Forces That Enable Flight

1. Forces That Enable Flight • Lift (upward force) • Thrust (forward force)

2. LIFT

3. Sources of Lift • Dynamic pressure above airfoil higher than below, thus static pressure below greater, pushes airfoil up • Some air deflected downward from lower surface, opposite (upward) reaction force is another source of lift

6. Airfoils • Wing • Bone at leading edge with feathers trailing creates convex shape • Forearm with secondaries attached is the main source of lift • Primary feathers • Asymmetric shape with stiff rachis leading and long barbs trailing creates convex shape

9. Thrust • Produced like propeller • Leading edge of wing cuts vertically, force produced is forward thrust • Large birds twist primary feathers as wing beats to produce thrust, forearm stays horizontal and continues to produce (increased) lift • Small birds create thrust with entire wing, plus additional thrust from primary feathers • Passive recovery upstroke

11. Flight Adaptations • Rigid wing and feather structure • Required to create air flow patterns that result in lift • Because wing form maintained, forces created result in lift rather than bending of body • Light weight • Weight counters lift

12. Application of Thrust • Momentum (mass x velocity) perpetuates thrust • Drag counters thrust • Profile drag (friction with air, the reason for streamlining) • Induced drag (turbulence)

14. Sources of Drag • Air flow over wing (induced) • Wing tips (induced) • Projections from body (profile) • Adaptations in forelimb eliminate sources of profile drag • Alula is the exception

16. Features used to maintain lift • Problem when wing is held at steep angle • Alula deflects air down over wing to maintain flow, prevent induced drag • Slots in wings perform same function, front primary directs air over primary behind • Air forced through slots expands on upper surface (reduced static pressure) to create more lift

18. Static Soaring • Use thermals, obstruction currents to attain altitude • Use gravity to generate speed, lift • Low wing loading (large wings relative to weight) promotes use of lift • Must be able to maneuver at slow speeds • Use alula, have slots in wings • Broad, rounded wings produce drag to reduce momentum

21. Soaring by Small Birds • Only possible at high speeds, because speed is the only source of momentum • Generate speed with thrust, use speed to generate lift • Low wing loading to promote use of lift • Long, narrow, pointed wings to reduce drag, maintain momentum

24. Dynamic Soaring • Use gradient in wind speed at surface ocean • Use wind, gravity to increase speed, gain momentum as glide downwind • Use momentum, wind to generate lift, gain height when turn into wind • Gain velocity as rise due to wind speed gradient, start cycle over again

25. Form for Dynamic Soaring • Dynamic soaring is based on perpetuating momentum • High wing loading increases momentum • Long, narrow, pointed wings to reduce drag, maintain momentum

28. Wing Form • Longer hand portion in species that rely more on thrust compared to soaring birds • High speed fliers have long, narrow, pointed wings (falcons) • Many birds have shorter, more rounded wings for maneuverability through cover

30. Wing Loading • Low in soaring birds (except dynamic soarers), high in diving birds • Higher in larger birds due to volume (weight) – area (wing size) relationship • Wing shape does not change with size • Migration selects for longer, narrower, more pointed wings

35. Tails • Used as brake and rudder • Longer tails generate more drag • Long, forked tail best for high-speed maneuvering, short tail best for high speed • Also maneuver by folding wings • Tails sometimes are display features

38. Landing • Slow down by braking with tail, feet (webbed especially) • Increase angle of wings, twist wings to reduce forward component of thrust • Turn into the wind, come in below perch • Some large birds have difficulty landing (too much momentum)

40. Take-off • Use gravity to generate speed for lift by jumping from perch or cliff • Use wind and speed to generate lift by running into wind • Dynamic soarers have biggest problem • Use thrust by rotating wings vertically • Short, broad wing = power take-off wing (with white muscle)

43. Hummingbirds: Avian Helicopters • Fly totally by thrust • Wing is mostly hand, little forearm • Rotate wing 180 degrees to produce force on both downstroke and upstroke • Figure 8 pattern of motion creates thrust perpendicular to plane of wingbeat • Can move in any direction • Can hover by producing upward thrust = weight