Lorena A. Barba group

A successful student-led (flipped) class on avian wings

High-speed wing of a purple martin. Credit: NatureSkills © 2013 Wilderness Awareness School.

High-speed wing of a purple martin. Credit: NatureSkills © 2013 Wilderness Awareness School.

A sign that I still need to trust students to be able and willing to lead their own learning is that I had 70 slides prepared for this class (most of them pictures, of course). A sign that this was a successful student-led (flipped) class is that I only got to project one slide—a single slide with four questions.

For two and a half hours (the once-a-week class meeting of MAE 6291 "Bio-Aerial Locomotion"), the students worked diligently, collaboratively, noisily and effectively in front of their computers. They arrived at all the material that I could have "covered" in a traditional lecture on this topic and wrote a wiki-style article with what they learned. I'm sure that they learned a lot more than if I had lectured, and no one fell asleep!

Some background

"Bio-Aerial Locomotion" is a new course at the George Washington University School of Engineering and Applied Science, aimed at first-year graduate students and senior undergraduates. This version of the course grew out of a lower-level undergraduate class that I taught at Boston University as part of the Introduction to Engineering series, but expanded and strengthened by the latest research in animal flight. (The first time around, the BU class included lectures, which were recorded and shared on iTunes U)

These are the course aims, taken from the Syllabus:

This course aims to motivate the subject of bio-inspired engineering, characterized by seeking examples in the biological world of the desired function in the engineered creation. In particular, we seek examples of aerial locomotion by flapping flight, gliding, soaring and directed aerial descent. We aim to provide a working knowledge of the mechanics of animal flight and an overview of evolutionary questions.

A class on avian wings

To introduce the topic, in the previous class I showed this delightful animated video about bird flight titled "Staying in the Air," by Emma Lumley. I also prepared a TED-Ed lesson around this video, and encouraged the students to test their knowledge later with the embedded quiz.



In that class, I projected photographs of different bird wings, and challenged the students to guess what type of wing it was. Discussions and questions ensued. (The photos on my slides came from the Nature Skills web page on bird wings.)

Between this class and the next, I posted in Piazza a summary of the four wing types and I assigned the following paper for reading:

Savile distinguishes four main types of bird wings:

  1. Elliptical wings — are short in length and use high beat frequency, for rapid take-off, quick acceleration and turning; their shape tends to create a uniform pressure distribution over the wing; they are found on passerines and birds that live in habitats with dense vegetation;
  2. High-aspect-ratio wings — are long and narrow (some albatrosses can have aspect ratio as high as 18) with no slotting, for high-speed flight and dynamic soaring; found in soaring seabirds;
  3. High-speed wings — have moderate to high aspect ratio, low camber, slender tips and no slotting (sometimes swept back); found in open-habitat birds, long-distance migrants and birds that feed in flight (hummingbirds);
  4. Slotted high-lift wings — have moderate aspect ratio, deep camber and high slotting; found in soaring birds like hawks and owls.

The class activity

Having assigned the paper by Savile (1957) for reading, the next class I projected the following questions on a slide and made four groups, tasked to find the answers to these questions:

This was a flipped class: the assigned reading (and TED-Ed lesson) presented the content, and in class we assimilated that content through an interactive class activity. (It is important to note that the flipped classroom model does not require videos; this class relied principally on the assigned reading.)

The four groups of three students each had to find out all they could to answer their question, collaboratively, and write an answer in our Piazza class. A restricted, read-only and anonymized version of the Piazza class is available, where you can see the students' work (and our other online interactions).

The students uncovered all the fundamental mechanics at play, including: the mechanism by which lifting wings generate wing-tip vortices; the way to explain the drag due to tip vortices; the impact of a finite wing on lift-to-drag ratio and thus the ability to fly long distances; the reasons why a high-aspect-ratio wing has a higher lift-to-drag ratio and the implication for bird migration; the reason slotted wings allow flight at lower speeds without stall; and even the relationship between sweep-back and leading-edge-vortices in high-speed wings!

My role during the class was to walk around and sit for a few minutes with each group, asking questions and commenting on the material they were finding online. At the end, I guided a whole-class discussion where each group presented what they had found out, and the rest of the students (and myself) asked questions.

Comments on Savile (1957)

The paper by Savile has only 116 citations on Google Scholar right now, which surprised me because it is not only a very nice paper, but it seems to be the first in which the aerodynamic characteristics of bird wings are related to birds' habitat, behavior and their evolution. The basic classification of bird wings that Savile presents in this paper is used to this day, with little variation.

In conversations with Prof. Michael Habib (paleontologist at USC and frequent correspondent of  mine on these topics), he speculated that there was simply a lack of interest in bird flight in the 1950s. Mike and I entered our comments on the paper using an online annotation tool, and shared this with the students.

I noticed this very interesting part in the paper: on page 218, Savile is talking about the high-speed wing and he touches upon the aerodynamic reasons for various features: the low camber, short chord, elliptical tip, etc. Then he says:

"Only the pronounced sweepback is seemingly unexplained by orthodox aircraft aerodynamics ... I suspect that the answer may lie in some peculiarity of airflow over small wings that does not show up in airplane prototypes or large-scale models."

This is an amazing comment for 1957! It presages the find that swifts use leading-edge vortices to enhance lift, as reported by Videler, et al. in Science (2004). My students found this connection on their own during the class activity and landed on the Science paper, which made me very pleased.

Mike also noted that Savile's suggestion that the alula may act as a slot is questionable. Today, other studies have suggested that the alula may act as a vortex generator or a stall detection mechanism. And I note that his explanation of lift on the wing is flawed. Still, these are minor criticisms and this paper is still very good!