Returning to the data provided by T-motor for their U7 units, we can see the issues we face during forthcoming flight-tests this month. In truth we're underpowered, though that would have been the case with any number of aircraft flying today: like fine wines most airframes improve their performance as the years go by whilst the power-plants improve both their power output and efficiency along the way.
Sadly the above table is all that T-motor provide, and although the motor is slated as a 22.20V design the figures are for 24V and thus we lose 10% at the get-go. The vehicle will have a gross weight of around 16 to 17 kilos once fitted out, or well under the 20kg budget. Nonetheless the motors have to run at a middling RPM that leaves a degree of slack for variations in thrust necessary to stabilise it in the event of wind gusts, or yet simply to steer it in three dimensions.
We can see from the table that at 65% the motors will therefore produce around 2kg of thrust apiece, and altogether more at 75%. This should be sufficient even given the losses attributable to propellers on the lower quad operating in the efflux of those on the upper.
I've included the operating temperatures for our general amusement, which T-motor describe as a lawsuit avoiding HOT. Accordingly I have gone for higher amperage speed controllers, as they are the aspects most likely to fail under the considerable currents involved. Interestingly too, they are MORE likely to fail when reducing the RPM of each motor, presumably because of the heat that the necessary resistance generates.
You can thus see how my Plan B ~ that involves operating the lower quad as a form of 'collective pitch' providing only thrust ~ is a very cunning plan indeed. Running at close to 100% these motors provide practically twice the lift they would otherwise, while at the same time protecting the most failure-prone component. This is an overwhelming argument in favour of four foot-level rotors, even were it the case that the upper quad supported a failure-redundant conventional X-8 octocopter.
The cost of the rebuild (discounting electrical equipment retained from the previous prototype along with the mannekin) was around £300 besides the time involved and travelling expenses. A drop in the ocean compared to spend to date, and source of comfort for those who chose the round-the-world cruise instead.
