BIOL 325 Thrust & Wing Loading

• Aerodynamic Resultant

• Net force exerted on the aerofoil by a passing fluid.

• Sum of drag and lift forces

Thrust

• Component of the aerodynamic resultant directed forwards

Aerodynamic resultant can be resolved into 2 components-

1. Thrust - force moving a propeller and whatever is attached to it forward

2. Force (torque) opposing blade motion (resistance that engine needs to overcome to turn propeller)

Pitch or blade angle

Angle between the cord of the propeller blade and the direction of rotation.

2 drag types acting on flying animals

1. Induced drag

2. Parasitic drag

Parasitic drag

• Also called form drag

• Drag due to animal’s shape

• Combination of pressure drag, friction drag across body surface, and interference drag

Interference drag

Drag caused by the mixing of airflow components between wing and the body

Induced drag

Any surface producing lift, will have it as it is drag due to production of lift

• Wingtip vortices

• At the wingtip, high pressure air beneath the wing can flow around the wing to the low pressure above

• Generated a vortex of air trailing behind each wingtip

Induced Drag

The wing and the wingtip vortices produce downwash behind the wing. The causes the effective AoA of the wing to decrease, causing lift to be directed downstream, which is the induced drag. It is induced by the downwash.

Drag (induced and parasitic) vs Airspeed

High airspeed, Induced drag decreases

High airspeed, parasitic drag increases

Aspect Ratio

Indication of how wide : how high

relationship of width : height

AR = span²/Planform area

dimensionless quantity 

low AR, short, stubby wings

high AR, long, large wings 

Aspect Ratio and Induced Drag

• low AR, short wings, suffer more form induced drag due to greater influence of downwash and vice versa. 

• Inversely proportional

• low AR, more Induced Drag & vice versa

AR values for birds and their needs 

• Low AR - 5.9: Rapid takeoff, not for extended flight (partridge)

• Medium AR - 6.5: good for hovering (hawk)

• High AR - 9.5: more lift than drag, less induced drag, good for gliding and soaring (gull) 

• More AR for birds that need to fly more. 

Wing Loading (def, unit, measure of what?, used for?)

• weight of the flying animal/machine divided by the total wing area 

• units - N/m²

• Measure of amt. of lift needs to be produced/ unit area of wing 

• used for quantitative comparison of lift a bird must produce per unit area of wing to support animal’s weight

• = m x g/ area

Low wing loading meaning 

high wing loading meaning 

• a bird (thrush) can take off easily and fly at lower speeds

• problem for take-off and landing, to get airborne and remain in air, needs to generate high amt. of lift, so, need high airspeed

Wing Area and scaling in terms of M^b (birds, hummingbird, bat)

• Birds - M^0.71 to M^0.78

• Hummingbirds - M¹

• Bats - M^0.49 to M^0.69 but small bats have larger wings relative to weight than birds

Wing Loading can be calculated with Wing area

• increases with increase in body weight for all birds and bats

• Wing loading = Weight/area → M¹/M^{0.71 to 0.79} = M^0.22 to M^0.29

• Large birds have high wing loading, need more lift, need more air speed, so more run up to take off (swan)