Xperimental Quad.. (this was tested out in 2012/13)

Knowing larger props and lower rpms are more efficient in generating lift and use less current. Here was  test setup where the major proportion of the lift is provided by efficient large diameter, low rpm props, and the stability and controls are through smaller dia, high rpm pros... the plan was to test it out initially with all electric motors .. and subsequently have the lift props be IC driven.
Concentric contra-rotating props would have been ideal.. however, to fast-forward experiment two contra-rotating props suitability placed in a symmetrical manner was used.

The motor setup was
4 X 2212 1400kv motors  with 7X4 prop.. these would be control motors
2 X D750 750kv motors driving 12x3.8 prop.. the lift motors

The 4 smaller motors were connected to a Flight Controller in the standard manner. The Rx outputs used were ch 1 to 6. The 2 lift motors were on Rx output ch 7 and 8 , activated with a switch.
Different setups were tried.. (benefits of OpenTX TX allowed custom mixes)
1. ch 7 & 8 output was mapped straight to ch 4 ..i.e throttle without any changes
2. ch 7 & 8 output was mapped to the input from a pot in the Tx
3. ch 7 & 8 output was a mix (50/50) of ch 4 (throttle) and the pot mentioned in point 2

All the airframe setups and lift motor setups were flown. for setup B the quad W/O the lifting motors had to be tuned first (due to the size issue).
From a stability and control aspect the best setup was Airframe setup C, followed by B and then  A.. B had the least flight time... (due to one of the lift props being the propwash of the other in one segment only.. vibrations).

Happy flying
Rahul
(myrahulroy.blogspot.com)

Interesting.
Wonder what would have happened if instead of 4x2212 plus 2xDT750, you had used  just 4 DT750s on the arms...

it would be a normal quad .. all props of same size and rpm.. though DT750

I wanted to use large dia and low rpm props to do most of the lifting .. and stability and control through smaller props... latter on scale up just the 2 lifting props to even bigger dia..20" while the other 4 props would remain the same....and subsequently use IC engine just for the lifting props.... long duration and load.