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There are two different varieties of angels—the naked, long-distance flier and the feathered, short-distance glider (The Four-Winged Angel). But both varieties have the following anatomical features that makes flight possible:

  • The bones are hollow, with criss-crossing struts or trusses for structural strength.

  • The human lungs make up 7% of the overall body volume. The lungs of an angel take up 15%. Gas does not get mixed during the inhalation/exhalation cycle as is the case in other mammals.

  • Though leg length in the proportion to the rest of the body is still human, arm length in proportion to the rest of the body is more akin to a more arboreal, more acrobatic ape—the gibbon.

  • The rib cage is barrel-shaped, like a Neandertal.

  • The sternum, or breastbone, has been modified into a keel.

  • The flocculus, an area of the brain dedicated to concentration, makes up 2% of the brain’s overall mass.

The flier’s wings, true to the name, are formed from skin connected by two or three of the five fingers of each hand that have each been elongated to triple or even triple-and-a-half the length of the body. The remaining two to three fingers don’t have nails but claws.

The glider has all five of its fingers on both hands at normal size, but to make up for that, all four limbs have been modified into feathered wings. Jutting from the posterior is a short but broad fan of tail feathers for better steering.

Are there any other anatomical features I might have missed that could make either angel flight-capable? Would the flier be a biped or a quadruped?

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    $\begingroup$ What is the wingspan? It should be proportional to the individual's weight and displacement, right? Also, the breast muscles. They have to be powerful $\endgroup$ – Sphoorthy Nutulapati Apr 23 '16 at 15:48
  • $\begingroup$ Also, after checking out the link in the post, kinda makes sense that since there are two pairs of wings, each would necessitate a separate set of muscles (The glider). Muscles used for beating wings around might not really be suited for ground-based movement. Might mean that the glider is a quad? $\endgroup$ – Sphoorthy Nutulapati Apr 23 '16 at 16:00
  • $\begingroup$ @SphoorthyNutulapati Which is where the keel and the barreled chest come in. Also, Microraptor was a biped. The last question was for the flier. $\endgroup$ – JohnWDailey Apr 23 '16 at 16:21
  • $\begingroup$ @XandarTheZenon I was referring to wing type. $\endgroup$ – JohnWDailey Apr 24 '16 at 2:07
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The realistic design of any winged creature should include the following considerations:

  • Wingspan is at least twice the height/length of the body, but more likely triple, and no more than quadruple.
  • The current extant mega-birds max out at under 5 ft. long/tall, with a wingspan of about 3x that size (e.g. albatross, condors, thunderbirds), and never reach 35 lbs. Extinct creatures of the pterosaur type were - at the most - only about double that size/span, but perhaps up to 15x heavier. It is unclear if and how their flight was achieved.
  • The wing placement on the flying body is balanced, or slightly nose-heavy; i.e. the center of the overall wing area is placed at the center of the balance point on the body, or up to 15% lower/behind that (e.g. the wings may attach to the ribcage, but must sweep down/back beyond the pelvis).
  • The pectoral muscles are leveraged with an avian ribcage/sternum (you already got that).
  • The relative size of the pectoral and supporting "shoulder" muscles will be enormous.
  • The extremities have virtually no muscles (e.g. the outer wings and the lower legs are only bone and sinew).
  • The anatomy of body structures are lightweight (e.g. low body fat, bones less dense, limbs and necks are long and thin) despite the result of a fragile makeup.
  • The biology of the creature minimizes carrying extra weight (e.g. laying eggs rather than carrying a pregnancy, eating smaller meals more often rather than gorging on a large meal, retaining moisture to require less drinking water, etc.).
  • Wings with feathers (or other controlled louvered structures) create more lift with less surface area.
  • Arbitrarily adding an extra set of wings only adds extra surface area, unless the wings overlap in a louvered pattern.
  • If the body of the creature is shaped as an aerofoil (i.e. a wing-like configuration) it's backside can also count toward wing surface area (this will result in your creature appearing more bird-like or pterosaur-ish).
  • The requisite size of the wing (or leanness of the body) is governed by the ratio of gross body weight to total wing surface area. A minimum real-world ratio is 1 sq. ft. of wing per lb. of weight; a more believable ratio is 7 sq. ft. per lb. (see https://en.wikipedia.org/wiki/Wing_loading ).
  • With larger (i.e. human-sized) creatures, take-off is more likely to be achieved by the airfoil lift (air passing over the wings) than by the brute beating of wings against gravity. This lift is most easily generated by the airflow of a running start, or via a drop/dive (from a cliff, tree or building). Pterosaurs may have launched by running/jumping using all 4 limbs (addressing question #2).

Given the above confines of real world physics, in a fantasy/sci-fi setting the following additional options should be considered:

  • Net body weight may be reduced slightly by filling the hollow bones with a lighter-than-air gas.

  • Alterations to the world's gravity and atmosphere can make plausible flight more accessible.

  • Creatures might have the ability to control/redirect the forces of gravity.

  • Creatures could propel themselves via the ability to generate limited force-fields (or telekinesis).

  • Magic is always an option. (In explaining "flying dragons," magic is the best option.)

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  • $\begingroup$ If adding another pair of wings is arbitrary, then why did Microraptor have the extra pair? $\endgroup$ – JohnWDailey Apr 28 '16 at 13:54
  • $\begingroup$ @JohnWDailey: To increase wing surface area. (Arbitrary, meaning the choice of location.) The microraptor's extra wings on the hind appendages increased wing surface area, and also functioned as stabilizers (in aircraft design the Q2 is another example of this). But for sheer lift there can be a synergistic increase if the extra wings are positioned in a way to guide airflow over the main wings, as with the dragonfly (in aircraft design Fowler Flaps are used to increase lift at lower speeds during takeoff; they only retract/close to facilitate high speed flight after takeoff). $\endgroup$ – Everett Steed Apr 30 '16 at 4:50

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