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Comment by foxylad | Hacker News Reader

I'm concerned that the test uses a static wing. If you drop a fishing float directly in front of an obstacle in a river, it will be moved with the water around the obstacle, not hitting it.

Air is a lot less dense than water, but my instinct is that unless the drone hit a very narrow sweet spot it would simply be swept around the wing. And even in that sweet spot, there will be a buffer of compressed air that would substantially slow the impact.

Could someone with more knowledge of fluid dynamics comment on the applicability of static tests like this to actual aircraft damage?

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Does a baseball move out of the way of the swinging bat? There might be some small effect, but it's not the dominant one. In this scenario the wing is just a slightly bigger bat, and the drone is a slightly bigger ball. It would of course take a very (un)lucky strike to collide dead on like this, but not that unreasonable. I do hope they test indirect strikes too though.

I think you're probably right, but note that even for a powerful hitter, bat speed is ~80 mph. This is significantly less than 238 mph used in the video as a typical airplane speed.

Air movement is a rounding error in this test.

Suppose the curvature of the wing's airflow could in fact push the drone 15cm up or down in the time it takes the wing to advance 30cm forward (a tiny fraction of a second). That would require such a high G load exerted on the drone, it would disintegrate even if it didn't hit the airplane.

But of course, the air density and curvature is not nearly enough to affect a drone so dramatically. Mooney M20's wing generates only about 15 pounds of lift (down force) per square foot of the wing at cruise speed.

If instead of the drone we had a feather (small low density item) and the speed was 5 knots then yes, that might have worked.

I do not have a fluid dynamics background, but my hunch is that at the speeds involved, and the fact that there is a column of air from the cannon already surrounding the projectile, that this makes this simulation close enough.

Ah, good point about the air column.

Keep mind water is much more dense than air. Also air is easily compressed compared to liquids.

So the higher density of water means it can transfer more inertia onto objects. Moreover, it has higher drag than air.

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