One for the physicists -

 

It would seem obvious that the lower rotor in a Y6 (or any vertical rotor set-up) is working in a whole different way to any rotor in a hexa set-up.  The fact that it's pushing down "pre-pushed" air suggests, intuitively, that it's not working as hard as it's upper neighbor and therefore not generating as much lift.  

 

So is a Y6 weaker than a hexacopter, with every other spec identical?

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Yes, the basic concepts of momentum theory treat the swept area as an "actuator disk" for calculations, including the inferential solidity ratio. But in the end, everything gets multiplied by the Figure of Merit (FM), which as we know, is the holy grail of blade design. That's where all the art goes, and at some level it IS an art; fluid dynamics is still an area where empirical data rules. Most of the formulas are in fact derived from the data, rather than used to predict the data as in most of physics. It's almost hilarious that most of the complex-looking calculus equations in graduate-level rotorcraft aerodynamics textbooks have "just because" (empirically derived) multipliers in them. There IS no substitute for testing!

That's why the work Dr. Selig and others are doing is in the area of refining the testing regimes. Until the electric-powered UAV boom, there was not much effort (no grant money) in researching lift in the sub-million Reynolds number ranges, where air doesn't behave the way we think it ought to.  It's funny, actually, that modelers resort to 5-10 thousand RPMs to get the Re numbers up so the scaled propellers start to work acceptably (or at all), precisely because little attention has been paid to low Re propulsive aerodynamics. Of course, part of this is because a good low Re propeller wouldn't look very cool mounted on the front of a model P-51.

I followed the MK link (after fixing it) and the graphs are revealing. Note that the propellers tested likely have a FM of about 20 given the present research. Imagine if that were 40, doubling the thrust? I have reason to believe that even 80 is achievable. What would it mean to our craft if we had four times the thrust per watt, without any added disk area and only a modest (but unfortunately necessary) increase in rotational inertia? It IS possible.

(I can't help but point out that the Germans' little 4X4 weed-whacker was the first to lift a live person precisely because they availed themselves of >>30C batteries to overcome the limitations of off-the-shelf propellers, rather than do the real development work in aerodynamics. Wait until they try to scale that thing - methinks they're in for a rude awakening.)

I agree that heat-cured layups are the best for CF, but we of limited budgets have to make do with room-temperature epoxy. A forward-sweeping blade sounds awesomely radical. Please post a picture.

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