Flight (Part 2)

Continuing on the topic of flight... The physics of flight include 4 forces. 2 of these (in bold) are generated by the flying animal (in this case a bat) and 2 of them (drag and gravity) are natural properties that the animal must negotiate (fight) in order to fly. 1. Lift (i.e. upward force) 2. Thrust (i.e. forward force) 3. Drag (backward force (resistance)) 4. Gravity (i.e. downward force)

LIFT

The shape of wings causes air to move over the wing-surface in a unique way. Specifically, air flows faster over the upper curved surface of the wing and slower over the cupped (concave) surface. This causes negative pressure on the upper side of the wing and thus the wing begins to rise. In principle this is what happens when as a child you cupped your hand and played with the wind outside the open car window. Remember how the air would suddenly PUSH your hand upward? That is 'lift'! Additionally that is about the time you were reminded to not put your arms out the window.

Take home: the shape of wings (think of a bird or bat wing) is what causes air to move in a way that LIFTS the wing upward. The opposing force is gravity... without lift (like you or I flapping our arms, sorry to say but we are not going to generate sufficient lift) gravity 'wins' and we stay stuck to the ground. Gravity is complex for despite the fact that physicists can tell us its value (9.81 m/s2) how it 'works' and why it exerts force on objects was perplexing even to Einstein who wrote extensively on the topic. Incidentally, Einstein apparently also noted "Gravity cannot be held responsible for people falling in love." ______________ (I will insert an awkward pause here as you ponder that and why he might have felt it necessary to declare such a fact, if he made this statement in English or German and if it would have been funny regardless). Back to flight: The wings of bats are more flexible than birds because the wing itself is formed by a thin layer of skin rather than stiff feathers. However, birds are able to more easily change their wing area by moving their wings closer to their bodies because bats require the full extension of their 'fingers' i.e. the wing to have it function because of its thin nature. In searching for an analogy the best I have for you follows. This is similar in a way to how airplanes have rigid wings (like bird wings) while a para glider that uses a flexible surface (like a bats wings) is less able to change the shape of the 'wings' or they will loose lift.

THRUST

Basically thrust is the power from a wing-stroke that pushes back against air and results in a force propelling the animal FORWARD. Indeed to understand this idea you must remember that air is made up of molecules and is in actuality a fluid (like water). Thus there is indeed something to 'push upon'.

I will conclude for now and leave you with another Einstein quote as a 'token' physicist

"Science i nothing more than a refinement of everyday thinking".

I refer you to this much more comprehensive explanation of flight (here) and hope you find this topic interesting.

The evolution of flight (Part 1)

Many animals are able to fly and most of these are insects. However, when one stops to consider which vertebrates fly (vertebrates are animals with an internal skeleton including a spine and something resembling a spinal chord) only 3 main groups are able to fly.

These are: birds, Pterosaurs and bats.

The wing surface formed in 3 different ways one for each of these groups (see above).

Both bats and Pterosaurs used a wing-surface created by finger elongation (the 'pinkie' in Pterosaurs and all fingers elongated except the 'thumb' in bats).

Meanwhile, birds fly using a surface created by modified scales (feathers) and a strong series of arm and finger bones that are the result numerous bones fusing. Additional details are left out for brevity but are extremely interesting such as skeletal lightening in birds.

Pterosaurs are extinct and so only two extant flying vertebrate groups (extant = animals that are still alive today) are bats and birds. (What is a Pterosaur? Click here!)

Why did flight evolve?

There are two main thoughts about this. Basically either animals were able to flap and lift UP to escape predators etc. OR they were living in trees and started to glide from tree to tree (like 'flying' squirrels) also to catch food (insects) or avoid predators.

Regardless, the sky is (and was) full of potential food and once they were able to fly bats quickly filled a new niche (a niche is the ecological role of an animal generally relating to the food it eats). Both bats and birds were able to eat flying insects but bats as nocturnal animals compete with a fewer set of birds. Indeed this successful niche 'nocturnal insectivore' has since been radically expanded to include other more unusual food-types such as frogs or fish!

You can read more about the evolution of flight here.

Click here for cool videos of bats flying in slow motion.

For my next I will try to summarize the physics required for flight.

Can pregnant bats fly?

I happened upon the above question incidentally and thought perhaps it is a good one to address since I study reproductive bats and many people who do not study bats might have the same question!

(to the right is a photo of a Jamaican fruit-bat late in her pregnancy)

The simple answer is yes, bats fly while pregnant. That said it is very energetically costly for them and they become increasingly awkward as the pregnancy progresses. Indeed they may be more likely to be eaten by predators as they become slower and less-maneuverable.

Not only do they fly while pregnant, but bats give birth to pups that are much larger relative to the mom's size than human mothers give birth to! So, they are still flying when the developing baby is larger relative to the mother than a human would be walking around with.

After bats give birth (upside-down which is yet another difficulty) the mother will occasionally fly with her ever-growing baby (most bats only have one pup per pregnancy although some have twins and more rarely- triplets). The fortunate bats like Mexican free-tailed bats (Tadarida brasiliensis) or other cave dwelling bats usually leave their young in the roost while they forage but some less-fortunate bats may have to move their young nightly or semi-regularly if they inhabit less-permanent roosts.

So next time you hear a pregnant friend or family member saying how their back hurts or it is hard to get around you can tell them to cheer up and remind them they are lucky that they are not a pregnant bat!

what is a bat wing anyway?

The scientific name for bats CHIROPTERA provides an answer to this question of 'what is a bat wing'?

Bats have very long finger bones and webbing between each finger except the thumb! (Note a Jamaican fruit bat thumb compared to my own to the left). Other mammals that glide (bats are the only true fliers among mammals) have webbing but just attached to different body parts like from the elbow to knees of some 'flying' squirrels (flying is in quotes because they are technically just gliding i.e. falling gracefully!)

Together webbing and log fingers is what comprises a bat wing. I have tried to demonstrate this in the form of a drawing (below).