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Grid fins are used "commonly" in aerospace - usually for things like control of guided bombs or spacecraft. The most obvious example is SpaceX's use on the Falcon 9, but the US and Russian militaries have been using them for decades.

My understanding is that there's a tradeoff with grid fins. They offer greater control authority (and/or lift) in a more compact space, and require less torque to turn/reposition than the equivalent wing would. The downside is that they have a lot more drag, and are almost useless at trans-sonic speeds.

My question is: Why aren't they used as wings (to produce lift) on low-speed aircraft and RC drones? I was unable to find any examples of grid fins being used as wings or lifting surfaces. It seems like if it were possible to create more lift in less space someone would have used it in a design. Am I missing something obvious here? I'm guessing yes. :)

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  • $\begingroup$ Are you really asking to use grid fins as wings (i.e. to produce lift)? Or did you actually mean to ask about using grid fins as control surfaces? $\endgroup$
    – Bianfable
    Dec 13 '20 at 14:21
  • $\begingroup$ Yes, I mean using grid fins as the primary/only lift mechanism. I've updated the question accordingly. $\endgroup$
    – ZECTBynmo
    Dec 13 '20 at 14:25
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  • $\begingroup$ Haha yes, like that, although I imagine there's a lot of improvement possible with real airfoils, etc. Are there examples of extreme multiplanes like that in the modern era? $\endgroup$
    – ZECTBynmo
    Dec 13 '20 at 15:17
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    $\begingroup$ At low speeds, at very small overall width, a heli might outperform any fixed-wing arrangement. $\endgroup$ Dec 13 '20 at 16:11
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No, not as/on aircraft, and there are good reasons. They're useful elsewhere, and should make good kites.

Prototype multiplanes flew, quadruplanes reached production, triplanes served operationally in WWI, and biplanes were retired from almost all uses (military and civilian) by early WWII. The biplanes that still fly for purposes other than display are used for low-speed work.

As speeds increase - and we almost always want to get people and goods somewhere quickly - the power lost to drag increases dramatically, in proportion to the cube of the airspeed. Any construction that increases drag is going to limit speed, payload, range or probably all of them for a given engine power. Biplanes (and to an even greater extent multiplanes) are inefficient at lifting because of the interference between the airflow over the lower wing and under the upper - now squeeze the wings closer together so you can fit more, and this gets worse.

The surfaces (or struts/wires) perpendicular to the lifting surfaces, making up the grid fin or the bracing of a multiplane don't contribute lift but do contribute drag.

Grid fins are sometimes useful: they're compact, more so if the aspect ratio means they can be stowed as by SpaceX and on missiles/glide bombs. SpaceX use the grid fins in landing, where drag isn't a downside so long as it doesn't cause too much heating.

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  • $\begingroup$ Thanks, that's super helpful. Just for my own interest, what if we didn't care at all about moving quickly, meaning we could operate as slow as necessary to make the "wings" reasonable? Might things make a bit more sense in that context? $\endgroup$
    – ZECTBynmo
    Dec 14 '20 at 17:52
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    $\begingroup$ Yes, if the desired speed is arbitrarily low. But if you can't fly into a normal headwind you can't go anywhere, so you'd be looking at a kite rather than an aircraft. And the grid would likely be horizontal lift members with vertical bracing, rather than diagonals. Some of the wind energy kite prototypes approach that, but even they seem to be heading towards well-separated wings $\endgroup$
    – Chris H
    Dec 15 '20 at 6:48

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