The Spacefaring Civilizations Are In Fact Advanced - Blog Posts

Yes, this does count as an aerospace question. And yes, their aerodynamics are quite bad. They would not fly very well with modern technology.

That "with modern technology" part is the key word, there. Because according to the lore, a lot of them can canonically fly very well.

Most canon speeds for 40k aircraft are 2000 kph or more, at about a mile above sea level. The speed of sound at that altitude is a bit over 1200 kph, for reference. Having 20,000+ km of range (eg they could fly from anywhere on earth to anywhere else on earth) isn't uncommon, either.

Many are also single-stage-to-orbit spacecraft (meaning that when they go to space, they take the entire spacecraft with them, and don't have boosters that they drop partway up like real world rockets), and all of those are reusable (meaning they can make multiple flights to and from orbit) and able to withstand reentry without any heat shielding beyond whatever armor they might have (and they are armored, with larger craft like the Thunderhawk being similar in durability to heavier tanks like Land Raiders).

There are a few conclusions you can draw from that.

Any of these aircraft would experience immense drag when flying (or when reentering the atmosphere). Not only does this mean that their airframes would need to be able to withstand the stress from that drag, they also need engines powerful enough to push them against it.

The combination of their range and single-stage-to-orbit capacity (and payload) means that they are powered by something far more energy-dense than any real world fuel.

I have a feeling that a space marine (talking about in armour here btw) might actualy be more earodynamic than any of the space marine planes

You know what? Yeah