Is this humanoid dragon realistic the way I’ve imagined it?

Question

In this world I’m dabbling with, the dominant species is a race of six limbed humanoid with two wings on its back slightly below the shoulder blades. They are very slender (“BMI” is between 17 & 18) with the wings having evolved as a pair of secondary limbs. Finishing them off is a pair of tail planes evolved from the fibula. The wings have what looks like a second pair of Pectoralis major under and behind the usual ones. Their bones are hollow and filled with methane, they also have a methane bladder which they use both for buoyancy and to spew fire.

They are primarily predators and have limited flight ability. Powered flight is limited to a minority of the population who have what’s considered to be genetic defects with surface symptoms similar to asthma and narcolepsy. This “disease” affects around one in every 1,000 and having it will only slightly increase the chance of your child having it. The wings are bat-like and of the broad soaring variety (like eagles or vultures). The blood is amber coloured from the Coboglobin pumping through their veins.

The larger cities are built to encourage updraft for the inhabitants to fly on. The higher levels are more expensive to live in (for obvious reasons) and are generally built near large bodies of water as fish is their primary food source. The rooftops are commonly public spaces which you can glide from to catch one of the very popular steam trains. As powered flight isn’t an option, rail travel is very popular as aircrafts are still in their infancy with the general tech level being that of a 1920-30s Europe.

Is this humanoid dragon realistic the way I’ve imagined it?

Answer 1

I don't think so.

Birds have their wings attached to huge pectoralis major muscles which takes large percentages of their total bodyweight, which in turn are attached to the sternum and parts of the ribs. This sternum also protrudes extremely compared to our flat chests, allowing for better power transfer to the wings. The muscles of your wings, located below the shoulder blades, would need to be attached mostly on the linea alba (that line that has your navel resting in it and divides your abdomal muscles). Even ignoring the weight that these muscles must have to have the capacity to perform flight, they are going to be pressing extremely hard on the intestines, the descending aorta (no ribs to take the pressure) and likely force a slight exhalation each time the wings need to be pulled down. That's not going to be a fun experience and would likely cause all kinds of hell on your organs if you fly for more than a few minutes at a time.

Worse, these muscle locations will also pull on the area's above and below them. This causes problems with where you can attach them, as they would pull the torso down and the hips upwards, forcing a bend position with each wingbeat. So you would only be able to attach them to the other side's wing-muscles. This allows very little control on how much force you put in each wing, so you'd be able to turn only very slowly or need to stop flapping your wings to steer. Without bones to attach it to and no bones for this specific bone to rest on, it's likely that the muscles would just rip themselves apart unless the stomach is absolutely huge to accomodate it all. Imagine having extremely large breasts on your stomach, but made out of muscle and bone and attached to eachother. That's pretty much what your "humanoid" would look like. And we haven't even started on the back problems this would cause, and the extra-large muscles and counterweight you would need to walk straight up comfortably.

I would relocate them. Ignoring that it's unlikely that a humanoid would fly in current earth conditions, having the muscles of the arms and wings be shared would help. A kinematic chain would allow the muscles of your arm to help with the wingbeats, basically the arm would simply be an extra attachment for the pectoralis major and have another muslce of similar size attach to the wings. Not very efficient compared to a direct connection, but far better. Most of the pectoralis major would likely bypass the arm entirely, reducing armstrength but also reducing the amount of extra muscle you need to transfer the muscle energy.