Brainstorming and Barnstorming: Basics of Flight

Transcription

1 Brainstorming and Barnstorming: Basics of Flight

2 Flight History First flight: The Wright Flyer 1903 Break Speed of Sound: Bell X-1A 1947 Land on Moon: Apollo Circumnavigate Earth on one tank of gas: Global Flyer 2005 We ve come a long way

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4 Major Topics Terminology and Theory Forces of Flight Aircraft Design

5 Basic Aircraft Terminology Airfoil: Cross sectional shape of a wing Leading Edge: Front edge of wing Trailing Edge: Back edge of wing Chord Line: Line connecting LE to TE Camber: Center line between top and bottom of wing High camber found on slow flying high lift aircraft

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7 Wing Layout Planform: Vertical projection of wing area Elliptical: good for high speed Straight: root stalls, but cheap to make Tapered: good stall characteristics Delta: used for supersonic flight

8 Wing Layout Sweep: Angle between the lateral axis and the wing (high speed aircraft) Taper: Chord decreases as you move to the wing tip Incidence: Angle between the longitudinal axis and the wing chord Angle of Attack: Angle between the wing and the relative wind

9 Wing Layout Twist: Bending of wing about lateral axis (helps prevent tip stall by changing angle of attack) Anhedral: Downward bend in wing (helps with stability) Dihedral: Upward bend in wing Corsair: WWII Fighter

10 Wing Layout Aspect ratio (AR)= Span^2/Wing Area More efficient for slow aircraft Typical Values Glider: Trainer: 7-9 Loadstar: 18.5 U2 spy plane: High AR SR-71: Low AR

11 6 degrees of freedom Three axes of an aircraft Longitudinal: Parallel to the fuselage Lateral: Parallel to the wing Normal: Perpendicular to the ground

12 Control Surfaces: Change Wing by altering the Angle of Attack Ailerons: horizontal surfaces located on wing tips Roll: rotation about the longitudinal axis Elevator: horizontal surface located on the tail Pitch: rotation about the lateral axis Rudder: vertical surface located on the tail Yaw: rotation about the normal axis

13 Stabilizing Surfaces: Balancing Moments Vertical Stabilizer: The vertical part of the tail which prevents unwanted yaw Horizontal Stabilizer: Horizontal portion of the tail (or the Canard) that prevents unwanted pitch

14 Flaps Change the shape of wing Increase Lift and Drag Used on takeoff and landing

15 Neutral Point: Location of resultant lift force CG: Center of gravity High Wing: Wing on top (very stable) Mid Wing: Wing in middle (acrobatic) Low Wing: Wing on bottom ( less drag)

16 Reynolds Number Reynolds Number (Re): ratio of inertial forces to viscous forces Re = (D*V*p)/mu D=characteristic length V=velocity p=density Mu= dynamic (absolute) viscosity A non-dimensionalized number that can be used to relate models to actual aircraft Determines whether a flow is laminar or turbulent in the Boundary Layer (laminar is good) Very useful for aircraft design

17 Reynolds Number S1223 at various Reynolds numbers Cl Angle of Attack (degrees) Re=61000 Re= Re= Re= Re= Re= Re= Re= Re= Re= Note the difference in stall characteristics for different Re

18 Boundary Layer No slip condition at surface (V=0) Effectively alters the shape of the airfoil Separation of the B.L. results in a stall Lead to major advances in aircraft design

19 Boundary Layer

20 Forces of Flight Lift Drag Thrust Weight For steady, level flight these four forces and the moments they generate must be in equilibrium. An airplane is a force and moment balancing machine.

21 Lift Controlled by Airspeed, angle of attack, altering airfoil, and altering the planform area Lift = ½ * p * V^2 * A *Cl P=density, V=velocity, A = wing area Cl=coefficient of lift How is lift actually generated???

