[AutumnBison (OP)]

In this image [nofollow] by jerica128, they created a hypothetical kind of winged humanoid called a "legnarian", and it was portrayed as having the following features to justify flight capability:

•     Longer necks, said to ease stress on the vertebrae and muscles when flying
•     Shorter torsos for larger core muscle stabilization
•        Longer hips for the same reason as #2

Impressive feats to have for a flying primate, but what about arboreal primates? In a fictional alternate Earth, would having those make them better climbers than they already are? And would they also be useful in primates with less conventional forms, niches or guilds, preferably comparable to those of carnivorans, pigs and even elephants?

[alancalverd]

Long necks have no aerodynamic advantage. A long tail, or a long neck furnished with vertical and horizontal stabilisers, reduces the force needed to maintain attitude and direction, but at the cost of increased weight and surface drag.

The length of the torso is unimportant. If you are going to fly by flapping, or even gliding, what matters is the quantity of muscle attached to the sternum, whci his why birds have a very deep sternum and humans can't fly by waving their arms.

Strong legs are used to propel man-powered flight. In various competitions it has proved easier to teach a racing cyclist to fly, than to train a glider pilot to generate 300 watts for more than a few seconds. But all these machines use an external propellor and gear system as no biological machine can have rotating parts.

You might consider rockets and pulse jets, as used by many aquatic animals, but you'd have a job to design a flying mammal that didn't look like a bat.

However if your fictional planet has a very dense atmosphere, you might find it already populated by whales and dolphins with gills instead of lungs.

[AutumnBison (OP)]

What about on non-flying primates?  Would those three features work?

[alancalverd]

The impressive characteristic of primates is their grasping hands and feet. Animals with long necks need them to feed, but primates carry the food to their mouths, so can wear a bigger head (hence bigger brain) with less musculature than swans or giraffes. 

Homo sapiens has proportionately longer legs than the other apes, that allow long-distance walking, striding over obstacles, and efficient bipedalism. But it comes at a cost - a huge amount of unconscious brain power  is involved in standing and walking upright.

A shorter trunk would compromise lung capacity or the ability of the digestive system to recycle water.

Gorillas and chimps have deeper skeletal ridges that provide anchors for more powerful upper body muscles: great for climbing trees and potentially amazing for playing wheelchair cricket or tennis, but they don't have the hips and brain capacity to play the able-bodied game.

Horses for courses! Design your environment, then optimise the species to exploit it.  Worked OK for God and Darwin!