FLIGHT FINAL Flashcards
What are the two ways the coefficient of lift can be increased?
- increase AoA
- shape of wing
What is relative wind?
velocity and direction of airflow experienced by flying animal due to combined motion of their bodies through air and local wind direction
What two things does relative wind influence?
- velocity with which air flows past a flying animal’s wings
- angle of attack
What is profile drag?
pressure drag + friction drag
How is the total aerodynamic force determined?
determined by integrating the pressure acting across the entire wing surface
What are the two components of aerodynamic force?
- lift component (perpendicular to relative wind)
- drag component (parallel to relative wind)
What is the resultant?
sum of lift and drag components
What is thrust?
force moving propeller forward (parallel to oncoming fluid flow)
component of aerodynamic resultant directed forwards
What is force (torque) opposing blade motion?
resistance that engine needs to overcome to turn propeller (perpendicular to oncoming fluid flow)
What is pitch angle?
angle between longitudinal axis (where airfoil is pointed) and horizon
angle between chord of propeller blade and direction of rotation
How is pitch angle related to AoA?
changes to maintain AoA to relative wind
some pitch is selected to produce positive AoA to produce favourable L/D ratio and generate thrust
What is parasitic drag?
drag due to animal’s shape – combination of pressure drag, friction drag across body surface, and interference drag
What is interference drag?
drag caused by mixing of airflow components between wing and body
What is induced drag?
drag due to lift production
What is a ‘fictitious’ bound vortex equivalent to?
circulation around wing due to higher air speed over than under aerofoil
How is induced drag produced?
- downwash behind training edge of wing
- downwash causes relative wind to be pushed down at the trailing edge – magnitude of downwash and downward deflection is most pronounced close to wingtip, but occurs behind entire wing too
- effect relative wind acting on aerofoil now has slight downward deflection at an angle between angle of wind in downwash and oncoming relative wind
- lift remains perpendicular to effective relative wind, therefore it is tilted rearwards
- this backwards-tilted effective lift now directs more force rearwards – INDUCED DRAG
SUMMARY:
- wing and wingtip vortices produce downwash behind wing
- causes effective AoA of wing to decrease
- causes lift to be directed downstream = induced drag
At low flight velocities, is there low or high induced drag?
high
- at low airspeed, high CL produced by high AoA is needed to generate sufficient lift for flight
- downwash angle is increase compared to relative wind, tilting lift vector back towards trailing edge – high induced drag
At high flight velocities, is there low or high induced drag?
low
- low AoA is sufficient for lift
Is parasite drag low or high at low airspeed?
relatively low
Is parasite drag low or high at high airspeed?
relatively high – increases with v^2
Do wings with a low aspect ratio suffer less or more from induced drag?
more – due to the greater influence of the downwash behind the trailing edges of the wing
What are wings with low aspect ratio good for? Not good for?
- short wings good for rapid takeoff
- not good for extended flight
What are wings with medium aspect ratio good for?
- good for hovering
- slotted primary feathers reduce vorticity at wing tips
What are wings with high aspect ratio good for?
- increased lift relative to drag
- reduced induced drag
- gliding and soaring
What does low wing loading mean?
- can takeoff easily
- fly at lower speeds
What does high wing loading mean?
- problem at takeoff and landing
- to get airborne and remain in air, wings need to generate high amount of lift – need high airspeed
Wing area in birds scales with body mass between what values?
M0.71 to M0.78
- higher than predicted, but still not sufficient to prevent wing loading from increases
Wing area in hummingbirds scales with body mass with what value?
M1
Wing area in bats scales with body mass between what values?
M0.49 to M0.69
- but small bats have larger wings relative to weight than birds
Bird wing loading scales with body mass between what values?
M0.22 to M0.29
What are the 3 energies for flight?
- potential energy (proportional to height)
- kinetic energy (proportional to airspeed squared)
- metabolic energy (can be used to product thrust by flapping)
What are the 2 energies for gliding?
- potential energy (proportional to height)
- kinetic energy (proportional to airspeed squared)
What do gliders do with their potential energy?
produce maximum lift
convert potential energy into aerodynamic work – trade height for forward motion and lift
What is the descent angle of gliders?
< 45º
What do parachutes do?
produce maximum drag
- as they descend through air, relative wind comes from below, and drag is produced in same direction as oncoming wind (ie. upwards)
What is the descent angle of parachuters?
> 45º
What does a downward glide path do?
- converts potential energy (height) to kinetic energy (velocity)
- changes direction of relative wind
- changes AoA
What is glide angle determined by?
due to L/D ratio (their AoA)
- high L/D ratio = smaller glide angle (shallower)
- low L/D ratio = larger glide angle (steeper)
What is the net force on a glider in a non-accelerating equilibrium glide?
0
What does L/D ratio tell us in terms of distance travelled?
L = distance travelled
overq
D = height lost
What affects the speed of a glide?
- AoA (sets L/D ratio, which sets glide angle)
- weight (heavier travels faster along same path as lighter)
How can a glider alter its airspeed vs. sinkspeeed?
by varying
- AoA
- wing camber (curvature of wing)
- wingspan
What is minimum airspeed? How do you determine minimum airspeed before stalling on glide polar?
slowest speed that can still produce lift without stalling the aerofoil
point at which airspeed is lowest
What does minimum sink speed tell you? How do you determine minimum sink speed on glide polar?
tells you airspeed you should use to maximize time spent in air
point at which sink speed is lowest
What does maximum distance speed tell you? How do you determine maximum distance speed on glide polar?
tells you airspeed to use to maximize distance travelled in air (max L/D ratio)
line from origin that touches tangent to curve
What are airbrakes and how do they work?
increase rate of descent
- increase total drag without increasing lift (as increasing AoA would)
- to maintain airspeed, glider tilts forward, increasing glide angle
- can continuously vary the glide ratio by varying brake deployment
What are birds’ version of airbrakes?
