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+ BIOL 325 > FINAL: Flight > Flashcards

FINAL: Flight Flashcards

1
Q

For an animal that is in an un-accelerating, equilibrium glide in still air…

A

vector sum of the aerodynamic resultant and weight must equal zero

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2
Q

What happens when an animal begins to glide within < 1 wingspan of the ground?

A

induced drag decreases, reducing the total drag acting on the animal

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3
Q

What can you determine from an animal’s L/D ratio?

A
  • angle of descent
  • distance travelled to height lost
  • ratio of CL to Cd
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4
Q

What happens to glide angle when drag is increased?

A

increases (becomes steeper)

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5
Q

What does equilibrium glide mean?

A

resultant is vertical

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6
Q

What is the angle of attack?

A

angle between chord and Fdrag

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7
Q

What happens when a wing stalls?

A

drag Fd increases

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8
Q

How does the total drag acting on a flying animal change with airspeed?

A

decreases with airspeed to some intermediate low point before increasing again

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9
Q

What is induced drag produced by?

A

lift-producing surfaces of a flying animal

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10
Q

Although being quite different in their shape, both aerofoils and Flettner rotors produce lift. Explain briefly (in terms that Bernoulli would understand) what is similar about how these two structures produce lift when moving through air.

A

Flettner rotors and aerofoils are similar in that they both produce lift by increasing the velocity of air moving over their top surface relative to the bottom (or one-side relative to the other)

as Bernoulli states, if ½ρv2 increases, Pstat decreases – by increasing the velocity of the air
moving over their top surface, the pressure decreases relative to the air moving underneath the wing/Flettner rotor

net pressure difference between the top and bottom of the aerofoil/flettner rotor = lift

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11
Q

What is lift oriented based on?

A

incoming flow

  • if airfoil is operating at non-zero AoA, then lift will not be opposite to weight which will always be pointing down
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12
Q

When does lift occur?

A

only occurs if there is a ‘bound vortex’ or non-zero circulation or rotational flow

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13
Q

Why can’t a symmetrical airfoil with a non-zero AoA generate lift?

A

there is equal pressure distribution above and below the airfoil, generating zero lift

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14
Q

What is the equal transit theory?

A

flow above the airfoil “meets up” with flow below the airfoil

proven INCORRECT

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15
Q

How can thrust be increased?

A

by tilting the resultant aerodynamic force vector forward – this can be done by:

increasing lift vector
- increase velocity
- increase lift coefficient by increasing AoA
- increase camber, which also increases AoA

reducing drag vector
- decrease drag coefficient (ie. by streamlining airfoil or increasing Re up to a certain point)

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16
Q

What is the stall angle?

A

angle of attack when the lift begins to drop

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17
Q

How do you determine optimal AoA from glide polar?

A

occurs where a line from the origin meets the polar plot ‘curve’

AoA is producing the most lift (CL) for the least drag (Cd)

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18
Q

How can lift coefficient be changed to increase lift force?

A

increase angle of attack

lift coefficient value is normalized for density, velocity, and reference area

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19
Q

What happens to induced drag if you reduce the aspect ratio while maintaining the same wing area?

A

increases

reducing aspect ratio of wing means that a larger portion of the wing is subject to downwash from wingtip vortices – as a result, larger portion of the wing will be affected by a reduction in it’s AoA, therefore induced drag overall will increase

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20
Q

What is lift?

A

force generated perpendicular to fluid flow

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21
Q

What is drag?

A

force generated parallel and in the same direction of fluid flow

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22
Q

What is the Magnus effect? How can it be used to generate lift?

A

effect by which a rotating object (cylinder/sphere, etc.) can generate lift

  • rotating object placed in a fluid flow accelerates the fluid on the side of the object where the fluid’s direction of travel and the object’s direction of rotation coincide
  • fluid flow that encounters the opposite side of the rotating object is slowed down
  • Bernoulli states that high velocity air has lower pressure, while low velocity air has higher pressure
  • therefore the object’s rotation combined with a fluid flow creates a pressure difference on opposite sides of the object, producing a force at right angles to the direction of fluid flow = lift
23
Q

What is the critical angle of attack?

