The quadcopter has taken off as a toy, and as a remote sensing tool. It has extraordinary stability.
I want to see it scaled-up to full-size. There are several factors involved here – one is that the longer the blades the less power is needed to fly, so the blades should be as long as possible, with two not three blades per propeller. Ducted fans tend to be heavier so I don’t recommend them unless you want to try for sub-millimetre gaps between the blades and duct.
On seeing the quadcopter, the standard helicopter with one large and one small propeller on helicopters reminds me of penny-farthing bicycle, with a motor fitted, a poor design so far as stability is concerned. A two propeller helicopter is analogous to a bicycle or motorcycle. A quadcopter is analogous to a cart, car, or, with legs, quadruped.
In analogy with a quadruped’s gait, I would set up a full-size quadcopter with the same rotational rate for each propeller. That would allow overlapping of propeller tip circles, giving a full width approaching 1.5 propeller lengths instead of >> 2 propeller lengths for toy quadcopters, making landing easier and maximising propeller length. What of interference between propellers? I’ve seen numerical simulations of the airflow around a helicopter rotor and the disturbance moves downward very rapidly, if it didn’t then one blade on the propeller would interfere with the next. The frame holding these propellers could be square, but better slightly rectangular to give a preferred direction of motion (possibly even trapezoidal).
The overlap on the propeller tip circles ties in with the quadcopter analogy of quadruped “gait”. For example one gait would be to have the left front propeller blade pointing forward at the same time as the rear right points 45 degrees from that, front left 90 degrees and rear left 135 degrees, bringing us back to 180 degrees (full circle for a two-bladed propeller) for the left front.
The success of the quadcopter makes me think of the possibility of a quad-plane. Not a biplane with one wing above another but having two wings in front of the other two. This is sort of like using a canard wing but much larger and with attached engines. To envisage the stability of a conventional aeroplane think of a ruler whose centre is balanced on a finger, any slight deviation and it will go unstable. A quadplane would be like a ruler balanced on two fingers some distance apart, very much more stable. In addition, shorter wings would be required which would allow large planes to land at small airports. Just an idea. I wouldn’t tend to line the engines on the front wings up with those on the hind wings, instead I’d be more inclined to go for a cheetah-like analogy with the engines on the larger hind wings outboard of those on the front wings. But it may be OK to have front and rear wings the same size with the same engines in the same positions, like gazelles.
Any comments? Pictures?