What do you mean? An African or European swallow? I don't know!Well, there are 75 swallow species spread throughout the world. Of these species, about 37 reside in Africa, with only the West African swallow and the South African cliff swallow bearing the continent's name. On the other hand, the barn swallow is also referred to as the European swallow, so it's pretty clear where to look for data. So, which one are we going to use?Given that there have been significantly more studies on barn swallows, we're going to take a look at the European swallow. Wait, what is airspeed velocity anyway?It's “the relative velocity between some object,” or, for our purposes, between the swallow and the air. so, how do we find it?We use the Strouhal number, which can be calculated for birds as wingbeat frequency multiplied by stroke amplitude all over cruising speed. Stroke amplitude can be found using the formula bsin(theta/2), where b is wingspan, or the length of the wings, and theta is the stroke angle, or the angle between the highest and lowest wingtips. Cruising speed is the speed of the bird through the air, and therefore is the airspeed Velocity. So, to find the airspeed velocity of a barn swallow, we must know its Strouhal number, wingspan, stroke angle, and wingbeat frequency. in order to maintain airspeed velocity, a swallow needs to beat its wings 43 times every second, right?Wrong. In fact, according to a 2002 study done on barn swallows and house martins flying in free flight and wind tunnels “Compared with other species of similar size, the swallow has quite low wingbeat frequency and relatively long wings.” The results of the study found that the wingbeat frequencies of barn swallows range from 2.5 to 8.4 times every second, not 43. For a nice estimate, we can use the median numbers of 6.95 and 7.07 beats per second. What about the other variables?Well, the wingspans of the two aforementioned swallows are 0.318 meters for Swallow 1 and 0.328 meters for Swallow 2. At this point, as a non-professional, I turned to the work of someone who really knows what they’re talking about. Professor Graham Taylor of Oxford University found the stroke angles of the two birds to be about 95 and 90 degrees, respectively. This allows us to calculate a stroke amplitude of about 0.23 meters for both birds. The last number we need is the Strouhal number. Lucky for us, Taylor found Strouhal numbers of 0.18 and 0.19, respectively for the birds. All that is left to do is to find the average airspeed velocity by adding the two resulting cruising speeds (8.89 meters per second and 8.86 meter per second) and dividing them by two. So, what is the airspeed velocity of an unladen swallow?About 8.8 meters per second, or 20 miles per hour. Now, you may cross the Bridge of Death (and there was much rejoicing). curious? Here's some more info!
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