A

angle between chord of aerofoil and relative wind that produces maximum lift (and coefficient of lift)

24
Q

STALL: What happens to the magnitude of lift and drag once the critical angle of attack is exceeded?

A

boundary layer of air flowing across top surface of wing begins to separate – this is caused by air in boundary layer over top of wing being unable to flow to trailing edge due to increasingly unfavourable pressure gradient behind the wing generated by the increasing AoA

once boundary layer separates from surface of wing, turbulent wake is formed behind the wing – this dramatically reduces lift and increases pressure drag produced by the wing (STALL)

25
Q

How much weight could a wing potentially carry in level flight?

A

to fly straight and level, lift must balance weight

26
Q

How much thrust is needed to be produced to maintain a constant airspeed?

A

to maintain a constant forward speed while flying requires a forward thrust that is equal to drag

27
Q

Can a symmetrical aerofoil at 0° angle of attack (as below) generate lift?

A

NO

  • air flowing past the aerofoil will produce the same pressure distribution on both the top and bottom surface – pressure will be high at the leading edge, then low on either side of the aerofoil where air must accelerate around the curved surface, slowly rising towards the trailing edge
  • with no net pressure difference between the top and bottom of the aerofoil there is no net force generated perpendicular to the direction of flow, thus no lift
28
Q

What are the 4 types of soaring?

A
  • slope
  • thermal
  • sea-anchor
  • dynamic
29
Q

What is induced drag? Why does it occur?

A

induced drag occurs as a byproduct of lift production, which produces low pressure
above the wing and high pressure below

  • at the wingtip, the high pressure air flows up
    and around the wingtip to the low pressure region above the wing, producing wingtip
    vortices
  • these vortices produce an area of downward moving air directly behind the trailing edge of the wing
  • this downwash causes the relative wind to be pushed down at the trailing edge of the wing
  • this creates an effective relative wind that is at an angle halfway between the angle of wind in the downwash and the oncoming relative wind
  • since lift is generated perpendicular to the effective relative wind, it is tilted rearwards
    and directs more force rearwards
  • this backwards directed force is the induced drag
30
Q

What is parasite drag?

A

the combined drag caused by the skin friction, pressure, and interference drag produced by the non-lift producing surfaces of a flying animal or aircraft

31
Q

Why are some large birds required to run a long distance before take-off is achieved? For
example, a swan running across the water or an albatross running down a hill.

A

because of their high wing loading

  • wing loading is a measure of the amount of lift that needs to be produced per unit area of the wing in order to balance weight
  • weight of flying/gliding animal (mg) divided by total wing area
  • high wing loading means more lift must be produced per unit wing area, and this can be achieved by flying faster, thereby increasing the airspeed over the wing, and increasing lift
  • the higher the wing loading, the harder it will be for the animal to take off and fly at low speeds

ie. swans have high wing loading, which means they suffer from this problem – by running across the lake, they generate high airspeed over their wings, which is necessary to generate the large amount of lift they require to get airborne

ie. albatross have high wing loading and must run down hills into oncoming wind in order to generate a fast enough airspeed to fly

32
Q

What forms of energy are used in powered flight?

A

three energies needed to maintain lift:

  • kinetic energy
  • potential energy
  • metabolic/chemical energy
33
Q

What forms of energy are used in gliding?

A
  • kinetic energy
  • potential energy
34
Q

What is the difference in the descent angle in a gliding organism and a parachuting organism? Why does this difference in descent angle exist?

A
  • descent angle of gliding organism < 45º
  • descent angle of parachuting organism > 45º
  • this difference is due to the fact that gliders are able to generate lift as they descend through air, gliding forwards
  • parachutes produce maximum drag vertically, in opposite direction to their descent path, to drift slowly to the ground
35
Q

What is the difference between a thermal vortex and a thermal column?

A

thermal vortex: bubble of warm air that rises from the ground
- forms a discrete ring-shaped vortex (toroidal vortex), with air rising up through the middle of the ring, and descending down the outside

thermal column: continuous column of rising warm air

both often produce cumulus clouds at their apex

36
Q

What happens to the sink rate of a glider when it banks (turns)? How does this affect
which thermals (size/strength) a glider could use to gain altitude?

