Lecture 18

.title[
# Lecture 18
]
.subtitle[
## Gliding into D = m x g cos(theta)
]
.author[
### Dr. Christopher Kenaley
]
.institute[
### Boston College
]
.date[
### 2025/03/20
]

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<link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/font-awesome/5.14.0/css/all.min.css">

- How birds generate forward thrust

- Evolution of flight

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# Review of lift

Along a streamline PE + KE + W + **E dissipated** = constant

`$$(P_2-P_1)/\rho+(u_2^2-u_1^2)/2=0$$` 
<img src="img/bernlift.png" width="650" />
]

LIFT: component perpendicular to direction of motion
DRAG: component parallel to direction of motion

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# Review of lift

LIFT: component perpendicular to direction of motion
DRAG: component parallel to direction of motion

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# But how do animals do it?

They don't have props or engines.

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# But how do animals do it?

In gliding/soaring
<img src="img/liftdrag.jpg" width="300" />
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Powered flight, i.e., flapping

If the glide angle is large enough to
shift the net aerodynamic force
forward, there is a forward force
component.
]

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# But how do animals do it?

Powered flight, i.e., flapping

Helicopters have to pitch the rotors to produce forward thrust
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# But how do animals do it?

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Over flap cycle:

* lift overcomes weight
  * thrust overcomes drag

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# But how do animals do it?

]

Over flap cycle:

* lift overcomes weight
  * thrust overcomes drag

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# But how do animals do it?

Wing is folded on upstroke (amongst other things):

* Reduced wing area
  * Drive less air
  * Reduced *drag* and lift
  * Typically undulating flight path

]
  
.pull-right[      
<img src="img/howfly.jpg" width="500" />

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.center[      
<img src="https://cdn.birdwatchingdaily.com/2020/07/Songbird-flight-paths.jpg" width="400" />

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# Is flight efficient?

Why somewhere in the middle?

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# Evolution of bird flight

* Ostrom's Bug-net Theory

REJECTED

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# Evolution of bird flight

* Top-down (arboreal - began gliding from trees)
  * Bottom-up (cursorial - running take-off)
  * WAC (Wing-Assisted climbing) 
  * WAIR (Wing-Assisted Incline Running)

<img src="https://www.researchgate.net/profile/Sankar-Chatterjee/publication/278708131/figure/fig3/AS:267540194656302@1440797961634/Four-models-for-the-origin-of-avian-flight-a-Running-for-the-cursorial-model-a.png
" width="250" />
]

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# Evolution of bird flight

* Top-down (arboreal - began gliding from trees)
  * Bottom-up (cursorial - running take-off)
  * WAC (Wing-Assisted climbing) 
  * WAIR (Wing-Assisted Incline Running)

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# Evolution of bird flight

## WAIR (Wing-Assisted Incline Running)

<img src="img/wair.png" width="700" />
]

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# Evolution of bird flight

## WAIR (Wing-Assisted Incline Running)

<img src="img/wair2.png" width="600" />
]

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# Thanks!

Slides created via the R package [**xaringan**](https://github.com/yihui/xaringan).