echoesof.science · Ages 15–16
Flight: adaptations and the lift equation
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Birds evolved from maniraptoran theropod dinosaurs about 150 million years ago; the Jurassic fossil Archaeopteryx combined teeth and a long bony tail with modern-style flight feathers. Today there are over 10,000 identified species.
Flight is costly: birds are endothermic with a high metabolic rate. Air sacs connected to the lungs give a one-way flow of air through the lungs, and gas exchange works by a cross-current system.
Mass is reduced by pneumatic (hollow) bones strengthened by struts called trabeculae, and by a keratin beak instead of heavy jaws and teeth. The flight muscles attach to a deep keel on the sternum: the pectoralis major powers the downstroke, and the supracoracoideus the upstroke.
Experiment: Modelling a wing with the lift equation
- Make a cambered wing: fold a 10 cm × 20 cm sheet so that the top surface is curved and the bottom flat, and tape the trailing edge. Push a straw vertically through the wing near its front third.
- Hang a thread vertically from the clamp through the straw and fix the bottom end, so the wing can slide up and down the thread.
- Measure the mass of the wing and calculate its weight (W = mg, with g = 9.8 N/kg). Measure its area A in m².
- Hold the fan level with the wing at 60 cm, 40 cm and 20 cm. Record whether the wing rises and how far. Repeat each distance three times.
- When the wing just lifts off, lift ≈ weight. Rearrange L = ½ρv²ACL to estimate the product v²CL needed, using ρ = 1.229 kg/m³.
- Turn the wing upside down and repeat. Does it still lift? What does this tell you about the 'equal transit' idea?
| Fan distance (cm) | Rise, try 1 (cm) | Rise, try 2 (cm) | Rise, try 3 (cm) | Mean (cm) |
|---|---|---|---|---|
| 60 | ||||
| 40 | ||||
| 20 | ||||
| 20 (wing upside down) |
Questions
Describe three adaptations of the bird skeleton and muscles for flight and explain each.
Calculate the lift on a wing of area 0.030 m² at 12 m/s with CL = 1.2 at sea level (ρ = 1.229 kg/m³).
Explain why the 'equal transit' explanation of lift is wrong and give the better explanation.
Using your results, describe how lift changed with fan distance and give two reasons why your estimate of CL is uncertain.
Sources for every fact on this sheet: echoesof.science/school/birds/sources/