Ka-50 vs Mi-8 engine power performance question

[Griffin]

Here are charts for the Ka-50 to climb speed depending on the weight of the helicopter.

 

Wheeere the heck did you get those?

[esb77]

To sum up the explanations so far:

 

How powerful a heli feels in hover is a function of the available excess positive lift.

 

The Mi-8's larger rotor is more efficient at converting power into lift than the Ka-50's smaller rotors.

 

Both helis have a similar amount of engine power.

 

So for a similar take off weight the Mi-8 will tend to have more available excess lift.

 

 

 

Also note that calculating the lift that a helicopter can generate is still fairly complicated, even if you simplify a lot of things. Engine power is a factor, but there are many others, some of which may have a greater effect on the final output.

 

Hope that makes it clear for anyone still confused.

[NeilWillis]

Finally, some sense!

[ShuRugal]

I ask again ... what does the rotor area tell someone when comparing helos? I still think, the more relevant information would be the total blade surface area (and blade profile).

 

Example: two helos with the same rotor diameter - i.e. same disc area - but different number of blades - which will more likely perform better?

 

Rotor area for a helicopter is almost directly analogous to wing area on a plane. With rotors turning at speed, the rotor disk of a helicopter behaves aerodynamically like a solid surface. From this, disk loading can be calculated the same way that wing-loading is (IE kg/m^2) and performance measures such as pitch and roll rates can be inferred.

 

Adding rotor blades to a helicopter has the same effect as adding blades to the prop of a plane: it enables a larger engine to put more power into the air, but at the cost of significant drag increase. chord and profile of the blades also factor into this: a fatter blade will take a bigger bite out of the air, but will have higher drag as well. This makes it possible to transfer more total engine power to the air, but less efficiently.

 

Practical upshot: if you want to increase the power output of a helicopter, the best thing to do is lengthen the blades. Once this has been done, if you still need to transfer more power, you make the blades fatter. Past a certain point, the control forces required to manipulate those fatter blades becomes too much, so you have to start adding more blades.

 

to answer your example scenario, it depends. Assuming that you were to take a helicopter engineered to be 2-bladed, and then stuck a 4-bladed head on its twin, the 4-blade bird will suffer apparant power loss, because the 4-bladed head is less efficient and the existing 2-bladed system was (presumably) engineered to take full advantage of the available engine power. However, if you take a 4-bladed bird and stick a 2-bladed head on it, then the 2-bladed head will lose power, because it won't be able to trasfer all the available engine power into the air. As for aerobatic performance (IE: pitch and roll rates) the 2 and 4 blade helicopter will perform close to the same in both examples.

Edited September 6, 2014 by ShuRugal