Blackeye

And I'm not saying that you can deduct all flight parameters by just looking at those stats. However the original questions was concerning the Ka-50 feeling underpowered compared to the Mi-8 hovering in 2,500m and 11,500kg load. Since I don't have first or second hand experience with either aircraft, all I can do is look at the relevant numbers. The Ka-50 has a 17% lower MTOW (which was actually exceeded in this test) and a smaller rotor area, which makes me think that the observed behavior is plausible.

Not sure why you consider comparing MOTWs spurious - it is after all the maximum weight the aircraft is supposed to take off with, regardless of design. Yes, rotor areas won't tell you everything but still are an indicator for the helicopters performance.

In general I'd say the coaxial design is more efficient and should be able to generate more lift out of the same engine power if you design your rotors accordingly. With the Ka-50 they chose to trade in some of that lifting capacity for higher speeds and smaller footprint. Not necessarily. It does eliminate some problems in (fast) forward flight like asymmetric lift and interference between main and tail rotor though. It's also yields a better maneuverability and is generally more efficient. Check out the BS2 flight manual chapter 5 for some more information. How you use that efficiency is up to the designers: Do you want a small, fast and agile helicopter? Then you have to sacrifice some lifting capacity. Do you want to lift more? Then you won't be able to fly that fast. However those decisions are not limited to coaxial designs.

I'm not sure Newtons Laws have a lot to do with this. It's been a while but I think it's down to thermodynamics: you have some place hot and transfer the heat to somewhere cold and use some of that energy to generate mechanical movement, i.e. a heat engine. The ideal representation of this is the carnot heat engine and according to Carnots theorem no heat engine can be more efficient than this theoretical device. The efficiency of a carnot heat engine solely depends on the absolute temperatures of the hot and cold reservoirs: n = 1 - Tc/Th For a piston engine these temperatures are strongly linked to the compression, i.e. a higher compression results in a larger difference between Tc and Th and thus a more efficient engine. (that's why Diesel engines are generally more efficient than Otto engines) Real engines of course have additional losses like friction.

Maximum take off weight, rotor diameter and area for both helicopters according to the manuals: Ka-50: 10,800 kg, 14.45 m , 164 m^2 (x2) Mi-8 : 13,000 kg, 21.29 m , 356 m^2 So the Ka-50 is supposed to be less powerful in that regard, probably because a high MTOW is not the primary goal when designing an attack helicopter - it should be faster and more maneuverable though.