22 Lift: Equal Transit Time (Wrong) Air splitting at LE must meet at TE Air on top has a longer path; must travel faster Example: Boeing 747 Weight: 775,000 lbs Airspeed: 550 mph or 810 ft/sec Distance across top: 1.059*bottom Density: 1/39 lb/ft^3 Wing Area: 5,500 ft^2

23 Boeing 747 Example Pressure difference: Punder-Pover=1/2*p*(Vbottom^2-Vtop^2) Punder-Pover=18.75 lbs/ft^2 Lift=P*A=(18.75 lbs/ft^2) * (5500 ft^2) Lift=103,000 lbs Weight=775,000 lbs Ooops!!! This theory says that air accelerates thereby causing a pressure gradient. This is completely wrong. A pressure gradient will cause a fluid to accelerates.

24 Einstein and Lift Einstein hired by the German Air Force He designed a wing based on the previously described theory It failed miserably He was still relatively successful

25 Lift is complicated!!!!!!!! Newton Vs Bernoulli Newton: deflection of air Bernoulli: Pressure gradient Coanda effect Circulation 3-D fluid flow is hard Pressure Gradient

26 Newton and Bernoulli A wing forces air down Thus air forces a wing up A change in the momentum of the fluid results in a force Air in motion creates a pressure difference around the wing Air being forced down

27 Coanda Effect Tendency of a fluid in motion to stick to an object Due to skin friction between fluid and surface The top of the wing also directs air down Experiment with a rolled up paper.

28 3-D effects of lift Spanwise flow High pressure on bottom Low pressure on top Air from bottom tries to move to top Wing Tip Vortex

29 Return to the lift equation Lift = ½ * p * V^2 * A * Cl Lift can be explained by the pressure gradient as indicated by the equation The gradient cannot solely be explained by air moving faster over the top of the wing What about this Cl factor????

30 Coefficient of Lift Magic number of lift; determined experimentally Constant for any size wing with same airfoil Accounts for unknowns Varies with angle of attack There is an angle where the wing produces zero lift Explains how a wing can fly upside down

31 Loss of Lift: Stall Every wing has a stall angle Stall angle is the angle of attack at which the wing loses lift Stall angle range from degrees What actually causes a stall???

32 Stall at high AoA Boundary layer separates from the surface (inertial vs viscous effects) Effectively changes wing shape Turbulence results that causes more drag and less lift

33 Drag: Form Drag: shape of object Skin Friction Drag: surface of object Induced Drag: component of lift Parasitic Drag = Form Drag + Skin Drag Total Drag = Induced Drag + Parasitic Drag Total Drag = ½ * p * V^2 * A * Cd Cd is the key and is determined experimentally just like Cl.

34 Form and Skin Friction Drag Form Drag Greatly affects slow flying planes Depends upon the frontal area Depends upon how streamlined What does it mean to be streamlined?? Examples of things that are streamlined Skin Friction Depends upon the surface roughness

35 Form Drag How do we know if an object is streamlined? Nature, wind tunnel testing, conformal mapping If these shapes are so aerodynamic, why aren t race cars shaped this way????

36 Induced Drag Equal to horizontal component of lift Therefore increases with AoA Actually caused by the wing tip vortex discussed earlier Reduced with use of a high AR wing Can be reduced with the use winglets

37 Tradeoff: Skin Friction vs Form Turbulators: prevent the B.L. from separating Increases skin friction Decreases form drag For slow aircraft; tradeoff is beneficial Found on sea animals, new swim suits, and golf balls

38 Turbulator Examples

39 Aircraft Stability Static Stability: When disturbed, the aircraft returns to original flight path Longitudinal, Lateral, Roll Dynamic Stability: Returns to original flight path without excessive oscillation

40 Longitudinal Stability Longitudinal Stability: Locate the Neutral Point behind CG Creates a correcting moment To move the Neutral Point backwards, increase the horizontal tail area

41 Lateral Stability Largely depends upon tail size CLA: Center of lateral area Size tail to locate the CLA 25-28% of tail length behind the CG Prevents Spiral Instability Side gust rotates plane One wing speeds up Creates more lift

42 Directional Stability Also depends upon tail size and CLA A high wing adds stability The plain acts like a pendulum Naturally returns to stable position

43 Aircraft Control Longitudinal, Lateral, and Directional Control surfaces generate forces These forces create moments that rotate the plane Proper location and sizing results in excellent control Stall must always be considered Ailerons are located at the wing tips

44 KSU Aero Design Team 2005 Ft. Worth, Texas 3 rd Place