- Taking cheap shots about their measures of efficiency is hypocritical. TFA says that the researchers did investigate the efficiency and calculated a 25% gain. Presumably their calculations will be in their research paper. You on the other hand have just given a YouTube video and battery capacities as your 'proof'. Heck, the researchers went as far as constructing a device to test their efficiency claims.

- The stability might be an issue for completely manual flight (but then, people fly ordinary helicopters just fine...) but with computer-assisted balancing, the problem can be much reduced. Even existing quad copters often have computer-aided stability.

I investigated and I say this is wrong. No backup either. = The point.

Now then again, I actually fly every of these so I pretty much know what the efficiency is.

YES they gain 25% efficiency with the tricopter that has a huge prop in the middle, I TRUST that. But the test is meaningless:

If you change the 3 props instead and put big props, with corresponding motors. Guess what. Efficiency is probably higher than the 25% difference. I actually design my own. In fact, I also design my own control boards (the software that is)

Quadcopters use accelerometers, barometers, gyroscopes and magnetometers (and GPS) to ensure stability. Guess what. Measurement from these devices isn't 0ms. Actuating the motor control isn't 0ms.

On regular 1 rotor head heli it isn't either. In fact, it's longer.

Result? it's more stable with 4 props than one prop. It has more inherent stability., even if it wasn't "computer assisted". Timecop did a fully gyro stabilized quad copter recently that shows exactly that, if any "actual proof" was needed. Gyro stabilized can be made with no computer whatsoever (like helicopters are/used to be - that's why they're very, very hard to pilot, real ones and RC ones).

The physics makes sense. larger rotors are more efficient because they have lower disk loading. At really low loadings you can lift a human with their own power.

The sikorski prize was claimed a few months ago.

http://www.youtube.com/watch?v=syJq10EQkog

you can get a 600 or 700 class that can do "3D" flying as in the video for much less than 5k. The entry point is below $1k.

their configuration makes sense to get the best features of both. it's not solving all problems but it's an improvement over quads for some tasks.

the one they're showing in the TFA video is not 1K. The motor, higher servo, blades, etc don't come for free.

Not to mention simplicity. I suppose it wouldn't matter as much for commercially manufactured rigs, but for hobbyists this thing is a lot more complicated.

Agreed, though the one leg analogy would more accurately be hanging from one arm, since the lift is provided from above.

Ehhh... that sounds like an example of the rocket pendulum fallacy: http://en.wikipedia.org/wiki/Pendulum_rocket_fallacy

Maybe there is something more complicated going on with helicopters that I am not aware of, but a helicopter would not be an inherently stable system due to it "hanging" from the rotor because the direction the rotors are providing thrust changes (all other things being equal), when the helicopter starts to tip one direction.

Seems you're right, though I think it would depend on the rigidity of the rotor with respect to the body of the helicopter.