A

when a glider banks to turn in a circle, sink rate increases

  • as the turning radius of a glider decreases, its bank angle must increase, and so will its sink rate
  • if a glider wants to circle within a rising thermal to gain altitude, the thermal must be wide enough that the glider can remain within it by adopting an angle of bank that does not increase its sink rate above the updraft velocity of the thermal
  • to remain in a thermal that is weak (low updraft velocity) or very narrow might require the glider to turn using a bank angle that produces a sink speed that exceeds the updraft velocity of the thermal – if this is the case, the glider will not be able to use this thermal to gain altitude
37
Q

Where in a thermal is the updraft speed of the wind the greatest? What does this suggest
about where a glider should try to fly in order to gain altitude the fastest? What would
prevent a glider from doing this?

A

highest updraft speed is in the center of a thermal

  • flying in this air would produce the fastest increase in altitude
  • but to remain soaring within the most rapidly rising air would require the glider to fly in very tight circles (ie. circle in a narrow radius, with a high bank angle and, therefore a high sink speed)
  • this would likely offset any benefit that could be gained from the most rapidly rising air and the glider would descend in the thermal
38
Q

How do you determine maximum time a gliding organism can spend aloft from a glide polar?

A

height divided by minimum sink speed

39
Q

How do you determine the minimum airspeed of an organism before stalling on a glide polar?

A

left-most point of the glide polar curve is the minimum glide speed

remember that before stalling = before rising due to lift (before sink speed decreases)

40
Q

How do you determine maximum L/D ratio of an organism on a glide polar?

A

take line through origin tangent to the glide polar curve
- airspeed = L
- sinkspeed = D

41
Q

How do you determine the glide angle of an organism for maximum distance on a glide polar?

A

maximum distance can be achieved when flying at the airspeed that gives max L/D ratio (taken from line running from origin to tangent of glide polar curve)

42
Q

On a glide polar, which organism would stay aloft the longest?

A

organism that has the lowest sink speed

43
Q

If a vulture is flying at the airspeed to achieve minimum sink speed and hits a thermal rising at 1 m/s, what is it’s rising rate?

A

thermal rising (updraft) speed - minimum sink speed = net rising rate

44
Q

As a bird is landing, what is likely to happen specifically because and when the bird is within one wingspan’s distance from the ground?

A

ground effect

  • ground blocks the wing tip vortices which
    reduces induced drag
  • air gets compressed underneath the wing which increases ram pressure
  • birds also tend to drop their feet and tail posture changes which increases profile drag
45
Q

What is centripetal force?

A

horizontal component of lift

46
Q

How do you determine which can stay aloft the longest on a graph of sink speed (y) vs. turning radius (x)?

A

lowest sink speed can stay aloft the longest

47
Q

How do you determine wing loading (relatively) on a graph of sink speed (y) vs. turning radius (x)?

A

wing loading is directly related to turn radius

lower turn radius at every bank angle (and if they have the same lift coefficient) = lower wing loading

48
Q

What is the pitch angle?

A
49
Q

According to Taylor et al., flapping animals tend to converge between what values for Strouhal number when they are operating efficiently?

A

0.2-0.4

50
Q

What features would reduce pressure drag in a bird flying at low speeds and high angles of attack?

A
  • alula – prevent flow separation, which will decrease pressure drag
  • coverts – prevent flow separation, which will decrease pressure drag
51
Q

What features would reduce induced drag in a bird flying at low speeds with low angles of attack?

A

slotted primaries – block wing tip vortices from forming, reducing downwash over more of the wing and thereby reducing induced drag

this is mainly significant only in slow flying birds that might need lower aspect ratio wings like many birds of prey, but less effective for fast flying seabirds

52
Q

Based on Hovering Lecture Slides, what would you expect the scaling coefficient of lift force relative to mass to be?

A

M1

according to Skandalis et al., wing area (S) scales to M1

all other variabless in FL (or CL) equation scales to M0, therefore FL = M1

53
Q

What features do insects and vertebrates have in common?

A

they all feature a reinforced (slightly thicker) leading edge which can prevent excessive spanwise bending and likely excessive sweep to maintain wing posture

54
Q

How do insect wings generate lift at high AoAs?

A

by using a stable leading-edeg vortex attached to the upper surface of the wing

+ BIOL 325 (14 decks)

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