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I do a lot of spelunking and I would like to know if anyone has some recommendation as to what type of drone would be the most useful in cave exploration with light and camera possibilities in total darkness.
I am only interested in drone usage where the cave chambers are quite spacious and not for usage in narrow passageways.
If it were me, there are two different kinds of drones I would want. A tiny whoop and a cinewhoop.
Tinywhoop is a loose definition of a very small drone that's about 3 inches wide with guards/ducts around its props and weighing anywhere from 20-50g. They are very durable and fly in places nothing else could. You can assemble them yourself (very easy) or buy one pre-assembled for around $100. You can add a small, powerful LED that allows some pretty fun exploration in dark and very tiny spaces without the risk of loosing a larger, more expensive piece of equipment. Example video of someone flying in their house with the lights out
A cinewhoop is a larger drone, more like 6-7 inches wide and weighing 200-400g, also with ducts. Typically it's called a cinewhoop because it's about the smallest thing you would want to fly that can comfortably carry a GoPro. You could equip this with a much more powerful light and have a real blast flying around. Like tavis mentioned the DJI FPV system would be an excellent setup for caves as it has great video quality and is very resistant to signal multipathing which is a very common affliction of analog video systems like what you would install on a tiny whoop. Multipathing is likely to be a very real issue inside a cave. If you could live with the analog break up though there are few 'starlight' type analog cameras that can operate in very dark environments. However, I don't think that would be nearly as enjoyable or as useful as a 1000+ lumen flashlight strapped to a DJI equipped cinewhoop. That would be awesome. Video here of DRL pilot NURK doing a better job of explaining what a cinewhoop is.
I would say your typical DJI consumer drone would not be well suited to a dark cave. Without GPS they would rely on downward-firing cameras for position hold which doesn't work in the dark. You could possibly rig up light to assist the cameras but I would guess that it still wouldn't work very well.
I would think one of the stable platforms with obstacle avoidance would work. DJI's newest drones have this feature although you would need to mount a very bright light on the drone for it to be useful in a dark cave. If you're a skilled pilot then I would recommend a very powerful mid-range platform and fit a very bright LED lamp for illumination. The DJI digital FPV system would also be essential to have if you're exploring a cave and examining the walls.
Flying in this sort of environment needs to be very carefully planned + controlled.
The radio frequencies typically used for drone control + video transmission are extremely poor at penetrating any thickness of rock.
Unless you're always able to keep to a very clear line of sight, I think you would be getting random fail-safes more often than not. Rock is also quite hard... likely a very unpleasant experience for a failsafe'd drone.
From what I know of the activity, it involves lots of squeezing yourself and your gear through small passages and there's often some water involved. Also, I would imagine, you wouldn't want to risk flying into walls (or people). So, if you need just a tool for exploring the walls and indentations of larger caverns, the best drone would be very compact in transportation, waterproof, stable and have a collision-avoidance system. A controllable camera gimbal is a big bonus for looking in different directions while hovering in place. That feature set suggests something in the Mavic line of DJI drones since they have all of it. They are compact, foldable, rugged, waterproof, stabilized, gimbal-equipped and collision-avoiding. If you want a recon drone, that would be it.
That said, while DJI drones can be flown indoors (and even have a special "tripod mode" that limits their forward speed), there seem to be some quirks with the position-stabilized modes there. Optical stabilization can fail in darker environments, the drone may wander around erratically for some seconds if it acquires some satellites through a window, for example. If you keep to the attitude mode, however, you should be fine.
All in all, if all you need is a camera platform/reconnaissance drone, get a Mavic (one of the smaller ones probably) and mount some powerful LED lights on it. Also, a good idea may be to get some prop guards with it despite the collision avoidance just to be extra safe. You don't want to lose that many dollars (or cut your buddy with a prop) in a cave.
Now, if you want to have fun with the drone, that's another deal entirely. Large caverns have been flown by freestyle and race pilots on several occasions with great success. Depending on the size of the cavern you expect to fly, you might want anything from a larger (75mm+) brushless micro quad to a 4-5 inch quadcopter for huge spaces. The most universal is probably something like an 85mm whoop. Prop guards are a good idea in any case, ducts are probably not if you're looking for performance. Conformal coating all electronics to protect them from moisture is a must. Still, be prepared for the possibility of losing the drone. Also, acrobatic drones are not very useful for looking over stuff in a hover due to the fixed camera uptilt.
A digital video system is good for rejecting multipath signals in enclosed spaces, but note that all current digital systems are way bulkier than analog ones, so I'd probably go with a good analog transmitter and Rapidfire or similar.
