I read a story the other day about pilots who were returning to flying after they’d taken time off for various reasons (raising kids, financial trouble, health issues, etc.). They were referred to in the article as “rusty pilots.” Aren’t we rusty pilots as well?
We devoted time to reading 4 blogs about bird flight, starting with “Designed to Fly” and ending with “Airborne.” Then we spent time away and took a short detour. Yours truly busied himself with writing an article about trash, setting up a new website, and tying up some loose ends. But now we’ve returned, we’re shaking off the rust, and we’re headed “back into flying.” But before we do, let’s review what we’ve learned.
Bird Flight Review
- What enables a bird to fly
air (no kidding)
the airfoil or (modified) teardrop shape of a bird’s body and wings
the smooth body surfaces created by feathers
the turning of oncoming air, especially around a bird’s wings
the flapping of wings–those upstrokes and downstrokes
- Important flight vocabulary
drag: the force opposing a bird’s motion through the air
lift: the force that keeps a bird aloft
gravity: the downward force that birds overcome
thrust: the force that propels a bird forward
airfoil: think: a sail, a wind turbine blade, an airplane wing, a bird’s wing!
A Closer Look at the Crow Photograph
I promised in “Flying Machines” that we’d take a closer look at our crow picture. Crows. A prominent bird expert, and my mentor, Wayne Petersen, has referred to them as “licorice in the sky.” Please read what he wrote about crows. It may encourage you to learn more about these fascinating birds.
The crow picture. This is the flight shot of a bird that looks more like it’s part of a hanging mobile. The crow doesn’t appear to be flying through space, as there’s so little negative space in the frame. Nonetheless, the photo does show us some things about birds and bird flight.
One thing that sticks out is the aerodynamic, airfoil shape of the bird’s body. Also note that the tail is closed and on the same axis as the body (reducing drag), unlike the tail in our eagle photo from “Airborne.” As you’ll recall, that eagle is braking to land on a branch. This explains why its “landing gear” is down. There’s no landing gear evident–and for good reason–in the crow shot above.
Primary and Secondary Bird Feathers
Another thing that stands out is the wing, which is at the top of the upstroke. You can almost anticipate the downstroke, the power stroke, which is coming next. Note that some of the flight feathers near the wingtip are open. Those flight feathers, among the set of 10 “primary” feathers on the bird’s outer wing, can be moved, twisted and turned. Here, those feathers are open to minimize drag. Sometimes, though, those feathers are open to help a bird maneuver, as in this Osprey photo.
The flight feathers closer to the bird’s body, and which can’t be moved, are the “secondary” flight feathers. Though this is counterintuitive, and our crow photo appears to indicate otherwise, these inner wing feathers and the inner wing itself move very little during flight. They are what are most responsible for generating lift.
The Angle of Attack of a Bird’s Wings
Here’s an aspect of bird flight we haven’t explored: the angle of attack or the angle of a bird’s wings in relation to the oncoming airflow. Birds fly with a low angle of attack, and that angle of attack has an effect on incoming air and on lift. Recall what I wrote in “Airborne”: “Lift is created when some of the air hitting the wings is deflected downward. That downward directed force creates an opposing upward force.” A low angle of attack allows that “downward directed force” to happen.
Again, think about an arm stuck out of a window of a fast-moving car. I mentioned this in “Airborne” as well. If the hand is held flat there will be some lift. If the hand is slowly rotated so the palm faces the air flow, lift will increase. (If it’s rotated further, of course, the hand and arm will snap backward–or go into a stall.)
Let’s trot out again our female Mallard duck. We’ve gotten a lot of mileage out of this photo. You can see that low angle of attack here.
Now, birds can adjust that angle. Look once again at the eagle photo. Notice the high angle of attack there. The eagle is adjusting its angle of attack so that it can go into a stall and land.
Adjusting the Angle of Attack
Here’s another adjustment example. These are photos of a Canada Goose stalling and landing in the Elizabeth River in Chesapeake. Notice how this goose adjusted its wings, and how the angle of attack changed as it landed. Who knew that birds use their wings, and not just their tails, to brake?
These photos made me think of the three-legged InSight spacecraft that reached the surface of the planet Mars recently, after going from 12,300 mph to zero in six minutes flat and using a parachute and braking engines to slow down. This isn’t exactly a “supersonic plunge” and landing here–but it’s close. And it’s interesting. Even though the eagle in “Airborne” never landed, it would have had to stop on a dime. This goose didn’t have to be anywhere near as precise.
And This Just In
My apologies. We never got around to exploring: “See a plane, thank a bird.” That’s likely because we spent so much time on “Maitre Corbeau” above. I expect that “see a plane, thank a bird” will be the theme, or one of the themes, in our next blog. Once again, please stay tuned.
Quip, Question, Quote
A brief exchange between 2 Boston birders:
Alice: “Our heated birdbath is frozen solid. The Blue Jays have taken to chipping away at the frozen water to get some moisture.”
Ida: “Water is more important than bird seed for the birds. You can get a birdbath heater for them. It works very well.”
Interesting idea that in winter, providing water for birds is more important than providing food.