- stick out body parts, which produces controllable amount of extra drag that steepens glide angle and increasing rate of descent
What does the ground effect do?
- increases lift when flying animal is within < 1 wingspan distance of the ground
- wingtip vortices partially blocked by ground, reducing downwash and reducing induced drag
- closer to the ground, air is compressed between wing and ground (ram pressure), which increases lift
gliding is easier, landing is harder
How does a steeper glide angle increase distance travelled?
by reducing the time spent in air that is moving in a direction opposite to the desired direction of travel
How does a shallower glide angle increase distance travelled?
- (tailwind) by increasing time spent in air that is moving the same direction to the desired direction of travel
- (rising air) by increasing time spent in air that is moving upwards and offsetting sinking speed
If the goal is to gain altitude as quickly as possible, what sink speed will always produce the fastest altitude gains.
minimum sink speed
What is soaring?
when local wind comes from below the animal (updraft), then speed of updraft will offset the sink rate of the glider
What are the 4 types of soaring?
- declivity or slope soaring
- thermal soaring
- sea-anchor soaring
- dynamic soaring
What is slope soaring?
gliding in sufficiently fast updraft does not require loss of altitude
- if vertical component of wind is greater than glider’s sink speed, it can gain altitude
ie. hillside forces wind to move upwards
What is the structure of a thermal vortex?
- air rises in centre
- air sinks outside centre
What is thermal soaring?
- glider circles in thermal in order to remain in rising air
- altitude gained within thermal is used to travel cross-country to a new thermal
What is a thermal column?
continuous plume of rising air – formed by essentially same processes as thermal vortex, except rising hot air column does not break into discrete bubbles
What is a thermal column?
continuous plume of rising air – formed by essentially same processes as thermal vortex, except rising hot air column does not break into discrete bubbles
What is a thermal column?
continuous plume of rising air – formed by essentially same processes as thermal vortex, except rising hot air column does not break into discrete bubbles
What must a bird do to remain within a rising thermal?
needs to fly in a circle by banking its wings at some angle to the horizontal
- increasing angle of bank to turn within thermal increases rate of descent
- increasing angle of bank to turn in a smaller radius reduces vertical component of lift
What is the angle of bank?
angle between lift force and vertical component of lift force (which balances weight)
What does bank angle do to sink speed?
any angle > 0º increases sink speed compared to level flight
as angle increases, rate of sink increases
How low/high does turn radius and sink speed need to be to remain circling within a small thermal?
- small turn radius
- high sink speed
How is turn radius related to wing loading?
lower wing loading = smaller turn radius
What is sea-anchor soaring?
drag feet through water, resisting the aerodynamic drag induced by the wind moving past its body
when sea-anchor drag = wing drag, bird is suspended motionless at a height where it can pick food from the surface of the water
What is dynamic soaring?
makes use of the velocity gradient of the wind above the ocean (also known as the shear wind field)
wind velocity increases rapidly with increasing height above ocean
What do winglets allow?
allow the low aspect ratio wings of the vulture to reduce induced drag, effectively making them function more like the long thin high aspect ratio wings of specialized soaring seabirds
What is thrust in flapping flight?
force that flying animals must produce (by flapping) to overcome drag and move forward through air
What is the total drag opposing thrust equal to?
induced drag + parasitic/form drag
How is thrust produced?
by imparting momentum to the air – accelerating air backwards to accelerate yourself forwards
Flapping Flight: Downstroke vs. Upstroke
- downstroke: tip of wing twists, angling the aerofoil shape of the distal part of the wing downward
- upstroke: wingtip twists, angling distal wing downward, thereby directing lift vector forwards as thrust
- proximal part of wing continues to produce lift during downstroke and upstroke
How does parasitic drag change with airspeed?
increases with airspeed
How does induced drag change with airspeed?
decreases with airspeed
At what airspeed is total drag a minimum?
at some intermediate airspeed
- as thrust must equal drag for a constant airspeed, optimum flight speed requiring the least thrust also occurs at this intermediate airspeed
What does the Strouhal number tell us?
relates to vortex shedding from heaving (flapping) aerofoils/hydrofoils
At what Strouhal number is the highest propulsive effiencies?
0.2-0.4
if efficiency is not a primary concern, St number can be way higher
Environment energy for flapping flight:
upwash generated by wingtip vortices are a potential source of upward moving air to be used by following birds (ie. formation flying)
intensity of vortex increases during power stroke
What do slotted primaries (as winglets) do?
makes low aspect ratio wing function like high aspect ratio wing
- reduce pressure gradient between top/bottom of wing at wingtip – REDUCES DOWNWASH AND INDUCED DRAG
What does the alula (protrusion on leading edge of wing) do?
allow bird wings to operate at lower airspeeds and higher AoA necessary to generate sufficient lift at these lower airspeeds
- at high speed, there is low AoA and therefore no risk of stall
- at normal low speed, there is high AoA and therefore wing will stall
What do covert feathers (cover top surface of wing) do?
function as ‘eddy-flaps’ that prevents sudden lost of lift that accompanies stall by preventing turbulent airflow at trailing edge of wing from progressing up and stalling the wing
- good for slow flight at high AoA
What is hovering flight?
ability to remain airborne in a fluid with zero or negligible mean relative velocity between the body and the fluid
ability to stay up in the air without moving relative to the surrounding air
How does hummingbird wing loading scale with body mass?
M0 – independent
How do insects generate lift?
bound leading edge vortex appears to prevent the wing from stalling at increasing angles of attack – functions to push oncoming air onto the upper surface of the wing
