How many coaxial rotors could you theoretically stack on top of a helicopter before the design becomes infeasible?

Question

All coaxial-rotor helicopters in existence today have only 2 counter rotating rotor blades. An example of such a helicopter is the Kamov Ka-50. Coaxial-rotor helicopters have one big advantage compared to single-rotor helicopters and that is that they can lift a lot more.

My question is: Has anyone ever tried putting 4 counter-rotating rotors on a helicopter in order to increase its lift capacity even more? Would placing 4 or more stacked rotors on top of a helicopter be feasible?

Answer 1

Coaxial rotors don't have an advantage in lifting power because they have more rotors. The main advantage in that respect is that they don't need a tail rotor to maintain yaw control, meaning all the engine power can drive the main rotors.

Putting two rotors on the same axis increases the complexity quite a bit, but lets you remove the tail rotor. Putting four rotors on the same axis increases the complexity a lot more, for basically no extra benefit.

But it has been tried. Kind of. If you count a rotor made of biplane wings as two rotors, anyway.

Answer 2

Coaxial-rotor helicopters have one big advantage compared to single-rotor helicopters and that is that they can lift a lot more

Coaxial rotor helicopter do not lift more than conventional helicopters: the same 7,700 kg (16,976 lb) of the Kamov Ka-50 can be easily lifted by the single rotor of the H175 for example.

And as normally wrongly thought, coaxial helicopters do not even have the advantage that, missing the tail rotor, all the engine power goes in the main rotors: aerodynamic interferences between the two rotors make the lower one loose some 10 to 15% efficiency; and since in a conventional helicopter the tail rotor eats some 10 to 15% of the power available, let's just call it even.

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Coaxial helicopters have only one advantage in respect to conventional helicopters: compactness.

Indeed, stacking two rotor one above the other, means that:

• the tail rotor with its tailboom is no more required and;
• the rotor diameter can be reduced, being the thrust the same.

Being more compact a higher manoeuvrability can be expected which is for sure important for military helicopters.

Anyway this brings some disadvantages:

1. a horizontal tailplane (and the relevant tailboom) is still needed for stability, so you don't really get rid of the whole tailboom;
2. diameter of the rotor cannot be reduced too much before running into performance degradations;
3. as said, aerodynamic interferences between the rotors make the lower rotor loose some 10 to 15% efficiency;
4. rotor head becomes packed with rods and other rotating mechanisms increasing (a lot) the total aerodynamic drag and mechanical complexity (picture source);
5. so many exposed mechanical part might fail and are for sure a target in military operations;
6. the second rotor atop shifts the CG upward which might reduce stability.
7. and for sure I'm forgetting something.

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For all these reasons a coaxial design is really important where compactness is at a premium. Packing more rotors one on top of the other will only worsen everything, especially points 3., 4. and 5., without bringing any improvement.